Acupuncture is a traditional Chinese medicine treatment method. The method is to insert the acupuncture needle into a certain depth of the body at a certain angle to achieve the purpose of treating diseases. In the treating process, clinical doctors often need to rely on their own experience and techniques to deliver needles to specific acupuncture points, and this operation requires high skills of doctors. Objectification and precision are the trends in the development of modernization of traditional Chinese medicine. The development of acupuncture-responsive hypersensitive depth sensors plays a vital role in the quantitative representation of the needle depth, which will contribute to the development of modernization of traditional Chinese medicine. 

In order to cope with this challenge, the Cryogenic Biomedicine Research Group at the Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences discovered a self-assembled deposition method to prepare an electrically anisotropic flexible liquid metal film. Ultra-thin Janus films of 1~49 μm can be successfully prepared based on the density difference of nano-gallium indium eutectic alloy (EGaIn) with crystal nanocellulose (CNC) and polyvinyl alcohol (PVA) during the natural evaporation of the mixed solution (as shown in Fig. 1). The prepared film is not electrically conductive on both sides without any treatment. However, the LM surface (where aggregates a large number of liquid metal micro-nanoparticles) of the liquid metal film can be conductive after applying concentrated vertical stress or shear friction. It is worth mentioning that it takes only 2 ns to achieve the conduction response for every two liquid metal nanoparticles by applying shear friction (as shown in Figure 2). The depth sensor prepared by using the LM Janus film has sensitive depth detection capability, which would help the doctor obtain the depth information of the needle at the first time by visualizing the display lamp during the training process. As a new type of electronic paper, this flexible liquid metal film also provides a new method for the manufacture of multi-layer complex circuits for intelligent micro-scale sensors (as shown in Figure 3). 

Moreover, the research team found that the two sides of the liquid metal Janus film have a significant difference in light reflection and thermal conductivity, which makes the liquid metal Janus film possible to be used as a light conversion switch or a temperature regulator (Fig. 4). The preparation process of such a two-sided anisotropic film with both electrical, optical and thermal properties does not require any complicated process and precision instrument assistance and is not only simple and easy to fabricate but also does not consume too much time, which makes the Janus film possible for mass production and application. 

This research has been published in the Materials Horizons with the title "Self-assembled ultrathin film of CNC/PVA–liquid metal composite as a multifunctional Janus material". (DOI: 10.1039/C9MH00280D). The co-first author of the thesis is Ph.D. candidate Zhang Pengju and associate professor Wang Qian, Professor Rao Wei from the Technical Institute of Physics and Chemistry and associate professor He Zhizhu from China Agricultural University are co-corresponding authors of this article. 

This research was supported by National Natural Science Foundation of China (No. 51605472, No. 51706236), National Key R&D Program of China (No. 2018YFC1705106), Beijing Municipal Science & Technology Commission research fund (No. Z171100000417004), TIPC Director Fund (No. Y6AL021R2X). 

Paper link: 

https://pubs.rsc.org/en/content/articlehtml/2019/mh/c9mh00280d 

Fig.1 The LM Janus film based acupuncture training model used for acupuncture practice.