Optical Fiber Arrays for Signal Splitting and Neuronal Recording
Optical fiber arrays are used in various applications including signal splitting. Typically the data is split from one source to many outputs using planar waveguide circuitry.
This requires a very precise alignment of the transmitting and receiving fibers. Historically this has been achieved by fire-sharpening the tips of the fibers. This is a slow and error-prone process.
Optical Performance
The optimum choice of optical fibers for this type of application must be taken into account to ensure the best possible performance. This includes both the required wavelength range and the modal dispersion (M-D). In addition, the mechanical properties of the fiber must also be suitable for this purpose. This is a critical requirement for 2D arrays in particular because the two ends are often inserted into a connector and can therefore experience significant mechanical stress.
For this reason, the insertion loss of the fibers should be as low as possible and the polarization mode dispersion should be as low as possible. To achieve this, the end face of the optical fibers must be polished to a very high level of surface quality. It is also a good idea to use a material that is as thermally stable as possible and has a low expansion coefficient. Glass, for example, is an excellent choice.
Another important factor is the method of coupling the fiber array to the IO device. This can be done either by surface coupling or by using a stepping block. Surface coupling is the preferred method because it results in a very low insertion loss and very little reflection, but it requires precise alignment of the optical fibers and a very high-quality polishing of the IO chip surface. In contrast, the stepping block method is much simpler and easier to process.
Biocompatibility
Obtaining neuronal activity over extended periods of time is a crucial goal in neuroscience, but this requires an invasive approach by implanting neural probes into living brain tissue. Current optical waveguide materials have a Young’s modulus far higher than the one 64-fiber-array of brain tissue, making it difficult for them to embed themselves in the brain. Furthermore, the rigid shuttle devices required to insert and retract such probes cause extra damage.
A more suitable material is carbon fiber electrode arrays, which are thinner and more flexible than metal wires or silicon probes. They produce a lower immune response in the CNS10,21,22 and can be produced with high channel counts using automated assembly methods23,24. However, the scalability of such systems is limited by the low number of channels that can be recorded at once.
Natural silk is a promising candidate for future brain implants because of its transparency and mechanical strength25,26. It also exhibits biocompatibility, being able to dissolve under physiological conditions28,29.
In this study, we designed and fabricated an epiretinal microelectrode array with a substrate made of PSPI and platinum (Pt) electrodes. The biocompatibility of the array was evaluated in vitro by an MTT assay and by direct contact tests between the electrode surface and cells. The results showed that the PSPI layer did not affect cell viability and that electrochemical impedance characteristics were appropriate for epiretinal stimulation. In addition, chronic implantation of the non-functional array into the eye of rabbits for up to six months showed no signs of retinal injury or mechanical compression of the Pt electrodes.
Electrical Performance
Neuroscientists strive to understand how neuronal activity correlates with behavior and perception, but recordings in freely-behaving animals have always been limited by the invasiveness of electrodes. Unlike metal wires and silicon probes, carbon fiber (CF) electrodes are thinner and more flexible, and they produce less immune response upon implantation10.
However, previous multi-channel carbon-fiber arrays had low channel counts and required time-consuming preparation of each recording site by fire-sharpening individual electrode tips.30 Moreover, these arrays used connectorized coupling between the CF and the device, which can introduce additional loss and reflection at the interface.
A key challenge in making multi-channel CF arrays is to achieve high steering efficiency, i.e., high directionality, with a relatively large distance between the electrodes. The maximum possible number of CF electrodes in an array is constrained by the minimum distance between two adjacent fibers. With standard SMF fibers having a cladding diameter of 125 m, the maximum number of CF electrodes in an array can be around 32.
To overcome this limitation, we adapted a 2-dimensional liquid-crystal spatial light modulator (LC-SLM) for telecommunications uses and applied it to our fiber-array platform. The LC-SLM has a silicon backplane with 12801024 pixels and operates at 1.55 m wavelength. The LC-SLM is placed on top of a flat V-Groove chip, which is optically polished and coated with antireflection coating (ARC). The end face of the LC-SLM can be coupled to the CF-array via a surface coupling method.
Reliability
Our team has developed a new approach to chronically recording and optogenetically manipulating neural population dynamics in freely-moving animals. This approach uses micro-fiber arrays to capture multiunit activity, enabling the investigation of naturally occurring patterns of dynamic interactions in 3D volumes with unprecedented spatial coverage and resolution.
Recordings of neuronal population activity are critical for understanding brain function and behavior. Current electrodes allow us to monitor these dynamics in brains of head-fixed mice, but are bulky and stiff, causing tissue damage. We have been working on an alternative to these traditional metal wire or manufacturing fiber optic passive components silicon probes, which is much thinner and more flexible and generates a lower immune response after implantation. Carbon fiber (CF) electrodes are one such option. Previous designs of CF electrodes had low channel counts and required time-consuming manual preparation of individual recording sites by fire-sharpening30.
The new 64-channel CF array described here offers high channel count, fast assembly, and compatibility with micro-drive devices. It consists of a tiny 16-channel plastic block holding the CFs, and a breakout PCB that holds four mating 20-pin Hirose connectors on one side and a 70-pin DF40 connector (Digi-key, H11618CT-ND) on the other.
The CFs are positioned in a precise V-groove on the PCB and their ends are connected to the breakout board via a pressurized contact system. HYC uses professional production processes for the FAs, which offer a wide range of options with regard to the number of channels, core spacing and core grinding angle.
