Silk-Mesoporous Silica-Based Hybrid Macroporous Scaffolds using Ice-Templating Method: Mechanical, Release, and Biological Studies.

Development of biocompatible, biodegradable, and drug-eluting macroporous three-dimensional scaffolds that mimic the extracellular matrix of cells remains an important challenge in tissue engineering. In this endeavor, we report the preparation of self-standing macroporous scaffold composed of the natural biopolymer silk fibroin and mesoporous silica particle using the ice-templating strategy. Using methanol as a physical cross-linker, we were able to make self-standing scaffolds with very high mesoporous silica content (∼75% by weight) and with varying mechanical properties (38 ± 1.0 to 181 ± 5.9 kPa). These macroporous scaffolds have ∼80% porosity with an average pore size of 60 µm. Scaffolds that encapsulated the small molecule doxorubicin (as a model drug) and macromolecule fluorescein isothiocyanate conjugate-bovine serum albumin (FITC-BSA) (as a model protein) were also prepared. We demonstrate that the release behavior of encapsulated molecules like doxorubicin (∼35% release) and FITC-BSA (∼47% release) is largely influenced by their interaction with the mesoporous silica particles and the silk fibroin. The biodegradability property of silk hybrid scaffolds is also determined in the presence of protease enzyme, which demonstrates ∼90% degradation in 21 d. Biological studies on ice-templated hybrid silk scaffolds demonstrate excellent biocompatibility, which indicates that hybrid scaffolds are promising candidate for therapeutically relevant repair and regeneration of soft tissues such as tendon and nascent bone.

Preparation of macroporous scaffolds with holes in pore walls and pressure driven flows through them.

Controlling the pore architecture in macroporous scaffolds has important implications for their use as reactor packings and as catalyst supports. We report the preparation of a macroporous structure, where the pore walls are perforated by holes. These materials are prepared by modification of the ice-templating protocol developed in our group. We freeze a dispersion of colloidal silica, polymer and cross-linker in a water/acetonitrile medium and allow crosslinking to proceed in the frozen state. The presence of a small fraction of acetonitrile (varying between 1.6% to 6.4%) results in the formation of holes in the pore walls. Increasing the acetonitrile concentration changes the pore size distribution, and produces smaller pores on average. This also results in an increasing fraction of the wall area being covered by small pores, of the order of a few microns in size. Perforation of the walls by pores does not change the overall porosity or modulus of the scaffolds. However, the introduction of pores leads to a drastic reduction in the pressure drop required to pump liquid through the scaffolds. The observed residence time distribution (RTD) in the scaffolds is represented by two plug flow reactors (PFRs) in parallel. The RTD results indicate that increasing the hole fraction in the pore walls results in increased channelling which explains the aforementioned decreased pressure drop during pressure driven flow.

Fire-Retardant, Self-Extinguishing Inorganic/Polymer Composite Memory Foams.

Polymeric foams used in furniture and automotive and aircraft seating applications rely on the incorporation of environmentally hazardous fire-retardant additives to meet fire safety norms. This has occasioned significant interest in novel approaches to the elimination of fire-retardant additives. Foams based on polymer nanocomposites or based on fire-retardant coatings show compromised mechanical performance and require additional processing steps. Here, we demonstrate a one-step preparation of a fire-retardant ice-templated inorganic/polymer hybrid that does not incorporate fire-retardant additives. The hybrid foams exhibit excellent mechanical properties. They are elastic to large compressional strain, despite the high inorganic content. They also exhibit tunable mechanical recovery, including viscoelastic "memory". These hybrid foams are prepared using ice-templating that relies on a green solvent, water, as a porogen. Because these foams are predominantly comprised of inorganic components, they exhibit exceptional fire retardance in torch burn tests and are self-extinguishing. After being subjected to a flame, the foam retains its porous structure and does not drip or collapse. In micro-combustion calorimetry, the hybrid foams show a peak heat release rate that is only 25% that of a commercial fire-retardant polyurethanes. Finally, we demonstrate that we can use ice-templating to prepare hybrid foams with different inorganic colloids, including cheap commercial materials. We also demonstrate that ice-templating is amenable to scale up, without loss of mechanical performance or fire-retardant properties.

Capillary uptake in macroporous compressible sponges.

The capillarity-driven uptake of liquid in swellable, highly porous sponges is of significant industrial importance. Sponges prepared using polymers and their composites with carbon nanotubes and graphene have been reported, with extraordinary solvent uptake capacities and with the ability to separate oil from water. However, the effect of systematic variation of sponge characteristics on solvent uptake has not been investigated. Here, we report experiments that study capillary uptake in a variety of flexible, centimetre-sized macroporous cylindrical sponges. We used ice-templating to prepare a series of model macroporous sponges in which the porosity, modulus and composition were systematically varied. We investigated two kinds of sponge: (a) those composed purely of cross-linked polymers and (b) those prepared as composites of inorganic particles and polymers. Both kinds of sponge are flexible and exhibit elastic recovery after large compressive deformation. All sponges were characterized thoroughly with respect to their pore microstructure and elastic modulus. When one end of a sponge is plunged into a large reservoir, water rises through capillary action against gravity. We observed a transition from an inertial capillary regime, where the liquid column height rose linearly with time, t, to a viscous capillary regime, where the liquid height rose with time t0.5. We showed that these results can be rationalized using analyses developed for rigid sponges. We combined differential momentum balance equations for uptake in rigid capillaries with the phenomenological Ergun-Forchheimer relations to account for the effect of the sponge microstructure. This approach works remarkably well in the viscous capillary regime and shows that capillary uptake is governed primarily by the total porosity and pore dimensions of soft sponges.