Date of Award

December 2016

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical and Chemical Engineering

Advisor(s)

Patrick T. Mather

Keywords

biodegradable, composites, elastomers, shape memory, thermoplastic

Subject Categories

Engineering

Abstract

There is a need for biodegradable thermoplastic elastomers for a variety of medical applications. This dissertation aims at (i) developing and characterizing biodegradable elastomers and (ii) developing and characterizing biodegradable smart composites, both to be used as biomaterials. First, in Chapter 1, an overview was given of the literature of relevant background information on polymers, biomaterials, and previous biodegradable elastomers and responsive elastomeric systems.

The first biodegradable elastomer developed and investigated, as presented in Chapter 2, is a novel thermoplastic polyurethane that has a biodegradable soft segment and a crystalline hard segment (POSS). By varying the composition of the poly(caprolactone)-based soft segment as well as the soft segment to hard segment ratio, the thermal and mechanical properties could be controlled. Increasing the comonomer (glycolide or d,l-lactide) content caused a decrease in the thermal and mechanical properties, while increasing POSS caused an increase in these properties. Overall, the synthesized polyurethanes had low moduli, high strain-to-failure, and high elasticity.

In Chapter 3, these polymers were degraded in vitro in phosphate-buffered saline (PBS) solution to investigate their properties throughout degradation. It was found that increasing the amount of comonomer (glycolide or d,l-lactide) in the soft segment increased the rate of degradation at 37 °C. The mechanical properties of all materials with comonomer decreased before 12 w, even if > 95% of the initial mass remained. Interestingly, different trends were seen in the 60 °C degradation study. All materials tested lost their mechanical properties by 4 w. While materials with d,l-lactide decreased over time, the material without comonomer saw a sharp decrease in mass after almost no mass loss during the first weeks of degradation.

The POSS-based polyurethane elastomers were then tested for cytocompatibility using a non-contact cell viability assay in Chapter 4. It was found that all of the materials developed in Chapter 2 had low cell viability. Therefore, it was hypothesized that residual tin catalyst (tin-POMS) from synthesis was causing this cytotoxicity. A study was performed systematically varying the tin catalyst used to synthesize PCL1k:POSS. Polymers were synthesized with 1 – 0.01 wt.% tin-POMs, 0.1 – 0.01 wt. % dibutyltin dilaurate, or processed with repeated dissolutions and precipitations to try to wash away the tin. It was found that the polyurethane synthesized from lowest concentration of tin-POMS catalyst (0.01 wt. %) had the best cell viability without compromising the mechanical properties.

The biodegradable, elastomeric polyurethanes and a thermoplastic semi-crystalline polymer, poly(caprolactone), were combined by dual-electrospinning and compaction to fabricate a smart composite. Chapter 5 presents the processing and characterization of these shape memory elastomeric composites, or SMECs. The composites had high elasticity and extensibility while having shape memory capabilities, or the ability to fix into a temporary shape and return to its original shape upon heating. These materials were tested extensively for thermal, mechanical and shape memory properties for different compositions. Increasing poly(caprolactone) generally increased thermal properties, mechanical properties, and fixing. Next, a single composition was degraded at different fixed strains. It was determined that up to 100% fixed strain had no effect on the SMEC degradation profile.

In order to produce lower-cost biomaterials, a second type of polyurethane was developed that utilized different length poly(caprolactone) diols as both the hard and soft segments and is presented in Chapter 6. These materials had a higher modulus than the POSS-based materials but had high elasticity. One composition, because of its high molecular weight and therefore high entanglements, had shape memory properties.

Finally, in Chapter 7, conclusions and future directions for this work was presented.

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