Hybrid implantable cannulas
For example, some optogenetic experiments require injecting viral material to form photosensitive proteins in target cells and subsequent illumination through the tip of the optical fiber. Doing this as a two-stage process with two different implantable devices may compromise the accuracy of the experiment. In this case, hybrid cannulas are widely used for injecting optical fiber along with liquid supply tubes (Fig. 3), which provides a greater probability of illuminating infected cells .
Implantable cannulas combining optical fiber and an implantable electrode are used for mixed optogenetic experiments involving prolonged electrophysiological stimulation of freely moving laboratory animals. This configuration ensures the simultaneous and selective introduction of stimulating tools into the experimental area . The simplest version of such a cannula is shown in Fig. 4.
Optical fiber and cable are connected through the combiner 8 into a common implantable tip 9. Optical (2) and electrical (3) contacts with the output parameters set to match the experimental purpose are fixed to the combiner by clamping rings 4, 5. The tip 9 of the device is injected into the TAPI-1 tissues of a laboratory animal.
Implantable optical electrode arrays
Another interesting mechanism involves simultaneous light delivery and data readout using the so-called optrodes, which are a device containing an optical stimulator (a glass light guide of 100–120 μm in diameter) and a glass-isolated platinum or tungsten microelectrode (of 60–80 μm in diameter). An optrode records electrical activity in optogenetic experiments. A combination of several microelectrodes allows to simultaneously record the activity of several neurons in the illuminated area. The effect from light scattering inside the tissue and the position mismatch between the light source and the detector recording the neuronal excitation/inhibition parameters are minimized .
As the constructions of implantable cannulas and electrodes were further enhanced, they were minimized and combined in a single device to readout a spatial and temporal picture of light activation or inhibition of a group of neurons. The device itself can be placed in specific areas of cortical or subcortical structures of the brain of freely moving laboratory animals in in vivo experiments .
The implant consists of a conical coaxial optical waveguide (optrode) integrated inside an implantable multi-electrode array (МЕА) to readout the experimental data (Fig. 5) [9,11,12].
An important function of optical electrode arrays is studying the spatial and temporal distribution of optically generated excitation waves .
Other medical and biological options of using optical light guides to stimulate excitable tissues
Attempts to restore the lost motor functions are of key importance for traumas or infectious neuromuscular diseases. Programmable electric pulse sequences are commonly used for this. Larger nervous fibers are more sensitive to electrical stimulation, which is why muscles have a tendency of contracting in the wrong order: first large, fast-twitch fibers, then small, slow-twitch ones. This causes convulsive movements, and as a result, fatigue follows quickly. Using optical stimulation methods, in particular, the above-described implantable optical electrode arrays makes it possible for light to penetrate deep into the nerves and ensures that all fibers comprising the nerve will receive adequate stimulation by short LED pulses. As a result of this stimulation, contractions in nervous fibers will therefore occur in the correct order, including the contractions similar to those that happen under normal conditions .
Another intriguing development is the possibility of opsin-encoding genes transfected into striated muscle tissue. This would allow to bypass nerve cell stimulation and directly stimulate muscle cells. However, this direction presents a significant complication in that the vertebrate retinal is separated from opsins and diffuses into other cells, such as retinal epithelial cells. As for invertebrates, their chromophore remains in place and is capable of repeated transformations from cis to trans without separating from the opsin. The African clawed frog (Xenopus laevis) is an interesting example of this, as its melanopsin is similar in this regard to invertebrate opsin and is largely suitable for experiments .
Hybrid implantable cannulas