240 H. Zeng
the intensity of the scattered photons as a function of the frequency difference, a
Raman spectrum is obtained. A laser is usually used as the excitation source for
Raman spectroscopy. Raman peaks are typically narrow and in many cases can be
attributed to the vibration of specific chemical bonds (or normal mode dominated
by the vibration of a functional group) in a molecule. As such, it is a “fingerprint”
for the presence of various molecular species and can be used for both qualitative
identification and quantitative determination.
62
Ramanspectroscopyhasarichhistoryofapplications in the characterization
of biomaterials such as polymers.
61
Raman spectroscopy is complimetary to IR
spectroscopy for measuring molecular vibrations, but has several advantages over
IR spectroscopy: (1) Little sample preparation or pretreatment is necessary since
scattering from the sample can be measured at the back-reflection configuration;
(2) The Raman slection rules are less restrictive than IR, so more vibrational modes
are observable; (3) Raman can be used to monitor biomaterial interactions with
water rich biological environments such as tissue, blood and other body fluids
since water is a weak Raman scatter and Raman siganl is measured at a wavelength
range closing to the visible or near IR laser line where water absorption is minimal
as well when compared to the mid-IR wavelength range. The major disadvantge
of Raman spectroscopy is the weakness of Raman scattering signal that leads to
length data acquisition time. But recent technical advancement has significantly
improved this situation, a Raman spectrum from biological tissues in vivo can
be acquired within one second.
63
This is very encouraging for characterizing
biomaterial in and its interactions with in vivo biological environments.
Raman spectroscopy makes microscopic and mapping analysis simpler than
with IR spectroscopy for the following reasons: (1) The laser beam is collimated
and can be easily focused to a micro-spot, the spatial resolution of micro-Raman
is on the order of micron or even sub-micron; (2) Raman is measured in the
visible or near IR wavelength range, so conventional microscope can be used
for Raman microscopy, while IR microscopy requires special optics; (3) Raman
scattering is inherectly a scattering process, and back scattering at 180
is partic-
ularly useful for mapping purposes because positioning of the beam is simple.
As a consequence of these advantages, Raman microscopy has been widely used
for the study of materials. With recent instrument advances, especially confocal
Raman microscopy, the use of micro-Raman spectroscopy and spectral imaging in
biomaterial researches is expanding. Example applications include contaminant
analysis, polymer degradation assessments, identification of phases in polymer,
and pathological tissue sample analysis.
61
It has also been used to analyze the
composition of bone and apatitic biomaterials,
64
image load-bearing surfaces in
artificial hip joints,
65
and monitor the effects of traetment protocols and subcuta-
neous implantation on bovine percardium.
66
10.3.4 Thermodynamic Methods
Thermodynamic methods can be used to obtain surface parameters related to the
interfacial free energy. These are the earliest developed methods for solid material
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