Biomedical Engineering Reference
In-Depth Information
time delay (on the order of minutes) in rabbit models. The approximate width of the aver-
age anterior chamber of a human eye is on the order of 1 cm. Therefore, an observed rota-
tion of about 4.562 millidegrees per optical pass can be expected for a normal blood glucose
level of 100mg/100ml, given a specific rotation of glucose at
633 nm of 45.62
cm
2
g
1
and thickness of 1 centimeter. The eye as a sensing site, however, is not without its share
of potential problems. For instance, potential problems with using the eye include corneal
birefringence and eye motion artifact.
As shown in Eq. (17.68), the rotation is directly proportional to the path length, and thus
it is critical that this length be determined or at least kept constant for each individual sub-
ject regardless of the sensing site. If the eye is used as the sensing site, the angle of incidence
on the surface of the cornea must be kept relatively constant for each patient so not only the
path length but alignment remains fixed each time a reading is taken. In most tissues,
including the eye, the change in rotation due to other chiral molecules such as proteins
needs to be accommodated in any final instrument. In addition, most other tissues also have
a birefringence associated with them that would need to be accounted for in a final polari-
metric glucose sensor.
It is the birefringence and retardation of the polarized light as well as polarized scatter-
ing of the tissue that are the signals rather than the noise when using polarized light for tis-
sue characterization. For example, a scanning laser polarimeter has been used to measure
changes in retardation of the polarized light impinging on the retinal nerve fiber layer. It
has been shown that scanning laser polarimetry provides statistically significant higher
retardation for normal eyes in certain regions over glaucoma eyes. Images generated from
the scattering of various forms of polarized light have also been shown to be able to differ-
entiate between cancer and normal fibroblast cells.
l ΒΌ
17.4.5 Micrometer and Nanometer Biosensing Applications
Developments in microtechnology and, in particular, nanotechnology are transforming
the fields of biosensors, prosthesis and implants, and medical diagnostics. In terms of med-
ical diagnostics, these devices are being used in combination with optical biosensing for
external, lab-on-a-chip, high-throughput screening for analyzing blood and other samples.
Inside the body many researchers and companies are developing optically based nanotech-
nology applications for cancer diagnosis and therapy.
One nanotechnology used in biomedical optics that has come to the forefront is that of
quantum dots. Quantum dots are devices capable of confining electrons in three dimen-
sions in a space small enough that their quantum (wave-like) behavior dominates over their
classical (particle-like) behavior. At room temperature, confinement spaces of 20-30 nm or
smaller are typically required. Once the electrons are confined, they repel one another,
and no two electrons can have the same quantum state. Thus, the electrons in a quantum
dot will form shells and orbitals highly reminiscent of the ones in an atom, and will exhibit
many of the optical, electrical, thermal, and chemical properties of an atom. Quantum dots
can be grown chemically as nanoparticles of semiconductor surrounded by an insulating
layer nearly colloidal in nature. These particles can also be deposited onto a substrate, such
as a semiconductor wafer patterned with metal electrodes, or they can be crystallized into
bulk solids by a variety of methods. Either substance can be stimulated with electricity or
