No abstract

Synthetic dispersants such as Corexit 9500A were used in large quantities (∼2 million gallons) to disperse the oil spilled in the ocean during the recent Deepwater Horizon event. These dispersant formulations contain a blend of surfactants in a base of organic solvent. Some concerns have been raised regarding the aquatic toxicity and environmental impact of these formulations. In an effort to create a safer dispersant, we have examined the ability of food-grade amphiphiles to disperse (emulsify) crude oil in seawater. Our studies show that an effective emulsifier is obtained by combining two such amphiphiles: lecithin (L), a phospholipid extracted from soybeans, and Tween 80 (T), a surfactant used in many food products including ice cream. Interestingly, we find that L/T blends show a synergistic effect, i.e., their combination is an effective emulsifier, but neither L or T is effective on its own. This synergy is maximized at a 60/40 weight ratio of L/T and is attributed to the following reasons: (i) L and T pack closely at the oil-water interface; (ii) L has a low tendency to desorb, which fortifies the interfacial film; and (iii) the large headgroup of T provides steric repulsions between the oil droplets and prevents their coalescence. A comparison of L/T with Corexit 9500A shows that the former leads to smaller oil droplets that remain stable to coalescence for a much longer time. The smaller size and stability of crude oil droplets are believed to be important to their dispersion and eventual microbial degradation in the ocean. Our findings suggest that L/T blends could potentially be a viable alternative for the dispersion of oil spills.

This communication describes a novel strategy to achieve programmable shape transformation of hybrid hydrogel sheets by modulating both the in-plane and out-of-plane mismatches in mechanical properties. Both our experimental and computational results demonstrate that the shape transformation of hybrid hydrogel sheets shows rich features (e.g., rolling direction, axis, chirality, etc.) and versatile tunability (e.g., via various external stimuli, material properties, pattern geometry, etc.). This work can provide guidance for designing soft materials that are able to undergo more precise and complex shape transformation.

External triggers such as pH or temperature can induce hydrogels to swell or shrink rapidly. Recently, these triggers have also been used to alter the three-dimensional (3-D) shapes of gels: for example, a flat gel sheet can be induced to fold into a tube. Self-folding gels are reminiscent of natural structures such as the Venus flytrap, which folds its leaves to entrap its prey. They are also of interest for applications in sensing or microrobotics. However, to advance the utility of self-folding gels, the range of triggers needs to be expanded beyond the conventional ones. Toward this end, we have designed a class of gels that change shape in response to very low concentrations of specific biomolecules. The gels are hybrids of three different constituents: (A) polyethylene glycol diacrylate (PEGDA); (B) gelatin methacrylate-co-polyethylene glycol dimethacrylate (GelMA-co-PEGDMA); and (C) N-isopropylacrylamide (NIPA). The thin-film hybrid is constructed as a bilayer or sandwich of two layers, with an A/B layer (alternating strips of A and B) sandwiched above a layer of gel C. Initially, when this hybrid gel is placed in water, the C layer is much more swollen than the A/B layer. Despite the swelling mismatch, the sheet remains flat because the A/B layer is very stiff. When collagenase enzyme is added to the water, it cleaves the gelatin chains in B, thus reducing the stiffness of the A/B layer. As a result, the swollen C layer is able to fold over the A/B layer, causing the sheet to transform into a specific shape. The typical transition is from flat sheet to closed hollow tube, and the time scale for this transition decreases with increasing enzyme concentration. Shape transitions are induced by enzyme levels as low as 0.75 U/mL. Interestingly, a shape transition is also induced by adding the lysate of murine fibroblast cells, which contains enzymes from the matrix metalloproteinase (MMP) family at levels around 0.1 U/mL (MMPs are similar to collagenase in their ability to cleave gelatin). We further show that transitions from flat sheets to other shapes such as helices and pancakes can be engineered by altering the design pattern of the gel. Additionally, we have made a rudimentary analog of the Venus flytrap, with two flat gels ("leaves") flanking a central folding gel ("hinge"). When enzyme is added, the hinge bends and brings the leaves together, trapping objects in the middle.