High density electrical interconnections in liquid crystal polymer (LCP) substrates for retinal and neural prosthesis applications

Retinal prostheses implemented by means of electrical stimulation of retinal ganglion cells have been previously demonstrated with 16 and 60 channel microstimulator arrays. Blind patients with severe retinal degeneration (e.g., retinitis pigmentosa (RP) have been able to use these devices to navigate and read large letters. However, to dramatically improve the effectiveness of such prostheses, and to enable a variety of neural stimulation implants, channel densities of 1000 per cm² and higher are highly desirable. This paper reports on a novel approach to an integrated bioelectronic package with high density electrical feedthroughs, capable of 1024 stimulator channels in a 5 mm × 5 mm area using liquid crystal polymer (LCP) substrates to enable implantable retinal prostheses. A novel fusion bonding process was demonstrated to achieve fine pitch interconnections with high adhesion strength and biocompatible metal-polymer interfaces. Helium leak rates of 1 × 10^-9 mbar-1/sec were measured for LCP samples without feedthroughs, representative of penetration through the bulk LCP film, and leak rates of <; 5 × 10^-8 mbar-1/sec were measured for feedthrough array samples, comparable to leak rates demonstrated for glass substrates with metallized vias.