Dynamics of one laser pulse surface nanofoaming of biopolymers

Abstract

Self standing films of the biopolymers gelatine, collagen, chitosan, irradiated with single nanosecond and femtosecond laser pulses easily yield on their surface, a nanofoam layer, formed by a cavitation and bubble growth mechanism. The laser foam has interesting properties that challenge the molecular features of the natural extracellular matrix and which make them good candidates for artificial matrix fabrication (nanoscopic fibers, large availability of cell adhesion sites, permeability to fluids due to open cell structure). As part of the mechanistic study, the dynamics of the process has been measured in the ns timescale by recording the optical transmission of the films at 632 nm during and after the foaming laser pulse. A rapid drop 100→0% taking place within the first 100 ns supports the cavitation mechanism as described by the previous negative pressure wave model. As modeled a strong pressure rise (~several thousands of bar) first takes place in the absorption volume due to pressure confinement and finite sound velocity, and then upon relaxation after some delay equal to the pressure transit time gives rise to a rarefaction wave (negative pressure) in which nucleation and bubble growth are very fast.