Thermal Analysis of Model Bio-Polymers: Poly-L-Lactic Acid and Shedded Snake Skins

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American Pharmaceutical Review


Biological polymers, or biopolymers, are of great interest in medical research. Compatibility of materials placed in the human body is important because of the known rejection of many thermoplastics. However, polypropylene, a classic thermoplastic, used in hernia repairs is typically not rejected. Researchers are constantly seeking biopolymers which are accepted by the human body. One very popular polymer that is being studied in our laboratories is poly-L-lactic acid. Poly-L-lactic Acid (PLLA) is an optically active, biocompatible and biodegradable polymer that has been widely investigated as an artificial cell scaffold material. In its most crystalline form, PLLA is highly anisotropic and is one of the most piezoelectric polymers known. Conversely, amorphous PLLA exhibits little, if any, piezoelectric behavior. Compression molded PLLA films can be endowed with varying amounts of crystalline character and piezoelectricity by uniaxially stretching the polymer in a hot air bath. Understanding the precise crystalline architecture of PLLA that results from tensile drawing is important for constructing cell scaffolds that have highly tailored biodegradation and cell guiding properties. In our work here, we investigate the changes in the thermal properties of PLLA at draw ratios between 1 and 5.5 using differential scanning calorimetry (DSC). The crystallinity of the compression molded undrawn starting material and drawn films are characterized using wide angle X-ray diffraction analysis. Our DSC results show an increase in percent crystallinity with increasing draw up to a draw ratio of 4.0. At greater draw ratios, there is a decrease in the crystalline character exhibited by PLLA. There is a growing interest in employing the transdermal pathway for administration of drugs. In order to develop knowledge of the drug transport properties or the effects of sun radiation on human skin, one needs a practical human skin model. We propose that shedded snake skins are biological polymers which fit the need and are readily available. We have developed in our laboratories a correlation in the thermal properties by TG, DTG and DTA between the human and shedded snake skins. Based on these correlations, we are studying the effects of chemicals and radiation on shedded snake skin as predictive model membranes for human skin behavior. The present study concerns the evaluation of various shedded snake skins by Thermal Mechanical Analysis (TMA). The low temperature TMA will be used to verify the effectiveness of sun screen creams and lotions.