Shape-Morphing Materials from Stimuli-Responsive Hydrogel Hybrids.

The formation of well-defined and functional three-dimensional (3D) structures by buckling of thin sheets subjected to spatially nonuniform stresses is common in biological morphogenesis and has become a subject of great interest in synthetic systems, as such programmable shape-morphing materials hold promise in areas including drug delivery, biomedical devices, soft robotics, and biomimetic systems. Given their ability to undergo large changes in swelling in response to a wide variety of stimuli, hydrogels have naturally emerged as a key type of material in this field. Of particular interest are hybrid systems containing rigid inclusions that can define both the anisotropy and spatial nonuniformity of swelling as well as nanoparticulate additives that can enhance the responsiveness and functionality of the material. In this Account, we discuss recent progress in approaches to achieve well-defined shape morphing in hydrogel hybrids. First, we provide an overview of materials and methods that facilitate fabrication of such systems and outline the geometry and mechanics behind shape morphing of thin sheets. We then discuss how patterning of stiff inclusions within soft responsive hydrogels can be used to program both bending and swelling, thereby providing access to a wide array of complex 3D forms. The use of discretely patterned stiff regions to provide an effective composite response offers distinct advantages in terms of scalability and ease of fabrication compared with approaches based on smooth gradients within a single layer of responsive material. We discuss a number of recent advances wherein control of the mechanical properties and geometric characteristics of patterned stiff elements enables the formation of 3D shapes, including origami-inspired structures, concatenated helical frameworks, and surfaces with nonzero Gaussian curvature. Next, we outline how the inclusion of functional elements such as nanoparticles can enable unique pathways to programmable and even reprogrammable shape-morphing materials. We focus to a large extent on photothermally reprogrammable systems that include one of a variety of additives that serve to efficiently absorb light and convert it into heat, thereby driving the response of a temperature-sensitive hydrogel. Such systems are advantageous in that patterns of light can be defined with very high spatial and temporal resolution in addition to offering the potential for wavelength-selective addressability of multiple different inclusions. We highlight recent advances in the preparation of light-responsive hybrid systems capable of undergoing reprogrammable bending and buckling into well-defined 3D shapes. In addition, we describe several examples where shape tuning of hybrid systems enables control over the motion of responsive hydrogel-based materials. Finally, we offer our perspective on open challenges and future areas of interest for the field.

Origami structures with a critical transition to bistability arising from hidden degrees of freedom.

Origami is used beyond purely aesthetic pursuits to design responsive and customizable mechanical metamaterials. However, a generalized physical understanding of origami remains elusive, owing to the challenge of determining whether local kinematic constraints are globally compatible and to an incomplete understanding of how the folded sheet's material properties contribute to the overall mechanical response. Here, we show that the traditional square twist, whose crease pattern has zero degrees of freedom (DOF) and therefore should not be foldable, can nevertheless be folded by accessing bending deformations that are not explicit in the crease pattern. These hidden bending DOF are separated from the crease DOF by an energy gap that gives rise to a geometrically driven critical bifurcation between mono- and bistability. Noting its potential utility for fabricating mechanical switches, we use a temperature-responsive polymer-gel version of the square twist to demonstrate hysteretic folding dynamics at the sub-millimetre scale.

Photopatternable Biodegradable Aliphatic Polyester with Pendent Benzophenone Groups.

Highly efficient photo-cross-linking reactions enable numerous applications in biomaterials. Here, a photopatternable biodegradable aliphatic polyester with benzophenone pendent groups was synthesized by copper-catalyzed alkyne-azide cycloaddition, affording polyesters that undergo UV-induced cross-linking to yield photopatterned films. Using this material, a self-folding multilayer structure containing polyester/hydrogel bilayer hinges was fabricated. Upon swelling of the hydrogel layer, the construct folds into a triangular tube, which subsequently unfolds due to lipase-catalyzed degradation of the polyester layer. The ability to precisely design such degradation-induced structural changes offers potential for biomaterials and medical applications, such as evolving and responsive 2D and 3D tissue engineering scaffolds.

Tuning innate immune activation by surface texturing of polymer microparticles: the role of shape in inflammasome activation.

Polymeric microparticles have been widely investigated as platforms for delivery of drugs, vaccines, and imaging contrast agents and are increasingly used in a variety of clinical applications. Microparticles activate the inflammasome complex and induce the processing and secretion of IL-1ß, a key innate immune cytokine. Recent work suggests that although receptors are clearly important for particle phagocytosis, other physical characteristics, especially shape, play an important role in the way microparticles activate cells. We examined the role of particle surface texturing not only on uptake efficiency but also on the subsequent immune cell activation of the inflammasome. Using a method based on emulsion processing of amphiphilic block copolymers, we prepared microparticles with similar overall sizes and surface chemistries but having either smooth or highly microtextured surfaces. In vivo, textured (budding) particles induced more rapid neutrophil recruitment to the injection site. In vitro, budding particles were more readily phagocytosed than smooth particles and induced more lipid raft recruitment to the phagosome. Remarkably, budding particles also induced stronger IL-1ß secretion than smooth particles through activation of the NLRP3 inflammasome. These findings demonstrate a pronounced role of particle surface topography in immune cell activation, suggesting that shape is a major determinant of inflammasome activation.

Kinetically trapped co-continuous polymer morphologies through intraphase gelation of nanoparticles.

We describe an approach to prepare co-continuous microstructured blends of polymers and nanoparticles by formation of a percolating network of particles within one phase of a polymer mixture undergoing spinodal decomposition. Nanorods or nanospheres of CdSe were added to near-critical blends of polystyrene and poly(vinyl methyl ether) quenched to above their lower critical solution temperature. Beyond a critical loading of nanoparticles, phase separation is arrested due to the aggregation of particles into a network (or colloidal gel) within the poly(vinyl methyl ether) phase, yielding a co-continuous spinodal-like structure with a characteristic length scale of several micrometers. The critical concentration of nanorods to achieve kinetic arrest is found to be smaller than for nanospheres, which is in qualitative agreement with the expected dependence of the nanoparticle percolation threshold on aspect ratio. Compared to structural arrest by interfacial jamming, our approach avoids the necessity for neutral wetting of particles by the two phases, providing a general pathway to co-continuous micro- and nanoscopic structures.

Dynamic display of biomolecular patterns through an elastic creasing instability of stimuli-responsive hydrogels.

Surfaces with physicochemical properties that can be modulated using external stimuli offer great promise for designing responsive or adaptive materials. Here, we describe biocompatible dynamic scaffolds based on thin hydrogel coatings that reversibly hide and display surface chemical patterns in response to temperature changes. At room temperature, the gel absorbs water, triggering an elastic creasing instability that sequesters functionalized regions within tight folds in the surface. Deswelling at approximately 37 degrees C causes the gel surface to unfold, thereby regenerating the biomolecular patterns. Crease positions are directed by topographic features on the underlying substrate, and are translated into two-dimensional micrometre-scale surface chemical patterns through selective deposition of biochemically functionalized polyelectrolytes. We demonstrate specific applications of these dynamic scaffolds--selective capture, sequestration and release of micrometre-sized beads, tunable activity of surface-immobilized enzymes and reversible encapsulation of adherent cells--which offer promise for incorporation within lab-on-a-chip devices or as dynamic substrates for cellular biology.

Surface creasing instability of soft polyacrylamide cell culture substrates.

Efforts to understand and engineer cell behavior in mechanically soft environments frequently employ two-dimensional cell culture substrates consisting of thin hydrogel layers with low elastic modulus supported on rigid substrates to facilitate culturing, imaging, and analysis. Here we characterize how an elastic creasing instability of the gel surface may occur for the most widely used soft cell culture substrate, polyacrylamide hydrogels, and show that stem cells respond to and change their behavior due to these surface features. The regions of stability and corresponding achievable ranges of modulus are elucidated in terms of the monomer and cross-linker concentrations, providing guidance for the synthesis of both smooth and creased soft cell substrates for basic and applied cell engineering efforts.

Spontaneous generation of amphiphilic block copolymer micelles with multiple morphologies through interfacial Instabilities.

We introduce a method for the formation of block copolymer micelles through interfacial instabilities of emulsion droplets. Amphiphilic polystyrene-block-poly(ethylene oxide) (PS-PEO) copolymers are first dissolved in chloroform; this solution is then emulsified in water and chloroform is extracted by evaporation. As the droplets shrink, the organic solvent/water interface becomes unstable, spontaneously generating a new interface and leading to dispersion of the copolymer as micellar aggregates in the aqueous phase. Depending on the composition of the copolymer, spherical or cylindrical micelles are formed, and the method is shown to be general to polymers with several different hydrophobic blocks: poly(1,4-butadiene), poly(-caprolactone), and poly(methyl methacrylate). Using this method, hydrophobic species dissolved or suspended in the organic phase along with the amphiphilic copolymer can be incorporated into the resulting micelles. For example, addition of PS homopolymer, or a PS-PEO copolymer of different composition and molecular weight, allows the diameter and morphology of wormlike micelles to be tuned, while addition of hydrophobically coated iron oxide nanoparticles enables the preparation of magnetically loaded spherical and wormlike micelles.

Interfacial tension of evaporating emulsion droplets containing amphiphilic block copolymers: effects of solvent and polymer composition.

Evaporating droplets of volatile organic solvent containing amphiphilic block copolymers may undergo hydrodynamic instabilities that lead to dispersal of copolymer micelles into the surrounding aqueous phase. As for related phenomena in reactive polymer blends and oil/water/surfactant systems, this process has been ascribed to a nearly vanishing or transiently negative interfacial tension between the water and solvent phases induced by adsorption of copolymer to the interface. In this report, we investigate the influence of the choice of organic solvent and polymer composition for a series of polystyrene-b-poly(ethylene oxide) (PS-PEO) diblock copolymers, by in situ micropipette tensiometry on evaporating emulsion drops. These measurements suggest that the sensitivity to the organic solvent chosen reflects both differences in the bare solvent/water interfacial tension as well as the propensity of the copolymer to aggregate within the organic phase. While instabilities coincident with an approach of the interfacial tension nearly to zero were observed only for copolymers with PEO content greater than 15 wt.%, beyond this point the interfacial behavior and critical concentration needed to trigger surface instability were found to depend only weakly on copolymer composition.

Photonic multilayer sensors from photo-crosslinkable polymer films.

Colorimetric temperature sensors are prepared from photo-crosslinkable polymers by sequentially spin-coating and crosslinking alternating layers of poly(N-isopropylacrylamide) and poly(para-methyl styrene). Layer thicknesses and copolymer chemistries are chosen to provide robust colorimetric temperature sensors that cover nearly the full visible spectrum.

Nanocomposite "superhighways" by solution assembly of semiconductor nanostructures with ligand-functionalized conjugated polymers.

Poly(3-hexyl thiophene) containing chain-end thiols or phosphonic acids is crystallized to yield nanowires with the functional groups at the wire edges. CdSe quantum dots and nanorods associate with these fibrils, leading to 'superhighways' that consist of alternating parallel lanes of conjugated polymer and CdSe.

Multifunctional nanoparticle-loaded spherical and wormlike micelles formed by interfacial instabilities.

Hybrid spherical and wormlike amphiphilic block copolymer micelles are formed through evaporation-induced interfacial instabilities of emulsion droplets, allowing the incorporation of pre-synthesized hydrophobic inorganic nanoparticles within the micelle cores, as well as co-encapsulation of different nanoparticles. This encapsulation behavior is largely insensitive to particle surface chemistry, shape, and size, thus providing a versatile route to fabricate multifunctional micelles.

Enhancement of anhydrous proton transport by supramolecular nanochannels in comb polymers.

Transporting protons is essential in several biological processes as well as in renewable energy devices, such as fuel cells. Although biological systems exhibit precise supramolecular organization of chemical functionalities on the nanoscale to effect highly efficient proton conduction, to achieve similar organization in artificial systems remains a daunting challenge. Here, we are concerned with transporting protons on a micron scale under anhydrous conditions, that is proton transfer unassisted by any solvent, especially water. We report that proton-conducting systems derived from facially amphiphilic polymers that exhibit organized supramolecular assemblies show a dramatic enhancement in anhydrous conductivity relative to analogous materials that lack the capacity for self-organization. We describe the design, synthesis and characterization of these macromolecules, and suggest that nanoscale organization of proton-conducting functionalities is a key consideration in obtaining efficient anhydrous proton transport.