High Strain and Biocompatible Screen Printed Nanocomposite Based Conductive Pdms Strain Sensors

Strain sensors capable of operating in high strain conditions remain a technical challenge. In particular, biocompatible strain sensor technology is needed that satisfies the following requirements: (1) construction from low modulus materials that approach values of soft biological tissues and (2) high strain operation (≥ 20%). We present a screen printed, nanocomposite-based conductive polydimethylsiloxane (CPDMS) strain sensor capable of 40% strain operation with a gauge factor (GF) >100. Strain sensing using CPDMS sensors containing multi-walled carbon nanotubes (MWNT), graphene nanoplatelets (GNP), or a mixture of both nanocarbon filler materials was demonstrated. The combination of high strain operation, high GF, and biocompatible construction pave the way for minimally invasive in vivo strain measurements. Strain sensors were characterized according to their conductivity, zero current resistance (ZCR), thermal coefficients of resistance (TCR), and gauge factor. INTRODUCTION Typical strain sensors are made from silicon, metal, or other hard materials using microelectromechanical systems (MEMS) technology [1-2]. These devices are efficient strain sensors; however, they are limited to low strain applications, and their modulus of elasticity does not match that of soft tissues [3], thus limiting their ability to be used in vivo for measuring the strain of soft tissues. Recent research has shown that polydimethylsiloxane mixed with nanocarbon filler (typically multi-walled carbon nanotubes) has potential as a piezoresistive material for strain gauge applications [4-8] but only low gauge factors (~12) were achieved [9]. Such composites become conductive once the filler concentration reaches the percolation threshold. The percolation threshold of CNTs is dependent on aspect ratio, diameter, degree of conglomeration and alignment and therefore ranges widely from 0.005 vol% to several vol% [10]. We investigated a GNP/MWNT blend mixed with PDMS to achieve a low percolation threshold with MWNTs while conductive networks above the percolation threshold were created using GNPs. GNPs are also lower in cost and improve the consistency of the prepolymer for screenprinting. The GNP/MWNT and PDMS composite form CPDMS that is sandwiched between and supported by two layers of transparent, medical grade PDMS. A medical grade PDMS (USP Class VI and ISO 10993-1) was selected for the polymer matrix to improve elongation properties of the resulting CPDMS for high strain operation, which, in our past work, was limited by fracture of the piezoresistive material under strain (failed at 1.5% strain [11]). PDMS-based strain sensors are low cost, simple to manufacture, and well suited for in vivo use because of their low Young’s modulus (closer match to that of organs and tissues than silicon) and biocompatibility. Our intended application is in vivo measurement of urinary bladder fullness which requires reliable operation up to ~20% strain. THEORY Gauge factor is a useful measure of merit for strain sensors and is equal to the normalized change in resistance divided by the strain: R R R R GF L L ε Δ Δ