Integrated optoelectronic microprobes.

Optogenetics opened not only new exciting opportunities to interrogate the nervous system but also requires adequate probes to facilitate these wishes. Therefore, a multidisciplinary effort is essential to match these technical opportunities with biological needs in order to establish a stable and functional material-tissue interface. This in turn can address an optical intervention of the genetically modified, light sensitive cells in the nervous system and recording of electrical signals from single cells and neuronal networks that result in behavioral changes. In this review, we present the state of the art of optoelectronic probes and assess advantages and challenges of the different design approaches. At first, we discuss mechanisms and processes at the material-tissue interface that influence the performance of optoelectronic probes in acute and chronic implantations. We classify optoelectronic probes by their property of delivering light to the tissue: by waveguides or by integrated light sources at the sites of intervention. Both approaches are discussed with respect to size, spatial resolution, opportunity to integrate electrodes for electrical recording and potential interactions with the target tissue. At last, we assess translational aspects of the state of the art. Long-term stability of probes and the opportunity to integrate them into fully implantable, wireless systems are a prerequisite for chronic applications and a transfer from fundamental neuroscientific studies into treatment options for diseases and clinical trials.

Investigations on different epoxies for electrical insulation of microflex structures.

The microflex interconnection (MFI) technique is often used to connect electrically and mechanically thin film ribbons or electrodes with a solid substrate like screen printed ceramics. For stabilization reasons epoxy is used to fix the MFI structure. As epoxy tends to form cracks when surrounded by water or electrolytes we are eager to find an epoxy which provides sufficient insulation between the single channels of the MFI structure also in a moist surrounding. Therefore we designed a device to investigate the insulating properties of different epoxies (Uhu Plus Endfest 300, Epo-Tek 353ND and 353ND-T) immersed in saline solution. For comparison reasons we use as well only silicone rubber (Nusil MED-1000) instead of epoxy. We performed the experiment for 23 weeks at 60 °C, which corresponds to 26 months at body temperature. The epoxy of preference is the Epo-Tek 353ND-T as it develops no failures and insulates all channel pairs of the MFI structures electrically over the whole period of experiment.

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.