Flexible Piezoresistive Sensors for Biomedical Application

This thesis aims to understand and engineer the performance-determining factors of flexible piezoresistive sensors (FPSs), and provides guidelines for achieving desirable sensor performances for biomedical applications through material design.;Firstly, a wearable sensor patch system that integrates FPS and epidermal electrocardiogram (ECG) sensors for cuffless blood pressure (BP) measurement is presented. By developing parametric models on the FPS sensing mechanism and optimizing operational conditions, a highly stable epidermal pulse monitoring method is established and beat-to-beat BP measurement from the ECG and epidermal pulse signals is demonstrated.;Secondly, I explored the potential of using regularly interlaced textile materials to achieve high-repeatability and low-hysteresis FPS. I found that the structural flexibility and surface regularity of knitted fabric structures could in general provide reproducible pressure response; however, response fluctuations and hysteresis are still present due to the inevitable inelastic deformation of the textiles. To address this limitation, I introduced carbon black (CB) particles and polyvinylidene fluoride (PVDF) in the knitted fabric as the electrical and mechanical interconnects, respectively, between the fibers. Through composition optimization, I reduced the pressure response variation to below 2% and decreased the hysteresis loop deviation to below 10% for single sensors. The performance variation among multiple sensors is as low as 5%, much smaller than the 33% variation of the sensors made with nonwoven fabrics. Utilizing the high sensor repeatability, I successfully realized sensor arrays and multi-site sensor network for monitoring superficial temporal artery pulse pressure and pulse wave velocity, which demonstrate the potential of using wearable sensing systems for multifunctional cardiovascular monitoring.;Finally, a three-dimensional graphene-polydimethylsiloxane (GP) hollow structure was realized, where the electrical conductivity and mechanical elasticity of the composite can be tuned separately by varying the graphene layer number and the polydimethylsiloxane (PDMS) composition ratio, respectively. Accordingly, the sensor sensitivity and linear range can be easily improved through a decoupled tuning process, reaching a sensitivity of 15.9 kPa-1 in a 60 kPa linear region. By optimizing the density of the graphene percolation network and thickness of the composite, we improved the stability and repeatability of the sensor output under bending, achieving a measurement error below 6% under bending radius variations from -25mm to +25mm. The potential applications of these sensors in pulse monitoring and robotic operations were explored.;In conclusion, this thesis correlates electrical and mechanical properties of composite materials with FPS performances. Highly sensitive and bending applicable sensors were achieved and applied to health monitoring systems. The fundamental findings and material strategies developed in this thesis provide guidelines for design and engineering of pressure sensors to fulfill application-specific requirements.