Research On Fabrication And Properties Of Flexible Self-powered E-skin Based On Polyacrylamide Ionic Hydrogel

Artificial electronic skin?e-skin?,which mimics the characteristics properties of human skin,has attracted extensive attention due to its broad application prospects in wearable electronics,medical monitoring,and artificial intelligence.Owing to the accompanying huge challenge of energy supply which severely restricts the flexibility and stretchability of the e-skin,this thesis developed a flexible transparent self-powered e-skin based on polyacrylamide?PAAm?ionic hydrogel-elastomer structure.Utilizing the ability of ionic hydrogel to couple ion current and electron current,the self-powered e-skin integrates the energy harvesting function and sensing function into one device.To ensure the performance of the device in extreme environments and the longevity of long-term storage,the adhesion between hydrogel and elastomer was improved by functional silane coupling.The ionic hydrogel was prepared by adding Li Cl and sodium alginate?SA?into PAAm hydrogel,referring to as PAAm-SA-Li Cl hydrogel.Ultrahigh transparency?average transmittance of 89.39% in the visible range?,excellent mechanical properties(?_c?14,E=15.85 k Pa,?=687.26 J·m^-2)and extremely low resistivity?0.13 ?·m?are achieved by the PAAm-SA-Li Cl hydrogel.Besides,the ionic hydrogel shows remarkable anti-dehydration performance,which retained 90.8% of its initial weight after exposed to the indoor environment?25 ?,RH=20%?for 3 days.The flexible and transparent PDMS e-skin and VHB e-skin were fabricated by wrapping and sealing the PAAm-SA-Li Cl hydrogel with polydimethylsiloxane?PDMS?films and 3M VHB?Very High Bond?tape films.Both the PDMS e-skin and VHB e-skin show good stretchability,which can be stretched up to 250% and 800% of the original length respectively.Single-electrode triboelectric nanogenerator?TENG?structure enable the e-skin to achieve mechanical energy harvesting.When nylon was used as dielectric friction layer,the soft VHB e-skin is capable of outputting an open-circuit voltage of up to 100 V,a short-circuit current of up to 1.5 ?A and a short-circuit charge quantity of up to 23 n C,while the corresponding values of the PDMS e-skin are 150 V,2 ?A and 45 n C.Compared with the VHB e-skin,the PDMS e-skin has better cyclic output capability,which shows no obvious degradation of output performance after ?3000 cycles of repeated contact-separation motion relative to a nylon film or human skin.The PDMS e-skin can realize strain and stress sensing of various principles,based on the electrical characteristics of the ionic hydrogel-elastomer structure.The ability of TENG to convert mechanical energy input into voltage signal enable the PDMS e-skin to sense external pressure from 1.5 k Pa to 60 k Pa and achieve a sensitivity of 0.017 k Pa^-1.Considering that the effective interface adhesion between the ionic hydrogel and the hydrophobic PDMS elastomer cannot be achieved,this thesis adopted oxygen plasma treatment and silane coupling treatment to modify the surface of PDMS.The oxygen plasma treatment can generate a hydrophilic layer on the surface of PDMS and improve the interface toughness between PDMS and hydrogel from 0.4 J·m^-2 to 3.0 J·m^-2.However the weak bond based on oxygen plasma treatment is not reliable and will completely fail after a period of time.By contrast,the functional silane modification can achieve effective adhesion between the PDMS and the hydrogel by chemically anchored,which can increase the interfacial toughness to 31.2 J·m^-2.The PDMS e-skin with enhanced interface shows significant improvement in anti-dehydration performance,which retained 97.6% of the initial weight after exposed to the indoor environment?25 ?,RH=20%?for 108 hours,while the untreated device retained 88.3% under the same condition.In addition,compared with unimproved PDMS e-skin,the improved device shows better electrical output durablity and outstanding storage life,which can still maintain stable peak output performance after ?5000 cycles of repeated contact-separation motion or 20 days of indoor storage.The improved PDMS e-skin with stable output performance and durability,allows energy harvesting and tactile sensing without interference of optical information's delivering and could thus have potential in fields such as soft robots and wearable flexible electronics.