Biomaterials for MEMS

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

1−3

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.

4−7

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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