Biomedical Engineering Reference
In-Depth Information
users, and general interest groups. These standards are available electronically and are a
valuable resource for both the new student and the experienced engineer.
Although many techniques are available for evaluating the composition and structure of
biomaterials, no single technique is capable of providing all of the needed information.
Thorough characterization requires the use of multiple analytical methods. For example,
if the material is crystalline, x-ray diffraction can be used for bulk product identification
(typically, powdered samples are analyzed). The x-ray beam is diffracted as it goes through
the rows of atoms, and a detector measures the reflected intensities as a function of angle
from the surface. The intensities at various angles provide a unique signature of the mate-
rial structure and can be compared to existing data files for product identification purposes.
Fourier transform infrared spectroscopy is a method that complements the structural
information gained by x-ray diffraction by providing information about the chemical
groups found within the structure. The material does not need to be crystalline and can
be gas, liquid, or solid. The sample is exposed to infrared radiation, and the molecular
vibrations induced by the radiation are observed. Radiation at frequencies matching the
fundamental modes of vibration is absorbed, causing oscillating dipoles perpendicular
tothesurface.Thesearedetectedasafunction of wavelength and provide a chemical
“fingerprint” that can be compared to existing databases to identify the material. Most
characterization methods capable of identifying an unknown substance involve bombard-
ing the material with some type of energy, quantitating the interaction with the material,
and then searching a database for similar results. Other techniques utilizing this basic
procedure include secondary ion mass spectroscopy (SIMS) and x-ray photoelectron spec-
troscopy (XPS), both of which are well suited for identifying the surface chemistry of a
biomaterial.
Scanning electron microscopy (SEM) is very useful for characterizing the two-dimensional
surface topography of a biomaterial. In SEM, a beam of high-energy electrons is scanned
across the sample, causing the material to emit secondary electrons. The intensity of the
secondary electrons primarily depends on the topography of the surface. An image can be
recreated by recording the intensity of the current generated from the secondary electrons.
The resolution of an SEM allows magnifications of up to 100,000
. When greater resolution
of a material surface is needed, atomic force microscopy (AFM) can be used. In AFM, an
atomically sharp tip attached to a cantilever is dragged across the surface of a material but
actually does not touch the material. The interactions of the atoms of the material being
analyzed with the tip cause either repulsion or attraction. The height adjustments or changes
in interatomic forces are recorded and used to construct images of the surface topography.
Under proper conditions, images showing individual atoms can be obtained.
Images in three dimensions can be obtained using computer aided x-ray tomography
(micro-CT) or nuclear magnetic resonance (NMR) imaging. An image obtained by micro-
CT of a biodegradable porous scaffold is shown in Figure 5.16. In both techniques, the
samples are scanned in all directions, and then the image is created mathematically by a
merging of all the directional information. Solid or porosity volume or volume fraction
can be measured nondestructively. Direct measurement of solid or pore characteristic
dimensions (width, diameter, thickness) and spacing (or period) of repeating structure
can be made. In micro-CT, image contrast is achieved via attenuation of x-radiation; thus,
