Floating light-activated microelectrical stimulators tested in the rat spinal cord

Abstract

Microelectrodes of neural stimulation utilize fine wires for electrical connections to driving electronics. Breakage of these wires and the neural tissue response due to their tethering forces are major problems encountered with long term implantation of microelectrodes. The lifetime of an implant for neural stimulation can be substantially improved if the wire interconnects are eliminated. Thus, we proposed a floating light-activated micro electrical stimulator (FLAMES) for wireless neural stimulation. In this paradigm, a laser beam at near infrared (NIR) wavelengths will be used as a means of energy transfer to the device. In this study, microstimulators of various sizes were fabricated, with two cascaded GaAs p-i-n photodiodes, and tested in the rat spinal cord. A train of NIR pulses (0.2 ms, 50 Hz) was sent through the tissue to wirelessly activate the devices and generate the stimulus current. The forces elicited by intraspinal stimulation were measured from the ipsilateral forelimb with a force transducer. The largest forces were around 1.08N, a significant level of force for the rat forelimb motor function. These in vivo tests suggest that the FLAMES can be used for intraspinal microstimulation even for the deepest implant locations in the rat spinal cord. The power required to generate a threshold arm movement was investigated as the laser source was moved away from the microstimulator. The results indicate that the photon density does not decrease substantially for horizontal displacements of the source that are in the same order as the beam radius. This gives confidence that the stimulation threshold may not be very sensitive to small displacement of the spinal cord relative to the spine-mounted optical power source.

Near-infrared light penetration profile in the rodent brain

“…2 The main objective of this study was to estimate the light energy that will be collected by the optical microstimulators at various depths of implantation inside the neural tissue. The dark regions in Fig.…”

“…[1][2][3] Microelectrode implants often fail either due to the chronic tissue response caused by the tethering forces of the wires or their breakage. 4,5 A wireless neural stimulator can solve the problems associated with wire interconnects.…”

Temperature elevation profile inside the rat brain induced by a laser beam

“…As a comparison, this optical power is about two orders of magnitude higher than the energy levels required to wirelessly activate neural tissue via photodiodebased floating devices implanted into the rat cervical spinal cord. 6 Nonetheless, the thermal effects should be tested for each specific application since the temperature map depends on many parameters such as the thermal properties of the tissue and the surrounding structures, the optical properties of the CNS region, and the laser beam size and shape.…”

“…6 This suggests that our microstimulators can be implanted deeper in neural tissue without exceeding the temperature limitation on the surface. In a simulation study, the maximum power for the same temperature increase was found as 325 and 250 mW∕cm 2 for the human gray and white matters, respectively.…”

“…Our laboratory is currently investigating the feasibility of wireless neurostimulation in the central nervous system (CNS) using optically activated microstimulators in the NIR wavelengths. [4][5][6] The project's motivation is to solve well-known problems caused by the microwires that connect electrode arrays to the outside world. 7 Microelectrode arrays often fail due to breakage of wire interconnects and the chronic tissue response caused by the tethering forces of the wires.…”