Date of Thesis

Spring 2021

Description

Contemporary research based on electrospinning fabrication has continued to grow as this technique has proven to be an efficient and highly reproducible method for nanofiber production. Electrospinning is the process of forming nanofibers from viscous polymer solutions exposed to electrostatic forces that drive continuous jet formation as solvent is removed and the jet is stretched. The use of dual-electrospinning, involving a two-solution delivery system, in reducing fabrication time and fabricating composite fibrous mats has continued the advancement of electrospinning-based research. This study aimed to utilize such advancements to demonstrate triple shape memory via two distinct stimuli exhibited by electrospun poly(vinyl acetate) (PVAc):poly(ε-caprolactone) (PCL):PVAc trilayer films.

The term shape memory (SM) refers to the ability of a material to be thermo-mechanically programmed into a temporary (or dormant) form and later triggered to recover from this secondary shape to its permanent shape through the application of an external stimuli and related action of rubber elasticity. We report in this thesis that isotropic PVAc:PCL:PVAc trilayers prepared by electrospinning exhibit triple SM, meaning that they can be programmed into and recovered from two distinct temporary shapes. Hydration of the films was found to reduce the PVAc glass transition to below body temperature as water molecules disrupt intramolecular bonding and mobilize the network chains. This reduction produces a rubbery material that has a well-separated glass transition temperature (of PVAc) and melt temperature (of PCL), thus allowing this material to exhibit triple SM using two different external stimuli: hydration and heat.

We report in this thesis hybrid heat-hydration triple SM behavior involving both heat and mass transport for PVAc:PCL:PVAc trilayer films. Heating the trilayers above the melting point of PCL, deforming the sample, and cooling results in a temporary shape. Subsequent hydration of the sample at body temperature recovers the PVAc fibers to an entropically favorable state that is balanced by resistance of the crystalline PCL phase. Reheating above the melting point recovers the PCL to an entropically favorable state. This double recovery is indicative of the simplest form of triple SM. A slightly more complex mechanism was developed where two temporary shapes were created and recovered partially via hydration and further by heat, and this mechanism was used to assess the triple SM capabilities of the trilayer films.

This study was designed to prove the concept of this novel hybrid heat-hydration triple SM mechanism and investigate the potential of this material design in biomedical applications. The study varied the PVAc to PCL percent composition ratios to determine if there were any preliminary trends in the hybrid heat-hydration triple SM behavior based on percent composition. The deformation strains used to form the temporary shapes were also varied in order to determine if the recovery performance could be tailored by this programming parameter. Quantitative SM assessment using a dynamic mechanical analyzer confirmed the existence of the hybrid heat-hydration triple SM mechanism as well as showed distinct SM trends that are dependent on the composition of the trilayers and the programming parameters used. In addition to assessing the SM capabilities of the trilayer films, materials characterization via differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) was performed to provide additional insight into the properties of these trilayers. Direct contact cytotoxicity assays using Madin-Darby Canine Kidney (MDCK) cells established that electrospun composites of PVAc and PCL were non-cytotoxic to MDCK cells, and hence these materials have the potential to be biocompatible and could potentially be considered for biomedical applications. Future biocompatibility studies (e.g., hemocompatibility assays, biodegradability assays, etc.) are required to determine biocompatibility with significant certainty. Along with additional biocompatibility studies, future investigations are encouraged to expand upon the SM data presented. These investigations could include parametric evaluations of the influence of the polymer(s), solution concentration, and other fabrication variables used. Nevertheless, results from this study provide valuable information on a hybrid process of two well-defined and well-understood phenomena while also investigating the potential of this material design in minimally invasive biomedical applications.

Keywords

Shape Memory, Electrospinning, Trilayers, Hybrid, Minimally Invasive Applications

Access Type

Honors Thesis (Bucknell Access Only)

Degree Type

Bachelor of Science in Biomedical Engineering

Major

Biomedical Engineering

Minor, Emphasis, or Concentration

Theatre & Dance

First Advisor

Patrick Mather

Second Advisor

Donna Ebenstein

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