Browsing by Author "Rojahn, Martin"

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    Encapsulation of a retina implant
    (2003) Rojahn, Martin; Werner, Jürgen H. (Prof. Dr. rer. nat. habil.)
    Novel micro-photodiodes with an open circuit voltage of V = 2.3 V for the in-vitro stimulation of retinal tissue are designed and manufactured in this work. Three material systems for the encapsulation of retina implants are tested in in-vitro cell cultures: (i) SiOx/SiNy, (ii) benzocyclobutene, (iii) polyimide 2611. Surface sensitive X-ray photoelectronspectroscopy (XPS) and volume- and surface sensitive Fast Fourier transmission infrared spectroscopy (FTIR) serve to analyse the chemical composition of the materials; impedance spectroscopy (Z(f)) is employed to observe the electronic behavior of the dielectric materials. XPS and FTIR demonstrate the progressive dissolution of the top SiNx layer of the SiOx/SiNy system by 1...2 nm/d in cell culture media. In contrast, neither XPS nor FTIR show a progressing degradation of the polymers BCB and polyimide after cell culture treatment. Z(f) demonstrates the layer´s high parallel resistance after cell culture tests. However, within the first 24 h of electrolyte treatment after the hot-oven sterilization at a temperature T = 105 C, Z(f) points to a diffusion-like process of electrolyte components into the surface layer of the polyimide. Based on a physical model of the polyimide layer with a columnar-like bulk structure and a high surface roughness, this work develops an electric equivalent circuit for the PI-electrolyte system in the frequency range f = 10^-3 ...10^7 Hz. The diffusion process leads to a smooth interface region, d = 3...10 nm, at the PI's surface where ions or molecules are embedded into the biphenyldianhydride/1,4phenylenediamine (BPDA/PPD) matrix of the polyimide. The "water" uptake does not, however, result in the break up of bonds in the BPDA/PPD backbone structure. As a result of the superior biostability, the polyimide 2611 will be the encapsulation material of the next generation of subretinal implants.