Apparatus for electrodermal activity measurement with current compensation

An apparatus for measuring electrodermal activity can include a first electrode in contact with a first portion and a second electrode in contact with a second portion of a stratum corneum, and in electronic communication with the second electrode through the stratum corneum. A processing module is electrically coupled to the first electrode and the second electrode and is operable to (a) bias the first electrode at a first voltage V+ and the second electrode at a second voltage V− (b) measure a current flowing between the first electrode and the second electrode, the current corresponding to the conductance of the stratum corneum, (c) subtract a compensation current from the measured current (d) measure a resulting current producing an amplified output voltage (e) measure a conductance of the stratum corneum, and (f) adjust at least one of the first voltage, the second voltage and the compensation current to desaturate the output voltage.

BACKGROUND

Embodiments described herein relate generally to devices, systems and methods for measuring electrodermal activity, and in particular to wearable devices and methods for measuring electrodermal activity of the skin of a user.

The human skin is composed of different layers of tissue. These layers of tissue perform several functions, for example, forming an interface between the internal and external parts of the body, serve as a protection mechanism, have a thermoregulatory function, and allow exchange of fluids through the skin. The human skin also includes sweat glands that produce sweat. The sweat includes various electrolytes which allow current to be conducted through the skin. For example, if two electrodes capable of producing free ions such as, for example, silver (Ag) electrodes are disposed on the skin, free ions can be electronically communicated between the two electrodes via the skin.

The conductance of skin, which is generally referred to as the electrodermal activity, is extremely low and is generally measured in Siemens (S). The conductance of the skin depends upon the thickness of the stratum corneum. The inner layer of the skin creates a potential barrier which changes in size and allows the current to flow in a less or more restricted way in the stratum corneum. The thinner the stratum corneum, the higher is the conductance. For example, the conductance of skin at the finger tips can be in the range of about 0.5 μS to about 50 μS, and the conductance of the skin at the wrist can be in the range of about 0.05 μS to about 80 μS. These variations can depend on many factors, including the physiology of an individual, temperature, skin structure, and autonomous nervous system (ANS) activity.

The electrodermal activity signal generally includes two interleaved signals; the tonic level and phasic level. The tonic level (also referred to herein as “tonic level conductance”) is the skin conductance in the absence of any external or environmental stimuli, is slow changing (i.e., low frequency), and is caused by the human physiological factors as described herein. The tonic level can have a range of about 0.05 μS to about 50 μS at the wrist of a user.

The phasic level (also referred to herein as “phasic level conductance”) is typically associated with short-term events and occurs in the presence of discrete environmental stimuli such as for example, sight, sound, smell, and cognitive processes that precede an event such as anticipation, decision making, etc. Phasic changes usually show up as abrupt increases in the skin conductance, or “peaks” in the skin conductance.

Systems and devices can also be used to measure heart rate variability (HRV) through the skin, or in the blood beneath the skin of the user. The HRV is defined as the beat-to-beat variations in heart rate. The larger the alterations, the larger the HRV. HRV is a known predictor of mortality of myocardial infarction and other pathological conditions may also be associated with modified (usually lower) HRV, including congestive heart failure, diabetic neuropathy, depression post-cardiac transplant, susceptibility to sudden infant death syndrome (SIDS), and poor survival in premature babies. HRV is also related to emotional arousal. HRV has been found to decrease during conditions of acute time pressure and emotional strain, elevated levels of anxiety, or in individuals reporting a greater frequency and duration of daily worry.

HRV includes two primary components: respiratory sinus arrhythmia (RSA) which is also referred to as high frequency (HF) oscillations, and low frequency (LF) oscillations. HF oscillations are associated with respiration and track the respiratory rate across a range of frequencies, and low frequency oscillations are associated with Mayer waves (Traube-Hering-Mayer waves) of blood pressure. The total energy contained by these spectral bands in combination with the way energy is allocated to them gives an indication of the heart rate regulation pattern given by the central nervous system, and an indication of the state of mental and physical health.

However, known methods for analyzing heart beat data to determine HRV and a psychophysical state of a person often fail to determine a true mental and physical state of the person. Some known HRV spectral analysis methods use non-parametric approaches (e.g., Fast Fourier transforms) or parametric approaches. These strategies rely on the approximation that the tachogram is “sampled” at a constant frequency. Such known methods are susceptible to missing beat data or high variability in the heart beat data. Furthermore, high activity can also lead to high variability in the heart beat data which cannot be analyzed properly by known methods.

Thus, there is a need for new systems, devices and methods that can measure skin conductance with high reliability, repeatability and do not suffer from electrolysis. Furthermore, there is also a need for new methods to analyze heart beat data and determine human well being through heart rate variability.

SUMMARY

Embodiments described herein relate generally to devices, systems and methods for measuring electrodermal activity, and in particular to wearable devices and methods for measuring electrodermal activity of the skin of a user. In some embodiments, an apparatus for measuring electrodermal activity can include a first electrode in contact with a first portion of stratum corneum of skin and a second electrode in contact with a second portion of stratum corneum. The first electrode can be in electronic communication with the second electrode through the stratum corneum. A processing module is electrically coupled to the first electrode and the second electrode. The processing module is operable to (a) bias the first electrode at a first voltage V+ and the second electrode at a second voltage V−, (b) measure a current flowing between the first electrode and the second electrode, the current corresponding to the conductance of the stratum corneum, (c) subtract a compensation current from the measured current, (d) measure a resulting current and produce an amplified output voltage, (e) measure a conductance of the stratum corneum, and (f) adjust at least one of the first voltage, the second voltage and the compensation current to desaturate the output voltage.

DETAILED DESCRIPTION

Embodiments described herein relate generally to devices, systems and methods for measuring electrodermal activity, and in particular to wearable devices and methods for measuring electrodermal activity of the skin of a user. Measurement of the two different frequency conductances that define the electrodermal activity of a human can be challenging. The tonic level has a wide range which can be difficult to encompass with conventional electrodermal activity monitors. Furthermore, the phasic level is fast changing and can be difficult to resolve with conventional electrodermal activity monitors.

Electrodes used for electrodermal sensing can also undergo electrolysis on the skin. As the current flows through the skin, the electrode (e.g., a Ag electrode) can lose metal ions which can get deposited on the skin. This can lead to corrosion of the electrode, and can also lead to skin irritation because of the metal ions.

Conventional electrodermal activity sensors can be DC current sensors or AC current sensors. DC current based electrodermal activity sensors generally give good performance in measuring both tonic level conductance and phasic level conductance but can suffer from electrolysis. In contrast, AC current based electrodermal activity sensors give good performance in measuring tonic level conductance and have little or no electrolysis but demonstrate poor performance in measuring phasic level conductance.

Embodiments of the systems, devices and methods described herein can provide a compensation mechanism for reliably measuring the tonic level and phasic levels of the conductance of the skin. The electrodermal activity measurement systems, devices and methods described herein provide several advantages over conventional electrodermal activity sensors including, for example: (1) capability of measuring electrodermal activity over a wide range that covers the entire range of expected tonic level conductances, (2) capability of measuring phasic level conductances with high resolution, (3) reduction in electrolysis of sensing electrodes, and (4) allowing real time electrodermal activity measurement by integration in a wearable device, for example, a wrist band.

In some embodiments, an apparatus for measuring electrodermal activity can include a first electrode in contact with a first portion of a stratum corneum of skin and a second electrode in contact with a second portion of the stratum corneum. The first electrode can be in electronic communication with the second electrode through the stratum corneum. A processing module is electrically coupled to the first electrode and the second electrode. The processing module is operable to (a) bias the first electrode at a first voltage V+ and the second electrode at a second voltage V−, (b) measure a current flowing between the first electrode and the second electrode, the current corresponding to the conductance of the stratum corneum, (c) subtract a compensation current from the measured current, (d) measure a resulting current and produce an amplified output voltage, (e) measure a conductance of the stratum corneum, and (f) adjust at least one of the first voltage, the second voltage and the compensation current to desaturate the output voltage.

In some embodiments, a wearable device for measuring electrodermal activity can include a housing configured to be removably associated with the skin of a user. A first electrode and a second electrode are included in the device such that at least a portion of the first electrode and the second electrode are disposed outside the housing. The first electrode is configured to contact a first portion of a stratum corneum of skin and the second electrode is configured to contact a second portion of the stratum corneum of the skin when the housing is associated with the user. A processing module is also disposed in the housing and coupled to the first electrode and the second electrode. The processing module is operable to (a) bias the first electrode at a first voltage V+ and the second electrode at a second voltage V−, (b) measure a current flowing between the first electrode and the second electrode, the current corresponding to the conductance of the stratum corneum, (c) subtract a compensation current from the measured current, (d) measure a resulting current and produce an amplified output voltage, (e) measure a conductance of the stratum corneum, and (f) adjust at least one of the first voltage, the second voltage and the compensation current to desaturate the output voltage. A communications module is also disposed in the housing and coupled to the processing module. The communications module can be configured to at least one of a display an electrodermal activity of the user and communicate electrodermal activity data from the processing module to an external device. A power source is also disposed in the housing and is configured to provide electrical power to the processing module and the communications module. In some embodiments, the wearable device can be a wrist band.

In some embodiments, a method for measuring electrodermal activity can include disposing a first electrode and a second electrode on a stratum corneum of a user. The first electrode is biased at a first voltage and the second electrode is biased at a second voltage. An output voltage proportional to the current flowing through the skin is measured. The method transforms the output voltage into a conductance level and determines if it is saturated or not. If the output voltage is saturated low, the compensation current is increased or the difference in voltage between the two electrodes is decreased to change the output voltage such that it is not saturated. If the output voltage is saturated high, the compensation current is decreased or the difference in voltage between the two electrodes is increased to change the output voltage such that it is not saturated. In some embodiments, the measured conductance is a tonic level conductance having a value in the range of about 0.05 μS to about 50 μS.

As used herein, the terms “about” and “approximately” generally mean plus or minus 10% of the value stated. For example, about 0.5 would include 0.45 and 0.55, about 10 would include 9 to 11, about 1000 would include 900 to 1100.

The embodiments described herein can be used to measure the electrodermal activity (i.e., conductance including tonic level and phasic level conductance) of a stratum corneum of a skin. For reference,FIG. 1Ashows a cross section of a skin of a human. The outermost layer of the skin is the stratum corneum. Below the stratum corneum is the stratum lucidum. A potential barrier exists between the stratum corneum and the stratum lucidum. The conductance of the skin varies, as shown by the arrow G from about 100 nS at a top surface of the stratum corneum to about 1 mS near a bottom surface of the skin which is a substantially equally potential surface. As shown inFIG. 1A, the stratum corneum can have a first thickness s1measured from a top surface of the stratum corneum to the potential barrier. When a pair of electrodes “a-a” are placed in electronic communication with the stratum corneum such that the distance between the electrodes is greater than the first thickness s1of the stratum corneum the stratum corneum can have a first conductance. The various factors affecting the conductance of the skin, for example, the physiology of an individual, temperature, skin structure, and autonomous nervous system (ANS) activity, do so by changing the thickness of the stratum corneum. The stratum corneum serves essentially as a potential barrier that changes in size and thickness. As shown inFIG. 1B, the thickness of the stratum corneum can increase to a second thickness s2substantially greater than s1. Change in thickness also changes the conductance of the stratum corneum. The thinner the stratum corneum, the higher the conductance. Thus, the pair of electrodes a-a when placed in electronic communication with the stratum corneum such that the distance between the electrodes a-a is greater than the second thickness s2of the stratum corneum, will measure a second conductance less than the first conductance. Thus, changes in conductance of the stratum corneum can be directly correlated to the physiological status of a user, for example, the ANS activity.

FIG. 2Ashows an exemplary electrodermal activity measurement showing changes in the tonic level and phasic level conductances of a human. The tonic level can be characterized as “a smooth underlying slowly-changing conductance level.” The phasic level conductance can be characterized as “rapidly changing peaks.” Tonic level conductance level can slowly vary over time in an individual depending upon his or her psychological state, hydration, skin dryness, and autonomic regulation. Tonic changes in the skin conductance level typically occur in a period from tens of seconds to minutes. Phasic level conductance measurements are typically associated with short-term events and occur in the presence of discrete environmental stimuli (sight, sound, smell, cognitive processes that precede an event such as anticipation, decision making, etc). Phasic changes usually show up as abrupt increases in the skin conductance, or “peaks” in the skin conductance.FIG. 2Bshows a typical electrodermal activity signal ranging from low to high values.

In some embodiments, an apparatus for measuring electrodermal activity can include a first electrode and a second electrode. Referring now toFIG. 3, an apparatus100for measuring the electrodermal activity includes a first electrode110a,a second electrode110b(collectively referred to as “the electrodes110”) and a processing module130. The first electrode110aand the second electrode110bcan be disposed on a stratum corneum SC of a skin of a target, such that the first electrode110aand the second electrode110bcan be in electronic communication through the stratum corneum SC and measure a conductance of the stratum corneum SC.

The electrodes110can include any suitable electrodes that can allow electronic communication through the stratum corneum SC and measure a conductance of the stratum corneum SC. For example, the first electrode110acan be brought into contact with a first portion of the stratum corneum SC of the skin, and the second electrode110bcan be brought into contact with a second portion of the stratum corneum SC, such that the first electrode110ais in electronic communication with the second electrode110bthrough the stratum corneum SC. The electrodes110can have any suitable shape. For example, the electrodes110can be discs, plates, or rods, a solid state microfabricated electrode (e.g., of the type used in MEMS devices), or a screen printed electrode. The electrodes110can have any suitable cross section, for example circular, square, rectangle, elliptical, polygonal, or any other suitable cross-section. In some embodiments, at least a portion of the electrodes110can be insulated with an insulating material, for example, rubber, TEFLON®, plastic, parylene, silicon dioxide, silicon nitride, any other suitable insulation material or combination thereof. The insulation material can, for example, be used to define an active area of the electrodes110. In some embodiments, the electrodes110can be subjected to a surface modification process to modify a surface area of the electrodes110, for example, to provide a larger surface area. Such surface modification processes can include, for example, etching (e.g., etching in an acidic or basic solution), voltage cycling (e.g., cyclic voltammetry), electrodeposition of nanoparticles, and/or any other suitable surface modification process or combination thereof

The electrodes110can be formed from any suitable material capable of electronic communication (i.e., ionic and electric communication) through the stratum corneum. Suitable materials can include, for example, silver (Ag), gold, platinum, palladium, iridium, carbon, graphite, carbon nanotubes, graphenes, conductive polymers, ceramics, alloys, any other suitable material or combination thereof. In some embodiments, the electrodes110can include Ag electrodes, for example, metallic plates coated with Ag. The Ag electrodes can dissociate into Ag+ions at the surface of the electrode allowing electronic communication through the stratum corneum. Ag can also prevent any damage to the stratum corneum and has inherent anti-bacterial properties that can prevent any bacterial growth on the stratum corneum in proximity of the electrodes110.

The processing module130is coupled to the first electrode110aand the second electrode110b.The processing module130can be operable to (a) bias the first electrode at a first voltage V+ and the second electrode at a second voltage V−, (b) measure a current flowing between the first electrode and the second electrode, the current corresponding to the conductance of the stratum corneum, (c) subtract a compensation current from the measured current, (d) measure a resulting current and produce an amplified output voltage, (e) measure a conductance of the stratum corneum, and (f) adjust at least one of the first voltage, the second voltage and the compensation current to desaturate the output voltage.

In some embodiments, the processing module130can include an electrical circuit (not shown) configured to polarize the first electrode110aat the first voltage and the second electrode110bat the second voltage. The electrical circuit can include a resistor and an amplifier, for example, an operational amplifier, a transimpedance amplifier, a voltage amplifier, a current amplifier, a transconductance amplifier, any other suitable amplifier or combination thereof. The electrical circuit can be further configured to measure a conductance (e.g., the tonic level conductance and/or the phasic level conductance of the stratum corneum SC) and an output voltage which corresponds to the conductance of the stratum corneum SC.

The processing module130can also include a compensation mechanism (not shown) configured to communicate a compensation voltage to the electrical circuit to modify the compensation current or modify the difference in voltage between the two electrodes. The compensation mechanism can be configured to optimally measure the current flowing between the first electrode and the second electrode, corresponding to the conductance of the stratum corneum. Furthermore, the compensation mechanism can be configured to adjust at least one of the first voltage and the second voltage, or to adjust the compensation current if the output voltage reaches a saturation value, for example a high saturation or a low saturation. Moreover, the compensation mechanism can be configured to adjust the compensation current if the conductance of the stratum corneum SC is too low. For example, the compensation mechanism can be configured to increase the compensation current if the output voltage reaches a saturation value or decrease the compensation current if the conductance of the stratum corneum is too low. In this manner, the compensation mechanism can serve as voltage feedback mechanism to maintain the output voltage at an optimal value.

In some embodiments, the processing module130can include a filtering circuit, for example, a low pass filter, a high pass filter, a band pass filter, any other suitable filtering circuit, or combination thereof, configured to substantially reduce signal noise. In some embodiments, the processing module130can include a processor, for example, a microcontroller, a microprocessor, an ASIC chip, an ARM chip, or a programmable logic controller (PLC). The processor can include signal processing algorithms, for example, band pass filters, and/or any other signal processing algorithms or combination thereof. In some embodiments, the processing module130can include a memory configured to store at least one of an electrodermal activity data, or a physiological status data, for example, ANS activity data. In some embodiments, the memory can also be configured to store a reference signature, for example, a calibration equation. In such embodiments, the processor can include algorithms which can be configured to correlate the measured electrodermal activity data to an ANS activity or any other physiological status parameter of the user. The memory can also include algorithms to maximize the signal to noise ratio of the electrodermal activity signal. In some embodiments, the processing module130can also include a generator of clock signals coupled to the processor. In some embodiments, the processing module130can also include an RFID or bluetooth chip configured to store or send information in real-time for example, the electrodermal activity data, and allow a near field communication (NFC) device to read the stored information.

In some embodiments, the processing module130can be configured to measure a compensated value of conductance from which a tonic level conductance is removed. In some embodiments, the processing module130can be configured to reverse a polarity of the at least one of the first electrode110and the second electrode110bafter a predetermined period of time to substantially reduce electrolysis. For example, reversing the plurality can urge any dissolved ions of the electrodes110, for example, Ag+ions to be reabsorbed into the electrodes110. This can reduce fouling of the electrodes110, increase shelf life, and/or prevent irritation of the skin. In some embodiments, the processing module can be configured to allow a tuning of the compensation current that is subtracted from the current flowing between the electrodes before the current is amplified. For example, the processing module130can be configured to allow a tuning of the current corresponding to the conductance of the stratum corneum SC in the range of about −1 μA to about 1 μA. The apparatus100can be configured to measure a conductance of a stratum corneum SC of any portion of the skin of the use, for example, the skin of a wrist of a user. In such embodiments, the processing module130can be configured to measure a tonic level conductance of the stratum corneum SC of the wrist in the range of about 0.05 μS to about 80 μS. In some embodiments, the apparatus100can be configured to measure a conductance of a stratum corneum of a finger of a user. In such embodiments, the processing module130can be configured to measure a tonic level conductance of the stratum corneum SC of the finger in the range of about 0.5 μS to about 50 μS. In some embodiments, the processing module130can be configured to measure a phasic level conductance of up to about 5 mS. In some embodiments, the apparatus100can be configured to measure the conductance of the stratum corneum with a resolution of 0.0001 μS.

In some embodiments, the apparatus100can also include a communications module (not shown) coupled to the processing module130. The communications module can be configured to display an electrodermal activity of the user or communicate electrodermal activity data from the processing module130to an external device, for example, a smart phone app, a local computer and/or a remote server. In some embodiments, the communications module includes a communication interface to provide wired communication with the external device, for example, a USB, USB 2.0, or fire wire (IEEE 1394) interface. In some embodiments, the communication interface can also be used to recharge a power source (not shown), for example, a rechargeable battery which can be included in the apparatus100. The power source can include for example, coin cells, Li-ion or Ni-Cad batteries of the type used in cellular phones. In some embodiments, the communications module can include means for wireless communication with the external device, for example, Wi-Fi, BLUETOOTH®, low powered BLUETOOTH®, Wi-Fi, Zigbee and the like.

In some embodiments, the communications module can include a display, for example, a touch screen display, configured to communicate information to the user for example, electrodermal activity, ANS activity, physiological activity of use, remaining battery life, wireless connectivity status, time, date, and/or user reminders. In some embodiments, the communications module can also include microphones and/or vibration mechanisms to convey audio and tactile alerts. In some embodiments, the communications module can include a user input interface, for example, a button, a switch, an alphanumeric keypad, and/or a touch screen, for example, to allow a user to input information into the dose measurement system100, for example, power ON the system, power OFF the system, reset the system, manually input details of a user behavior, manually input details of apparatus100usage and/or manually initiate communication between the apparatus100and a remote device.

In some embodiments, the apparatus can also include various physiological sensors, for example, a heart beat sensor (e.g., a photoplethysmography sensor), an accelerometer, a temperature sensor, a blood oxygen sensors, a glucose sensor, a barometer, a gyroscope, any other physiological sensor or combination thereof. In such embodiments, the processing module130can be configured to process signals form each sensor to determine a physiological status of the user. In some embodiments, data processing of the signal received from each sensor can be performed on an external device, for example, a smart phone, a tablet, a personal computer, or a remote server. Furthermore, the communications module can be configured to communicate the physiological data from each of the sensors to the user, for example, via a display included in the apparatus or the external device. Such physiological data can include, for example, electrodermal activity (e.g., skin conductance), heart rate, heart rate variability, metabolic equivalent of task (MET), a stress level, a relaxation level, a movement or activity level, a temperature, a heat flux, and/or an ANS activity (e.g., an arousal or excitement).

In some embodiments, the apparatus can include a housing (not shown) which can be configured to removably associate with the stratum corneum SC of the user. The housing can define an internal volume within which the electrodes110, the processing module130, the communications module, and the power source, and/or any other components included in the apparatus100can be disposed. At least a portion of the first electrode110aand the second electrode110bcan be disposed outside the housing. The electrodes110can be configured such that the first electrode110contacts a first portion of the stratum corneum SC and the second electrode110bcontacts a second portion of the stratum corneum SC when the housing is associated with the skin of the user.

The housing can be formed from a material that is relatively lightweight and flexible, yet sturdy. The housing also can be formed from a combination of materials such as to provide specific portions that are rigid and specific portions that are flexible. Example materials include plastic and rubber materials, such as polystyrene, polybutene, carbonate, urethane rubbers, butene rubbers, silicone, and other comparable materials and mixtures thereof, or a combination of these materials or any other suitable material can be used. The housing can have a relatively smooth surface, curved sides, and/or otherwise an ergonomic shape.

In some embodiments, the apparatus100can have a small form factor such that the apparatus100is wearable (i.e., can be worn on a body part of a user). For example, in some embodiments, the apparatus100can be a wrist band. In such embodiments, a flexible strap, for example, leather strap, a rubber strap, a fiber strap, or a metal strap can be coupled to the housing and configured to secure the housing to the body part of the user. Furthermore, the housing can have a small form factor. In some embodiments, the strap can be hollow such that the strap defines an internal volume. In such embodiment, any one of the sensors included in the apparatus100, for example, the electrodes110configured to measure electrodermal activity can be disposed in the internal volume defined by the strap. At least a portion of the electrodes110can be disposed outside the housing to contact the stratum corneum SC of the skin of the user. In some embodiments, the apparatus100can be a head band, an arm band, a foot band, an ankle band, or a ring. In some embodiments, the apparatus110can be a glove configured to be worn on a hand of the user.

In use the apparatus100can be disposed on the skin of a user such that the first electrode110acontacts a first portion of the stratum corneum SC of the skin (e.g., the skin of a wrist of the user), and the second electrode110bcontacts a second portion of the stratum corneum SC. The processing module130can bias the first electrode at a first voltage and the second electrode at a second voltage different than the first voltage, and measure a skin current flowing through the stratum corneum. A compensation current can be subtracted from the skin current to obtain an input current. The compensation current can be set by a compensation voltage, for example, a compensation voltage provided by the compensation mechanism. The processing module130can transform the input current to measure an output voltage and a conductance of the stratum corneum SC (e.g., derived from the output voltage). The processing module130can determine if the output voltage is saturated or unsaturated. If the output voltage is saturated, for example, saturated high or saturated low, the processing module130can adjust the first voltage, the second voltage and/or the compensation current (e.g., by adjusting the compensating voltage) to desaturate the output voltage. The apparatus100can be configured to perform real time measurements of the electrodermal activity and/or any other physiological parameters such that a physiological status of the user can be determined. This information can be used to generate a physiological profile of the user over a period of time.

Having described above various general principles, several embodiments of these concepts are now described. These embodiments are only examples, and many other configurations of systems, devices and methods for measuring electrodermal activity are contemplated.

In some embodiments, an apparatus for measuring electrodermal activity can include a wearable device configured to be worn on the wrist of a user. Referring now toFIGS. 4-7, a wearable device200for measuring electrodermal activity includes a housing202, a first strap206aand a second strap206b,a first electrode210a,a second electrode210b(collectively referred to as the “electrodes210”), a processing module230, a communications module250, and a power source270. The wearable device200is configured to be worn on the wrist of the user, analogous to a watch and to measure at least an electrodermal activity of the stratum corneum of a skin on the wrist of the user.

The housing202defines an internal volume204configured to house the processing module230, the communications module250and the power source270. The housing202can be formed from a material that is relatively lightweight and flexible, yet sturdy. The housing202also can be formed from a combination of materials such as to provide specific portions that are rigid and specific portions that are flexible. Example materials include plastic and rubber materials, such as polystyrene, polybutene, carbonate, urethane rubbers, butene rubbers, silicone, and other comparable materials and mixtures thereof, or a combination of these materials or any other suitable material can be used. The housing202can have a relatively smooth surface, curved sides, and/or otherwise an ergonomic shape. While shown as being a monolithic structure, in some embodiments, the housing202can include a base and a cover such that the base is removably coupled to the cover to define the internal volume204. In such embodiments, the base can be removed to access the components disposed in the housing204(e.g., the replace the power source270).

A first strap206aand a second strap206b(collectively referred to as the “straps206”) are coupled to a first side and a second side of the housing202, respectively. The straps206can be formed from any suitable material such as, for example, leather, rubber, fiber, polyurethane, or metal. The straps206can include a coupling mechanism, for example, a hole and pin, clamp, notches, grooves, indents, detents, magnets, Velcro, bands, or any other suitable coupling mechanism to couple the straps206to each other. In this manner, the strap206can be removably secured on the wrist of the user such that the electrodes210can be associated with stratum corneum of the wrist of the user. Each strap206defines an internal volume208which is coupled to the housing202via an opening205defined in a side wall of the housing202. The opening can allow the processing module230to be electrically coupled to the electrodes210via electrical couplings, for example, electrical leads, that can pass through the opening205between the internal volume204of the housing202, and the internal volume208of the strap206. The electrodes210are disposed in the internal volume208defined by the first strap206asuch that at least a portion of each of the electrodes210is disposed outside the internal volume. In this manner, the electrodes210are configured to contact the stratum corneum of the skin of the user when the wearable device200is associated with the wrist of the user.

The electrodes210can includes any suitable electrodes that can allow electronic communication through the stratum corneum and measure a conductance of the stratum corneum. For example, the first electrode210acan be brought into contact with a first portion of the stratum corneum of the skin, and the second electrode210bcan be brought into contact with a second portion of the stratum corneum of the skin, such that the first electrode210ais in electronic communication with the second electrode210bthrough the stratum corneum. The electrodes210can have any suitable shape. While shown as having at least one surface which is curved, the electrodes210can have any suitable shape For example, the electrodes210can be discs, plates, or rods, a solid state microfabricated electrode (e.g., of the type used in MEMS devices), or a screen printed electrode. The electrodes210can have any suitable cross section, for example circular, square, rectangle, elliptical, polygonal, or any other suitable cross-section. In some embodiments, at least a portion of the electrodes210can be insulated with an insulating material, for example, rubber, TEFLON®, plastic, parylene, silicon dioxide, silicon nitride, any other suitable insulation material or combination thereof. The insulation material can, for example, be used to define an active area of the electrodes210. In some embodiments, the electrodes210can be subjected to a surface modification process to modify a surface area of the electrodes210for example, to provide a larger surface area. Such surface modification processes can include, for example, etching (e.g., etching in an acidic or basic solution), voltage cycling (e.g., cyclic voltammetry), electrodeposition of nanoparticles, and/or any other suitable surface modification process or combination thereof

The electrodes210can be formed from any suitable material capable of electronic communication (i.e., ionic and electric communication) through the stratum corneum. Suitable materials can include, for example, silver (Ag), gold, platinum, palladium, rhodium, iridium, carbon, graphite, carbon nanotubes, graphenes, conductive polymers, ceramics, alloys, any other suitable material or combination thereof. In some embodiments, the electrodes210can include Ag electrodes, for example, metallic plates coated with Ag. The Ag electrodes can dissociate into Ag+ions at the surface of the electrode allowing that can exchange ions with the electrolytes included in the sweat produced on the stratum corneum, thereby allowing electronic communication through the stratum corneum. Ag can also prevent any damage to the stratum corneum and has inherent anti-bacterial properties that can prevent any bacterial growth on the stratum corneum in proximity of the electrodes210.

The processing module230is disposed in the internal volume204defined by the housing202. The processing module230includes an electrical circuit232and a compensation mechanism234. The electrical circuit232can include an amplifier A, for example, an operational amplifier, a transimpedance amplifier, a voltage amplifier, a current amplifier, a transconductance amplifier, a transimpedance amplifier, any other suitable amplifier or combination thereof. The electrical circuit232also includes an analog to digital converter (ADC). The electrical circuit232can be configured to measure and output voltage VOUTand obtain the conductance of the stratum corneum from the output voltage VOUTas described herein. The compensation mechanism234can include at least a digital to analog converter. The compensation mechanism can be configured to read the output voltage VOUTand set a compensation voltageVBACcorresponding to a compensation current Icompas described herein.

In some embodiments, the processing module230can also be configured to reverse a polarity of the at least one of the first electrode210aand the second electrode210bafter a predetermined period of time to substantially reduce electrolysis. For example, reversing the plurality can urge any dissolved ions of the electrodes210, for example, Ag+ions to reabsorb into the electrodes210. This can, for example, reduce fouling of the electrodes210, increase shelf life, and/or prevent irritation of the skin.

FIG. 6shows a circuit diagram of the processing module230that can be used for current compensation and polarity inversion. As shown inFIG. 6, the electrodes210can be in contact with the skin, for example, the stratum corneum of the skin. The stratum corneum acts as a variable resistor disposed between the electrodes210. The conductance of the stratum corneum changes as the thickness of the stratum corneum changes, for example, because of a change in the physiological status of the user.

The power source270can be used to provide a positive voltage V+ at a first node1and a negative voltage V− at a third node3. In this configuration, the first electrode210areceives the positive voltage V+ and the second electrode210breceives the negative voltage V−. A polarity inversion mechanism, for example, a directional switch, can be used to divert the positive voltage towards a second node2and the negative voltage towards a fourth node4. As shown inFIG. 7, this reverses the polarity of the electrodes210, such that the first electrode210ais now biased at the negative voltage V− and the second electrode210bis biased at the positive voltage V+.

As shown inFIG. 6, the digital to analog converter (DAC) included in the compensation mechanism234is configured to subtract a compensation current Icompfrom entering the amplifier A. Thus the input current Iinentering the amplifier A is;
Iin=Iskin−Icomp

The DAC produces a voltage VDACsuch that the compensation current Icomp=f(VDAC), where f is a quasilinear function.

The amplifier A is responsible for amplifying the current Iinfor a given gain G and transform the input current Iininto the output voltage VOUT. The output voltage VOUTis used to obtain a conductance of the stratum corneum. The processing module230also includes an analog to digital converter (ADC) configured to convert the analog signal to a digital signal. The ADC can have any suitable resolution, for example, 10 bits, 12 bits or 16 bits. The gain G of the amplifier A can be fixed and chosen to meet the range requirements of the conductance levels of skin such that the output voltage VOUTafter the gain G is,
Vout=G(Iskin−f(VDAC))

FIG. 7shows an overall schematic of the processing module230. The control unit CU included in the compensation mechanism234sets a value of the compensation voltage VDACand reads the output voltage VOUTof the electrical circuit232. Since the gain G of the amplifier A is substantially high to magnify the weak conductance signal obtained from the electrodes210, VOUTtends to saturate towards a maximum value VMAXor 0.When this happens, the control unit CU acts on the compensation voltage VBACin order to de-saturate the output voltage VOUT. For instance, if the skin conductance keeps increasing the output voltage VOUTwill saturate. The compensation mechanism234can then increase the compensation current Icompto reduce the output voltage VOUTto a readable range.

This concept is further illustrated inFIG. 8. The top panel ofFIG. 8shows real time conductance of the stratum corneum which includes the tonic and the phasic levels. The middle panel shows the output voltage VOUTmeasured by the electrical circuit232, and the bottom panel shows the compensation voltage VDACset by the control unit CU. The electrodes210can be initialized at an initial value of the compensation voltage VDAC.The magnitude of the output voltage VOUTcan be measured by the compensation mechanism234. As soon as the conductance increases and the output voltage VOUTincreases and eventually reaches its saturation value (e.g., about 3.3 volts). In this scenario a substantial amount of current is flowing through the stratum corneum. To avoid saturation, the compensation mechanism234can compensate for the current by increasing the compensation voltage VDAC. This allows a higher current to flow away from the amplifier A and thereby, leads to desaturation of the output voltage VOUT. On the other hand when the conductance decreases the output voltage VOUTalso decreases until the output voltage VOUTfalls below a predetermined threshold, for example, the electrical circuit232fails to read the output voltage VOUT. In this scenario, the compensation mechanism234can decrease the compensation voltage VDAC, thereby allowing more current to flow towards the amplifier A and increasing the magnitude of the output voltage VOUT. In this manner, the compensation mechanism232can be configured to dynamically set the compensation value for the tonic level conductance that is subtracted from the real conductance level. Thus, when the wearable device100is in a stable state, the compensation voltage VDACis proportional to the current tonic level conductance of the user. The compensation mechanism234can measure the entire range of tonic level conductances associate with the stratum corneum of the wrist of the user, for example, in the range of about 0.05 μS to about 80 μS. In some embodiments, the compensation mechanism can allow a fine tuning of the current in the range of about −1 μA to about 1 μA.

Furthermore, the compensation mechanism234allows for the subtraction of the tonic level from the real time conductance such that the output voltage VOUTrepresents the phasic value of the electrodermal activity. Thus the phasic level conductance can be measured with high resolution, for example, by an analog to digital converter (ADC) included in the processing module230. In some embodiments, the phasic level can be measured with a resolution of about 0.0001 μS.

In this manner, the current compensation enables the range to be increased by focusing on a dynamic portion of the total range. The compensation mechanism234dynamically sets the compensation current to fit the tonic level conductance while the amplifier A and the ADC observe the phasic level conductance. The gain G provided by the amplifier A and the high resolution of the ADC enables the signal to be resolved with high resolution. Furthermore, the switching mechanism reduces electrolysis of the electrodes by allowing polarity inversion of the electrodes at predetermined intervals.

While shown as including the electrical circuit232and the compensation mechanism234, the processing module230can include any other components. In some embodiments, the processing module230can include a filtering circuit, for example, a low pass filter, a high pass filter, a band pass filter, any other suitable filtering circuit, or combination thereof, configured to substantially reduce signal noise. In some embodiments, the processing module230can include a processor, for example, a microcontroller, a microprocessor, an ASIC chip, an ARM chip, or a programmable logic controller (PLC). The processor can include signal processing algorithms, for example, band pass filters, low pass filters, any other signal processing algorithms or combination thereof. In some embodiments, the processing module230can include a memory configured to store at least one of an electrodermal activity data, or a physiological status data, for example, ANS activity data. In some embodiments, the memory can also be configured to store a reference signature, for example, a calibration equation. In such embodiments, the processor can include algorithms which can be configured to correlate the measured electrodermal activity data to an ANS activity or any other physiological status parameter of the user. The memory can also include algorithms to maximize the signal to noise ratio of the electrodermal activity signal. In some embodiments, the processing module230can also include a generator of clock signals coupled to the processor. In some embodiments, the processing module230can also include an RFID chip configured to store information, for example, the electrodermal activity data, and allow a near field communication (NFC) device to read the stored information.

In some embodiments, the processing module230can be configured to measure a compensated value of conductance from which a tonic level conductance is removed. In some embodiments, the processing module230can be configured to allow a tuning of the current corresponding to the conductance of the stratum corneum in the range of about −1 μA to about 1 μA. In some embodiments, the processing module230can be configured to measure a tonic level conductance of the stratum corneum of the wrist in the range of about 0.05 μS to about 80 μS.

The communications module250is coupled to the processing module230. The communications module250can be configured to display an electrodermal activity of the user or communicate electrodermal activity data from the processing module230to an external device, for example, a smart phone app, a local computer and/or a remote server. In some embodiments, the communications module250includes a communication interface to provide wired communication with the external device, for example, a USB, USB 2.0, or fire wire (IEEE 1394) interface.

In some embodiments, the communications module250can include means for wireless communication with the external device, for example, Wi-Fi, BLUETOOTH®, low powered BLUETOOTH®, Wi-Fi, Zigbee and the like. In some embodiments, the communications module250can include a display, for example, a touch screen display, configured to communicate information to the user, for example, electrodermal activity, ANS activity, physiological activity of the user, remaining battery life, wireless connectivity status, time, date, and/or user reminders. In some embodiments, the communications module250can also include microphones and/or vibration mechanisms to convey audio and tactile alerts. In some embodiments, the communications module250can include a user input interface, for example, a button, a switch, an alphanumeric keypad, and/or a touch screen, for example, to allow a user to input information into the wearable device200, for example, power ON the system, power OFF the system, reset the system, manually input details of a user behavior, manually input details of the wearable device200usage and/or manually initiate communication between the wearable device and the external device.

The power source270is coupled to the processing module230and the communications module250and configured to supply electrical power to the processing module230and the communications module250. The power source can include for example, coin cells, Li-ion or Ni-Cad batteries of the type used in cellular phones. In some embodiments, the communications module250can also be used to recharge the power source270, for example, by providing power to the power source270from an external source through a communications lead. In some embodiments, the power source270can be recharged using inductive coupling.

FIG. 9shows an exemplary method300for measuring electrodermal activity including the tonic level and the phasic level over a wide range, for example, in the range of about 0.05 μA and 80 μA. The method300can be used with any electrodermal activity measurement system, for example, the apparatus100, the wearable device200, or any other apparatus or device described herein. The method300involves disposing a first electrode and a second electrode on the stratum corneum302. The electrodes can include the electrodes110,210or any other electrode described herein. The first electrode is biased at a first voltage V+and the second electrode is biased at a second voltage V−304. For example, the first electrode can be positively charged and the second electrode can be negatively charged or vice versa. A compensation current Icompis subtracted from a current Iskin. flowing through the stratum corneum to obtain an input current308. For example, a compensation mechanism (e.g., the compensation mechanism234or any other compensation mechanism described herein) can be used to set a compensation voltage that is transformed into the compensation current to be subtracted from the skin current Iskin. The input current Iin, is transformed into an output voltage which is measured310. For example, a transimpedance amplifier (e.g., a transimpedance amplifier included in the electrical circuit232or any other electrical circuit described herein) can be used to transform the input current into the output voltage VOUT. The output voltage is related to a conductance of the stratum corneum and is used to measure the conductance of the stratum corneum. The method then determines if the output voltage VOUTis saturated low312. For example, the output voltage VOUTcan be communicated to a compensation mechanism (e.g., the compensation mechanism234or any other compensation mechanism described herein) which can determine if the output voltage VOUTis saturated low (i.e., reached a minimum value). In this scenario, the first voltage V+ can be decreased, the second voltage V− can be increased, or the compensation current Icompis increased314to change the output voltage VOUTsuch that the output voltage VOUTis not saturated low.

If the output voltage VOUTis not saturated low, the method determines if the output voltage VOUTis saturated high316, i.e., reached very high values. For example, if the conductance of the stratum corneum is too high, the output voltage VOUTcan drop to very high values. If the output voltage VOUTis saturated high, the first voltage V+ can be increased, the second voltage V− can be decreased, and/or the compensation current Icompcan be decreased318to change the output voltage VOUTsuch that the output voltage VOUTis not saturated high. If the output voltage is not saturated low or high, the method continues to measure the output voltage VOUT. In this manner, the method enables continuous monitoring and control of the output voltage VOUTsuch that the conductance of the stratum corneum can be measured over a wide range.

In some embodiments, a wearable device can include an electrodermal activity sensor and a heart beat sensor. Referring now toFIGS. 10 and 11, a wearable device400includes a housing402, a first strap406a,a second strap406b,a first electrode410a,a second electrode410b(collectively referred to as the “electrodes410”), a pair of heart beat sensors420, a processing module430, a communications module450, and a power source470. The wearable device400is configured to be worn on the wrist of a user, analogous to a watch and to measure an electrodermal activity of the stratum corneum skin as well as the heart beat variability of the user.

The housing402defines an internal volume404configured to house at least a portion of the heart beat sensors420, the processing module430, the communications module450and the power source470. The housing470can be substantially similar to the housing470described with respect to the wearable device200, and is therefore not described in further detail herein.

The first strap406aand the second strap406b(collectively referred to as the “straps406”) are coupled to a first side and a second side of the housing402respectively. The straps406define an internal volume408. At least a portion of the electrodes410can be disposed in the internal volume408. The straps406can be substantially similar to the straps406described with reference to the wearable device200, and are therefore not described in further detail herein.

The electrodes410can include any suitable electrodes that can allow electronic communication through the stratum corneum and measure a conductance of the stratum corneum. The electrodes410can be configured to measure an electrodermal activity of the stratum corneum of the user. The electrodes410can be substantially similar to the electrodes210described with respect to the wearable device200, and are therefore not described in further detail herein.

The heart beat sensors420can be disposed in the internal volume defined by the housing402. The heart beat sensors420can be any suitable sensors. In some embodiments, the heart beat sensors420can include electrodes such as those included in EKG monitors. In some embodiments, the heart beat sensors420can include optical sensors. For example, the heart beat sensors can include a light emitter and a light receiver that can convert reflected light form the skin, or blood below the skin into an electrical signal corresponding to the heart beat of the user. In some embodiments, the light emitter can include an LED diode. In some embodiments, the light receiver can include a photodiode or a phototransistor. The electrical signal measured by the light detector which corresponds to the light reflected from the skin, can be communicated to the processing module430for calculating a heart rate of the user. In some embodiments, the wearable device400can also include optical filters, for example, monochromators to dynamically select a wavelength of the reflected light. In some embodiments, the monochromators can be tunable Fabry-Perot filters.

The processing module430is disposed in the internal volume404defined by the housing402. The processing module430includes an electrical circuit432and a compensation mechanism434. The electrical circuit432and the compensation mechanism434can be substantially similar to the electrical circuit232and the compensation mechanism234described with respect to the wearable device200, and are therefore not described in further detail herein. In some embodiments, the processing module430can include a circuit to control the electrical power communicated to the light emitter, for example, to control a luminosity of the light emitted by the light emitter included in the heart beat sensors420.

In some embodiments, the apparatus can also include various physiological sensors, for example, a heart beat sensor (e.g., a photoplethysmography sensor), an accelerometer, a temperature sensor, a blood oxygen sensors, a glucose sensor, any other physiological sensor or combination thereof. In such embodiments, the processing module430can be configured to process signals form each sensor to determine a physiological status of the user. In some embodiments, data processing of the signal received from each sensor can be performed on an external device, for example, a smart phone, a tablet, a personal computer, or a remote server. Furthermore, the communications module can be configured to communicate the physiological data from each of the sensors to the user, for example, via a display included in the apparatus or the external device. Such physiological data can include, for example, electrodermal activity (e.g., skin conductance), heart rate, heart rate variability, metabolic equivalent of task (MET), a stress level, a relaxation level, a movement or activity level, a temperature, a heat flux, and/or an ANS activity (e.g., an arousal or excitement).

In some embodiments, the processing module430can include algorithms to determine a well being index (WBI) of the user from the HRV data.FIG. 12shows a method that can be incorporated into an algorithm to determine a WBI of the user from the heart rate variability data. In the first step, the inter beat interval (IBI) time series is processed to identify and delete wrongly recognized and ectopic beats. In the next step, a spectral analysis of the corrected time series is performed to assess the total power, the high frequency power and the low frequency power of the cardiac rhythm. In the third step, the values obtained are given as input to the WBI function together with other cardiac parameters, and in the fourth step a WBI of the user is determined.

As described herein, the IBI time series is processed in the first step. The first column of the time series shown inFIG. 12includes the time stamps at which the heart beat occurred. The second column includes the time interval between each subsequent heart beat. In other words, the first column is a cumulative sum of the first column. The obtained IBI time series is split into subsequent windows of five minutes.

Errors in the location of the instantaneous heart beat can translate into errors in the calculation of the HRV. HRV is highly sensitive to artifact and errors in 2% to 5% of the data can result in unwanted biases in HRV calculations. To ensure accurate results, it is critical to manage artifacts and ectopic heart beats appropriately prior to performing any HRV analysis. To ensure accuracy, the method applies four parallel filters to the five minute IBI windows. The filters are applied to the second column of the IBI time series, assigning progressive natural numbers to the beats, as shown inFIG. 13.

The first filter includes a plain selection filter. The tachogram is initially filtered by a low-pass numerical filter. Beats falling outside a confidence region A centered in the filtered curve are discarded. The mean IBI equals the mean values of the IBI calculated in the window.

The second filter is a one-step selection filter. A plain selection filter is first applied, with a confidence region B centered in the filtered curve, as described herein with respect to the plain selection filter.FIG. 14shows a visual representation of the one-step selection filter. The selected beats are discarded only if subsequent points fall outside the confidence region in an opposite fashion which can happen when one beat is misrecognized.

The third filter is a two-step selection filter. A plain selection is first applied, with a confidence region C centered in the filtered curve, as described herein with respect to the plain selection filter.FIG. 15shows a visual representation of the two-step selection filter. The selected beats are discarded only if points with a two beat distance fall outside the confidence region in an opposite fashion.

The fourth selection filter is a gross selection filter. First, the unit of measure of the tachogram is transformed from seconds to a heart rate measured in beats per minute (bpm), according to the following equation;
Heart rate (hr)=60/IBI

Then a polynomial is fitted to the transformed tachogram in a least squares sense. Finally, beats that fall outside a confidence region D centered in the fitted curve are discarded.

As described herein, after the IBI time series is filtered, spectral analysis is performed on the data. Before performing the spectral analysis, the heart beat signal is detrended by applying the following equation to the second column of the windowed IBI time series;
detrended signal=constant_detrend (Hamming_window(linear_detrend(signal)))

The signal is linearly detrended before multiplying it by a hamming window of the same length. Next, a constant detrend is applied to subtract the zero frequency component. In this manner, the non-autonomic regulation of the heart rate, for example, due to vigorous exercise or voluntary physical activity is removed from the signal. Applying a hamming window to the IBI time series before the spectral analysis can thereby enhance spectral information.

A Lomb normalize periodogram is obtained which is dimensionless, and can be expressed in terms of the power spectral density (PSD) as follows:
PSD=Lomb_periodogram/integral(Lomb_periodogram)*variance(detrended signal)

The PSD equation can be applied only if the time-domain signal to be transformed has zero mean value. The integral in the equation can be a trapezoidal numerical integral.

Next, low frequency (LF), high frequency (HF), and total power (TP) values can be obtained by numerically integrating the PSD in the standard bands of 0.04 Hz to 0.15, from 0.15 Hz to 0.4 Hz, and from 0.4 Hz to maximum frequency, respectively.

Finally, the WBI function is determined. The goal of the WBI is to encourage healthy behaviors among individuals. Thus, exertion as well as meditation and relaxation are awarded a high index value. On the contrary, stressful situations that limit the HRV are given a low score. The WBI can be determined using the following equation:
WBI=f1(HF/LF)+f2(TP)+f3(meanHR,HRmax)

where meanHR is the mean heart rate in bpm during the five minute window of interest, and HRmax is the maximum heart rate of the subject. In some embodiments, the Haskell and Fox formula, or any other suitable formula can be used to determine the HRmax.

The WBI provides a daily comprehensive value that indicates the quality of the day of the user from an HRV point of view. For example, a healthy nutritional regime increases the quality of sleep and wakefulness, and in turn the magnitude of HRV can urge the user towards such healthy behavior. Thus an increase of the daily WBI through weeks, months and years would indicate the effectiveness of the method described herein.

In this manner, the method described herein can allow the tracking of the level of the psychophysical health over a period of time. Awareness of a user's own well being level can provide the user encouragement as well as guidance to enhance daily interactions and quality of life. Furthermore, the method described herein can be incorporated in devices, for example, the wearable device300to help the user cope with stressful situations by providing compensational feedback, other than supporting healthy behaviors such as, for example, healthy eating and exercise.

While various embodiments of the system, methods and devices have been described above, it should be understood that they have been presented by way of example only, and not limitation. Where methods and steps described above indicate certain events occurring in certain order, those of ordinary skill in the art having the benefit of this disclosure would recognize that the ordering of certain steps may be modified and such modification are in accordance with the variations of the invention. Additionally, certain of the steps may be performed concurrently in a parallel process when possible, as well as performed sequentially as described above. The embodiments have been particularly shown and described, but it will be understood that various changes in form and details may be made.

For example, although various embodiments have been described as having particular features and/or combination of components, other embodiments are possible having any combination or sub-combination of any features and/or components from any of the embodiments described herein. In addition, the specific configurations of the various components can also be varied. For example, the size and specific shape of the various components can be different than the embodiments shown, while still providing the functions as described herein.