Page 331 - Vibrational Spectroscopic Imaging for Biomedical Applications
P. 331
Raman Imaging of Str ess Patterns in Biomaterials 305
draft of the confocal probe configuration and the laser penetration
depths for 90 percent Raman emission in normal and confocal probe
configurations are given in Fig. 10.1 and Table 10.1, respectively. In
Table 10.1, penetration depth for Raman confocal and normal probe
∗
∗
(z and z , respectively) refres to the threshold depth value for 90 percent
c n
Raman intensity emission. PS coefficients and data scatter recorded
during calibration refer to the Raman band indicated in brackets.
10.3 Raman Effect in Biological and
Synthetic Biomaterials
10.3.1 Spectral Features
According to group theory, the phonon modes of solids decompose
as irreducible representations, which split into selected vibrational
modes. Such modes can be classified depending on the propagation
direction of the selected mode with respect to the selected polariza-
tion direction [(e.g., longitudinal optic (LO) or transverse optic (TO)].
Selected Raman bands can be then used for stress assessments,
according to the knowledge of their respective PS coefficients.
Hydroxyapatite crystals possess a point group of C . The space
6
6
group CP6( ) specifies the unit cell symmetry for hydroxyapatite
6 3
2
more correctly than C ( P /m)6 , since the OH ions are not located on
6 h 3
22
the mirror plane σ . The phosphate and OH ions are located on sites
h
6
of C and C symmetries for C space groups, respectively. The factor
1 3 6
group (or correlation) analysis based on these sites predicts types and
numbers of symmetry species due to the crystal field splitting. It has
6
been reported that the space group of C symmetry is consistent with
6
the results from the Raman experiments on synthetic hydroxyapatite
at low temperature. Factor group analyses have been based on the
following irreducible representation, with a total of five OH-
related bands expected for space group of P6. 22
+
Γ= AE + E (10.1)
1 2
Figure 10.2 shows a typical spectrum of cortical bone (bovine
femur), in which the pertinent vibrational modes has been labeled,
although not all the theoretically predicted vibrational bands could
be observed in the collected spectra.
Alumina materials are of high-technological interest as synthetic bio-
materials. They possess the corundum structure, which belongs to the
6
23
D space group with two molecular Al O groups per unit cell. The
3d 2 3
aluminum atoms are octahedrally coordinated with two layers of oxy-
gen atoms, the octahedron being severely distorted. The irreducible rep-
resentations for the optical modes in the crystal can be expressed as
Γ= 2A + A2 + A3 + A2 + E5 + E4 (10.2)
1g 1u 2g 2u g u