A Conformal Sensor for Wireless Sweat Level Monitoring

Pinghung Wei, Briana Morey, Timothy Dyson, Nick McMahon, Yung-Yu Hsu, Sasha Gazman, Lauren Klinker, Barry Ives, Kevin Dowling, Conor Rafferty

MC10 Inc. Cambridge, USA

pwei@mc10inc.com

Abstract— A conformal, wearable and wireless system for continuously monitoring the local body sweat loss during exercise is demonstrated in this work. The sensor system includes a sweat absorber, an inter-digitated capacitance sensor, and a communication hub for data processing and transmission. Experimental results show that the sensor has excellent sensitivity and consistent response to sweat rate and level. A 150% variation in the sensor capacitance is observed with 50µL/cm2 of sweat collected in the absorber. During wear tests, the sensor system is placed on the subject’s right anterior thigh for measuring the local sweat response during exercise (eg. running), and the measured sweat loss (147µL) was verified by the weight change within the absorbent material (144mg). With a conformal and wireless design, this system is ideal for applications in sport performance, dehydration monitoring, and health assessment.

I. INTRODUCTION Sweat loss is the major mechanism for dehydration during physical activities, particularly in warm/hot conditions [1]. Deterioration in physiological function through cardiovascular and thermoregulatory strain occurs when fluid losses reach 2% of total body mass [2]. Fatigue related symptoms such as cardiovascular strain, negative changes in muscle metabolism, and reduced motivations could be induced due to dehydration. Therefore a sensing system that can alert the athletes or personnel regarding the body hydration state is important to prevent the impaired performance. Most reported hydration measurements are based on characterizing skin properties using electrical impedance [3], optical techniques, or thermal conductivity [4, 5]. Among those methods, the hydration level is being evaluated based on the skin properties, while the users have to maintain stationary during the measurements. Thus the assessment is not applicable for athletes (e.g. runners and bikers) who require real-time hydration monitoring during activities. Here we present a wearable and conformal system for continuously sweats monitoring with a sweat collector, inter-digitated capacitive sensor, and an active electronic system for signal processing and data recording. The data can be uploaded onto a Near Field Communication (NFC) enabled Android phone for further processing.

II. DESIGN

A. Sysstem Overview Fig. 1 shows a schematic of the wireless sweat sensing

system, which consists of a sweat absorber (cellulose GB005, GE Whatman), an inter-digitated capacitance sensor, and a communication hub for data processing. The sensor and communication hub are attached conformably against the skin using biocompatible adhesive. The communication hub is encapsulated within silicone for moisture protection. The sweat collector and the sensor are assembled within a wound dressing for skin attachment.

B. Sensor Design The sweat sensor was developed that exploits extremely thin copper-clad Kapton sheets with etched patterns of inter-digitated electrodes. The fabrication process was shown in Figure 2. First a 5 µm thick of Cu was electroplated onto a 12.5µm Kapton sheet. In step 2 and 3, Cu was patterned and following by laminating a 12.5 µm coverlay over the top

The research is funded by Air Force Research Laboratory (AFRL) FA8650-09-D-5037 DO #0013

Fig. 1: Schematic of the sweat sensing system, Top: top view, Bottom: cross-sectional view

978-1-4673-4642-9/13/$31.00 ©2013 IEEE

Fig. 2: Fabrication and Assembly Process of the Sweat Sensor.

Fig. 3: a) Top: a fabricated capacitive sweat sensor, b) Bottom: sweat sensor assembled in the TegadermTM

surface, with contact pads etched open. In step 5, the sensor structure is defined with laser cutting. A fabricated sweat sensor was shown in Figure 3a. Steps 5 to 7 in Fig. 2 demonstrate the integration process for the sweat sensor for further wear tests. First the sensor is attached to scotch tape with size matching the sensor dimension (1 x 1 cm2), step 5. The attachment of scotch tape enhances the uniformity of interface between sensor and absorber (cellulose sheet). Next in Step 6, the sensor is transferred onto the sweat absorber (1.4 x 1.4 cm2, cellulose sheet) with the coverlay side facing the absorber. A thin layer of Loctite 4011 epoxy is applied around the edge of scotch tape to improve the adhesion between sensor and absorber. Finally, in step 7, the sensor is assembled on TegadermTM (3M Nexcare Tegaderm Film) with the breathable and transparent adhesive film attaching to the sensor. A completely assembled sweat sensor with the absorber and Tegaderm TM adhesive film is shown in Figure 3b.

C. Sensing and Logic Circuit Fig. 4 illustrates the function blocks of the system. The

charging system includes an integrated circuit (IC) for power management and a rechargeable, thin-film, solid-state battery. The energy used to recharge the battery is harvested via the radio frequency (RF) system (green), which also has communication capabilities and an EEPROM that provides data storage functionality. Data is stored on the EEPROM, and can be uploaded to a cell phone or other device via Near Field Communication (NFC). In this instance, the data is collected by the microcontroller and the analog sensing block. The microcontroller then stores the data, representing sweat loss, on the EEPROM for transmission. The microcontroller is also partially responsible for power management, in that it controls the timing of analog sampling and communication, sleeping when neither action is required. Overall the system manages power, periodically collects data with user defined intervals, and stores the data so that it can be transmitted when an NFC-enabled device is in range.

Fig. 4: A system level block diagram.

III. RESULTS:

A. Sensor Calibration Testing and calibration of the sensor is shown in Fig. 5. The sensor was calibrated with 0.7X Phosphate Buffer Solution (PBS) to mimic the ionic concentration of human sweat. The capacitance of the sensor is measured with an LCR meter (Agilent U1733C) while fluid is delivered continuously into the absorber with a high precision automatic pump. Several fluid delivering rates (13µL/min, 6.5µL/min, and 3.25µL/min) were performed to test the consistency of sweat collector. As can be seen in Fig. 6a, the rate of capacitance increasing is proportional to the fluid delivering rate.

Fig. 6b demonstrates a linear relationship can be obtained for capacitance versus fluid volume with similar behavior and consistent responses seen across 4 tested sensors. The linear equation was extrapolated for further wear tests.

Fig. 5: Testing Setup for measuring sensor response vs. fluid volume.

Fig. 6: Testing Setup for measuring sensor response vs. fluid volume.

B. Wear Test A series of on-body wear tests were performed to evaluate the function of the total integrated system in a potential use case scenario. Participants wore the encapsulated communications hub with the sweat sensor and connector during bouts of exercise ranging from 30 to 60 minutes. Sensor readouts were taken with NFC-enabled smart phones and data was analyzed. From these preliminary field tests we were able to evaluate the system in terms of accuracy and consistency of the capacitive measurement for sweat loss, comfort and durability of the wearable system during

vigorous body movement. Fig. 7 summarizes the procedure and validation protocol for field testing.

Fig. 7: Validation protocol of the sweat sensing system.

During wear testing (Fig. 8b), the system is attached to the subject’s right anterior thigh due to its excellent correlation with full-body sweat loss [6]. The measured capacitance was read with a NFC-enabled phone and converted into fluidic volume based on the calibration curve (Fig. 6b). Fig. 8a shows that the fluid collected (147µL) in the absorber predicted by the electrical signal rises steadily with exercise. It correlates well with the measured weight gain (144mg) of the absorbent patch.

Fig. 8: a) Measured sweat level in the absorbent during the exercise, b) sweat sensing system placed on the subject’s right thigh

. IV. CONCLUSION

In this paper, we developed a conformal, wearable and wireless system for continuously monitoring the local body sweat loss during exercise. The sensor system includes a sweat absorber, an inter-digitated capacitance sensor, and a communication hub for data processing and transmission.

REFERENCES [1] B. Murray, “Hydration and Physical Performance”, J. Am. Coll. Nutr., vol. 26, no. 5, pp. 542-548, 2007. [2] SN. Cheuvront, 3rd R Carter, MN Sawka, “Fluid Balance and Endurance Exercise Performance”, Curr. Sports Med. Rep., Vol. 2, pp. 202-208, 2003 [3] X. Huang, WH. Yeo, Y. Liu, JA. Rogers, “Epidermal Differential Impedance Sensor for Conformal Skin Hydration Monitoring”, Biointerphases, Vol 7, pp.52-58, 2012 [4] JW. Choi, SH Kwon, CH Huh, KC. Park, SW. Youn, “The influences of skin visco-elasticity, hydration level and aging on the formation of wrinkles: a comprehensive and objective approach,” Skin Research and Technology, 2012. [5] T. Frodin, P. Helander, L. Molin, M. Skogh, “Hydration of human stratum corneum studied in vivo by optothermal infrared spectrometry, electrical capacitance measurement, and evaporimetry”, Acta Derm Venereol, vol. 68, No. 6, pp. 461-467, 1988. [6] CJ Smith, G. Havenith, “Body Mapping of Sweating Patterns in Male Athletes in Mild Exercise-Induced Hyperthermia”. Eur. J. Appl. Physiol., Vol. 111, pp. 1391-1404, 2011.

Consistency of sweat sensors were tested demonstrated. A 150% variation in the sensor capacitance is observed with 50µL/cm2 of sweat collected in the absorber. Wear tests were performed with protocol to verify the sensor performance versus the weight changes within the sweat collector. With a conformal and wireless design, this system is ideal for applications in sport performance, dehydration monitoring, and health assessment.

REFERENCES [1] B. Murray, “Hydration and Physical Performance”, J. Am. Coll. Nutr., vol. 26, no. 5, pp. 542-548, 2007. [2] SN. Cheuvront, “Fluid Balance and Endurance Exercise Performance”, Curr. Sports Med. Rep., Vol. 2, pp. 202-208, 2003 [3] X. Huang, “Epidermal Differential Impedance Sensor for Conformal Skin Hydration Monitoring”, Biointerphases, Vol 7, pp.52-58, 2012 [4] CJ Smith, “Body Mapping of Sweating Patterns in Male Athletes in Mild Exercise-Induced Hyperthermia”. Eur. J. Appl. Physiol., Vol. 111, pp. 1391-1404, 2011 [5] X. Huang, “Epidermal Differential Impedance Sensor for Conformal Skin Hydration Monitoring”, Biointerphases, Vol 7, pp.52-58, 2012 [6] [4] CJ Smith, “Body Mapping of Sweating Patterns in Male Athletes in Mild Exercise-Induced Hyperthermia”. Eur. J. Appl. Physiol., Vol. 111, pp. 1391-1404, 2011 [1] B. Murray, “Hydration and Physical Performance”, J. Am. Coll. Nutr., vol. 26, no. 5, pp. 542-548, 2007. [2] SN. Cheuvront, “Fluid Balance and Endurance Exercise Performance”, Curr. Sports Med. Rep., Vol. 2, pp. 202-208, 2003 [3] X. Huang, “Epidermal Differential Impedance Sensor for Conformal Skin Hydration Monitoring”, Biointerphases, Vol 7, pp.52-58, 2012 [4] CJ Smith, “Body Mapping of Sweating Patterns in Male Athletes in Mild Exercise-Induced Hyperthermia”. Eur. J. Appl. Physiol., Vol. 111, pp. 1391-1404, 2011 [5] X. Huang, “Epidermal Differential Impedance Sensor for Conformal Skin Hydration Monitoring”, Biointerphases, Vol 7, pp.52-58, 2012 [6] [4] CJ Smith, “Body Mapping of Sweating Patterns in Male Athletes in Mild Exercise-Induced Hyperthermia”. Eur. J. Appl. Physiol., Vol. 111, pp. 1391-1404, 2011 [2] SN. Cheuvront, “Fluid Balance and Endurance Exercise Performance”, Curr. Sports Med. Rep., Vol. 2, pp. 202-208, 2003 [3] X. Huang, “Epidermal Differential Impedance Sensor for Conformal Skin Hydration Monitoring”, Biointerphases, Vol 7, pp.52-58, 2012 [4] CJ Smith, “Body Mapping of Sweating Patterns in Male Athletes in Mild Exercise-Induced Hyperthermia”. Eur. J. Appl. Physiol., Vol. 111, pp. 1391-1404, 2011 [5] X. Huang, “Epidermal Differential Impedance Sensor for Conformal Skin Hydration Monitoring”, Biointerphases, Vol 7, pp.52-58, 2012 [6] [4] CJ Smith, “Body Mapping of Sweating Patterns in Male Athletes in Mild Exercise-Induced Hyperthermia”. Eur. J. Appl. Physiol., Vol. 111, pp. 1391-1404, 2011