Laboratory of Organic Electronics, Linköping University, Norrköping, Sweden
Purpose: To design minimally-invasive ultrathin photovoltaic devices for neuromodulation. The devices should be highly stable in physiological conditions, and deliver capacitive stimulation pulses in a spatially localized manner.
Methods: We use commercial crystalline organic pigments processed into thin films (60 nm) via vacuum sublimation. These thin films can be patterned into arbitrary pixels using conventional microfabrication techniques. The deposition procedure proceeds at room temperature, therefore ultrathin plastic foils can be used as substrates. Validation of device stimulation performance is done using combinations of microelectrochemical measurements, single-cell electrophysiology, ex vivo retinal explants, and in vivo nerve stimulation.
Results: The final design concept is the organic electrolytic photocapacitor (OEPC) – a device that mimics biphasic current-pulse neurostimulation and thus transduces an optical signal into directly-evoked action potentials in neurons. The devices operate in the tissue transparency window of the near-infrared, where light can penetrate deeply through skin and bone. Clear modulation of voltage-gated ion channels is evidenced in single-cell models. Embryonic light-insensitive retinas can be directly stimulated, using 660 nm light 1-5 ms light pulses. 50 µm diameter stimulation photoelectrodes are the smallest used that still achieve threshold for stimulation. Integrated onto ultrathin plastics, OEPCs can be wrapped around peripheral nerves for chronic in vivo stimulation.
Conclusion: The OEPC platform is one of the most minimalistic photovoltaic stimulation solutions reported to-date, and can be a versatile future approach to retinal stimulation.