Chameleon-Inspired Strain-Accommodating Smart Skin.

Stimuli-responsive color-changing hydrogels, commonly colored using embedded photonic crystals (PCs), have potential applications ranging from chemical sensing to camouflage and anti-counterfeiting. A major limitation in these PC hydrogels is that they require significant deformation (>20%) in order to change the PC lattice constant and generate an observable chromatic shift (∼100 nm). By analyzing the mechanism of how chameleon skin changes color, we developed a strain-accommodating smart skin (SASS), which maintains near-constant size during chromatic shifting. SASS is composed of two types of hydrogels: a stimuli-responsive, PC-containing hydrogel that is patterned within a second hydrogel with robust mechanical properties, which permits strain accommodation. In contrast to conventional "accordion"-type PC responsive hydrogels, SASS maintains near-constant volume during chromatic shifting. Importantly, SASS materials are stretchable (strain ∼150%), amenable to patterning, spectrally tunable, and responsive to both heat and natural sunlight. We demonstrate examples of using SASS for biomimicry. Our strategy, to embed responsive materials within a mechanically matched scaffolding polymer, provides a general framework to guide the future design of artificial smart skins.

Glycosylation of a model proto-RNA nucleobase with non-ribose sugars: implications for the prebiotic synthesis of nucleosides.

The emergence of nucleosides is an important, but poorly understood, element of the origins of life. We show that 2,4,6-triaminopyrimidine (TAP), a possible ancestral nucleobase of RNA, is glycosylated in water by non-ribose sugars in yields comparable to those previously reported for its reaction with ribose. The various sugars surveyed include ketoses and aldoses; tetroses, pentoses, and hexoses and are neutral, anionic, or cationic. Though they vary greatly in structure and properties, the data show that all sugars tested form glycosides with TAP. The structures of the eight TAP glycosides formed with glucose and two of its derivatives, glucose-6-phosphate and N-acetylglucosamine, were found to be ß-pyranosides with the glycosylation site on TAP varying with sugar identity. Our results suggest that prebiotic nucleoside formation would not have been restricted to ribose if ancestral RNA (or proto-RNA) utilized TAP and/or other proto-nucleobases with similar reactivities, and that the ability to form higher-order structures may have influenced proto-RNA monomer selection.

Integrin α3ß1 Binding to Fibronectin Is Dependent on the Ninth Type III Repeat.

Fibronectin (Fn) is a promiscuous ligand for numerous cell adhesion receptors or integrins. The vast majority of Fn-integrin interactions are mediated through the Fn Arg-Gly-Asp (RGD) motif located within the tenth type III repeat. In the case of integrins αIIbß3 and α5ß1, the integrin binds RGD and the synergy site (PHSRN) located within the adjacent ninth type III repeat. Prior work has shown that these synergy-dependent integrins are exquisitely sensitive to perturbations in the Fn integrin binding domain conformation. Our own prior studies of epithelial cell responses to recombinant fragments of the Fn integrin binding domain led us to hypothesize that integrin α3ß1 binding may also be modulated by the synergy site. To explore this hypothesis, we created a variety of recombinant variants of the Fn integrin binding domain: (i) a previously reported (Leu → Pro) stabilizing mutant (FnIII9'10), (ii) an Arg to Ala synergy site mutation (FnIII9(R)→(A)10), (iii) a two-Gly (FnIII9(2G)10) insertion, and (iv) a four-Gly (FNIII9(4G)10) insertion in the interdomain linker region and used surface plasmon resonance to determine binding kinetics of integrin α3ß1 to the Fn fragments. Integrin α3ß1 had the highest affinity for FnIII9'10 and FnIII9(2G)10. Mutation within the synergy site decreased integrin α3ß1 binding 17-fold, and the four-Gly insertion decreased binding 39-fold compared with FnIII9'10. Cell attachment studies demonstrate that α3ß1-mediated epithelial cell binding is greater on FnIII9'10 compared with the other fragments. These studies suggest that the presence and spacing of the RGD and synergy sites modulate integrin α3ß1 binding to Fn.

Ultrasoft, highly deformable microgels.

Microgels are colloidally stable, hydrogel microparticles that have previously been used in a range of (soft) material applications due to their tunable mechanical and chemical properties. Most commonly, thermo and pH-responsive poly(N-isopropylacrylamide) (pNIPAm) microgels can be fabricated by precipitation polymerization in the presence of the co-monomer acrylic acid (AAc). Traditionally pNIPAm microgels are synthesized in the presence of a crosslinking agent, such as N,N'-methylenebisacrylamide (BIS), however, microgels can also be synthesized under 'crosslinker free' conditions. The resulting particles have extremely low (<0.5%), core-localized crosslinking resulting from rare chain transfer reactions. AFM nanoindentation of these ultralow crosslinked (ULC) particles indicate that they are soft relative to crosslinked microgels, with a Young's modulus of ∼10 kPa. Furthermore, ULC microgels are highly deformable as indicated by a high degree of spreading on glass surfaces and the ability to translocate through nanopores significantly smaller than the hydrodynamic diameter of the particles. The size and charge of ULCs can be easily modulated by altering reaction conditions, such as temperature, monomer, surfactant and initiator concentrations, and through the addition of co-monomers. Microgels based on the widely utilized, biocompatible polymer polyethylene glycol (PEG) can also be synthesized under crosslinker free conditions. Due to their softness and deformability, ULC microgels are a unique base material for a wide variety of biomedical applications including biomaterials for drug delivery and regenerative medicine.

Modulation of the deswelling temperature of thermoresponsive microgel films.

We demonstrate fine-tuning of the deswelling temperatures of thermoresponsive microgels within a biologically relevant range (30-40 °C). This was achieved by copolymerizing N-isopropylacrylamide and N-isopropylmethacrylamide (NIPAm and NIPMAm, respectively) in varying ratios; the parent homopolymers are well-known thermoresponsive polymers. Polyelectrolyte layer-by-layer (LbL) assemblies of these microgels retain the temperature response properties as demonstrated by temperature-dependent light scattering. Furthermore, films composed of more than one type of microgel building block were shown to have multiple temperature responses similar to those observed for the individual building blocks, permitting further tailoring of the temperature responsive interface. Additional experiments with mixed composition films, investigating multiple assembly processes, show that the location of the microgels within the film does not interfere with the temperature response. This suggests that microgels within the polyelectrolyte assembly behave independently of neighboring microgels with respect to their thermally induced deswelling.