Oscillations, Waves, and Interactions - GWDG

218 A. Vogel, I. Apitz, V. Venugopalan
Figure 1. Transmission electron micrograph of human skin (dermis) showing collagen fibres
sectioned both longitudinally and transversely. Magnification 4900×. The fibres consist of individual
fibrils that are embedded in a ground substance with high water content (Reprinted
with permission from Ref. [8]. Copyright 1988 Blackwell Science).
the initial phase transitions, and its rapid movement produces a recoil pressure that
may result in secondary material ejection. We will confine the presentation to pulsed
ablation at tissue surfaces that does not involve plasma formation. Ablation processes
within transparent tissues or within cells that involve nonlinear absorption have been
considered in recent reviews [6,7].
2 Tissue composition, and properties relevant for ablation
Soft biological tissues consist of cells that reside in and attach to an extracellular
matrix (ECM). By mass, the composition of most soft tissues is dominated by water
(55-99%) and collagen (0-35%). In “cell-continuous” tissue such as liver and epithelia,
the ECM fraction is quite small and consists mostly of cell adhesion proteins. By
contrast, “matrix-continuous tissues” that include the corneal stroma, dermis, cartilage,
and tendon have a very small cellular fraction and are almost entirely ECM. In
matrix-continuous tissues (see Fig. 1), the ECM consists largely of collagen, with the
collagen content being as high as 35% [6,9]. A primary ECM function is the maintenance
of the tissue’s structural integrity. As a result, the ECM inhibits both tissue
vaporization and material removal that represent the desired outcomes of ablation
processes.
In general, the optical absorption properties of tissue are dominated by the absorption
of proteins, DNA, melanin, hemoglobin, and water. The absorption spectra
of these tissue constituents are presented in Fig. 2. In non-turbid samples, optical