LIG-OSS: Integrated Laser-Induced-Graphene Sensor and Open-Source Silicon Chip for an Affordable and Robust Wearable Sensing System with Precise Temperature, Humidity, and Strain Sensing Capability

This paper presents a multifunctional wearable sensing system that integrates flexible Laser-Induced-Graphene (LIG) based sensors and an Open-Source Analog Front-End (AFE) chip. The LIG sensors are fabricated on polyimide (PI) Flexible Printed Circuit Board (FPCB) through CO
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infrared laser direct-write method. The LIG sensors provide repeatable high- precision temperature sensing, humidity measurement, and strain detection capability. The temperature sensing characterization shows the resistive LIG sensor has a sensitivity of -0.0493 %/°C, the linear fit R-square factors > 0.9973 across -40 °C to 125 °C. The capacitive humidity sensor achieves a normalized response of 2260% at 95% relative humidity (RH). Our proposed AFE chip contains a hybrid folded-cascode Operational Amplifier (OPAMP) with 92 dB gain and 12.5 MHz Gain Bandwidth Product (GBW), and a 14-bit 10 MHz Successive Approximation Register Analog-to-Digital Converter (SAR ADC). Designed using open-source analog flow and fabricated in GF180 OpenPDK, the AFE chip serves as a flexible and universal readout platform, adaptable for various sensing applications. A real-time demonstration of finger bending detection, with readings output by the AFE Chip, is performed to validate the system functionalities and robustness. The temperature, humidity, strain sensing capability provide by the wearable system are attractive for personal healthcare application. This work underscores the integration of the LIG sensors and the AFE chip, developed using open-source tools which facilitate an affordable prototyping for a multifunctional flexible wearable sensing system.