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An accurate and robust EMG signal recording is necessary in EMG signal-based human–machine interfaces to allow prosthesis control for amputees using their residual muscle. In 2017, this research team previously proposed an electrode device using the kirigami structure for the intimate integration of the electronic device and biological tissues. (Y. Morikawa et al., 10.1002/adhm.201701100).

The remarkable potential of the kirigami structure stems from its high stretchability, including its high strain ratio and the small force required for stretching the device. The kirigami structure can be stretched with a low strain force and its mechanical characteristics are similar to soft biological tissues, such as in the brain and muscles. However, it is challenging to obtain an accurate and robust bio-signal recording without displacement of the electrode. Device displacement occurs when the kirigami device is applied to biological tissues, such as the heart and muscles, which undergo large deformation.

A research team lead by Associate Professor Takeshi Kawano from TUT’s Department of Electrical and Electronic Information Engineering and EIIRIS has developed a donut-shaped kirigami device for EMG recordings to solve the issue of device displacement during muscle deformation.

The donut-shaped kirigami structure is able to transform from a 2D donut shape to a 3D cylindrical shape. The cylindrical shape is suitable for numerous spherically or columnar-shaped deformable biological tissues (e.g. upper limb, lower limb, finger, abdomen, and heart). The donut-shaped kirigami device carries out the fixation mechanism to the target tissues and reduces the device displacement during tissue deformation with minimized stress to the biological tissue. The recording capability of the proposed device was confirmed through EMG signal recording from the hind limb of a mouse, thus raising the prospect of using the device for an EMG-based human-machine-interface in the future.

"The first demonstration using our conventional sheet-shaped kirigami device could not follow the deformation of a beating heart. We discussed the device structure, which enables the device to follow deformable tissues. In the preliminary experiment, we used a paper, which was patterned into the proposed donut-shape of the kirigami by the box cutter, and we demonstrated its stretchable and deformable capabilities for the muscle. However, it was uncertain whether the micro-scale donut-kirigami device would show these device properties or not. We explored them through fabricating the device by using the microfabrication process and device characterizations, and were able to confirm that the fabricated device exhibited the expected deformation beyond expectations," explains the first author of the article, Ph.D. candidate Yusuke Morikawa.

The donut-shaped kirigami device still needs further improvements in terms of durability and the dense array of the microelectrodes. Moreover, the influence of the device implantations on the biological tissues when used for a long period still remains to be clarified. However, it is expected that the proposed device will be applicable to an EMG based human-machine-interface and so contribute to the improvement of the quality of life of amputees.

This work was supported by Grants-in-Aid for Scientific Research (B) (No. 17H03250, No. 16H05434), for Young Scientists (A) (No. 26709024), on Innovative Areas (Research in a proposed research area) (15H05917), and Strategic Advancement of Multi-Purpose Ultra-Human Robot and Artificial Intelligence Technologies program from NEDO. Y.M. was supported by the Leading Graduate School Program R03 of MEXT. R.N. was supported by Takeda Science Foundation. K.K. was supported by Toyota Physical & Chemical Research Institute Scholars.