An optoelectronic neural interface approach for precise superposition of optical and electrical stimulation in flexible array structures.

Optical stimulation of genetically modified nerve cells has become one of the state-of-the-art methods in neuroscience. This so-called optogenetic approach allows cell-type specific activation in comparison to more generalized electrical stimulation. Combinations of both stimulation modalities would be desirable to investigate effects in detail and specify differences. This work presents the design of a miniaturized optoelectronic device that allows optical and electrical activation at the same spot. Indium tin oxide (ITO), which is transparent to visible light, has been chosen as electrode material. Light emitting diodes were assembled on a polyimide substrate with integrated interconnection lines, directly behind the electrodes to compare optical with electrical stimulation. The optical transparency of the ITO-polyimide layer stack was investigated and showed sufficient transmission in the required wavelength range. ITO electrodes with diameters up to 1000 µm were electrochemically characterized using electrical impedance spectroscopy (EIS). Several diameters did show comparable results to platinum, a commonly used electrode material. Fully assembled devices were used in combination an ex vivo setting with genetically modified retina to demonstrate the functionality of this approach. Retinal ganglion cells were excited by both, optical and electrical stimulation at the same spot and signals were recorded via standard microelectrode arrays (MEA) as reference. The simultaneous stimulation and recording of directly evoked action potentials indicates a similar mode of action of the two stimulation modalities. Further engineering work is needed to transfer the presented and proven concept into devices for chronic implantation, might it be in animal or first-in-human studies.

Experimental Characterization of Optoacoustic Phantoms in Gel Wax and Polyvinyl Alcohol for Blood Pressure Measurements.

An experimental evaluation and characterization of two transparent phantoms of arms, to be used as non-invasive experimental models to measure the blood pressure continuously, was performed in an in vitro study. The first phantom was made of polyvinyl alcohol (PVA) gel and the second phantom is based on gel wax. For the first time, a PVA and gel wax phantom for the arm, respectively, were developed, including tissue, radial artery, and ulnar and radius bones. The optoacoustic parameters of various samples for both phantoms were characterized and systematically compared, obtaining a maximum acoustic transmission of T=(78.3±1.9)% at 921 nm in the PVA hydrogel, due to its high transparency and homogeneity. The gel wax phantom possesses similar optical properties as the PVA hydrogel and presents acoustic characteristics similar to those of the soft tissue. Thus, both phantoms are well-suited as in vitro models for the development of new methods in optical-acoustic medical diagnosis.

Implantable Glass Waveguides and Coating Materials for Chronic Optical Medical Applications.

An innovative fabrication process of glass waveguides on silicon substrates for miniaturized implants is presented. Thin glass was bonded on oxidized silicon wafers and patterned using wet etching. Multimode waveguides with different shapes and a low surface roughness as well as low scattering of light were successfully fabricated. For efficient coupling of light and accurate alignment, KOH-grooves were etched in the silicon with respect to the glass waveguides to attach optical fibers from external light sources. Towards higher biostability, several coating materials were evaluated in accelerated in vitro tests in 60°C PBS for the first time over a long period of time regarding their optical properties. Ti02, SiC, polyimide, Parylene C and SU-8 showed a very stable optical transmittance after 320 days in accelerated aging while PECVDSi3N4 showed significant changes within the first days.