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. 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 engineering1, vehicles for drug delivery2, actuators for optics and fluidics3, and model extracellular matrices for biological studies4. The scope of hydrogel applications, however, is often severely limited by their mechanical behaviour5. 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 hydrogels6,7 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 cartilage9 and ∼10,000 J m−2 for natural rubbers10. Intense efforts are devoted to synthesizing hydrogels with improved mechanical properties11,12,13,14,15,16,17,18; 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.

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Highly stretchable and tough hydrogels

Tissue engineering applications commonly encompass the use of three-dimensional (3D) scaffolds to provide a suitable microenvironment for the incorporation of cells or growth factors to regenerate ...

/pdf/three-dimensional-scaffolds-for-tissue-engineering-1huqh8s1ln.pdf

Three-dimensional scaffolds for tissue engineering applications : role of porosity and pore size

Recent advances on protein–polysaccharide electrostatic complex and coacervates' formation, structure and properties are presented. The first section emphasises on the thermodynamics of complexes formation, the contribution of enthalpic and entropic effects is specifically discussed. Then, their structure is described at different length scales and recent findings on the phase-ordering dynamics of complex and coacervates formation are covered. Finally, the most relevant functional properties of protein–polysaccharide complexes and coacervates for food applications are described.

Protein–polysaccharide complexes and coacervates

Abstract Recent advances on protein–polysaccharide electrostatic complex and coacervates' formation, structure and properties are presented. The first section emphasises on the thermodynamics of complexes formation, the contribution of enthalpic and entropic effects is specifically discussed. Then, their structure is described at different length scales and recent findings on the phase-ordering dynamics of complex and coacervates formation are covered. Finally, the most relevant functional properties of protein–polysaccharide complexes and coacervates for food applications are described.

Ainhoa Murua

Papers

Cell microencapsulation technology: towards clinical application.

Microcapsules and microcarriers for in situ cell delivery.

In vitro characterization and in vivo functionality of erythropoietin-secreting cells immobilized in alginate-poly-L-lysine-alginate microcapsules.

Xenogeneic transplantation of erythropoietin-secreting cells immobilized in microcapsules using transient immunosuppression.

Low molecular weight oligochitosans for non-viral retinal gene therapy