A Neuron-readable Artificial Photoreceptor Composed of Photodeformable Liquid Crystal Polymers and Piezoelectric Materials

Artificial photoreceptors have been extensively developed to help the patients with serious eye diseases by conversion from light into electric signals. The researchers focus on the development of intelligent materials and novel mechanism in the new-generation photoreceptors to simplify the camera visual systems like Argus II. Photovoltaic materials such as cadmium sulfide nanorods, single-crystalline silicon photodiodes output photoelectric signals by the generation and migration of the photocarrier, but the reactive oxygen generated in photoelectrical conversion process is concerned in cytotoxicity and limits the organism application. Pyroelectric materials such as ZnO, poly(vinylidene fluoride-co-hexafluropropylene) are used for electric power output under NIR by the spontaneous polarization change with temperature; however, the photothermal effect of NIR limits the miniaturization of devices.<br/>The conversion of light-stress-electric signals can be realized by the combination of photodeformable materials and piezoelectric materials, which provides an opportunity for the construction of novel artificial photoreceptors without above-mentioned disadvantages. Azobenzene moieties are usually used as a trigger to generate photo-induced stress in the polymers by trans-cis isomerization upon exposure to light irradiation. Combined with azobenzene polymers, the piezoelectric polymers convert the stress into electric signals due to the density change of dipoles. However, the existing systems still suffer from poor output signals, restricting signal transduction to cells.<br/>Azobenzene-containing liquid crystal polymers (LCPs) provide a feasible solution to improve the photo-induced stress and output large electric signals, because the microscopic geometry variation of azobenzene moieties can be amplified into a macroscopic large deformation of the entire LCP materials owing to the alignment change of LC mesogens. Much effort has been made to develop photoresponsive soft actuators with the azobenzene-containing LCPs by ultilizing their quick and various deformation, including contraction/expansion, bending, twisting, oscillating, etc. Due to the cooperative effect of the LC mesogens, as long as 1 mol% of azobenzene moieties reach the photostationary state upon illumination, the generated alignment change can lead to the deformation of the whole LCP systems; therefore, photodeformable LCPs are an excellent candidate for the construction of the photoreceptor featuring a large photo-induced stress.<br/>Here, a neuron-readable artificial photoreceptor with significant voltage output is constructed by using LCPs and polyvinylidene fluoride trifluoroethylene (P(VDF-TrFE)). The significant voltage output originates from light-stress-electricity conversion, where the photo-induced stress is attributed to the change of mesogens alignment in the photodeformable LCPs and subsequently converted into strong electric signals by the P(VDF-TrFE) layer. The photo-induced open-circuit voltage reaches up 0.79 ± 0.02 V, which is, to our knowledge, 19 times higher than the maximum voltage (0.04 V) that has been reported to date. Hence, such artificial photoreceptor successfully transduces photo-induced electric signals to cells and tissues, communicates with the neurons and triggers spiking activities in blind retinas. Besides, visual image recognition is demonstrated in a pixelated matrix by analyzing electric signals of each unit. This artificial photoreceptor opens new opportunities for the combination of the photodeformability and piezoelectricity, providing an avenue to develop neuron-readable artificial retinas and implantable sensors.