Surface foaming of collagen, chitosan and other biopolymer films by KrF excimer laser ablation in the photomechanical regime

Collagen, an important material made of a protein of the extracellular matrix, was extracted from rat tail tendons by acetic acid dissolution, and dry glassy films (15% water content) with smooth surfaces were casted from the solution with a thickness of ∼25 μm. Collagen and similar biopolymer films surface were exposed to single pulses of radiation of the KrF laser with increasing fluence. A white damaged area appears on the treated surface at a threshold of 0.5 J/cm2 with a single pulse and becomes more visible for higher fluence. SEM and profilometry of the ablated surface displays an important swelling (∼5–7 μm) and a microscopic foam structure indicative of the laser induced expansion of the excited material. This process is due to the explosive ablation (a critical phenomenon also called explosive boiling or phase explosion) of the irradiated material and produces upon laser heating a high concentration of bubbles within the polymer because of the relatively large absorption depth (20 μm) and the presence of a large amount of water in the material. Further bubble colliding and bursting produces a three-dimensional polymer micro-foam with interconnected pores. This is the first this result has been reported. The laser induced micro-foam may display some interesting properties for new applications.