Strain Stiffening Hydrogels through Self‐Assembly and Covalent Fixation of Semi‐Flexible Fibers

Abstract

Biomimetic, strain‐stiffening materials are reported, made through self‐assembly and covalent fixation of small building blocks to form fibrous hydrogels that are able to stiffen by an order of magnitude in response to applied stress. The gels consist of semi‐flexible rodlike micelles of bisurea bolaamphiphiles with oligo(ethylene oxide) (EO) outer blocks and a polydiacetylene (PDA) backbone. The micelles are fibers, composed of 9–10 ribbons. A gelation method based on Cu‐catalyzed azide–alkyne cycloaddition (CuAAC), was developed and shown to lead to strain‐stiffening hydrogels with unusual, yet universal, linear and nonlinear stress–strain response. Upon gelation, the X‐ray scattering profile is unchanged, suggesting that crosslinks are formed at random positions along the fiber contour without fiber bundling. The work expands current knowledge about the design principles and chemistries needed to achieve fully synthetic, biomimetic soft matter with on‐demand, targeted mechanical properties.

Biomimetic Strain‐Stiffening Self‐Assembled Hydrogels

“…Moreover, the critical stress of the stiffening σ c showed a linear dependence of c , that is, σ c ≈ c 1.8 , which enables the rescaling of the K ′ versus σ curves to G 0 and σ c , respectively. We found that all the rescaled curves corresponding to different c overlapped at a single master curve, indicating that all gel samples undergo a common stiffening mechanism …”

“…Kouwer and colleagues presented the first example of synthetic strain‐stiffening hydrogels that are formed by the entanglement of semi‐flexible polyisocyanopeptides (PICs) bundles, closely mimicking the characteristic mechanical properties of biological intermediate filaments . The other example was described very recently by Sijbesma . In their work, strain‐stiffening hydrogels are prepared by covalently crosslinking of semi‐flexible rod‐like micelles that are formed through self‐assembly and covalent fixation of oligo(ethylene oxide) grafted bisurea bolaamphiphiles.…”

“…However, the resulting hydrogels do not show stiffening with applied strain (Figure S2), which can be explained by the formation of bundles (Figure S1). While bundling would significantly enhance the stiffness of the fibers, as a result, the physical crosslinks are not sufficiently strong to permit these stiff bundles to enter their athermal stiffening regimes …”

“…Theoretical models developed to understand the stiffening mechanism suggest that strain-stiffening, in principle, can be observed in all semi-flexible polymers crosslinked materials for which the bending modulus of the assemblies is comparable to the thermal energy, k B T. [8] The lack of observation in synthetic systems can be ascribed to the challenge of preparing self-assembled structures composed of semi-flexible fibers that are thin enough and sufficiently crosslinked that allow the matrix structures to access their stiffening regimes before rupture. [9] So far, very few examples of synthetic semi-flexible filaments leading to strain-stiffening materials have been reported. Kouwer and colleagues presented the first example of synthetic strain-stiffening hydrogels that are formed by the entanglement of semi-flexible polyisocyanopeptides (PICs) bundles, closely mimicking the characteristic mechanical properties of biological intermediate filaments.…”

Mechanochemical Adhesion and Plasticity in Multifiber Hydrogel Networks

“…Several strategies have been used to recapitulate such mechanochemical behavior in materials. For instance, strain‐stiffening synthetic hydrogels have been engineered through the synthesis of polymers with semiflexible backbones, which organize into a mesh‐like architecture and mechanical loading induces polymer extension and packing . Additionally, mechanochemical polymers have been designed with latent mechanophores that are activated under destructive shear forces and undergo crosslinking to increase bond density to higher levels than before bond breakage (i.e., strengthening) or create mechanoradicals in response to bond scission, that promote growth of polymer networks and increased crosslinking (i.e., toughening) .…”