Biomedical Engineering Reference
In-Depth Information
varying topography. Mechanical means are then used to remove all excess proteins, except
that deposited in lithographically fabricated trenches [81,82].
The challenge of maintaining BR in a functional and stable state is only partly satisfied
by the purple membrane. Although it does impart some level of protection from weather-
ing, it is unable to maintain the appropriate moisture level for proper function.
Encapsulation of the protein in a suitable polymer matrix can often fix the moisture at a
functional level, and also ensure enhanced stability. The matrix must allow efficient inter-
rogation of the protein (i.e., it must be optically transparent), as well as adequate coupling
to the semiconductor surface. Selection of a matrix can be difficult because the protein
prefers an aqueous environment and water can often be fatal to semiconductors.
The ability to couple the protein signal output to the microelectronic substrate will vary
for different proteins, and will depend upon the nature of the output signal: optical, chem-
ical, or electrical. Specific orientation of the protein may be required. For BR, orientation is
facilitated by a net charge difference (-3) across the purple membrane. Application of the
appropriate voltage will orient the purple membrane and deposit it on the substrate sur-
face [83]. This is particularly important for applications that rely on BR's photoelectric
effect, which will be cancelled out in a random orientation of PM fragments. Effective ori-
entation will maximize the voltage signal resulting from photoexcitation, thereby ensur-
ing effective coupling for signal transduction. Assuming that the signal is strong enough,
optical coupling is only important if the detection scheme is wavelength dependent.
Several examples of devices utilizing BR (in purple membrane form) as a light to voltage
transducer exist in the literature, and a few will be discussed herein, as they are relevant
to BR's sensor capabilities.
14.3.1.1 Microelectronic Devices Employing Bacteriorhodopsin for Enhanced Function
There have been several devices proposed and in some cases constructed, which utilize
BR to enhance function by integration of the protein into a semiconductor device. This
latter challenge—integration and stabilization of BR in a semiconductor environment—
must be met to make BR-based sensors a commercially viable reality. Work along these
lines has progressed over the last few years, resulting in several microelectronic
devices that utilize BR to modulate circuit response through the photoelectric effect.
Several such devices have been fabricated by Bhattacharya and coworkers [32], includ-
ing a BR field-effect transistor (FET) photoreceiver and a MODFET with BR as a light-
activated gate. In the latter case, purple membrane was oriented and selectively
deposited at the gate of a GaAs-based modulation-doped FET; the gate was activated
by the photovoltage that developed across the purple membrane in response to light,
thereby allowing current flow (Figure 14.6). The photo-induced current was transient
with a picosecond-scale rise time and a slower decay—the voltage modulation was
produced only with changes in the actinic light intensity, indicating that the resulting
photoreceiver cannot be used with continuous illumination. However, the transient
voltage response produced by the protein facilitates any application that requires
detection of dynamic changes in light intensity, that is, any situation where the object
of interest is characterized by modulated light levels. This sort of sensitivity is referred
to as differential responsivity, and examples include edge detection and motion sensi-
tivity [30,32,84].
Artificial vision and construction of an artificial retina based on BR has been a long-term
goal for a number of researchers [84,85]; BR is a strong candidate for such architectures
due not only to its characteristic ability of differential responsivity, but also its obvious
similarity to the visual rhodopsins. Differential responsivity is a necessary behavior for
any optical system designed to mimic human vision. From a physiological perspective, the
