The Fabrication Of Multi-response Flexible Electronic Skin

Skin is one of the most important organs of human body,and has a variety of unique properties such as high flexibility,stretchability,tactile sensing and self-healing ability.The tactile sensing of skin is an important approach for human to perceive the changes of force and temperature,the shapes and textures of objects and other physical/chemical signals in the external environment.Inspired by human skin,flexible electronic skin(e-skin)that is comparable to the tactile sensing ability of human skin emerged,demonstrating broad prospects in the fields of intelligent robots,wearable medical devices and human-machine interaction.Based on the recent works on electronic skins,we find that researchers all over the world have developed a series of pioneering works.Electronic skins based on different materials,device structures and sensing mechanisms have been developed successfully,demonstrating excellent advantages such as flexibility,stretchability,high sensitivity,fast response and good stability.However,there are still some problems in the fabrication of high performance electronic skins that need to be solved from the viewpoint of practical applications:(1)It is necessary to consider more about the soft substrate materials,for example,fabrication cost,softness,wearing comfort,and biocompatibility.(2)The introduction of hierarchical micronanostructures plays an important role in improving the performances of e-skin devices.Nevertheless,the preparation of micronanostructures usually involves complex fabrication process,strict experimental conditions and high cost.Unlike the mature silicon-based Micro Electro Mechanical Systems(MEMS)technology,the lack of standardized fabrication process for flexible electronic skin has emerged as the bottleneck that restricts its practical applications.(3)At present,most of the research works are focused on the optimization of performance for single-signal detection,such as improving the sensitivity and linearity,while there are few reports on multi-functional electronic skins that can distinguish the responses of multiple signals.The incapability of multi-signal detection limits the practical applications of electronic skins since they cannot obtain comprehensive information.Based on the above-mentioned issues,our research works mainly focus on the preparation of flexible materials,the tunable property of active materials,fabrication of bionic hierarchical micronanostructures and multi-signal response of electronic skins.We have achieved a series of electronic skins that can detect multiple signals such as pressure,proximity,humidity and skin metabolites.The main results are listed as follows:1.Paper-based multifunctional flexible electronic skin.Considering the cost,breathability,and flexibility of substrate materials,we chose flexible copying tissues and porous lens papers as substrates to prepare two kinds of electronic skins,respectively.The former used disc-ring shaped graphite patterns as electrodes,and freeze-dried graphene oxide/paper cellulose fibers composite foam as the elastic dielectric layer,respectively.The resultant capacitive sensor shows high sensitivity and fast response.Meanwhile,the detection of non-contact proximity has been realized through the special design of disc-ring electrode structures.In the latter case,porous lens papers with good breathability were used as both substrates for electrodes and the dielectric layer.After coating gold electrode arrays with the help of a shadow mask,the all-paper-based electronic skin was fabricated.In addition to the detection of pressure and proximity,the porosity of the lens paper promotes the mass transfer and exchange of guest molecules,so that the electronic skin also enables humidity detection.Moreover,after gold coating,a surface enhanced Raman scattering(SERS)substrate formed naturally due to the presence of micronanostructures on the paper fibers,which makes it possible to detect metabolites in human sweat.2.Multifunctional electronic skin based on bionic micronanostructures.Materials in nature have demonstrated perfect hierarchical micronanostructures.Using the surface structures of natural materials as templates is a shortcut for preparing hierarchically structured electronic skins.Taking natural reed leaves and arthropod compound eyes as templates,we prepared polydimethylpolysiloxane(PDMS)substrates with bionic micronanostructures by soft lithography.Then the capacitive electronic skin sensors were fabricated after coating gold electrodes.The sensors not only allow multi-signal detection(including pressure,proximity,and deformation),but also work as SERS chips to detect metabolites in sweat,revealing great potential in frontier fields such as human health monitoring.3.All-graphene flexible MEMS devices.With the help of controllable photoreduction of graphene oxide,we explored the application of reduced graphene oxide in the field of MEMS sensors.Firstly,a controllable photoreduction technology was used to induce reduction gradient in the normal direction of a graphene oxide film.Due to the gradient of oxygen-containing functional groups,the as formed graphene film becomes smart under the actuation of moisture.Subsequently,we used the femtosecond laser direct writing technology to fabricate planar and three-dimensional patterning of reduced graphene oxide circuits,revealing great potential for programmable wiring towards device integration.In the third part,we used the above-mentioned technologies,including the patterning of reduced graphene oxide electrodes and the laser structuring of graphene top electrode,to prepare all-graphene pressure and humidity sensors,demonstrating reasonable high sensitivities towards different signals.