Figure 4: Optical stimulation of neurons cultured onto an ITO/rr-P3HT:PCBM device.

(a) Schematic representation of the optical stimulation paradigm including the localization of the stimulus in a region surrounding the patched neuron (scale bar, 10 μm). (b) Scheme of the photosensing interface, with the neuronal network grown on top of the polymer active layer during patch-clamp recordings. (c) Online monitoring of pH of the extracellular solution during the experiments in the presence (black, n=4) or absence of photostimulation (red, n=4). Data are presented as means±s.e.m. (d) Action-potential generation in response to a photostimulation pulse (50 ms). (e) Example of spike train generated with 20 ms pulses repeated at 1 Hz (upper panel). Peristimulus time histogram (PSTH) count was computed and normalized by considering spike trains in all recorded neurons (bottom panel; n=10, bins 20 ms). The right plot shows (means±s.e.m.) the latency to the spike peak with respect to the light onset computed by averaging all spikes in the train obtained from all recorded neurons and the jitter calculated as the s.d. of spike latencies measured across all recorded neurons (n=10). (f) Spatial properties of the photostimulating interface. A grid of nine spots (diameter 20 μm, spacing 30 μm) was overlaid to a patched neuron and spikes were counted. PSTHs, arranged in a similar grid, represent the spike counts normalized for the total number of sweeps in all recorded neurons (n=10, bins 10 ms). Each histogram represents the count of the spikes recorded at the soma by the corresponding stimulation spot. (g) Model of the polymer/electrolyte and electrolyte/neuron interface, where C_i and R_i represent the capacitance and the resistance of the double layer at the interface of electrolyte and polymer, respectively. V_b represents the bias voltage traditionally applied for the cell stimulation (in our case V_b=0), R_s the electrolyte resistance and V_m the measured membrane potential. Green bars and traces represent light pulses in all panels.