3D Printing Of Soft Elastomer For Liquid-state Stretchable Electronics

Recently,with the increasing demand for wearables electronics,stretchable electronics has developed quickly,exceeding the restriction from traditional rigid devices.Among various systems,liquid-state devices based on different kinds of functional liquids exhibit unique advantages.Due to the excellent fluidity and deformability of these functional liquids,they are able to endure repetitive deformations including twisting,stretching and bending,while maintaining stable electrical connection,showing their outstanding durability and reliability.Based on different functional liquids,current liquid electronic devices are divided into mainly four types,including conductive ink based devices,liquid metal based devices,ionic liquid based devices,and the devices based on both liquid metal and ionic liquids.The general fabricating methods for liquid-state devices include:(1)photolithography for patterning the required microchannels;(2)transferring the channel pattern to a soft substrate by soft etching;(3)generating the microchannel by bonding the patterned layer with soft encapsulating layer;(4)injecting functional liquids to functionalize the devices.However,the series of preparation methods are not only time consuming but also labor intensive,and it usually generates a variety of harmful wastes.The widespread adaption to such attractive form of device is hindered by the lack of robust fabrication approach to precisely and efficiently assemble liquid-state materials into functional systems.In order to figure out the above issues,this project put forward a direct ink writing platform for the fabrication of three-dimensional elastomeric structures.This article firstly concentrates on the rheological properties required for printing ink,the theoretical theories and implementation strategies for adjusting the rheological properties.The printing characteristics of inks with different rheological properties were also compared.Based on the creation of the ideal printing ink,we employed an environmentally friendly and convenient preparation methods and various elastic features with complex architectures were generated without using sacrificial materials,which consist of overhanging parts,suspended structures,and embedded channels.The development of the printing platform allows facile creation of elastomeric sensors and various complex functional devices with strain-and pressure-sensing capabilities by simply filling the embedded microchannels with liquid metal.The liquid-state stretchable electronics developed here may find potential applications in many advanced areas including biomedical instruments,wearable devices,and soft robotics.The main achievements are summarized as follows:1.Starting from the rheological adjustment mechanism of polymer slurry,we firstly adjusted the rheological properties of liquid silicone elastomer by adding nano-silica particles.After modifying,a viscous silicone ink with high viscosity,strong shear-thinning behavior and high shear yield stress was created.The viscous ink can be directly printed to fabricate self-supporting complicated architectures.Additionally,isotropic mechanical property was demonstrated by measuring the mechanical responses,which also facilitate the design of the printing paths for further applications.2.The 3D printing was further employed to fabricate the soft microfluidic chip with an embedded microchannel.This manufacturing platform makes full use of the unique rheological properties of the viscous ink and make sure that the microchannels will not collapse during printing and cross-linking.The sensors were functionalized after injecting the liquid metal.The resistance of the microchannel was defined by the sensor structure and the conductance of liquid metal,which lays a theoretical foundation for its application as a sensor.3.We further employed the soft microfluidic chip and liquid metal to fabricate strain sensors and pressure sensors: the resistance of the strain sensor increased almost linearly with the tensile strain up to about 50% and its gauge factor was ~2.1.It is still able to operate in applications requiring much higher extensibility.The pressure sensor was prepared consisting of four sensing components forming an equivalent Wheatstone bridge.The additional cavity structure effectively improved the linearity and sensitivity of the pressure.The sensitivity of the optimized pressure sensor was 0.29 k Pa-1,and the detection limit was lower than 50 Pa.4.A wearable smart glove consisting of 5 strain sensors connected in series was successfully fabricated through DIW.The smart glove was capable of capturing various hand gestures by recording the potentials via external circuit,showing the unique advantages in the fabrication of liquid-state stretchable electronic devices.