Flexible Nitinol-based Implants with Integrated Electrode Arrays for Bioelectric Sensing and Stimulation

Bioelectronic medicine uses smart implants and diagnostic tools to treat diseases and injuries by recording, stimulating or blocking of neural signals. The fabrication of mechanically flexible bioelectronic implants for permanent application in the human body with a high number of individual electrodes is challenging. In this publication, we used superelastic NiTi as a frame structure to fabricate highly flexible devices that are even suitable for minimally invasive catheter applications. These systems are based on a multilayer structure of NiTi/Oxide/Pt/Oxide, deposited and structured by microsystem technology processes, with NiTi acting as the flexible substrate of 1-100 µm thickness and Pt as the electrode layer. The oxide serves as insulation between NiTi and Pt and as top layer encapsulation. Multilayer structures with different oxides were investigated in-vitro with regard to their suitability as permanent or temporary implants with special focus on their electrical characteristics and the electrode performance for sensing and stimulation applications, dissolution in the body and biocompatibility. Among the investigated oxides are SiO_x, YSZ (Yttrium-stabilized zirconia) and TaO_x, which show heterogeneous characteristics regarding the above-mentioned requirements for implants and diagnostic tools. Structures with YSZ exhibit a good overall performance, in particular concerning their stability in body fluids and bioelectric stimulation capabilities. Advantages and disadvantages of the different oxides and the future potentials and challenges of this technology are discussed.