Development of Flexible Multilayer Circuit Board Fabrication Technology by Combining Laser Micromachining with Platinum Foil and Microwelding for Biomedical Applications

Implantable devices utilize flexible substrates due to their ability to conform to the complex shapes and movements of living organisms. However, these devices have a limit to the available power and necessitate low-resistance wiring to handle the extra power consumed on the substrate side. Moreover, complex wiring technology with multilayer wiring is essential to enhance the functionality of implantable devices. Previous studies primarily employed sputtering and printing methods for the fabrication of flexible substrate wiring. However, the wiring resistance is tens to hundreds Ω, causing large power loss. In this study, we aimed to create flexible, low-resistance multilayer circuit boards with resistance less than 1 Ω for implantable devices. Polydimethylsiloxane was used as the substrate material, while platinum was used as the wiring material. The wiring pattern was formed by laser micromachining using an ultrashort pulse laser, and interlayer connections were achieved using Pt Vias fabricated by microwelding. The multilayer circuit board fabricated low impedance wires of 0.25 Ω or less. Furthermore, the wiring demonstrated excellent insulation between wires, even after a 3-hour exposure test in a simulated biological environment, with no short-circuiting issues. Regarding mechanical properties, no significant changes were observed after subjecting the circuit boards to 500 cycles of repeated bending in a bending test with a 6-mm radius. In conclusion, these results indicate that the flexible multilayer circuit boards are well suited for various implantable devices that require low resistance.