This grant provides funding for investigating laser modifying the surface crystallinity of biodegradable polymers. Thermal processes, including rapid melting at the surface and subsequent high quench rates, in pulsed laser irradiation in conjunction with slow crystallization kinetics of the polymers produce a surface layer with reduced crystallinity. Since hydrolytic degradation rates are a strong function of polymer crystallinity, the process can be utilized to alter degradation profiles of such polymers. The project will focus on poly (a-hydroxy acid) polymers, especially poly (L-lactide) (PLLA), as they are USFDA approved, are crystalline if solvent cast, and have desirable mechanical properties. Depth profiling of crystallinity and degradation testing will be conducted to relate changes along the depth with mass loss measurements. Effects of laser processing parameters on chemical and molecular weight changes will be investigated. Primary characterizations include Differential Scanning Calorimetry (DSC), Wide Angle X-ray Diffractometer, and Fourier Transform Infrared spectroscopy in Attenuated Total Reflectance (FTIR-ATR). Numerical models will incorporate heat transfer with microstructure evolution to predict degradation, weight loss and erosion rate.
If successful, the technology could impact a wide range of applications such as fixation devices, sutures, tissue engineering, drug delivery, pesticide dissemination, and packaging. In particular, drug delivery systems based on bulk eroding polymers can benefit from the ability of designing a desirable degradation profile and thus a controlled drug releasing profile. In conjunction with optimum device shape and size design, the technology can potentially help tailor drug release rates in delivery devices to enable desired therapeutic effects. Significantly advanced understanding of laser processing of homopolymer PLLA, will provide insight into laser interactions with other semicrystalline biodegradable copolymers.
The degradation profile (degradation rate vs. time) of biodegradable polymers depend on their crystallinity. The crystallinity alternation by laser irradiation is investigated. It is shown that melt-mediated crystallinity reduction (Fig. 1) is made possible by the high cooling rate associated with the laser irradiation and the slow crystallization kinetics of these polymers. Degradation tests in physiological environments are carried out to investigate the effect of laser induced crystallinity reduction on molecular weight reduction over degradation period (Fig. 2). When more energetic laser sources (e.g., excimer lasers) are involved, potential chemical changes to the polymers are investigated (Fig. 3) and ways to mitigate such changes are proposed. Furthermore, a crystallinity gradient and thus degradation gradient from polymer surfaces is made possible by optimizing laser irradiation parameters. Experimental and numerical investigation is carried out to quantify these relationships (Fig. 4). Specifically, poly (α-hydroxy acid) polymers, especially poly (L-lactide) (PLLA) are focused on as they are USFDA approved, are crystalline if solvent cast, and have desirable mechanical properties. It is investigated in the context of potential drug delivery applications, where a drug-embedded polymer device is implanted and the time release period is in the order of a few months. The effect of drug loading in the polymer on the laser-induced crystallinity modification is investigated and better understood (Fig. 5). The advance made in the project will enable innovative designs in various drug delivery applications. Significantly improved understanding of laser processing of homopolymer PLLA will provide insight into laser interactions with other semicrytalline biodegradable polymers and copolymers and resultant structural changes. Surface structure changes via laser irradiation in conjunction with optimum device shape and size design can be potentially used to tailor drug release rates in delivery devices using bulk eroding polymers, which is of great benefit to the field of medicine and society at large. Typical mass loss profiles of biodegradable polymers show an initial incubation period during which the polymer matrix undergoes water absorption followed by degradation and linear bulk erosion. This is disadvantageous in applications such as tissue engineering where faster adsorption kinetics and hence faster degradation may be preferred. Other fields can also benefit, such as interactions of proteins/cells with such surfaces, packaging, and pesticide delivery.
