Fabrication of flexible pressure sensors with microstructured polydimethylsiloxane dielectrics using the breath figures
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exible touch sensors with high sensitivity show promise in biomedical diagnostics and for artificial “electronic skin” for robotics or prosthetic devices. For “electronic skin” applications, there exists a need for low-cost, scalable methods for producing pixels that sense both medium (10–100 kPa) and low pressures (,10 kPa). Here, the “breath figures” (BFs) method, a simple, self-assembly-based method for producing honeycomb-structured porous polymer films, was used to prepare pattern compressible, and microstructured dielectric layers for capacitive pressure sensors. Porous polystyrene BFs films served as molds for structuring polydimethylsiloxane dielectrics. Pressure sensing devices containing the BFs-molded dielectrics consistently gave pressure response with little hysteresis, high sensitivities at lower applied pressures, and improved sensitivity at higher pressures. Analysis of microstructure geometries and pressure sensor performance suggests that structures with higher aspect ratios (height-to-width) produce less hysteresis, and that less uniform, more polydisperse structures yield a more linear pressure response.
I. INTRODUCTION
Contributing Editor: Tao Xie a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.334
electrodes (er), or (ii) a change in the distance between the electrodes (d). Capacitive sensors therefore require a dielectric material that is easily compressed and responds quickly to application and release of pressure. In addition to the use of foams as dielectric materials for sensing moderatepressures,11 elastomers have been incorporated into capacitive pressure sensors. In particular, polydimethylsiloxane (PDMS) shows promise as a flexible, biocompatible material for dielectric films.6,7,12–17 However, unstructured PDMS thin films possess high viscoelasticity, which prevents rapid response and relaxation and leads to poor sensitivity. As a result, our group pursued a strategy of microstructuring the PDMS to introduce air into the dielectric layer and enable elastic deformation of the material. Dielectrics with microstructured lines or pyramid patterns were fabricated using silicon molds that had been photolithographically patterned and etched in potassium hydroxide.13 However, the pressure sensors produced exhibited high sensitivities (up to 0.55 kPa1) only within the narrow range of 0–7 kPa.13,15 We subsequently found that the sizes of and spacing between pyramids could be adjusted, but the methods used possessed similar limitations.15 For the large-scale fabrication of such sensors, it is desirable to have a simple, low-cost, tunable method for producing microstructured compressible layers with the desired sensitivity over a wide range of pressures. The breath figures (BFs) patterning technique represents a simple, robust method for producing microstructured
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Ó Materials Research Society 2015
Flexible, stretchable pressure sensors with high sensitivity show promise in applications including touch screens, biomedical diagnostic technologies such
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