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Large Area One-Step Facile Processing of Microstructured Elastomeric Dielectric Film for High Sensitivity and Durable Sensing over Wide Pressure Range Sujie Chen, Bengang Zhuo, and Xiaojun Guo* National Engineering Laboratory of TFT-LCD Materials and Technologies, Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai 200240, China S Supporting Information *
ABSTRACT: Once the requirement of sensitivity has been met, to enable a flexible pressure sensor technology to be widely adopted as an economic and convenient way for sensing diverse human body motions, critical factors need to be considered including low manufacturing cost, a large pressure detection range, and low power consumption. In this work, a facile approach is developed for one-step processing of a large area microstructured elastomer film with high density microfeatures of air voids, which can be seamlessly integrated into the process flow for fabricating flexible capacitive sensors. The fabricated sensors exhibit fast response and high sensitivity in the low pressure range to be able to detect very weak pressure down to 1 Pa and perform reliable wrist pulse monitoring. Compared to previous work, more advantageous features of this sensor are relatively high sensitivity being maintained in a wide pressure range up to 250 kPa and excellent durability under heavy load larger than 1 MPa, attributed to the formed dense air voids inside the film. A smart insole made with the sensor can accurately monitor the real-time walking or running behaviors and even a small weight change less than 1 kg under a heavy load of a 70 kg adult. For both application examples of wrist pulse monitoring and smart insole, the sensors are operated in a 3.3 V electronic system powered by a Li-ion battery, showing the potential for power-constrained wearable applications. KEYWORDS: large area, microstructured PDMS film, pressure sensor, wrist pulse monitoring, smart insole sensors to achieve the high sensitivity,24,26,44 make these sensors difficult for the targeted wearable applications, which require low voltage operation and are severely powerconstrained. Capacitive pressure sensors with a piezoelectric,51,52 ferroelectric,53 or elastomeric dielectric layer16−19,43−50 sandwiched between two flexible electrodes have been widely investigated for the simple structure, low-cost, and ease of processing. The required voltage signal to sense the capacitance change during pressure sensing can also be very low, resulting in negligible dynamic power consumption ideal for wearable applications. Among various pressure sensing dielectric materials for capacitive pressure sensors, polydimethylsiloxane (PDMS) elastomer is a promising material of choice, for its excellent flexible and elastic properties and biomedical compliance with human tissues.44 When an external pressure is applied to the elastomeric dielectric capacitive sensors, the induced deformation of the dielectric layer (e.g., reduction the dielectric layer thickness) causes changes of the capacitance to measure the applied force. The sensing performance of such
1. INTRODUCTION Demands of future human-centric electronic technologies for wearable healthcare, patient rehabilitation, and biomedical prostheses have brought profound research interests on flexible pressure sensors, capable of being embedded into clothes,1,2 shoes,3,4 or directly wrapped on skin surfaces5,6 for real-time monitoring of diverse human body motions and health/ wellness. To satisfy the critical requirements of these applications, flexible pressure sensors with high sensitivity in the wide pressure range, low voltage and low power consumption are required. In the past, improving the sensitivity to detect weak pressure signals has been the research focus. For this purpose, sensor devices in various structures including resistors, capacitors, and field-effect transistors (FETs) have been developed using nanostructure materials,7−14 elastomeric polymers and their composites,15−20 predefined micro- or nanostructures.21−50 However, although some of these sensors presented very high sensitivity, they were operated within a very limited detection range in the low pressure regime and required complicated fabrication procedures, which are difficult to be scaled up. Moreover, large dc current consumed in some reported resistive type sensors,8,20,22,34,37 and high operation voltage of a few tens volts required to operate the FET based © 2016 American Chemical Society
Received: April 30, 2016 Accepted: July 18, 2016 Published: July 18, 2016 20364
DOI: 10.1021/acsami.6b05177 ACS Appl. Mater. Interfaces 2016, 8, 20364−20370
Research Article
ACS Applied Materials & Interfaces devices is thus determined by the mechanical properties of the elastomer dielectric layer. Unsatisfactorily, the PDMS film, typically having a modulus of ∼1 MPa, cannot provide acceptable sensitivity for reliable sensing in the low pressure regime (e.g., in the
