Biomedical Engineering Reference
In-Depth Information
17.5.4
Experimental Setup and Results
The photoreceptor array is tested experimentally by detecting a moving light spot
generated by a 2D laser scanning system, as illustrated in Figure 17.23. A tunable
argon/krypton laser system is used as the light source, and a two-axis acousto-optic
deflector (AA.DTS.XY-250) scans the laser beam across the array surface. A two-channel
function generator is connected to the deflector driver and controls the deflector's RF fre-
quency. The beam deflection angle is adjusted by varying the amplitude of the signal
being fed into the deflector driver, whereas the signal frequency controls the sweeping
speed directly. The photoreceptor array and detection circuitry are located inside a
shielded enclosure with an opening to expose the array. Output data are observed with a
four-channel oscilloscope.
Figure 17.24 shows signals that correspond to the different motion detection stages as a
laser beam scans a pair of bR pixels from left to right. The top two traces indicate the out-
put signals produced by the left and right bR photoreceptor amplifiers. Traces three and
four are produced by two voltage comparators. Each comparator is set to trigger at a 0.5
V threshold and produces a 5 V binary signal as its output. These are then read by the
microcontroller's digital input pins. The input signals are then delayed internally for a
period of 200 ms, as indicated by traces five and six. The overlap between the two delayed
pulses produces a region that is proportional to the light spot velocity. This region is inte-
grated to produce a ramp-like profile over time, as indicated by the final trace. However,
the integrated value must be scaled to an appropriate PWM duty cycle to drive an external
motor. Thus, the effective power delivered to the motor will be proportional to the
observed light spot velocity. Direction of motor rotation is determined by the sign of the
integrated value.
17.6
Conclusions
17.6.1
Overview
This chapter presents a novel photoreceptor array that is fabricated by immobilizing bR
onto a flexible plastic substrate with a patterned conductive coating. As a retinal protein,
bR shows great potential in many optical and photoelectrical applications due to its high
sensitivity, large dynamic range, fast response time, small spatial size, and long-term sta-
bility. Extending electronics to flexible substrates introduces a new approach that has
promising future in lightweight and durable sensing devices with curved or free-form
geometries. The possibility of combining bioelectronics and flexible electronics is investi-
gated in this work; the ultimate goal is to develop an artificial retina that can mimic func-
tions and geometries inherent in biological vision systems.
In this work, PM patches, obtained from wild-type bR, are deposited onto a PET sub-
strate coated with a patterned ITO layer using the EPS technique. A flexible 4
4 pixel
array is carefully assembled and packaged. Positive spectral absorption results and the
stable photoelectric response generated from each individual pixel prove EPS to be a
viable fabrication technique.
Designing suitable signal processing circuitry for the proposed array is critical to its suc-
cess. An equivalent circuit model for a bR photoreceptor is presented. Modeling the dried-
bR film in this study is a vital step in integrating it with signal processing circuitry. The
underlying physical mechanisms of photoelectric transduction are studied to ensure
