Ultra-sensitive and tunable microfluidic strain sensor with novel mechanotransduction method for the continuous monitoring of skin deformation

Wearable skin-mounted strain sensors for human motion tracking have the potential to assist in the prevention, diagnosis, and treatment of medical conditions related to musculoskeletal system by tracking human motion. Previous work at Dr. Araci’s lab at Santa Clara University demonstrated the advantages of dilatometry to achieve ultra-high gauge factor (GF) in a microfluidic strain sensor. However, despite its high performance, the used passive readout method hinders its implementation for continuous monitoring of skin strain. In this thesis, various strategies were adapted to achieve continuous electrical measurement in dilatometric sensors, including the design of a hybrid nanocomposite-microfluidic device and the direct resistance measurement from the dilatometric response. As a result, a novel mechanotransduction mechanism based on capillary corner flow was established. This approach utilizes ultra-high electrical resistance modification due to the capillary flow of conductive ionic liquids in response to the elastomeric deformation of silicone microchannels. Modular dilatometric strain sensors implementing this method were developed and characterized here, achieving directional specificity and tunable performance. The devices’ characteristics were tuned by controlling the liquid reservoir (LR) dilatation response and the morphology of the corner flow. A linear relation (R2 = 0.989) between liquid displacement and GF was found, hence the GF can be independently controlled by increasing the number of parallel LR channels (GFs of 97, 171, and 236 for 1X, 2X, and 4X, respectively). Moreover, varying the aspect ratio (AR) of the probe channel and the contact angles (CAs) between the three tested ionic liquids (IL1: [BMIM][Otf], IL2: [BMIM][N(CN)2], IL3: [EMIM][N(CN)2]) and the microchannels surface, the corner flow morphology can be controlled. A GF of 3000 in the dynamic range of 3-5% for CA=77.6° and 171 in the 3-20% range for CA=28.8° were obtained. Furthermore, probe channel AR~1 was determined to be the optimal condition for increasing the GF. Devices with IL1 presented a linear response (2.8% - 22% strain), small hysteresis (1.5%) and reproducible measurements for hundreds of cycles (2% error). Finally, the directional specificity and high sensitivity were demonstrated for distinguishing wrist and facial activity types and the subtle differences in a facial muscle strengthening exercise