Feasibility of tracking phospholipid permeation into skin using infrared and raman microscopic imaging

The feasibility of monitoring the permeation of chain perdeuterated 1,2-dipalmitoylphosphatidylcholine (DPPC-d₆₂) and 1-palmitoyl-d ₃₁, 2-oleoylphosphatidylcholine (P-d₃₁OPC) vesicles into pigskin using infrared (IR) microscopic imaging and confocal Raman microscopy was demonstrated. The former technique permits the examination of the relative concentration of molecular species (e.g., endogenous and exogenous lipids and proteins) over spatial areas, ∼1 mm, with a spatial resolution of ∼10-12 μm. In contrast, Raman microscopy allows the confocal examination of tissue at depths up to 100 μm with a pixel size of about 2-3 μm³. Spectral signal/noise, however, is reduced from IR and significantly smaller areas are generally monitored. The permeation of the gel phase DPPC-d ₆₂ was limited to ∼5-15 μm, whereas the liquid-crystalline phase P-d₃₁OPC permeated to substantially greater depths (35-100 μm), at times ranging up to 24 h after application. The results are generally in accord with literature values. In addition, the state of the P-d ₃₁OPC (intact vesicles or molecularly dispersed with skin constituents) was evaluated from the spatial dependence of the deuteriopalmitate chain conformational order. Upon permeation, the chains became more ordered. The advantages and limitations of these imaging technologies are discussed.