Microfabrication process for long-term reliable neural electrode arrays using liquid crystal polymer (LCP)

Abstract

Liquid crystal polymer (LCP) has gained attention as a new substrate material for implantable neural interfaces for its low moisture absorption and permeability. Unlike the conventional polymers such as polyimide, parylene and silicone elastomers, LCP has unique characteristics that it comes in film-type products, multiple films are bonded by thermal pressing, and the optical transmittance is relatively low. Therefore, special considerations during the microfabrication process are required to realize its good barrier property fully as well as high precision and uniformity. A series of microfabrication processes is presented in this study to create implantable neural electrode arrays using LCP particularly focusing on improving its long-term reliability under an aqueous condition, and the effectiveness of each process step is evaluated. A LCP-based generic 25-channel neural electrode array was fabricated as a proof-of-concept with thermoformed curvature for conformably fitting to a target tissue or organ and a surface finish of either electroplated gold or iridium oxide. Electroplated and wavy metal patterning could allow the thermal lamination of a cover layer at higher pressure levels without damaging the metallization, which in turn contributed to strengthening the interlayer adhesion and enhancing the longevity of the device. Accelerated lifetime tests showed that this device survived for 158 days in 87 °C saline, roughly suggesting an equivalent lifetime of 14 years at body temperature. The use of silicone elastomer as a temporary adhesive of LCP film on a carrier wafer provided good flatness comparable to that of a glass wafer. Laser-thinning could effectively thin down a 50 μm-thick array by half and doubled its flexibility. Laser-ablation combined with a special alignment scheme created channel openings without affecting the surface morphology of the exposed metal with uniform impedance values. The proposed microfabrication process can serve as a core technology for various applications of LCP-based biomedical devices.