A CMOS Integrated Sweat Monitoring System for Medical Applications

Recently, demand for sweat measuring sensors has been increasing, especially for detection of heat stroke, dehydration and pathology of neurological diseases. Currently commercially available sensors are immobile, as they are large and require cables and hoses. Previous studies have presented a design that combines a custom chip analog to digital converter (ADC), discrete capacitive humidity and thermal sensors in a single board. In addition to the ADC, this study integrates a thermal sensor and a time to digital converter (TDC) into a chip. We evaluated the custom chip by comparing the readings of our chip with the market available discrete chip of ADT-7410 inside a environment chamber. Our custom chip size is 2.6 mm₂ with a 0.18 $\mu$m CMOS process including TDC and thermal sensor. Our system produces results comparable to those of a commercially available system, while being much smaller.

I. Introduction

Monitoring human sweat in daily life attracts attention nowadays because human sweat includes a lot of information, for example neuropathy, heatstroke, dehydration, etc [1, 2] . To measure human sweat, a humidity sensor is needed. There are two types of humidity sensors: resistor-type and capacitor-type. A resistor-type humidity sensor typically has a small size but its accuracy is not enough for monitoring human sweat. Therefore, we propose a human sweat monitoring system based on capacitor-type humidity sensor for wearable applications. This system is based on the first proto-type design of human sweat monitoring system presented in [3, 4] , which achieves a compact size with equivalent accuracy compared to the conventional sweat sensor of SKN-2000 [5]. However, the last design of monitoring system did not integrate time-to-digital converter (TDC) nor a thermal-sensor since it was just the first proto-type. Thus, the previous design requires off-chip TDC and thermos sensor which are realized on a FPGA board.