Non-invasive monitoring of biomarkers in biological fluids through the use of wearable sensors gained significant interest in the past several decades [1].  Research efforts have been focused on the development of chemical and biochemical sensors for a range of metabolites, including glucose, lactate, ethanol, trace metals and various ions, targeting several bodily fluids such as tears, saliva, and sweat [2].  Work presented here will discuss the development of a sensor system, capable of real-time monitoring of lactate levels in sweat.  Such a device is desired in order to correlate increasing levels of lactate to fatigue levels of a person performing physically demanding activities [3].  Most existing systems employ traditional batteries as power sources for sensors.  The system described here utilizes a light weight biofuel cell as the power source giving a complete bio-friendly sensor system.

The system is comprised of three main elements: a biosensor, a biofuel cell, and electronics interface.  The biosensor detects lactate levels in sweat via enzymatic reaction of Lactate Dehydrogenase (LDH).  Sensor patch features a three electrode design assembled onto generic athletic tape and covered with medical gauze, providing both flexibility and sweat collection capability.  LDH is immobilized at the working electrode via conductive carbon-based ink.  The electrode surface is then further modified by vapor-deposited tetramethyl orthosilicate (TMOS) coating in order to provide greater stability to the sensor against temperature and pH fluctuations.  Sensor calibration time period was relatively quick (5 minutes) and response was immediate with addition of lactate.  Test results for the sensor patch showed an open circuit potential of ~0.06V vs. Ag/AgCl and an equilibrated current of approximately 30µA when held at 0.3V vs. Ag/AgCl.  The patch also demonstrated a sensitivity of 0.2µA/mM lactate when tested in a range of 5-100mM lactate.

A glucose-based biofuel cell provides the power to operate the biosensor.  The anode, driving glucose oxidation, is comprised of immobilized Glucose Dehydrogenase (GDH) on a high surface area carbon felt (CF) electrode.  Prior to enzyme immobilization, CF electrode surface was modified with electrochemically deposited polymethylene green (PMG) as well as chemically tethered Nicotinamide Adenine Dinucleotide (NAD).  An oxygen-reducing cathode was employed as counter electrode.  The biofuel cell generated an open circuit potential of 0.7V vs. Ag/AgCl, a total current of 81.0mA, and total power of 16.7mW.

The patch sensor is coupled to the biofuel cell via external electrical components: an energy harvester and a micropotentiostat.  The energy harvester component is connected to the biofuel cell. Its primary function is to continuously provide stable output voltage to the micropotentiostat.  Thus it is constantly drawing power from the biofuel cell and up-converting the voltage from 0.7 to 3V.  The patch sensor is connected to the micropotentiostat.  A constant potential of 0.3V vs. Ag/AgCl is needed to operate the sensor, which is supplied by the micropotentiostat.  Additionally, it is used to provide a read-out of an electrical signal, i.e. current, generated during sensor operation. 

System performance was demonstrated, employing artificial sweat.  Results showed linear sensor response with increasing lactate content of the artificial sweat solution. 

[1]	A. J. Bandodkar and J. Wang, "Non-Invasive Wearable Electrochemical sensors: A Review," Trends in Biotechnology, vol. 32, no. 7, pp. 363-371, 2014.
[2]	J. Kim, W. R. de Araujo, I. A. Samek, A. J. Bandodkar, W. Jia, B. Brunetti, T. R. Paixao and J. Wang, "Wearable temporary tattoo sensor for real-time trace metal monitoring in human sweat," Electrochemistry Communications, vol. 51, pp. 41-45, 2015.
[3]	S. Jadoon, S. Karim, M. R. Akram, A. K. Khan, M. A. Zia, A. R. Siddqi and G. Murtaza, "recent Developments in Sweat Analysis and Its Applications," International Journal of Analytical Chemistry, pp. 1-7, 2015.