Biomedical Engineering Reference
In-Depth Information
150 ms
Light on
Light off
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30 ms
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0
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(a)
(c)
Time (ms)
(b)
BK7 glass
Light
Transperent
ITO conducting
ground layer
Oriented bR
polymer
thin film
CID array
BK7
side
Glass
plates
Charge-
sensitive
semiconductor
element
Internal electronics permit
random access of individual
charge sensitive elements
Individual
array element
amplifier
FIGURE 14.7
A hybrid semiconductor bacteriorhodopsin-based artificial retina is fast and inexpensive, and mimics the motion
sensitivity of the human retina. By utilizing differential responsivity (a), the artificial retina is capable of both edge
enhancement and motion detection. (b) Device schematic illustrating the oriented bacteriorhodopsin layer and the
CID array. The protein is encapsulated in a polymer for stability and longevity. The array is modified so that the
charge-sensitive semiconductor elements (SCCE) are in direct contact with the protein film. A transparent indium
tin oxide (ITO) conductive coating on BK7 glass is placed over the top, sealing the protein from the environment
along with two other glass plates. Each SCCE has an individual amplifier that provides 100 photon per pixel per
second sensitivity (photon pixel-1s-1). The differential responsivity of the protein photocurrent generates a final
image which is selective of those elements that have changed within the last 3-100 ms. Thus this artificial retinal
simulates the motion-sensitivity characteristics of its biological counterpart. (c) Photograph of actual device.
14.3.2
Bacteriorhodopsin as a Sensor of its Chemical Environment
The complex photophysical properties that characterize BR make it a natural candidate for
architectures that rely on modulations of either light or voltage. In this regard, all previ-
ous applications have revolved around BR's ability to respond to light of the appropriate
wavelength, thereby eliciting a photochromic response. In essence, the protein senses the
actinic light and responds by entering the photocycle. In the applications discussed earlier,
