Infrared neural stimulation (INS) is a novel technology that holds promise for increasing the number of independent channels in neuroprostheses. A complementary technology to electrical stimulation (ES), INS has been used in the cochlea, vestibular system, peripheral nerve, brain, and other excitable tissues to provide precisely targeted, artifact-free, stimulation of nerves. The long term goal of the proposed research is to improve the performance of neural prostheses. For instance, cochlear implants have been very successful in restoring hearing, but still face limitations in terms of their performance in noisy environments or the ability of the user to enjoy music. This is primarily due to the limited number of effective frequency channels resulting from electrical cross-talk between electrodes. The preliminary feasibility and safety of neuroprostheses based upon INS has been demonstrated by Northwestern University and Lockheed Martin by coupling light into the cochlear spiral ganglion of cats with a remote laser and optical fibers. However, the practical implementation of INS requires substantial miniaturization, with the key missing piece being an optical source that can be incorporated into an implantable device. Vertical Cavity Surface Emitting Lasers (VCSELs) hold great promise for providing the combination of size and performance that are required. The Phase I project will therefore establish the feasibility of the VCSEL for meeting the optical power, power efficiency and physical size requirements. A Phase II project would develop suitable packaging for implantation and perform a proof-of concept demonstration of efficacy and safety in animals. Vixar has demonstrated the highest power 1860nm VCSELs to date, but a gap remains between the status and the requirements for INS. By combining several new design features of the VCSEL we expect to achieve a 50mW output power per channel, a power conversion efficiency of >25 percent with short pulses, in a chip size of 0.25mm x 0.25mm x 0.125mm. These original design features include the choices of epitaxial layers structures and mask features that provide improved current capture and confinement for conversion to light, optimized mirror structure to allow for greater emission efficiency of generated light, and a tailored contact and layer doping profile to reduce electrical resistance. This Phase I project will therefore establish the feasibility of using a VCSEL device in INS, setting the stage for the Phase II project, which will address thermal management, packaging and the proof-of concept.
Public Health Relevance: This research will develop a miniaturized IR laser that will form the basis of optical stimulation based neuroprostheses, with dramatically improved spatial selectivity. The size, power and efficiency of the device would make possible implantable devices which overcome some of the issues of electrically based stimulation devices, for instance current spreading that limits the ability to select a particular nerve, or limits the numbr of independent channels in cochlear implants. In the example of cochlear implants, this innovation would improve the quality of the implants, particularly in noisy environments or in appreciating music.
