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

Hydrogel: Preparation, characterization, and applications: A review

The widespread use of controlled molecular-level motion in key natural processes suggests that great rewards could come from bridging the gap between the present generation of synthetic molecular systems, which by and large rely upon electronic and chemical effects to carry out their functions, and the machines of the macroscopic world, which utilize the synchronized movements of smaller parts to perform specific tasks. This is a scientific area of great contemporary interest and extraordinary recent growth, yet the notion of molecular-level machines dates back to a time when the ideas surrounding the statistical nature of matter and the laws of thermodynamics were first being formulated. Here we outline the exciting successes in taming molecular-level movement thus far, the underlying principles that all experimental designs must follow, and the early progress made towards utilizing synthetic molecular structures to perform tasks using mechanical motion. We also highlight some of the issues and challenges that still need to be overcome.

Synthetic molecular motors and mechanical machines.

Stimuli-reponsive polymers and their bioconjugates

Chitosan-based hydrogels for controlled, localized drug delivery

https://imechanica.org/files/Qiu_environmental_sensitive_hydrogel.pdf

Environment-sensitive hydrogels for drug delivery

Hydrogels having improved elasticity and mechanical strength properties are obtained by subjecting a hydrogel formulation containing a strengthening agent to chemical or physical crosslinking conditions subsequent to initial gel formation. Superporous hydrogels having improved elasticity and mechanical strength properties are similarly obtained whenever the hydrogel formulation is provided with a foaming agent. Interpenetrating networks of polymer chains comprised of primary polymer(s) and strengthening polymer(s) are thereby formed. The primary polymer affords capillary-based water sorption properties while the strengthening polymer imparts significantly enhanced mechanical strength and elasticity to the hydrogel or superporous hydrogel. Suitable strengthening agents can be natural or synthetic polymers, polyelectrolytes, or neutral, hydrophilic polymers.

/pdf/hydrogels-having-enhanced-elasticity-and-mechanical-strength-23wmrpl1lv.pdf

Hydrogels having enhanced elasticity and mechanical strength properties

The objective of this study was to improve the mechanical properties of superporous hydrogels (SPHs), which were used to develop gastric retention devices for long-term oral drug delivery. The main approach used in this study was to form an interpenetrating polymer network by incorporating a second polymer network inside an SPH structure. Polyacrylonitrile was used as the second network inside an SPH. Mechanical properties including compression strength and elasticity were significantly improved, up to 50 times as compared with the control SPHs. The enhanced mechanical properties were a result of the scaffold-like fiber network structures formed inside the cell walls of SPHs. The fast swelling property of SPHs was not affected by the incorporation of the second polymer network because the interconnected pore structures were maintained. Gastric retention devices based on superporous IPN hydrogels (SPIHs) with the improved mechanical properties are expected to withstand compression pressure and mechanical frictions in the stomach better than the control SPHs.

/pdf/superporous-ipn-hydrogels-having-enhanced-mechanical-270g6aoij3.pdf

Yong Qiu

Papers

Environment-sensitive hydrogels for drug delivery

Hydrogels having enhanced elasticity and mechanical strength properties

Superporous IPN hydrogels having enhanced mechanical properties.

Modulated Drug Delivery