Skin Electroporation: Modeling Perspectives - Porous Media: Applications in Biological Systems and Biotechnology - page 347

EFG

0 25 50 75 100 125 (°C )

FIGURE 9.6
Representative heat versus temperature curve of SC lipids in thermal phase
transition.

important in a thermodynamic-based understanding of the SC . Here we pro-
vide a short description of lipid thermal behavior to better understand the
physics underlying the thermal influences during the electroporation LTR
development.
The lipid structure is known to become destabilized at elevated temper-
atures. It is generally accepted that there exist four main endothermic tran-
sitions in the SC in the temperature range 40 ◦ C-130 ◦ C, which have been
independently confirmed by differential scanning calorimetry (Golden et al.
1986, 1987; Cornwell et al. 1996; Al-Saidan et al. 1998; Tanojo et al. 1999).
Figure 9.6 shows a representative heat versus temperature in which the SC
endothermic transitions are evident as the peaks and have been identified
using the convention followed by Silva et al. (2006a,b). Of great interest to
thermally assisted destabilization of the SC 's lipid barrier function is phase
change E, which has primarily been attributed to the disordering of the
lamellar lipid phase (Cornwell et al. 1996). Phase change E has been doc-
umented in numerous differential scanning calorimetry studies at endother-
mic peak temperatures from 65 ◦ Cto72 ◦ C (Silva et al. 2006b). Using X-ray
diffraction microscopy it has been shown that the lipid lamellar structure
in the SC is evident up to temperatures about 60 o C and then disappears
within 10 ◦ C (White et al. 1988; Bouwstra et al. 2003). Polarized light ther-
mal microscopy of lipids extracted from the SC directly shows an overall
fluidization of lipid structures at 60 ◦ C (Silva et al. 2006a). Further evidence
of lipid chain melting and increased permeation at this phase change is pro-
vided in the reports by Golden et al. (1987) and Potts and Francoeur (1990),
where it is shown that the SC becomes more permeable to water flux abruptly
at 70 ◦ C.
The remaining two high-temperature phase transitions, F and G, are of
less direct importance to skin permeability. The enthalpic peaks of phase