Biomedical Engineering Reference
In-Depth Information
5 (64). This charge asymmetry results in a permanent dipole moment across the membrane,
directed from the cytoplasmic side to the extracellular side. EPS occurs when the PM sus-
pension is inserted between two parallel electrodes while an electric field is applied. The PM
patches are oriented and transported onto the anode due to the more negatively charged
cytoplasmic side. This provides a simple and effective means of achieving high degrees of bR
orientation during film formation. Note that either the cytoplasmic side or extracellular side
of the PM can be attached to the electrode by selecting the appropriate suspension pH. The
EPS method produces thick PM films where the film thickness is a function of the applied DC
electric field, its duration, and PM concentration.
17.2.3.2 Fabrication Conditions
Fabrication conditions, such as the electric field intensity, exposure time, and humidity,
affect the photoelectric response and topology of bR films. A large aggregation of bR mol-
ecules is observed at low electric field intensities. These aggregates cannot be oriented due
to their hydrophobic behavior and thus form poorly oriented films, resulting in reduced
photoelectric responses. High electric field intensities reduce aggregation and create more
uniform bR films. However, signals measured from bR films formed at high electric field
strengths are found to be relatively small. This may be caused by protein degradation (65).
The long exposure time under an electric field can also cause decreased photoresponse
because of degradation. Therefore, an intermediate electric field produces the best results.
It has been reported that larger photocurrents are generated by the bR film when dried
under higher humidity; therefore, humidity must be carefully controlled during the dry-
ing process. Relative humidity in the range of 50 ~ 60% typically yields the best results
(66). Constant humidity can be achieved by drying the film in the presence of vapor gen-
erated by a saturated salt solution.
17.2.3.3 Fabrication Process
Figure 17.3 illustrates the fabrication process used to create the flexible 16-pixel array. One
hundred microliters of PM suspension is ejected from a pipette evenly over the patterned
surface. A 1.0-mm plastic spacer (not shown) is placed around the array to separate the
two electrodes. An external power supply is connected such that the bottom electrode
becomes the anode and the top becomes the cathode. An electric field of 40 v/cm is
applied for 5 min. The top electrode is then lifted and the remaining bulk water is care-
fully removed with a pipette. The film is allowed to dry by placing it in a humidity-regu-
lated chamber for 12 h. When dry, a PET film with a continuous ITO coating is then
aligned with the dried bR pixels and sealed carefully along the edge by using fast-curing
epoxy resin. A photograph of the bR photoreceptor array on flexible PET substrate is
shown in Figure 17.4. Two flexible ribbon cables are connected to the array via two flat
connectors to the readout electronics. Such a connection maintains device flexibility while
keeping good alignment.
17.2.3.4 Absorption Spectrum of the Flexible Bacteriorhodopsin Film
To verify deposition quality, the absorption spectrum of the dried bR film is compared
against the absorption spectra of the PM suspension and ITO-coated substrate, respec-
tively. An ultraviolet-visible spectrophotometer (Varian, Cary 50Bio) is used to record the
absorption spectra. The absorption peak of the PM suspension in the visible light range
was found at 570 nm (Figure 17.5a), whereas no peaks appeared in the visible light range
for the ITO-coated PET film (Figure 17.5b). The spectrum of bR-deposited flexible film
was obtained by subtracting the absorption spectrum of the ITO-PET substrate from the
