Biomedical Engineering Reference
In-Depth Information
model correctness. The switched integrator design is selected as the most appropriate
front-end circuitry design over two other common designs. The integrating behavior of
the chosen design significantly reduces output noise by averaging the input noise gener-
ated by the light source, photoreceptor, and amplifier. Furthermore, the switched integra-
tor design provides adjustable gain-bandwidth and low power consumption, making it a
practical solution for creating highly integrated bR photoreceptor array.
The photoelectric response of the proposed bR photoreceptor array is characterized
experimentally to provide a means of performance evaluation. The following characteris-
tics are addressed: noise, linearity and dynamic range, spectral response, response time,
pixel uniformity, and responsivity under mechanical bending. The SNR is logarithmically
dependent with the light power. The highest SNR value, 35.37 dB, is obtained from an
illumination power of 22 mW at the wavelength of 568 nm. The photoelectric response of
an individual pixel is approximately linear over the light power range of 200
W-12 mW.
bR photoreceptors respond mostly to visible light, where the spectral response peak is
located at 568 nm. Depending on the selected integrating time, signal response times
within the millisecond range can be achieved. The overall array is also evaluated in terms
of response uniformity of each pixel and responsivity under mechanical bending.
Uniformity of the photoresponse among all pixels is greater than 71% of the average value
465.25. Good photoresponse characteristics, without any cracking or delamination, are
achieved even when the bending radius is 10 mm.
Inherent image processing capabilities of the proposed bR array are demonstrated in a
motion detection application. Reichardt's delay-and-correlate algorithm is implemented
in hardware to detect both the speed and direction of a moving light spot. Binary pulse
correlation identifies direction of motion, where the pulse overlapping area determines the
speed. Such a motion detection scheme can be exploited by real-time machine vision sys-
tems. Differential photosensitivity inherent in bR films provides the principal mechanism
for motion detection. This unique property simplifies the signal processing stage and
effectively reduces the need for differential circuitry used by conventional spatiotemporal
detection systems.
17.6.2
Limitations and Recommendations
From a system design perspective, protein-based photoreceptor arrays on the flexible sub-
strates have great advantages over their conventional silicon-based counterparts. High-
resolution and high-speed vision is possible, although many challenges and limitations
must be overcome. Several limitations have to be addressed before bR photoreceptor array
can become commercially viable. The reminder of this section discusses a number of rec-
ommendations.
The primary challenge in developing a bR photoreceptor array is to develop effective
fabrication and packaging techniques. In this work, PM patches are immobilized onto a
flexible substrate by EPS. This process gives rise to some unavoidable problems, such as
film fragility and nonuniform thickness. Thus, it is difficult to control the interface qual-
ity at the film-electrode boundary. LB deposition is a method widely used to form highly
ordered and uniform thin films of organic molecules. However, bR film created in this
manner is also fragile under bending conditions. Encapsulating oriented PM patches
within a polymer gel and covering the bR film with an aqueous electrolyte gel provide
two viable alternatives to the above methods. Nonetheless, these two methods have crit-
ical requirements for packaging as simple shelter cannot maintain device lifetime.
Moreover, additional materials may influence the optical and electrical properties of the
hybrid bR devices.
