Biomedical Engineering Reference
In-Depth Information
FIGURE 17.4
Photograph of the 16-pixel bR photoreceptor array on flexible PET substrate.
17.3
Array Circuit Design
17.3.1
Equivalent Circuit Model of the Individual Pixel
During photoisomerization, charge separation inside the bR film causes changes in its sur-
face charge density (67). This alters the surface alignment of bR molecules in contact with
the solid substrate; therefore, the physical model of dried-bR film must take into account
the interactions between the surface polarization and the surface charge changes to accu-
rately describe the physical process. Rather than employing the permanent dipole
moment to describe time-dependent surface charge changes, the transition dipole moment
is more appropriate in this instance. The reason lies in the fact that the former is caused by
the surface charge asymmetry in PM, and the latter is induced by temporal charge translo-
cation in bR. Increased dipole moment relates directly to retinal structural changes and
likely drives the accompanying torsional motion (68).
The electric dipole moment provides a useful means of measuring the effects of charge
separation between atoms and molecules; however, the distances between charges are
typically too small to be easily measured. In electrostatic theory, a dipole is defined as two
opposite point charges that are separated by a distance. No current flows; only static
charge is present. However, the effect of time-dependent dipole moment changes must be
considered in the model because it is invalid to assume that charge changes instanta-
neously. Thus, the current dipole concept is introduced; it is defined by two current-car-
rying points separated by a distance. This is a fundamental model for representing
endogenous signal sources in biology (69). The potential field generated by a current
dipole in a continuous uniform medium is given as
I
11
(17.1)
d
4
rr
1
2
