Biomedical Engineering Reference
In-Depth Information
17.1.5
Flexible Electronics Technology
Microelectronics are commonplace in nearly all aspects of daily life, but interests continue
to grow toward creating high-performance, low-cost, and large-area photoelectric devices
that are difficult to achieve with the conventional technologies. Examples include the large
flat panel displays, solar cell arrays, and x-ray sensors large enough to image the whole
body. Spreading electronics over complex surfaces to monitor or control the surface char-
acteristics has attracted both scientific and commercial interest. Flexible electronics tech-
nology presents a new approach that may provide solutions to such expanding needs (44).
Extending electronics to flexible substrates encompasses many potential advantages.
Flexible electronics can reduce the space, weight, complexity, and cost of a system by inte-
grating electronics into the physical structure. Plastic substrates are particularly attractive
because they enable construction of lightweight and durable devices that can potentially
be rolled up or folded when not in operation (45).
Fabricating devices on flexible plastic substrates provides diverse possibilities. Using the
surface areas of large structures within an intelligent sensor network for structural health
monitoring illustrates a promising application area (46). Local signal processing of sensor
data, such as amplification, routing, and switching, can also be employed by incorporating
transistor-based electronics onto flexible substrates (47). Compared with standard printed
circuit board solutions, such integration can save space and reduce weight. While current
interests lie in replacing existing technologies, there are many opportunities for developing
new applications and capabilities. High-scale integration of flexible electronics can be used
to create intelligent skin surfaces for air and land vehicles (48). New classes of photosensor
arrays can also be developed for visual navigation or medical imaging.
17.1.6
Motivation of This Work
Within the past several decades, scientists and engineers strive to uncover the mysteries
of biological vision and attempt to design artificial vision systems to mimic natural func-
tions (49-51). Biological vision has evolved over several millions of years into very effi-
cient sensors that provide some unique features. Biological photoreceptors can be
extremely sensitive; for example, the retina of human eyes and that of several other verte-
brates can respond to individual photons (52). These photoreceptors can also respond to
signals over a remarkably wide range of intensities. Human eyes, for example, exhibit a
dynamic range that is ten orders in magnitude (53). Typically, sensing elements arranged
in high spatial densities yield high-resolution visual information. Transmitting this wealth
of information to the brain requires a large degree of data compression. Spatial and tem-
poral compression is accomplished by the preprocessing capabilities of front-end sensing
elements and their highly parallel and interconnected neural pathways. In addition, other
preprocessing capabilities include edge detection and contrast enhancement. Unlike pla-
nar silicon-based image arrays, nearly all biological vision systems exhibit curved geome-
try. Insect eyes, for example, typically have their photoreceptors situated on the convex
side of the surface. Alternatively, all vertebrate have their photoreceptors arranged within
the inner concave surface of their eyes. These geometries can provide various advantages,
such as increased field of view or enhanced focusing and tracking capabilities.
As stated earlier, bR exhibits photoresponse characteristics similar to that of
rhodopsin
,
including high sensitivity, large dynamic range, high spatial resolution, and preprocessing
capabilities. Most recently-proposed bR sensor designs are composed of planar 2D arrays that
are fabricated on solid substrates. Flexible electronics offers a great opportunity to develop bR-
based vision devices with curved or free-form geometries. In this work, complementary fea-
tures of bioelectronics and flexible electronics produce the primary motivation for integrating
these two technologies. This combination can grant artificial system with the benefits of
