Sensitivity Enhancement of Soft Capacitive Pressure Sensors Using a Solvent Evaporation-Based Porosity Control Technique

Summary

A simple and cost-efficient fabrication method based on the solvent evaporation technique is presented to optimize the performance of a soft capacitive pressure sensor, which is enabled by porosity control in the dielectric layer using different mass ratios of the molding PDMS/toluene solution.

Abstract

Soft pressure sensors play a significant role in developing "man-machine" tactile sensation in soft robotics and haptic interfaces. Specifically, capacitive sensors with micro-structured polymer matrices have been explored with considerable effort because of their high sensitivity, wide linearity range, and fast response time. However, the improvement of the sensing performance often relies on the structural design of the dielectric layer, which requires sophisticated microfabrication facilities. This article reports a simple and low-cost method to fabricate porous capacitive pressure sensors with improved sensitivity using the solvent evaporation-based method to tune the porosity. The sensor consists of a porous polydimethylsiloxane (PDMS) dielectric layer bonded with top and bottom electrodes made of elastic conductive polymer composites (ECPCs). The electrodes were prepared by scrape-coating carbon nanotubes (CNTs)-doped PDMS conductive slurry into mold-patterned PDMS films. To optimize the porosity of the dielectric layer for enhanced sensing performance, the PDMS solution was diluted with toluene of different mass fractions instead of filtering or grinding the sugar pore-forming agent (PFA) into different sizes. The evaporation of the toluene solvent allowed the fast fabrication of a porous dielectric layer with controllable porosities. It was confirmed that the sensitivity could be enhanced more two-fold when the toluene to PDMS ratio was increased from 1:8 to 1:1. The research proposed in this work enables a low-cost method of fabricating fully integrated bionic soft robotic grippers with soft sensory mechanoreceptors of tunable sensor parameters.

Introduction

In recent years, flexible pressure sensors have been drawing attention due to their indispensable application in soft robotics^1,2,3, "man-machine" haptic interfaces^4,5, and health monitoring^6,7,8. Generally, the mechanisms for pressure sensing include piezoresistive^1,4,7, piezoelectric^2,

Protocol

1. Fabrication of the soft capacitive pressure sensor with a porous PDMS dielectric layer

1. Manufacturing of the porous PDMS dielectric layer
   1. Prepare the sugar/erythritol porous template following the steps below.
      1. Filter the sugar with sample sieves with apertures of 270 µm and 500 µm. Choose sugar with a particle diameter in the range of 270-500 µm.
         NOTE: A larger or smaller sugar particle size is also acceptable as long as the uniformity is wi.......

Discussion

This work proposes a simple method based on solvent evaporation to control the porosity, and a series of experimental results have proved its feasibility. Although the porous structure has been widely used in the flexible capacitive pressure sensor, the porosity control still needs further optimization. Unlike existing methods for changing the particle size of the PFA^11,12,13,18,

Materials

Name	Company	Catalog Number	Comments
3D printer	Zhejiang Qidi Technology Co., Ltd	X-MAX
3D printing metarials	Zhejiang Qidi Technology Co., Ltd	3D Printing Filament PLA 1.75 mm
Carbon nanotubes (CNTs)	XFNANO	XFM13
Data acquisition (DAQ)	National Instruments	USB6002
Double side tape	Minnesota Mining and Manufacturing (3M)	3M VHB 4910	1 mm thick
Electrode metal mold	Guangdong Shunde Molarobot Co., Ltd		This metal mold is a round metal plate with a flat bottom round groove and an embossed electrode pattern of 0.2 mm thick in the middle of the groove.
Erythritol	Shandong Sanyuan Biotechnology Co.,Ltd.
Isopropyl Alcohol (IPA)	Sinopharm chemical reagent Co., Ltd	80109218
LabVIEW	National Instruments	LabVIEW 2019
LCR meter	Keysight	EA4980AL
Metal wire	Hangzhou Hongtong WIRE&CABLE Co., Ltd.	2UEW/155
Microscope	Aosvi	T2-3M180
Numerical modeling software	COMSOL	COMSOL Multiphysics 5.6
Polydimethylsiloxane (PDMS)	Dow Chemical Company	SYLGAR 184 Silicone Elastomer Kit	Two parts (base and curing agent)
Sealing film	Corning	PM-996	parafilm
Si wafer	Suzhou Crystal Silicon Electronic & Technology Co.,Ltd	ZK20220416-03	Diameter (mm): 50.8 +/- 0.3 Type/Orientation: P/100 Thickness (µm): 525 +/- 25
Silver conductive paint	Electron Microscopy Sciences	12686-15
Stepping motor	BEIJING HAI JIE JIA CHUANG Technology Co., Ltd	57H B56L4-30DB
Sugar/erythritol template metal mold	Guangdong Shunde Molarobot Co., Ltd		This metal mold is a 5 mm thick square metal plate with a flat bottom square groove of 2.5 mm deep.
Toluene	Sinopharm chemical reagent Co., Ltd	10022819