3D-structured soft bioelectronic devices with crack-free metal patterns

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Three-dimensionally (3D) structured soft electronic devices that fit the contour of target organs or tissues are preferred for the stable implantation of biomedical devices. However, the metal patterns deposited on soft materials, especially polydimethylsiloxane (PDMS), can generate microcracks or disconnections following an expansion of the substrate. A way to generate 3D structured soft devices is using fluid injection into the unbonded area of a selectively bonded 2D structure comprised of PDMS and parylene layers. This work outlines the development of 3D soft bioelectronic devices, including the fabrication processes optimized for stable metal patterns even after substrate expansion via fluid injection. The generation of cracks in metal patterns was significantly affected by the sputtered material for plasma mask during the selective bonding process and the thickness of the intermediate parylene layer used underneath the sputtered metal. An RF-sputtered titanium mask was chosen to create crack-free metal patterns on the intermediate parylene layer based on PDMS substrate. Moreover, the thickness of the intermediate parylene layer was optimized using finite element analysis for stable and intact metal patterns after fluid injection. The developed soft bioelectronic device successfully demonstrated the ability to record and stimulate the peripheral nerve in vivo as cuff electrodes. The optimized processes developed in this study not only enable soft 3D electronic devices applicable to internal organs or tissues with 3D curvatures, but also suggest a promising way to fabricate flexible electronics using conventional micromachining processes such as photolithography and metal sputtering on soft and expandable substrates.