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We show that the addition of solid particles to droplet–matrix blends of immiscible polymers induces massive changes in the rheology and the flow-induced structure even at loadings as low as 0.1 vol. %. Experiments were conducted using blends of polyethylene oxide (PEO, dispersed phase), polyisobutylene (PIB, continuous phase), and 470 nm monodisperse silica particles with two different surface wettabilities. Rheological experiments were conducted under molten conditions, while the morphology was characterized at room temperature using scanning electron microscopy. We are able to image the morphology at both lengthscales: The >20 μm lengthscale of the dispersed phase, as well as the submicron lengthscale of the particles. Rheological experiments along different trajectories in the ternary particle/PEO/PIB composition diagram reveal that addition of ∼1 vol. % particles that are preferentially wetted by the PIB induces a large increase in steady shear viscosity, severe shear-thinning, and yield-like behavior. However if the particles are equally wetted by PEO and PIB, these effects are greatly diminished. Remarkably, addition of very low loadings (∼0.1 vol. %) of particles reduces the viscosity under some conditions regardless of wettability. These rheological changes are interpreted in terms of three observations from morphological studies: That particles greatly enhance coalescence at low volume loadings, that particles jam the interface at higher loadings, and that particles bridge across drops and glue them together into large clusters. The first two of these effects occur regardless of particle wettability, whereas the last occurs only with particles that are preferentially wetted by the continuous phase.

We explore the microfluidic generation of bubbles in a flow-focusing junction using a pressure-controlled device rather than the more common flow rate-controlled devices. This device is a prototype for extending microfluidic drop generation methods to molten polymers. We show that the bubble generation process is highly sensitive to pressure: small changes in pressure induce large changes in bubble size and bubble formation frequency. A simple resistance circuit model can explain this pressure dependence. Briefly, we show that bubble generation is possible only within a finite pressure range. Near the ends of this pressure range, the ratio of the flow rates of the dispersed to continuous phase is highly sensitive to pressure, and therefore so also is the bubble generation process. The circuit model offers a way to use existing models of drop generation (which are based on flow rate-controlled operation) to predict pressure-controlled operation. We also examine drop formation using a highly viscous polymer as the dispersed phase. Drops are formed far downstream of the flow-focusing junction, and they are far smaller than the microfluidic channel dimensions. These results suggest that existing microfluidic drop generation methods may be exploited to make complex particles from thermoplastic polymers.

We provide some new insights into the kinetics and mechanism of sol-gel transition as it pertains to regenerated silk fibroin, which is the principle structural protein of silkworm silk fiber. Silk fibroin was dissolved in lithium bromide and dialyzed against deionized water to prepare a regenerated fibroin solution. This solution was found to be unstable at lower pH and transformed into a colloidal gel. The kinetics and mechanism of the sol-gel transition were investigated using rheology and light scattering. We show that gelation proceeds in two steps. In the first step, a weak gel is formed almost immediately upon lowering the pH, while in the second step further gelation proceeds rapidly after a long induction time to form a self-similar structure.

Biologic scaffolds composed of extracellular matrix (ECM) are commonly used repair devices in preclinical and clinical settings; however the use of these scaffolds for peripheral and central nervous system (CNS) repair has been limited. Biologic scaffolds developed from brain and spinal cord tissue have recently been described, yet the conformation of the harvested ECM limits therapeutic utility. An injectable CNS-ECM derived hydrogel capable of in vivo polymerization and conformation to irregular lesion geometries may aid in tissue reconstruction efforts following complex neurologic trauma. The objectives of the present study were to develop hydrogel forms of brain and spinal cord ECM and compare the resulting biochemical composition, mechanical properties, and neurotrophic potential of a brain derived cell line to a non-CNS-ECM hydrogel, urinary bladder matrix. Results showed distinct differences between compositions of brain ECM, spinal cord ECM, and urinary bladder matrix. The rheologic modulus of spinal cord ECM hydrogel was greater than that of brain ECM and urinary bladder matrix. All ECMs increased the number of cells expressing neurites, but only brain ECM increased neurite length, suggesting a possible tissue-specific effect. All hydrogels promoted three-dimensional uni- or bi-polar neurite outgrowth following 7 days in culture. These results suggest that CNS-ECM hydrogels may provide supportive scaffolding to promote in vivo axonal repair.

The ECM of mammalian tissues has been used as a scaffold to facilitate the repair and reconstruction of numerous tissues. Such scaffolds are prepared in many forms including sheets, powders, and hydrogels. ECM hydrogels provide advantages such as injectability, the ability to fill an irregularly shaped space, and the inherent bioactivity of native matrix. However, material properties of ECM hydrogels and the effect of these properties upon cell behavior are neither well understood nor controlled. The objective of this study was to prepare and determine the structure, mechanics, and the cell response in vitro and in vivo of ECM hydrogels prepared from decellularized porcine dermis and urinary bladder tissues. Dermal ECM hydrogels were characterized by a more dense fiber architecture and greater mechanical integrity than urinary bladder ECM hydrogels, and showed a dose dependent increase in mechanical properties with ECM concentration. In vitro, dermal ECM hydrogels supported greater C2C12 myoblast fusion, and less fibroblast infiltration and less fibroblast mediated hydrogel contraction than urinary bladder ECM hydrogels. Both hydrogels were rapidly infiltrated by host cells, primarily macrophages, when implanted in a rat abdominal wall defect. Both ECM hydrogels degraded by 35 days in vivo, but UBM hydrogels degraded more quickly, and with greater amounts of myogenesis than dermal ECM. These results show that ECM hydrogel properties can be varied and partially controlled by the scaffold tissue source, and that these properties can markedly affect cell behavior.

We examine the effects of interfacially active particles in blends of immiscible homopolymers with a droplet/matrix morphology. The effect of particles on drop size is not monotonic: at low volume fractions (∼0.1%), particles greatly increase the size of the dispersed phase by promoting flow-induced coalescence regardless of which phase wets the particles preferentially. At higher particle loadings (∼1 vol%), the particles completely cover the interface and reduce the size of the dispersed phase. Particles that are preferentially wetted by the continuous phase are also able to glue together two drops by bridging across them. In this case the morphology consists of clusters of particle-bridged drops, and the resulting blends show solid-like rheology. At low particle loadings, most of the particles are sandwiched between drops, leaving the rest of the interface particle-free. At high particle loadings, the entire surface is nearly covered with particles, the drops are faceted, and the morphology resembles a foam structure. In contrast to these results, particles that are preferentially wetted by the dispersed phase or equally wetted by both phases are not capable of bridging, and these blends do not show drop clusters. These results offer new insights on the behavior of ternary systems composed of two fluids and one particulate species.