Design Of Stretchable Silicone Non-Aqueous Gels And Their Applications For On-Skin Wearable Electronics

Flexible wearable electronic devices can operate normally and reliably under dynamic bending and even stretch conditions,which have been widely applied in personal health monitoring and management,portable energy harvesting,soft robots and human-machine interface interactions,attracting great attention from researchers.Among the flexible wearable electronic devices,skin-adherent wearable electronic devices are one of the important applications,which can be attached to human skin to collect signals and transmit data.Currently,there are two approaches to manufacturing skin-adherent wearable electronic devices.The first approach is to achieve structural flexibility,and the second approach is to find intrinsic soft materials.Achieving structural flexibility includes developing one-dimensional electronic fibers,two-dimensional films,and fractal designs.Intrinsic soft materials mainly include silicone elastomers and hydrogels.Silicone elastomers have good stretchability and low dielectric constant,and are common flexible electronic packaging materials.In comparison,hydrogels are even softer,with a Young’s modulus matching the modulus of human skin（0.5-2 MPa）,but hydrogels still face problems such as poor long-term water retention,poor resistance to high and low temperatures,and inability to directly integrate circuits.The research objective of this paper is to develop a skin-adherent wearable electronic material that is intrinsically soft,stretchable,has high water retention,low dielectric constant,and combines the advantages of hydrogels and silicone elastomers.（1）A network is formed by cross-linking the molecular chains of silicone polymers.After comparing the volatility,surface energy,and compatibility of different solutions with the polymer network,polydimethylsiloxane（silicone oil）inert liquid is finally selected as the swelling liquid to prepare a non-aqueous gel with a maximum liquid content of 92.7 wt%.The non-aqueous gel has good anti-freezing and anti-drying ability.It remains transparent and stretchable at-20℃and has a 100%liquid retention rate at 120℃.The stretching stress,strain,and Young’s modulus of the non-aqueous gel were studied in detail by controlling the content of silicone oil.As the proportion of swelling liquid increases,the stretching ratio of the non-aqueous gel increases.The effect of silicone oil viscosity on the mechanical strength of the non-aqueous gel was also explored,and the experimental results showed that the mechanical properties were better when 1000 cst silicone oil was used as the swelling liquid.（2）Based on the successful preparation of the non-aqueous gel,the polymer topology network structure of the non-aqueous was analyzed.Starting from the polymer swelling process and the uniform cross-linking of molecular chains,the cross-linking network of the polymer was reconstructed.The problem of poor mechanical properties caused by small network defects of the polymer was mainly solved by solvent displacement strategy,and silicone gel with both flexibility and high tensile strength was successfully produced.The mechanical properties of the solvent-displaced gel were tested,and its fracture strain was 3.6 times that of untreated silicone rubber,and its tensile strength was significantly improved.（3）Using non-aqueous gels for the encapsulation of wearable electronic devices,the dielectric properties of the non-aqueous gels were tested first.The electrical conductivity of the non-aqueous gel did not increase with increasing frequency.When the frequency reached 80,000 Hz,the electrical conductivity of EO1000（85%）was1.98×10^-7S/m,proving its excellent suitability for the encapsulation of electronic components.This paper also studied how to design conventional conductive metal wires as fractal structures,making it possible for rigid metals to become stretchable.The metal material was embedded in the non-aqueous gel as a stretchable circuit to create a flexible electrode that had good skin adhesion and fatigue resistance,which is particularly suitable for wearable electronic devices that require high skin fitting,such as real-time monitoring of human heart rate signals.In order to functionalize the non-aqueous gel,nano-Ti O₂was used to composite with the non-aqueous gel,which effectively blocked UV radiation.In addition,electronic components and flexible circuits were integrated into the nano-composite gel to build a UV monitoring alarm device,proving that non-aqueous gel materials have broad application prospects in the field of wearable electronics.