Laser induced foaming and chemical modifications of gelatine films

This paper reports on the investigation of the microfoaming and chemical modifications following single-pulse laser irradiation conducted at 248 nm (KrF excimer laser, 20 ns fwhm) and at 266 and 355 nm (Nd:YAG laser 4th and 3rd harmonic, 6 ns fwhm) of gelatine films. Fluorescence emissions of the films were studied by laser induced fluorescence and spectrofluorimetry and the emission lifetimes were measured by time-correlated single photon counting. The main fluorophores responsible of the observed emissions are due to aromatic amino acids like tyrosine and derived crosslinked products, possibly formed during the laser foaming. It is observed that laser irradiation with a single ns UV pulse induces modifications on the relative intensity of emissions depending on the irradiation wavelength and type of film. It was earlier anticipated that the foaming mechanism can be viewed as a cavitation phenomenon mainly induced by the laser launched tensile wave at the film surface. Spectroscopic investigations confirm that the major accompanying chemical modifications are related with the relative increase of the fluorescence of dityrosine moieties and other products derived of the photoxidation of tyrosine. These modifications are related to the structural disruption induced by foaming and to the increase of temperature in the irradiated region. This approach opens up the possibility to gain information on the molecular structure of the new nanofoamy material, as well as on its microscopic distribution with emerging fluorescence-based confocal systems allowing high spatial resolution. The results shown have important consequences for the laser processing of biopolymers of interest in biomedical applications since the resulting laser foam exhibits properties close to those of the natural extracellular matrix.