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The flow in a pipe of wormlike micellar solutions is examined using a simple model that consists of the codeformational Maxwell constitutive equation and a kinetic equation that accounts for the breaking and reformation of micelles. The model needs six parameters, all of which are extracted from single independent rheological experiments. One of the parameters, the shear-banding intensity parameter is associated with the stress plateau in the shear-banding region. The stress plateau is set in our model by the criterion of equal extended Gibbs free energy of the bands. The model predicts a Newtonian (parabolic profile) flow at low-shear rates or low-pressure gradients, followed by shear thinning up to a critical rate where instabilities and long transients appear. At this critical shear rate, a shear-banding flow region arises near the pipe wall. The model indicates that tube lengths up to 400 diameters are required to obtain fully developed flow, where a pluglike profile at the center of the tube coexists with a region supporting a much higher shear rate next to the wall. Shear-banding flow is present up to a second critical shear rate. At shear rates larger than the second critical rate, the parabolic velocity profile is recovered, except near the center of the tube where a small shear-banding flow region remains because the stress at that radial position is equal to the plateau stress. This is a consequence of the linear dependence of the shear stress with the pipe radius. The predictions of the model are compared with experimental results from the literature.

Hyaluronic acid (HA) and gelatin (Gel) are major components of the extracellular matrix of different tissues, and thus are largely appealing for the construction of hybrid hydrogels to combine the favorable characteristics of each biopolymer, such as the gel adhesiveness of Gel and the better mechanical strength of HA, respectively. However, despite previous studies conducted so far, the relationship between composition and scaffold structure and physico-chemical properties has not been completely and systematically established. In this work, pure and hybrid hydrogels of methacroyl-modified HA (HAMA) and Gel (GelMA) were prepared by UV photopolymerization and an extensive characterization was done to elucidate such correlations. Methacrylation degrees of ca. 40% and 11% for GelMA and HAMA, respectively, were obtained, which allows to improve the hydrogels’ mechanical properties. Hybrid GelMA/HAMA hydrogels were stiffer, with elastic modulus up to ca. 30 kPa, and porous (up to 91%) compared with pure GelMA ones at similar GelMA concentrations thanks to the interaction between HAMA and GelMA chains in the polymeric matrix. The progressive presence of HAMA gave rise to scaffolds with more disorganized, stiffer, and less porous structures owing to the net increase of mass in the hydrogel compositions. HAMA also made hybrid hydrogels more swellable and resistant to collagenase biodegradation. Hence, the suitable choice of polymeric composition allows to regulate the hydrogels´ physical properties to look for the most optimal characteristics required for the intended tissue engineering application.

The kinetics of the sphere-to-rod transition was studied in aqueous micelle solutions of triblock copolymer poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) pluronic P103 (PEO(17)PPO(60)PEO(17)). This transition was triggered by a temperature jump from the sphere phase to the rod phase and monitored with dynamic light scattering. The combination of the scattering intensity and the hydrodynamic radius were used to show that the micelles grow steadily as rods throughout the growth process. The transition was found to exhibit a single exponential behavior even in the case of large deviations from equilibrium. The linear increase in the decay rate with increasing copolymer concentration shows that the transition is dominated by a mechanism involving fusion and fragmentation of proper micelles. The decays of the sphere-to-rod transition were simulated for two pathways: random fusion fragmentation and successive addition of spherical micelles to rods. We show that micelle growth most likely occurs via random fusion-fragmentation. The second order rate constant for fusion and the fragmentation rate are calculated for the case of random fusion-fragmentation.