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.

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Rheological behavior of the first eight generations of bulk polyamidoamine (PAMAM) dendrimers, having nominal molecular weights from about 500 to over 116 000, was investigated under steady shear, shear creep, and dynamic oscillatory shear within the temperature range from T g + 15 °C to Tg + 105 °C. It was found that these dendrimers exhibit (a) constant viscosity at small deformations regardless of the type of stress applied and (b) temperature-/generation-dependent non-Newtonian response at higher shear rates/frequencies. The latter was characterized by finite moduli of elasticity at all generations and by onset of complex-viscosity thinning at some generation-dependent critical temperature and shear frequency. These results represent the first observation of elasticity in one dendrimer family, and they indicate that at rest in bulk these dendrimers collapse, deform, and organize into transient, secondary (i.e., hydrogen)-bonded, quasi-network supramolecular microstructure. They also reveal a distinct change from single-relaxation-mode to a multirelaxation-mode Maxwell-type behavior at generation 4, which is consistent with the closure of dendrimer molecular surface upon itself and the earlier proposed soft interior-dense shell model of intramolecular dendrimer morphology. Further support for this model resulted from an analysis of dendrimer free volume, which exhibited significant contribution not only from the dendrimer end groups but also from their interior building blocks. To account for these observations, a model is proposed that involves dynamics of structural elements that are smaller than the overall dendrimer molecules.

ABSTRACT:One emerging market for electrically conductive resins is for bipolar plates for use in fuel cells. Adding carbon fillers to thermoplastic resins increases composite electrical conductivity and viscosity. Current technology often adds as much of a single type of carbon filler as possible to achieve the desired conductivity, while still allowing the carbon-filled thermoplastic matrix material to be extruded and molded into a bipolar plate. In this study, varying amounts of two different types of carbon, one carbon black and one synthetic graphite, were added to Vectra A950RX liquid crystal polymer. The resulting single filler composites were then tested for electrical conductivity and rheological properties. The electrical conductivity followed that typically seen in polymer composites with a percolation threshold at 4 vol % for carbon black and at 15 vol % for synthetic graphite. Over the range of shear rates studied, the viscosity followed a shear-thinning power law model with power-law exponent (n Ϫ 1) ϭ Ϫ0.5 for neat Vectra A950RX and (n Ϫ 1) ϭ Ϫ0.7 for highly filled composite materials. Viscosity increased with increasing filler volume fraction for all shear rates. The viscosity-enhancement effect was more rapid for the composites containing carbon black when compared with those containing synthetic graphite.