Patent Publication Number: US-2023147750-A1

Title: Wearable physiological monitoring devices

Description:
INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS 
     The present application claims priority to U.S. Provisional Application No. 63/235,412, filed Aug. 20, 2021, U.S. Provisional Application No. 63/234,983, filed Aug. 19, 2021, and U.S. Provisional Application No. 63/235,008, filed Aug. 19, 2021. All of the above-listed applications and any and all other applications for which a foreign or domestic priority claim is identified in the Application Data Sheet as filed with the present application, are hereby incorporated by reference under 37 CFR 1.57. 
    
    
     FIELD OF THE DISCLOSED EMBODIMENTS 
     The subject matter of the present application is in the field of wearable devices having at least one sensor for sensing a physiological parameter of a user. 
     BACKGROUND OF THE DISCLOSED EMBODIMENTS 
     Wearable devices, including smartwatches, rings, auricular devices, glasses, can provide multiple functions. In addition to providing conventional time and calendar functions, smartwatches also provide an interface to a smartphone to enable a user to communicate via the smartphone without having to hold the smartphone. A smartwatch can include a 3-axis accelerometer to detect motion and orientation of the user to thereby monitor the user&#39;s steps and other actions. A smartwatch may also detect physiological parameters of the user via one or more sensory interfaces on a surface of the smartwatch adjacent to a portion of a user&#39;s limb (e.g., the user&#39;s wrist). For example, a smartwatch can include an optical heart rate sensor to detect the user&#39;s heartrate, an SpO 2  monitor to measure blood oxygen levels, a bioimpedance sensor to measure respiratory rate, heart rate, water level and the like. 
     SUMMARY 
     A need exists for a way to charge a smartwatch without losing the ability to monitor the user&#39;s physiological parameters while the smartwatch is charging. A need also exists for a fitness tracker that has only single or a small set of functionality that can be chosen by a user. If additional functionality is desired by a user, one or more additional fitness trackers can be chosen by the user and placed on a wrist or ankle adjacent to a first fitness tracker. A need also exists for a fitness tracker that is configured to be decorative which can include aesthetically pleasing and less bulky than conventional smartwatches. For wearable devices capable of providing multiple functionalities, there is a need for some functionalities to be displayed more frequently than other functionalities. For example, it may be beneficial for a smartwatch to continually display the time of day and/or date so that a user may view the time of day and/or date without performing an action to cause the smartwatch to display the time of day. It may also be beneficial to display certain parameters in an always on display. For example, step counts may be always displayed. 
     In one aspect, a wearable device for monitoring one or more physiological parameters of a user can comprise a base housing portion configured for placement on a portion of the user&#39;s body, the base housing portion having at least one sensor for sensing a physiological parameter when the wearable device is in use, wherein the base housing portion does not include a display; a first electronic subsystem within the base housing portion configured to process one or more signals generated by the at least one sensor and to generate a wireless communication signal based on the sensed signals; a first battery within the base housing portion and configured to provide power to the first electronic subsystem; a removable housing portion comprising a display and attachable to a top surface of the base housing portion; and a second electronic subsystem within the removable housing portion, the second electronic subsystem powered by a second battery, the second battery selectively couplable to an external charger when the removable housing portion is removed from the base housing portion, wherein, when the removable housing portion is attached to the base housing portion, the second battery within the removable housing portion is configured to charge the first battery within the base housing portion. 
     The wearable device of any of the preceding paragraphs and/or any of the apparatuses, systems, or devices disclosed herein can include one or more of the following features. In some cases, the display of the removable housing portion can occupy an entire top surface of the removable housing portion. In some cases, the display of the removable housing portion can display a dynamic hour hand and a dynamic minute hand, the dynamic hour hand and the dynamic minute hand can provide an indication to the user of time. In some cases, the display of the removable housing portion can display at least one numerical value indicative of a physiological parameter of the user. In some cases, the base housing portion can further include a first slot comprising a first opening and a second opening smaller than the first opening; and a second slot comprising a first opening and a second opening smaller than the first opening. In some cases, the removable housing portion can further include a first engagement post including a first end and a second end, wherein the first end can be larger than the second end; and a second engagement post including a first end and a second end, wherein the first end can be larger than the second end. In some cases, the first and second slots of the base housing portion can receive the first and second engagement posts of the removable housing portion when the removable housing portion is attached to the base housing portion. In some cases, the first openings of the first and second slots can receive the first ends of the first and second engagement posts when the removable housing portion is attached to the base housing portion. In some cases, the second openings of the first and second slots can receive the second ends of the first and second engagement posts when the removable housing portion is attached to the base housing portion and rotated along a surface of the base housing portion. In some cases, the wearable device can further include a band including a first end and a second end. In some cases, the first end of the band can be attached to a first end of the base housing portion and the second end of the band can be attached to a second end of the base housing portion. In some cases, the base housing portion can wirelessly communicate with the removable housing portion. In some cases, the second electronic subsystem can receive and process the wireless communication signal generated by the first electronic subsystem. 
     In another aspects, a method of charging a wearable device can include detaching a removable housing portion of the wearable device from a base housing portion of the wearable device, the removable housing portion enclosing a main battery therein, the base housing portion enclosing an auxiliary battery therein; positioning the removable housing portion of the wearable device on a charging source to charge the main battery; maintaining the base housing portion in sensory communication with a user while the removable housing portion is detached, the base housing portion receiving power from the auxiliary battery; reattaching the removable housing portion to the base housing portion after charging the main battery; and charging the auxiliary battery with energy from the main battery when the removable housing portion is attached to the base housing portion. 
     The method of any of the preceding paragraphs herein can include one or more of the following steps and/or features. In some cases, the base housing portion can wirelessly communicate with the removable housing portion while the removable housing portion is removed from the base housing portion. In some cases, detaching the removable housing portion of the wearable device from the base housing portion of the wearable device can include rotating the removable housing portion about a surface of the base housing portion and removing a first engagement post and a second engagement post of the removable housing portion from a first slot and a second slot of the base housing portion. In some cases, reattaching the removable housing portion to the base housing portion after charging the main battery can include inserting a first engagement post and a second engagement post of the removable housing portion to a first slot and a second slot of the base housing portion and rotating the removable housing portion along a surface of the base housing portion. In some cases, maintaining the base housing portion in sensory communication with the user while the removable housing portion is detached can include securing a band to a portion of the user&#39;s body, the band attached to a first end and a second end of the base housing portion. In some cases, the method can include using at least one sensor of the wearable device to sense a physiological parameter of the user when the wearable device is in use. 
     In another aspects, a wearable device for monitoring one or more physiological parameters of a user can include a main body for placement on a user&#39;s limb, the main body including a display and a first input electrode and a second input electrode positioned on a bottom surface of the main body; a first battery inside the main body; and a removable housing comprising a second battery, a top surface, and an opening along the top surface, said removable housing attachable to a top surface of the main body, the removable housing including a plurality of attachment legs configured to engage to the main body to removably secure the removable housing to the main body, the removable housing including at least a first output electrode positioned on an inner surface of a first attachment leg of the plurality of attachment legs and a second output electrode positioned on an inner surface of a second attachment leg of the plurality of attachment legs, the first output electrode positioned on the first attachment leg such that the first output electrode contacts the first input electrode when the removable housing is secured to the main body and such that the second output electrode on the second attachment leg contacts the second input electrode when the removable housing is secured to the main body, wherein, when the removable housing is attached to the main body, the second battery of the removable housing can charge the first battery within the main body via the first and second input electrodes and the first and second output electrodes; and wherein the opening along the top surface of the removable housing can permit visualization of the display of the main body when the removable housing is attached to the main body. 
     The wearable device of any of the preceding paragraphs and/or any of the apparatuses, systems, or devices disclosed herein can include one or more of the following features. In some cases, the removable housing portion does not include a display. In some cases, the wearable device can include a band including a first end and a second end, wherein the first end attaches to a first end of the main body and the second end attaches to a second end of the main body. In some cases, the main body can include a third input electrode and a fourth input electrode positioned on the bottom surface of the main body; the removable housing can include a third output electrode positioned on an inner surface of a third attachment leg of the plurality of attachment legs and a fourth output electrode positioned on an inner surface of a fourth attachment leg of the plurality of attachment legs; and the third output electrode can contact the third input electrode and the fourth output electrode can contact the fourth input electrode when the removable housing is secured to the main body. In some cases, the main body can include at least one sensor for sensing a physiological parameter of the user when the wearable device is in use. 
     In another aspects, a method of charging a wearable device while the wearable device is being worn by a user can include attaching a charged removable battery pack to a main body of the wearable device by positioning a plurality of engagement surfaces against respective portions of the main body of the wearable device; engaging a first output electrode of the removable battery pack with a first input electrode of the main body of the wearable device to provide a first electrical contact between the first output electrode and the first input electrode; engaging a second output electrode of the removable battery pack with a second input electrode of the main body of the wearable device to provide a second electrical contact between the second output electrode and the second input electrode; and charging a battery within the main body of the wearable device with power from a battery within the removable battery pack by propagating electrical energy via the first electrical contact and the second electrical contact. 
     The method of any of the preceding paragraphs herein can include one or more of the following steps and/or features. In some cases, the first output electrode can be positioned on a first engagement surface of the plurality of engagement surfaces and the second output electrode can be positioned on a second engagement surface of the plurality of engagement surfaces. In some cases, the method can include engaging a third output electrode of the removable battery pack with a third input electrode of the main body of the wearable device to provide a third electrical contact between the third output electrode and the third input electrode; and engaging a fourth output electrode of the removable battery pack with a fourth input electrode of the main body of the wearable device to provide a fourth electrical contact between the fourth output electrode and the fourth input electrode. In some cases, the first output electrode can be positioned on a first engagement surface of the plurality of engagement surfaces; the second output electrode can be positioned on a second engagement surface of the plurality of engagement surfaces; the third output electrode can be positioned on a third engagement surface of the plurality of engagement surfaces; and the fourth output electrode can be positioned on a fourth engagement surface of the plurality of engagement surfaces. In some cases, attaching the charged removable battery pack to the main body of the wearable device by positioning the plurality of engagement surfaces against respective portions of the main body can include attaching a plurality of legs against respective portions of the main body. In some cases, the method can include securing a band to a portion of the user&#39;s body, the band attached to a first end and a second end of the base housing portion. In some cases, the method can include using a sensor of the main body to sense at least one physiological parameter of the user when the wearable device is in use. 
     In another aspects, a system of at least a first fitness tracker and a second fitness tracker, each fitness tracker capable of wirelessly communicating a respective physiological parameter of a user to a remote device can include a first enclosure of the first fitness tracker, the first enclosure including a first electronic subsystem comprising at least one sensor configured to sense a first physiological parameter of the user when the first fitness tracker is in use, the first electronic subsystem can wirelessly transmit signals to the remote device responsive to the first physiological parameter, the first enclosure couplable to a first band configured to attach to a portion of the user&#39;s body; and a second enclosure of the second fitness tracker, the second enclosure including a second electronic subsystem including at least one sensor configured to sense a second physiological parameter of the user when the second fitness tracker is in use, the second electronic subsystem can wirelessly transmit signals to the remote device responsive to the second physiological parameter, the second enclosure couplable to a second band configured to attach to a portion of the user&#39;s body; and wherein the first enclosure includes at least a first engagement surface, the first engagement surface having at least a first selected contour; and wherein the second enclosure includes at least a second engagement surface, the second engagement surface having at least a second selected contour, the second selected contour selected with respect to the first selected contour such that the second engagement surface of the second enclosure interlocks with the first engagement surface of the first enclosure. 
     The system of at least a first fitness tracker and a second fitness tracker of any of the preceding paragraphs and/or any of the apparatuses, systems, or devices disclosed herein can include one or more of the following features. In some cases, the portion of the user&#39;s body can include a limb. In some cases, the first band can include a first magnet and a second magnet; the second band can include a third magnet and a fourth magnet; the first and third magnets can have opposing poles, the first and third magnets can attach to each other and secure at least a portion of the first band to at least a portion of the second band; and the second and fourth magnets can have opposing poles, the second and fourth magnets can attach to each other and secure at least a portion of the first band to at least a portion of the third band. In some cases, the first band can include a fifth magnet and a sixth magnet; the second band includes a seventh magnet and an eighth magnet; the fifth and seventh magnets can have opposing poles, the fifth and seventh magnets can attach to each other and secure at least a portion of the first band to at least a portion of the second band; and the sixth and eighth magnets can have opposing poles, the second and fourth magnets can attach to each other and secure at least one portion of the first band to at least one portion of the third band. In some cases, the at least one first selected contour and the at least one second selected contour can be the same. In some cases, the first physiological parameter can be motion of the user. In some cases, the second physiological parameter can be a heartrate of the user. In some cases, the first physiological parameter can be a heartrate of the user. In some cases, the system includes a third fitness tracker positioned between the first fitness tracker and the second fitness tracker, the third fitness tracker including a third enclosure including a third electronic subsystem including at least one sensor configured to sense a third physiological parameter of the user when the third fitness tracker is in use, the third electronic subsystem can wirelessly transmit signals to the remote device responsive to the third physiological parameter, the third enclosure couplable to a third band that can attach to a portion of the user&#39;s body; and the third enclosure including at least a third engagement surface and a fourth engagement surface, the third engagement surface including at least a third selected contour, the third selected contour selected with respect to the first selected contour such that the third engagement surface of the third enclosure can interlock with the first engagement surface of the first enclosure, the fourth engagement surface having at least a fourth selected contour, the fourth selected contour selected with respect to the second selected contour such that the fourth engagement surface of the third enclosure can interlock with the second engagement surface of the second enclosure. In some cases, the first, second, and third physiological parameters can be different from each other. In some cases, the wirelessly transmitted signal of the first and second electronic subsystems can include an alert. In some cases, the wirelessly transmitted signal of the first and second electronic subsystems can be received and processed by the remote device. In some cases, the remote device can include a smartphone. In some cases, the first and second electronic subsystems can receive and process user information from the remote device. 
     In another aspects, a method of sensing at least two physiological parameters of a user can include positioning a first fitness tracker on a portion of the user&#39;s body with a first band, the first fitness tracker sensing a first physiological parameter of the user and wirelessly communicating signals responsive to the first physiological parameter to a remote device, the first fitness tracker including a first engagement surface; positioning a second fitness tracker on a portion of the user&#39;s body with a second band, the second fitness tracker sensing a second physiological parameter of the user and wirelessly communicating signals responsive to the second physiological parameter to the remote device, the second fitness tracker including a second engagement surface, the second engagement surface positioned adjacent to and interlocked with the first engagement surface to form the appearance of a unified enclosure for the first and second fitness trackers. 
     The method of any of the preceding paragraphs herein can include one or more of the following steps and/or features. In some cases, the first engagement surface can have at least a first selected contour and the second engagement surface can have at least a second selected contour. In some cases, the first selected contour and the second selected contour can be the same. In some cases, the first physiological parameter can be motion of the user. In some cases, the second physiological parameter can be a heartrate of the user. In some cases, the first physiological parameter can be a heartrate of the user. In some cases, the method can include positioning a third fitness tracker on a portion of the user&#39;s body with a third band, the third fitness tracker positioned between the first fitness tracker and the second fitness tracker, the third fitness tracker sensing a third physiological parameter of the user and wirelessly communicating signals responsive to the third physiological parameter to the remote device, the third fitness tracker including a third engagement surface and a fourth engagement surface, the third engagement surface positioned adjacent to and interlocked with the first engagement surface, the fourth engagement surface positioned adjacent to and interlocked with the second engagement surface, the first, second and third fitness trackers forming the appearance of a unified enclosure for the first and second fitness trackers. 
     In another aspects, a system can include a first wearable device including a first housing and a first strap configured to secure the first housing to a user&#39;s body, the first housing including a first sensor; and a second wearable device including a second housing and a second strap configured to secure the second housing to the user&#39;s body, the second housing including a second sensor that can be different than the first sensor; when the first and second wearable devices are secured to the user&#39;s body adjacent one another, a first portion of the first housing can contact a second portion of the second housing, said first and second portions can have complimentary shapes. 
     The system of any of the preceding paragraphs and/or any of the apparatuses, systems, or devices disclosed herein can include one or more of the following features. In some cases, the first sensor can sense a first physiological parameter of the user when the first wearable device is in use, and the second sensor can sense a second physiological parameter of the user when the second wearable device is in use. In some cases, the first physiological parameter can different than the second physiological parameter. In some cases, the first and second physiological parameters can include at least one of a blood oxygen saturation, a heart rate, a temperature, and a motion of the user. In some cases, each of the first portion of the first housing and the second portion of the second housing can be curved. In some cases, the first and second housings can be removably connect to each other. In some cases, the first strap can include a first plurality of magnets; the second strap can include a second plurality of magnets; each magnet of the first plurality of magnets and a corresponding magnet of the second plurality of magnets can have opposing poles; the first plurality of magnets and the second plurality of magnets can attach to each other and secure at least a portion of the first strap to at least a portion of the second strap. In some cases, the first and second straps can abut one another when the first and second portions of the first and second wearable housing contact one another. In some cases, the first and second housings can form a unified enclosure when the first and second housing are in contact with each other. In some cases, the first wearable device can include a first electronic subsystem configured to wirelessly communicate with a remote device; and the second wearable device can include a second electronic subsystem configured to wirelessly communicate with the remote device. In some cases, the system includes a third wearable device including a third housing and a third strap configured to secure the third housing to the user&#39;s body, the third housing including a third sensor that can be different than the first sensor; when the first, second, and third wearable devices are secured to the user&#39;s body adjacent one another, the first portion of the first housing can contact a third portion of the third housing and the second portion of the second housing can contact a fourth portion of the third housing, said first, second, third, and fourth portions can have complimentary shapes. In some cases, the first and third straps can abut one another when the first and third portions of the first and third wearable housings contact one another; and the second and third straps can abut one another when the second and fourth portions of the second and third wearable housings contact one another. In some cases, the first, second, and third housings can form a unified enclosure when the first, second, and third housing are in contact with each other. 
     In another aspects, the system can include a first wearable device including a first housing, the first housing comprising a first sensor; and a second wearable device including a second housing, the second housing comprising a second sensor that is different than the first sensor; the first and second wearable devices can be secured to a user&#39;s body adjacent one another, a first portion of the first housing can contact a second portion of the second housing, said first and second portions can have complimentary shapes; and the first and second housings can form a unified enclosure when the first and second housings are in contact with each other. 
     The system of any of the preceding paragraphs and/or any of the apparatuses, systems, or devices disclosed herein can include one or more of the following features. In some cases, each of the first portion of the first housing and the second portion of the second housing can be curved. In some cases, the system can include a third wearable device including a third housing, the third housing including a third sensor; when the first, second, and third wearable devices are secured to the user&#39;s body adjacent one another, the first portion of the first housing can contact a third portion of the third housing and the second portion of the second housing can contact a fourth portion of the third housing, said first, second, third, and fourth portions can have complimentary shapes; and the first, second, and third housings can form a unified enclosure when the first, second, and third housings are in contact with each other. 
     In another aspects, a system can include a first wearable device including a first housing and a first strap to secure the first housing to a user&#39;s body, the first housing including a first sensor; a second wearable device including a second housing and a second strap to secure the second housing to the user&#39;s body, the second housing including a second sensor that can be different than the first sensor; when the first and second wearable devices are secured to the user&#39;s body adjacent one another, portions of the first and second housings can contact one another along an abutment juncture having a serpentine shape. 
     In another aspects, a wearable device including at least a first operational mode and a second operational mode can include a first screen display to display images representing data and graphic information in the first operational mode; and a second screen display overlaying the first screen display to display low refresh rate images in the second operational mode, the second screen display can be transparent in the first operational mode such that images displayed by the first display are visible through the second display in the first operational mode. 
     The wearable device of any of the preceding paragraphs and/or any of the apparatuses, systems, or devices disclosed herein can include one or more of the following features. In some cases, the wearable device can include a switch to transition the wearable device from the first operational mode to the second operational mode and vice versa. In some cases, the wearable device can include at least one sensor to detect a physiological parameter of a user when the wearable device is in use. In some cases, the physiological parameter can include at least one of a blood oxygen saturation, a heart rate, a temperature, and a motion of the user. In some cases, the first screen display can include an OLED display. In some cases, the second screen display can include a reflective display including a plurality of ink particles. In some cases, the wearable device can include an electric circuitry to selectively charge the ink particles of the reflective display in the second operational mode. In some cases, a shape of the first screen display and a shape of the second screen display are the same. In some cases, the wearable device consumes more power when the wearable device is in the first operational mode than when the wearable device is in the second operational mode. In some cases, the wearable device can include a band for securing the wearable device to a portion of a user&#39;s body, the band including a first end and a second end, wherein the first end can attach to a first end of the wearable device and the second end can attach to a second end of the wearable device. 
     In another aspects, a method of operating a smartwatch in two operational modes can include activating an e-ink display screen in a first operational mode, the smartwatch sending commands to the e-ink display screen in the first operational mode to cause the e-ink display screen to display images representing an analog watch; and switching the smartwatch to a second operational mode, the smartwatch deactivating the e-ink display screen in the second operational mode to cause the e-ink display to become transparent, the smartwatch activating an interactive display screen to display information on the interactive screen and to receive tactile inputs via the interactive display screen, the information visible through the transparent e-ink display screen in the second operational mode. 
     One aspect of the embodiments disclosed herein is a smartwatch that includes a housing having a lower base portion and an upper removable portion. The base portion is secured to a band configured to engage a portion of a user&#39;s limb. A lower surface of the base portion includes at least one sensor configured to contact the user&#39;s skin and to sense a physiological parameter. A first electronic subsystem within the base portion processes sensed signals from the at least one sensor and selectively generates a wireless communication signal in response to the sensed signals. The first electronic subsystem is powered by a first battery within the base portion. A second electronic subsystem within the removable portion is powered by a second battery, which is selectively couplable to an external charger when the removable portion is removed from the base portion. The second battery charges the first battery when the removable portion is attached to the base portion. 
     Another aspect in accordance with embodiments disclosed herein is a smartwatch that includes a base housing portion secured to a band. The band is configured to engage a portion of a user&#39;s limb. The base housing portion has a lower surface. The lower surface has at least one sensor configured to contact the user&#39;s skin and to sense a physiological parameter. A first electronic subsystem within the base housing portion is configured to process sensed signals from the at least one sensor and to selectively generate a wireless communication signal in response to the sensed signals. A first battery within the base portion is configured to provide power to the first electronic subsystem. A removable housing portion is attachable to the base housing portion. The removable housing portion has a visual display. The removable housing portion includes a second electronic subsystem. The second electronic system is powered by a second battery. The second battery is selectively couplable to an external charger when the removable housing portion is removed from the base portion. The second battery within the removable housing portion charges the first battery within the base housing portion when the removable housing portion is attached to the base housing portion. In certain embodiments, the base housing portion wirelessly communicates with removable housing portion. 
     Another aspect of the embodiments disclosed herein is a method of charging a smartwatch. The method includes detaching a removable housing portion of the smartwatch from a base housing portion of the smartwatch. The removable housing portion encloses a main battery therein. The base housing portion encloses an auxiliary battery therein. The method further includes positioning the removable housing portion of the smartwatch on a charging source to charge the main battery. The method further includes maintaining the base housing portion in sensory communication with a user while the removable housing portion is detached. The base housing portion receives power from the auxiliary battery when the removable housing portion is detached from the base housing portion. The method further includes reattaching the removable housing portion to the base housing portion after charging the main battery. The method further includes charging the auxiliary battery with energy from the main battery when the removable housing portion is attached to the base housing portion. In certain embodiments, the base housing portion wirelessly communicates with the removable housing portion while the removable housing portion is removed from the base housing portion. 
     Another aspect of the embodiments disclosed herein is a smartwatch including a smartwatch body and a removable battery pack. The smartwatch body houses an internal battery. The smartwatch body includes at least first input electrode positioned on a first portion of the smartwatch body and a second input electrode positioned on a second portion of the smartwatch body. The removable battery pack is selectively attachable to the smartwatch body. The removable battery pack has a plurality of engagement surfaces that engage the smartwatch body to removably secure the removable battery pack to the smartwatch body. The removable battery pack includes at least first output electrode and a second output electrode. The first output electrode is positioned on the removable battery pack such that the first output electrode contacts the first input electrode of the smartwatch when the removable battery pack is secured to the smartwatch body and such that the second output electrode contacts the second input electrode of the smartwatch body when the removable battery pack is secured to the smartwatch body. The removable battery pack provides electrical power via the contacts between the output electrodes and the input electrodes to charge the internal battery within the smartwatch body. In certain embodiments, the first output electrode is positioned on a first engagement surface of the plurality of engagement surfaces, and the second output electrode is positioned on a second engagement surface of the plurality of engagement surfaces. In certain embodiments, the smartwatch body further includes a third input electrode and a fourth input electrode. The removable battery pack further includes a third output electrode positioned on a third engagement surface of the plurality of engagement surfaces and includes a fourth output electrode positioned on a fourth engagement surface of the plurality of engagement surfaces. The third output electrode contacts the third input electrode and the fourth output electrode contacts the fourth input electrode when the removable battery pack is secured to the smartwatch body. 
     Another aspect of the embodiments disclosed herein is a method of charging a smartwatch while the smartwatch is being worn by a user. The method includes attaching a charged removable battery pack to a body of the smartwatch by positioning a plurality of engagement surfaces against respective portions of the body of the smartwatch. The method further includes engaging a first output electrode of the removable battery pack with a first input electrode of the body of the smartwatch to provide a first electrical contact between the first output electrode and the first input electrode. The method further includes engaging a second output electrode of the removable battery pack with a second input electrode of the body of the smartwatch to provide a second electrical contact between the second output electrode and the second input electrode. The method further includes charging a battery within the smartwatch body with power from a battery within the removable battery pack by propagating electrical energy via the first electrical contact and the second electrical contact. In certain embodiments, the first output electrode is positioned on a first engagement surface of the plurality of engagement surfaces, and the second output electrode is positioned on a second engagement surface of the plurality of engagement surfaces. In certain embodiments, the method further includes engaging a third output electrode of the removable battery pack with a third input electrode of the body of the smartwatch to provide a third electrical contact between the third output electrode and the third input electrode; and engaging a fourth output electrode of the removable battery pack with a fourth input electrode of the body of the smartwatch to provide a fourth electrical contact between the fourth output electrode and the fourth input electrode. In certain embodiments, the first output electrode is positioned on a first engagement surface of the plurality of engagement surfaces; the second output electrode is positioned on a second engagement surface of the plurality of engagement surfaces; the third output electrode is positioned on a third engagement surface of the plurality of engagement surfaces; and the fourth output electrode is positioned on a fourth engagement surface of the plurality of engagement surfaces. 
     One aspect of the embodiments disclosed herein is a fitness tracker having an enclosure secured to a band configured to engage a portion of a user&#39;s limb. The enclosure houses at least one sensor to sense a physiological parameter of the user. The enclosure is configured to have a shape selected to provide an aesthetic appearance that may have the appearance of a gem or other item of jewelry. The enclosure has at least one peripheral surface having preselected contours. The contours are selected such that the contours of the peripheral surface of a first enclosure of a first fitness tracker are engageable with the contours of the peripheral surface of a second enclosure of an adjacent fitness tracker such that the two enclosures appear as a single enclosure. Each enclosure includes a respective electronic system that communicates wirelessly with a remote device such as a smartphone. 
     Another aspect in accordance with embodiments disclosed herein is a system of at least a first fitness tracker and a second fitness tracker. Each fitness tracker is capable of wirelessly communicating a respective physiological parameter of a user to a remote device. The systems includes a first enclosure of the first fitness tracker. The first enclosure is couplable to a first band configured to attach to a limb of a user. A first electronic subsystem within the first enclosure is configured to sense a first physiological parameter of the user when attached to the user. The first electronic subsystem is configured to wirelessly transmit signals responsive to the first physiological parameter. An upper portion of the first enclosure has at least a first engagement surface. The first engagement surface has at least one selected contour. A second enclosure of the second fitness tracker is couplable to a second band configured to attach to the limb of the user. A second electronic subsystem within the second enclosure is configured to sense a second physiological parameter of the user when attached to the user. The second electronic subsystem is configured to wirelessly transmit signals responsive to the second physiological parameter. An upper portion of the second enclosure has at least a second engagement surface. The second engagement surface has at least a second selected contour. The second selected contour is selected with respect to the first selected contour such that the second engagement surface of the second enclosure interlocks with the first engagement surface of the first enclosure. In certain embodiments of the system, the at least one first selected contour and the at least one second selected contour are the same. In certain embodiments of the system, the first physiological parameter is motion of the user, and the second physiological parameter is a heartrate of the user. In certain embodiments of the system, the first physiological parameter is a heartrate of the user. 
     In certain embodiments, the system further includes a third fitness tracker positionable between the first fitness tracker and the second fitness tracker. The third fitness tracker includes a third enclosure couplable to a third band configured to attach to the limb of the user. A third electronic subsystem within the third enclosure is configured to sense a third physiological parameter of the user when attached to the user. The third electronic subsystem is configured to wirelessly transmit signals responsive to the third physiological parameter. An upper portion of the third enclosure has at least a third engagement surface and a fourth engagement surface. The third engagement surface has at least a third selected contour. The third selected contour is selected with respect to the first selected contour such that the third engagement surface of the third enclosure interlocks with the first engagement surface of the first enclosure. The fourth engagement surface has at least a fourth selected contour. The fourth selected contour is selected with respect to the second selected contour such that the fourth engagement surface of the third enclosure interlocks with the second engagement surface of the second enclosure. 
     Another aspect in accordance with embodiments disclosed herein is a method of sensing at least two physiological parameters of a user. The method includes positioning a first fitness tracker on a limb of a user with a first band. The first fitness tracker senses a first physiological parameter of the user and wirelessly communicates signals responsive to the first physiological parameter to a remote device. The first fitness tracker has a first upper enclosure having a first engagement surface. The method further includes positioning a second fitness tracker on the limb of a user with a second band. The second fitness tracker senses a second physiological parameter of the user and wirelessly communicates signals responsive to the second physiological parameter to the remote device. The second fitness tracker has a second upper enclosure having a second engagement surface. The second engagement surface of the second enclosure is positioned adjacent to and interlocks with the first engagement surface of the first enclosure to form the appearance of a unified upper enclosure for the first and second fitness trackers. In certain embodiments of the method, the first engagement surface has at least a first selected contour and the second engagement surface has at least a second selected contour. In certain embodiments of the method, the first selected contour and the second selected contour are the same. In certain embodiments of the method, the first physiological parameter is motion of the user, and the second physiological parameter is a heartrate of the user. In certain embodiments of the method, the first physiological parameter is a heartrate of the user. 
     In certain embodiments, the method further includes positioning a third fitness tracker on the limb of a user with a third band. The third fitness tracker is positioned between the first fitness tracker and the second fitness tracker. The third fitness tracker senses a third physiological parameter of the user and wirelessly communicates signals responsive to the third physiological parameter to the remote device. The third fitness tracker has a third upper enclosure having a third engagement surface and a fourth engagement surface. The third engagement surface of the third enclosure is positioned adjacent to and is interlocked with the first engagement surface of the first enclosure. The fourth engagement surface of the third enclosure is positioned adjacent to and is interlocked with the second engagement surface of the second enclosure. The first, second and third enclosures form the appearance of a unified upper enclosure for the first, second and fitness trackers. 
     One aspect of the embodiments disclosed herein is a smartwatch having a first interactive screen display that displays fitness information and other information generated by an internal processor in response to user commands. The smartwatch has a second display overlaying the interactive display, the second display displaying low refresh rate and/or low resolution metrics, such as time of day using symbols representing a time display of a conventional wristwatch. The second display can be based on low power consumption technologies, such as e-paper technology. The second display hides the first interactive screen display in a time display mode. A user may switch the smartwatch to a smartwatch mode to activate the interactive screen display and to cause the second display to become effectively transparent to enable the user to view the interactive screen display and to enter commands via the interactive screen display. 
     Another aspect in accordance with embodiments disclosed herein is a smartwatch having at least a first operational mode and a second operational mode. The smart watch includes a first screen display that displays images representing data and graphic information in the first operational mode. The smartwatch further includes a second screen display overlaying the first screen display. The second screen display includes e-ink technology. The second screen display is configured to display images representing an analog watch in the second operational mode. The second screen display is configured to be transparent in the first operational mode such that images displayed by the first screen display are visible through the second screen display when the smartwatch is in the first operational mode. 
     Another aspect in accordance with embodiments disclosed herein is a method of operating a smartwatch in two operational modes. The method includes activating an e-ink display screen in a first operational mode. The smartwatch sends commands to the e-ink display screen in the first operational mode to cause the e-ink display screen to display images representing the hands of an analog watch. The method further includes switching the smartwatch to a second operational mode. The smartwatch deactivates the e-ink display screen in the second operational mode to cause the e-ink display to become transparent. The smartwatch activates an interactive display screen to display information on the interactive screen and to receive tactile inputs via the interactive display screen. The information displayed by the interactive screen display is visible through the transparent e-ink display screen in the second operational mode. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
       The foregoing aspects and other aspects of the disclosure are described in detail below in connection with the accompanying drawings in which: 
         FIG.  1    illustrates an upper perspective view of a smartwatch showing a plurality of visual displays on an upper surface, wherein the upper surface also provides a tactile (touch sensitive) interface;  FIG.  1    further illustrating a smartphone in wireless communication with the smartwatch; 
         FIG.  2    illustrates a lower perspective view of the smartwatch of  FIG.  1    showing sensor interfaces on a bottom surface of the smartwatch; 
         FIG.  3    illustrates an upper exploded perspective view of the smartwatch of  FIGS.  1  and  2    showing the smartwatch separated into a first (lower base) portion and a second (upper removable) portion; 
         FIG.  4    illustrates a lower exploded view of the smartwatch of  FIGS.  1  and  2    showing the lower surface of the upper removable housing portion of the smartwatch; 
         FIG.  5    illustrates a perspective view of an exemplary charging unit; 
         FIG.  6    illustrates the upper removable housing portion of the smartwatch of  FIGS.  1  and  2    positioned on a charging unit and illustrates the lower base housing portion of the smartwatch displaced from the upper removable housing portion; 
         FIG.  7    illustrates a block diagram of the electronic circuitry of the smartwatch, which includes a first electronic subsystem in the lower base housing portion of the smartwatch, the first electronic subsystem powered by an auxiliary battery, the electronic circuitry of the smartwatch further including a second electronic subsystem in the upper removable housing portion of the smartwatch, the second electronic subsystem powered by a main battery, the electronic circuitry illustrated with the upper removable housing portion of the smartwatch attached to the lower base housing portion and with the first electronic subsystem receiving power from the second electronic subsystem; 
         FIG.  8    illustrates a block diagram of the second electronic subsystem separated from the first electronic subsystem when the upper removable housing portion of the smartwatch is detached from the lower base housing portion of the smartwatch, the electronic circuitry illustrated with the first electronic subsystem in wireless communication with the second electronic subsystem; 
         FIG.  9    illustrates an upper perspective view of an alternative embodiment of a smartwatch having a removable battery pack, the view in  FIG.  9    showing the smartwatch and the removable battery pack prior to attachment of the removable battery pack to the smartwatch; 
         FIG.  10    illustrates a lower perspective view of the smartwatch and removable battery pack of  FIG.  9   ; 
         FIG.  11    illustrates the removable battery pack of  FIGS.  9  and  10    as viewed in  FIG.  10    with the removable battery pack rotated to show electrodes on two of the attachment legs; 
         FIG.  12    illustrates the removable battery pack of  FIGS.  9  and  10    rotated 180 degrees with respect to the view of  FIG.  11    to show electrodes on the two other attachment legs; 
         FIG.  13    illustrates a bottom view of the smartwatch of  FIGS.  9  and  10    showing the four electrodes on the smartwatch; 
         FIG.  14    illustrates an upper perspective view of the smartwatch and removable battery pack of  FIGS.  9  and  10    showing the removable battery pack attached to the smartwatch; 
         FIG.  15    illustrates a top view of the smartwatch and removable battery pack showing the removable battery pack attached to the smartwatch as in  FIG.  14   ; 
         FIG.  16    illustrates a lower perspective view of the smartwatch and the removable battery pack with the removable battery pack attached to the smartwatch as in  FIG.  13   ; 
         FIG.  17    illustrates a bottom plan view of the smartwatch and the removable battery pack of  FIG.  16    showing the attachment legs positioned over the electrodes of the smartwatch; 
         FIG.  18    illustrates a charging circuit and a battery within the smartwatch showing the connections to the removable battery pack via the external electrodes; 
         FIG.  19    illustrates an upper perspective view of a first fitness tracker attached to a band that enables the first fitness tracker to be attached to the limb (not shown) of a user, the fitness tracker communicating wirelessly to a smartphone; 
         FIG.  20    illustrates a lower perspective view of the first fitness tracker of  FIG.  19    showing a sensor interface on a bottom surface of the fitness tracker; 
         FIG.  21    illustrates a simplified electronic circuit within the first fitness tracker of  FIGS.  19  and  20   , the electronic circuit coupled to a sensor; 
         FIG.  22    illustrates an alternative simplified electronic circuit within the first fitness tracker of  FIGS.  19  and  20   , the electronic circuit coupled to a motion tracking subsystem; 
         FIG.  23    illustrates an upper perspective view of a second fitness tracker having an enclosure configuration that engages with the enclosure configuration of the first fitness tracker of  FIGS.  19  and  20   , the second fitness tracker shown spaced apart from the first fitness tracker prior to engagement of the two fitness trackers; 
         FIG.  24    illustrates a top plan view of the first and second fitness trackers of  FIG.  23    prior to engagement of the two fitness trackers; 
         FIG.  25    illustrates an upper perspective view of the first and second fitness trackers of  FIGS.  23  and  24    positioned on a limb (not shown) of a user with the two fitness trackers positioned adjacent to each other with the upper portion of the second fitness tracker engaging the upper portion of the first fitness tracker; 
         FIG.  26    illustrates a top plan view of the first fitness tracker and the second fitness tracker with the two upper portions engaged as illustrated in  FIG.  25   ; 
         FIG.  27    illustrates an upper perspective view of a third fitness tracker having an upper portion having a first selected contour on a first peripheral surface and having a second selected contour on a second peripheral surface; 
         FIG.  28    illustrates a top plan view of the third fitness tracker of  FIG.  27   ; 
         FIG.  29    illustrates an upper perspective view of the third fitness tracker of  FIGS.  27  and  28    positioned between the first fitness tracker of  FIGS.  19  and  20    and the second fitness tracker of  FIG.  23    with the upper portion of the third fitness tracker engaging the upper portions of the first fitness tracker and the second fitness tracker, each fitness tracker communicating wirelessly with a smartphone; 
         FIG.  30    illustrates a top plan view of the third fitness tracker positioned between the first and second fitness tracker as shown in  FIG.  29   ; 
         FIG.  31    illustrates three fitness trackers wherein each of the three fitness trackers has an upper portion corresponding to the upper portion of the third fitness tracker of  FIG.  29   , the three fitness trackers positioned on a limb (not shown) of a user the upper portions of the three fitness trackers engaged, each of the three fitness trackers communicating wirelessly with a smartphone; 
         FIG.  32    illustrates an exploded upper perspective view of an embodiment of a smartwatch having an interactive screen display and an e-ink (e-paper) screen display, the exploded view illustrating the e-ink screen display prior to positioning the e-ink display over the interactive screen display; 
         FIG.  33    illustrates the smartwatch of  FIG.  32    with the e-ink screen display installed as an overlay over the interactive display, the smartwatch shown in the analog watch display mode; and 
         FIG.  34    illustrates the smartwatch of  FIG.  33    with the e-ink screen display in the transparent mode such that the data and graphics information of the interactive screen display is visible through the e-ink screen display. 
     
    
    
     DESCRIPTION OF ILLUSTRATED EMBODIMENTS 
     As used throughout this specification, the words “upper,” “lower,” “longitudinal,” “upward,” “downward,” “proximal,” “distal,” and other similar directional words are used with respect to the views being described. 
     Physiological parameter sensors are particularly useful in monitoring a user while the user is sleeping to determine whether the user is experiencing unusual conditions that may indicate a medical condition (e.g., sleep apnea) that should be evaluated. 
     Although decreasing size and decreasing power consumption of digital and analog electronics allows the number of functions performed by a smartwatch to increase, the electronics must be powered by a battery, and the battery must be charged to maintain the available power. A smartwatch may be charged using a wired interconnection to a charging source or by using a magnetic induction charger. In either case, the smartwatch is effectively “tethered” to the charger during the charging process. Such charging generally requires the user to remove the smartwatch to connect the smartwatch to a charging cord or to a magnetic induction charger. In either case, the smartwatch is no longer able to monitor the user&#39;s position and orientation or able to monitor the user&#39;s physiological parameters while the smartwatch is being charged. If, for example, the user chooses to charge the smartwatch at night when the user is less likely to use the smartphone connection features of the smartwatch, the smartwatch is no longer monitoring the user&#39;s physiological parameter while the user is sleeping. Accordingly, changes in the parameters that occur when the user is charging the smartwatch might be missed. 
       FIG.  1    illustrates an upper perspective view of a smartwatch  100  attached to a wristband  102 . The wristband allows a user (not shown) to secure the smartwatch to a portion of a limb, such as the wrist of the user. The smartwatch may be in wireless communication with a smartphone  104  or another interactive device.  FIG.  2    illustrates a lower perspective view of the smartwatch of  FIG.  1   . 
     The smartwatch  100  has an upper surface  110  and a lower surface  112 . When the smartwatch is secured to the limb portion of the user using the wristband  102 , the lower surface of the smartwatch contacts the skin of the limb portion. The upper surface of the smartwatch is configured as an interactive display and tactile (touch sensitive) input device. Such surfaces are well known in the art and are not described in detail herein. An example of a display is shown in  FIG.  1    wherein a first portion  120  of the display represents the dial and the hands of an analog watch; a second portion  122  of the display provides a numeric representation of a first physiological parameter (e.g., the temperature) of the user wearing the smartwatch; a third portion  124  of the display provides a numeric display of a second physiological parameter (e.g., the heartrate) of the user, and a fourth portion  126  of the display provides a numeric representation of a third physiological parameter of the user. The dial and hands of the watch and the three numeric displays visual displays on a liquid crystal display (LCD) screen or other display device. The display device may be reconfigured to represent different information (e.g., the numeric dialing pad of a telephone, a text messaging screen or the like). 
     As is known in the art, the information displayed on the upper surface  110  of the smartwatch  100  changes in accordance with the operational mode of the smartwatch. The operational mode may be changed by the user or changed automatically as determined by events detected by the smartwatch. For example, the user may select a mode in which the smartwatch operates as a remote input/output device for the smartphone  104 , and the upper surface displays information related to the smartphone interface (e.g., the display is configured as a numeric dialing pad or configured as a text messaging screen). The smartwatch may detect a change in a sensed physiological parameter and may automatically display detailed information regarding the sensed parameter. In many cases, the upper surface displays icons in areas of the upper surface that the user can touch to provide inputs to the smartwatch (e.g., touching the area of a number of a displayed numeric dialing pad causes the smartwatch to send a signal to the smartphone as part of a command to the smartphone). 
     As illustrated in  FIG.  2   , the lower surface  112  of the smartwatch  100  can include one or more sensors that interact with the skin of the user&#39;s wrist to obtain physiological parameters indicative of the physical condition of the user. The lower surface can include one or more of a temperature sensor, a bioimpedance sensor, a blood oxygenation sensor, an electrocardiogram (ECG) sensor or the like. For example, an oxygen saturation (SpO2) sensor  130  is illustrated with a central light source (e.g., a light-emitting diode (LED))  132  surrounded by a plurality of photodetectors  134 . The light emitted by the LED is sensed by the photodetectors and is processed within the smartwatch to determine a percentage of oxygen saturation. A second sensor  136  may also be provided with a similar structure to the first sensor or with a different structure. 
     The smartwatch  100  can also include at least one mechanical pushbutton (mode switch)  140  positioned on peripheral surfaces of the watch. The mechanical pushbuttons may be activated, for example, to turn off and turn on the displays of the upper surface  110 , to change operational modes, or to initiate or control other options. In some cases, the mode switch  140  can act as a privacy switch. That is, activating the mode switch  140  can cause the one or more sensors of the smartwatch  100  to at least momentarily stop sensing physiological parameters of the user and/or cause the display of the smartwatch  100  to at least momentarily stop displaying information indicative of at least one physiological parameter of the user. 
     Generally, a smartwatch includes a single housing unit having the upper surface  110  and the lower surface  112 ; however, in the illustrated embodiment, the smartwatch  100  of  FIGS.  1  and  2    can include a first housing portion  200  and a second housing portion  202 , which are shown in the upper perspective exploded view of  FIG.  3    and the lower perspective exploded view of  FIG.  4   . In some cases, the first housing portion does not include a display. The first housing portion may also be referred to herein as the lower housing portion, the base housing portion or the base portion. The second housing portion may also be referred to herein as the upper housing portion, the removable housing portion or the removable portion. The base (lower) housing portion is attached to the wristband  102  as described above. The removable (upper) housing portion is removably attached to the base housing portion such that the removable housing portion can be easily attached to and easily removed from the base housing portion as described below. 
     Various attachment techniques may be used to removably attach the removable housing portion  202  to the base housing portion  200  of the smartwatch  100 . For example, in the illustrated embodiment, a lower surface  210  of the removable housing portion (see  FIG.  4   ) can include two engagement posts  212  that extend perpendicularly from the lower surface and which can include respective enlarged end portions  214 . The engagement posts are spaced apart diametrically with respect to center of the lower surface. 
     An upper surface  220  of the base (lower) housing portion  200  can include a first arcuate slot  222  and a second arcuate slot  224 . The arcuate slots are spaced apart by a distance corresponding to the spacing of the engagement posts. Each arcuate slot has an upper portion  230  and a lower portion  232 . The upper portion of each slot has respective first enlarged end opening  234  that is sized to receive the enlarged end portion of one of the engagement posts. The upper portion of each slot has a respective second narrower end opening  236 . The lower portion of each slot has a continuous arcuate width. A first end  240  of the lower portion of each slot (see the first arcuate slot  222 ) has the same size as the first enlarged end opening of the upper portion of the slot. A second end  242  of the lower portion of each slot (see the second arcuate slot  224 ) also has the same size as the first end of the lower portion of the slot. 
     The removable housing portion  202  is attached to the base housing portion  200  by aligning the enlarged end portions  214  of the engagement posts  212  with the first enlarged end openings  234  of the arcuate slots  222 . The enlarged end portions are inserted into the enlarged openings until the lower surface  210  of the removable housing portion is flush with the upper surface  220  of the base housing portion. The removable housing portion is rotated (e.g., approximately 30 degrees counterclockwise) until each engagement post is positioned against the respective second narrower end opening  236  of the upper portion  230  of each respective arcuate slot. 
     When positioned in the described manner, the enlarged end portions  214  of the engagement posts  212  are positioned below the narrower second end openings  236  of the upper portions  230  of the arcuate slots  222 ,  224 . The enlarged end portions of the engagement posts are precluded from moving vertically. Thus, the lower surface  210  of the removable housing portion  202  is secured tightly against the upper surface  220  of the base housing portion  200 . The tight fit and the resulting friction between the two surfaces prevents the removable housing portion from being removed from the base housing portion without deliberate effort by the user. To remove the removable housing portion from the base housing portion, the user rotates the removable housing portion in the opposite direction (e.g., clockwise) to realign the enlarged end portions of the engagement posts with the first ends of the arcuate slots such that the enlarged end portions can be extracted from the arcuate slots. 
     Other techniques (not shown) may also be used to removably secure the removable housing portion  202  to the base housing portion  200 . For example, the two housing portions may be magnetically coupled. The two housing portions may be engaged with respective threads around the perimeter of each housing portion. Tabs and engagement surfaces may be provided on the two housing portions to enable the two portions to be snapped together. 
     Removing the removable housing portion  202  from the base housing portion  200  of the smartwatch  100  enables the user to position the removable housing portion in communication with a charging source without removing the base housing portion from the user&#39;s wrist.  FIG.  5    illustrates a charging unit  300  suitable for charging the removable housing portion of the smartwatch. The charging unit receives power via a power cable  310 , which may be connected to a wall adapter (not shown) or to a USB port (not shown). 
     The charging unit  300  has a flat upper surface  320  ( FIG.  5   ) that receives the removable housing portion  202  as shown in  FIG.  6   . In the illustrated embodiment, the upper surface of the charging unit can include a first arcuate slot  330  and a diametrically opposed second arcuate slot  332 . The two arcuate slots are similar to the arcuate slots  222 ,  224  of the upper surface  220  of the base housing portion  200  (see  FIG.  3   ). The arcuate slots of the charging unit are spaced apart by a corresponding distance as the arcuate slots of the base housing portion. 
     Each arcuate slot  330 ,  332  has an upper portion  340  and a lower portion  342 . The upper portion of each slot has respective first enlarged end opening  344  that is sized to receive the enlarged end portion of one of the engagement posts. The upper portion of each slot has a respective second narrower second end opening  346 . The lower portion of each slot has a continuous arcuate width. A first end  350  of the lower portion of each slot (see the first arcuate slot  330 ) has the same size as the enlarged upper end opening. A second end  352  of the lower portion of each slot (see the second arcuate slot  332 ) also has the same size as the first end of the lower portion of the slot. 
     The removable housing portion  202  of the smartwatch  100  is coupled to the charging unit  300  by positioning the lower surface  210  of the removable housing portion over the upper surface  320  of the charging unit with the enlarged end portion  214  of each engagement post  212  aligned with a respective first enlarged end opening  344  of the arcuate slots  330 ,  332  of the upper surface of the charging unit. The enlarged end portions of the engagement posts are inserted into the enlarged first ends of the arcuate slots. The removable housing portion is then rotated (e.g., approximately 30 degrees counterclockwise) to secure the removable housing portion to the charging unit. 
     The charging unit  300  operates to magnetically couple electrical energy to the removable housing portion  202  in a conventional manner. Alternatively, the arcuate slots  330 ,  332  may also be metalized electrodes and the engagement posts  212  of the removable housing portion  202  may be metalized electrodes so that the charging unit may couple electrical energy to the removable housing portion by electrical conduction. After charging is completed, the removable housing portion is removed from the charging unit by rotating the removable housing portion in the opposite direction (e.g., clockwise). 
     In further alternative embodiments, the removable housing portion  202  of the smartwatch  100  may not have the illustrated engagement posts  212 . The charging unit  300  may have an outer peripheral raised rim (not shown) that centers the removable housing portion on the charging unit to assure alignment of the magnetic field generated by the charging unit with the magnetic receiving unit within the removable housing portion. 
       FIG.  6    illustrates the removable housing portion  202  of the smartwatch  100  positioned on the charging source  300  while the base housing portion  200  remains secured to the wristband  102  so that the lower portion can remain in contact with the user&#39;s wrist (not shown). Accordingly, the user is free to move about without being tethered to the charger. 
     Although the removable housing portion  202  of the smartwatch  100  is positioned on the charging unit  300  and is physically separated from the base housing portion  200  in  FIG.  6   , the smartwatch retains a substantial portion of the functionality of the smartwatch. This continued functionality is provided by parsing the electronics of the smartwatch into at least first electronic subsystem  400  and a second electronic subsystem  402  as illustrated in  FIG.  7   . The first electronic subsystem can include the electronics housed within the base (lower) housing portion. The second electronic subsystem can include the electronics housed within the removable (upper) housing portion. The second electronic subsystem is coupled to the upper surface  110  of the smartwatch  100 , which functions as a display and tactile display interface. The parsing of the first and second electronic subsystems in  FIG.  7    is only one example of the parsing of the overall electronics into the two subsystems. Some of the electronic components shown in one of the two subsystems may be moved to the other subsystem. Some of the electronic components shown in one of the two subsystems may also be duplicated in the other subsystem. 
     In the example of  FIG.  7   , the second (upper) electronic subsystem  402  can include an input/display electronics subsystem  410  that interacts with the interactive display and tactile (touch sensitive) upper surface  110 . The input/display electronic subsystem may also interact with an audio output (e.g., a piezoelectric speaker)  414  and an audio input (microphone)  416 . 
     The second electronic subsystem  402  can also include at least one remote wireless communication interface  420  that enables the second electronic subsystem to communicate with the smartphone  104  ( FIG.  1   ) or another remote device. For example, the wireless communication interface may be a Bluetooth® interface. The wireless communication interface can also include a WiFi interface. The second electronic subsystem can also include a dedicated local wireless interface  426  to the first electronic subsystem  400 . In the illustrated embodiment, the dedicated local interface to the first electronic subsystem is a second wireless communication (local) interface, such as a Bluetooth® low energy (BLE). The two wireless interfaces may be two components of a single wireless interface. 
     The second electronic subsystem  402  can also include a second electronic subsystem processor  430  that communicates with the input/display electronics subsystem  410  and the wireless communication interface  420  in a conventional manner. The second electronic subsystem processor is programmed to interact with the display and inputs of the upper surface  110 , to perform routines in response to user commands, to monitor sensory information from the second electronic subsystem  402  and to communicate with the smartphone  104  ( FIG.  1   ) or other remote devices. 
     The second electronic subsystem  402  can also include a main battery  440  and a battery charging system  442  and an upper housing voltage generator  444 . In the illustrated embodiment, the battery charging system converts magnetic energy from a built-in antenna  446  to a DC voltage for charging the battery. The main battery provides one or more DC voltages to the components within the second electronic subsystem via the upper housing voltage generator. The upper housing voltage generator provides at least one DC voltage output to the first electronic subsystem  400  via the engagement posts  212  as described below. The battery charging system is couplable to an external source of power to receive energy and to convert the energy to suitable voltage to charge the main battery. For example, in the illustrated embodiment, the battery charging system is couplable to the magnetic induction charging unit  300  as shown in  FIG.  5    and as is described below with respect to  FIG.  8   . The battery charging system receives electromagnetic energy from the charger and converts the electromagnetic energy to a suitable DC electric current to charge the main battery. In other embodiments, the battery charging system can include a DC voltage input that receives DC electric current, which is converted to a suitable DC electric current to charge the main battery. For example, the engagement posts  212  may provide an input voltage to the voltage generator that can be used for charging the main battery. Magnetic induction charging and DC electric current charging are well known to the art. 
     As further illustrated in  FIG.  7   , the first electronic subsystem  400 , which is housed in the base (lower) housing portion  200 , can include a first electronic subsystem processor  500 , a sensor interface  510 , a wireless communication interface  520  and an auxiliary battery  530 . The auxiliary battery provides one or more DC voltages to the components in the first electronic subsystem via a lower housing voltage generator  540 . The first electronic subsystem processor is coupled to the mode switch  140 . The first electronic subsystem can include a piezoelectric buzzer (alert device)  550  or other perceptible alert system. The perceptible alert system notifies the user of an incoming message or other alert. 
     The auxiliary battery receives power from the second electronic subsystem  402  via a power input interface  560 . The power input interface is coupled to the first arcuate slot  222  and the second arcuate slot  224 . When the base housing portion  200  is attached to the removable housing portion  202  as shown in  FIG.  1   , electrical energy from the upper housing voltage generator  444  in the second electronic subsystem  402  is electrically coupled to the arcuate slots via the engagement posts  212  of the base housing portion  200  to thereby provide electrical energy to the power input interface to charge the auxiliary battery. In alternative embodiments, separate pairs of electrodes may be provided on the removable housing portion and the base housing portion to provide charging current from the removable housing portion to the auxiliary battery in the base housing portion. 
     In the illustrated embodiment, the lower housing voltage generator  542  receives electrical energy from the auxiliary battery  530 . When the removable housing portion  202  is attached to the base housing portion  200 , the battery is charged and is then maintained fully charged from the electrical energy from the removable housing portion. The auxiliary battery provides electrical energy to the lower housing voltage generator. When the removable housing portion is removed from the base housing portion, the auxiliary battery continues to provide electrical energy to the first electronic subsystem. 
     In an alternative embodiment (not shown), the lower housing voltage generator  542  may receive electrical energy directly from the removable housing portion  202  when the removable housing portion is attached to the base housing portion. The auxiliary battery may only provide electrical energy to the base housing portion when the removable housing portion is detached as described below. In such an embodiment, diodes (not shown) can be included to assure that the auxiliary battery is only providing power to the first electronic subsystem  400  when the removable housing portion is detached from the base housing portion. 
     As shown in  FIG.  7   , the first electronic subsystem  400  in the base housing portion  200  communicates with the second electronic subsystem  402  in the removable housing portion  202  via the wireless communication interface  520  in the first electronic subsystem communicating with the local wireless interface  426  of the second electronic subsystem. The first electronic subsystem receives data and commands from the second electronic subsystem and transmits processed sensor information to the second electronic subsystem. 
       FIG.  8    illustrates a block diagram of the first electronic subsystem  400  separated from the second electronic subsystem  402  when the upper removable housing portion  202  of the smartwatch  100  is detached from the lower base housing portion  200  as shown in  FIG.  6   . The electronic circuitry illustrated with the first electronic subsystem remains in wireless communication with the second electronic subsystem via the wireless communication interface  520  communicating the local wireless interface  426 . The auxiliary battery  530  within the first electronic subsystem is no longer receiving power from the second electronic subsystem because the engagement posts  212  are no longer engaged with the arcuate slots  222 ,  224 . The absence of power being received by the power input interface  560  may be communicated to the first electronic subsystem processor  500  to cause the first electronic subsystem processor to enter a low power operating mode. For example, the first electronic subsystem processor may access the sensors  130 ,  136  at a lower repetition rate to conserve power. The first electronic processor may only activate the wireless communication interface  520  to communicate detected abnormalities or other changes instead of communicating continuous readings to the second electronic subsystem. When the upper removable housing portion is directly coupled to the lower base housing portion, the user may use the tactile input and display features of the upper surface  110  to select the sensing features to remain active when the upper removable housing portion is detached and the lower base housing portion is in the lower power mode. 
     If an abnormality or other warning occurs while the removable housing portion  202  is disconnected from the base housing portion  200 , the user may receive a warning via the alert device  550 . The user may also receive an alert via a haptic feedback that can be included in the base housing portion  200 . Accordingly, the user is able to monitor selected physiological parameters while the main battery  440  in the second electronic subsystem is charging. If an alert occurs, the user can quickly detach the removable housing portion from the charging unit  300 , reattach the two housing portions, and resume monitoring of physiological parameters using the full capabilities of the smartwatch  100 . 
       FIGS.  9 - 18    illustrate an alternative embodiment that allows a smartwatch  600  to be charged while the user is wearing the watch. The smartwatch is attached to a wristband  602 , which is shown partially broken away in the drawings. As described below, the smartwatch is chargeable using a magnetic charger, such as the charging unit  300  described above with respect to  FIG.  5   . As further described below, the smartwatch is also chargeable from a removable battery pack  610 , which is shown separate from the smartwatch in  FIGS.  9 - 13   , and which is shown attached to the smartwatch in  FIGS.  14 - 17   . 
     As shown in  FIG.  9   , the smartwatch  600  can include an upper surface  620 , a lower surface  630 , and a peripheral wall  640 . The peripheral wall extends between the upper surface and the lower surface. 
     The upper surface  620  of the smartwatch  600  may correspond to the upper surface  110  of the previously described smartwatch  100 . As previously described, the upper surface provides an interactive display  650 , which may display time of day and other information. The interactive display may also function as an input/output device. 
     The lower surface  630  of the smartwatch  600  encloses a conventional magnetic coupling system (see  FIG.  18   ), which is positioned in the body of the smartwatch proximate to a circular area  652  of the lower surface. The circular area of the lower surface of the smartwatch may be placed on the charging unit  300  ( FIG.  5   ) to charge the smartwatch in a conventional manner. 
     The peripheral wall  640  of the smartwatch  600  can include a middle wall portion  660 , which extends for a selected distance generally midway between the upper surface  620  and the lower surface  630 . The middle wall portion forms the largest circumference of the smartwatch. The peripheral wall can further include an upper wall portion  670 , which slopes inwardly from the middle wall portion and extends to the upper surface  620 . In the illustrated embodiment, at least a portion  672  of the upper wall portion is frustoconical. 
     The peripheral wall  640  can further include a lower wall portion  680 , which slopes inwardly from the middle wall portion  660  to the lower surface  630 . In the illustrated embodiment, at least a portion  682  of the lower wall portion is frustoconical. 
     The smartwatch  600  can include a first control device  690  extending from the middle portion  660  of the peripheral wall  640  of the smartwatch and can include a second control device  692  extending from the middle portion of the peripheral wall of the smartwatch. In the illustrated embodiment, the two control devices are diametrically opposed from each other on opposite sides of the smartwatch; however, the control devices may be located elsewhere about the peripheral wall. In the illustrated embodiment, the first control device is a wheel, which may be rotated to move among items on the interactive display  650  of the upper surface  620 . In the illustrated embodiment, the second control device is a pushbutton, which may be used, for example, to activate and deactivate the display and to select functions (e.g., modes) of the smartwatch. Additional controls or fewer controls can be included in accordance with the functionality of the smartwatch. 
     The smartwatch  600  can include a plurality of electrodes on the frustoconical portion  682  of the inwardly sloped lower wall portion  680 . In the illustrated embodiment, the smartwatch can include a first electrode  700 , a second electrode  702 , a third electrode  704  and a fourth electrode  706 . In alternative embodiments, the smartwatch can include only two electrodes. In the illustrated embodiment, the four electrodes are spaced apart angularly by approximately 45 degrees. The electrodes are connected to an internal battery charging circuit (battery charger)  710 , which is illustrated in  FIG.  18   . As further illustrated in  FIG.  18   , the battery charging circuit also receives charging energy from an antenna (e.g., a coil)  712  positioned proximate to the circular area  652  of the lower surface  630  of the smartwatch. Only two of the four electrodes are needed to provide electrical energy to the charging circuit. In the illustrated embodiment, the first electrode and the third electrode are interconnected to provide a relatively positive voltage to the charging circuit, and the second electrode and the fourth electrode are interconnected to provide a relatively negative voltage. In alternative embodiments, only two electrodes may be used to provide the charging voltage, and the other two electrodes may be used to communicate between the removable battery pack  610  and the smartwatch. The removable battery pack  610  can also be referred to as a removable housing. In some cases, the battery pack  610  does not include a display. 
     In the illustrated embodiment, each of the four electrodes  700 ,  702 ,  704 ,  706  is circular; however, the electrodes may have different sizes and shapes. The electrodes can include conventional material such as copper or copper with gold or silver plating. 
     In the illustrated embodiment, the removable battery pack  610  can include an upper body portion  750 . The upper body portion has the shape of a rectangular parallelepiped in the illustrated embodiment; however, other shapes may also be used. The width of the removable battery pack is selected such that when the removable battery pack is positioned on the smartwatch as shown in  FIGS.  14 - 17   , a portion of the smartwatch extends beyond the sides of the removable battery pack so that the first control  690  and the second control  692  are accessible. 
     The upper body portion  750  of the removable battery pack  610  encloses a battery (not shown) and electronic circuitry (not shown) that controls the charging and discharging of the battery within the removable battery pack. 
     The upper body portion  750  of the removable battery pack  610  has an upper surface  760  and a lower surface  762 . The upper surface of the upper body portion can include a circular portion  780 . In some cases, the circular portion  780  can include an opening along an upper surface  760  of the battery pack  610 . The opening along the upper surface  760  of the battery pack  610  can permit visualization of the interactive display  650  when the battery pack  610  is attached to the smartwatch  600 . Beneficially, this can allow users to visualize the interactive display  650  while the battery pack  610  is attached to the smartwatch  600 . An internal antenna (e.g., coil) is positioned within the upper body portion proximate to the circular portion. The removable battery pack is charged by positioning the removable battery pack on a charging unit, such as the charging unit  300  of  FIG.  5   , with the circular portion of the upper surface of the removable battery pack proximate to the top surface of the charging unit. The removable battery pack is charged in a conventional manner. 
     The lower surface  762  of the upper body portion  750  of the removable battery pack  610  is flat in the illustrated embodiment. A first lower attachment portion  800  extends from one end of the lower surface, and a second lower attachment portion  810  extends from an opposite end of the lower surface. The first lower attachment portion can include a first inner surface  802 . The second lower attachment portion can include a second inner surface  812 . The first and second inner surfaces are sized and shaped to conform to the contours of the upper wall portion  670  of the smartwatch  600 . Accordingly, when the removable battery pack is positioned over the smartwatch as shown in  FIGS.  14 - 17   , the lower surface of the upper body portion of the removable battery pack rests against the upper surface  620  of the smartwatch, and the first and second inner surfaces rest against respective portions of the upper wall portion  670  of the smartwatch. 
     The first attachment portion  800  has a first attachment leg  820  extending from a first side and has a second attachment leg  822  extending from a second side. Similarly, the second attachment portion  810  has a third attachment leg  824  extending from a first side and has a fourth attachment leg  826  extending from a second side. Each of the four attachment legs curves inwardly toward the center of the removable battery pack. 
     The first attachment leg  820  has a first attachment leg inner surface  830  ( FIG.  12   ). The second attachment leg  822  has a second attachment leg inner surface  832  ( FIG.  12   ). The third attachment leg  824  has a third attachment leg inner surface  834  ( FIG.  11   ). The fourth attachment leg  826  has a fourth attachment leg inner surface  836 . The four attachment legs are configured so that the respective inner surfaces conform to the middle wall portion  660  and the lower wall portion  680  of the smartwatch  600 . The four attachment legs are resilient such that when the removable battery pack  610  is positioned over the smartwatch, the four attachment legs flex to allow the attachment legs to expand outwardly to pass over the larger diameter middle wall portion of the smartwatch. When the lower surface  762  of the removable battery pack is positioned against the upper surface  620  of the smartwatch, the resilience of each of the four attachment legs causes the respective inner surface of each attachment leg to press against a corresponding portion of the frustoconical portion  682  of the lower wall portion  680  of the smartwatch as shown in  FIGS.  14 ,  16  and  17   . 
     As shown in  FIG.  12   , the first attachment leg inner surface  830  of the removable battery pack  610  has a first battery pack electrode  850  positioned thereon. The second attachment leg inner surface  832  has a second battery pack electrode  852  positioned thereon. As shown in  FIG.  11   , the third attachment leg inner surface  834  of the removable battery pack has a third battery pack electrode  854  positioned thereon. The fourth attachment leg inner surface  836  has a fourth battery pack electrode  856  positioned thereon. When the removable battery pack is secured to the smartwatch  600  as illustrated in  FIGS.  14 - 17   , the resilience of each attachment leg  820 ,  822 ,  824 ,  826  provides sufficient pressure to force each battery pack electrode against a respective one of the smartwatch electrodes  700 ,  702 ,  704 ,  706  to provide electrical contact between the electrodes. The removable battery pack can also include temperature monitoring sensor to monitor overheating during the charging. 
     In the illustrated embodiment, the first, second, third and fourth battery pack electrodes  850 ,  852 ,  854 ,  856  of the removable battery pack  610  are sized and shaped to engage the first, second, third and fourth electrodes  700 ,  702 ,  704 ,  706 , respectively, of the smartwatch  600 . For example, the removable battery pack electrodes are illustrated as circular electrodes in  FIGS.  11  and  12    having sizes similar to the sizes of the smartwatch electrodes. In alternative embodiments, the four battery pack electrode may have sizes and shapes that differ from the smartwatch electrodes. For example, the removable battery pack electrodes may be larger than the smartwatch electrodes to accommodate tolerance in the fit of the removable battery pack with the smartwatch. In one example (not shown), the removable battery pack electrodes can include rectangular bands that extend at least partially across the respective inner surfaces  830 ,  832 ,  834 ,  836  of the attachment legs  820 ,  822 ,  824 ,  826 . In some embodiments, only two electrodes are included on the smartwatch  600  for charging. Further, the removable battery pack may also have only two electrodes that correspond to the smartwatch electrodes. In some instances, the removable battery pack  610  can include four electrodes, but the smartwatch  600  can only include two electrodes so that the removable battery pack  610  can be placed in any orientation on the smartwatch  600  as long as the two electrodes on both components are in contact. 
     As shown in a block diagram  900  of  FIG.  18   , the internal battery charging system of the smartwatch  600  can include an internal battery  910  and the battery charger  710 . The charging system receives electrical energy from the antenna (e.g., coil)  712  to enable the battery within the smartwatch to be charged using a magnetically coupled charger such as the charging unit  300  ( FIG.  5   ). The charging system is also electrically connected to the four electrodes  850 ,  852 ,  854 ,  856  to enable the battery to be charged from the removable battery pack  610 . In the illustrated embodiment, the first electrode  850  is electrically connected to the third electrode  854  to receive a relatively positive voltage (e.g., +), and the second electrode  852  is electrically connected to a relatively negative voltage (e.g., —). By connecting the pairs of electrodes in this manner, the removable battery pack can be positioned on the smartwatch as shown or rotated by 180 degrees to provide the correct voltage polarity to the smartwatch. 
     When the removable battery pack  610  is attached to the smartwatch  600  as shown in  FIGS.  13 - 17   , the internal battery  910  is charged from the removable battery pack. The smartwatch remains fully operable to monitor the physiological parameters as described above. If the wearer of the smartwatch receives an alert as the result of the physiological monitoring or for other reasons (e.g., an incoming call or text message), the wearer can quickly remove the removable battery pack from the smartwatch and have complete access to the interface  650  on the upper surface  620  of the smartwatch. 
     In the embodiment of  FIGS.  9 - 18   , the internal battery  910  of the smartwatch  600  is the main battery, and the removable battery pack  610  is the auxiliary battery. 
       FIGS.  19  and  20    illustrate an upper perspective view and a lower perspective view, respectively, of a first fitness tracker, or wearable device,  1100  attached to a band  1102 . The band may also be referred to herein as the strap. The band allows a user (not shown) to secure the fitness tracker to a portion of a limb, such as the wrist or the ankle of the user. In the following description, the band referred to as a wristband  1102 . The fitness tracker may be in wireless communication with a smartphone  1104  or another interactive device. 
     The fitness tracker  1100  has an upper portion  1110  and a lower portion  1112 , which are attached to each other or which may be formed as an integrated component. The upper portion has a contoured shape as described below. The lower portion may have a conventional geometric shape such as the illustrated rectangular parallelepiped, a cylinder, or the like. The lower portion is secured to the wristband  1102 . For example, the lower portion may extend through the wristband as illustrated. Alternatively, the wristband may be attached to peripheral surfaces of the wristband. 
     The upper portion  1110  of the fitness tracker  1100  has an upper surface  1120 . The lower portion  1112  of the fitness tracker has a lower surface  1122 . When the fitness tracker is secured to the limb portion of the user using the wristband  1102 , the lower surface of the fitness tracker contacts the skin of the limb portion. In the illustrated embodiment, the upper surface has a contour, a texture and a color that may be selected for aesthetic reasons. Unlike conventional fitness trackers, the upper surface in the illustrated embodiment is not interactive and does display information or accept tactile inputs from the user. In alternative embodiments, the upper surface may have a display and a tactile input device while retaining the disclosed contour. 
     As illustrated in  FIG.  20   , the lower surface  1122  of the fitness tracker  1100  can include at least one sensor  1130  that interacts with the skin of the user&#39;s wrist (or ankle) to obtain physiological parameters indicative of the physical condition of the user. For example, the sensor may be a blood oxygenation sensor as illustrated in  FIG.  20   . The sensor may also be a heartrate sensor, a temperature sensor, a bioimpedance sensor, an electrocardiogram (ECG) sensor or other physiological parameters. Such sensors are well known in the art and are not described in detail herein. In certain embodiments the lower surface of the fitness tracker may not have an exposed sensor. For example, the fitness tracker may have internal components configured to operate as a motion tracker (e.g., a step counter). The motion tracker may also sense orientation of the user, direction of movement and the spatial position of the user (e.g., using an internal GPS). For the purpose of the description herein, the tracking of motion, orientation, location and the like is also identified as the sensing of a physiological parameter of the user. 
     The fitness tracker  1100  can include internal circuitry that is configured to perform a fitness-related sensing function. For example,  FIG.  21    illustrates an electronic circuit  1200  that senses a physiological parameter of the user via the sensor  1130  on the lower surface  1122  of the fitness tracker. The sensor is controlled by a processor  1210  that sends commands or signals to and receives sensed information from the sensor via a sensor interface  1220 . The structures and operations of various types of physiological sensors are well known and are not described herein. 
     The electronic circuit  1200  can further include a wireless interface  1240 , such as a Bluetooth® interface or another suitable interface. The processor  1210  receives input commands and inquiries from the smartphone  1104  ( FIG.  19   ) and sends information to the smartphone via the wireless interface. The user may use the smartphone to set up the fitness tracker and to provide information about the user to enable the processor to evaluate the sensed information from the sensor  1130 . In the illustrated embodiment, the fitness tracker can include an alert device  1250  (e.g., a piezoelectric vibrator or the like) that can be activated by the processor to inform the user that an event has occurred. The user can then use an application on the smartphone to access the information within the fitness tracker to determine the cause of the alert. 
     The electronic circuit  1200  can further include an internal battery  1260  that provides electrical power to the processor  1210 , the sensor interface  1220  and wireless interface  1240  via a voltage generator  1262 . The battery can be recharged via a charging antenna  1264  within the fitness tracker  1100  by placing the fitness tracker on a magnetic induction charger (not shown) or connecting the fitness tracker to a source (not shown) of DC electrical energy. 
     In some cases, only one of two or more fitness trackers (e.g., fitness tracker  1100  and fitness tracker  1330 ) includes a processor  1210 . For example, the fitness tracker  1100  can include a sensor for sensing a first physiological parameter of a user and a processor for receiving and processing the sensed information from the sensor. The fitness tracker  1300  can include a sensor for sensing a second physiological parameter of a user different than the first physiological parameter. Instead of including a processor like the fitness tracker  1100  for receiving and processing the sensed information from the sensor, the fitness tracker  1300  can wirelessly transmit the sensed information from the sensor to the fitness tracker  1100  and/or the smartphone  1104  using the wireless interface. The wireless interface of fitness tracker  1100  can receive the sensed information from the fitness tracker  1300  and transmit it to the processor. The processor of the fitness tracker  1100  can receive and process the sensed information from the fitness tracker  1300 . After receiving and processing the sensed information from the fitness trackers  1100 ,  1300  the wireless interface of the fitness tracker  1100  can send the processed sensed information to the smartphone  1104 . In some cases, the smartphone  1104  can provide a visual representation of the sensed information of fitness trackers  1100 ,  1300 . In some cases, none of the fitness trackers include a processor or the processor is not configured to process the sensed information from the fitness trackers. In such cases, the sensed information from the fitness trackers is transmitted to the smartphone  1104  where a processor of the smartphone  1104  can receive and process the sensed information from the fitness trackers  1100 ,  1300 . The smartphone  1104  can provide a visual representation of the sensed information from the fitness trackers  1100 ,  1300 . 
     In certain embodiments, the lower surface  1122  of the fitness tracker  1100  may not have a sensor. For example,  FIG.  22    illustrates an electronic circuit  1270  that operates as a step counter or other motion tracking device. The fitness tracker can include an internal motion sensor  1280  that senses the orientation and motion of the user. The motion sensor may be used, for example, to determine the physical activity of the user. Motion sensors are well known in the fitness tracking art and are not described in detail herein. 
     As described herein, each fitness tracker  1100  has a dedicated purpose and does not include features that the user does not want or need. As discussed above, one embodiment of the fitness tracker can be configured with an internal motion sensor to count steps and other activities. Another embodiment of the fitness tracker can be configured as an optical sensor. Still another embodiment of the fitness tracker can be configured as a temperature sensor. Still another embodiment of the fitness tracker can be configured as an electrical sensor. Other embodiments of the fitness tracker can be configured with other fitness tracking features. A user wanting only one feature can purchase a fitness tracker having the desired feature without paying the additional costs for unwanted features. A user wanting an additional feature can purchase an additional fitness tracker having the additional feature. In some cases, each fitness tracker  1100  does not include more than one sensor. 
     As discussed above, by providing embodiments of the fitness tracker  1100  with each embodiment having only a single feature, a user only needs to purchase an embodiment of the fitness tracker having a desired feature. A user may add a second feature by purchasing another embodiment of the fitness tracker having the second feature. For example, a user may purchase a first fitness tracker  1100  the blood oxygenation sensor  1130  and the electronics circuit  1200  of  FIG.  21    and may also purchase a second fitness tracker  1300  ( FIGS.  23  and  24   ) having the motion sensor electronic circuit  1270  of  FIG.  22   . Rather than having to wear two visually independent fitness trackers, the contours of the upper portion  1110  of the fitness tracker disclosed herein allows the user to wear the two fitness trackers as a single interlocked unit that has a visual appearance of a single fitness tracker. As shown in  FIGS.  23  and  24   , the second fitness tracker  1300  is rotated 180 degrees with respect to the first fitness tracker  1100 . When the second fitness tracker is positioned adjacent to the first fitness tracker as shown in  FIGS.  25  and  26   , the upper portions of the two fitness trackers interlock, and the two fitness trackers have the appearance of a single device. Beneficially, the interlocking of two or more fitness trackers (e.g., fitness tracker  1100  and fitness tracker  1300 ) can reduce the amount of space that the fitness trackers occupy on, for example, the wrist of a user. This can allow a user to wear more fitness trackers than would otherwise be possible to wear on, for example, a wrist if the fitness trackers did not interlock with each other. 
     The interlocking of the upper portions  1110  of the fitness tracker  1100  and the fitness tracker  1300  illustrated in  FIGS.  25  and  26    is enabled by selecting the contours of the upper portion of the fitness tracker to have symmetry along at least one plane as shown in the top plan view of  FIG.  24   . As shown in  FIG.  24   , the upper surface  1120  of the upper portion of each fitness tracker has a first side  1400  a second side  1402 , a third side  1404  and a fourth side  1406 . 
     The first side  1400  and the second side  1402  have selected contours. In the illustrated embodiment, the first side has a single smooth outwardly projecting (convex) arcuate contour  1410 . The second side has a first contour  1420 , which is convex in the illustrated embodiment, and has a second contour  1422 , which is concave in the illustrated embodiment. The third side  1404  interconnects the upper ends of the first side and the second side and may have an arcuate contour as shown. The fourth side  1406  interconnects the lower ends of the first side and the second side and may have an arcuate contour as shown. 
     The curvatures (e.g., the radiuses and the arcuate lengths) of the first (convex) contour  1420  and the second (concave) contour  1422  of the second side  1402  of the upper surface  1120  of the upper portion  1110  of the fitness tracker  1100  are selected to be substantially the same. In the illustrated embodiment, the first and second contours are disposed equidistantly from a horizontal plane  1424 , wherein “horizontal” is defined for the view in  FIG.  25   , with the first (concave) contour positioned above the horizontal plane and with the second (convex) contour positioned below the horizontal plane. 
     As shown in  FIGS.  23  and  24   , the upper portion  1110  of the second fitness tracker  1300  is identical to the upper portion  1110  of the first fitness tracker  1100 . Thus, the second fitness tracker can be rotated 180 degrees with respect to the first fitness tracker such that the first fitness tracker and the second fitness tracker can be positioned adjacent to each other as shown in  FIGS.  25  and  26   . When positioned as shown in  FIGS.  25  and  26   , the first (convex) contour  1420  of the second fitness tracker engages the second (concave) contour  1422  of the second fitness tracker. The second (convex) contour of the second fitness tracker engages the first (concave) contour of the first fitness tracker. The resulting pair of adjacent upper surfaces  1120  of the two fitness trackers has the appearance of a single upper surface. When interlocked as shown in  FIGS.  25  and  26   , portions of the first fitness tracker  1100  and the second fitness tracker  1300  can contact one another along an abutment juncture having a serpentine shape. 
     When attached to the limb of a user, the two adjacent fitness trackers  1100 ,  1300  communicate with the smartphone  1104  independently as illustrated in  FIG.  25   . 
     As further illustrated in  FIG.  23   , the wristband  1102  of each fitness tracker  1100 ,  1300  can include a plurality of optional embedded magnets which are positioned to engage each when the two wristbands are adjacent. For example, a first magnet  1430  may have an exposed N pole and a second magnet  1432  may have an exposed S pole. When the two fitness trackers are aligned as shown in  FIG.  24   , the poles of opposite polarity are aligned and hold the wristbands together. The force of the magnets is selected to be sufficient to hold the wristbands together; however, the user may easily separate the magnets to separate the wristbands. The wristbands can also include a third magnet  1434  with an exposed N pole and a fourth magnet  1436  with an exposed S pole. 
     The illustrated first (convex) contour  1420  and the second (concave) contour  1422  are examples of contours that enable one of two fitness trackers having identical contours to be rotated with respect to the other fitness tracker such that the contours of the rotated fitness tracker engage the contours of the other fitness tracker. Other contours can also be used as long as the contour has a symmetry that allows the illustrated engagement. In further alternative embodiments, the contours of an engagement side of one fitness tracker may differ from the contours of an engagement side of another fitness tracker. For example, one fitness tracker may have an engagement surface with first and second convex contours with a concave contour interposed between the two convex contours. A second fitness tracker may have an engagement surface with first and second concave contours with a convex contour interposed between the two concave contours. When the two fitness trackers are engaged, the two convex contours of the first fitness tracker engage the two convex contours of the second fitness tracker, and the concave contour of the first fitness tracker engages the convex contour of the second fitness tracker. 
     If a user wants to add a third feature to the first and second features described above, the user can purchase a third fitness tracker  1500  with the desired third feature. The third fitness tracker is illustrated in  FIGS.  27  and  28   . Unlike the first fitness tracker  1100  and the second fitness tracker  1300 , the third fitness tracker  1500  has a modified shape that enables the third fitness tracker to fit between and interlock with the first fitness tracker and the second fitness tracker. The third fitness tracker is mounted on or attached to a wristband  1502  as described above. 
     The third fitness tracker  1500  has an upper portion  1510  and a lower portion (not shown). The lower portion of the third fitness tracker extends through the wristband as previously described. The upper portion of the third fitness tracker has an upper surface  1520 , which has a first (left) side  1530 , a second (right) side  1532 , a third (upper) side  1534 , and a fourth (lower) side  1536 . 
     The second side  1532  of the upper surface  1520  of the upper portion  1510  of the third tracker  1500  has a first contour  1540 , which is convex in the illustrated embodiment, and has a second contour  1542 , which is concave in the illustrated embodiment. The first (convex) contour and the second (concave) contour of the second side of the upper surface of the third fitness tracker correspond in radius, arcuate length and position to the first (convex) contour  1420  and the second (concave) contour  1422  of the upper surface  1120  of the upper portion  1110  of the first fitness tracker  1100 . The first and second contours are disposed equidistantly from a horizontal plane  1544 , with the first (concave) contour positioned above the horizontal plane and with the second (convex) contour positioned below the horizontal plane as described above with respect to the first fitness tracker  1100 . 
     Unlike the first side  1400  of the upper surface  1120  of the upper portion  1110  of the first fitness tracker  1100 , the first side  1530  of the upper surface  1520  of the upper portion  1510  of the second fitness tracker has a third (concave) contour  1550  positioned above the horizontal plane  1544  and has a fourth (convex) contour  1552  positioned above the horizontal plane. The third contour and the fourth contour have radiuses and arcuate lengths corresponding to the radiuses and arcuate lengths of the first contour and the second contour. 
     As illustrated in  FIG.  28   , the third side  1534  interconnects the upper ends of the first side  1530  and the second side  1532  and may have an arcuate contour as shown. The fourth side  1536  interconnects the lower ends of the first side and the second side and may have an arcuate contour as shown. 
     When the third fitness tracker  1500  is positioned on the wrist of a user between the first fitness tracker  1100  and the second fitness tracker  1300  as illustrated in  FIGS.  29  and  30   , the upper portion  1510  of the third fitness tracker interlocks with the upper portions  1110  of the first fitness tracker and the second fitness tracker. The third (concave) contour  1550  of the third fitness tracker is aligned with and interlocks with the first (convex) contour  1420  of the first fitness tracker. The fourth (convex) contour  1552  of the third fitness is aligned with and interlocks with the second (concave) contour  1422  of the first fitness tracker. The first (convex) contour  1540  of the third fitness tracker is aligned with and interlocks with the second (concave) contour  1422  of the second fitness tracker. The second (concave) contour  1542  of the third fitness tracker is aligned with and interlocks with the first (convex) contour  1420  of the second fitness tracker. 
     It should be understood that the third fitness tracker  1500  is rotationally symmetrical about an axis  1560  ( FIG.  27   ), which is perpendicular to the upper surface  1520  and which is centered on the upper surface. This symmetry enables the third fitness tracker to be rotated 1180 degrees such that the first (convex) contour  1540  of the third fitness tracker engages the second (concave) contour  1422  of the first fitness tracker; the second (concave) contour  1542  of the third fitness tracker engages the first (convex) contour  1420  of the first fitness tracker; the third (concave) contour  1550  of the third fitness tracker engages the first (convex) contour  1420  of second fitness tracker; and the fourth (convex) contour of the third fitness tracker engages the second (concave) contour of the second fitness tracker. The resulting interlocks of the upper portions of the fitness trackers with the third fitness tracker rotated 180 degrees would have the same visual appearance as illustrated in  FIGS.  29  and  30   . 
     Additional fitness trackers  1500  with differing sensing features can be interposed between the first fitness tracker  1100  and the second fitness tracker  1300  to expand the capabilities of the combined interlocked fitness trackers. 
     Two or more of the third fitness trackers  1500  can be positioned on the wrist of a user without using either the first fitness tracker  1100  or the third fitness tracker  1300 . For example,  FIG.  31    illustrates three interlocked third fitness trackers. 
       FIG.  32    illustrates an exploded upper perspective view of an embodiment of a smartwatch  2100  attached to a wristband  2102 . The smartwatch has a first screen display  2110  and second screen display  2120 . In  FIG.  32   , the second screen display is shown spaced apart from the first screen display for illustration only.  FIGS.  33  and  34    illustrate the second screen display installed as an overlay over the first screen display. In  FIG.  33    the second screen display is activated to represent an analog watch as described below. In  FIG.  34   , the second screen display is deactivated to provide visual access to the first screen display as described below. 
     The first screen display  2110  is an interactive screen display that displays data and graphic information in response to user commands. For example, the first screen display may be an OLED screen that directly displays the data and graphic information. The first screen display is touch sensitive such that a user can touch various icons (not shown) on the first screen display to initiate commands or to respond to information on the first screen display. For example, the first screen display may be an OLED screen that directly displays the data and graphic information. 
     The second screen display  2120  is an e-paper display, which may also be referred to as an e-ink display. Such displays are well-known in the art and are not described in detail herein. Basically, an e-paper display can include tiny capsules filled with charged ink particles. When a proper electrical voltage is applied to selected particles, the ink darkens to mimic the appearance of an image on paper. Once an image is formed, the image is maintained without any requirement for additional power until a change in the image is made. 
     The second screen display  2120  is positioned as overlay over the first interactive screen display  2110 . The second screen display is electrically connected to circuitry within the smartwatch  2100 . In the illustrated embodiment, commands are sent from the smartwatch to the second screen display to configure outer image portions  2122  of the display to represent the dial of an analog watch, to configure a second image portion  2124  to represent the hour hand of the watch and to configure a third image portion  2126  to represent the minute hand of the analog watch. Additional image portions (not shown) can be configured to represent images of some or all of the numerals representing the hours around the dial of the analog watch, to represent an image of the second hand of the analog watch, to represent an images of letters identifying the day of the week and alphanumeric images to represent the date. 
     Typically, the smartwatch  2100  only needs to communicate with the second screen display  2120  to erase and re-display the image  2126  of the minute hand and the image  2124  of the hour hand as the time changes. If day and date are displayed, the smartwatch only needs to update the images representing that information once per day. Since power is only required to erase and rewrite selected images on the second screen display, the second screen display inherently requires very little power for operation. Accordingly, the time of day is constantly available for viewing by the user while the smartwatch is in the analog watch mode. In some embodiments, the smartwatch can include a temporary backlighting mode so that the user may be able to view the images of the watch hands in low lighting conditions. In the analog watch mode of operation, the interactive screen display  2110  is deactivated, and very little power is required to operate the smartwatch. 
     The smartwatch  2100  can include a mode switch  2130  that a user may activate to switch the operational mode of the smartwatch from the analog watch mode describe above to an interactive display mode and to switch the smartwatch back to the analog display mode. The operation of the smartwatch in the interactive display mode is illustrated in  FIG.  34   . 
     In  FIG.  34   , the second screen display  2120  is effectively turned off by erasing the images representing the analog watch. Accordingly, the second screen display is illustrated as being transparent in  FIG.  34   . The images of the watch dial and the hands are shown in phantom in  FIG.  34    to indicate that the images are not visible with the second screen display is deactivated in this operational mode. 
     In  FIG.  34   , the first screen display  2110  is turned on to display selected images in accordance with an application selected by the user. For example,  FIG.  34    represents an example of the images displayed when the user operates the smartwatch as a fitness tracker. The smartwatch has one or more sensors (not shown) on a lower surface of the smartwatch that are positioned in contact with the skin of the user when the wristband  2102  is secured to the user&#39;s wrist. The first screen display displays a first numeral  2150  representing a first sensed physiological parameter of the user, displays a second numeral  2152  representing a second physiological parameter of the user and displays a third numeral  2154  representing a third physiological parameter of the user. The first screen display may also display an image  2160  of a graphical representation of a physiological parameter of the user. 
     Since the second screen display  2120  is transparent, the user is able to view the information displayed on the first screen display  2110 . The second screen display is sufficiently thin that the user may touch the exposed surface of the second screen display to tactilely interact with the first screen display in a similar manner to interacting with a smartphone through a screen protector. 
     When the user completes his or her interactions with the features of the selected application of the smartwatch  2100 , the user may return to the analog watch mode by engaging the mode switch  2130 . Some applications of the smartwatch may be programmed to return the smartwatch to the analog watch mode when the user exits the applications. 
     Terminology 
     Conditional language used herein, such as, among others, “can,” “might,” “may,” “for example,” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain examples include, while other examples do not include, certain features, elements and/or states. Thus, such conditional language is not generally intended to imply that features, elements and/or states are in any way required for one or more examples or that one or more examples necessarily include logic for deciding, with or without author input or prompting, whether these features, elements and/or states are included or are to be performed in any particular example. The terms “comprising,” “including,” “having,” and the like are synonymous and are used inclusively, in an open-ended fashion, and do not exclude additional elements, features, acts, operations, and so forth. Also, the term “or” is used in its inclusive sense (and not in its exclusive sense) so that when used, for example, to connect a list of elements, the term “or” means one, some, or all of the elements in the list. Further, the term “each,” as used herein, in addition to having its ordinary meaning, can mean any subset of a set of elements to which the term “each” is applied. 
     Disjunctive language such as the phrase “at least one of X, Y, or Z,” unless specifically stated otherwise, is otherwise understood with the context as used in general to present that an item, term, etc., may be either X, Y, or Z, or any combination thereof (for example, X, Y, and/or Z). Thus, such disjunctive language is not generally intended to, and should not, imply that certain examples require at least one of X, at least one of Y, or at least one of Z to each be present. 
     Unless otherwise explicitly stated, articles such as “a” or “an” should generally be interpreted to include one or more described items. Accordingly, phrases such as “a device configured to” are intended to include one or more recited devices. Such one or more recited devices can also be collectively configured to carry out the stated recitations. For example, “a processor configured to carry out recitations A, B and C” can include a first processor configured to carry out recitation A working in conjunction with a second processor configured to carry out recitations B and C. 
     While the above detailed description has shown, described, and pointed out novel features as applied to various examples, it will be understood that various omissions, substitutions, and changes in the form and details of the devices or algorithms illustrated can be made without departing from the spirit of the disclosure. As will be recognized, the inventions described herein can be embodied within a form that does not provide all of the features and benefits set forth herein, as some features can be used or practiced separately from others. 
     As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all the matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.