Microelectrode pH sensors based on micropatterned silver nanowire/PAA hydrogel composite

Abstract

Owing to exceptional merits such as high flexibility, lightweight, great stretchability and conformability, flexible electronics is growing towards a promising platform for personal wearable electronics, in which wearable and flexible biochemical sensors are going to be adopted to replace cumbersome and costly medical instruments for the healthcare monitoring and diagnostics. This project aims to develop a microelectrode pH biosensor to monitor ambient pH which is one of the signatures of the physiological health status in body fluids, such as sweat, tears as well as saliva. Responsive hydrogels have become one of the essential building blocks for functional biomedical microelectromechanical system (BIOMEMS) devices for a great many biomedical applications. Here, we choose pH-responsive hydrogel, poly acrylic acid (PAA), and use an in-house optical maskless photolithography platform to photopolymerize the hydrogel to fabricate pH sensors. Because of significant swelling in solutions with different pH values, PAA hydrogel adheres to the substrate with difficulty while it is used to make sensors working in solutions for long-term operation. To solve the problem, we developed a special stretchable microelectrode using silver nanowire (AgNW) / polydimethylsiloxane (PDMS) composite. When the PAA hydrogel is deposited on the electrode, a diffusion layer between PAA hydrogel and PDMS can be formed to promote the adhesion of PAA. The conductivity of the electrode was optimized by the contents of AgNWs in the PDMS composite. To fabricate microelectrode pH sensors, we mixed acrylic acid (AA) monomers with AgNWs to make pH-sensitive hydrogel composite. When such hydrogel composites are soaked into the different pH solution, the swelling of the hydrogel composite would change the spatial distribution of AgNWs in the hydrogel network and then cause the changes of composite's conductivity. With the in-house optical maskless photolithography platform, the hydrogel composite was in situ photo-polymerized and micropatterned on the microelectrode for sensor fabrication. The effects of the microstructure on the micropatterned hydrogel composite on the performances such as response time and sensitivity of that pH sensor would be systematically researched through the measurements. Experimental results revealed that the response time of that pH sensor depend on size of microstructures. Due to the accelerated diffusion, the sensor with microstructure showed much fast response than that without microstructure. Compared with other pH sensors, the fabricated sensors showed much better performance in terms of response time.