Implantable neurostimulation devices play a key role in treating many injuries and diseases by providing a direct therapeutic link to the nervous system. This enables brain stimulation for treatment of Parkinson’s disease and epilepsy, nerve guidance and regeneration to remedy spinal cord injury, and retinal prosthetic devices that could cure blindness.^[1] To address such issues, new bioelectronic systems that can deliver electrical stimulation to nerve cells are required. Although silicon microelectronics and metal electrodes have been the historic gold standard for bioelectronic interfaces, the main obstacles to further translation of these devices include a low biocompatibility that reduces in vivo lifetimes, a mechanical rigidity that is poorly matched with soft tissue, causing inflammation and ineffective electrical contact, and a requirement for costly external power supplies to deliver current.^[2] These issues result in indiscriminate tissue activation, with a consequent lack of spatial selectivity.^[3]

In this work, we report our recent efforts to simultaneously address these issues by combining soft carbon-based organic semiconductors and nanoscale science to build bioelectrodes that allow optical neurostimulation without external power. Our approach creates bioelectronic interfaces from organic semiconductors that can be formed into customized nanoparticles with established solution-based chemistry methodologies. This approach enables the stimulating electrodes to be combined with targeted pharmaceutical factors in the fabrication procedure, which subsequently optimise connections to the neural network when released in-vivo.^[4]

We will discuss how we tuned the optoelectronic properties of the organic nanoparticles to cover red, green, and blue wavelengths, allowing spectrally selective platforms for neurostimulation. These semiconductors are turned into electroactive inks, and subsequently fabricated into pixelated arrays using inkjet printing. This approach establishes a new low-cost manufacturing methodology that is applicable to other organic materials and can be used for a variety of bioelectronic devices, creating a new manufacturing paradigm for healthcare.

We demonstrate both the anatomical and functional biocompatibility of neural tissue with our organic bioelectronic systems using immunolabelling with neuronal marker MAP2 and visualisation with epifluorescence microscopy to detect neurons cultured on the organic conductors. We demonstrate the controlled release of drugs from the organic nanoparticles, aiding in precise spatial delivery of pharmaceutical factors. Finally, we employ whole-cell patch clamp electrophysiology recordings to demonstrate an exciting result; purely optical neurostimultation of dorsal root ganglion nerve cells. We demonstrate that the organic conductors can trigger changes in the nerve cell membrane potentials via a capacitive coupling mechanism, the efficacy of which can be improved by judicious selection of the device architecture.^[5]

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