Characterization of Biomaterials 225
three-dimensional imaging of surfaces from micron to nanometer molecular scales,
may assist the biomaterials investigator to more accurately determine the surface
properties of his/her biomaterial and characterize molecular level interactions that
might occur at the interface of the biomaterial with tissue and biofluids.
10.2 BULK ANALYSIS METHODS
Bulk analyses of biomaterials include chemical composition analysis, measure-
ments of porosity, creep, stress relaxation, and fatigue testing, assessment of
mechanical properties such as tensile strength, elongation, and elastic modulus,
determination of hardness and density, and thermal property measurements. Most
bulk analysis methods have been well developed. For example, infrared absorp-
tion spectroscopy is widely used for chemical composition analysis, X-ray diffrac-
tion for 3-D structure determination, and thermal properties can be determined
by differential scanning calorimetry (DSC), differential thermal analysis (DTA),
thermal gravimetric analysis (TGA), and dynamic-mechanical-thermal analysis
(DMTA). The details of these methods as well as the mechanical property analysis
methods have been well documented in numerous textbooks
13
and will not be
discussed in this chapter. We will focus our discussions on two recently developed
new technologies for obtaining 3D information of biomaterials at micron and
nanometer resolutions respectively.
10.2.1 X-ray Micro-computed Tomography
X-ray micro-computed tomography (also called microCT or μCT) is similar to
conventional CT systems used in medical diagnoses and research. Unlike these
systems, which typically have a maximum spatial resolution of about 0.5 mm,
advanced μCT is capable of achieving a spatial resolution up to 0.3 μm i.e. about
three orders of magnitude lower.
4
To image in such a high resolution, the size
of the specimen has to be reduced to about a few cubic millimeters. Most μCTs
are performed with synchrotron radiations that allow quantitative high spatial
resolution images to be generated with high signal-to-noise ratio.
47
The super
performance of μCT is resulted from the unique advantages of synchrotron X-
rays: (1) a high photon flux permits measurements at high spatial resolution; (2)
the tunable capability allows measurements at different energies; (3) the highly
monochromatic synchrotron radiation eliminates beam hardening effects; and
(4) parallel beam acquisition allows the use of exact tomographic reconstruction
algorithms.
4
The spatial resolution of a CT image is dependent on the number of parallel
beam projections and the number of data points in each projection. In μCT,
hundreds of two-dimensional (2D) projection radiographs are taken of the spec-
imen at many different angles. The information contained in each radiograph
is a projection of the absorption density distribution in the sample along the
direction of X-ray beam onto the plane perpendicular to the direction of the X-ray
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