Photovoltaic implants introduce a new wireless method for precise electrical stimulation of tissue using the conversion of light into electrical energy, employing thin layers of organic semiconductors on a flexible substrate. With their remarkable potential for biomedical applications, these implants offer a versatile platform capable of delivering targeted electrical stimuli in the brain, facilitating the development of neuronal networks, and fostering regeneration and neuroprotection following traumatic brain injury [1]. Applications include devices that can be placed on the surface of the brain, or in the form of cuffs wrapped around nerves, which have already been showcased in numerous studies [2,3].

However, these principles may be applied in many other areas. They hold promise for use in electrical nerve conduits. By simultaneously providing guidance and stimulation to regenerating nerves, these conduits can help promote axonal regrowth, reconnection, and functional recovery. The precise control over electrical parameters through light enables tailored interventions, allowing for targeted treatment strategies in cases of nerve damage, peripheral neuropathies, or even spinal cord injuries [4]. On the other hand, the use of optoelectronic devices as capacitive sensors opens up avenues for monitoring tissue properties, neuronal activity and nerve growth, and for investigating brain-diseases in both research and clinical settings.

Our studies emphasize general essential requirements for these different clinical applications. Notably, our focus lies on fabrication methods of new devices, biocompatibility, the distribution of the electric field in biological tissue, and their ability to influence cellular growth.

To assess biocompatibility, Chick Chorioallantoic Membrane (CAM) assays were conducted and tissue was sampled for immunohistochemical studies. Furthermore, colorimetric LDH cytotoxicity assays were used to quantify possible adverse effects of the devices when in contact with primary cell cultures of cortical neurons [5].

In addition to quantifying the biocompatibility of these photovoltaic implants, investigating their generated electric field and long-term stability is imperative to optimize their performance. Therefore, a custom measurement system was employed to characterize the spatial distribution of the electric field in an aqueous solution during stimulation over the span of several hours to several days. By precisely mapping the electric field, we gained insights into the long-term stability of the device, and efficacy and intensity of the stimulation to optimize precise targeting of specific areas with low light intensities while minimizing potential damage to the surrounding tissue.

Furthermore, we utilized in vitro experiments to observe the effects of photovoltaic stimulation on the intrinsic electrical activity, signal transmission and morphology of neuronal cell cultures. Voltage-sensitive dyes provide real-time feedback on the neuronal response to optoelectronic stimulation. Long-term observations of morphological changes using a microscopy setup were used to investigate the phenomenon of electrotropism to give insights into directional cell growth patterns and structural alterations in response to electrical stimulation, to further support the potential of these electrodes to guide and manipulate cellular growth [6].

Through our studies encompassing biocompatibility assessments, electric field mapping, and in vitro experiments, we aim to optimize fabrication steps, increase their performance and promoting research on optoelectronics in biomedical engineering.