Biomedical Engineering Reference
In-Depth Information
tool to measure the anisotropic optical properties of materials. A graphi-
cal representation of this 'measurement tool' is known as the Michel Levy
chart. The chart shows the interrelationships between thickness, birefrin-
gence and interference color.
Using the equation
r
= 1000 ×
t
(
n
2 −
n
1) (Equation [1.7]), an experienced
analyst can use the chart to estimate birefringence from the observed inter-
ference color and conversely determine retardation for a specimen by plug-
ging in the difference between two measured RIs (
n
2,
n
1) and the measured
thickness. The Michel Levy chart can be used to identify an unknown sub-
stance. It can also be used to confi rm the identities of several substances
present in a mixed preparation of similar-sized particles.
As we have seen, the modern polarized light microscope is a powerful
technique to determine the chemical and optical properties of many clas-
ses of biomaterials. The precise determination of these properties allows
the biomaterials researcher to make predictions of the fi nal performance
parameters of biomaterials such as cell adhesion, biodegradability and bio-
compatibility when the materials are implanted in the body.
Surface characterization using refl ected light differential interference
contrast (DIC) microscopy
Differential interference contrast (DIC) is a light microscopy technique that
can be considered an extension of polarized light microscopy. DIC micros-
copy can be carried out using either transmitted light or refl ected light.
Refl ected light DIC microscopy can be used to study the surface morpholo-
gies of fi bers, fi lms and molded parts/devices made from biomaterials at mag-
nifi cations approaching the lower limits of scanning electron microscopy.
The images produced also have a similar three dimensional quality to those
produced by the SEM. Many times samples should fi rst be examined using
this imaging technique so that the analyst can get an idea of a material's sur-
face morphology before characterization is carried out using more advanced
instruments. In this section we will exclusively discuss refl ected light DIC
microscopy. Next we will discuss in detail how DIC images are produced.
Image production in refl ected light DIC microscopy
Opaque specimens, such as thick fi lms, molded plastic parts and metallic
components, are usually fairly refl ective and do not absorb or transmit a
signifi cant amount of the incident light. Surface details such as slopes, val-
leys and other undulations create optical path differences, which are trans-
formed by refl ected light DIC microscopy into amplitude or intensity
variations. These amplitude or intensity variations reveal the topographi-
cal profi le of the specimen's surface. Unlike the situation with transmitted
light DIC microscopy, the image created in refl ected light DIC can often be
