Biomedical Engineering Reference
In-Depth Information
biological vision and its unique geometries. However, there are numerous engineering chal-
lenges that must be overcome to transform this material into reliable technology. Current
efforts are directed toward refining material properties and fabrication technology to produce
desired thin-film structures. Design of high-performance processing circuitry for bR-based
photoreceptor array is also a critical task. Applying bR to high-level vision tasks is an emerg-
ing area of research. Most proposed devices exploiting the photoelectric properties of bR only
exploit amplification to process the detected signals. Successful incorporation of bR onto flex-
ible substrates is also dependent on whether the desired visual task can be realized.
17.2
Flexible Photoreceptor Array Based on Bacteriorhodopsin Film
17.2.1
Purple Membrane Preparation
Purple membrane patches containing bR are extracted from wild-type
H. salinarium
by
using a standard procedure (54). To remove sucrose and salt ions, the PM suspension is
purified by diluting with bidistilled water and then isolated by centrifugation. The ultra-
centrifuge operates at 22,400 rpm at 4°C for 30 min with a 50-Ti rotor. After the rinsing
cycle, the supernatant is carefully removed and the PM pellet is resuspended in bidistilled
water. This centrifugation process is repeated two more times and the final PM pellet is
suspended in bidistilled water to achieve a desired concentration of 16 mg/ml. This sam-
ple is then softly sonicated for 10 s to break up particle aggregation. Upon the completion
of three rinse cycles, the pH of the PM suspension is roughly 6.7, which eliminates the
need for acidification or alkalization of the sample.
Studies have shown that the salt concentration within the PM suspension influences the
electric signal generated by the reconstituted bR films (55). A film that contains a high salt
concentration will have a lower resistance, resulting in decreased signal-to-noise ratio
(SNR) during signal measurement. Therefore, the low salt concentration is necessary for
acceptable signal measurement. The salt concentration of a PM suspension is monitored
by measuring its ionic strength in term of conductivity. The final conductivity value is
maintained less than 10
S. If the conductivity is still too high, additional centrifugation is
required. Note that extra and unnecessary deionization cycles will produce undesirable
results. More specifically, cation loss occurs in the photochromic group of the bR molecule
and the PM patches become blue in color. Upon absorption of light, the retinals of the blue
membrane will isomerize from all-
trans
to both 13-
cis
and 9-
cis
, where the 9-
cis
retinal is a
long-lived photoproduct (56). Such a membrane exhibits lower quantum efficiency and is
thus not suitable for photoelectric applications.
17.2.2
Flexible Polyethylene Terephthalate Film With Patterned Indium Tin Oxide
Coating
17.2.2.1 Overview of Materials and Techniques
Emerging applications such as foldable display, large-area solar cells, wearable monitoring
systems, and conformal sensor sheets have motivated electronic devices to be grown on
flexible substrates. Plastics continue to replace traditional substrate materials due to their
ease of manufacture, chemical resistance, and wide availability. Therefore, it would be
advantageous to utilize plastic substrates for electronic devices. Plastics can form thin films
to cover large areas, while only requiring low-temperature manufacturing techniques.
Some commonly used plastic substrates include polyethylene terephthalate (PET), poly-
ethylene naphthalate, polycarbonate, and polyimide (57; 58).
