Responsive hydrogels applied in the biomedical area show great potential as synthetic extracellular matrix mimics and as host medium for cell growth. The hydrogels often lack the characteristic mechanical properties that are typically seen for natural gels. Here, we demonstrate the unique responsive and mechanical properties of hydrogels based on oligo(ethylene glycol) functionalized polyisocyanopeptides. These stiff helical polymers form gels upon warming at concentrations as low as 0.006 %-wt polymer, with materials properties almost identical to those of their intermediate filaments, a class of cytoskeletal proteins. Using a combination of macroscopic rheology and molecular force microscopy the hierarchical relationship between the macroscopic behaviour of theses peptide mimics has been correlated with the molecular parameters.

The stiffness of hydrogels is crucial for their application. Nature’s hydrogels become stiffer as they are strained. This stiffness is not constant but increases when the gel is strained. This stiffening is used, for instance, by cells that actively strain their environment to modulate their function. When optimized, such strain-stiffening materials become extremely sensitive and very responsive to stress. Strain stiffening, however, is unexplored in synthetic gels since the structural design parameters are unknown. Here we uncover how readily tuneable parameters such as concentration, temperature and polymer length impact the stiffening behaviour. Our work also reveals the marginal point, a well-described but never observed, critical point in the gelation process. Around this point, we observe a transition from a low-viscous liquid to an elastic gel upon applying minute stresses. Our experimental work in combination with network theory yields universal design principles for future strain-stiffening materials.

Ionic liquid crystals combine the unique solvent properties of ionic liquids with self-organization found for liquid crystals. We report a detailed analysis of the structure-property relationship of a series of new imidazolium-based liquid crystals with an extended aromatic core. Investigated parameters include length and nature of the tails, the length of the rigid core, the lateral substitution pattern, and the nature of the counterion. Depending on the molecular structure, two mesophases were observed: a bilayered SmA2 phase and the more common monolayered SmA phase, both strongly interdigitated. Most materials show mesophases stable to high temperatures. For some cases, crystallization could be suppressed, and room-temperature liquid crystalline phases were obtained. The mesomorphic properties of several mixtures of ionic liquid crystals were investigated. Many mixtures showed full miscibility and ideal mixing behavior; however, in some instances we observed, surprisingly, complete demixing of the component SmA phases. The ionic liquid crystals and mixtures presented have potential applications, due to their low melting temperatures, wide temperature ranges, and stability with extra ion-doping.

6503wileyonlinelibrary.com mechanical properties. The large morphological and volume change at the LCST of PNIPAM-based gels [ 10 ] make mechanical studies meaningless. A series of studies on the Hofmeister effect on supramolecular hydrogels showed a dependence of the mechanical properties, but this was merely induced by the hugely different morphologies found for the different assembly conditions of the gels. [11][12][13] Here, we describe for the fi rst time that we can use the Hofmeister effect to controllably manipulate the mechanical properties of polymer hydrogels. As expected, the addition of salts directly affects the gelation temperature and, for our thermoresponsive hydrogel, this changes the stiffness of the gel over two orders of magnitude at a constant temperature, for instance 37 °C. We construct hydrogels that range from very soft to stiff, but all at identical concentration. To achieve such change in mechanical response, one commonly needs to vary parameters such as concentration, morphology and bundle (or fi ber) diameter and stiffness. [ 14 ] Some of these are diffi cult to control, while others simultaneously change many important network characteristics; for instance, the concentration changes the biomedically relevant pore size and stiffness of the gel (as well as nonlinear gel properties). The salts, however, are able to shift the mechanics, without changing porosity and network morphology.The polymers we use are ethylene glycol-functionalized polyisocyanides (PICs, Figure 1 b), which form a thermoreversible gel upon heating when dissolved in water. [ 15 ] At the LCST, the polymers become hydrophobic and form a network of entangled semi-fl exible bundles of polymer chains. The mechanical properties of the PIC hydrogels mimic those of biological gels, [ 15 ] including the nonlinear mechanics at large stress (or strain), which is markedly different than that of other synthetic hydrogels. The mechanics of PIC hydrogels is readily tuned by changing concentration, temperature, and polymer length. [ 16 ] Here, we add two additional parameters to this list: the nature of the salt (more precisely the anion) and its concentration.For a long time, the molecular basis of the Hofmeister series was related to the effect of ions on the bulk structure of water. [ 17 ] More recently, however, it was shown that salts generally do not affect the bulk water structure. [ 18,19 ] Instead, more recent theories hypothesize that direct interactions between ions and macromolecules and their fi rst hydration shell can explain the effects fi rst described by Hofmeister. [ 7,[20][21][22][23] The Hofmeister effect is usually more pronounced for anions than for cations and some ions have a stronger effect than others. The general order of the anions, termed the Hofmeister series is shown in Figure 1 a. [ 7,24 ] The ions on the The mechanical properties of hydrogels are commonly modifi ed by changing the concentration of the molecular components. This approach, however, does not only change hydrogel mechanics, but also th...

The mechanical properties of cells and the extracellular environment they reside in are governed by a complex interplay of biopolymers. These biopolymers, which possess a wide range of stiffnesses, self-assemble into fibrous composite networks such as the cytoskeleton and extracellular matrix. They interact with each other both physically and chemically to create a highly responsive and adaptive mechanical environment that stiffens when stressed or strained. Here we show that hybrid networks of a synthetic mimic of biological networks and either stiff, flexible and semi-flexible components, even very low concentrations of these added components, strongly affect the network stiffness and/or its strain-responsive character. The stiffness (persistence length) of the second network, its concentration and the interaction between the components are all parameters that can be used to tune the mechanics of the hybrids. The equivalence of these hybrids with biological composites is striking.

Bundling of single polymer chains is a crucial process in the formation of biopolymer network gels that make up the extracellular matrix and the cytoskeleton. This bundled architecture leads to gels with distinctive properties, including a large-pore-size gel formation at very low concentrations and mechanical responsiveness through nonlinear mechanics, properties that are rarely observed in synthetic hydrogels. Using small-angle X-ray scattering (SAXS), we study the bundle formation and hydrogelation process of polyisocyanide gels, a synthetic material that uniquely mimics the structure and mechanics of biogels. We show how the structure of the material changes at the (thermally induced) gelation point and how factors such as concentration and polymer length determine the architecture, and with that, the mechanical properties. The correlation of the gel mechanics and the structural parameters obtained from SAXS experiments is essential in the design of future (synthetic) mimics of biopolymer networks.

Over the past 50 years, numerous roads towards carbon-based materials have been explored, all of them being paved using mainly one functional group as the brick: acetylene. The acetylene group, or the carbon-carbon triple bond, is one of the oldest and simplest functional groups in chemistry, and although not present in any of the naturally occurring carbon allotropes, it is an essential tool to access their synthetic carbon-rich family. In general, two strategies towards the synthesis of π-conjugated carbon-rich structures can be employed: (a) either the acetylene group serves as a building block to access acetylene-derived structures or (b) it serves as a synthetic tool to provide other, usually benzenoid, structures. The recently discovered copper-catalysed azide-alkyne cycloaddition (CuAAC) reaction, however, represents a new powerful alternative: it transforms the acetylene group into a five-membered heteroaromatic 1H-1,2,3-triazole (triazole) ring and this gives rise to new opportunities. Compared with all-carbon aromatic non-functional rings, the triazole ring possesses three nitrogen atoms and, thus, can serve as a ligand to coordinate metals, or as a hydrogen bond acceptor and donor. This Feature Article summarises examples of using the triazole ring to construct conjugation- and/or function-related heteroaromatic materials, such as tuneable multichromophoric covalent ensembles, macrocyclic receptors or responsive foldamers. These recent examples, which open a new sub-field within organic materials, started to appear only few years ago and represent "a few more bricks" on the road to carbon-rich functional materials.

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