Biomedical Engineering Reference
In-Depth Information
chain packing and/or increasing chain conformational disorder. The bottom
half of the figure attempts to diagram the progressive loosening of chain pack-
ing in going from orthorhombic to hexagonal to liquid crystalline bilayers.
Underlying the SC is the viable epidermis (see Fig. 15.1). The thickness of
this layer ranges from
m (eyelids) to
>
1 mm (palms). A major overall
function of the epidermis is to generate the SC. The principal cell in this
region is the keratinocyte, which differentiates upon migration toward the
SC. The epidermis is highly stratified, starting from the basal layer, in which
proliferation followed by differentiation of the cells takes place. The layer
directly beneath the SC is the stratum granulosum and is the region where
keratinocytes are morphologically transformed from a rounded to a flattened
shape.
The underlying layer of the skin, the dermis, ranges in thickness from
∼
40
μ
600
m to 4 mm. This region consists of connective tissue and specialized
structures (including blood vessels) with collagen comprising 75% of the dry
weight. Once a substance reaches the dermis, it is generally assumed to have
access to the systemic circulation.
μ
15.2.2 Experimental Considerations - Confocal Raman
Microscopy for the Study of Skin
Early studies by Puppels, Caspers, and their co-workers demonstrated the
utility of Raman microscopy for the study of skin [9, 10]. The major advan-
tage of the approach is that spectra may be collected in a confocal manner,
thus permitting acquisition of both molecular composition and structural in-
formation as a function of depth from full thickness samples. Caspers et al.
acquired in vitro spectra from 6
m thick skin sections as well as in vivo spec-
tra with laser excitation wavelengths of 850 and 730 nm focused
μ
mbelow
the skin surface. From their initial experiments, a commercial instrument has
resulted in which in vivo depth profiles of water [11, 12] or pharmacological
agents (e.g., retinol) [13] can be readily generated. In addition, the Puppels
group and several other groups [14-20] have produced a variety of studies ap-
plying multivariate statistics for diagnosis of pathological states of skin from
Raman spectra.
Technically, the heterogeneity of skin and the presence of refractive index
gradients likely impose some constraints to the accurate determination of
imaging parameters. However, uncertainties in the determination of spatial
resolution and axial location in transparent samples from which spectra are
extracted, such as those described by Everall [21-23] and others [24, 25],
are probably not important for highly opaque skin samples. We estimate the
axial resolution to be 2-3
∼
85
μ
m with the 785 nm excitation wavelength used in
the current measurements. A study from this lab has suggested that errors in
depth measurements are less than
μ
∼
15% which is probably adequate for most
current purposes.
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