Intracortical Neural Stimulation With Untethered, Ultrasmall Carbon Fiber Electrodes Mediated by the Photoelectric Effect

Objective:
Neural stimulation with tethered, electrically activated probes is damaging to neural tissue and lacks good spatial selectivity and stable chronic performance. The photoelectric effect, which converts incident light into electric potential and heat, provides an opportunity for a tetherless stimulation method. We propose a novel stimulation paradigm that relies on the photoelectric effect to stimulate neurons around a free-floating, ultrasmall (7-8 
μ
m diameter) carbon fiber probe.
Methods:
A two-photon microscope induced photo-stimulation with a near-infrared laser. Chronoamperometry and chronopotentiometry were used to characterize the electrochemical properties of photo-stimulation, while the fluorescence of Rhodamine-B was used to quantify temperature changes.
Results:
Photo-stimulation caused a local cathodic potential pulse with minimal leakage current. Stimulation induced voltage deflections of 0.05–0.4 V
in vitro
, varying linearly with the power of the laser source (5–40 mW). Temperature increases in the immediate vicinity of the electrode were limited to 2.5 °C, suggesting that this stimulation modality can be used without inducing heat damage. Successful stimulation was supported
in vivo
by increased calcium fluorescence in local neurons at stimulation onset in a transgenic GCaMP-3 mouse model. Furthermore, cells activated by photo-stimulation were closer to the electrode than in electrical stimulation under similar conditions, indicating increased spatial precision.
Conclusion:
Our results support the hypothesis that the proposed photoelectric method for neural stimulation is effective.
Significance:
Photoelectric stimulation is precise and avoids the need for a potentially destructive tether, making it a promising alternative to electrical stimulation.