Catalyst-Loaded Capsules that Spontaneously Inflate and Violently Eject their Core.

We present a design for polymer capsules that exhibit a range of unusual autonomous behaviors when exposed to a chemical fuel. The capsules have a physically gelled core (alginate-Ca2+) loaded with catalytic (silver) particles and a shell composed of a chemically cross-linked gel. In the presence of the fuel (H2O2), a catalytic reaction occurs, which generates oxygen (O2) gas. The gas collects in a zone between the core and the shell, and the resulting gas pressure causes the elastic shell to stretch. This makes the capsule inflate in a process reminiscent of a swelling pufferfish. As the capsule inflates, the polymer chains in the shell continue to stretch until a breaking point is reached, whereupon the shell ruptures. Three rupture modes are documented: gentle, moderate, and violent. The latter involves the gelled core being forcefully ejected out of the shell in a manner similar to the ejection of needles out of nematocysts on jellyfish. The extent and duration of inflation can be tuned by altering the core and shell composition; for example, shells that are more densely cross-linked swell less and rupture faster. Also, instead of a catalytic reaction, capsule inflation can be achieved by combining reactants, one in the capsule and the other in the external solution, that together generate a different gas (e.g., CO2).

Single-Molecule Orthogonal Double-Click Chemistry-Inorganic to Organic Nanostructure Transition.

Thiasilacyclopentane (TSCP) and azasilacyclopentane (ASCP) heteroatom cyclics have proven capable of rapidly converting hydroxylated surfaces to functionalized surfaces in inorganic click reactions. In this work, we demonstrate that the use of these reagents can be extended to "simultaneous double-clicking" when both inorganic and organic substrates are present at the onset of the reaction. The simultaneous double-click depends on a first ring-opening click with an inorganic substrate that is complete in ∼1 s at 30 °C and results in the reveal of a cryptic mercaptan or secondary amine group, which can then participate in a second click with an organic substrate. TSCPs and ASCPs can take part in tandem double-click reactions in which the organic substrate is added to the reaction mixture after the initial inorganic click reaction is completed. Additionally, ASCPs with exocyclic functionality, specifically N-alkenyl-, N-aminoalkyl, and N-alkynyl-ASCPs, are shown to be options for tandem double-clicking in which functionalization proceeds in two independent steps and the sequence of the double-click reaction can be reversed.

Programming the Shape Transformation of a Composite Hydrogel Sheet via Erasable and Rewritable Nanoparticle Patterns.

Hydrogels with shapes that can be adapted to their environment have attracted great attention from both academia and industry. We report herein a new and robust strategy to reprogram the light-induced shape transformation of a thermoresponsive composite hydrogel sheet with erasable and rewritable patterns of iron oxide nanoparticles as photothermal agents. Numerous distinct and reversible shape transformations are achieved from a single hydrogel sheet by repeatably writing in the sheet with different nanoparticle patterns. The shape transformations were verified by finite element modeling. The present strategy is simple, fast, and efficient in reprogramming the shape change of the thermoresponsive hydrogel material. The composite hydrogel sheet may find applications in soft robotics, tissue engineering, and controlled release.

A shape-shifting composite hydrogel sheet with spatially patterned plasmonic nanoparticles.

We report a simple and reliable approach to fabricate composite hydrogel sheets with spatially patterned regions of plasmonic gold nanoparticles using a combination of contact printing and diffusion-controlled galvanic replacement reaction. In response to near-infrared laser irradiation, the localized increase in temperature induced the controlled shape deformation of the composite hydrogels, due to the combined effect of photothermal heating of the loaded gold nanoparticles and the thermal responsiveness of the hydrogel matrix. The same hydrogel can be designed to exhibit different modes of shape deformation depending on the direction of light irradiation, which has rarely been reported previously. The composite hydrogels may find applications in biomedicine and soft robots.

Nature-Inspired Hydrogels with Soft and Stiff Zones that Exhibit a 100-Fold Difference in Elastic Modulus.

Many biological materials, such as the squid beak and the spinal disc, have a combination of stiff and soft parts with very different mechanical properties, for example, the elastic modulus (stiffness) of the stiffest part of the squid beak is about 100 times that of the softest part. Researchers have attempted to mimic such structures using hydrogels but have not succeeded in synthesizing bulk gels with such large variations in moduli. Here, we present a general approach that can be used to form hydrogels with two or more zones having appreciably different mechanical characters. For this purpose, we use a technique developed in our lab for creating hybrid hydrogels with distinct zones. For the soft zone of the gel, we form a polymer network using a conventional acrylic monomer [ N, N'-dimethylacrylamide (DMAA)] and with laponite (LAP) nanoparticles as the cross-linkers. For the stiff zone, we combine DMAA, LAP, and a methacrylated silica precursor ([3-(methacryloyloxy)-propyl]trimethoxy-silane). When this mixture is polymerized, nanoscale silica particles (∼300 nm in diameter) are formed, and these serve as additional cross-links between the polymer chains, making this network very stiff. The unique character of each zone is preserved in the hybrid gel, and different zones are covalently linked to each other, thereby ensuring robust interfaces. Rheological measurements show that the elastic modulus of the stiff zone can be more than 100 times that of the soft zone. This ratio of moduli is the highest reported to date in a single, continuous gel and is comparable to the ratio in the squid beak. We present different variations of our soft-stiff hybrid gels, including multizone cylinders and core-shell discs. Such soft-stiff gels could have utility in bioengineering, such as in interfacing stiff medical implants with soft tissues.