Page 327 - Vibrational Spectroscopic Imaging for Biomedical Applications

Raman Imaging of Str ess Patterns in Biomaterials 301

biomaterials. Provided that standardized analytical references and
protocols are established for confocal microscopy techniques, the
Raman microspectroscopy also allows visualization as to how
stresses and strains intensify or relax at the very surface or in the
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bulk of structures and devices. From a purely spectroscopic point of
view, stress assessments deal with the wavenumber shift of selected
Raman bands upon stress. This phenomenon is known as the
piezo-spectroscopic effect (henceforth referred to as PS effect after
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Grabner). For this effect to be quantitatively exploited, the probed
biomaterial should be Raman sensitive. Luckily, many biomaterials
(e.g., hydroxyapatite, alumina, zirconia, etc.) and biomedical devices
(e.g., artificial joints, artificial bones, etc.) of interest possess intense
and sharp-featured Raman spectra. Therefore, they can be character-
ized with respect to their internal residual stress state without requir-
ing any alteration or manipulation of the material. 1,2,13,14
Throughout our Raman microspectroscopy studies of biomateri-
als, we have analyzed microscopic fracture mechanisms in natural
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biomaterials (e.g., cortical bone ) and synthetic biomaterials (e.g.,
alumina, zirconia and related composites 13,14,16 ). In both natural and
synthetic biomaterials, a conspicuous amount of toughening was
found to arise from microscopic mechanisms operating both at the
crack tip and along the crack wake. A crack-tip mechanism conspicu-
ously enhanced the resistance of cortical bone to fracture initiation.
The microscopic mechanism responsible for stress relaxation at the
crack tip in the natural bone material inspired the development of
new synthetic biomaterials toughened by extrinsic mechanisms. Fur-
thermore, an understanding of how cracks grow in bone is important
for locating the main cause of clinical stress fractures and for devel-
oping new methods of crack healing.
Nowadays, synthetically prepared ceramic materials, particularly
alumina and zirconia, are widely used in joint replacement resulting
in reduction in wear particles as compared to metallic and polymeric
biomaterials. 17,18 Obviously, surgeons are concerned about the risk of
using a brittle ceramic material for heavily loaded artificial joints (e.g.,
hip and knee joints). In answer to such concerns, Raman spectros-
copy may provide definitive solutions to several of the problems
relating to the chemical and structural reliability of biomaterials com-
monly employed in arthroplasty. Quality control regimens based on
Raman techniques can be systematically applied to identify surface
and subsurface residual stress fields, as well as phase transforma-
tions and chemical alteration of biomaterials prior to implantation in
the patient. Therefore, the potential for manufacturing errors and
other material reliability-related problems can be greatly reduced.
In this chapter, we demonstrate that Raman microspectroscopy
possesses considerable potential in biomaterials science because it
gives a chance to raster samples with high-spatial resolution and to
characterize nondestructively their micromechanical characteristics