Fused silica microlenses for hermetic packages as part of implantable optrodes.

The request for stable and reliable devices is tremendous in the field of optogenetics. So far, no device which is called optrode, encapsulating the needed light source hermetically, can be found. We therefore introduce a novel optrode concept consisting of polyimide, silicone as well as a silicon- and fused silica-based hermetic package. One of the main features of the hermetic package is the integration of custom-made microlenses. These microlenses are fabricated using thermal reflow of photoresist. Chosen parameters for remelting the photoresist AZ9260 are 2 min @ 160 °C. An additional dry etching step is introduced to transfer the resist pattern into a fused silica substrate. We were able to fabricate lenses in diameters ranging from 25 µm to 1300 µm. The focal lengths of the etched lenses vary from 630 µm to 5500 µm for lens diameter ranging from 200 µm to 900 µm. Deviations of the transferred pattern to an ideal sphere range from 0.055 % and -0.151 % to 0.040 % and -0.003 % (300 µm and 700 µm lens diameter) and can be neglected.

Mechanical deformation of thin film platinum under electrical stimulation.

Thin-film-based electrodes used to interact with nervous tissue often fail quickly if used for electrical stimulation, impairing their translation into long-term clinical applications. We initiated investigations about the mechanical load on thin-film electrodes caused by the fact of electrical stimulation. Platinum electrodes of Ø 300µm on a polyimide carrier were subjected to approximately 50 000 asymmetrical, biphasic stimulation pulses in vitro. The electrode's surface was investigated optically by means of white-light interferometry. The structural expansion for the metallic surface subjected to stimulation was measured to reach roughly 30%. The study points towards a failure mechanism of thin-films being of mechanical nature, inherent to the unavoidable electrochemical processes involved (change in lattice constants) during electrical stimulation at the electrode's surface. Based on further scientific facts, we set 3 hypotheses for the exact mechanisms involved in the failure of thin-films used for electrical stimulation, opening a new door for research and improvement of novel neuroprosthetic devices.