Faith A. Morrison

Fabrication and characterization of tunable polysaccharide hydrogel blends for neural repair.

Hydrogels are an important class of biomaterials that have the potential to be used as three-dimensional tissue engineering scaffolds for regenerative medicine. This is especially true in the central nervous system, where neurons do not have the ability to regenerate due to the prohibitory local environment following injury. Hydrogels can fill an injury site, replacing the growth-prohibiting environment with a more growth-permissive one. In this study, dextran and chitosan were incorporated into a methylcellulose and agarose hydrogel blend. This created several thermally sensitive polysaccharide hydrogel blends that had tunable mechanical and surface charge properties. Cortical neurons were cultured on the hydrogels to determine the blend that had the greatest neuron compatibility. Our results show that softer, more positively charged polysaccharide hydrogel blends allow for greater neuron attachment and neurite extension, showing their promise as CNS regeneration scaffolds.

A protocol for rheological characterization of hydrogels for tissue engineering strategies.

Hydrogels are studied extensively for many tissue engineering applications, and their mechanical properties influence both cellular and tissue compatibility. However, it is difficult to compare the mechanical properties of hydrogels between studies due to a lack of continuity between rheological protocols. This study outlines a straightforward protocol to accurately determine hydrogel equilibrium modulus and gelation time using a series of rheological tests. These protocols are applied to several hydrogel systems used within tissue engineering applications: agarose, collagen, fibrin, Matrigel™, and methylcellulose. The protocol is outlined in four steps: (1) Time sweep to determine the gelation time of the hydrogel. (2) Strain sweep to determine the linear-viscoelastic region of the hydrogel with respect to strain. (3) Frequency sweep to determine the linear equilibrium modulus plateau of the hydrogel. (4) Time sweep with values obtained from strain and frequency sweeps to accurately report the equilibrium moduli and gelation time. Finally, the rheological characterization protocol was evaluated using a composite Matrigel™-methylcellulose hydrogel blend whose mechanical properties were previously unknown. The protocol described herein provides a standardized approach for proper analysis of hydrogel rheological properties.

Characterization of exfoliated graphite nanoplatelets/polycarbonate composites: electrical and thermal conductivity, and tensile, flexural, and rheological properties

Exfoliated graphite nanoplatelets (GNP) can be added polymers to produce electrically conductive composites. In this study, varying amounts (2–15 wt%) GNP were added to polycarbonate (PC) and the resulting composites were tested for electrical conductivity (1/electrical resistivity), thermal conductivity, and tensile, flexural, and rheological properties. The percolation threshold was approximately 4.0 vol% (6.5 wt%) GNP. The addition of GNP to polycarbonate increased the composite electrical and thermal conductivity and tensile and flexural modulus. The 8 wt% (5.0 vol%) GNP in polycarbonate composite had a good combination of properties for electrostatic dissipative applications. The electrical resistivity and thermal conductivity were 4.0 × 107 ohm-cm and 0.37 W/m · K, respectively. The tensile modulus, ultimate tensile strength, and strain at ultimate tensile strength were 3.5 GPa, 58 MPa, and 3.5%, respectively. The flexural modulus, ultimate flexural strength, and strain at ultimate flexural strength were 3.6 GPa, 108 MPa, and 5.5%, respectively. Ductile tensile behavior is noted in pure polycarbonate and in samples containing up to 8 wt% GNP. PC and GNP/PC composites show shear-thinning behavior. Viscosity of the composite increased as the amount of GNP increased dueto a volume-filling filler effect. The viscosity of the GNP/PC composites are well described by a Kitano-modified Maron-Pierce model.

Effects of Carbon Fillers on Rheology of Polypropylene-based Resins

Varying amounts of three different carbons (carbon black, synthetic graphite particles, and carbon nanotubes) are added to polypropylene and the resulting single filler composites are tested for viscosity. In addition, the effects of single fillers and combinations of different carbon fillers are studied via a factorial design. Each single filler and all the combinations of different fillers cause a statistically significant increase in composite viscosity. For synthetic graphite/polypropylene composites, the viscosity increase is due to a volume filling effect. Composites containing carbon black and/or carbon nanotubes show viscosity increases above those containing only synthetic graphite.

The influence of shear on the ordering temperature of a triblock copolymer melt

The effect of shear on the ordering temperature of a triblock copolymer melt of polystyrene‐polybutadiene‐polystyrene (SBS) is examined by in situ small angle neutron scattering (SANS). Results obtained by SANS are compared to the rheologically determined order–disorder transition temperature, TRODT=115±5 °C. The SANS measurements from a Couette geometry shear cell are then used to construct a ‘‘dynamical phase diagram’’ based on characteristic changes in the scattering with temperature and shear rate, γ. A shear rate dependent ordering temperature, Tord(γ), is identified as the system is sheared isothermally from the disordered state. The scattering behavior is shown to be highly strain dependent. We compare our findings on the shear rate dependence of the ordering transition in triblock materials with previous observations on diblock copolymer materials and theoretical expectations for the shear rate dependence of the order–disorder transition temperature. A simple scaling argument leads to a good des...

Shear-induced changes in the order-disorder transition temperature and the morphology of a triblock copolymer

A summary of our work on a triblock copolymer under steady shear is presented. The experiments were conducted using small-angle neutron scattering (SANS), as a function of shear rate and temperature, and transmission electron microscopy (TEM) on quenched specimens. The triblock copolymer is composed of polystyrene-d 8 /polybutadiene/polystyrene-d 8 , and the ordered microstructure normally consists of hexagonally packed cylinders. Two temperature-dependent, characteristic shear rates, γ C1 and γ C2 , are identified from the scattering results. The first characteristic shear rate identifies the shear rate required to go from a disordered state to an ordered state, while the second characteristic shear rate is interpreted as the shear rate necessary to produce a different ordered morphology. This latter transition was previously identified as being analogous to a martensitic transition in metal alloys. The supporting SANS and TEM evidence for the new ordered phase is presented. Dynamical aspects of the structural transition between different ordered triblock morphologies are discussed using the model of a martensitic-like transformation.

Gelation Point In Borosilicate Sols From Rheological Experiments

Dynamic oscillatory experiments are used to monitor the gelation of the borosilicate systems prepared through the sol-gel process from metal alkoxides. The rheological experiments show that tan δ = G”/G’ is independent of frequency at the gel point in agreement with the results of others on organic gelling systems. The dynamic moduli at the gel point followed power-law behavior with respect to frequency. The power-law exponent is found to be ∼0.70. The apparent fractal dimension, dp, of the network cluster at the gel point is determined. The d F values for the samples ranged from 2.5 to 3.8 depending on the final structure of the evolved products at the gel point. The large values (d F > 3) exclude a simple geometric interpretation of the results. The effect of processing parameters, such as composition of reactants and temperature, on the resulting microstructures near the gel point is discussed.

Rheology of Phase Separated Block Copolymer Melts

Flow can transfer the microphase separated morphology of block copolymers into a state of global order. For tri-block copolymers with cylindrical domains, this has been demonstrated by Folkes et al. (1973) and Hadziioannou et al. (1979). who established the existence of ‘single crystals’ of hexagonally ordered cylinders with uniform director throughout a sheared sample. The director is aligned with the shear direction. In this study, we investigate the flow induced ordering of cylindrical domain structure at increasing levels of shear.