HUMIDITY-BASED SWEAT RATE SENSING DEVICES
Embodiments of the disclosed invention provide wearable devices that use a humidity sensor to measure sweat rate generated from an area of skin. A sensing chamber is continuously filled with a sweat sample, which forms a droplet and alters the humidity measured within the chamber. Once the sweat sample droplet expands to the edge of the chamber, the droplet contacts a wick and is drawn away, so the chamber can fill with a subsequent droplet. The device uses a droplet volume and the time required to reach a maximum humidity to calculate a sweat rate. A pump is used to draw old sweat sample out of the wick to allow extended device operation. Some embodiments also include capacitive sensors to perform back up measurements. Another set of embodiments includes alternatively shaped sensing chambers configured to reduce sample volumes or improve function. A method for determining sweat rate based on humidity sensor measurements is also included.
1 . A device, comprising:
a fluid-impermeable substrate for defining a device volume and interacting with a device wearer's skin;
a sweat collector;
a sensing chamber in fluidic communication with the sweat collector through an inlet, wherein the sensing chamber comprises a humidity sensor, a partially sealed space having a volume, and a droplet formation area surrounding the inlet, wherein the droplet formation area has a hydrophobic coating;
a wick in fluidic communication with the sensing chamber;
a humidity dissipation volume in fluidic communication with the wick; and
a pump comprising a membrane, and an absorbent material, and wherein the membrane has a first side adjacent to the humidity dissipation volume and a second side adjacent to the absorbent material
2 . The device of claim 1 , further comprising a pump humidity sensor that is in fluidic communication with the absorbent material.
3 . The device of claim 1 , wherein the humidity dissipation volume has a volume of zero.
4 . The device of claim 1 , further comprising one or more secondary sensors, wherein the one or more secondary sensors is chosen from the following: a sensor for measuring an ambient temperature, a sensor for measuring a skin temperature, a sensor for measuring a device internal temperature, a volumetric sweat rate sensor, a micro-thermal flow rate sensor, a discrete volume dosing system sweat rate sensor, a galvanic skin response sensor, a sweat conductivity sensor, an impedance skin contact sensor, and a capacitive sensor.
5 . The device of claim 1 , further comprising:
a first capacitive sensor, comprising an insulator at least partially covering the wick, wherein the insulator has a first side that is adjacent to the wick and a second side; two conductive traces arranged in concentric circles centered on the sensing chamber and situated on the second side of the insulator, and electronics for measuring a capacitance across the conductive traces.
6 . The device of claim 5 , further comprising:
a second capacitive sensor, comprising two conductive traces arranged in concentric circles centered on the sensing chamber and situated on the second side of the insulator, and electronics for measuring a capacitance across the conductive traces, wherein the first capacitive sensor is located between the second capacitive sensor and the sensing chamber.
7 . The device of claim 5 , wherein the insulator and the membrane are the same component.
8 . A device, comprising:
a fluid-impermeable substrate for defining a device volume and interacting with a device wearer's skin;
a sweat collector;
a sensing chamber in fluidic communication with the sweat collector through an inlet, wherein the sensing chamber comprises a humidity sensor, a partially sealed space having a chamber volume, and a hydrophobic coating defining a sample volume;
a wick in fluidic communication with the sensing chamber and partially separated from the sample volume by the hydrophobic coating;
a humidity dissipation volume in fluidic communication with the wick; and
a pump comprising a membrane, and an absorbent material, and wherein the membrane has a first side adjacent to the wick and a second side adjacent to the absorbent material.
9 . The device of claim 8 , further comprising a pump humidity sensor that is in fluidic communication with the absorbent material.
10 . The device of claim 8 , wherein the humidity dissipation volume has a volume of zero.
11 . The device of claim 8 , further comprising one or more secondary sensors, wherein the one or more secondary sensors is chosen from the following: a sensor for measuring an ambient temperature, a sensor for measuring a skin temperature, a sensor for measuring a device internal temperature, a volumetric sweat rate sensor, a micro-thermal flow rate sensor, a discrete volume dosing system sweat rate sensor, a galvanic skin response sensor, a sweat conductivity sensor, an impedance skin contact sensor, and a capacitive sensor.
12 . The device of claim 8 , further comprising:
a first capacitive sensor, comprising an insulator at least partially covering the wick, wherein the insulator has a first side that is adjacent to the wick and a second side; two conductive traces arranged in concentric circles centered on the sensing chamber and situated on the second side of the insulator, and electronics for measuring a capacitance across the conductive traces.
13 . The device of claim 12 , further comprising:
a second capacitive sensor, comprising two conductive traces arranged in concentric circles centered on the sensing chamber and situated on the second side of the insulator, and electronics for measuring a capacitance across the conductive traces, wherein the first capacitive sensor is located between the second capacitive sensor and the sensing chamber.
14 . The device of claim 8 , wherein the sample volume has one of the following shapes: a cylinder, and a shape comprised of a first cylinder having a first radius, and a second cylinder having a second radius, wherein the second radius is larger than the first radius, and the first cylinder is between the inlet and the second cylinder.
15 . The device of claim 8 , wherein the sensing chamber further comprises a containment wall having an internal surface, a hydrophobic coating on the internal surface, and wherein the containment wall is shaped to facilitate the movement of condensed vapor to the wick.
16 . A method of measuring a sweat rate, comprising:
receiving a plurality of sweat samples in a sensing chamber, wherein the sensing chamber has a chamber volume and each of the plurality of sweat samples has a sample volume;
using a sensor to take a plurality of humidity measurements of the chamber volume;
determining a maximum humidity measurement and a baseline humidity measurement;
correlating the baseline humidity measurement to an empty time when a sweat sample is not in the chamber, and correlating the maximum humidity measurement to a filled time when a sweat sample is in the sensing chamber;
determining a fill duration, as a difference between the empty time and the filled time; and
calculating a sweat rate from the fill duration and the sample volume.
17 . The method of claim 16 , further comprising:
using a first capacitance sensor to measure a first wetting time in a wick, wherein the first wetting time is when a sweat sample is near enough to interact with the first capacitance sensor;
comparing the first wetting time to the empty time; and
determining that a sweat sample has exited the sensing chamber and entered the wick.
18 . The method of claim 17 , further comprising:
using a second capacitance sensor located between the first capacitance sensor and an outer edge of the wick to measure a second wetting time in the wick, wherein the second wetting time is when a sweat sample is near enough to interact with the second capacitance sensor; and determining that a device lifespan has elapsed as of the second wetting time.