Travis S. Bailey

Large Pore Size Nanoporous Materials from the Self-Assembly of Asymmetric Bottlebrush Block Copolymers

Asymmetric polystyrene-polylactide (PS-PLA) bottlebrush block copolymers have been shown to self-assemble into a cylindrical morphology with large domain spacings. PLA cylinders can be selectively etched out of the shear-aligned polymer monoliths to generate nanoporous materials with an average cylindrical pore diameter of 55 nm. The remaining bottlebrush backbone provides a functional, hydrophilic coating inside the nanopores. This methodology significantly expands the range of pore sizes attainable in block copolymer based nanoporous materials.

Control of pore hydrophilicity in ordered nanoporous polystyrene using an AB/AC block copolymer blending strategy

Ordered nanoporous plastics with hydrophilic pore surfaces were prepared by the degradative removal of polylactide from a self-organised, multi-component composite containing two block copolymers: polystyrene-polylactide and polystyrene-polyethylene oxide. The solid-state characterization of blends containing up to 12 wt.% polyethylene oxide was consistent with nanoscopic cylinders of mixed polyethylene oxide and polylactide hexagonally packed in a polystyrene matrix. Orientation of these materials through simple channel die processing resulted in good cylinder alignment. Subsequent methanolysis/hydrolysis of the polylactide component gave nanoporous polystyrene with polyethylene oxide coated pores. The resulting nanoporous materials were able to imbibe water, in contrast to nanoporous polystyrene with no polyethylene oxide component.

Highly distensible nanostructured elastic hydrogels from AB diblock and ABA triblock copolymer melt blends

A new structural motif for the generation of highly distensible, highly elastic, nanostructured hydrogels is presented. Based on the swelling of vitrified melt-phase blends of sphere-forming polystyrene–poly(ethylene oxide) diblock and polystyrene–poly(ethylene oxide)–polystyrene triblock copolymers, the equilibrium swelling ratio (3.8–36.9 g H2O per g polymer) and dynamic elastic modulus (G′ = 1700–160 000 Pa) of these novel hydrogel systems were found to be remarkably tunable through simple manipulation of temperature (10–50 °C) and triblock copolymer content (3.3–72.0 mol%). Mechanical properties were found to be almost exclusively a function of triblock copolymer content, independent of temperature induced changes in swelling ratio. The resulting hydrogels were highly elastic at all swelling ratios with G′/G″ ≈ 102 for the range of triblock copolymer concentrations examined. Hydrogel samples exhibited excellent preservation of dry polymer shape upon swelling, with complete recovery of both shape and mechanical performance following repeated compression–decompression cycles.

Imidazolium-containing, hydrophobic-ionic-hydrophilic ABC triblock copolymers: synthesis, ordered phase-separation, and supported membrane fabrication

Novel ABC triblock copolymers containing hydrophobic, imidazolium ionic liquid (IL)-based ionic, and non-charged hydrophilic blocks were synthesized by direct sequential, ring-opening metathesis polymerization (ROMP) of three chemically immiscible norborene monomers. The resulting ABC triblock copolymers were found by small-angle X-ray scattering to phase-separate into different nanostructures in their pure melt states, depending on their block sequence and compositions. Supported composite membranes of these triblock copolymers were successfully fabricated with defect-free, ≤20 microns thick top coatings. Preliminary CO2/light gas transport studies demonstrated the potential of this new type of IL-based block copolymer material for gas separation applications.

Effects of degeneration on the compressive and tensile properties of human meniscus

Healthy menisci function within the joint to prevent the underlying articular cartilage from excessive loads. Understanding how mechanical properties of menisci change with degeneration can drive future therapeutic studies to prevent this degeneration. Thus, the goal of this study was to characterize both compressive and tensile moduli of human menisci with varying degrees of gross damage due to osteoarthritis (OA). Twenty four paired menisci were collected from total knee joint replacement patients and the menisci were graded on a scale from 0-4 according to level of gross meniscal degeneration with 0=normal and 4=full tissue maceration. Each meniscus was then sectioned into anterior and posterior regions and subjected to indentation relaxation tests. Samples were sliced into 1mm thick strips, made into dumbbells using a custom punch, and pulled to failure. Significant decreases in instantaneous compressive modulus were seen in the lateral posterior region between grades 0 and 1 (36% decrease) and in the medial anterior regions between grades 1 and 2 (67% decrease) and 1 and 3 (72% decrease). Changes in equilibrium modulus where seen in the lateral anterior region between grades 1 and 2 (35% decrease), lateral posterior region between grades 0-2 (41% decrease), and medial anterior regions between grades 1 and 2 (59% decrease), 1 and 3 (67% decrease), 2 and 4 (54% decrease), and 3 and 4 (42% decrease). No significant changes were observed in tensile modulus across all regions and degenerative grades. The results of this study demonstrate the compressive moduli are affected even in early stages of gross degeneration, and continue to decrease with increased deterioration. However, osteoarthritic menisci retain a tensile modulus similar to that of previously reported healthy menisci. This study highlights progressive changes in meniscal mechanical compressive integrity as level of gross tissue degradation increases, and thus, early interventions should focus on restoring or preserving compressive integrity.

Mechanical viability of a thermoplastic elastomer hydrogel as a soft tissue replacement material

Hydrogels are a class of synthetic biomaterials composed of a polymer network that swells with water and as such they have both an elastic and viscous component making them ideal for soft tissue applications. This study characterizes the compressive, tensile, and shear properties of a thermoplastic elastomer (TPE) hydrogel and compares the results to published literature values for soft tissues such as articular cartilage, the knee meniscus, and intervertebral disc components. The results show the TPE hydrogel material is viscoelastic, strain rate dependent, has similar surface and bulk properties, displays minimal damping under dynamic load, and has tension-compression asymmetry. When compared to other soft tissues it has a comparable equilibrium compressive modulus of approximately 0.5MPa and shear modulus of 0.2MPa. With a tensile modulus of only 0.2MPa though, the TPE hydrogel is inferior in tension to most collagen based soft tissues. Additional steps may be necessary to reinforce the hydrogel system and increase tensile modulus depending on the desired soft tissue application. It can be concluded that this material could be a viable option for soft tissue replacements.

Dynamic compression of human and ovine meniscal tissue compared with a potential thermoplastic elastomer hydrogel replacement: DYNAMIC COMPRESSION OF HUMAN AND OVINE MENISCAL TISSUE

Understanding how human meniscal tissue responds to loading regimes mimetic of daily life as well as how it compares to larger animal models is critical in the development of a functionally accurate synthetic surrogate. Seven human and eight ovine cadaveric meniscal specimens were regionally sectioned into cylinders 5 mm in diameter and 3 mm thick along with 10 polystyrene-b-polyethylene oxide block copolymer-based thermoplastic elastomer (TPE) hydrogels. Samples were compressed to 12% strain at 1 Hz for 5000 cycles, unloaded for 24 h, and then retested. No differences were found within each group between test one and test two. Human and ovine tissue exhibited no regional dependency (p < 0.05). Human samples relaxed quicker than ovine tissue or the TPE hydrogel with modulus values at cycle 50 not significantly different from cycle 5000. Ovine menisci were found to be similar to human menisci in relaxation profile but had significantly higher modulus values (3.44 MPa instantaneous and 0.61 MPa after 5000 cycles compared with 1.97 and 0.11 MPa found for human tissue) and significantly different power law fit coefficients. The TPE hydrogel had an initial modulus of 0.58 MPa and experienced less than a 20% total relaxation over the 5000. Significant differences in the magnitude of compressive modulus between human and ovine menisci were observed, however the relaxation profiles were similar. Although statistically different than the native tissues, modulus values of the TPE hydrogel material were similar to those of the human and ovine menisci, making it a material worth further investigation for use as a synthetic replacement.