A Smart Skin for Hydrogels That Enables Switchable Solute Release.

Many applications of hydrogels rely on their ability to deliver encapsulated solutes, such as drugs; however, small hydrophilic solutes rapidly leak out of gels by diffusion. A need exists for a way to regulate solute release out of gels─to ensure zero release until a desired time (the OFF state) and thereafter for the release to be switched ON at a high rate. This should ideally be a repeatable switch; i.e., the gel should be cyclable repeatedly between the ON and OFF states. Such perfect, cyclical ON-OFF release of solutes from gels is demonstrated for the first time through a "smart skin" that is synthesized rapidly (in ∼10 min) around an entire gel. The thin (∼100 µm) and transparent polymer skin is endowed with redox-responsive properties through the use of urethane and acrylate monomers, one of which contains a thioether group. Initially, the skin is hydrophobic (water contact angle 102°), and it completely prevents hydrophilic solutes from leaking out of the gel. When contacted with oxidants such as hydrogen peroxide (H2O2), the thioethers are converted to sulfoxides, making the skin hydrophilic (water contact angle 42°) and thereby turning ON the release of solutes. Conversely, solute release can be turned OFF subsequently by adding a reducing agent such as vitamin C that reverts the sulfoxides to thioethers and thus returns the skin to its hydrophobic state. The release rate in the ON state can be tuned via the skin thickness as well as the oxidant concentration. The ability to regulate solute delivery from gels using smart skins is likely to prove significant in areas ranging from separations to agriculture and drug delivery.

How a Gel Can Protect an Egg: A Flexible Hydrogel with Embedded Starch Particles Shields Fragile Objects Against Impact.

Hydrogels are networks of polymer chains that are swollen in water. In recent years, several routes have been devised to make hydrogels that are flexible and bendable. This work investigates whether such flexible gels can be wrapped around brittle or fragile objects (such as an egg or a fruit) and protect the objects against impact. We study gels made by either physical cross-linking (e.g., gelatin) or chemical cross-linking (e.g., acrylamide) and the same gels with various particulate additives. None of the bare gels are protective, and nanoparticles like iron oxide or silica do not help. However, the addition of starch granules to the above gels greatly enhances their protective abilities. When a load strikes a gelatin gel containing 20% starch, the peak impact force is reduced by 25% when compared to a bare gel without the starch. Correspondingly, the coefficient of restitution (COR) is also lowered by the presence of starch (i.e., a ball bounces less on a starch-bearing gel). We correlate the impact-absorbing effects of starch granules to their ability to shear-thicken water. When starch granules are gelatinized by heat, they no longer give rise to shear-thickening, and in turn, their protective ability in a gel is also eliminated. Our research can guide the rational design of protective coatings or armor for fragile objects, which could be applied in the sports, defense, and consumer sectors.

A Simple Way to Synthesize a Protective "Skin" around Any Hydrogel.

In nature, various structures such as fruits and vegetables have a water-rich core that is covered by a hydrophobic layer, i.e., their skin. The skin creates a barrier that prevents chemicals in the external environment from entering the core; at the same time, the skin also ensures that the water in the core is preserved and not lost by evaporation. Currently, for many applications involving hydrogels, especially in areas such as soft robotics or bioelectronic interfaces, it would be advantageous if the gel could be encased in a skin-like material. However, forming such a skin around a gel has proved challenging because the skin would need to be a hydrophobic material with a distinct chemistry from the hydrophilic gel core. Here, we present a simple solution to this problem, which allows any hydrogel of arbitrary composition and geometry to be encased by a thin, transparent "skin." Our synthesis technique involves an inside-out polymerization, where one component of the polymerization (the initiator) is present only in the gel core, while other components (the monomers) are present only in the external medium. Accordingly, a thin polymeric layer (∼10-100 µm in thickness) grows outward from the core, and the entire process can be completed in a few minutes. We show that the presence of the skin prevents the gel from swelling in water and also from drying in air. Likewise, hydrophilic solutes in the gel core are completely prevented by the skin from leaking out into the external solution, while harsh chemicals (e.g., acids, bases, and chelators) or harmful microbes are prevented from entering the gels. The properties of the skin are all tunable, including its thickness and its mechanical properties. When the monomer used is urethane diacrylate, the resulting polyurethane skin is elastomeric, transparent, and peelable from the core gel. Conversely, when polyethylene glycol dimethacrylate is used as the monomer, the skin is hard and brittle (glass-like). The ability to grow a skin readily around any given hydrogel is likely to prove useful in numerous applications, such as in maintaining the electrical functionality of gel-based wires or circuit elements.

Phase-Selective Gelation of the Water Phase in an Oil-Water Mixture: An Approach Based on Oil-Activated Nanoparticle Assembly in Water.

Phase-selective gelation refers to the selective gelation of one phase in an immiscible mixture. Thus far, all such examples have involved a molecular gelator forming nanofibers in (and thus gelling) the oil phase in an oil/water mixture. Here, for the first time, we report the counterpart to the above phenomenon, i.e., selective gelation of the water phase in an oil/water mixture (while leaving the oil undisturbed). This has been a challenging problem because moieties that gel water tend to be either amphiphilic or oil-soluble; thus, if combined with an oil/water mixture, they invariably form an emulsion. Our approach solves this problem by exploiting the tunable self-assembly of laponite (LAP) nanoparticles. Initially, LAP nanoparticles (25 nm disks) are dispersed in water, where they remain unaggregated due to the steric stabilization provided by a triblock copolymer (Pluronic P123) adsorbed on their surface. Thus, the dispersion is initially a low-viscosity sol. When an immiscible oil such as hexadecane is introduced above the sol, the mixture remains biphasic, and both phases remain unaffected. Next, an organic acid such as butanoic acid (BA) is added to the oil. The BA is oil-soluble but also has limited solubility in the water. Over about 30 min, some of the BA enters the water, whereupon it "activates" the self-assembly of LAP particles into a three-dimensional "house-of-cards" network. Ultimately, the water phase is converted into a homogeneous gel with a sufficient yield stress: the aqueous gel holds its weight in the inverted vial while the oil phase remains a thin liquid that can be poured out of the vial. On the whole, the concept advanced here is about activating nanoparticle assembly in water through an adjacent, immiscible phase. This concept could prove useful in conducting certain separations or reactions in the laboratory as well as in enhanced oil recovery.