Infrared light activated floating micro stimulators for neuro-prosthetic applications

The most common failures in neural stimulation implants are due to interconnect complications such as tissue response, lead migration, and lead breakages. The challenge in eliminating interconnects lies in minimizing device size to maintain spatial selectivity required in the CNS. One approach to this problem is a current generating device that can be stimulated by an external signal, such as light or sound. Here, we report the design, construction and testing of rnicrophotodiode devices that can be stimulated remotely with near-infrared (NIR) light to generate current that can be injected locally into the peripheral nervous system. The use of near-infrared (NIR) light to activate microphotodiodes was investigated. The chip size of the prototype device is 300μm by 500μm, and the small stimulation area necessitates a contact material capable of delivering a minimum charge injection rate of 0.5 mC/cm2. The charge transfer properties of iridium oxide, platinum, and titanium nitride were analyzed, and titanium nitride was found to have a stable charge injection rate above 0.5 mC/cm2. The volume conductor response of the diode showed a primarily capacitive transfer of energy into the tissue. Three diode geometries were implanted in a peripheral nerve, and an EMG signal was recorded in response to laser stimulation of two diode types. The diodes with the largest active area achieved successful stimulation despite size differences in contact area; this suggests the importance of active area size for stimulation. Further characterization of diode performance in vivoestablished an optimum pulse width for minimum light energy needed for diode activation. This optimum pulse width increased as implantation depth increased. For an implantation depth of 3.5 mm, the energy threshold was 0.53 mJ/cm2 which is 30 times below the maximum permissible exposure for λ = 830 nm. The total energy required for stimulation at a given pulse width increased as tissue depth increased