Photomechanical waves

FIGURE 15.16 Photomechanical wave delivery devices. A drug reservoir backed with a laser target material (e.g., black polystyrene) is placed on the skin. Laser-induced photomechanical waves are supposed to increase the permeability of the stratum comeum allowing the facilitated passage of drug molecules from the reservoir into the skin. (From Cross, S.E. and Roberts, M.S., Curr. Drug Deliv., 1, 81, 2004. With permission.)... 

Finally, in a related approach, Lee et al. have used photomechanical waves to enhance the skin delivery of 5-aminolevulinic acid (ALA), a 5-amino acid... 

A.G. Topical drug delivery in humans with a single photomechanical wave. Pharm. Res. 1999,16, 1717-1721. 

Lee, S., McAuliffe, D.J., Flotte, T.J., Kollias, N. and Doukas, A.G. (2001) PermeabiUzation and recovery of the stratum corneum in vivo the synergy of photomechanical waves and sodium lauryl sulphate . Lasers Surg. Med. 29, 145-150. 

Generally, peak pressure amplitudes range from a few MPa at the ablation threshold up to several hundred MPa at high laser fluences [104-108]. During propagation through the substrate, these high-amplitude waves may induce structural modifications at areas away from the ablation spot. Thus, in contrast to the photochemical effects which are confined to the laser-irradiated area, the photomechanical effects of UV ablation can be much more delocalized. 

All data obtained for TP strongly suggest that photochemical reactions play an important role during UV laser ablation, but also that photothermal processes are important. This is confirmed by the presence of the thermal N2 products in the TOF curves. Photothermal processes will also always be present if the polymer decomposes exothermically during a photochemical decomposition and if the quantum yields of the photochemical reaction is not equal to one (which is most of time the case). The ablation of polymers will therefore always be a photophysical process (a mixture of photochemical and photothermal processes), where the ratio between the two mechanisms is a function of the irradiation wavelength and the polymer. In addition, photomechanical processes, such as pressure produced by trapped gaseous ablation products or shock and acoustic waves in the polymer, take place and can lead to a damage of the polymer and are most important for picosecond pulses. 

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