Application Of Kirigami-Structured Liquid Metal Electrode In Multifunctional Electronic Skin

Human-machine interfaces have aroused extensive attention recently due to the ability to exchange information between humans and electronics.Electronic skin is an important human-machine interface that can imitate the mechanical properties and sensing functions of human skin.It has mechanical flexibility and stretchability similar to skin,and can respond to external stimuli in real time,which exhibits prospects in smart robots,virtual and augmented reality,healthcare monitoring,and wearable electronics.The stretchability and electrical conductivity of electronic skin are its most basic properties and are the principal elements to be considered when designing electronic skin.Common electrode materials in electronic skins include metal films,metal nanoparticles,carbon nanotube and liquid metal(LM),etc.The electronic skin constructed from these conductive materials has excellent mechanical and electrical properties and has been widely used in flexible electronics.LM has been regarded as an ideal electrode material to fabricate electronic skin,owing to its extreme deformability,high conductivity,nontoxicity,and recyclability.The current LM electrodes are mainly sealed and printed,however,the LM is easy to leak when the LM electrode sealed with elastomer is stretched,and the LM layer cannot be exposed to direct contact with the skin,which greatly reduces performance in electrophysiological signal monitoring.The printed LM electrodes are difficult to transfer and recycle.Also,the printed LM layer often fractures under stretching of the substrate,which will severely lower its conductivity.Thus,developing a self-supporting and conductor-exposing LM thin film with reliable stretchability is highly desired to solve these problems of conventional LM electrodes.In this paper,we report a new type of LM electrode that we call a kirigami-structured LM paper(KLP),which is self-supporting,conductor-exposing,stretchable,ultrathin,and recyclable for multifunctional electronic skin.The KLP is fabricated by kirigami cutting of a LM composite paper with three types of structures including uniaxial,biaxial,and square spiral.beam theory was used to theoretically analyze the three different kirigami patterns.The stress-strain experiment was carried out with uniaxial KLP,and the results showed that when y=2.5 mm,x=2 mm,and L_c=18mm,the maximum elongation was 140%,and the fracture stress was 300 k Pa,indicating that uniaxial KLP has better performance of stretchability.By testing its electrical properties,it is found that the resistance of KLP remains unchanged in different deformation processes,which is of great significance for signal transmission in practical applications of stretchable electronics.Compared with previously reported LM electrodes,the KLP simultaneously integrated conductor-exposing,self-supporting,stretchable(strain>20%),ultrathin(thickness<50μm),and recyclable advantages.As a multifunctional electronic skin,high-quality electrophysiological signals such as electrocardiogram(ECG),electromyogram(EMG)and electroencephalogram(EEG)can be obtained from the human body.The measured electrocardiogram is comparable to that measured by commercial gel electrodes,showing clear PQRST waves,and the collected electromyogram can reflect the signals generated by muscle movement in real time.The KLP was used to record real-time EEG signals when a human subject engaged in different mental states.The results showed that the EEG signals exhibited significant differences in frequency and intensity in three mental states.The double-layer feature of the KLP enables it to operate as a self-powered electronic skin on the basis of the TENG principle.The experimental results show that under the external force of 6 Hz and 20 N,its open-circuit voltage and short-circuit current reach 540 V and 1.1μA,respectively,and its output performance does not decrease significantly after 3000 cycles of continuous operation.On the basis of the self-powered electronic skin,we further developed a smart dialing communication system,the function of calling a mobile phone can be realized by touching the KLP attached to the human skin.Compared with conventional sealed or printed LM electrodes,the KLP simultaneously integrated self-supporting,conductor-exposing,stretchable,ultrathin,and recyclable characteristics.Such KLPs will provide opportunities for electronic skins in healthcare monitoring and intelligent control,as well as smart robots,virtual reality,on-skin personal electronics,etc.