Research On Stencil Lithography Of Liquid Metal On PDMS Substrates Based On Pre-stretching

Gallium-based liquid metals have many advantages,such as high electrical conductivity,low viscosity,low melting point and low toxicity,and have recently been used in many fields.Especially,owing to the intrinsic stretchability,gallium-based liquid metal wires fabricated on a flexible substrate,such as polydimethylsiloxane(PDMS),have been increasingly used for flexible and stretchable electronics.Till now,several methods have been developed for patterning liquid metal wires including microfluidic injection,vacuum filling,direct-write,laser patterning,selective surface wetting and stencil lithography.Compared with other methods,stencil lithography is a simple,universal and high throughput method.However,the resolution of stencil lithography for patterning liquid metals is relatively low,typically larger than 200 ?m.Therefore,a high-resolution stencil lithography method based on a pre-stretching process for patterning liquid metal wires on PDMS substrates is proposed.First,the PDMS substrate was axially stretching several times its original length.Second,a stencil mask was covered on the surface of the stencil mask,liquid metal was cast on the surface of the stencil mask,and a portion of liquid metal flowed through the stencil mask and formed the wire pattern on the PDMS substrate.Third,excess liquid metal on the surface of the stencil mask was removed.Fourth,the PDMS substrate was released and restored back to its original length,and the width and spacing of liquid metal wires on the PDMS substrate were deceased the same times.Hence,high-resolution liquid metal wires patterned on the PDMS substrate can be achievedTwo issues have to be dissolved when liquid metal wires are patterned on PDMS substrates by the proposed method.One was the low adhesion between liquid metals and PDMS.To enhance the adhesion,the surface of the PDMS substrate was modified by UV-ozone before patterning liquid metals.The other was the removing of excess liquid metals on the surface of the stencil mask.Accordingly,a simple method was put forward.The PDMS substrate and the stencil mask were immersed in NaCl solution,and applying an external electric field can easily remove excess liquid metals.The resolution that the proposed method can be achieved was systematically studied.First,the stretchability of PDMS directly affects the resolution;in order to improve the stretchability of PDMS,the curing parameter and the ratio of the PDMS prepolymer and curing agent were optimized.When the weight ratio was 30: 1,a PDMS substrate could be easily stretched 3 times its original length.Second,the effect of the pre-stretching process of PDMS substrates on the resolution was investigated.The experimental results showed that the times that the width and spacing of liquid metal wires were decreased were approximately equal to the times that the PDMS substrate was pre-stretched.Hence,the pattern of liquid metal wires can be precisely controlled.Third,the effect of the thickness of the stencil mask on the resolution was analyzed.Liquid metal wires with a resolution of about 30 ?m were successfully fabricated when the PDMS substrate was pre-stretched 3 times and a stencil mask with a thickness of 10 ?m was used.Finally,to demonstrate the proposed method in this study,a flexible strain sensor for monitoring the movement of fingers was fabricated and tested,and the sensor exhibited excellent stretchability,stability and sensitivity.