Degradable polyester sponges with tunable degradability made by sustainable methods

This dissertation is aimed at the preparation and degradation of poly(L-lactide) (PLLA) sponge for tissue engineering. Three sub-aims, which are built up upon each other, could be branched from the main aim. The result of each sub-aim led to a new research field and resulted in sustainable processing. The first sub-aim is sponge preparation, including thorough hydrophilization. Surface modification methods were used for the hydrophilization of the sponges in order to conquer the inherent hydrophobicity of PLLA. Dip-coating is a well-known method for surface modification. In this work, dip-coating of a surfactant, Tween 20, was used in a sustainable way for sponge hydrophilization. The successful and efficient hydrophilization was demonstrated by hydrophilic dye staining. The strong hydrophilization was persistent even after storage in aqueous medium for a long time, favoring cell proliferation and penetration. A cell test showed the good cell compatibility of the hydrophilic sponges. Contrarily, low cell compatibility was observed in hydrophobic sponges. The hydrophilic PLLA sponges resolved the problem of the deep infiltration of cells into the sponge bulk, which was a remarkable challenge in the tissue engineering application of sponges. Hydrophilic PLLA sponges are favorable for not only biomedical applications, such as tissue engineering and drug delivery, but also other applications, such as agricultural materials. Besides hydrophilization, the next challenge is to resolve the problem of the low degradation of PLLA to adapt to the requisites for tissue engineering application with different tissues. Hereby, the second sub-aim was proposed. In this sub-aim, an enzyme proteinase K, which was found to degrade PLLA effectively, was identified as a highly promising additive for PLLA to accelerate its degradation rate. However, the thermal processing of enzymes with PLLA might be a major issue due to their thermal sensitivity (such as with proteinase K). Therefore, new methods were established by cooperating with other groups to understand the structural changes of proteinase K at different temperatures, focusing mainly on the enzymatic activity of proteinase K. The result showed that the latter could retain its enzymatic activity in bulk state up to 130 °C. Nonetheless, proteinase K lost most of its activity once the temperature reached 150 °C or higher. The thermal processing temperature of PLLA is usually higher than 180 °C, therefore, the result clearly indicates that the encapsulation of proteinase K into PLLA during thermal processing will give rise to a limited success. This finding from the second sub-aim led to the third sub-aim: the development of the processing of proteinase K with baroplastic polymers at ambient temperatures. Utilizing baroplastic degradable polymers, it was shown that proteinase K could be processed in bulk with well-known poly(L-lactide)-block-poly(ethylene glycol) block copolymers. It was found that poly(L-lactide)-block-poly(ethylene glycol)-block-poly(L-lactide) triblock copolymers with a specific ratio range of an LA/EG repeating unit (170/182 – 214/182) showed excellent baroplastic processability together with the formation of mechanically stable films. Following the baroplastic concept, polymer film could be processed with proteinase K at 37 °C under 10 MPa or higher pressure with a negligible loss of enzymatic activity. The preservation of protein functionality could be verified after processing with baroplastic by the encapsulation of a yellow fluorescent protein, yellow fluorescent protein for energy transfer (YPet), which was provided by Prof. Möglich’s group. When being processed at 135 °C with baroplastic, YPet lost its yellow fluorescence. By contrast, the yellow fluorescence was maintained when processed at 37 °C. The degradation test of baroplastic film in proteinase K buffer solution showed a rather slow degradation rate, which was in accordance with the sludge water degradation result. Additionally, baroplastic displayed little toxicity against cells and Daphnia, indicating the biocompatibility of the baroplastic material in the environment. The overall result of the thesis transported a comprehensive understanding of the processing and tailored degradation of PLLA-related materials, which is promising for applications where tailored degradation is required, such as tissue engineering and agricultural materials.