Micro/nano Cracks-based Flexible MEMS Pressure Sensors For Wearable Device Applications

Wearable device is a type of portable consumer electronic product with specific functions,which can be used for monitoring human health?such as heart rate,pulse,sleep quality,etc.?,recognizing robot gestures and even speech patterns.These features benefit from various flexible sensors integrated inside it.Flexible pressure sensor is one of the most core components of wearable device.Because of its light weight,easy processing,low cost,wide application and many other advantages,it has attracted wide attention.Flexible pressure sensor generally consists of a flexible substrate,a sensing layer,a dielectric insulating layer and electrodes.The core sensing layer determines the sensor's performance.Sensitivity is the most important reference indicator for judging the pressure sensor's performance.The working range determines the applicable scene of the flexible pressure sensor.The shorter the response time,the smaller the response hysteresis of the sensor to the external pressure stimulus.And the repeatability determines the sensor's stability.In order to improve the sensitivity and relevant performance of the sensor,a great number of explorations have been made on structural design and material selection of the sensing layer.Interestingly,some researchers have utilized cracks that have been seen as defects.Once the sensor is under pressure,micro/nano cracks will generate in the structural layer,leading to resistance's rapid increase and high sensitivity.However,in terms of cost or process complexity,the sensing layer materials for cracking reported have disadvantages,limiting their large-scale applications.Besides,there is also room for improvement in structural optimization design of the sensing layer.Herein,by MEMS micromachining process,two flexible pressure sensors were fabricated successfully,including one based on micro/nano cracks of patterned electroplated Ni film and the other based on photoresist micro/nano cracks filled with composite conductive network.As for the former flexible pressure sensor,the test results show that it can reach the sensitivity_max of 24.2482 kPa^-1 at 30.84 kPa and can respond to pressure quickly?response time is¹8 ms?.It has a lower cost than the traditional flexible pressure sensor which uses cracking of Pt film for sensing.In the preparation of the second flexible pressure sensor,the positive photoresist film's directional cracking is achieved by introducing pattern with tips into the film and changing the sample temperature suddenly.The resulting micro/nano cracks?width:300 nm⁶00 nm?connect the isolated units together into a microchannel network.By electrodeposition technique,Ni and multilayer graphene oxide were filled in the microchannel network to form the“Ni/multilayer graphene oxide/Ni”sandwich structure as the sensing conductive network layer.When the sensor is under pressure,strain effect of the bottom Ni film,sliding and misalignment of the intermediate multilayer graphene oxide,and nano cracks generated by the top wrinkle thin Ni layer can jointly enhance the resistive effect of the sensor and improve its sensitivity.Test results show that the flexible MEMS pressure sensor can achieve sensitivity_max of 4953.15 kPa^-1 at 52.27 kPa,which is better than many sensors reported previously.Response time of the sensor is approximately 47 ms.In addition,the sensor's excellent stability was verified by the repeatability test result after more than 11000 cycles.Finally,the flexible pressure sensor was applied for demonstration experiments,and results of pulse,smile and swallowing test have demonstrated its potential for application in the fields of wearable devices and health monitoring.