Long-term implantation and cortical stimulation with photoactive organic semiconductors in a rat model

Neurostimulation employing photoactive organic semiconductors presents a promising alternative to conventional techniques, enabling targeted action and wireless control through light. This study investigates the efficacy of stimulation of the neuronal networks in vivo using organic electrolytic photocapacitors (OEPC) and explores the interactions between the device and neural tissue following a long-term implantation.OEPC implants were positioned subdurally on the somatosensory cortex of adult rats, with the craniectomy site sealed by a transparent window. Three weeks post-implantation, animals underwent stimulation with red laser light pulsed at 20 Hz for 30 minutes. Immunoreactivity of c-Fos in brain sections served to identify recent neuronal activity. Furthermore, glial and immune cell markers were analysed, alongside characterisation of explanted devices through scanning electron microscopy and voltage transient measurement.Photostimulation of the somatosensory cortex elicited an increase in c-Fos expression in neurons at the stimulation site and in connected brain regions of both the ipsilateral and contralateral hemisphere. Chronic implantation of OEPC in the rat brain did not induce foreign body reaction in glial and immune cells. The devices demonstrated sustained functionality even after three weeks of implantation.The observed upregulation of c-Fos implies a significant impact of OEPC-based stimulation on neural responsiveness. The absence of detrimental effects on both brain tissue and the device encourages the continued use of OEPC as an implant. These findings not only highlight its potential as a novel mode of neurostimulation, but also open a vast field for further exploration into the mechanistic foundations of this phenomenon.