PATENT DOCUMENT

Publication Number: US-11432766-B2
Application Number: US-201816118282-A
Country: US
Kind Code: B2

Title: Wearable electronic device with electrodes for sensing biological parameters

Abstract:
An electronic device, such as a watch, has a housing to which a carrier is attached. The carrier has a first surface interior to the electronic device, and a second surface exterior to the electronic device. A set of electrodes is deposited on the exterior surface of the carrier. An additional electrode is operable to be contacted by a finger of a user of the electronic device while the first electrode is positioned against skin of the user. The additional electrode may be positioned on a user-rotatable crown of the electronic device, on a button of the electronic device, or on another surface of the housing of the electronic device. A processor of the electronic device is operable to determine a biological parameter of the user based on voltages at the electrodes. The biological parameter may be an electrocardiogram.

Claims:
What is claimed is: 
     
       1. An electronic watch, comprising:
 a housing; 
 a crown comprising:
 a crown body; and 
 a shaft connected to the crown body and passing through the housing; 
 
 a transparent carrier connected to the housing and defining a first portion of a back exterior surface of the electronic watch; 
 a transparent cover connected to the housing and defining at least a portion of a front exterior surface of the electronic watch; 
 a touch-sensitive display at least partially within the housing and viewable through the transparent cover; 
 a first electrode comprising a conductive coating positioned on the transparent carrier, the conductive coating defining a second portion of the back exterior surface of the electronic watch and configured to contact a body of a user when the electronic watch is being worn; 
 a second electrode on the crown body; and 
 a processor within the housing and operationally connected to the first electrode and the second electrode; wherein: 
 the first electrode is configured to measure a first voltage; 
 the second electrode is configured to measure a second voltage; 
 the processor is configured to determine an electrocardiogram using the first voltage and the second voltage; and 
 the touch-sensitive display is configured to display the electrocardiogram. 
 
     
     
       2. The electronic watch of  claim 1 , wherein:
 the touch-sensitive display is further configured to display a graphic other than the electrocardiogram; and 
 the touch-sensitive display is further configured to change from displaying the graphic to displaying the electrocardiogram. 
 
     
     
       3. The electronic watch of  claim 2 , wherein the touch-sensitive display is further configured to change from displaying the graphic to displaying the electrocardiogram in response to a crown input. 
     
     
       4. The electronic watch of  claim 1 , wherein:
 the crown is configured to rotate about an axis of rotation and translate along the axis of rotation; and 
 the axis of rotation extends along a center of the shaft. 
 
     
     
       5. The electronic watch of  claim 1 , wherein the crown further comprises:
 an electrically insulating split around the crown body; and 
 a trim around the electrically insulating split; wherein:
 the electrically insulating split is configured to electrically insulate the trim from the crown body; and 
 the crown body comprises the second electrode. 
 
 
     
     
       6. The electronic watch of  claim 1 , wherein the shaft and the crown body are integrally formed with one another. 
     
     
       7. The electronic watch of  claim 1 , further comprising an insulating coating on at least one of the crown body, the shaft, or the housing. 
     
     
       8. An electronic watch, comprising:
 a housing; 
 a carrier attached to a rear of the housing and formed from an optically transparent material and attached to the housing, the carrier defining:
 an inner surface; and 
 an outer surface; 
 
 a conductive coating positioned on the outer surface of the carrier and defining a first electrode on the carrier; 
 a crown extending through the housing and configured to translate and rotate, comprising a second electrode; and 
 a processor operable to determine a biological parameter of a user based on voltages measured at the first electrode and the second electrode; wherein: 
 the voltages are measured while the user is in contact with the first electrode and the second electrode. 
 
     
     
       9. The electronic watch of  claim 8 , further comprising:
 a collar receiving the crown; 
 a first collar coating on the collar and configured to reduce friction between the collar and the crown; and 
 a second collar coating on the first collar coating and configured to obscure the first collar coating. 
 
     
     
       10. The electronic watch of  claim 8 , wherein:
 the crown comprises:
 a shaft extending through an opening in the housing; and 
 a crown body integrally formed with the shaft, exterior to the housing, and configured to be rotated by the user; 
 
 the conductive coating extends along a portion of the inner surface of the carrier, around an edge of the carrier, and along a peripheral portion of the outer surface of the carrier; 
 the second electrode is a surface of the crown body; and 
 the crown body and the shaft are in electrical communication with the processor. 
 
     
     
       11. The electronic watch of  claim 8 , further comprising:
 a transparent cover attached to the housing; and 
 a display viewable through the transparent cover; wherein
 rotating the crown changes a graphic on the display in a first manner; 
 translating the crown changes the graphic in a second manner; and 
 the biological parameter is an electrocardiogram. 
 
 
     
     
       12. The electronic watch of  claim 8 , wherein one of rotating or translating the crown initiates determining the biological parameter. 
     
     
       13. The electronic watch of  claim 8 , further comprising a touch-sensitive display at least partially within the housing; and wherein:
 touching the touch-sensitive display initiates determining the biological parameter. 
 
     
     
       14. The electronic watch of  claim 13 , wherein:
 the biological parameter is a first biological parameter; and 
 the electronic watch further comprises an optical sensor subsystem configured to sense a second biological parameter through the carrier. 
 
     
     
       15. The electronic watch of  claim 14 , wherein:
 the first biological parameter is an electrocardiogram; and 
 the second biological parameter is a heart rate. 
 
     
     
       16. The electronic watch of  claim 8 , further comprising a touch-sensitive display; wherein:
 the biological parameter is an electrocardiogram; and 
 the electrocardiogram is shown on the touch-sensitive display. 
 
     
     
       17. A method for determining and displaying an electrocardiogram by an electronic watch, comprising:
 measuring a first voltage at a first electrode on a crown of the electronic watch; 
 measuring a second voltage at a second electrode on a transparent carrier of the electronic watch, the second electrode defined by a conductive coating disposed on an outer surface of the transparent carrier, the conductive coating defining at least a portion of a back exterior surface of the electronic watch; 
 determining, by a processor of the electronic watch, the electrocardiogram using the first voltage and the second voltage; and 
 displaying the electrocardiogram on a display of the electronic watch. 
 
     
     
       18. The method of  claim 17 , further comprising:
 sensing a touch on the crown of the electronic watch; wherein: 
 initiating measuring the first voltage and measuring the second voltage occurs in response to sensing the touch. 
 
     
     
       19. The method of  claim 17 , further comprising:
 sensing a touch on the display of the electronic watch; wherein: 
 initiating measuring the first voltage and measuring the second voltage occurs in response to sensing the touch. 
 
     
     
       20. The method of  claim 17 , further comprising:
 measuring a third voltage at a third electrode on the transparent carrier of the electronic watch; and 
 the operation of determining, by the processor of the electronic watch, the electrocardiogram using the first voltage and the second voltage comprises determining, by the processor of the electronic watch, the electrocardiogram using the first voltage, the second voltage, and the third voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application No. 62/554,196, filed on Sep. 5, 2017, and entitled “Wearable Device with Electrodes for Sensing Biological Parameters,” and U.S. Provisional Patent Application No. 62/644,886, filed on Mar. 19, 2018, and entitled “Wearable Device with Electrodes for Sensing Biological Parameters,” the contents of which are incorporated herein by reference as if fully disclosed herein. 
    
    
     FIELD 
     The described embodiments relate generally to an electronic watch or other wearable electronic device. More particularly, the described embodiments relate to techniques for providing, on a watch or other wearable electronic device, electrodes for sensing biological parameters. The electrodes may be variously provided on a surface of an optical component, crown, button, or housing member of the watch or other wearable electronic device. 
     BACKGROUND 
     A wearable electronic device may include a set of sensors for determining a set of biological parameters of a user that wears the wearable electronic device. Circuitry associated with the set of sensors may generate, for example, electrical signals or measurements corresponding to voltages at, forces applied to, or amounts of light incident on, the sensors. The various signals or measurements may be correlated to, or used to derive, various biological parameters of the user, such as a heart rate of the user. 
     SUMMARY 
     Embodiments of the systems, devices, methods, and apparatuses described in the present disclosure are directed to an electronic watch or other wearable electronic device having a set of electrodes that may be used to sense or determine biological parameters of a user that wears the wearable electronic device. The biological parameters may include, for example, an electrocardiogram (ECG) of the user. 
     One embodiment takes the form of an electronic watch, comprising: a housing; a crown comprising: a crown body; and a shaft connected to the crown body and passing through the housing; a carrier connected to the housing; a transparent cover connected to the housing; a touch-sensitive display at least partially within the housing and viewable through the transparent cover; a first electrode on the carrier; a second electrode on the crown body; and a processor within the housing and operationally connected to the first electrode and the second electrode; wherein: the first electrode is configured to measure a first voltage; the second electrode is configured to measure a second voltage; the processor is configured to determine an electrocardiogram using the first voltage and the second voltage; and the touch-sensitive display is configured to display the electrocardiogram. 
     Another embodiment takes the form of an electronic watch, comprising: a housing; a carrier attached to the housing; a first electrode on the carrier; a crown extending through the housing and configured to translate and rotate, comprising a second electrode; and a processor operable to determine a biological parameter of a user based on voltages measured at the first electrode and the second electrode; wherein: the voltages are measured while the user is in contact with the first electrode and the second electrode. 
     Yet another embodiment takes the form of a method for determining and displaying an electrocardiogram by an electronic watch, comprising: measuring a first voltage at a first electrode on a crown of the electronic watch; measuring a second voltage at a second electrode on a carrier of the electronic watch; determining, by a processor of the electronic watch, the electrocardiogram using the first voltage and the second voltage; and displaying the electrocardiogram on a display of the electronic watch. 
     In addition to the example aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: 
         FIG. 1A  shows a functional block diagram of a wearable electronic device; 
         FIG. 1B  shows an example of an electronic device having a set of electrodes disposed thereon; 
         FIGS. 2A-2C  show an example of an electronic watch that incorporates a set of electrodes; 
         FIG. 3  shows another example of an electronic watch that incorporates a set of electrodes; 
         FIGS. 4A-4D  show an additional example of an electronic watch that incorporates a set of electrodes on a carrier; 
         FIGS. 5A-5E  illustrate an example of coatings that may be deposited on the interior and exterior surfaces of the carrier shown in  FIGS. 4A-4C ; 
         FIG. 6  shows a cross-section of the carrier shown in  FIG. 5B ; 
         FIG. 7  shows a cross-section of the carrier shown in  FIGS. 5C and 5D ; 
         FIG. 8  shows an example layer construction of an ITO-based electrode; 
         FIGS. 9A-9C  show alternative electrical connections between an electrode on an exterior surface of a carrier that forms part of a housing of an electronic device and an electrical contact interior to the electronic device; 
         FIGS. 10A-10D  show alternative carrier configurations, and alternative attachments or connections of carriers to other housing members of an electronic device; 
         FIG. 11A  is a cross-section of an example crown assembly; 
         FIG. 11B  is a cross-section of another example crown assembly; 
         FIGS. 12A &amp; 12B  show another example of a crown assembly; 
         FIGS. 13 &amp; 14  show cross-sections of additional examples of crown assemblies; 
         FIGS. 15-22  show various examples of button assemblies; 
         FIG. 23  shows a schematic of an electronic device that may be used for acquiring an ECG or other biological parameter from a user of the electronic device; 
         FIG. 24  shows an example method of determining a biological parameter of a user wearing a watch or other wearable electronic device; 
         FIG. 25  shows a sample electrical block diagram of an electronic device such as a watch or other wearable electronic device; 
         FIG. 26A  illustrates a sample electronic watch displaying a list; 
         FIG. 26B  illustrates the sample electronic watch of  FIG. 26A , with an updated list in response to a crown input; 
         FIG. 27A  illustrates a sample electronic watch displaying a graphic; 
         FIG. 27B  illustrates the sample electronic watch of  FIG. 27A  with the graphic updated in response to a crown input; 
         FIG. 28A  illustrates a sample electronic watch displaying a first graphic; and 
         FIG. 28B  illustrates the sample electronic watch of  FIG. 28A  displaying a second graphic in response to a crown input. 
     
    
    
     The use of cross-hatching or shading in the accompanying figures is generally provided to clarify the boundaries between adjacent elements and also to facilitate legibility of the figures. Accordingly, neither the presence nor the absence of cross-hatching or shading conveys or indicates any preference or requirement for particular materials, material properties, element proportions, element dimensions, commonalities of similarly illustrated elements, or any other characteristic, attribute, or property for any element illustrated in the accompanying figures. 
     Additionally, it should be understood that the proportions and dimensions (either relative or absolute) of the various features and elements (and collections and groupings thereof) and the boundaries, separations, and positional relationships presented therebetween, are provided in the accompanying figures merely to facilitate an understanding of the various embodiments described herein and, accordingly, may not necessarily be presented or illustrated to scale, and are not intended to indicate any preference or requirement for an illustrated embodiment to the exclusion of embodiments described with reference thereto. 
     DETAILED DESCRIPTION 
     Reference will now be made in detail to representative embodiments illustrated in the accompanying drawings. It should be understood that the following description is not intended to limit the embodiments to one preferred embodiment. To the contrary, it is intended to cover alternatives, modifications, and equivalents as can be included within the spirit and scope of the described embodiments as defined by the appended claims. 
     The following disclosure relates to techniques for distributing a set of electrodes over a set of surfaces of a wearable electronic device, such as an electronic watch, and to techniques for electrically isolating the electrodes from other components of the device and/or mitigating effects of environmental factors when sensing voltages or signals indicative of one or more biological parameters of a user who is in contact with the electrodes, and to techniques for routing the voltages or signals within the device. 
     Embodiments further may take the form of an electronic watch, or other portable and/or wearable device, configured to detect an electrocardiogram (“ECG”) of a person wearing or otherwise interacting with the electronic device. As one non-limiting example, a person may wear an electronic watch that has two external electrodes configured to be touched by the user. A first electrode may be placed on a rear surface of the watch and be in contact with skin on the wrist of the person. A second electrode may be defined by or on a crown of the watch and may be configured to be touched by a finger (or other body part) of the person. 
       FIG. 1A  shows a functional block diagram of a wearable electronic device  100 . In some examples, the device  100  may be an electronic watch or electronic health monitoring device. The wearable electronic device  100  may include one or more input devices  102 , one or more output devices  104 , and a processor  106 . Broadly, the input devices  102  may detect various types of input, and the output devices  104  may provide various types of output. The processor  106  may receive input signals from the input devices  102 , in response to inputs detected by the input devices. The processor  106  may interpret input signals received from one or more of the input devices  102  and transmit output signals to one or more of the output devices  104 . The output signals may cause the output devices  104  to provide one or more outputs. Detected input at one or more of the input devices  102  may be used to control one or more functions of the device  100 . In some cases, one or more of the output devices  104  may be configured to provide outputs that are dependent on, or manipulated in response to, the input detected by one or more of the input devices  102 . The outputs provided by one or more of the output devices  104  may also be responsive to, or initiated by, a program or application executed by the processor  106  and/or an associated companion device. 
     In various embodiments, the input devices  102  may include any suitable components for detecting inputs. Examples of input devices  102  include audio sensors (e.g., microphones), optical or visual sensors (e.g., cameras, visible light sensors, or invisible light sensors), proximity sensors, touch sensors, force sensors, mechanical devices (e.g., crowns, switches, buttons, or keys), vibration sensors, orientation sensors, motion sensors (e.g., accelerometers or velocity sensors), location sensors (e.g., global positioning system (GPS) devices), thermal sensors, communication devices (e.g., wired or wireless communication devices), resistive sensors, magnetic sensors, electroactive polymers (EAPs), strain gauges, electrodes, and so on, or some combination thereof. Each input device  102  may be configured to detect one or more particular types of input and provide a signal (e.g., an input signal) corresponding to the detected input. The signal may be provided, for example, to the processor  106 . 
     The output devices  104  may include any suitable components for providing outputs. Examples of output devices  104  include audio output devices (e.g., speakers), visual output devices (e.g., lights or displays), tactile output devices (e.g., haptic output devices), communication devices (e.g., wired or wireless communication devices), and so on, or some combination thereof. Each output device  104  may be configured to receive one or more signals (e.g., an output signal provided by the processor  106 ) and provide an output corresponding to the signal. 
     The processor  106  may be operably coupled to the input devices  102  and the output devices  104 . The processor  106  may be adapted to exchange signals with the input devices  102  and the output devices  104 . For example, the processor  106  may receive an input signal from an input device  102  that corresponds to an input detected by the input device  102 . The processor  106  may interpret the received input signal to determine whether to provide and/or change one or more outputs in response to the input signal. The processor  106  may then send an output signal to one or more of the output devices  104 , to provide and/or change outputs as appropriate. Examples of suitable processors are discussed in more detail below with respect to  FIG. 25 . 
     In some examples, the input devices  102  may include a set of electrodes. The electrodes may be disposed on one or more exterior surfaces of the device  100 . The processor  106  may monitor for voltages or signals received on at least one of the electrodes. In some embodiments, one of the electrodes may be permanently or switchably coupled to a device ground. The electrodes may be used to provide an ECG function for the device  100 . For example, a 2-lead ECG function may be provided when a user of the device  100  contacts first and second electrodes that receive signals from the user. As another example, a 3-lead ECG function may be provided when a user of the device  100  contacts first and second electrodes that receive signals from the user, and a third electrode that grounds the user to the device  100 . In both the 2-lead and 3-lead ECG embodiments, the user may press the first electrode against a first part of their body and press the second electrode against a second part of their body. The third electrode may be pressed against the first or second body part, depending on where it is located on the device  100 . 
       FIG. 1B  shows an example of an electronic device  110  (here, an electronic watch) having a set of electrodes  112 ,  114  disposed thereon. The device  110  may be an example of the wearable electronic device described with reference to  FIG. 1A , or may be an example of an electronic device that is not wearable. In some embodiments, the set of electrodes  112 ,  114  may be provided on one surface of the device  110 . In other embodiments (as shown), the set of electrodes  112 ,  114  may include electrodes provided on different surfaces of the device  100 , such as a first electrode  112  provided on a first surface  116  of the device  110 , and a second electrode  114  provided on a second surface  118  of the device  110 . Providing electrodes on different surfaces of a device may make it easier for a user to place different body parts in contact with different electrodes. For example, a user may place one or more of the electrodes (e.g., the first electrode  112 ) in contact with their wrist, and may touch another one or more of the electrodes (e.g., the second electrode  114 ) with a finger of their opposite hand. Alternatively, the user may press the electrodes  112 ,  114  against different parts of their body. A processor  120  of the device  110 , or a processor remote from the device  110 , may determine, from the voltages or signals (e.g., from stored digital samples or values representing the voltages or signals), the biological parameter(s) of the user. The biological parameter(s) may include, for example, an electrocardiogram (ECG) of the user, an indication of whether the user is experiencing atrial fibrillation, an indication of whether the user is experiencing premature atrial contraction or premature ventricular contraction, an indication of whether the user is experiencing a sinus arrhythmia, and so on. 
     In some embodiments, one or two thin film electrodes may be PVD deposited on an exterior surface of a structure that forms part of a housing of an electronic device. The surface may be any transparent, semi-transparent, translucent, or opaque surface made out of an amorphous solid, glass, a crystal or crystalline material (such as sapphire or zirconia), plastic, or the like. In the case of a watch (i.e., a type of electronic device), an additional electrode may be positioned on a user-rotatable crown of a watch body, on a button of the watch body, or on another surface of a housing that defines the watch body. 
     When an electrode is formed on a carrier that forms part of a housing of an electronic device, the electrode may be connected to an electrical contact within the electronic device by depositing the electrode material such that it wraps around an edge or perimeter of the carrier, and onto an interior surface of the carrier. The electrical contact may be on the interior surface of the carrier. In other embodiments, the electrode may be formed on the exterior surface of the carrier, and a thru-carrier via that is filled or coated with a conductive material may connect the electrode to an electrical contact within the electronic device. The carrier may be any appropriate structure that supports the electrodes, on which the electrodes are formed, or to which the electrodes are attached. In certain embodiments described herein, the carrier is an optically transparent material having a dome shape. It should be appreciated that the carrier may have different shapes (flat, stepped, parallelepiped, and so on) and may be made from different materials, including opaque materials. 
     Generally, the term “attached” means that two elements, objects, structures, or objects are separate but affixed or retained to one another, whether removably, as with an electronic device attached to a user by a band, or fixedly, as with two elements that are affixed to one another with a mechanical fastener not meant to be decoupled (a screw, bolt, or the like), by an adhesive, by plating or depositing one material on another (as with an electrode deposited on the carrier), and so on. The term “connected” means that two elements may be attached to one another, or may be two parts of a unitary whole (as with a shaft and crown body formed from the same material as a single piece). Thus, while two elements that are attached to one another are necessarily connected to one another, the reverse is not necessarily true. For example, two elements may be formed as a single piece or part and thus connected to one another, although they are not attached to one another. 
     When an electrode is provided on a crown of an electronic device, the crown may be conductive or have a conductive surface, and the conductive portion of the crown may be coupled to a conductive rotatable shaft that extends through an opening in a device housing. An end of the shaft interior to the housing, or a conductive shaft retainer interior to the housing, may be in mechanical and electrical contact with a spring-biased conductor that carries electrical signals between the shaft or shaft retainer and a circuit, thereby providing electrical communication between the crown and the circuit. 
     A processor of an electronic device (e.g., the processor  120 ) may be operable to determine a biological parameter of a user based on voltages at various electrodes (e.g., at the set of electrodes  112 ,  114 ). In some cases, the biological parameter may be an ECG of a user of the electronic device. For example, when a watch has a first electrode on an exterior surface of a carrier and a second electrode on a crown, the user&#39;s fastening of the watch to their wrist may place the first electrode in contact with skin on the user&#39;s wrist. To acquire an ECG, the user may touch a conductive portion of the crown with a finger on their opposite hand. For example, the carrier or housing of the watch may touch a wrist adjacent one hand, and the crown may be touched with a finger of the opposite hand. In some cases, the watch may have a third electrode, also on the exterior surface of the carrier, which grounds the user to the watch. The third electrode may be used to reject noise from ECG signals. The electrodes may be positioned on different surfaces, or different portions of surfaces, in various embodiments. 
     The electrode(s) on the exterior surface of the carrier may be positioned at the periphery of the carrier, or otherwise positioned to enable an optical sensor subsystem to emit and receive light through the carrier. The light may be emitted into, and reflected from, a user&#39;s skin to determine other biological parameters of the user, such as a heart rate, blood pressure, pulse, blood oxygenation, glucose level, and so on. 
     These and other embodiments are discussed with reference to  FIGS. 1-25 . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes only and should not be construed as limiting. 
       FIGS. 2A-2C  show an example of an electronic watch  200  that incorporates a set of electrodes. The watch  200  may be an example of the wearable electronic device  100  or  110  described with reference to  FIG. 1A or 1B . The watch  200  may include a watch body  202  and a watch band  204 . The watch body  202  may include an input or selection device, such as a crown  210  or a button  212 .  FIG. 2A  shows an isometric view of the watch body&#39;s front face.  FIG. 2B  shows an example cross-section of the crown  210 .  FIG. 2C  shows an isometric view of the watch body&#39;s rear face. In  FIGS. 2A &amp; 2C , only a portion of the watch band  204  is shown (i.e., only the portions of the watch band  204  that attach to the watch body  202 ). 
     The watch body  202  may include a housing  206 . The housing  206  may include a front side housing member  206   a  that faces away from a user&#39;s skin when the watch  200  is worn by a user (see  FIG. 2A ), and a back side housing member  206   b  (or rear cover) that faces toward the user&#39;s skin (see  FIG. 2C ). Alternatively, the housing  206  may include a singular housing member, or more than two housing members. The one or more housing members may be metallic, plastic, ceramic, crystal, or other types of housing members (or may include combinations of such materials). 
     As shown in  FIG. 2A , a transparent cover  208  may be attached to a front side of the watch body  202  (i.e., facing away from a user&#39;s skin), over or within an opening in the housing  206 , and may protect a display positioned at least partially within the housing  206 . The display may be viewable by a user through the cover  208 . In some embodiments, the display may depict an ECG waveform of a person who is wearing or otherwise using the watch  200 . In some cases, the cover  208  may be part of a display stack, which display stack may include a touch sensing or force sensing capability. The display may be configured to depict a graphical output of the watch  200 , and a user may interact with the graphical output (e.g., using a finger or stylus that touches or hovers over the cover  208 , or using the crown  210  or button  212 ). As one example, the user may select (or otherwise interact with) a graphic, icon, indicator, message, or the like (collectively, “graphic”) presented on the display by touching or pressing on the display at the location of the graphic. In some embodiments, the user may receive confirmation of their selection by means of haptic output provided by the watch body  202  through the display or cover  208 . The exterior surface of the cover  208  may therefore function as a means for receiving input (i.e., function as an input device) and a means for providing output (i.e., function as an output device). The cover  208  may be attached to the housing  206  or part of the housing  206  (e.g., connected to the housing). In some embodiments, the cover  208  may be considered part of the housing  206  because it forms part of an outer shell that defines an interior volume (or houses internal components) of the watch body  202 . In some examples, the cover  208  may be or include a crystal, such as a sapphire crystal. Alternatively, the cover  208  may be formed of glass, plastic, or other materials. 
     The watch body  202  may include at least one input device or selection device, such as a crown  210 , scroll wheel, knob, dial, button  212 , or the like, which input device may be operated by a user of the watch  200 . In some embodiments, the crown  210 , scroll wheel, knob, dial, button  212 , or the like may be conductive, or have a conductive surface, and a signal route may be provided between the conductive portion of the crown  210 , scroll wheel, knob, dial, button  212 , or the like and a circuit (including a processor) within the watch body  202 . 
     The operation of determining and/or displaying a user&#39;s ECG may be initiated by rotating the crown  210 , translating the crown, tilting the crown, touching the crown, and so on. Likewise the operation of determining the ECG may be initiated by interacting with a touch-sensitive cover  208  or display of the electronic watch. As discussed above, the display may be partially or fully within the housing of the electronic watch. 
     Turning primarily to  FIG. 2B , it is shown that the housing  206  may include an opening through which a shaft  224  extends. A crown  210  may be connected to the shaft  224 , and may be accessible to a user exterior to the housing  206 . The crown  210  may be manipulated by a user to rotate or translate the shaft, as indicated by arrows  218  and  220 . Such manipulations are examples of crown inputs. The shaft may be mechanically, electrically, magnetically, and/or optically coupled to components within the housing  206 . In some embodiments, the crown  210  may be part of a crown assembly, as described with reference to  FIG. 11, 12A, 12B, 13 , or  14 . 
     A user&#39;s manipulation of the crown  210  (and thus the shaft  224 ) may be used to manipulate or select various textual or graphical elements displayed by the watch  200 , to adjust a volume of a speaker, to turn the watch  200  on or off, and so on. In some embodiments, the crown  210  may be manipulated (e.g., rotated or pressed) to select or activate a health monitoring function of the watch  200  (e.g., an ECG or other heart monitoring function). For example, a user may rotate the crown to select an ECG application, and may press the crown to activate the ECG application (e.g., initiate determination and display of a wearer&#39;s ECG). Alternatively, a user&#39;s touch or press of the crown (or touch or press of the crown for a predetermined period of time) may activate the ECG application and cause a heart rhythm of the user to be displayed. As yet another example, the user may interact with the touch-sensitive display to select and/or activate the ECG application. By way of example, a user&#39;s activation of an ECG application is indicated by the watch&#39;s display of the ECG  222  in  FIG. 2A . Alternatively, the user&#39;s selection of an ECG application may be indicated by another graphic or text displayed by the watch  200 . Generally, the watch  200  (and specifically its display) may change from displaying some graphic or text to displaying the ECG  222  once the ECG (or its corresponding application, or function) is initiated, selected, or determined. 
     As shown in  FIG. 2B , the crown  210  may be connected to the shaft  224  (and may be unitary with the shaft), and the shaft  224  may extend through an opening in the housing  206 . In some embodiments, the shaft  224  may be separated from the housing  206  by a bushing or other component, or retained to the housing  206  by a retention mechanism. The shaft  224  may rotate or translate with respect to the housing  206 , as indicated by arrows  218  and  220 , thereby providing one or more crown inputs to a processor of the electronic device  200 . A first sensor  226  within the housing  206  may sense aspects of shaft movement such as direction of rotation, speed of rotation, rotational acceleration, or angular position of the shaft  224 . In some embodiments, the first sensor  226  may be an optical sensor positioned adjacent the shaft  224 , such that light  227  is emitted onto, and reflected from, the shaft  224 . Light  227  may be reflected from the shaft by a pattern of surface features (such as scallops, grooves, indentation, projections, or the like) or by byproducts of machining the shaft, such as bumps, scratches, irregularities, and so on. The pattern and speed of light  227  reflected onto the optical sensor  226  maybe used to determine a direction and/or speed of rotation of the shaft  224 . In other embodiments, different sensors may be used to detection direction and/or speed of rotation of the shaft  224 , including mechanical sensors, electrical sensors, capacitive sensors, brush contacts, magnetic sensors, and so on. 
     A second sensor  228  within the housing  206  may sense aspects of shaft movement such as translation or direction of translation. In some embodiments, the second sensor  228  may be a tactile switch, optical sensor, magnetic sensor, capacitive sensor or the like positioned at an end of the shaft  224 . 
     A third sensor  230  within the housing  206  may sense when a user is touching the crown  210 , or may sense signals (e.g., a heart rhythm) received by the crown  210  when a user touches the crown  210 . In some embodiments, the third sensor  230  may be electrically coupled to the crown  210  or shaft  224 . In some cases, the sensors  226 ,  228 ,  230  may be provide signals or information to the processor  214 , or may be partly or wholly integrated with the processor  214  or other components of the watch  200 . In some embodiments, two or more of the sensors  226 ,  228 ,  230  may be combined into a multipurpose sensor. In some embodiments, one or more of the sensors  226 ,  228 ,  230  may not be provided. In some embodiments, the functions of one of the sensors  226 ,  228 ,  230  may be distributed among multiple sensors, or additional crown sensors may be provided. 
     Any or all of the first sensor  226 , second sensor  228 , and third sensor  230  may be attached to or otherwise supported by one or more internal supports  232 , as shown in  FIG. 2B . 
     Turning primarily to  FIG. 2C , the housing  206  may include structures for attaching the watch band  204  to the watch body  202 . In some cases, the structures may include elongate recesses or openings through which ends of the watch band  204  may be inserted and attached to the watch body  202 . In other cases (not shown), the structures may include indents (e.g., dimples or depressions) in the housing  206 , which indents may receive ends of spring pins that are attached to or threaded through ends of a watch band to attach the watch band to the watch body  202 . 
     The watch band  204  may be used to secure the watch  200  to a user, another device, a retaining mechanism, and so on. 
     As previously mentioned, the watch  200  may include a set of electrodes. The set of electrodes may be configured, in some cases, as described with reference to  FIG. 1A or 1B . The set of electrodes may be used by a processor  214  that is internal to the watch body  202 , to sense biological parameters (e.g., an ECG) of a person who wears the watch  200  and presses the electrodes against their skin. In some embodiments, the set of electrodes may include a rear-facing electrode  216  on the back of the watch body  202  (e.g., on the back side housing member  206   b ). The set of electrodes may also include an electrode on the crown  210  and/or an electrode on the button  212 . 
     The rear-facing electrode  216  may be formed (e.g., printed, plated, or otherwise deposited) on the back side housing member  206   b . If the back side housing member  206   b  is non-conductive, the rear-facing electrode  216  may be formed directly on the back side housing member  206   b  and connected to circuitry internal to the watch body  202  (e.g., the processor  214 ) by, for example, conductive vias formed through the back side housing member  206   b . If the back side housing member  206   b  is conductive, the rear-facing electrode  216  may be separated from the back side housing member  206   b  by an insulator or insulating layer, and conductive vias formed through the back side housing member  206   b  may likewise be insulated from the back side housing member  206   b . Alternatively, the back side housing member  206   b  may have an opening to which the rear-facing electrode  216  is mated. In some embodiments, the opening may define a ledge in the back side housing member  206   b , and the rear-facing electrode  216  may rest on the ledge (and in some cases may be separated from the back side housing member  206   b  by an insulator (e.g., a seal) or an insulating layer). 
     The electrode(s) on the crown  210  or button  212  may be conductive surfaces of the crown  210  or button  212 . In some cases, the crown  210  or button  212  may be conductive over its entire exterior surface. In other cases, the crown  210  or button  212  may have conductive portions (e.g., cores or inserts). When the front side housing member  206   a  is conductive, the crown  210  or button  212  (or the conductive components thereof) may be insulated from the front side housing member  206   a  by an insulator (e.g., a set of seals, non-conductive coatings, and so on). 
     In some embodiments, one of the crown  210  or button  212  may have an electrode thereon, and a user wearing the watch  200  on one of their wrists may touch the electrode on the crown  210  or button  212  with a finger of their opposite hand. The processor  214  may then use the electrodes to acquire an ECG for the user. In other embodiments, both the crown  210  and the button  212  may have an electrode thereon, and a user wearing the watch  200  on one of their wrists may touch the electrodes on the crown  210  and button  212  with a finger of their opposite hand. In still other embodiments, the entirety of the back side housing member  206   b  (or even the entirety of the housing  206 ) may be an electrode. In these latter embodiments, electrical isolation may be provided between the housing  206  and the crown  210  and/or between the housing  206  and the button  212 . 
     In some examples, the watch  200  may lack the display, the crown  210 , or the button  212 . For example, the watch  200  may include an audio input or output interface, a touch input interface, a haptic (force) input or output interface, or other input or output interface that does not require the display, crown  210 , or button  212 . The watch  200  may also include the afore-mentioned input or output interfaces in addition to the display, crown  210 , or button  212 . When the watch  200  lacks the display, the front face of the watch  200  may be covered by the cover  208 , or by a metallic or other type of housing member (e.g., the opening for the cover  208  may not exist, and the front side housing member  206   a  may extend over the area defined by the cover  208 ). In these embodiments, the electrode(s) on the crown  210  or button  212  may be replaced by (or supplemented with) an electrode on the front face of the watch body  202 . A user may touch the front-facing electrode with a finger, similarly to how they would touch an electrode on the crown  210  and/or button  212 . Alternatively, a user could place the front-facing electrode in contact with, for example, an opposite wrist, part of their leg, or their torso or forehead. 
     In some embodiments, the watch  200  may lack the rear-facing electrode  216 , and each of the crown  210  and the button  212  may have a conductive surface that serves as an electrode. In these embodiments, the watch  200  may need to be removed from a user&#39;s wrist to enable the user to press different parts of their body against the crown and button electrodes. In some embodiments, the crown  210  or the button  212  may be moved to an opposite side of the watch body  202 , thereby increasing the separation between the crown  210  and the button  212  and making it easier for a user to press different parts of their body against the crown and button electrodes. 
     Other electronic devices that may incorporate a set of electrodes include other wearable electronic devices, other timekeeping devices, other health monitoring or fitness devices, other portable computing devices, mobile phones (including smart phones), tablet computing devices, digital media players, or the like. 
     Because the voltages or signals provided at, propagated from, or monitored at the various electrodes of a set electrodes may be low voltage or have low amplitudes, the materials, positions, electrical connections to, and electrical routing paths for the set of electrodes can have a significant impact on a processor&#39;s ability to discern useful signals representing an ECG or other biological parameter of a person wearing the watch  200  or a similar device (e.g., one of the other watches or electronic devices described herein). The materials, positions, electrical connections to, and electrical routing paths for the set of electrodes can determine how well the electrodes receive voltages/signals from the person&#39;s skin (e.g., a signal-to-noise ratio (SNR) of a device-to-user interface through which the voltages/signals pass); how well voltages/signals are transferred between the electrodes and internal components of the watch  200  (e.g., a voltage/signal propagation SNR); and how well the electrodes operate in the face of environmental factors, such as temperature, humidity, moisture, electromagnetic radiation, dust, and so on. Techniques described in the present disclosure may improve the usability of a set of electrodes under some or all of these conditions. 
       FIG. 3  shows another example of an electronic watch  300  that incorporates a set of electrodes. The watch  300  may be an example of the wearable electronic device  100  or  110  described with reference to  FIG. 1 or 1B , and may include many of the components of the watch  200  described with reference to  FIGS. 2A-2C . The watch  300  may include a watch body  202  and a watch band  204 .  FIG. 3  shows an isometric view of the watch body&#39;s rear face. Only a portion of the watch band  204  is shown (i.e., only the portions of the watch band  204  that attach to the watch body  202 ). 
     The watch  300  in  FIG. 3  differs from the watch  200  of prior figures in that it has a different set of internal components, a different back side housing member  302 , and a different set of elements that are provided or exposed on the back side housing member  302 . For example, the watch  300  may include a sensor subsystem that includes both electrical and optical components. The electrical components may include one or more electrodes  304 ,  306  formed on the back side housing member  302 . In some cases, each of the electrodes  304 ,  306  may have a circular shape and may be PVD deposited on the back side housing member  302 . Alternatively, only one, or more than two electrodes may be formed on the back side housing member  302 , or the electrodes  304 ,  306  may be positioned over (or inset into) openings in the back side housing member  302 . 
     The optical components of the sensor system may include a set of one or more windows  308 ,  310 ,  312 ,  314  in the back side housing member  302 . Each of the windows  308 ,  310 ,  312 ,  314  may pass at least one wavelength of light. In some cases, each of the windows  308 ,  310 ,  312 ,  314  may have a semicircular shape. The windows may alternatively have other shapes. The windows may be formed of crystal, glass, plastic or another material that passes at least one wavelength of light emitted or received by the sensor subsystem. 
     In some embodiments, the back side housing member  302  may be or include a transparent cover (e.g., a cover including a crystal, such as a sapphire crystal, or glass, or plastic, or the like), and may be substantially flat or planar (as shown) or may be curved or otherwise non-planar. A mask (e.g., an ink mask and/or dark mask) may be applied to the transparent cover to define the windows  308 ,  310 ,  312 ,  314 . The electrodes  304 ,  306  may be formed on top of the mask or over openings in the mask. 
     In some embodiments, the back side housing member  302  may be an opaque substrate, such as a metal or plastic substrate, and one or more transparent windows  308 ,  310 ,  312 ,  314  may be fitted to openings in the substrate. The transparent windows may be fitted to the openings internally to (or externally from) the watch body  202 . The electrodes  304 ,  306  may be fitted to additional openings that enable the electrodes  304 ,  306  to protrude outward from the external surface of the back side housing member  302 , or the electrodes  304 ,  306  may be formed on the surface of the back side housing member  302  and the electrically connected to components internal to the watch body  202  by conductive vias or other elements formed through the surface of the back side housing member  302 . 
     By way of example,  FIG. 3  shows the electrodes  304 ,  306  aligned along a first axis that divides the back side housing member  302  into two halves, and shows the windows  308 ,  310 ,  312 ,  314  aligned along a second axis, perpendicular to the first axis, that divides the back side housing member  302  into a different two halves. In this manner, the electrodes  304 ,  306  and windows  308 ,  310 ,  312 ,  314  may form four circular areas on the exterior surface of the back side housing member  302 , with the circular areas that contain the windows  308 ,  310 ,  312 ,  314  appearing bifurcated. 
     In use, each pair of windows  308 / 310 ,  312 / 314  forming a circular area may include a first window under which one or more light emitters are positioned, and a second window under which one or more light receivers are positioned, with an optional set of one or more light blocking walls positioned between the one or more light emitters and the one or more light receivers (or around the light emitter(s), or around the light receiver(s)). 
       FIGS. 4A-4D  show an additional example of an electronic watch  400  that incorporates a set of electrodes. The watch  400  may be an example of the wearable electronic device  100  or  110  described with reference to  FIG. 1A or 1B , and may include many of the components of the watch  200  described with reference to  FIGS. 2A-2C . The watch  400  may include a watch body  202  and a watch band  204 .  FIG. 4A  shows an isometric view of the watch body&#39;s rear face. Only a portion of the watch band  204  is shown (i.e., only the portions of the watch band  204  that attach to the watch body  202 ). 
     Similarly to the watch  300  described with reference to  FIG. 3 , the watch  400  may include a sensor subsystem that includes both electrical and optical components. However, the electrical and optical components of the watch  400  may be arranged differently than the electrical and optical components of the watch  300 . 
     Referring primarily to  FIG. 4A , a light-transmissive element such as a carrier  404  (e.g., a rear-facing or skin-facing carrier) may be coupled to or otherwise attached to a back side housing member  402  of the watch  400 , and in some cases may be considered to form a part of the housing  206  of the watch body  202 . The carrier  404  may have a first surface  406  that is interior to the watch body  202  (see  FIG. 4C ) and a second surface  408  that is exterior to the watch body  202  (see  FIG. 4A ). The carrier  404  may be dome-shaped or otherwise non-planar, as shown in  FIGS. 4A-4C , such that the second surface  408  protrudes or extends away from a back member  402  of the watch  400 . This is best illustrated in  FIGS. 4B and 4C . 
     By way of example, the carrier  404  is shown as having a round perimeter and fitted to a round opening in the back side housing member  402 . In other examples, the carrier  404  may have a perimeter that is square, oval, or some other shape. Similarly, the opening in the back side housing member  402  may be square, oval, or some other shape. The perimeter of the carrier  204  and the perimeter of the opening need not have the same size or shape (e.g., the perimeter of the opening in the back side housing member  402  may be smaller or differently shaped than the perimeter of the carrier  404 ). In some examples, the carrier  404  may be a sapphire crystal. Alternatively, the carrier  404  may be formed from (or replaced by) a light-transmissive element formed of glass, plastic, or another material. The carrier  404  may be transparent to all wavelengths of light or just some wavelengths (and even one wavelength) of light. 
     The exterior surface  408  of the carrier  404  may have a set of electrodes (e.g., first and second (or rear-facing) electrodes  412 ,  414 ) thereon, although in some embodiments a single electrode or more than two electrodes may be used. In some embodiments, the electrodes  412 ,  414  may be PVD deposited on the carrier  404 . Example constructions of the electrodes  412 ,  414  and masks  422  are described with reference to  FIGS. 5A-5E, 6-8, 9A-9C , &amp;  10 A- 10 D. In some embodiments, the electrodes  412 ,  414  may be opaque. In other examples, the electrodes  412 ,  414  may be formed of a transparent material, as described with reference to  FIG. 6 , and the optical sensor subsystem  416  may transmit/receive light through the electrodes  412 ,  414 . The optical sensor subsystem  416  may be, for example, an optical heart rate sensor. 
     In some cases, the first and second electrodes  412 ,  414  may be arc-shaped (e.g., semi-circle-shaped), and may be positioned around a central opening  418  and concentric ring of openings  420  formed in the masks  422 . The first and second electrodes  412 ,  414  may extend to the edge of the carrier  404 , and in some cases may wrap around the perimeter of the carrier  404  to the interior surface  406  of the carrier  404 , or be connected to conductive vias formed in the carrier  404 , or otherwise be electrically connected to elements within the watch body  202  that receive a signal sensed by one or both of the first and second electrodes  412 ,  414 . In some cases, the first and second electrodes  412 ,  414  may be electrically insulated from the back side housing member  402  (e.g., by a non-conductive gasket or adhesive), or the back side housing member  402  may be non-conductive. In some cases, the first and second electrodes  412 ,  414  may be formed of, or include, stainless steel (SUS) or diamond like carbon (DLC). 
     The electrodes  412 ,  414  may be positioned (e.g., at the periphery of the carrier  404  or in other locations) so as not to interfere with optical communication between an optical sensor subsystem  416  interior to the watch body  202  (see  FIG. 4C ) and a medium (e.g., skin) exterior to the watch body  202 . The optical communication may occur through the carrier  404 , and in some cases may occur through a number of openings  418 ,  420  formed in one or more masks  422  applied to the carrier  404 . The optical sensing subsystem is discussed in more detail, below. 
       FIG. 4B  shows an elevation of the watch body  202  shown in  FIG. 4A . The exterior of the watch body  202  is defined primarily by the housing  206 , the transparent cover  208 , and the carrier  404 . The carrier  404  supports the rear-facing electrodes  412 ,  414  (e.g., as described with reference to  FIGS. 4A, 4C, 5C, 5D, 5E, 6-8, 9A-9C , &amp;  10 A- 10 D). The element  430  may represent the crown  210  or button  212 . For ease of explanation, it is noted that the positions of the electrodes  412 ,  414  on the carrier  404  have been rotated 90 degrees with respect to their positions in  FIGS. 4A &amp; 4C . 
     The watch body  202  may be abutted to a user&#39;s wrist  432  or other body part, and may be adhered to the user by the watch band  204  or another element. When abutted to a user&#39;s wrist  432 , the electrodes  412 ,  414  on the carrier  404  may contact the user&#39;s skin. The user may touch a conductive portion of the element  430  with a finger  434 . The user may touch the element  430  in various ways, depending on where the element  430  is conductive, and depending on the user&#39;s preference. In some cases, the user may touch the element  430  while also touching their wrist  432 . However, high skin-to-skin impedance tends to reduce the likelihood that signals will travel from the electrodes  412 ,  414 , through their wrist  432  to their finger  434 , and subsequently to the element  430  (or vice versa). In some cases, the user may touch the element  430  while also touching the housing  206 , which may be okay if the housing  206  is not conductive. 
       FIG. 4C  shows an exploded view of components that may be attached to the interior surface  406  of the carrier  404  shown in  FIGS. 4A &amp; 4B . When the watch body  202  shown in  FIGS. 4A &amp; 4B  is assembled, the components shown in  FIG. 4C  may reside within the housing  206 . By way of example,  FIG. 4C  shows the components in relation to the back side housing member  402  (i.e., in relation to a skin-facing housing member). 
     In some cases, the interior components shown in  FIG. 4C  may be attached to (and in some cases directly on) the interior surface  406  of the carrier  404 . The interior surface  406  may sometimes be referred to as a first surface of the carrier  404 . The components attached to the carrier  404  may include a lens  436 , a light filter  438 , one or more adhesives  440 ,  442 , the optical sensor subsystem  416 , circuitry or a processing subsystem  444 , a magnet  446 , or a magnetic shield  448 . 
     The lens  436  may abut, be attached to (and optionally, directly on), or be formed on the first or interior surface  406  of the carrier  404 . By way of example, the lens  436  is aligned with the center of the carrier  404 . In some cases, the interior or exterior surface  406 ,  408  of the carrier  404  may have a mask  422  thereon (e.g., an ink mask or dark mask, and in some cases a plurality of masks). The mask  422  may define an opening  418  (e.g., a first opening or central opening) that allows light of at least one wavelength to pass through the carrier  404 , and the lens  436  may be aligned with the opening  418 . In some cases, the lens  436  may be or include a Fresnel lens, a spherical lens, a diffuser film, or the like. 
     In some cases, the light filter  438  may include one or more segments  450 , and each segment  450  may be attached to (e.g., laminated to) the interior surface  406  of the carrier  404  and positioned on the interior surface (e.g., adjacent or around the lens  436 ) to prevent a set of one or more light receivers of the optical sensor subsystem  416  from receiving a portion of the light that is emitted by a set of one or more light emitters of the optical sensor subsystem  416 . The set of light emitters and set of light receivers are not shown in  FIG. 4C , and may be attached to an underside of the optical sensor subsystem  416 . When the carrier  404  includes the mask  422 , the mask  422  may further define a second opening  420   a , or a set of openings  420  including the second opening  420   a . The second opening  420   a  or set of openings  420  may be positioned adjacent or around the first opening  418 . In these embodiments, the segments  450  of the light filter  438  (or a light filter ring or other light filter configuration) may be aligned with (e.g., may cover) each of the openings in the set of openings  420 . 
     As an example,  FIG. 4C  shows a mask  422  that defines a set of eight radial openings  420  around a central opening  418 . Each segment  450  of the light filter  438  may block (e.g., absorb) a portion of light emitted by a set of light emitters that is attached to the optical sensor subsystem  416 , which portion of light reflects from a surface too close to (or within) the carrier  404  (e.g., the exterior surface  408  of the carrier  404 , imperfections within the carrier  404 , or a medium too close to the carrier  404 ), such that the reflected light is not useful in a sensing operation for which the optical sensor subsystem  416  is designed. For example, when the optical sensor subsystem  416  is configured to determine a biological parameter of a user, light reflected from the carrier  404 , or from the outer layer of skin of the user, may not have any relation to the biological parameter determined and may not be useful. Accordingly, the filter may be configured to filter out light frequencies associated with light reflected from the carrier and/or a skin surface, allowing light reflected from deeper skin layers, blood vessels, and the like to be received by the receiver(s). In some examples, the light filter  438  or segments  450  thereof may include at least one of a light control film, a light polarizer, an anti-reflective film, a reflective film, or a light absorber. Accordingly and as one non-limiting example, the optical sensor subsystem  416  may act as an optical heart rate detector. 
     In some embodiments, the mask  422  may represent multiple masks, and different masks may allow different wavelengths of light to pass through the carrier  404 , as described for example with reference to  FIGS. 5A-5E &amp; 6 . 
     The optical sensor subsystem  416  may include a substrate  452  on which the set of one or more light emitters (e.g., LEDs) and the set of one or more light receivers (e.g., photodetectors, such as photodiodes) are attached. The light emitter(s) and light receiver(s) may be attached or positioned on the substrate  452  to emit and receive light through the carrier  404 . The sensor subsystem  416  may be attached to the carrier  404  by one or more adhesives  440 / 442 , such as pressure sensitive adhesives (PSAs) or heat-activated films (HAFs). In some cases, the set of light emitters may be centrally attached on the substrate  452 , and a first wall may be attached to (e.g., formed on or bonded to) an underside of the substrate  452  surrounding the set of light emitters. The first wall may be attached to the interior surface  406  of the carrier  404  using a first adhesive  440 . The set of light receivers may be attached on the substrate  452  around the set of light emitters, between the first wall and a second wall attached to (e.g., formed on or bonded to) the underside of the substrate  452 . The second wall may be attached to the interior surface  406  of the carrier  404  using a second ring of adhesive  442 . 
     The substrate  452  of the optical sensor subsystem  416  may include various contacts, pads, traces, or other conductive structures  454  that enable the processing subsystem  444  to be electrically coupled to the set of light emitters and set of light receivers of the optical sensor subsystem  416 . The processing subsystem  444  may include a substrate  456  (e.g., a printed circuit board (PCB)) that is attached to the optical sensor subsystem  416 , and thereby to the carrier  404 , via the conductive structures  454  and/or additional adhesive between the substrates  452 ,  456  of the optical sensor subsystem  416  and the processing subsystem  444 . The substrates  452 ,  456  may also or alternatively be connected using mechanical fasteners (e.g., screws). The processing subsystem  444  may activate the light emitters and light receivers to perform a sensor function (e.g., to determine a heart rate). In some cases, the processing subsystem  444  may be attached to another structure within the watch body, and may be electrically connected to the conductive structures  454  of the optical sensor subsystem  416  by a flex circuit or other conductors. 
     In some embodiments, the substrate  456  of the processing subsystem  444  may have a hole  458  therein, and the magnet  446  may be aligned with the hole  458  and abutted to (or attached to) the substrate  452 . In some cases, the magnet  446  may be adhesively bonded to the substrate  452  of the optical sensor subsystem  416 . The magnet  446  may inductively couple to a battery charger used for charging a battery included within the watch body, which battery may power components of the watch including the components of the optical sensor subsystem  416  and the processing subsystem  444 . 
     The magnetic shield  448  may abut (or be attached to) the magnet  446 . In some cases, the magnetic shield  448  may be adhesively bonded to the magnet  446 . The magnetic shield  448  may direct magnetic flux associated with the magnet  446  toward and out the carrier  404  to improve inductive battery charging performance for a battery included within the watch body  202 . 
     Direct or indirect connection of the components shown in  FIG. 4C  to the interior surface  406  of the carrier  404  can reduce the height of the components when stacked. 
       FIG. 4D  shows the sensor subsystem  416  attached to the carrier  404  shown in  FIGS. 4A-4C .  FIG. 4B  also shows a flex circuit  460 , surrounding the sensor subsystem  416 , which may provide electrical connections between the electrodes  412 ,  414  and the sensor subsystem  416  while also providing a ground that operates as an electrical noise mitigation barrier (or E-shield) between the sensor subsystem  416  and the electrodes  412 ,  414 . The electrodes  412 ,  414  may be connected to the electrical contacts  462 ,  464 , which electrical contacts  462 ,  464  are on the interior surface of the carrier  404  and connected to both traces in the flex circuit  460  and the electrodes  412 ,  414 . Traces in the flex circuit  460  may be connected to the electrical contacts  462 ,  464  via a conductive epoxy, and may connect the electrical contacts  462 ,  464  to the sensor subsystem  416 . A processor may be part of the sensor subsystem  416 , and the processor may be connected to another processor or other circuitry via a flex circuit  466 . 
       FIGS. 5A-5E  illustrate an example of coatings that may be deposited on the interior and exterior surfaces of the carrier  404  shown in  FIGS. 4A-4C . As shown in  FIG. 5A , a first mask  500  (e.g., a first ink mask) that is opaque to infrared (IR) and visible light may be deposited (e.g., PVD deposited) on the interior surface  406  of the carrier  404 . In some examples, the first mask  500  may include an inner ring  500   a  and an outer ring  500   b  that define a central first opening  418  and a concentric second opening  506  (i.e., a second opening  506  that is concentric with the first opening  418 ). The central first opening  418  may be positioned over light emitters of the optical sensor subsystem  416  described with reference to  FIG. 4C  (and over the optional lens  436 ), and the concentric second opening  506  may be positioned above light receivers of the sensor subsystem  416 . The inner ring  500   a  of the first mask may prevent the light receivers from receiving light that is unlikely to have passed through a user&#39;s skin after passing through the central first opening  418 . The outer ring  500   b  of the first mask  500  may in some cases be provided for cosmetic reasons, and in some cases may not be provided. 
       FIG. 5B  shows a second mask  508  (e.g., a second ink mask) that is opaque to visible light but transparent to IR light. The second mask  508  may be deposited (e.g., PVD deposited) on the interior surface  406  of the carrier  404 . The second mask  508  may be deposited on the carrier  404  over the concentric second opening  506  in the first mask  500 , and may overlap the inner and outer rings  500   a ,  500   b  of the first mask, as shown in  FIG. 6 . The second mask  508  may define a plurality of visible light openings  420  above respective light receivers of the optical sensor subsystem  416 , while allowing IR light to pass through the entirety of the concentric second opening  506 . This may increase the amount of IR light received by the light receivers. The second mask  508  may also define an optional opening above a condensation detector  512 . In some cases, the second mask  508  may look visually similar to the first mask  500  (e.g., both masks may be dark masks, such that it may be impossible or difficult for a user to visually distinguish the first and second masks  500 ,  508 ). 
       FIGS. 5C and 5D  show an example of PVD deposited first and second electrodes  514   a ,  514   b  on the interior and exterior surfaces  406 ,  408  of the carrier  404 . The first and second electrodes  412 ,  414  may be arc-shaped and positioned at the periphery of the carrier  404 . The first and second electrodes  412 ,  414  may be sized based on factors such as: providing a sufficient area to provide good electrical contact between the electrodes  412 ,  414  and skin (which may improve electrical sensor efficiency); providing electrodes  412 ,  414  of a size that do not substantially interfere with an antenna or other electrical structures of a device (which may improve wireless communication efficiency); or providing electrodes  412 ,  414  positioned to allow optical communication through the carrier  404  (which may improve optical communication efficiency). The first and second electrodes  412 ,  414  may be separated from one another by a pair of gaps  518   a ,  518   b.    
     The first and second electrodes  412 ,  414  may be deposited on both the interior and exterior surfaces  406 ,  408  of the carrier  404  and may wrap around the edge (or perimeter) of the carrier  404 . The material used to form the first and second electrodes  412 ,  414  may be patterned to form electrical contacts  520   a ,  520   b  (e.g., tabs) on the interior surface  406  of the carrier  404 . The first and second electrodes  412 ,  414  may overlap the first mask  500  (or outer ring  500   b  of the first mask) on the interior surface  406  of the carrier  404 , such that the first mask  500  is positioned between the first and second electrodes  412 ,  414  and the interior surface  406  of the carrier  404 . Thus, the material used to form the electrodes  412 ,  414  may need to have properties that enable the material to adhere to a carrier surface (e.g., a sapphire surface) and a mask (e.g., an ink mask). The material or materials used to form the electrodes  412 ,  414  may also have properties, singularly or in combination, such as: a low impedance and good conductivity (e.g., a low DC resistance); a low electrode-to-skin impedance; a high hardness to reduce scratching of the electrodes  412 ,  414 ; a higher elastic modulus than the carrier  404  (e.g., to mitigate the likelihood that a crack in an electrode  412 ,  414  propagates through the carrier  404 ); compatibility with a HAF or other adhesive; and good biocompatibility (e.g., not likely to cause an adverse reaction to a user of a device). In some embodiments, the electrodes  412 ,  414  may include aluminum titanium nitride (AlTiN) or chromium silicon carbonitride (CrSiCN). AlTiN and CrSiCN hold up well to abrasion and corrosion and tend not to place undue stresses on a sapphire carrier. 
       FIG. 5E  shows an example deposition of adhesive on the interior surface  406  of the carrier  404 . The adhesive may be deposited in inner and outer rings  440 ,  442 , as described with reference to  FIG. 4C . The inner ring  440  of adhesive may be positioned on the inner ring  500   a  of the first mask. The outer ring  442  of adhesive may be positioned on the second mask  508 , outward from the plurality of openings  420  in the second mask  508 . In some cases, the adhesive may include a PSA or HAF. 
       FIG. 6  shows a cross-section of the carrier  404  shown in  FIG. 5B , and illustrates an overlap between the first and second masks  500 ,  508 . The second mask  508  may overlap the first mask (e.g., outer ring  500   b ) on the interior surface  406  of the carrier  404  such that the first mask  500  is positioned between the second mask  508  and the interior surface  406  of the carrier  404 . 
       FIG. 7  shows a cross-section of the carrier  404  shown in  FIG. 5C or 5D , and illustrates an overlap between the first electrode  412  (or second electrode) and the first mask  500  (e.g., outer ring  500   b  of the first mask). The first electrode  412  may overlap the first mask  500  (or outer ring  500   b  of the first mask) on the interior surface  406  of the carrier  404 , such that the first mask  500  is positioned between the first electrode  412  and the interior surface  406  of the carrier  404 . 
     In some embodiments, the electrodes  412 ,  414  shown in  FIGS. 5C, 5D, and 7  may be formed using indium titanium oxide (ITO) or another transparent material. In these embodiments, the electrodes  412 ,  414  may be transparent to light emitted by a sensor subsystem positioned below the carrier  404 , and thus the electrodes  412 ,  414  may extend over a greater portion (or all) of the exterior surface  408  of the carrier  404 .  FIG. 8  shows an example layer construction of an ITO-based electrode. As shown, the stack  800  may include a layer  802  of aluminum oxide (Al 2 O 3 ) on the carrier, a layer  804  of ITO on the layer  802  of aluminum oxide, a first layer  806  of silicon dioxide (SiO 2 ) on the layer  804  of aluminum oxide, a layer  808  of silicon nitride (Si 3 N 4 ) on the first layer  806  of silicon dioxide, a second layer  810  of silicon dioxide on the layer  808  of silicon nitride, and a layer  812  of diamond like carbon, or another hard coating, on the second layer  810  of silicon dioxide. Alternatively, the layer  808  of silicon nitride and second layer  810  of silicon dioxide may not be deposited, or only the layer  804  of ITO may be deposited, or just the layer  804  of ITO layer and first layer  806  of silicon dioxide may be deposited. Other variations in the number or types of layers may also be used to form the stack  800 . Each of the layers may be transparent to IR or visible light. 
       FIGS. 9A-9C  show alternative electrical connections between an electrode (e.g., an electrode on an exterior surface of a carrier that forms part of a housing of an electronic device) and an electrical contact interior to the electronic device. In some examples, the carriers shown in cross-section in  FIGS. 9A-9C  may have circular perimeters. In other examples, the carriers may have perimeters that are oval-shaped, square-shaped, rectangular-shaped, and so on. The techniques described with reference to  FIGS. 9A-9C  can be applied to carriers having various perimeter shapes, to carriers having different compositions, and so on. In some embodiments, the features shown in  FIGS. 9-9C  may be replicated to electrically connect more than one electrode on an exterior surface of an electronic device to components interior to the electronic device. 
     In  FIG. 9A , the electrode  900  may be a thin film electrode that is PVD deposited on a surface  902  of a carrier  904 . The surface  902  on which the electrode  900  is deposited may be a surface of the carrier  904  that is exterior to an electronic device (i.e., the electrode  900  may be deposited on an exterior surface  902  of the carrier  904 ). As shown, a conductive material used to form the electrode  900  may be deposited on the carrier  904  such that the material wraps around an edge  906  or perimeter of the carrier  904  to form an electrical contact  908  on a surface  910  of the carrier  904  that is interior to the electronic device (i.e., on an interior surface  910  of the carrier  904 ). In some cases, the electrical contact  908  may be a tab that traces a much smaller arc about the periphery of the carrier  904  than the electrode  900  (as shown, for example, in  FIGS. 5C, 5D, and 5E ). In other cases, the electrical contact  908  may be arc-shaped and trace an arc that is similar in size to an arc traced by the electrode  900  on the exterior surface  902  of the carrier  904 . 
     In some cases, the conductive material(s) used to form the electrode  900  and electrical contact  908  may be deposited on the exterior surface  902 , edge  906 , and interior surface  910  of the carrier  904  in a single operation (or single set of operations in which the material(s) are deposited on the exterior surface  902 , edge  906 , and interior surface  910  of the carrier  904 ). In other cases, the material(s) used to form the electrode  900  may be deposited on the edge  906  or interior surface  910  of the carrier  904  in operations that are performed separately from one or more operations in which the electrode  900  is deposited on the exterior surface  902  of the carrier  904 . In these latter examples, the material(s) may be deposited such that the materials overlap. In some cases, a set of one or more materials used to form the electrode  900  may differ from a set of one or more materials deposited on the edge  906  or interior surface  910  of the carrier  904 . 
     In some embodiments, the conductive material(s) deposited on the exterior surface  902 , edge  906 , and interior surface  910  of the carrier  904  may include a layer of SUS or a layer of DLC. In other embodiments, only the electrode  900  or edge  906  of the carrier  904  may be coated with a layer of stainless steel or DLC. In some examples, the conductive material(s) may include a PVD deposited layer of AlTiN or CrSiCN. 
     In some embodiments, one or more masks (e.g., one or more ink masks) may be applied to the interior surface of the carrier (e.g., as described with reference to  FIGS. 5A-5E, 6 , &amp;  7 ). In these embodiments, one or more of the conductive materials used to form the electrode  900  and electrical contact  908  may be applied over the mask(s). The conductive material(s), and the manner in which the conductive material(s) are deposited on the carrier  904 , may therefore be selected to ensure adhesion of the conductive material(s) to the carrier  904  and to the ink or other material used to form the mask(s). 
     As shown in  FIG. 9A , a peripheral band of the interior surface  910  of the carrier  904  may be attached to a recessed ledge  912  in another housing member  914  of the electronic device (e.g., with the carrier  904  overlapping the housing member  914 ). In such an embodiment, the carrier  904  may be attached to the housing member  914  using an adhesive  916 , such as a heat-activated film (HAF). The adhesive  916  or conductive material(s), and the manner in which the carrier  904  is attached to the housing member  914 , may therefore be selected to ensure adhesion of the carrier  904  to the housing member  914 . 
     The electrical contact  908  may have a great enough width (e.g., a great enough width along a radius of the carrier  904 ) that the electrical contact  908  extends past the recessed ledge  912  in the housing member  914  when the carrier  904  is attached to the housing member  914 , making the electrical contact  908  accessible interior to the electronic device. In some cases, a flex circuit, other flexible conductor, or other conductive element may be soldered or otherwise electrically connected to the electrical contact  908 , to enable a signal to be received from or applied to the electrode  900 . 
     In  FIG. 9B , the electrode  918  may be a thin film electrode that is PVD deposited on a surface  920  of a carrier  922 . The surface  920  on which the electrode  918  is deposited may be a surface of the carrier  922  that is exterior to an electronic device (i.e., the electrode  918  may be deposited on an exterior surface  920  of the carrier  922 ). Prior to or after depositing the electrode  918  on the carrier  922 , a thru-carrier via  924  may be drilled or otherwise cut into the carrier  922 . The thru-carrier via  924  may extend from the exterior surface  920  of the carrier  922  (or electrode  918 ) to the interior surface  926  of the carrier  922 . The via  924  may be coated or filled with a conductive material  928  such as stainless steel (SUS), and the conductive material  928  may be covered by, overlap, or otherwise electrically connect to the electrode  918 . In some cases, the conductive material  928  in the via  924  may be molded or glued in the via  924 . The conductive material  928  may provide an electrical contact  930  on a surface  926  of the carrier  922  that is interior to an electronic device (i.e., on an interior surface of the carrier). In some cases, the conductive material  928  in the via  924  may overlap a portion of the interior surface  926  of the carrier  922 , or may be connected to another conductive element deposited on the interior surface  926  of the carrier  922 . 
     The conductive material(s) used to form the electrode  918  and deposited in the via  924  may be the same or different. In some embodiments, the conductive material(s) used to form the electrode  918  may include a layer of stainless steel (SUS) or a layer of diamond like carbon (DLC). In some examples, the conductive material(s) may include a PVD deposited layer of Aluminum Titanium Nitride (AlTiN) or Chromium Silicon Carbon Nitride (CrSiCN). 
     As shown in  FIG. 9B , a peripheral band of the interior surface  926  of the carrier  922  may be attached to a recessed ledge  932  in another housing member  934  of the electronic device (e.g., with the carrier  922  overlapping the housing member  934 ). In such an embodiment, the carrier  922  may be attached to the housing member  934  using an adhesive  936 , such as a HAF. The adhesive  936 , and the manner in which the carrier  922  is attached to the housing member  934 , may therefore be selected to ensure adhesion of the carrier  922  to the housing member  934 . 
     When the carrier  922  is attached to the housing member  934 , the via  924  may be positioned such that it overlaps the recessed ledge  932  or is interior to the recessed ledge  932 , making the electrical contact  930  accessible interior to the electronic device. In some cases, a flex circuit, other flexible conductor, or other conductive element may be soldered or otherwise electrically connected to the electrical contact  930 , to enable a signal to be received from or applied to the electrode  918 . 
     In  FIG. 9C , the electrode  938  may be a metallic arc-shaped element positioned between a carrier  940  and another housing member  942  of an electronic device. In some embodiments, the electrode  938  may be a singular metallic ring-shaped electrode (e.g., one electrode). In these embodiments, a peripheral band of a surface of the carrier  940  that is interior to the electronic device (e.g., an interior surface  944  of the carrier  940 ) may be attached to a recessed ledge  946  in the electrode  938  (e.g., with the carrier  940  overlapping the electrode  938 ), and the electrode  938  may be attached to a recessed ledge  950  in the housing member  942  (e.g., with the electrode  938  overlapping the housing member  942 ). In other embodiments, the electrode  938  may be arc-shaped and may be one of two or more arc-shaped electrodes between the carrier  940  and the housing member  942 . In these embodiments, a peripheral band of the interior surface of the carrier  940  may be attached to recessed ledges  946  in multiple arc-shaped electrodes  938  (e.g., with the carrier  940  overlapping the electrodes  938 ), and each of the arc-shaped electrodes  938  may be attached to the recessed ledge  950  in the housing member  942  (e.g., with the electrodes  938  overlapping the housing member  942 ). In some cases, the multiple arc-shaped electrodes  938  may be electrically isolated from each other by flexible seals or gaskets, or by rigid separators (e.g., rigid extensions of the housing member  942 , which rigid extensions may include extensions of the recessed ledge  950  to which the electrodes  938  are attached. In some cases, the electrode  938  may be attached to the housing member  942  before the carrier  940  is attached to the electrode  938 . 
     The electrode  938  may be formed of a conductive material including a layer of stainless steel (SUS) or DLC. In some examples, the conductive material may include a PVD deposited layer of Aluminum Titanium Nitride (AlTiN) or Chromium Silicon Carbon Nitride (CrSiCN). A surface  954  of the electrode  938  interior to the electronic device may provide an electrical contact for connecting components interior to the electronic device to the electrode  938 . 
     In some cases, an edge  956  or perimeter of the carrier  940  may be abutted directly to the electrode  938 , and an edge  958  of the electrode  938  may be abutted directly to the housing member  942 . In other cases, an adhesive, seal, gasket, or filler may fill a gap between the carrier  940  and the electrode  938  or a gap between the electrode  938  and the housing member  942 . 
     As shown in  FIG. 9C , a peripheral band of the interior surface  944  of the carrier  940  may be attached to a recessed ledge  946  in the electrode  938 , and a peripheral band of an interior surface  954  of the electrode  938  may be attached to the housing member  942 . In such an embodiment, the carrier  940  may be attached to the electrode  938  or the electrode  938  may be attached to the housing member  942  using an adhesive  948  or  952 , such as a HAF. The adhesive  948  or  952 , and manner in which the respective elements are attached, may therefore be selected to ensure adhesion of the carrier  940  to the electrode  938  (or adhesion of the electrode  938  to the housing member  942 ). 
     When the electrode  938  is attached to the housing member  942 , the electrode  938  may be positioned such that it overlaps the recessed ledge  950  or is interior to the recessed ledge  950 , making the electrode  938  accessible interior to the electronic device. In some cases, a flex circuit, other flexible conductor, or other conductive element may be soldered or otherwise electrically connected to the electrode  938 , to enable a signal to be received from or applied to the electrode  938 . 
       FIGS. 10A-10D  show alternative carrier profiles, and alternative attachments (e.g., structural attachments) of carriers to other housing members of an electronic device. In some examples, the carriers may have circular perimeters. In other examples, the carriers may have perimeters that are oval-shaped, square-shaped, rectangular-shaped, and so on. The techniques described with reference to  FIGS. 10A-10D  can be applied to carriers having various perimeter shapes, to carriers having different compositions, and so on. Each of  FIGS. 10A-10D  shows an exterior surface (e.g., a back surface) of an electronic device, such as an electronic watch, and a cross-section of the exterior surface of the electronic device. 
     In  FIG. 10A , a carrier  1000  is attached to another housing member  1002  of an electronic device. The carrier  1000  includes an exterior surface  1004  that forms a part of the exterior surface of the electronic device, and an interior surface  1006  that faces components interior to the electronic device. 
     A peripheral band of the interior surface  1006  of the carrier  1000  may be attached to a recessed ledge  1064  in the housing member  1002  (e.g., with the carrier  1000  overlapping the housing member  1002 ). In such an embodiment, the carrier  1000  may be attached to the housing member  1002  using an adhesive, such as a HAF. The adhesive, and the manner in which the carrier  1000  is attached to the housing member  1002 , may be selected to ensure adhesion of the carrier  1000  to the housing member  1002 . 
     The carrier  1000  may be variously configured, but in  FIG. 10A , an inner portion  1008  of the interior surface  1006  of the carrier  1000 , such as a portion of the carrier interior to a number of thru-carrier vias  1010 , is flat. An outer portion  1012  of the interior surface  1006 , such as a portion outside the number of thru-carrier vias  1010 , is concave or inwardly-sloped (downwardly-sloped in the figure) with respect to the inner portion  1008  of the interior surface  1006 . In contrast, the exterior surface  1004  of the carrier  1000  may be convex. Thus, the thickness of the carrier  1000  may vary to some degree from its center axis to its perimeter. 
     In some embodiments, one or more arc-shaped electrodes  1014  (e.g., two electrodes) may be positioned around the exterior surface  1004  of the carrier  1000 , inward from the perimeter of the carrier  1000 . In other embodiments, the electrodes  1014  may have other shapes or may extend to or around the perimeter. The electrodes  1014  may be PVD deposited thin film electrodes. In some cases, the electrodes  1014  may be connected to interior components of the electronic device by the thru-carrier vias  1010 , which in some cases may be drilled or formed through the flat inner portion  1008  of the interior surface  1006  of the carrier  1000 . In other cases, the electrodes  1014  may be connected to interior components of the electronic device by conductive material that wraps around the edge or perimeter of the carrier  1000 , or in any of the ways shown in  FIGS. 9A-9C . 
     In some cases, components such as a sensor subsystem may be attached to the inner, flat portion of the interior surface  1006  of the carrier  1000 . 
     In  FIG. 10B , a carrier  1016  is attached to another housing member  1018  of an electronic device. The carrier  1016  includes an exterior surface  1020  that forms a part of the exterior surface of the electronic device, and an interior surface  1022  that faces components interior to the electronic device. 
     A peripheral band of the interior surface  1022  of the carrier  1016  may be attached to a recessed ledge  1024  in the housing member  1018  (e.g., with the carrier  1016  overlapping the housing member  1018 ). In such an embodiment, the carrier  1016  may be attached to the housing member  1018  using an adhesive, such as a HAF. The adhesive, and the manner in which the carrier  1016  is attached to the housing member  1018 , may be selected to ensure adhesion of the carrier  1016  to the housing member  1018 . 
     The carrier  1016  may be variously configured, but in  FIG. 10B , the carrier  1016  has a uniform thickness. The interior surface  1022  of the carrier  1016  may be concave, and the exterior surface  1020  of the carrier  1016  may be convex. To provide a flat surface for connecting components (e.g., a sensor subsystem) to the carrier  1016 , a secondary carrier  1026  having a convex exterior surface  1028  and a flat interior surface  1066  may be attached to an inner portion of the interior surface  1022  of the carrier  1016 . The secondary (or interior) carrier  1026  may be attached to the primary (or exterior) carrier  1016  using a transparent adhesive. In some cases, the secondary carrier  1026  may be attached to the primary carrier  1016  interior to a number of thru-carrier vias  1030 . 
     In some embodiments, one or more arc-shaped electrodes  1032  (e.g., two electrodes) may be positioned around the exterior surface of the carrier  1016 , inward from the perimeter of the carrier  1016 . In other embodiments, the electrodes  1032  may have other shapes or may extend to or around the perimeter. The electrodes  1032  may be PVD deposited thin film electrodes. In some cases, the electrodes  1032  may be connected to interior components of the electronic device by the thru-carrier vias  1030 . In other cases, the electrodes  1032  may be connected to interior components of the electronic device by conductive material that wraps around the edge or perimeter of the carrier  1016 , or in any of the ways shown in  FIGS. 9A-9C . 
       FIGS. 10C &amp; 10D  each show a carrier attached to another housing member of an electronic device. The carrier includes an exterior surface that forms a part of the exterior surface of the electronic device, and an interior surface that faces components interior to the electronic device. In each of  FIGS. 10C &amp; 10D , the carrier has a flat interior surface and a convex exterior surface. The carriers shown in  FIGS. 10C &amp; 10D  may be easier to manufacture than the carriers shown in  FIGS. 10A &amp; 10B . 
     In  FIG. 10C , a carrier  1034  is attached to another housing member  1036  of an electronic device. The carrier  1034  includes an exterior surface  1038  that forms a part of the exterior surface of the electronic device, and an interior surface  1040  that faces components interior to the electronic device. 
     A peripheral band of the interior surface  1040  of the carrier  1034  may be attached to a recessed ledge  1042  in the housing member (e.g., with the carrier  1034  overlapping the housing member  1036 ). In such an embodiment, the carrier  1034  may be attached to the housing member  1036  using an adhesive, such as a HAF. The adhesive, and the manner in which the carrier  1034  is attached to the housing member  1036 , may be selected to ensure adhesion of the carrier  1034  to the housing member  1036 . 
     In some embodiments, one or more arc-shaped electrodes  1044  (e.g., two electrodes) may be positioned around the perimeter of the exterior surface  1038  of the carrier  1034 . In other embodiments, the electrodes  1044  may have other shapes or other positions on the exterior surface  1038  of the carrier  1034 . The electrodes  1044  may be PVD deposited thin film electrodes. In some cases, the electrodes  1044  may be connected to interior components of the electronic device by conductive material that wraps around the edge or perimeter of the carrier  1034 , or in any of the ways shown in  FIGS. 9A-9C . 
     In some cases, components such as a sensor subsystem may be attached to the interior surface  1040  of the carrier  1034 . 
     In  FIG. 10D , a carrier  1060  is attached to another housing member  1046  of an electronic device (e.g., an electronic watch), similarly to how the carrier  1034  is attached to another housing member  1036  in  FIG. 10C . For example, a peripheral band of the interior surface  1048  of the carrier  1060  may be attached to a recessed ledge  1050  in the housing member  1046  (e.g., with the carrier  1060  overlapping the housing member  1046 ). The carrier  1060  may be attached to the housing member  1046  using an adhesive, such as a HAF. The adhesive, and the manner in which the carrier  1060  is attached to the housing member  1046 , may be selected to ensure adhesion of the carrier  1060  to the housing member  1046 . 
     The housing member  1046  to which the carrier  1060  is attached may be attached to yet another housing member (e.g., a second housing member  1052 ). A peripheral band of the interior surface  1054  of the first housing member  1046  may be attached to a recessed ledge  1056  in the second housing member  1052  (e.g., with the first housing member  1046  overlapping the second housing member  1052 ). The first housing member  1046  may be attached to the second housing member  1052  using an adhesive, such as a HAF. The adhesive, and the manner in which the first housing member  1046  is attached to the second housing member  1052 , may be selected to ensure adhesion of the first housing member  1046  to the second housing member  1052 . In some cases, the first housing member  1046  may electrically insulate the electrodes  1058  on the carrier  1060  from the second housing member  1052 , or may provide a transition between incompatible materials, or may provide a support for the carrier  1060 , or may facilitate assembly of the housing of the electronic device. 
     In some embodiments, one or more arc-shaped electrodes  1058  (e.g., two electrodes) may be positioned around the perimeter of the exterior surface of the carrier  1060 . In other embodiments, the electrodes  1058  may have other shapes or other positions on the exterior surface  1062  of the carrier  1060 . The electrodes  1058  may be PVD deposited thin film electrodes. In some cases, the electrodes  1058  may be connected to interior components of the electronic device by conductive material that wraps around the edge or perimeter of the carrier, or in any of the ways shown in  FIGS. 9A-9C . 
     In some cases, components such as a sensor subsystem may be attached to the interior surface  1048  of the carrier  1060 . 
     Turning now to the implementation of an electrode on a crown,  FIG. 11A  shows an example elevation of a crown assembly  1100 . The crown assembly  1100  may be an example of a crown assembly included in any of the electronic devices described with reference to  FIG. 1A, 1B, 2A-2C, 3, 4A , or  4 B. The crown assembly  1100  may include a crown  210 . 
     The crown  210  may be mechanically and electrically connected to a shaft  1102  that extends through an opening in a housing. By way of example, the housing is shown to be the housing  206  of the watch body  202  described with reference to  FIGS. 2A-2C, 3, 4A , &amp;  4 B. The crown  210  may be integrally formed from a single piece of material that includes the shaft  1102  (that is, they may be connected to one another), or the shaft  1102  may be semi-permanently attached to a crown body  1104  using a fastening means such as solder, threads, or an adhesive. The crown  210  (or at least the crown body  1104 ) may be external to the housing  206 , and may be rotated and/or translated by a user of the electronic device incorporating the crown assembly  1100 . Further, although the crown body  1104  and shaft  1102  are shown as integrally formed in  FIGS. 11A and 11B , it should be appreciated that they may be separate pieces that are joined together. 
     The crown assembly  1100  may further include a shaft retainer  1106  receiving an end of the shaft  1102 . The shaft retainer  1106  may be mechanically and/or electrically connected to the shaft  1102  (e.g., using solder, threads, or an adhesive), interior to the housing  206 . The shaft retainer  1106  may retain the crown  210  in position in relation to the housing  206 . 
     One or more insulators  1108  (e.g., electrical insulators) may electrically insulate the crown  210  and/or shaft  1102  from the housing  206 . The term “insulator” encompasses both a single insulator and multiple insulators taken as a set. The insulator  1108  is generally shown in  FIG. 11  as an annular seal having an L-shaped profile, functioning as a collar for the shaft. The shaft  1102  and crown body  1104  may translate, moving toward and away from the housing  206  while the insulator  1108  and shaft retainer  1106  move with the shaft  1102 . In some embodiments, the insulator  1108  may be stationary relative to the housing  206  while the shaft  1102 , crown body  1104  and shaft retainer  1106  move. 
     The insulator  1108  or seal electrically insulates the shaft  1102  from the housing  206 , and may also electrically insulate the housing  206  from the shaft retainer  1106 . In some embodiments, the insulator  1108  may alternatively include more than one element and/or be positioned elsewhere within the crown assembly  1100 . For example, the insulator  1108  may include an element, layer, or coating  1108   a  applied to a surface of the housing  206  that faces an underside of the crown  210 , or to other elements that face the underside of the crown  210 . 
     A tactile switch  1110  may be axially aligned with the shaft  1102  and positioned at an end of the shaft  1102  opposite the crown body  1104 . By way of example, the tactile switch  1110  may be attached on a substrate  1112 . The tactile switch  1110  may be actuated (e.g., switched between two or more states) as the shaft  1102  translates along an axis of the shaft  1102  to provide a crown input. A spring-biased conductor  1114  may be mechanically and electrically connected to at least one of the shaft  1102  or the shaft retainer  1106 , and in  FIG. 11  is shown to be connected to the shaft retainer  1106 . The spring-biased conductor  1114  may be biased to electrically contact the shaft  1102  and/or shaft retainer  1106  during all phases of rotation and translation of the crown  210 , and may electrically connect the shaft  1102  and/or shaft retainer  1106  to a circuit  1116  (e.g., a processor). When the crown is translated by a user, the spring-biased conductor  1114  may deform to maintain electrical contact with the shaft  1102  and/or shaft retainer  1106 . 
     The crown assembly  1100  may further include an optical encoder  1118 . The optical encoder  1118  may be used to detect rotation and/or translation of the shaft  1102  or shaft retainer  1106 . In some embodiments, the circuit  1116  and optical encoder  1118  may be attached to the same substrate as the tactile switch  1110 . 
     In some embodiments, the entirety of the crown  210  or crown body  1104  may be conductive and function as an electrode. The conductive crown body  1104  may be electrically connected to the circuit  1116  via the shaft  1102 , shaft retainer  1106 , and spring-biased conductor  1114 . In other embodiments, only a portion of the crown  210  or crown body  1104  may be conductive and function as an electrode, and the conductive portion of the crown body  1104  may be electrically connected to the circuit  1116  via the shaft  1102 , shaft retainer  1106 , and spring-biased conductor  1114 . 
     Because the signals received by or propagated from the crown  210  may be low voltage or low amplitude signals, the materials, positions, electrical connections to, and electrical routing paths for an electrode formed on or by the crown  210  can have a significant impact on the ability of the circuit  1116  to discern useful signals representing an ECG or other biological parameter of a person wearing an electronic device including the crown assembly  1100 . The materials, positions, electrical connections to, and electrical routing paths for the crown assembly  1100  can also determine how well the crown assembly  1100  receives voltages/signals from a person&#39;s skin (e.g., a SNR of a device-to-user interface through which the voltages/signals pass); how well voltages/signals are transferred between the crown  210  and internal components of an electronic device (e.g., a voltage/signal propagation SNR); and how well the crown assembly  1100  operates in the face of environmental factors, such as temperature, humidity, moisture, electromagnetic radiation, dust, and so on. In some cases, the insulator  1108  may be positioned to prevent moisture from electrically shorting the crown  210  to the housing  206 , or the housing  206  may be grounded to provide electrical shielding for some or all of the signals propagated through the crown assembly  1100 , or the interfaces between the shaft  1102  and the shaft retainer  1106 , or between the shaft retainer  1106  and the spring-biased conductor  1114 , may be configured to increase SNR and reduce signal attenuation. 
     In some embodiments, the crown  210  may include a coating, layer, or the like  1120  (“coating  1120 ”) that electrically isolates the crown  210 . The coating may thus function as, have similar properties to, and/or be made from the same or similar materials as, the insulator  1108 . Generally, the coating  1120  prevents electrical connection between the crown  210  and the housing  206 , for example when water is present in the space between the crown and housing. 
     In the absence of the coating  1120 , water, sweat, or another conductor may electrically bridge or short the crown  210  to the housing  206 , presuming both the crown and housing are made from (or incorporate) an electrical conductor such as metal. In the event the crown  210  or some portion thereof serves as an electrode for measuring an electrocardiogram, shorting the crown  210  to the housing  206  (and thus to the skin of a wearer) may render the crown inoperable or the electrocardiogram unreliable. 
     The coating  1120  serves as a barrier against shorting the crown  210  to the housing  206  or other electrically conductive material or body, thereby ensuring the electrical functionality of the crown  210 . In some embodiments the coating  1120  coats only those surfaces of the crown that oppose or face the housing; in other embodiments and as shown in  FIG. 11A , the coating  1108   a  may extend across one or more sides of the crown  210 . Further, the coating  1120  may extend onto a top or outer surface of the crown  210  (e.g., the surface of the crown that does not oppose or face the housing  206 ), although typically at least a portion of the outer surface is not covered by the coating in order to define an area of the crown that can electrically couple to a wearer&#39;s finger. 
     The coating  1120  also may extend at least partly down a shaft of the crown  210 , as discussed in more detail with respect to  FIG. 11B . 
     In some embodiments, a second coating  1122  may be applied to the housing  206  instead of, or in addition to, the coating  1120  on the crown  210 . This housing coating  1122  serves the same function as the crown coating  1120 , namely electrically insulating the housing  206  from the crown  210  when water or other electrical conductors are in the gap between the housing and crown. The housing coating  1122  may extend across the insulator  1108  (or a non-insulating collar), in some embodiments. Likewise, the housing coating  1122  may extend between the housing  206  and insulator  1108  in some embodiments, or may be an extension of the insulator. 
     In some embodiments, the coatings  1120 ,  1122  may improve wear or provide a cosmetic function (e.g., provide an accent or visually conceal a surface of the shaft, crown, and/or housing) for the crown assembly  1100 . Alternatively, the housing  206 , or that portion of the housing  206  that faces the crown  210 , may be formed from a material that operates as an electrical insulator (e.g., plastic, ceramic, or the like). In some embodiments, one or more of the insulators  1108  (including element, layer, or coating  1108   a  or  1120 ) may by overmolded liquid crystal polymer (LCP) elements or coatings, which can provide very good separation resistance (high separation resistance) between moving or other parts. LCP layers may also be used in place of polyimide layers in flex circuits and other elements, and may not require the use of temperature or moisture-sensitive adhesives. LCP elements, layers, and coatings absorb less moisture than polyimides in high temperature and high humidity environments, which can be useful in maintaining high separation resistance between components when sensing low voltage signals (e.g., biometric signals of a person) under varying conditions (e.g., under non-controlled conditions outside a doctor&#39;s office or hospital). LCP elements, layers, and coatings also maintain good separation resistance high temperatures. Other elements, layers, or coatings that may be used to provide electrical isolation include silicone or acrylic elements, layers, or coatings, or other elements having a high surface resistance. Other examples of insulators or insulator positions are described with reference to  FIGS. 11B, 12A, 12B, 13 , &amp;  14 . 
       FIG. 11B  shows another example crown  210  extending through a housing  206  of an electronic watch. As with other embodiments described herein, the crown  210  may be used to provide multiple types of input. For example, the crown may rotate about an axis of rotation (typically extending along a center length of the shaft  1102 , from an exterior of the housing  206  to an interior of the housing) to provide a first type of crown input, translate along the axis of rotation (e.g., move towards and/or away from the housing  206 ) to provide a second type of crown input, and be touch-sensitive to provide a third type of crown input. The first and second types of input may control graphical output on the electronic watch&#39;s display, as described below in more detail with respect to  FIGS. 26A-28B . The third type of input may be a measurement of an electrical signal (such as voltage, capacitance, current, or the like), or facilitating measurements of a differential in an electrical signal, to provide an ECG of a user. That is and as discussed in greater detail herein, the third type of input may be the crown  210  functioning as one of two electrodes in an electrical circuit configured to measure a user&#39;s ECG. Typically, although not necessarily, the second electrode is positioned a back of the electronic watch. This second electrode and its functionality is discussed in more detail elsewhere in this document. 
     The crown  210  may be formed from multiple elements attached together, as discussed in more detail below, or may be a single piece and connected to one another. The crown  210  generally includes a crown body  1104  coupled to (or formed with) a shaft  1102 . The shaft  1102  of the crown may extend through the housing  206  and is typically received in, or passes through, a collar  1124 . The collar  1124  may restrict tilting of the shaft  1102  and crown body  1104 . Further, the collar  1124  may permit translation of the shaft  1102  and crown body  1104  toward and away from the housing and rotation of the shaft and crown body  1104  about the axis of rotation. The collar  1124  may be the same as, or similar to, the shaft retainer  1106  and/or insulator  1108  of  FIG. 11A . 
     One or more O-rings  1134  are fitted about the shaft  1102  and within the collar  1124 . The O-rings  1134  may be received within grooves, depressions, or the like within one or both of the shaft  1102  and collar  1124 . The O-rings form a watertight seal and likewise reduce or eliminate contaminants passing into an interior of the housing  206  through the gap  1136  between the crown  210  and housing  206 . The O-rings  1134  may also permit the shat to rotate and/or translate while restricting (or helping to restrict) how far the shaft  1102  translates. 
     As with the embodiment shown in  FIG. 11A , the crown  210  may short to the housing  206  if water or another conductor is present in the gap  1136  between the crown and housing and the two are not electrically insulated from one another. Shorting the crown  210  to the housing  206  results in the electronic watch unreliably measuring and displaying an ECG of a user or not functioning at all. 
     Accordingly and similar to the embodiment of  FIG. 11A , an underside of the crown  210  may be coated with an electrically insulating coating  1132 . The coating  1132  may prevent water or another contaminant from acting as a short or ground path between the crown  210  and housing  206 . As discussed with respect to  FIG. 11A  and shown in that figure, such a coating may be applied to the housing  206  in addition to, or instead of, the crown  210 . 
     The crown  210  shown in  FIG. 11B  is not formed from a single piece of material but instead is formed from multiple elements. The crown body  1104  and shaft  1102  may be formed from or as a single piece of material, for example metal or another suitable conductor, and provide an electrical path between an object touching the crown body  1104  and a sensor within the housing  206 , such as the third sensor  230  discussed above. An insulating split  1146  may separate the crown body from a trim  1148 ; the trim  1148  may be annular, square, or any other suitable shape. The trim and split  1146  may provide various aesthetic looks as well as functional properties, such as different wear resistance, environmental resistance, and the like as compared to the crown body  1104 . Accordingly, the trim  1148  may be made from the same material or a different material as the crown body  1104  and the shaft  1102 . 
     Insofar as the split  1146  is an electrical insulator, the coating  1132  need not extend across the split or onto any portion of the trim  1148  (although it can in some embodiments). Thus, the coating  1132  may stop at an edge of the crown body  1104  abutting the split  1146 . This may reduce manufacturing and assembly complexity of embodiments, as well as provide cost savings. 
     In addition to, or instead of, providing a coating  1132  on the crown  210  or housing  206 , the collar  1124  may be coated. For example, an electrically insulating coating  1126  may be deposited on the collar  1124  and serve to electrically insulate the collar from the housing  206  and/or crown  210 . This may be useful when the collar is made from an electrically conductive material and the crown  210  may be shorted to the collar  1124 , in addition to or instead of to the housing  206 . 
     As one non-limiting example, capillary action may retain water (or another liquid) in a portion of the gap between the collar  1124  and crown  210  while the part of the gap  1136  between the crown and housing  206  is sized to permit water to drain out. Thus, in such an embodiment the crown  210  may be at risk of electrically shorting to the collar  1124  but not the housing  206 . It should be appreciated that in some embodiments the housing  206  and/or crown  210  may be coated as well as the collar  1124 . It should likewise be appreciated that any or all of the electrically insulating coatings described herein may attenuate noise with respect to a signal conducted from the crown body  1104  through the shaft  1102  to a sensor, thereby providing more accurate and/or faster readings of a biological parameter such as an ECG. 
     Although the coating  1126  (and coating  1132 ) has been discussed as an electrical insulator, it should be appreciated that the coating(s) may provide other properties in addition to, or instead of, electrical insulation. For example, the coating  1126  may reduce friction between the collar  1124  and shaft  1102  as the shaft  1102  rotates and/or translates. The coating  1126  may reduce wear on either or both of the collar  1124  and shaft  1102  as another example. 
     Further, in some embodiments the gap  1136  between the crown  210  and housing  206  may be large enough that the collar  1124  may be visible. In order to obscure the first collar coating and/or the collar, a second collar coating  1128  may be applied over the first, insulating collar coating  1126 . The second collar coating  1128  may be darker or otherwise visually conceal the first collar coating  1128  and collar  1124  from the naked eye. 
     As yet another option, the second coating  1128  may also provide environmental resistance and/or resist wear, tear, and/or friction between the collar  1124  and shaft  1102 , as described above. Thus, the first collar coating  1126  may be an electrical insulator while the second collar coating  1128  may be chosen for its other material properties and/or resistances. Similarly, the first collar coating may be chosen for its material properties and/or resistances (including functioning as an electrical insulator) and the second collar coating may be used to obscure the first collar coating. 
     Any or all of the coatings described herein may be deposited in a number of ways, including electrophoretic deposition or other manners that are suitable and known in the art. Likewise, any or all of the coatings may incorporate materials such as titanium dioxide, Teflon, or the like to provide or enhance properties such as resistance to wear, lowering of friction between adjacent elements, and the like. In some embodiments the first collar coating  1126  (or any other coating) may be approximately 10-30 microns thick or even 5-50 microns. The second collar coating  1128  (or any other coating) may be thinner on the order of 3-5 microns or even 2-10 microns. 
     More detailed examples of the crown assembly  1100  described with reference to  FIG. 11A  are shown in  FIGS. 12A, 12B, 13 , &amp;  14 . 
     Turning now to  FIGS. 12A &amp; 12B , there is shown an example of a crown assembly  1200 , as may be included in any of the electronic devices described with reference to  FIGS. 1A, 1B, 2A-2C, 3, 4A , or  4 B.  FIG. 12A  shows an exploded view of the crown assembly  1200 , and  FIG. 12B  shows an assembled cross-section of the crown assembly  1200 , as viewed from the front or rear face of an electronic device such as the watch body  202  described with reference to  FIGS. 2A-2C, 3, 4A , &amp;  4 B. 
     The crown assembly  1200  is an example of the crown assembly  1100  shown in  FIG. 11 , and includes components corresponding to the crown  210 , shaft retainer  1106 , insulator  1108 , tactile switch  1110 , substrate  1112 , spring-biased conductor  1114 , circuit  1116 , and optical encoder  1118 . 
     The crown assembly  1200  may include a conductive rotatable shaft  1202  configured to extend through an opening in a housing  1242  (see  FIG. 12B ), such as the housing described with reference to  FIG. 2A, 2C, 3, 4A, 4B , or  11 . A user-rotatable crown  1204  may be mechanically attached to the shaft  1202  exterior to the housing  1242 . The crown  1204  may be rotated by a user of an electronic watch, to in turn rotate the shaft  1202 . In some cases, the crown  1204  may also be pulled or pushed by the user to translate the shaft  1202  along its axis. The crown  1204  may be electrically connected to a circuit within the housing  1242 , but electrically isolated from the housing  1242 . 
     The crown  1204  may be electrically connected to the shaft  1202 . In some cases, at least part of the crown  1204  and at least part of the shaft  1202  may be molded, machined, or otherwise formed together (e.g., from a same material, such as a conductive ceramic or stainless steel). 
     In some embodiments, the crown  1204  may be formed of a conductive ceramic or stainless steel (or have a conductive ceramic or stainless steel core). The core may be coated in a PVD deposited layer of SUS or DLC, or an electro-deposited (ED) layer of AlTiN or CrSiCN, and may function as an electrode. In some embodiments, the crown  1204  may have a conductive crown body  1244  surrounded by a ring  1246  of non-conductive material (or other insulator). See,  FIG. 12B . The non-conductive ring  1246  may help prevent shorting of the crown  1204  to the housing  1242 . The ring  1246  of non-conductive material may in some cases be surrounded by another ring  1248  of conductive material. In the configuration shown, the ring  1248  may optionally contact and electrically short to a grounded housing  1242  when the crown is pressed, thereby helping to electrically shield the conductive crown body  1244  of the crown  1204  from some sources of interference. In some embodiments, the crown  1204  may have a conductive surface covered by a thin non-conductive coating. The non-conductive coating may provide a dielectric for capacitive coupling between the conductive surface and skin of a user of the crown  1204  (or an electronic watch or other device that includes the crown assembly  1200 ). In the same or different embodiments, the crown  1204  may have a non-conductive coating on a surface of the crown  1204  facing the housing  1242 . 
     A shaft retainer  1206  may be mechanically connected to the shaft  1202 , interior to the housing  1242  (e.g., interior to a watch body housing), after the shaft is inserted through the opening in the housing  1242  with the crown  1204  positioned exterior to the housing  1242 . In some cases, the shaft retainer  1206  may include a nut, and the shaft  1202  may have a threaded male portion that engages a threaded female portion of the nut. In some cases, the shaft retainer  1206  may be conductive, or have a conductive coating thereon, and mechanical connection of the shaft retainer  1206  to the shaft  1202  may form an electrical connection between the shaft retainer  1206  and the shaft  1202 . In an alternative embodiment (not shown), the shaft retainer  1206  may be integrally formed with the shaft  1202 , and the shaft  1202  may be inserted through the opening in the housing  1242  from inside the housing  1242  and then attached to the crown  1204  (e.g., the crown  1204  may screw onto the shaft  1202 ). 
     In some embodiments, a collar  1208  may be aligned with the opening in the housing  1242 , and a collar retainer  1210  may be coupled to the collar  1208  to retain the collar  1208  to the housing  1242  from a side of the housing  1242  opposite a side of the housing  1242  in which the collar  1208  is inserted. In some embodiments, the collar retainer  1210  may be coupled to the collar  1208  via threads on a male portion of the collar  1208  and corresponding threads on a female portion of the collar retainer  1210 . Optionally, a gasket  1212  (e.g., an I-ring) made of a synthetic rubber and fluoropolymer elastomer (e.g., Viton), silicone, or another compressible material may be placed over the collar  1208  prior to insertion of the collar  1208  through the opening, and attachment of the collar retainer  1210  to the collar  1208  may compress the gasket  1212 . The compressed gasket  1212  may provide stability to the collar  1208  and collar retainer  1210 , or provide a moisture barrier between the collar  1208  and the housing  1242 . The collar  1208  and collar retainer  1210  may be attached to one another, and thereby to the housing  1242 , prior to insertion of the shaft  1202  through the collar  1208 . Another gasket  1214  (e.g., a Y-ring) made of Viton, silicone, or another compressible material may be placed over the collar  1208 , before or after insertion of the collar  1208  through the opening, but before the shaft  1202  is inserted through the collar  1208 . The second gasket  1214  may provide a moisture barrier between the crown  1204  and the housing  1242  or the crown  1204  and the collar  1208 . 
     Also prior to inserting the shaft  1202  through the collar  1208 , and in some cases prior to inserting the collar  1208  into the opening in the housing  1242 , an insulator  1216  may be inserted into or deposited on the interior of the collar  1208 , or placed around or deposited on the shaft  1202 . The insulator  1216  may also be inserted, placed, or deposited as the shaft  1202  is inserted into the collar  1208 . In some cases, the insulator  1216  may include a non-conductive sleeve or bushing (e.g., a plastic sleeve) inserted (e.g., press-fit) into the collar  1208  (e.g., into a portion of the collar  1208  positioned interior to the housing  1242 ). The insulator  1216  may also or alternatively include a non-conductive sleeve overmolded on the collar (e.g., molded within the opening in the collar  1208  and over a surface of the collar  1208  facing the crown  1204 ). In some cases, the insulator  1216  may be an overmolded liquid crystal polymer (LCP) insulator  1216 . The insulator  1216  may also or alternatively include a non-conductive coating on the collar  1208  (e.g., on an inner surface of the collar  1208 ), or a non-conductive coating on the shaft  1202 , or a set of one or more non-conductive gaskets surrounding the shaft  1202 . When the shaft  1202  is inserted into the collar  1208 , the insulator  1216  may be positioned between the shaft  1202  and the collar  1208  and help to insulate a conductive portion of the shaft  1202  (or the entire shaft  1202 ) from the collar  1208 . 
     Another insulator  1218  may be positioned between the shaft retainer  1206  and the collar retainer  1210 . For example, a non-conductive (e.g., plastic) washer, plate, or shim may be attached to the interior of the collar retainer  1210 , between the shaft retainer  1206  and the collar retainer  1210 . In some cases, the non-conductive washer may be carried by a plate  1220 , such as a plate formed of stainless steel (e.g., the insulator  1218  may be an overmolded LCP insulator  1218 ). In these cases, the non-conductive washer may be attached to the interior of the collar retainer  1210  by welding (e.g., laser welding) the plate  1220  to the collar retainer  1210 . The non-conductive washer or other element may provide a bearing surface for the shaft retainer  1206 . 
     As shown in  FIGS. 12A &amp; 12B , one or more O-rings  1222 ,  1224  or other gaskets may be placed over the shaft  1202  before the shaft  1202  is inserted into the collar  1208 . The O-rings  1222 ,  1224  may be formed of a synthetic rubber, fluoropolymer elastomer, silicone, or another compressible material. The O-rings  1222 ,  1224  may maintain the shaft  1202  in a position that is centered within the collar  1208 . In some cases, the O-rings  1222 ,  1224  may provide a seal between the shaft  1202  and the collar  1208 . The O-rings  1222 ,  1224  may also function as an insulator between the shaft  1202  and the collar  1208 . In some embodiments, the O-rings  1222 ,  1224  may be fitted to recesses in the shaft  1202 . Additionally, a low-friction ring  1306  or filler may be placed around the top of the collar  1208 , between the crown  1204  and the collar  1208 . Alternatively, the low-friction ring  1250  or filler may be attached to the crown  1204 , between the crown  1204  and the collar  1208 . In some embodiments, the shaft  1202  may be smooth (not shown) and rotate within a thicker or closer fitting insulator  1216  without use of the O-rings  1222 ,  1224 . 
     In some embodiments, a bracket  1226  may be attached (e.g., laser welded) to the collar retainer  1210  or another element within the housing  1242 . The bracket  1226  may support a spring-biased conductor  1228  and maintain the spring-biased conductor  1228  in mechanical and electrical contact with the shaft retainer  1206  (or in some cases with an end of the shaft  1202 , such as when the shaft extends through the shaft retainer (not shown)). As shown, the spring-biased conductor  1228  may include a shear plate that is spring-biased about an axis  1238 , which axis  1238  is perpendicular to and radially outward from a second axis  1240  of the shaft  1202 . By way of example, the shear plate is shown to be circular, although the shear plate could also have other shapes. In some embodiments, the surface of the shear plate that abuts the shaft retainer  1206  or shaft end may be hardened (e.g., with a PVD deposited coating of cobalt chromium (CoCr or hard chromium)) to mitigate the likelihood of the shaft retainer  1206  or shaft end wearing through the shear plate after multiple rotations or translations of the shaft  1202 . The shear plate (and in some cases the entirety of the spring-biased conductor  1228 ) may be plated with gold or another material to improve electrical conductivity (e.g., prior to coating the shear plate with a hardener). In some cases, the spring-biased conductor  1228  may be formed (e.g., stamped or bent) from a piece of metal (e.g., stainless steel). In other cases, the spring-biased conductor  1228  may be formed in other ways. The length and thickness of the shear plate, perpendicular to the axis of the shaft  1202 , can be optimized to provide a balance between a high enough spring constant to ensure good electrical contact between the shear plate and the shaft retainer  1206  or shaft end (even during rotation of the shaft  1202 ), on one hand, and a low enough spring constant to mitigate the likelihood that the shaft retainer  1206  or shaft end will wear through the shear plate (or through a coating thereon). A flat or relatively flat shear plate can reduce the dimension of the crown assembly  1200  along the axis  1240  of the shaft  1202 . 
     In some embodiments, a majority or entirety of the shaft  1202 , shaft retainer  1206 , or crown  1204  may be coated with a non-conductive coating, but for an external conductive surface of the crown  1204  and a portion of the shaft  1202  or shaft retainer  1206  that contacts the spring-biased conductor  1228 . 
     When the shaft  1202  is translatable, translation of the shaft  1202  into the housing  1242  (e.g., into the housing of a watch body) may cause the spring-biased conductor  1228  (or the shear plate thereof) to deform. However, the spring bias of the spring-biased conductor  1228  may cause the spring-biased conductor  1228  (or the shear plate thereof) to maintain electrical contact with the shaft retainer or shaft end, regardless of whether the shaft  1202  is in a first position or a second position with reference to translation of the shaft  1202 . The spring-biased conductor  1228  may be electrically connected to a circuit, such as a circuit formed on or in a substrate  1230  such as a flex circuit or printed circuit board (PCB). In some cases, the spring-biased conductor  1228  may be surface-attached to the circuit substrate  1230  (such as soldered or otherwise mechanically connected, for example by using a surface-mount technology process), which circuit substrate  1230  may be supported by the rigid support member (or sub-housing frame member)  1226 . A conductive grease may be deposited between the shaft retainer  1206  or shaft  1202  and the shear plate or other member of the spring-biased conductor  1228 . The circuit may be in electrical communication with the crown  1204  via the spring-biased conductor  1228 , the shaft retainer  1206 , and the shaft  1202  (or when an end of the shaft  1202  protrudes through the shaft retainer  1206 , the circuit may be in electrical communication with the crown  1204  via the spring-biased conductor  1228  and the shaft  1202 ). 
     A tactile (tac) switch  1252 , such as a dome switch, may be electrically connected to the circuit and mechanically connected to the circuit substrate  1230 . In some cases, the tac switch  1252  may be surface-attached to the circuit substrate  1230  (such as soldered or otherwise mechanically connected). The shear plate of the spring-biased conductor  1228  may be positioned between the shaft retainer  1206  and the tac switch  1252 . The tac switch  1252  may be actuated or change state in response to translation of the shaft  1202 . Thus, when a user presses on the crown  1204 , the shaft  1202  may translate into the housing  1242  (e.g., into the housing of a watch body) and actuate the tac switch  1252 , placing the tac switch  1252  in one of a number of states. When the user releases pressure on the crown  1204  or pulls the crown  1204  outward from the housing  1242 , the tac switch  1252  may retain the state in which it was placed when pressed, or advance to another state, or toggle between two states, depending on the type or configuration of the tac switch  1252 . 
     The circuit to which the tac switch  1252  and spring-biased conductor  1228  are electrically connected may be part of, or electrically connected to, one or more circuits that carry portions of an optical encoder  1232  and other circuit elements, such as an interface  1234  to the electrodes described with reference to  FIGS. 5C, 5D, 5E, 6-8, 9A-9C , &amp;  10 A- 10 D, or a processor that receives and processes signals received from or provided to the crown  1204  or other electrodes. By way of example,  FIG. 12A  shows a circuit  1236  (e.g., a flex circuit or PCB) to which a set of one or more light emitters and light detectors of an optical encoder  1232  is connected. The light emitter(s) may illuminate an encoder pattern or other rotating portion of the optical encoder  1232 , which encoder pattern or other rotating portion of the optical encoder  1232  may be carried on (e.g., formed on, printed on, etc.) the shaft retainer  1206 . The light detector(s) may receive reflections of the light emitted by the light emitter(s), and a processor may determine a direction of rotation, speed of rotation, angular position, translation, or other state(s) of the crown  1204  and shaft  1202 . 
     The spring-biased conductor  1228  may be connected to a processor. The processor may be attached or coupled to one or more of the circuits shown in  FIG. 12A . The processor may determine whether a user is touching the crown  1204 , or determine a biological parameter of the user based on a signal received from or provided to the user via the crown  1204 , or determine other parameters based on signals received from or provided to the crown  1204 . In some cases, the processor may operate the crown and electrodes described in  FIGS. 5C, 5D, 5E, 6-8, 9A-9C, 10A-10D, 11, 12A , &amp;  12 B as an electrocardiogram and provide an ECG to a user of a watch including the crown and electrodes. 
     In an alternate embodiment of the crown assembly  1200  shown in  FIGS. 12A &amp; 12B , the spring-biased conductor  1228  may include a conductive brush that is biased to contact a side of the shaft  1202  or a side of the shaft retainer  1206 . The conductive brush may maintain electrical contact with the shaft  1202  or shaft retainer  1206  through rotation or translation of the shaft  1202 , and may be electrically connected to a circuit such as the circuit that supports the tac switch  1352 . 
       FIG. 13  shows a cross-section of a crown assembly  1300  as viewed from an edge of a watch body (e.g., an edge to which a watch band might be attached). The crown assembly  1300  shown in  FIG. 13  differs from the crown assembly  1200  shown in  FIGS. 12A &amp; 12B  in that the crown  1302  has a somewhat different configuration. For example, the crown  1302  has a conductive crown body  1304  surrounded by a non-conductive ring  1306 . The non-conductive ring  1306  may be attached to the conductive crown body  1304  by an adhesive  1308 . Alternatively, the ring  1306  may be conductive, and may be insulated from the conductive crown body  1304  by the adhesive  1308  (e.g., when the adhesive  1308  is non-conductive) or electrically connected to the conductive crown body  1304  (e.g., when the adhesive  1308  is conductive). 
     As shown in  FIG. 13 , the crown assembly  1300  may be positioned adjacent a transparent cover  1310  (e.g., a carrier) under which a display may be attached, such that the display is at least partially or fully within the housing. In some cases, the display may be a touch-sensitive display. In some embodiments, the display may also be a force-sensitive display.  FIG. 13  shows one example of a force sensor  1312  for a force-sensitive display, in which a compressible gasket  1314  is bounded by first and second capacitive plates  1316 ,  1318  and positioned between the carrier  1310  and the housing  1242 . 
       FIG. 14  shows a cross-section of a crown assembly  1400 , as may be included in any of the electronic devices described with reference to  FIG. 1A, 1B, 2A-2C, 3, 4A , or  4 B. Similarly to the crown assemblies  1200  and  1300  shown in  FIGS. 12A, 12B , &amp;  13 , the crown assembly  1400  is an example of the crown assembly  1100  shown in  FIG. 11 . 
     The crown assembly  1400  is similar to the crown assembly  1200  in that its crown  1402  has a conductive crown body  1404  surrounded by an inner ring  1406  of non-conductive material and an outer ring  1408  of conductive material. The conductive crown body  1404  may be formed of a conductive ceramic or stainless steel, and may be coated in a PVD deposited layer of SUS or DLC, or an ED layer of AlTiN or CrSiCN, and may function as an electrode. The non-conductive inner ring  1406  may help prevent shorting of the crown  1402  to the housing  1242 , and may be formed of a plastic or elastomer, for example. The conductive outer ring  1408  may be formed of the same or different material(s) as the conductive crown body  1404 . 
     The non-conductive inner ring  1406  may extend from an outer surface of the crown  1402  to under a portion of the conductive crown body  1404 . In this manner, the non-conductive inner ring  1406  may prevent the conductive crown body  1404  from contacting the collar  1208  when the crown  1402  is translated toward the housing  1242 . 
     The conductive outer ring  1408  may extend from an outer surface of the crown  1402  to under a portion of the non-conductive inner ring  1406 . In this manner, if the housing  1242  is grounded and the conductive outer ring  1408  contacts the housing  1242 , the conductive outer ring  1408  may be grounded to the housing  1242 . 
     In contrast to the crown assemblies  1200  and  1300 , the crown assembly  1400  has an insulator  1410  (e.g., a non-conductive element, layer, or coating) applied to at least one surface of the collar  1208  (e.g., to at least a portion of the surface or surfaces that face an underside of the conductive crown body  1404 ). The insulator  1410  may further prevent the conductive crown body  1404  from contacting the collar  1208  when the crown  1402  is translated toward the housing  1242  and may provide increased separation resistance between the collar  1208  and the crown  210 . In some embodiments, the insulator  1410  may include a layer of plastic that is overmolded (e.g., LCP overmolded) on at least a portion (or all) of the collar  1208  that faces the crown  210  (or a plastic element that is placed over or adhered to at least a portion (or all) of the collar  1208 , or a coating that is applied to at least a portion of the collar  1208 ). In some embodiments, the plastic may extend to adjacent surfaces of the housing  206 , or into the central opening in the collar  1208 . In some embodiments, the insulator  1410  may include a coating (e.g., an electro-deposited (ED) acrylic-based polymer coating). Alternatively or additionally, an insulator (e.g., an element, layer, or coating) may be applied to the underside of the conductive crown body  1404 , or to surfaces of the shaft  1202  that face the collar  1208  and/or housing  206 . Alternatively, the collar  1208  may be formed from plastic or another material that is non-conductive or otherwise electrically isolates the conductive crown body  1404  of the crown  1402 , or the shaft  1202 , from other conductive components of the crown assembly  1500 . 
     In any of the crown assemblies  1200 ,  1300 ,  1400  described in the present disclosure, the crown  1204 ,  1302 , or  1402  may alternately be a monolithic structure and not include additional conductive or non-conductive rings, or may include a single non-conductive (e.g., plastic) ring surrounding a conductive central portion. 
     Turning now to the implementation of an electrode on a button,  FIG. 15  shows an example cross-section of a button assembly  1500 . The button assembly  1500  may be an example of a button assembly included in any of the electronic devices described with reference to  FIG. 1A, 1B, 2A-2C, 3, 4A , or  4 B. 
     The button assembly  1500  may include a conductive button cap  1502 . The conductive button cap  1502  may be retained within an opening in a housing by a button cap retention assembly  1504 . The button cap retention assembly  1504 , or parts thereof, may be conductive. By way of example, the housing is shown to be the housing  206  of the watch body  202  described with reference to  FIGS. 2A-2C, 3, 4A , &amp;  4 B. The button cap retention assembly  1504  may be attached to the housing  206  and extend through the opening in the housing  206 . In some embodiments, the button cap retention assembly  1504  may include a first component  1506  that is inserted through the opening from one side of the housing  206 , and a second component  1508  that fastens to the first component  1506  on the other side of the housing  206  (e.g., by threads, screws, solder, or an adhesive). 
     A set of one or more insulators  1510  (e.g., electrical insulators) may electrically insulate the button cap retention assembly  1504  from the housing  206 . The insulator  1510  may also electrically insulate the conductive button cap  1502  from the housing  206 . Although the insulator  1510  is generally shown in  FIG. 15  as a singular annular seal around the perimeter of the button assembly  1500 , the insulator  1510  may alternatively include more than one element or be positioned elsewhere within the button assembly  1500 , as described with reference to  FIGS. 16A, 16B, 17A, 17B, 18A , &amp;  18 C. 
     The conductive button cap  1502  may translate toward and away from the housing  206 , and may be in electrical contact with the button cap retention assembly  1504  during all phases of translation. When the conductive button cap  1502  is pressed by a user and translates toward the housing  206 , a tactile switch  1512  may be actuated (e.g., switched between two or more states). A shaft  1514  may extend from an interior surface of, or be formed integrally with, the conductive button cap  1502  and a depressible surface of the tactile switch  1512 . The tactile switch  1512  and shaft  1514 , or other elements not shown in  FIG. 15  (e.g., springs), may bias the conductive button cap  1502  in an outwardly translated position. 
     The conductive button cap  1502  may function as an electrode, and an electrical signal may be routed between the conductive button cap  1502  and a circuit  1516 , at least in part, via the button cap retention assembly  1504 . In some embodiments, the button cap retention assembly  1504 , tactile switch  1512 , and circuit  1516  may be attached to a common substrate  1518 . 
     Because the signals received by or propagated from the conductive button cap  1502  may be low voltage or low amplitude signals, the materials, positions, electrical connections to, and electrical routing paths for an electrode formed on or by the conductive button cap  1502  can have a significant impact on the ability of the circuit  1516  to discern useful signals representing an ECG or other biological parameter of a person wearing an electronic device including the button assembly  1500 . The materials, positions, electrical connections to, and electrical routing paths for the button assembly  1500  can also determine how well the button assembly  1500  receives voltages/signals from a person&#39;s skin (e.g., a SNR of a device-to-user interface through which the voltages/signals pass); how well voltages/signals are transferred between the conductive button cap  1502  and internal components of an electronic device (e.g., a voltage/signal propagation SNR); and how well the button assembly  1500  operates in the face of environmental factors, such as temperature, humidity, moisture, electromagnetic radiation, dust, and so on. In some cases, the insulator  1510  may be positioned to prevent moisture from electrically shorting the conductive button cap  1502  to the housing  206 , or the housing  206  may be grounded to provide electrical shielding for some or all of the signals propagated through the button assembly  1500 . 
     More detailed examples of the button assembly  1500  described with reference to  FIG. 15  are shown in  FIGS. 16A, 16B, 17A, 17B, 18A , &amp;  18 B. 
     Turning now to  FIGS. 16A &amp; 16B , there is shown an example of a button assembly  1600  that may be included in any of the electronic devices described with reference to  FIG. 1A, 1B, 2A-2C, 3, 4A , or  4 B.  FIG. 16A  shows an exploded view of the button assembly  1600 , and  FIG. 16B  shows an assembled cross-section of the button assembly  1600 , as viewed from the front or rear face of an electronic device such as the watch body  202  described with reference to  FIGS. 2A-2C, 3, 4A, and 4B . 
     The button assembly  1600  is an example of the button assembly  1500  shown in  FIG. 15 , and includes components corresponding to the conductive button cap  1502 , button cap retention assembly  1504 , insulator  1510 , and tactile switch  1512 . 
     The button assembly  1600  may be at least partially within an opening  1602  in a housing  1604  (e.g., an opening in the housing described with reference to  FIG. 2A, 2C, 3, 4A , or  4 B), and attached to the housing or an internal structure. In some cases, and as shown, the housing  1604  may include a cavity  1606  ( FIG. 16B ) defined by a sidewall  1608  and a ledge  1610 . The ledge  1610  may surround the opening  1602 , and the sidewall  1608  may surround the ledge  1610 . 
     The button assembly  1600  may include a conductive button cap  1612 . The conductive button cap  1612  may be retained by a button cap retention assembly  1614 , and may be translatable toward and away from the housing  1604 . The button cap retention assembly  1614  may extend through the opening  1602  and be attached to the housing  1604 . In some examples, the button cap retention assembly  1614  may include a bracket  1616  that overlaps the ledge  1610  interior to the housing  1604 , and a retainer  1618  that overlaps the ledge  1610  exterior to the housing  1604 . The retainer  1618  may be mechanically attached to the bracket  1616  by a set of screws  1620  or other mechanical fastener. The screws  1620  may be inserted into through-holes in the bracket  1616  and screwed into threaded holes in the retainer  1618 , clamping the ledge  1610  between the bracket  1616  and the retainer  1618 . 
     The conductive button cap  1612  may have an exterior surface  1622 , a sidewall or set of sidewalls  1624  parallel to the sidewall  1608  of the cavity  1606 , and an inward facing lip or set of lips  1626  ( FIG. 16B ) that extends between the retainer  1618  and the ledge  1610  and toward a center axis of the conductive button cap  1612 . A set of one or more coil springs  1628  or other spring-biased members may be positioned between an outer surface of the retainer  1618  and an underside of the conductive button cap  1612 , and may bias the conductive button cap  1612  in an outward state of translation. 
     The button cap retention assembly  1614 , and in particular the retainer  1618 , may have a through-hole  1629  defined therein, with an axis of the through-hole  1629  extending perpendicular to the opening  1602  in the housing  1604 . A shaft  1630  may be positioned within the through-hole  1629 , and may translate toward and away from the housing  1604 . The shaft  1630  may be mechanically connected to the conductive button cap  1612 , or may be biased to contact the conductive button cap  1612 . By way of example, the shaft  1630  may be non-conductive. In a state of rest, the shaft  1630  and conductive button cap  1612  may be biased in an outward state of translation (i.e., away from the opening  1602 ) by the coil springs  1628  and/or a spring-biased tactile switch  1632 . When a user presses the conductive button cap  1612  toward the housing  1604 , the press may overcome the bias provided by the coil springs  1628  and/or tactile switch  1632 , and pressure on the conductive button cap  1612  may be transferred to the shaft  1630 , which translates toward the housing  1604  and presses on the tactile switch  1632  to change the state of the tactile switch  1632  (e.g., from ON to OFF or vice versa, from one functional state to another, etc.). The tactile switch  1632  may be aligned with an axis of the shaft  1630  and attached to the bracket  1616  using an adhesive  1634  (e.g., a conductive PSA). 
     In some embodiments, a gasket  1636  (e.g., an O-ring) may be positioned between the shaft  1630  and the through-hole. The shaft  1630  may have a circumferential groove  1638  in which a portion of the gasket  1636  is seated so that the gasket  1636  moves in a predictable way in response to movement of the shaft  1630 . In some examples, the gasket  1636  may be non-conductive. 
     The button assembly  1600  may further include a set of electrical insulators (i.e., one or more electrical insulators), which set of electrical insulators may electrically insulate the button cap retention assembly  1614  from the housing  1604 , and electrically insulate the conductive button cap  1612  from the housing  1604 . For example, the button assembly  1600  may include a first electrical insulator, such as a sleeve  1640  (or set of shims), positioned between the conductive button cap  1612  and the sidewall  1608  (or set of sidewalls) of the cavity  1606  in the housing  1604 . In some cases, the sleeve  1640  may include a closed-shape sidewall and an inward facing lip  1642  ( FIG. 16B ). In other cases, the sleeve  1640  may not include the inward facing lip  1642  or have a sidewall that does not define a closed shape. In other cases, the first electrical insulator may be a planar perimeter gasket (e.g., an insulator including the lip  1642  but not the sidewall). A second electrical insulator may include an adhesive  1644  (e.g., an adhesive ring) applied to a surface of the retainer  1618  facing the housing  1604 , or to the outer surface of the ledge  1610  within the cavity  1606 . In some cases, the adhesive  1644  may include a PSA. A gasket or seal  1646 , external to the housing  1604 , may be bonded to the adhesive  1644 . The adhesive  1644  and seal  1646  may be compressed when the screws  1620  are tightened to clamp the housing  1604  between the bracket  1616  and retainer  1618  of the button cap retention assembly  1614 . A third electrical insulator may include a spacer  1648 , internal to the housing  1604 , positioned between the bracket  1616  and the housing  1604 . The third electrical insulator, in conjunction with the first and/or second electrical insulator, may electrically insulate the conductive button cap retention assembly  1614  (e.g., the bracket  1616  and the retainer  1618 ) from the housing  1604 . The first electrical insulator may electrically insulate the conductive button cap  1612  from the housing  1604 . In some embodiments, additional or different electrical insulators may electrically insulate the conductive button cap  1612  or button cap retention assembly  1614  from the housing  1604 . 
     In use, a signal may be applied to, or received from, the button cap retention assembly  1614  (e.g., to/from the bracket  1616 ) via a circuit (e.g., a flex circuit or other circuit element  1650 ) that is electrically connected to the bracket  1616  (e.g., via a conductive adhesive  1652 ). A signal may travel through the conductive button cap  1612 , coil springs  1628 , retainer  1618 , screws  1620 , and bracket  1616  via a first electrical path, or through the conductive button cap  1612 , retainer  1618 , screws  1620 , and bracket  1616  via a second electrical path. Although the second electrical path may be broken when the conductive button cap  1612  is pressed by a user, the conductive button cap  1612  may remain in electrical contact with the button cap retention assembly  1614  during all states of translation (e.g., via the first electrical path). 
     With reference to  FIGS. 17A &amp; 17B , there is shown another example of a button assembly  1700  that may be included in any of the electronic devices described with reference to  FIG. 1A, 1B, 2A-2C, 3, 4A , or  4 B.  FIG. 17A  shows an exploded view of the button assembly  1700 , and  FIG. 17B  shows an assembled cross-section of the button assembly  1700 , as viewed from the front or rear face of an electronic device such as the watch body  202  described with reference to  FIGS. 2A-2C, 3, 4A , &amp;  4 B. 
     The button assembly  1700  is an example of the button assembly  1500  shown in  FIG. 15 , and includes components corresponding to the conductive button cap  1502 , button cap retention assembly  1504 , insulator  1510 , and tactile switch  1512 . 
     The button assembly  1700  may be at least partially within an opening  1702  in a housing  1704  (e.g., an opening in the housing described with reference to  FIG. 2A, 2C, 3, 4A , or  4 B), and may be attached to any the housing or an internal structure. In some cases, and as shown, the housing  1704  may include a cavity  1706  ( FIG. 17B ) defined by at least one sidewall (e.g., a single sidewall  1708  or set of sidewalls) and a ledge  1710 . The ledge  1710  may define the opening  1702 , and the sidewall  1708  may surround the ledge  1710 . 
     The button assembly  1700  may include a conductive button cap  1712  (or button cap having a conductive portion). The conductive button cap  1712  may be retained by a button cap retention assembly  1714  (or button retainer), and may be translatable toward and away from the housing  1704 . The button cap retention assembly  1714  may extend through the opening  1702  and be connected or otherwise attached to the housing  1704 . In some examples, the button cap retention assembly  1714  may include a bracket  1716  that overlaps the ledge  1710  interior to the housing  1704 , and a retainer  1718  that overlaps the ledge  1710  exterior to the housing  1704 . The retainer  1718  may be mechanically attached to the bracket  1716  by a set of screws  1720  or other mechanical fastener. The screws  1720  may be inserted into through-holes in the bracket  1716  and screwed into threaded holes in the retainer  1718 , clamping the ledge  1710  between the bracket  1716  and the retainer  1718 . 
     The conductive button cap  1712  may have an exterior surface  1722 , a sidewall or set of sidewalls  1724  parallel to the sidewall  1708  of the cavity  1706 , and an inward facing lip or set of lips  1726  ( FIG. 19 ) that extends between the retainer  1718  and the ledge  1710  and toward a center axis of the conductive button cap  1712 . A set of one or more coil springs  1728  or other spring-biased members may be positioned between an outer surface of the retainer  1718  and an underside of the conductive button cap  1712 , and may bias the conductive button cap  1712  in an outward state of translation. 
     The button cap retention assembly  1714 , and in particular the retainer  1718 , may have a through-hole  1729  defined therein, with an axis of the through-hole  1729  extending perpendicular to the opening  1702  in the housing  1704 . A shaft  1730  may be positioned within the through-hole  1729 , and may translate toward and away from the housing  1704 . The shaft  1730  may be mechanically connected to the conductive button cap  1712 , or may be biased to contact the conductive button cap  1712 . In some cases, the shaft  1730  may be mechanically and electrically connected to the conductive button cap  1712 . In a state of rest, the shaft  1730  and conductive button cap  1712  may be biased in an outward state of translation (i.e., away from the opening  1702 ) by the coil springs  1728  and/or a spring-biased tactile switch  1732 . In some cases, a shim  1734 , such as a non-conductive shim, may be attached to an end of the shaft  1730  facing the tactile switch  1732 . When a user presses the conductive button cap  1712  toward the housing  1704 , the press may overcome the bias provided by the coil springs  1728  and/or tactile switch  1732 , and pressure on the conductive button cap  1712  may be transferred to the shaft  1730 , which translates toward the housing  1704  and presses on the tactile switch  1732  to change the state of the tactile switch  1732  (e.g., from ON to OFF or vice versa, from one functional state to another, etc.). The tactile switch  1732  may be aligned with an axis of the shaft  1730  and attached to the bracket  1716  using an adhesive  1736  (e.g., a conductive PSA). 
     In some embodiments, a gasket  1738  (e.g., an O-ring) may be positioned between the shaft  1730  and the through-hole. The shaft  1730  may have a circumferential groove  1740  ( FIG. 17B ) in which a portion of the gasket  1738  is seated so that the gasket  1738  moves in a predictable way in response to movement of the shaft  1730 . In some examples, the gasket  1738  may be conductive. 
     The button assembly  1700  may further include a set of electrical insulators (i.e., one or more electrical insulators), which set of electrical insulators may electrically insulate the button cap retention assembly  1714  from the housing  1704 , and electrically insulate the conductive button cap  1712  from the housing  1704 . For example, the button assembly  1700  may include a first electrical insulator, such as a sleeve  1742  (or set of shims), positioned between the conductive button cap  1712  and the sidewall  1708  (or set of sidewalls) of the cavity  1706  in the housing  1704 . In some cases, the sleeve  1742  may include a closed-shape sidewall and an inward facing lip  1744  ( FIG. 18B ). In other cases, the sleeve  1742  may not include the inward facing lip  1744  or have a sidewall that does not define a closed shape. In other cases, the first electrical insulator may be a planar perimeter gasket (e.g., an insulator including the lip  1744  but not the sidewall). A second electrical insulator may include an adhesive  1746  (e.g., an adhesive ring) applied to a surface of the retainer  1718  facing the housing  1704 , or to the outer surface of the ledge  1710  within the cavity  1706 . In some cases, the adhesive  1746  may include a PSA. A gasket or seal  1748 , external to the housing  1704 , may be bonded to the adhesive  1746 . The adhesive  1746  and seal  1748  may be compressed when the screws  1720  are tightened to clamp the housing  1704  between the bracket  1716  and retainer  1718  of the button cap retention assembly  1714 . A third electrical insulator may include a spacer  1750 , internal to the housing  1704 , positioned between the bracket  1716  and the housing  1704 . The third electrical insulator, in conjunction with the first and/or second electrical insulator, may electrically insulate the conductive button cap retention assembly  1714  (e.g., the bracket  1716  and the retainer  1718 ) from the housing  1704 . The first electrical insulator may electrically insulate the conductive button cap  1712  from the housing  1704 . In some embodiments, additional or different electrical insulators may electrically insulate the conductive button cap  1712  or button cap retention assembly  1714  from the housing  1704 . 
     In use, a signal may be applied to, or received from, the button cap retention assembly  1714  (e.g., to/from the bracket  1716 ) via a circuit (e.g., a flex circuit or other circuit element) that is electrically connected to the bracket  1716  (e.g., as described with reference to  FIG. 17A ). A signal may travel through the conductive button cap  1712 , shaft  1730 , conductive gasket  1738 , retainer  1718 , screws  1720 , and bracket  1716  via a first electrical path. The signal may also travel through the conductive button cap  1712 , coil springs  1728 , retainer  1718 , screws  1720 , and bracket  1716  via a second electrical path, or through the conductive button cap  1712 , retainer  1718 , screws  1720 , and bracket  1716  via a third electrical path. Although the third electrical path may be broken when the conductive button cap  1712  is pressed by a user, the conductive button cap  1712  may remain in electrical contact with the button cap retention assembly  1714  during all states of translation (e.g., via the first and second electrical paths). 
       FIGS. 18A &amp; 18B  show another example of a button assembly  2000  that may be included in any of the electronic devices described with reference to  FIG. 1A, 1B, 2A-2C, 3, 4A , or  4 B.  FIG. 18A  shows an exploded view of the button assembly  1800 , and  FIG. 18B  shows an assembled cross-section of the button assembly  1800 , as viewed from the front or rear face of an electronic device such as the watch body  202  described with reference to  FIGS. 2A-2C, 3, 4A , &amp;  4 B. 
     The button assembly  1800  is an example of the button assembly  1500  shown in  FIG. 15 , and includes components corresponding to the conductive button cap  1502 , button cap retention assembly  1504 , insulator  1510 , and tactile switch  1512 . 
     The button assembly  1800  may be at least partially within an opening  1802  in a housing  1804  (e.g., an opening in the housing described with reference to  FIG. 2A, 2C, 3, 4A , or  4 B), and may be attached to the housing or an internal structure such as a support. In some cases, and as shown, the housing  1804  may include a cavity  1806  ( FIG. 18B ) defined by at least one sidewall (e.g., a single sidewall  1808  or set of sidewalls) and a ledge  1810 . The ledge  1810  may define the opening  1802 , and the sidewall  1808  may surround the ledge  1810 . 
     The button assembly  1800  may include a conductive button cap  1812  (or button cap having a conductive portion). The conductive button cap  1812  may be retained by a button cap retention assembly  1814  (or button retainer), and may be translatable toward and away from the housing  1804 . The button cap retention assembly  1814  may extend through the opening  1802  and be connected or otherwise attached to the housing  1804 . In some examples, the button cap retention assembly  1814  may include a bracket  1816  that overlaps the ledge  1810  interior to the housing  1804 , and a retainer  1818  that overlaps the ledge  1810  exterior to the housing  1804 . The retainer  1818  may be mechanically attached to the bracket  1816  by a set of screws  1820  or other mechanical fastener. The screws  1820  may be inserted into through-holes in the bracket  1816  and screwed into threaded holes in the retainer  1818 , clamping the ledge  1810  between the bracket  1816  and the retainer  1818 . 
     The conductive button cap  1812  may have an exterior surface  1822 , a sidewall or set of sidewalls  1824  parallel to the sidewall  1808  of the cavity  1806 , and an inward facing lip or set of lips  1826  ( FIG. 18B ) that extends between the retainer  1818  and the ledge  1810  and toward a center axis of the conductive button cap  1812 . A set of one or more coil springs  1828  or other spring-biased members may be positioned between an outer surface of the retainer  1818  and an underside of the conductive button cap  1812 , and may bias the conductive button cap  1812  in an outward state of translation. 
     The button cap retention assembly  1814 , and in particular the retainer  1818 , may have a through-hole  1829  defined therein, with an axis of the through-hole  1829  extending perpendicular to the opening  1802  in the housing  1804 . A shaft  1830  may be positioned within the through-hole  1829 , and may translate toward and away from the housing  1804 . The shaft  1830  may be mechanically connected to the conductive button cap  1812 , or may be biased to contact the conductive button cap  1812 . In some cases, the shaft  1830  may be mechanically and electrically connected to the conductive button cap  1812 . In a state of rest, the shaft  1830  and conductive button cap  1812  may be biased in an outward state of translation (i.e., away from the opening  1802 ) by the coil springs  1828  and/or a spring-biased tactile switch  1832 . In some cases, a spring-biased conductor (e.g., a conductive shear plate  1834 ) may extend between the tactile switch  1832  and an end of the shaft  1830  facing the tactile switch  1832 . When a user presses the conductive button cap  1812  toward the housing  1804 , the press may overcome the bias provided by the coil springs  1828  and/or tactile switch  1832 , and pressure on the conductive button cap  1812  may be transferred to the shaft  1830 , which translates toward the housing  1804  and presses on the tactile switch  1832  to change the state of the tactile switch  1832  (e.g., from ON to OFF or vice versa, from one functional state to another, etc.). The tactile switch  1832  may be aligned with an axis of the shaft  1830  and attached to the bracket  1816  using an adhesive  1836  (e.g., a conductive PSA). 
     In some embodiments, a gasket  1838  (e.g., an O-ring) may be positioned between the shaft  1830  and the through-hole. The shaft  1830  may have a circumferential groove  1840  in which a portion of the gasket  1838  is seated so that the gasket  1838  moves in a predictable way in response to movement of the shaft  1830 . In some examples, the gasket  1838  may be non-conductive. 
     The button assembly  1800  may further include a set of electrical insulators (i.e., one or more electrical insulators), which set of electrical insulators may electrically insulate the button cap retention assembly  1814  from the housing  1804 , and electrically insulate the conductive button cap  1812  from the housing  1804 . For example, the button assembly  1800  may include a first electrical insulator, such as a sleeve  1842  (or set of shims), positioned between the conductive button cap  1812  and the sidewall  1808  (or set of sidewalls) of the cavity  1806  in the housing  1804 . In some cases, the sleeve  1842  may include a closed-shape sidewall and an inward facing lip  1844  ( FIG. 18B ). In other cases, the sleeve  1842  may not include the inward facing lip  1844  or have a sidewall that does not define a closed shape. In other cases, the first electrical insulator may be a planar perimeter gasket (e.g., an insulator including the lip  1844  but not the sidewall). A second electrical insulator may include an adhesive  1846  (e.g., an adhesive ring) applied to a surface of the retainer  1818  facing the housing  1804 , or to the outer surface of the ledge  1810  within the cavity  1806 . In some cases, the adhesive  1846  may include a PSA. A gasket or seal  1848 , external to the housing  1804 , may be bonded to the adhesive  1846 . The adhesive  1846  and seal  1848  may be compressed when the screws  1820  are tightened to clamp the housing  1804  between the bracket  1816  and retainer  1818  of the button cap retention assembly  1814 . A third electrical insulator may include a spacer  1850 , internal to the housing  1804 , positioned between the bracket  1816  and the housing  1804 . The third electrical insulator, in conjunction with the first and/or second electrical insulator, may electrically insulate the conductive button cap retention assembly  1814  (e.g., the bracket  1816  and the retainer  1818 ) from the housing  1804 . The first electrical insulator may electrically insulate the conductive button cap  1812  from the housing  1804 . In some embodiments, additional or different electrical insulators may electrically insulate the conductive button cap  1812  or button cap retention assembly  1814  from the housing  1804 . 
     The shear plate  1834  may be formed from a conductive sheet that is stamped, molded, or otherwise shaped to form an open (shown) or closed (not shown) shape conductive perimeter  1852  and an elevated tab  1854  (e.g., a tab having an end positioned in a different plane than the conductive perimeter  1852 ). The conductive perimeter  1852  may be positioned between the bracket  1816  and spacer  1850 , such that the conductive perimeter  1852  and shear plate  1834  are electrically insulated from the housing  1804 . The shear plate  1834  deforms in response to translation of the shaft  1830 . 
     In use, a signal may be applied to, or received from, the button cap retention assembly  1814  (e.g., to/from the bracket  1816 ) via a circuit (e.g., a flex circuit or other circuit element) that is electrically connected to the bracket  1816  (e.g., as described with reference to  FIG. 16B ). A signal may travel through the conductive button cap  1812 , shaft  1830 , shear plate  1834 , and bracket  1816  via a first electrical path. The signal may also travel through the conductive button cap  1812 , coil springs  1828 , retainer  1818 , screws  1820 , and bracket  1816  via a second electrical path, or through the conductive button cap  1812 , retainer  1818 , screws  1820 , and bracket  1816  via a third electrical path. Although the third electrical path may be broken when the conductive button cap  1812  is pressed by a user, the conductive button cap  1812  may remain in electrical contact with the button cap retention assembly  1814  during all states of translation (e.g., via the first and second electrical paths). 
       FIG. 19  shows an example elevation of a button assembly  1900 . The button assembly  1900  may be an example of a button assembly included in any of the electronic devices described with reference to  FIG. 1A, 1B, 2A-2C, 3, 4A , or  4 B. 
     The button assembly  1900  may include a conductive button cap  1902 . The conductive button cap  1902  may be retained within an opening in a housing by a button cap retention assembly  1904  (e.g., a button retainer). The button cap retention assembly  1904 , or parts thereof, may be conductive. By way of example, the housing is shown to be the housing  206  of the watch body  202  described with reference to  FIGS. 2A-2C, 3, 4A , &amp;  4 B. The button cap retention assembly  1904  may be attached to the housing  206  and extend through the opening in the housing  206 . In some embodiments, the button cap retention assembly  1904  may include a first component  1906  that is inserted through the opening from one side of the housing  206 , and a second component  1908  that fastens to the first component  1906  on the other side of the housing  206  (e.g., by threads, screws, solder, or an adhesive). 
     A set of one or more insulators  1910  (e.g., electrical insulators) may electrically insulate the conductive button cap  1902  from the button cap retention assembly  1904 . The insulator  1910  may also electrically insulate the conductive button cap  1902  from the housing  206 . Although the insulator  1910  is generally shown in  FIG. 19  to include a non-conductive liner  1910   a  on an underside of the conductive button cap  1902 , a non-conductive sleeve  1910   b  positioned between the conductive button cap  1902  and the housing  206 , and a non-conductive sleeve  1910   c  positioned between the button cap retention assembly  1904  and a shaft  1914 , the insulator  1910  may alternatively include more or fewer elements, which elements may be positioned in different locations within the button assembly  1900 , as described with reference to  FIGS. 20, 21 , &amp;  22 . 
     The conductive button cap  1902  may translate toward and away from the housing  206 , and may be insulated from the button cap retention assembly  1904  during all phases of translation. When the conductive button cap  1902  is pressed by a user and translates toward the housing  206 , a tactile switch  1912  may be actuated (e.g., switched between two or more states). The shaft  1914  may extend between an interior surface of the conductive button cap  1902  and a depressible surface of the tactile switch  1912 . The tactile switch  1912  and shaft  1914 , or other elements not shown in  FIG. 19  (e.g., springs), may bias the conductive button cap  1902  in an outwardly translated position. 
     The conductive button cap  1902  may function as an electrode, and an electrical signal may be routed between the conductive button cap  1902  and a circuit  1916 , at least in part, via the shaft  1914 . In some embodiments, the button cap retention assembly  1904 , tactile switch  1912 , and circuit  1916  may be attached to a common substrate  1918 . 
     Because the signals received by or propagated from the conductive button cap  1902  may be low voltage or low amplitude signals, the materials, positions, electrical connections to, and electrical routing paths for an electrode formed on or by the conductive button cap  1902  can have a significant impact on the ability of the circuit  1916  to discern useful signals representing an ECG or other biological parameter of a person wearing an electronic device including the button assembly  1900 . The materials, positions, electrical connections to, and electrical routing paths for the button assembly  1900  can also determine how well the button assembly  1900  receives voltages/signals from a person&#39;s skin (e.g., a SNR of a device-to-user interface through which the voltages/signals pass); how well voltages/signals are transferred between the conductive button cap  1902  and internal components of an electronic device (e.g., a voltage/signal propagation SNR); and how well the button assembly  1900  operates in the face of environmental factors, such as temperature, humidity, moisture, electromagnetic radiation, dust, and so on. In some cases, the insulator  1910  may be positioned to prevent moisture from electrically shorting the conductive button cap  1902  to the housing  206 , or the housing  206  may be grounded to provide electrical shielding for some or all of the signals propagated through the button assembly  1900 . 
     More detailed examples of the button assembly  1900  described with reference to  FIG. 19  are shown in  FIGS. 20, 21 , &amp;  22 . 
     Referring now to  FIG. 20 , there is shown an assembled cross-section of another example of a button assembly  2000  that may be included in any of the electronic devices described with reference to  FIG. 1A, 1B, 2A-2C, 3, 4A , or  4 B. The button assembly  2000  may be at least partially within an opening  2002  in a housing  2004  (e.g., an opening in the housing described with reference to  FIG. 2A, 2C, 3, 4A , or  4 B), and may be attached to the housing or an internal structure such as a support. In some cases, and as shown, the housing  2004  may include a cavity  2006  defined by at least one sidewall (e.g., a single sidewall  2008  or set of sidewalls) and a ledge  2010 . The ledge  2010  may define the opening  2002 , and the sidewall  2008  may surround the ledge  2010 . 
     The button assembly  2000  may include a conductive button cap  2012  (or button cap having a conductive portion). The conductive button cap  2012  may be retained by a button cap retention assembly  2014  (or button retainer), and may be translatable toward and away from the housing  2004 . The button cap retention assembly  2014  may extend through the opening  2002  and be connected or otherwise attached to the housing  2004 . In some examples, the button cap retention assembly  2014  may include a bracket  2016  that overlaps the ledge  2010  interior to the housing  2004 , and a retainer  2018  that overlaps the ledge  2010  exterior to the housing  2004 . The retainer  2018  may be mechanically attached to the bracket  2016  by a set of screws  2020  or other mechanical fastener. The screws  2020  may be inserted into through-holes in the bracket  2016  and screwed into threaded holes in the retainer  2018 , clamping the ledge  2010  between the bracket  2016  and the retainer  2018 . 
     The conductive button cap  2012  may have an exterior surface  2022 , a sidewall or set of sidewalls  2024  parallel to the sidewall  2008  of the cavity  2006 , and an inward facing lip or set of lips  2026  that extends between the retainer  2018  and the ledge  2010  and toward a center axis of the conductive button cap  2012 . A set of one or more coil springs  2028  or other spring-biased members may be positioned between an outer surface of the retainer  2018  and an underside of the conductive button cap  2012 , and may bias the conductive button cap  2012  in an outward state of translation. 
     The button cap retention assembly  2014 , and in particular the retainer  2018 , may have a through-hole defined therein, with an axis of the through-hole extending perpendicular to the opening  2002  in the housing  2004 . A shaft  2030  may be positioned within the through-hole, and may translate toward and away from the housing  2004 . The shaft  2030  may be mechanically and electrically connected to the conductive button cap  2012 , or may be biased to contact the conductive button cap  2012 . In a state of rest, the shaft  2030  and conductive button cap  2012  may be biased in an outward state of translation (i.e., away from the opening  2002 ) by the coil springs  2028  and/or a spring-biased tactile switch  2032 . In some cases, a shim  2034 , such as a non-conductive shim, may be attached to an end of the shaft  2030  facing the tactile switch  2032 . When a user presses the conductive button cap  2012  toward the housing  2004 , the press may overcome the bias provided by the coil springs  2028  and/or tactile switch  2032 , and pressure on the conductive button cap  2012  may be transferred to the shaft  2030 , which translates toward the housing  2004  and presses on the tactile switch  2032  to change the state of the tactile switch  2032  (e.g., from ON to OFF or vice versa, from one functional state to another, etc.). The tactile switch  2032  may be aligned with an axis of the shaft  2030  and attached to the bracket  2016  using an adhesive  2036  (e.g., a non-conductive PSA). 
     In some embodiments, a gasket  2038  (e.g., an O-ring) may be positioned between the shaft  2030  and the through-hole. In some cases, the gasket  2038  may be positioned between a first non-conductive liner  2044  and a second non-conductive liner  2046 . In some examples, the gasket  2038  may be non-conductive. 
     The button assembly  2000  may further include a set of electrical insulators (i.e., one or more electrical insulators), which set of electrical insulators may electrically insulate the conductive button cap  2012  from the button cap retention assembly  2014  and housing  2004 . For example, the button assembly  2000  may include a first electrical insulator, such as a sleeve  2042  (or set of shims), positioned between the conductive button cap  2012  and the sidewall  2008  (or set of sidewalls) of the cavity  2006  in the housing  2004 . In some cases, the sleeve  2042  may include a closed-shape sidewall. In other cases, the sleeve  2042  may also include an inward facing lip, or may not have a sidewall that defines a closed shape. A second electrical insulator may include a non-conductive liner  2044  between an interior surface of the conductive button cap  2012  and the button cap retention assembly  2014 . In some cases, the non-conductive liner  2044  may be press-fit or adhesively bonded to the interior surface of the conductive button cap  2012 . Alternatively, the non-conductive liner  2044  may be press-fit or adhesively bonded to an exterior surface of the retainer  2018 . In some embodiments, the non-conductive liner  2044  may extend into the through-hole, between the shaft  2030  and the button cap retention assembly  2014  (e.g., between the shaft  2030  and the retainer  2018 ). A third electrical insulator may include a second non-conductive liner  2046 , positioned in the through-hole between the shaft  2030  and the retainer  2018 , below the gasket  2038 . The second electrical insulator, in some cases in conjunction with the third electrical insulator, may electrically insulate the conductive button cap  2012  from the button cap retention assembly  2014  (e.g., from the retainer  2018 ). The first electrical insulator may electrically insulate the conductive button cap  2012  from the housing  2004 . In some embodiments, additional or different electrical insulators may electrically insulate the conductive button cap  2012  from the button cap retention assembly  2014  or housing  2004 . 
     A conductive flexure  2048  may be coupled to, but insulated from, the bracket  2016 , and positioned (e.g., angled) to contact the end of the shaft  2030  that faces the tactile switch  2032 . The conductive flexure  2048  may be spring-biased to contact the end of the shaft  2030 , and may be spring-biased to remain in contact with the end of the shaft  2030  during all states of translation of the shaft  2030 . 
     In use, a signal may be applied to, or received from, the conductive button cap  2012  via a circuit (e.g., a flex circuit or other circuit element) that is electrically connected to the conductive flexure  2048 . A signal may travel through the conductive button cap  2012 , shaft  2030 , and conductive flexure  2048 . 
       FIG. 21  shows an assembled cross-section of another example of a button assembly  2100  that may be included in any of the electronic devices described with reference to  FIG. 1A, 1B, 2A-2C, 3, 4A , or  4 B. The button assembly  2100  may be at least partially within an opening  2102  in a housing  2104  (e.g., an opening in the housing described with reference to  FIG. 2A, 2C, 3, 4A , or  4 B), and may be attached to the housing or an internal structure such as a support. In some cases, and as shown, the housing  2104  may include a cavity  2106  defined by at least one sidewall (e.g., a single sidewall  2108  or set of sidewalls) and a ledge  2110 . The ledge  2110  may define the opening  2102 , and the sidewall  2108  may surround the ledge  2110 . 
     The button assembly  2100  may include a conductive button cap  2112  (or button cap having a conduction portion). The conductive button cap  2112  may be retained by a button cap retention assembly  2114  (or button retainer), and may be translatable toward and away from the housing  2104 . The button cap retention assembly  2114  may extend through the opening  2102  and be connected or otherwise attached to the housing  2104 . In some examples, the button cap retention assembly  2114  may include a bracket  2116  that overlaps the ledge  2110  interior to the housing  2104 , and a retainer  2118  that overlaps the ledge  2110  exterior to the housing  2104 . The retainer  2118  may be mechanically attached to the bracket  2116  by a set of screws  2120  or other mechanical fastener. The screws  2120  may be inserted into through-holes in the bracket  2116  and screwed into threaded holes in the retainer  2118 , clamping the ledge  2110  between the bracket  2116  and the retainer  2118 . 
     The conductive button cap  2112  may have an exterior surface  2122 , a sidewall or set of sidewalls  2124  parallel to the sidewall  2108  of the cavity  2106 , and an inward facing lip or set of lips  2126  that extends between the retainer  2118  and the ledge  2110  and toward a center axis of the conductive button cap  2112 . 
     The button cap retention assembly  2114 , and in particular the retainer  2118 , may have a through-hole defined therein, with an axis of the through-hole extending perpendicular to the opening  2102  in the housing  2104 . A shaft  2128  may be positioned within the through-hole, and may translate toward and away from the housing  2104 . The shaft  2128  may be mechanically and electrically connected to the conductive button cap  2112 , or may be biased to contact the conductive button cap  2112 . In a state of rest, the shaft  2128  and conductive button cap  2112  may be biased in an outward state of translation (i.e., away from the opening  2102 ) by a conductive flexure  2130  or other spring-biased member positioned between the bracket  2116  and an end of the shaft  2128  that faces the bracket  2116 . 
     The button cap retention assembly  2114 , and in particular the retainer  2118 , may also have a second through-hole formed therein, with an axis of the second through-hole extending perpendicular to the opening  2102  in the housing  2104 . A piston  2132  may be positioned within the through-hole, and may translate toward and away from the housing  2104 . In some cases, a shim  2134 , such as a non-conductive shim, may be attached to an end of the piston  2132  facing a spring-biased tactile switch  2136 . When a user presses the conductive button cap  2112  toward the housing  2104 , the press may overcome the bias provided by the conductive flexure  2130  and/or tactile switch  2136 , and pressure on the conductive button cap  2112  may be transferred to the piston  2132 , which translates toward the housing  2104  and presses on the tactile switch  2136  to change the state of the tactile switch  2136  (e.g., from ON to OFF or vice versa, from one functional state to another, etc.). The tactile switch  2136  may be aligned with an axis of the piston  2132  and attached to the bracket  2116  using an adhesive  2138  (e.g., a non-conductive PSA). 
     In some embodiments, a first gasket  2140  (e.g., an O-ring) may be positioned between the shaft  2128  and the first through-hole, and a second gasket  2142  (e.g., an O-ring) may be positioned between the piston  2132  and the second through-hole. In some cases, the first gasket  2140  may be positioned between a first non-conductive liner  2150  and a second non-conductive liner  2152 . In some cases, the piston  2132  may have a circumferential groove  2146  in which a portion of the second gasket  2142  is seated so that the second gasket  2142  moves in a predictable way in response to movement of the piston  2132 . In some examples, the first and second gaskets  2140 ,  2142  may be non-conductive. 
     The button assembly  2100  may further include a set of electrical insulators (i.e., one or more electrical insulators), which set of electrical insulators may electrically insulate the conductive button cap  2112  from the button cap retention assembly  2114  and housing  2104 . For example, the button assembly  2100  may include a first electrical insulator, such as a sleeve  2148  (or set of shims), positioned between the conductive button cap  2112  and the sidewall  2108  (or set of sidewalls) of the cavity  2106  in the housing  2104 . In some cases, the sleeve  2148  may include a closed-shape sidewall. In other cases, the sleeve  2148  may also include an inward facing lip, or may not have a sidewall that defines a closed shape. A second electrical insulator may include a non-conductive liner  2150  between an interior surface of the conductive button cap  2112  and the button cap retention assembly  2114 . In some cases, the non-conductive liner  2150  may be press-fit or adhesively bonded to the interior surface of the conductive button cap  2112 . Alternatively, the non-conductive liner  2150  may be press-fit or adhesively bonded to an exterior surface of the retainer  2118 . In some embodiments, the non-conductive liner  2150  may extend into the through-hole, between the shaft  2128  and the button cap retention assembly  2114  (e.g., between the shaft  2128  and the retainer  2118 ). A third electrical insulator may include a second non-conductive liner  2152 , positioned in the through-hole between the shaft  2128  and the retainer  2118 , below the gasket  2140 . The second electrical insulator, in some cases in conjunction with the third electrical insulator, may electrically insulate the conductive button cap  2112  from the button cap retention assembly  2114  (e.g., from the retainer  2118 ). The first electrical insulator may electrically insulate the conductive button cap  2112  from the housing  2104 . In some embodiments, additional or different electrical insulators may electrically insulate the conductive button cap  2112  from the button cap retention assembly  2114  or housing  2104 . 
     In use, a signal may be applied to, or received from, the conductive button cap  2112  via a circuit (e.g., a flex circuit or other circuit element) that is electrically connected to the conductive flexure  2130 . A signal may travel through the conductive button cap  2112 , shaft  2128 , and conductive flexure  2130 . 
       FIG. 22  shows an assembled cross-section of another example of a button assembly  2200  that may be included in any of the electronic devices described with reference to  FIG. 1A, 1B, 2A-2C, 3, 4A , or  4 B. The button assembly  2200  may be at least partially within an opening  2202  in a housing  2204  (e.g., an opening in the housing described with reference to  FIG. 2A, 2C, 3, 4A , or  4 B), and may be attached to the housing or an internal structure such as a support. In some cases, and as shown, the housing  2204  may include a cavity  2206  defined by at least one sidewall (e.g., a single sidewall  2208  or set of sidewalls) and a ledge  2210 . The ledge  2210  may define the opening  2202 , and the sidewall  2208  may surround the ledge  2210 . 
     The button assembly  2200  may include a conductive button cap  2212  (or button cap having a conductive portion). The conductive button cap  2212  may be retained by a button cap retention assembly  2214  (or button retainer), and may be translatable toward and away from the housing  2204 . The button cap retention assembly  2214  may extend through the opening  2202  and be connected or otherwise attached to the housing  2204 . In some examples, the button cap retention assembly  2214  may include a bracket  2216  that overlaps the ledge  2210  interior to the housing  2204 , and a retainer  2218  that overlaps the ledge  2210  exterior to the housing  2204 . The retainer  2218  may be mechanically attached to the bracket  2216  by a set of screws  2220  or other mechanical fastener. The screws  2220  may be inserted into through-holes in the bracket  2216  and screwed into threaded holes in the retainer  2218 , clamping the ledge  2210  between the bracket  2216  and the retainer  2218 . 
     The conductive button cap  2212  may have an exterior surface  2222 , a sidewall or set of sidewalls  2224  parallel to the sidewall  2208  of the cavity  2206 , and an inward facing lip or set of lips  2226  that extends between the retainer  2218  and the ledge  2210  and toward a center axis of the conductive button cap  2212 . 
     The button cap retention assembly  2214 , and in particular the retainer  2218 , may have a through-hole defined therein, with an axis of the through-hole extending perpendicular to the opening  2202  in the housing  2204 . A shaft  2228  may be positioned within the through-hole, and may translate toward and away from the housing  2204 . The shaft  2228  may be mechanically and electrically connected to the conductive button cap  2212 , or may be biased to contact the conductive button cap  2212 . In a state of rest, the shaft  2228  and conductive button cap  2212  may be biased in an outward state of translation (i.e., away from the opening  2202 ) by a conductive spring (e.g., a coil spring  2230 ) or other spring-biased member positioned between the bracket  2216  and an end of the shaft  2228  that faces the bracket  2216 . 
     The button cap retention assembly  2214 , and in particular the retainer  2218 , may also have a second through-hole formed therein, with an axis of the second through-hole extending perpendicular to the opening  2202  in the housing  2204 . A piston  2232  may be positioned within the through-hole, and may translate toward and away from the housing  2204 . When a user presses the conductive button cap  2212  toward the housing  2204 , the press may overcome the bias provided by the coil spring  2230  and/or tactile switch  2234 , and pressure on the conductive button cap  2212  may be transferred to the piston  2232 , which translates toward the housing  2204  and presses on the tactile switch  2234  to change the state of the tactile switch  2234  (e.g., from ON to OFF or vice versa, from one functional state to another, etc.). The tactile switch  2234  may be aligned with an axis of the piston  2232  and attached to the bracket  2216  using an adhesive  2236  (e.g., a non-conductive PSA). 
     In some embodiments, a first gasket  2238  (e.g., an O-ring) may be positioned between the shaft  2228  and the first through-hole, and a second gasket  2240  (e.g., an O-ring) may be positioned between the piston  2232  and the second through-hole. In some cases, the first gasket  2238  may be positioned between a first non-conductive liner  2248  and a second non-conductive liner  2250 . In some cases, the piston  2232  may have a circumferential groove  2244  in which a portion of the second gasket  2240  is seated so that the second gasket  2240  moves in a predictable way in response to movement of the piston  2232 . In some examples, the first and second gaskets  2238 ,  2240  may be non-conductive. 
     The button assembly  2200  may further include a set of electrical insulators (i.e., one or more electrical insulators), which set of electrical insulators may electrically insulate the conductive button cap  2212  from the button cap retention assembly  2214  and housing  2204 . For example, the button assembly  2200  may include a first electrical insulator, such as a sleeve  2246  (or set of shims), positioned between the conductive button cap  2212  and the sidewall  2208  (or set of sidewalls) of the cavity  2206  in the housing  2204 . In some cases, the sleeve  2246  may include a closed-shape sidewall. In other cases, the sleeve  2246  may also include an inward facing lip, or may not have a sidewall that defines a closed shape. A second electrical insulator may include a non-conductive liner  2248  between an interior surface of the conductive button cap  2212  and the button cap retention assembly  2214 . In some cases, the non-conductive liner  2248  may be press-fit or adhesively bonded to the interior surface of the conductive button cap  2212 . Alternatively, the non-conductive liner  2248  may be press-fit or adhesively bonded to an exterior surface of the retainer  2218 . In some embodiments, the non-conductive liner  2248  may extend into the through-hole, between the shaft  2228  and the button cap retention assembly  2214  (e.g., between the shaft  2228  and the retainer  2218 ). A third electrical insulator may include a second non-conductive liner  2250 , positioned in the through-hole between the shaft  2228  and the retainer  2218 , below the gasket  2238 . The second electrical insulator, in some cases in conjunction with the third electrical insulator, may electrically insulate the conductive button cap  2212  from the button cap retention assembly  2214  (e.g., from the retainer  2218 ). The first electrical insulator may electrically insulate the conductive button cap  2212  from the housing  2204 . In some embodiments, additional or different electrical insulators may electrically insulate the conductive button cap  2212  from the button cap retention assembly  2214  or housing  2204 . 
     In use, a signal may be applied to, or received from, the conductive button cap  2212  via a circuit (e.g., a flex circuit or other circuit element) that is electrically connected to the coil spring  2230 . A signal may travel through the conductive button cap  2212 , shaft  2228 , and coil spring  2230 . 
       FIG. 23  shows a schematic  2300  of an electronic device, such as an electronic watch, that may be used for acquiring an ECG or other biological parameter from a user of the electronic device. In some cases, the electronic device may include a watch body. As shown, the electronic device may include a first electrode  2302  on a carrier  2304 , an optional second electrode  2306  on the carrier  2304 , and a third electrode  2308  on the surface of a user-rotatable crown  2310  (or alternatively, on the surface of a button). The third electrode  2308  may be operable to be contacted by a finger of a user while the first electrode  2302  (and optional second electrode  2306 ) are positioned against a user&#39;s skin (e.g., against the wrist of the user). A processor  2312 , which in some cases may be provided in an integrated circuit (IC), an application-specific integrated circuit (ASIC), a field programmable gate array (FPGA), a system in package (SIP), a system on a chip (SOC), etc., may be operable to acquire an ECG from the user, or determine another biological parameter of, the user. The ECG or other biological parameter may be determined based on voltages at the first, optional second, and third electrodes  2302 ,  2306 ,  2308  while the user is in contact with the first, optional second, and third electrodes  2302 ,  2306 ,  2308 . 
     In some cases, voltages may be sensed at just the first and third electrodes  2302 ,  2308 . In other cases, the second electrode  2306  may be grounded to the electronic device, thereby the user to the electronic device, and the voltage at the second electrode  2306  (i.e., the ground voltage) may be used to remove noise generated by the electronic device or other environmental sources from the signals measured at the first and third electrodes  2302 ,  2308 . This may result in more accurate readings (or processing) of the first and third voltages. 
     As shown, a signal or voltages at the first electrode  2302  may be amplified by a first amplifier  2314 , and a signal or voltages at the third electrode  2308  may be amplified by a second amplifier  2316 . 
       FIG. 24  shows an example method  2400  of determining a biological parameter of a user wearing an electronic watch or other wearable electronic device, such as a watch or wearable electronic device described herein. 
     At block  2402 , a ground voltage is optionally applied to a user via a first electrode on the electronic device. The first electrode may be on an exterior surface of a carrier that forms part of a housing of the electronic device. The operation(s) at  2402  may be performed, for example, by the processor described with reference to  FIG. 24 , using one of the electrodes described with reference to  FIGS. 1B, 2A-2C, 3, 4A-4C, 5D, 5E, 6-8, 9A-9C, 10A-10D , &amp;  23 . 
     At block  2404 , a first voltage or signal is sensed at a second electrode on the electronic device. The second electrode may also be on the exterior surface of the carrier. The operation(s) at  2404  may be performed, for example, by the processor described with reference to  FIG. 24 , using one of the electrodes described with reference to  FIGS. 1B, 2A-2C, 3, 4A-4C, 5D, 5E, 6-8, 9A-9C, 10A-10D, 11, 12A, 12B, 13, 14, 15, 16A ,  16 B,  17 A,  17 B,  18 A,  18 B,  19 ,  20 ,  21 ,  22 , &amp;  23 . 
     At block  2406 , a second voltage or signal is sensed at a third electrode on the electronic device. The third electrode may be on a user-rotatable crown of the electronic device, or on a button of the electronic device, or on another surface of the housing of the electronic device. In some embodiments, the ground voltage is applied and the first voltage or signal is sensed on a wrist of one arm of the user, and the second voltage or signal is sensed on a fingertip of the user (with the fingertip belonging to a finger on a hand on the other arm of the user). The operation(s) at  2406  may be performed, for example, by the processor described with reference to  FIG. 24 , using one of the electrodes described with reference to  FIGS. 1B, 2A-2C, 3, 4A-4C, 5D, 5E, 6-8, 9A-9C, 10A-10D, 11, 12A, 12B, 13, 14, 15, 16A ,  16 B,  17 A,  17 B,  18 A,  18 B,  19 ,  20 ,  21 ,  22 , &amp;  23 . 
     At block  2408 , the biological parameter of the user may be determined from the optional ground voltage, the first voltage or signal, and the second voltage or signal. The ground voltage may provide a reference for the first and second voltages or signals, or may otherwise be used to reject noise from the first and second voltages or signals. When the first and second voltages are obtained from different parts of the user&#39;s body, the biological parameter may be an electrocardiogram for the user. The operation(s) at  2408  may be performed, for example, by the processor described with reference to  FIG. 25 . 
       FIG. 25  shows a sample electrical block diagram of an electronic device  2500 , which electronic device may in some cases take the form of any of the electronic watches or other wearable electronic devices described with reference to  FIGS. 1-23 , or other portable or wearable electronic devices. The electronic device  2500  can include a display  2505  (e.g., a light-emitting display), a processor  2510 , a power source  2515 , a memory  2520  or storage device, a sensor  2525 , and an input/output (I/O) mechanism  2530  (e.g., an input/output device, input/output port, or haptic input/output interface). The processor  2510  can control some or all of the operations of the electronic device  2500 . The processor  2510  can communicate, either directly or indirectly, with some or all of the components of the electronic device  2500 . For example, a system bus or other communication mechanism  2535  can provide communication between the processor  2510 , the power source  2515 , the memory  2520 , the sensor  2525 , and the input/output mechanism  2530 . 
     The processor  2510  can be implemented as any electronic device capable of processing, receiving, or transmitting data or instructions. For example, the processor  2510  can be a microprocessor, a central processing unit (CPU), an application-specific integrated circuit (ASIC), a digital signal processor (DSP), or combinations of such devices. As described herein, the term “processor” is meant to encompass a single processor or processing unit, multiple processors, multiple processing units, or other suitably configured computing element or elements. 
     It should be noted that the components of the electronic device  2500  can be controlled by multiple processors. For example, select components of the electronic device  2500  (e.g., a sensor  2525 ) may be controlled by a first processor and other components of the electronic device  2500  (e.g., the display  2505 ) may be controlled by a second processor, where the first and second processors may or may not be in communication with each other. In some cases, the processor  2510  may determine a biological parameter of a user of the electronic device, such as an ECG for the user. 
     The power source  2515  can be implemented with any device capable of providing energy to the electronic device  2500 . For example, the power source  2515  may be one or more batteries or rechargeable batteries. Additionally or alternatively, the power source  2515  can be a power connector or power cord that connects the electronic device  2500  to another power source, such as a wall outlet. 
     The memory  2520  can store electronic data that can be used by the electronic device  2500 . For example, the memory  2520  can store electrical data or content such as, for example, audio and video files, documents and applications, device settings and user preferences, timing signals, control signals, and data structures or databases. The memory  2520  can be configured as any type of memory. By way of example only, the memory  2520  can be implemented as random access memory, read-only memory, Flash memory, removable memory, other types of storage elements, or combinations of such devices. 
     The electronic device  2500  may also include one or more sensors  2525  positioned almost anywhere on the electronic device  2500 . The sensor(s)  2525  can be configured to sense one or more type of parameters, such as but not limited to, pressure, light, touch, heat, movement, relative motion, biometric data (e.g., biological parameters), and so on. For example, the sensor(s)  2525  may include a heat sensor, a position sensor, a light or optical sensor, an accelerometer, a pressure transducer, a gyroscope, a magnetometer, a health monitoring sensor, and so on. Additionally, the one or more sensors  2525  can utilize any suitable sensing technology, including, but not limited to, capacitive, ultrasonic, resistive, optical, ultrasound, piezoelectric, and thermal sensing technology. In some examples, the sensors  2525  may include one or more of the electrodes described herein (e.g., one or more electrodes on an exterior surface of a carrier that forms part of a housing for the electronic device  2500  and/or an electrode on a crown, button, or other housing member of the electronic device). 
     The I/O mechanism  2530  can transmit and/or receive data from a user or another electronic device. An I/O device can include a display, a touch sensing input surface, one or more buttons (e.g., a graphical user interface “home” button), one or more cameras, one or more microphones or speakers, one or more ports such as a microphone port, and/or a keyboard. Additionally or alternatively, an I/O device or port can transmit electronic signals via a communications network, such as a wireless and/or wired network connection. Examples of wireless and wired network connections include, but are not limited to, cellular, Wi-Fi, Bluetooth, IR, and Ethernet connections. 
     As discussed above, graphics displayed on the electronic devices herein may be manipulated through inputs provided to the crown.  FIGS. 26A-28B  generally depict examples of changing a graphical output displayed on an electronic device through inputs provided by force and/or rotational inputs to a crown assembly of the device. This manipulation (e.g., selection, acknowledgement, motion, dismissal, magnification, and so on) of a graphic may result in changes in operation of the electronic device and/or graphical output displayed by the electronic device. Although specific examples are provided and discussed, many operations may be performed by rotating and/or applying force to a crown such as the examples described above. Accordingly, the following discussion is by way of example and not limitation. 
       FIG. 26A  depicts an example electronic device  2600  (shown here as an electronic watch) having a crown  2602 . The crown  2602  may be similar to the examples described above, and may receive force inputs along a first lateral direction, a second lateral direction, or an axial direction of the crown. The crown  2602  may also receive rotational inputs. A display  2606  provides a graphical output (e.g., shows information and/or other graphics). In some embodiments, the display  2606  may be configured as a touch-sensitive display capable of receiving touch and/or force input. In the current example, the display  2606  depicts a list of various items  2661 ,  2662 ,  2663 , all of which are example graphics. 
       FIG. 26B  illustrates how the graphical output shown on the display  2606  changes in a first manner as the crown  2602  rotates, partially or completely (as indicated by the arrow  2660 ). Rotating the crown  2602  causes the list to scroll or otherwise move on the screen, such that the first item  2661  is no longer displayed, the second and third items  2662 ,  2663  each move upwards on the display, and a fourth item  2664  is now shown at the bottom of the display. This is one example of a scrolling operation that can be executed by rotating the crown  2602 . Such scrolling operations may provide a simple and efficient way to depict multiple items relatively quickly and in sequential order. A speed of the scrolling operation may be controlled by the amount of rotational force applied to the crown  2602  and/or the speed at which the crown  2602  is rotated. Faster or more forceful rotation may yield faster scrolling, while slower or less forceful rotation yields slower scrolling. The crown  2602  may receive an axial force (e.g., a force inward toward the display  2606  or watch body) to select an item from the list, in certain embodiments. 
       FIGS. 27A and 27B  illustrate an example zoom operation. The display  2706  depicts a picture  2766  at a first magnification, shown in  FIG. 27A ; the picture  2766  is yet another example of a graphic. A user may apply a translating force (e.g., a force along the z-axis) or a lateral force (e.g., a force along the x-axis) to the crown  2702  of the electronic device  2700  (illustrated by arrow  2765 ), and in response the display may change a graphic in a second manner, such as zooming into the picture  2766  so that a portion  2767  of the picture is shown at an increased magnification. This is shown in  FIG. 27B . The direction of zoom (in vs. out) and speed of zoom, or location of zoom, may be controlled through force applied to the crown  2702 , and particularly through the direction of applied force and/or magnitude of applied force. Applying force to the crown  2702  in a first direction may zoom in, while applying force to the crown  2702  in an opposite direction may zoom out. Alternately, rotating or applying force to the crown  2702  in a first direction may change the portion of the picture subject to the zoom effect. In some embodiments, applying an axial or translating force (e.g., a force along the z-axis) to the crown  2702  may toggle between different zoom modes or inputs (e.g., direction of zoom vs. portion of picture subject to zoom), or otherwise change the displayed graphic in a second manner. In yet other embodiments, applying force to the crown  2702  along another direction, such as along the y-axis, may return the picture  2766  to the default magnification shown in  FIG. 27A . 
       FIGS. 28A and 28B  illustrate possible use of the crown  2802  to change an operational state of the electronic device  2800  or otherwise toggle between inputs. Turning first to  FIG. 28A , the display  2806  depicts a question  2868 , namely, “Would you like directions?” As shown in  FIG. 28B , a lateral force may be applied to the crown  2802  (illustrated by arrow  2870 ) to answer the question. Applying force to the crown  2802  provides an input interpreted by the electronic device  2800  as “yes,” and so “YES” is displayed as a graphic  2869  on the display  2806 . Applying force to the crown  2802  in an opposite direction may provide a “no” input. Both the question  2868  and graphic  2869  are examples of graphics. 
     In the embodiment shown in  FIGS. 28A and 28B , the force applied to the crown  2802  is used to directly provide the input, rather than select from options in a list (as discussed above with respect to  FIGS. 26A and 26B ). 
     As mentioned previously, force or rotational input to a crown of an electronic device may control many functions beyond those listed here. The crown may receive distinct force or rotational inputs to adjust a volume of an electronic device, a brightness of a display, or other operational parameters of the device. A force or rotational input applied to the crown may rotate to turn a display on or off, or turn the device on or off. A force or rotational input to the crown may launch or terminate an application on the electronic device. Further, combinations of inputs to the crown may likewise initiate or control any of the foregoing functions, as well. 
     In some cases, the graphical output of a display may be responsive to inputs applied to a touch-sensitive display (e.g., displays  2606 ,  2706 ,  2806 , and the like) in addition to inputs applied to a crown. The touch-sensitive display may include or be associated with one or more touch and/or force sensors that extend along an output region of a display and which may use any suitable sensing elements and/or sensing techniques to detect touch and/or force inputs applied to the touch-sensitive display. The same or similar graphical output manipulations that are produced in response to inputs applied to the crown may also be produced in response to inputs applied to the touch-sensitive display. For example, a swipe gesture applied to the touch-sensitive display may cause the graphical output to move in a direction corresponding to the swipe gesture. As another example, a tap gesture applied to the touch-sensitive display may cause an item to be selected or activated. In this way, a user may have multiple different ways to interact with and control an electronic watch, and in particular the graphical output of an electronic watch. Further, while the crown may provide overlapping functionality with the touch-sensitive display, using the crown allows for the graphical output of the display to be visible (without being blocked by the finger that is providing the touch input). 
     As another example, and of the inputs described in  FIGS. 26A-28B  may be used to select, initiate, or display an ECG, or otherwise begin the operation of determining an ECG or launching an ECG application. 
     As described above, one aspect of the present technology is the gathering and use of data available from various sources, including the gathering and use of biological parameters of a user, to monitor or improve the user&#39;s health or fitness. The present disclosure contemplates that in some instances, this gathered data may include personal information data that uniquely identifies a specific person, or can be used to contact, locate, or identify a specific person. Such personal information data can include demographic data, location-based data, telephone numbers, email addresses, twitter IDs, home addresses, data or records relating to a user&#39;s health or level of fitness (e.g., vital sign measurements, medication information, exercise information), date of birth, or any other identifying or personal information. 
     The present disclosure recognizes that the use of such personal information data, in the present technology, can be used to the benefit of users. For example, the personal information data can be used to aid a user in monitoring or improving their health or fitness (e.g., biological parameters or health and fitness data may be used to provide insights into a user&#39;s general wellness, or may be used as positive feedback to individuals using technology to pursue wellness goals). 
     The present disclosure contemplates that the entities responsible for the collection, analysis, disclosure, transfer, storage, or other use of such personal information data will comply with well-established privacy policies and/or privacy practices. In particular, such entities should implement and consistently use privacy policies and practices that are generally recognized as meeting or exceeding industry or governmental requirements for maintaining personal information data private and secure. Such policies should be easily accessible by users, and should be updated as the collection and/or use of data changes. Personal information from users should be collected for legitimate and reasonable uses of the entity and not shared or sold outside of those legitimate uses. Further, such collection/sharing should occur after receiving the informed consent of the users. Additionally, such entities should consider taking any needed steps for safeguarding and securing access to such personal information data and ensuring that others with access to the personal information data adhere to their privacy policies and procedures. Further, such entities can subject themselves to evaluation by third parties to certify their adherence to widely accepted privacy policies and practices. In addition, policies and practices should be adapted for the particular types of personal information data being collected and/or accessed and adapted to applicable laws and standards, including jurisdiction-specific considerations. For instance, in the US, collection of or access to certain health data may be governed by federal and/or state laws, such as the Health Insurance Portability and Accountability Act (HIPAA); whereas health data in other countries may be subject to other regulations and policies and should be handled accordingly. Hence different privacy practices should be maintained for different personal data types in each country. 
     Despite the foregoing, the present disclosure also contemplates embodiments in which users selectively block the use of, or access to, personal information data. That is, the present disclosure contemplates that hardware and/or software elements can be provided to prevent or block access to such personal information data. For example, in the case of biological parameters or conditions identified therefrom, the present technology can be configured to allow users to select to “opt in” or “opt out” of participation in the collection of personal information data during registration for services or anytime thereafter. In another example, users can select not to provide health or fitness-associated data to the providers of applications or services, or can prevent the transmission of such data from the device on which it is collected or outside a collection of devices that are personal to a user from which the data is obtained. In yet another example, a user can select to limit the length of time health or fitness data, or biological parameters from which such data is derived, is maintained. In addition to providing “opt in” and “opt out” options, the present disclosure contemplates providing notifications relating to the access or use of personal information. For instance, a user may be notified upon downloading an app that their personal information data will be accessed and then reminded again just before personal information data is accessed by the app. 
     Moreover, it is the intent of the present disclosure that personal information data should be managed and handled in a way to minimize risks of unintentional or unauthorized access or use. Risk can be minimized by limiting the collection of data and deleting data once it is no longer needed. In addition, and when applicable, including in certain health related applications, data de-identification can be used to protect a user&#39;s privacy. De-identification may be facilitated, when appropriate, by removing specific identifiers (e.g., date of birth, etc.), controlling the amount or specificity of data stored (e.g., collecting location data at a city level rather than at an address level), controlling how data is stored (e.g., aggregating data across users), and/or other methods. 
     Therefore, although the present disclosure broadly covers use of personal information data to implement one or more various disclosed embodiments, the present disclosure also contemplates that the various embodiments can also be implemented without the need for accessing at least some personal information data. That is, the various embodiments of the present technology are not rendered inoperable due to the lack of a portion of such personal information data. For example, biological parameters can be ascertained or stored without associating the biological parameters with information identifying a particular user from which they are obtained, or with a bare minimum amount of personal information, such as non-personal information already available to service providers or publicly available information. 
     The foregoing description, for purposes of explanation, uses specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of the specific embodiments described herein are presented for purposes of illustration and description. They are not targeted to be exhaustive or to limit the embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings.

Metadata:
Filing Date: 20180830
Publication Date: 20220906
Grant Date: 20220906
Priority Date: 20170905
Inventors: PANDYA, SAMEER
CLAVELLE, Adam T.
DE JONG, ERIK G.
WITTENBERG, MICHAEL B.
HARRISON-NOONAN, TOBIAS J.
MELCHER, MARTIN
ZHANG, ZHIPENG
ROACH, STEVEN C.
CARDINALI, STEVEN P.
Assignee: APPLE INC
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Family ID: 63592548