Development of microstructured PDMS pressure sensors: Simulations, experiments, and OTFT applications

Abstract

Elastomer and plastic based pressure sensors are in demand for the next generation of health monitoring devices. Signal transduction is often done by microengineering resistive, capacitive, or piezoelectric active layers. Pressure sensitive active layers can be fabricated using various microstructuring methods, which increases sensitivity and the time resolution of sensors. In addition, integration of the microstructures with thin-film transistors further enhances their performance by amplifying or transforming the pressure response. One common fabrication method for the sensors is the moulding of pyramidal PDMS microstructures. These can be used as a deformable dielectric for which capacitance changes with pressure. This process is highly repeatable and allows for the control of microstructure geometry. By modifying the geometry or material parameters, the performance of the sensors can be tuned for targeted applications. Therefore, it is important to understand the impact each parameter has on initial sensor output, sensitivity, and dynamic range. Mathematical models and finite-element method simulations have been previously shown to predict capacitive pressure sensor response; however, studies are often limited to few geometries. Here, a generalized simulation model using COMSOL Multiphysics has been developed to predict sensor output with various geometric and material parameters. Studies have been conducted by varying pyramid base width and spacing from 10-100 microns, lamination layers of 7--105 microns, pyramid penetration of 0.1--10 microns, and elastic modulus of 1--3 MPa. Simulations are compared with fabricated PDMS pressure sensors, and the ability to control the sensor output is demonstrated by modifying the pyramid width and spacing from 25 to 75 microns. The sensitivity can be tuned from 0.008--0.025 1/kPa and initial capacitance from 19.4 to 44.5 pF/cm2. Furthermore, we have optimized the processing of organic thin-film transistors (OTFT) in order to fabricate a monolithic pressure sensor. The microstructured PDMS is integrated with a DPP-DTT OTFT, this transforms the output signal from capacitance to current, and amplifies the sensitivity. The semiconductor channel is p-doped due to oxygen trapping of electrons, which leads to an "always-on" device. The application of pressure increases the field-effect and removes carriers from the channel. We have demonstrated a sensitivity of -0.070 1/kPa in the range of 0-3 kPa, and a significant reduction in device variability. Additional simulations were done to compare ideal resistive and capacitive sensing performance for the future optimization of monolithic thin-film transistor-based pressure sensors.