Tissue Engineering Approach Aids Cartilage Repair Strategies

An extremely pliable, robust, biocompatible, and self-healing hydrogel could have the potential to become a next-generation cartilage repair-treatment option for human joint defects.

Researchers at Harvard University (Cambridge, MA, USA) have developed the synthesized hydrogel by using a mixture of polyacrylamide and alginate to create a complex network far stronger than gels formed from polyacrylamide or alginate alone. The hybrid polymer is capable of maintaining its enhanced toughness and elasticity over multiple stretches, and is able to stretch to many times its original length due to the chemical structure of the network, which allows the whole structure to pull apart very slightly over a large area, instead of the gel cracking.

The synthetic hydrogel polymer forms ionically and covalently crosslinked networks; and although they contain almost 90% water, the hydrogels can be stretched beyond 20 times their initial length, with high fracture energies. Even for samples that contain notches, a stretch of 17 times is possible. The researchers 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. 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 can however heal by re-zipping. The study was published in the September 6, 2012, issue of Nature.

“These gels may serve as model systems to explore mechanisms of deformation and energy dissipation, and expand the scope of hydrogel applications,” concluded lead author Prof. Zhigang Suo, PhD, and colleagues of the School of Engineering and Applied Sciences. “A hydrogel of this superior stretchability, toughness and recoverability is likely to become the perfect scaffold, providing great promise for developing a novel cartilage repair strategy using a tissue engineering approach.”

Hydrogels are used as scaffolds for tissue engineering, vehicles for drug delivery, actuators for optics and fluidics, and model extracellular matrices for biological studies. The scope of hydrogel applications, however, is often severely limited by their mechanical behavior, since most do not exhibit high stretching ability. An alginate hydrogel, for example, ruptures when stretched to about 1.2 times its original length. Some synthetic elastic hydrogels have achieved stretches in the range 10-20, but these values are markedly reduced in samples containing notches. Additionally, most hydrogels are brittle.

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