NEW SURFACE AND BULK SHAPE MEMORY EFFECTS IN POLYMERS FOR BIOMEDICAL APPLICATIONS
Date of Award
Doctor of Philosophy (PhD)
Biomedical and Chemical Engineering
Patrick T. Mather
Michelle M. Blum
Active Cell Culture, Amphiphilic Copolymers, Safer Needle Devices, Shape Memory Polymers, Surface Wrinkling
Shape memory polymers (SMPs) is a class of polymeric materials that are capable of changing shapes under an external stimulus such as heat, light, solvents and electromagnetic induction. In this dissertation, we explore both the bulk and surface shape memory effect of polymers, understand the material properties, and develop potential applications.
In the first case (Chapter 2, Chapter 3, and Chapter 4), wrinkles were developed utilizing the surface SM effect and were applied for cell culture. Wrinkling has been generally understood as a stress-driven instability phenomenon. In our bilayer system, the wrinkle formation is enabled by compressive buckling of a gold coating that is first applied to an SMP that has been fixed with a temporary uniaxial strain. The bilayer system is then recovered thermally to generate the wrinkles. The fundamentals of wrinkle characteristics and formation were investigated in Chapter 2. Gold film thickness and applied prestrain of the SMP substrate show a direct impact on wrinkle amplitude and wavelength. Our wrinkle system also features a time-evolved and temperature-dependent wrinkle formation.
On the pre-wrinkled substrates prepared with various prestrains, the degree of cell alignment (Chapter 3) was found to increase until saturation occurred, which can be attributed to both the increase in wrinkle orientation and the increase in crack density. Cell behavior on pre-wrinkled substrates and on actively wrinkling substrates was compared. Namely, for active substrates, wrinkle formation was induced during cell culture via increasing the medium temperature. In the active cell culture study, cell alignment can be activated by triggering wrinkle formation during active cell culture. Inspired by the cell application in Chapter 3, a functionally graded wrinkled surface was developed in Chapter 4 utilizing a Tg gradient shape memory polymer, which can allow for rapid and high-throughput investigations for a cell mechanobiology study.
Chapters 5-6 present two safer needle designs and fabrication methods that utilize the bulk shape memory effect. The heat-curling needle developed in Chapter 5 can curl upon heating, disabling the sharp needle to ensure a safe disposal. The sharp needle tip was achieved by freeze-fracturing the extruded polymer shaft in a torsional mode. Chapter 6 takes another route to create a safer needle that employs a water-triggered shape memory polymer. The concept of this safe needle device is to build a blunt needle that can be released later to shield the sharp needle once the injection is competed. Here, the release of the blunt needle is activated by the SMP that is triggered by water contact. An electrospinning technique was used to fabricate fibers featuring a high surface-to-volume ratio and tunable porosity, which can allow a fast water penetration. Electrospun poly(vinyl acetate) PVAc was selected for this application due to its unique water shrinkage behavior and good shape memory properties.
Finally, Chapter 7 presents a biodegradable amphiphilic co-network system containing poly(ethylene glycol) (PEG) and poly(ε-caprolactone) (PCL). By changing the relative composition of the two components, tunable thermal and thermomechanical properties can be achieved. Both co-networks and homo-networks exhibited good shape memory properties. Upon water contact, co-networks turned into hydrogels due to the hydrophilic nature of PEG. Further microstructure was revealed by x-ray scattering analysis. Moreover, the biodegradability was assessed in lipase-containing medium.
Overall, different biomedical applications were developed using shape memory polymers, which feature great potential for practical applications due to their high performance, tunable thermal and mechanical properties, and low cost. Some recommendations for future research and development are discussed in Chapter 8.
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Yang, Pine, "NEW SURFACE AND BULK SHAPE MEMORY EFFECTS IN POLYMERS FOR BIOMEDICAL APPLICATIONS" (2014). Dissertations - ALL. 175.
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