Effect of excimer laser irradiation on crystallinity and chemical bonding of biodegradable polymer

The biodegradable polymer such as poly(l-lactic acid) (PLLA) is promising in drug delivery applications because its chemical structure allows it to hydrolyze into non-toxic substances in the human body. In such applications, drugs are embedded in a polymer matrix and released at the rate at which it degrades. PLLA degradation rate is a strong function of its crystallinity. Thus, control over crystallinity allows for continuous modification of drug release profiles. The excimer laser is used in this study to induce surface crystallinity changes because its spatially uniform intensity profile is favorable for surface treatment. The effects of excimer laser irradiation on the surface morphology, crystallinity, and chemical modifications are investigated via optical microscopy, wide-angle X-ray diffraction, and X-ray photoelectron spectroscopy. A model is developed to numerically examine the spatial and temporal temperature profiles, as well as the amount of chemical modifications. It is found that PLLA crystallinity decreases as a function of laser fluence, and that the amount of chemical modifications is minimized by annealing before laser treatment and reducing laser fluences, which decrease free radical mobility and thus the dissociation quantum yield. A working window is demonstrated in which PLLA crystallinity decreases with no measurable chemical modifications, and the working window can be enlarged by decreasing free radical mobility.