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Hydrogels with improved mechanical properties, made by combining polymer networks with ionic and covalent crosslinks, should expand the scope of applications, and may serve as model systems to explore mechanisms of deformation and energy dissipation. A long stretch for hydrogels Hydrogels are used in flexible contact lenses, as scaffolds for tissue engineering and in drug delivery. Their poor mechanical properties have so far limited the scope of their applications, but new strong and stretchy materials reported here could take hydrogels into uncharted territories. The new system involves a double-network gel, with one network forming ionic crosslinks and the other forming covalent crosslinks. The fracture energy of these materials is very high: they can stretch to beyond 17 times their own length even when containing defects that usually initiate crack formation in hydrogels. The materials' toughness is attributed to crack bridging by the covalent network accompanied by energy dissipation through unzipping of the ionic crosslinks in the second network. Hydrogels are used as scaffolds for tissue engineering 1 , vehicles for drug delivery 2 , actuators for optics and fluidics 3 , and model extracellular matrices for biological studies 4 . The scope of hydrogel applications, however, is often severely limited by their mechanical behaviour 5 . Most hydrogels do not exhibit high stretchability; for example, an alginate hydrogel ruptures when stretched to about 1.2 times its original length. Some synthetic elastic hydrogels 6 , 7 have achieved stretches in the range 10–20, but these values are markedly reduced in samples containing notches. Most hydrogels are brittle, with fracture energies of about 10 J m −2 (ref. 8 ), as compared with ∼1,000 J m −2 for cartilage 9 and ∼10,000 J m −2 for natural rubbers 10 . Intense efforts are devoted to synthesizing hydrogels with improved mechanical properties 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 ; certain synthetic gels have reached fracture energies of 100–1,000 J m −2 (refs 11 , 14 , 17 ). Here we report the synthesis of hydrogels from polymers forming ionically and covalently crosslinked networks. Although such gels contain ∼90% water, they can be stretched beyond 20 times their initial length, and have fracture energies of ∼9,000 J m −2 . Even for samples containing notches, a stretch of 17 is demonstrated. We attribute the gels’ toughness to the synergy of two mechanisms: crack bridging by the network of covalent crosslinks, and hysteresis by unzipping the network of ionic crosslinks. Furthermore, the network of covalent crosslinks preserves the memory of the initial state, so that much of the large deformation is removed on unloading. The unzipped ionic crosslinks cause internal damage, which heals by re-zipping. These gels may serve as model systems to explore mechanisms of deformation and energy dissipation, and expand the scope of hydrogel applications.

Thermal transitions of polyisocyanide single molecules to polymer bundles and finally networks lead to hydrogels mimicking the properties of biopolymer intermediate-filament networks; their analysis shows that bundling and chain stiffness are crucial design parameters for hydrogels. Biomimetic polymer networks This paper describes a new class of water-soluble, relatively stiff polymers that bundle in a controlled manner on heating to produce very stiff fibres. These fibres, in turn, form hydrogels that very closely mimic components of the cell cytoskeleton, intermediate filaments. Synthesis involves the thermal transition of polyisocyanide polymers from single molecules to bundles of polymer chains. Networks made with this material demonstrate a stress-stiffening behaviour that is usually absent in synthetic polymer gels, and their mechanical properties can be modified by altering the chemical structure of the polymer, offering greater versatility than biopolymer networks. Mechanical responsiveness is essential to all biological systems down to the level of tissues and cells 1 , 2 . The intra- and extracellular mechanics of such systems are governed by a series of proteins, such as microtubules, actin, intermediate filaments and collagen 3 , 4 . As a general design motif, these proteins self-assemble into helical structures and superstructures that differ in diameter and persistence length to cover the full mechanical spectrum 1 . Gels of cytoskeletal proteins display particular mechanical responses (stress stiffening) that until now have been absent in synthetic polymeric and low-molar-mass gels. Here we present synthetic gels that mimic in nearly all aspects gels prepared from intermediate filaments. They are prepared from polyisocyanopeptides 5 , 6 , 7 grafted with oligo(ethylene glycol) side chains. These responsive polymers possess a stiff and helical architecture, and show a tunable thermal transition where the chains bundle together to generate transparent gels at extremely low concentrations. Using characterization techniques operating at different length scales (for example, macroscopic rheology, atomic force microscopy and molecular force spectroscopy) combined with an appropriate theoretical network model 8 , 9 , 10 , we establish the hierarchical relationship between the bulk mechanical properties and the single-molecule parameters. Our results show that to develop artificial cytoskeletal or extracellular matrix mimics, the essential design parameters are not only the molecular stiffness, but also the extent of bundling. In contrast to the peptidic materials, our polyisocyanide polymers are readily modified, giving a starting point for functional biomimetic hydrogels with potentially a wide variety of applications 11 , 12 , 13 , 14 , in particular in the biomedical field.

Different polymers can be used in combination to produce coexisting nanoparticles of different symmetry and tailored to co-assemble into well-ordered binary and ternary hierarchical structures. Patched-up assemblies There is considerable practical interest in developing the tools to fabricate multicomponent artificial systems that mimic the hierarchical ordering seen in the natural world — complex biomaterials can be assembled from the simple but precisely defined molecular building blocks. André Gröschel and colleagues have developed a bottom-up approach that's a step in that direction. Previously they designed simple linear polymers that self-assemble in solution to produce monodisperse nanoparticles with well-defined interaction anisotropies — surface 'patches' that direct self-assembly of the particles into larger structures. Now they show that different polymers can be used in combination to produce coexisting nanoparticles of different symmetry and tailored to co-assemble into well-ordered binary and ternary hierarchical structures. Possible applications of this approach range from smart materials to photonics. The concept of hierarchical bottom-up structuring commonly encountered in natural materials provides inspiration for the design of complex artificial materials with advanced functionalities 1 , 2 . Natural processes have achieved the orchestration of multicomponent systems across many length scales with very high precision 3 , 4 , but man-made self-assemblies still face obstacles in realizing well-defined hierarchical structures 5 , 6 , 7 , 8 , 9 , 10 , 11 . In particle-based self-assembly, the challenge is to program symmetries and periodicities of superstructures by providing monodisperse building blocks with suitable shape anisotropy or anisotropic interaction patterns (‘patches’). Irregularities in particle architecture are intolerable because they generate defects that amplify throughout the hierarchical levels. For patchy microscopic hard colloids, this challenge has been approached by using top-down methods (such as metal shading or microcontact printing), enabling molecule-like directionality during aggregation 12 , 13 , 14 , 15 , 16 . However, both top-down procedures and particulate systems based on molecular assembly struggle to fabricate patchy particles controllably in the desired size regime (10–100 nm). Here we introduce the co-assembly of dynamic patchy nanoparticles—that is, soft patchy nanoparticles that are intrinsically self-assembled and monodisperse—as a modular approach for producing well-ordered binary and ternary supracolloidal hierarchical assemblies. We bridge up to three hierarchical levels by guiding triblock terpolymers (length scale ∼10 nm) to form soft patchy nanoparticles (20–50 nm) of different symmetries that, in combination, co-assemble into substructured, compartmentalized materials (>10 μm) with predictable and tunable nanoscale periodicities. We establish how molecular control over polymer composition programs the building block symmetries and regulates particle positioning, offering a route to well-ordered mixed mesostructures of high complexity.

Kinetic control of the self-assembly of the π-conjugated oligomer S -chiral oligo( p -phenylenevinylene) (SOPV) reveals two competing pathways, leading to a kinetically favoured metastable product and a thermodynamically favoured stable product with opposite helicity, but the addition of a chiral tartaric acid changes the assembly process to produce only the desired metastable product. Flexibility in self-assembly Synthetic supramolecular polymers offer a range of potentially useful properties and are finding application in, for instance, organic electronics, as coating materials and as adhesives. The properties and performance of such materials depend on the organization of the molecular building blocks, which in turn depends on the pathways involved in their self-assembly. Korevaar et al . have taken a detailed look at the formation of supramolecular polymers and uncovered two parallel and competing pathways leading to assemblies with opposite helicity. One of these assemblies, discovered only through detailed kinetic analysis, is metastable but can be stabilized to force aggregation down the previously unfavoured pathway. This raises the prospect that competing pathways can be tuned to obtain supramolecular assemblies with a desired morphology, thus enabling the optimization of self-assembled functional materials. Self-assembly provides an attractive route to functional organic materials, with properties and hence performance depending sensitively on the organization of the molecular building blocks 1 , 2 , 3 , 4 , 5 . Molecular organization is a direct consequence of the pathways involved in the supramolecular assembly process, which is more amenable to detailed study when using one-dimensional systems. In the case of protein fibrils, formation and growth have been attributed to complex aggregation pathways 6 , 7 , 8 that go beyond traditional concepts of homogeneous 9 , 10 , 11 and secondary 12 , 13 , 14 nucleation events. The self-assembly of synthetic supramolecular polymers has also been studied and even modulated 15 , 16 , 17 , 18 , but our quantitative understanding of the processes involved remains limited. Here we report time-resolved observations of the formation of supramolecular polymers from π-conjugated oligomers. Our kinetic experiments show the presence of a kinetically favoured metastable assembly that forms quickly but then transforms into the thermodynamically favoured form. Quantitative insight into the kinetic experiments was obtained from kinetic model calculations, which revealed two parallel and competing pathways leading to assemblies with opposite helicity. These insights prompt us to use a chiral tartaric acid as an auxiliary to change the thermodynamic preference of the assembly process 19 . We find that we can force aggregation completely down the kinetically favoured pathway so that, on removal of the auxiliary, we obtain only metastable assemblies.

Here we describe the development of a water-responsive polymer film. Combining both a rigid matrix (polypyrrole) and a dynamic network (polyol-borate), strong and flexible polymer films were developed that can exchange water with the environment to induce film expansion and contraction, resulting in rapid and continuous locomotion. The film actuator can generate contractile stress up to 27 megapascals, lift objects 380 times heavier than itself, and transport cargo 10 times heavier than itself. We have assembled a generator by associating this actuator with a piezoelectric element. Driven by water gradients, this generator outputs alternating electricity at ∼0.3 hertz, with a peak voltage of ∼1.0 volt. The electrical energy is stored in capacitors that could power micro- and nanoelectronic devices.

Coffee rings have hidden depths When a drop of coffee dries, a halo of particles accumulates at the drop's edge. This 'coffee-ring effect', first described formally in a Nature paper in 1997, is a common occurrence when a solution of suspended colloidal particles evaporates. Far from being just a household curiosity, it has turned out to have relevance for many applications in which a uniform particle deposition is required, such as inkjet printing, assembly of photonics components and manufacture of DNA chips. In this issue, Peter Yunker and colleagues show that ellipsoidal particles suppress the coffee-ring effect. Attractive interparticle interactions between ellipsoids are sufficiently strong to counteract the forces that drive spherical particles towards the drop's edge as the drop evaporates. The coffee-ring effect can be restored for ellipsoids in solution containing surfactant, and 'designed' mixtures of spheres and ellipsoids can lead to uniform deposition. When a drop of liquid dries on a solid surface, its suspended particulate matter is deposited in ring-like fashion. This phenomenon, known as the coffee-ring effect 1 , 2 , 3 , is familiar to anyone who has observed a drop of coffee dry. During the drying process, drop edges become pinned to the substrate, and capillary flow outward from the centre of the drop brings suspended particles to the edge as evaporation proceeds. After evaporation, suspended particles are left highly concentrated along the original drop edge. The coffee-ring effect is manifested in systems with diverse constituents, ranging from large colloids 1 , 4 , 5 to nanoparticles 6 and individual molecules 7 . In fact—despite the many practical applications for uniform coatings in printing 8 , biology 9 , 10 and complex assembly 11 —the ubiquitous nature of the effect has made it difficult to avoid 6 , 12 , 13 , 14 , 15 , 16 . Here we show experimentally that the shape of the suspended particles is important and can be used to eliminate the coffee-ring effect: ellipsoidal particles are deposited uniformly during evaporation. The anisotropic shape of the particles significantly deforms interfaces, producing strong interparticle capillary interactions 17 , 18 , 19 , 20 , 21 , 22 , 23 . Thus, after the ellipsoids are carried to the air–water interface by the same outward flow that causes the coffee-ring effect for spheres, strong long-ranged interparticle attractions between ellipsoids lead to the formation of loosely packed or arrested structures on the air–water interface 17 , 18 , 21 , 24 . These structures prevent the suspended particles from reaching the drop edge and ensure uniform deposition. Interestingly, under appropriate conditions, suspensions of spheres mixed with a small number of ellipsoids also produce uniform deposition. Thus, particle shape provides a convenient parameter to control the deposition of particles, without modification of particle or solvent chemistry.

Despite recent advances in synthetic nanometer-scale tubular assembly, conferral of dynamic response characteristics to the tubules remains a challenge. Here, we report on supramolecular nanotubules that undergo a reversible contraction-expansion motion accompanied by an inversion of helical chirality. Bent-shaped aromatic amphiphiles self-assemble into hexameric macrocycles in aqueous solution, forming chiral tubules by spontaneous one-dimensional stacking with a mutual rotation in the same direction. The adjacent aromatic segments within the hexameric macrocycles reversibly slide along one another in response to external triggers, resulting in pulsating motions of the tubules accompanied by a chiral inversion. The aromatic interior of the self-assembled tubules encapsulates hydrophobic guests such as carbon-60 (C 60 ). Using a thermal trigger, we could regulate the C 60 -C 60 interactions through the pulsating motion of the tubules.

We report a strategy to create photodegradable poly(ethylene glycol)-based hydrogels through rapid polymerization of cytocompatible macromers for remote manipulation of gel properties in situ. Postgelation control of the gel properties was demonstrated to introduce temporal changes, creation of arbitrarily shaped features, and on-demand pendant functionality release. Channels photodegraded within a hydrogel containing encapsulated cells allow cell migration. Temporal variation of the biochemical gel composition was used to influence chondrogenic differentiation of encapsulated stem cells. Photodegradable gels that allow real-time manipulation of material properties or chemistry provide dynamic environments with the scope to answer fundamental questions about material regulation of live cell function and may affect an array of applications from design of drug delivery vehicles to tissue engineering systems.

In this review, we survey recent literature (2009–2013) on hydrogels that are mechanically tough and adhesive. The impact of published work and trends in the field are examined. We focus on design concepts, new materials, structures related to mechanical performance and adhesion properties. Besides hydrogels made of individual polymers, concepts developed to toughen hydrogels include interpenetrating and double networks, slide ring polymer gels, topological hydrogels, ionically cross-linked copolymer gels, nanocomposite polymer hydrogels, self-assembled microcomposite hydrogels, and combinations thereof. Hydrogels that are adhesive in addition to tough are also discussed. Adhesive properties, especially wet adhesion of hydrogels, are rare but needed for a variety of general technologies. Some of the most promising industrial applications are found in the areas of sensor and actuator technology, microfluidics, drug delivery and biomedical devices. The most recent accomplishments and creative approaches to making tough and sticky hydrogels are highlighted. This review concludes with perspectives for future directions, challenges and opportunities in a continuously changing world.

Hierarchical self-assembly offers a powerful strategy for producing molecular nanostructures. Although widely used, the mechanistic details of self-assembly processes are poorly understood. We spectroscopically monitored a nucleation process in the self-assembly of p-conjugated molecules into helical supramolecular fibrillar structures. The data support a nucleation-growth pathway that gives rise to a remarkably high degree of cooperativity. Furthermore, we characterize a helical transition in the nucleating species before growth. The self-assembly process depends strongly on solvent structure, suggesting that an organized shell of solvent molecules plays an explicit role in rigidifying the aggregates and guiding them toward further assembly into bundles and/or gels.