Bioelectronic photodetector and flexible photoreceptor array based on bacteriorhodopsin film

A new paradigm has recently emerged from classical machine vision, where biologically inspired vision hardware and software strive to mimic the functionality and efficiency of natural vision systems. Limitations in current silicon-based vision systems have prompted researchers to look toward biological vision for means of improving functionality and performance. Many biological photoreceptors demonstrate preprocessing capabilities and respond to a broad dynamic range of light intensities; they are typically very energy efficient and can be arranged across curved geometries, while requiring inexpensive fabrication techniques. Thus, machine vision can be further advanced by developing innovative non-silicon based technologies, creating a new generation of imaging arrays and sensor designs.; This dissertation illustrates a new approach in developing a biologically inspired photoreceptor array by exploiting the synergy between dried bacteriorhodopsin (bR) film and flexible transparent-electrode technology. The goal of this project is to prove the viability of integrating bR into innovative sensing applications using standard engineering methodologies. To accomplish this, the bR sensing element is modeled theoretically by applying three levels of abstraction. When responding to an incident photon, the internal photochemical reaction is modeled using rate equations. Electrostatic theory transforms this microscopic phenomenon into a macroscopic response measurable at the electrode boundary. An electrical equivalent circuit model then correlates the intrinsic response to the extrinsic response as measured by the readout electronics. Investigating the origin of bR's characteristic differential response forms the foundation of this work. Experimental and simulated findings indicate that the intrinsic response is attributed to charge displacement and recombination; whereas, the measured extrinsic differential response is caused by electrical coupling between the film and measurement circuitry. Device physical properties, light source conditions, load resistance and capacitance all have dramatic influence over the observed response characteristics.; The proposed theoretical model is proven both experimentally and analytically through simulation. Experiments are conducted using photodetector and array prototypes. Sensing elements are constructed by depositing wild-type bR onto flexible indium-tin-oxide (ITO)-coated polyethylene terephthalate (PET) substrates using electrophoretic sedimentation technique (EPS). The performance of a 4x4 photoreceptor array and amplification circuitry are characterized experimentally in terms of signal-to-noise ratio (SNR), linearity, dynamic range, spectral response, temporal response, interpixel uniformity and responsivity under mechanical bending. Designing an appropriate amplification architecture is vital to measuring the pico-ampere photocurrent generated by a bR photodetector. Consequently, a switched integrator circuit proves to be the most effective solution due to noise-filtering capabilities and programmable gain. Beneficial features of the bR photodetector are demonstrated by implementing a visual motion detection system based on Reichardt's delay-and-correlate model. In this application, the differential photosensitivity of bR performs temporal preprocessing at the material level, eliminating the need for circuitry used by a conventional motion detection system. Based on the work presented, bR shows great potential for becoming a viable engineering material for use in durable and lightweight vision systems. Numerous robotic, industrial and medical applications could benefit from the advantages of biologically inspired vision systems that exhibit curved or spherical geometries.; Keywords. Bacteriorhodopsin (bR), flexible bR-based photoreceptor array, differential photoresponse, electrical equivalent circuit model, switched integrator, signal-to-noise ratio (SNR), motion detection system.