Intraspinal stimulation with light activated micro-stimulators

Chronic tissue response to microelectrode implants stands in the way as a major challenge to development of many neural prosthetic applications. The long term tissue response is mostly due to the movement of interconnects and the resulting mechanical stress between the electrode and the surrounding neural tissue. Wireless microstimulators are a potential solution to the problem. As a method of energy transfer to the microstimulator, we propose to use a laser beam at near infrared (NIR) wavelengths. Microstimulators of various sizes were fabricated with two cascaded GaAs PIN photodiodes. The voltage field of the device was measured in saline solution as a response to an NIR laser source. The voltage in medium had a peak of around 190 mV above the cathodic contact and stayed flat for the duration of 0.2 ms pulse. In rats, microstimulators were inserted into the ventral horn of cervical spinal cord. A train of NIR pulses (0.2 ms, 100 Hz) were applied to wirelessly activate the devices. The forces generated due to stimulation of the motor neurons were measured from the ipsilateral forelimb with a force transducer. The largest force generated was around 0.9 N. The volume conductor and force measurements suggest that the floating light activated micro-electrical stimulator (FLAMES) approach is feasible for intraspinal stimulation.