Researchers at the Harvard University School of Engineering and Applied Sciences in Cambridge, Mass., have developed a new hydrogel that can be stretched, is biocompatible, and is self-healing. This has the possibility of becoming an effective ligament repair option in osteoarthritis (OA).
“Hydrogels are used as scaffolds for tissue engineering, vehicles for drug delivery, actuators for optics and fluidics, and model extracellular matrices for biological studies,” according to Zhigang Suo, Allen E. and Marilyn M. Puckett Professor of Mechanics and Materials at Harvard, and colleagues.1 “The scope of hydrogel applications is often severely limited by their mechanical behavior.”
The group synthesized hydrogels from polymers forming ionically (alginate) and covalently (polyacrylamide) cross-linked networks. The hydrogels obtained using this method can be stretched up to 20 times their original length and have very high fracture energies. Even when the gels were notched, a stretch of 17 times was still achieved.
Synergy Brings Toughness
“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,” the researchers note. “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 rezipping.”1
If a sample is notched and stretched, the polyacrylamide bridges the crack and acts to stabilize the deformation. This lets the alginate network unzip over a larger region. The unzipping reduces stress on the polyacrylamide network ahead of the notch.
The research suggests that combining weak and strong crosslinks can greatly increase the fracture energy of hydrogels. “The combination of relatively high stiffness, high toughness, and recoverability of stiffness and toughness, along with an easy method of synthesis, make these materials ideal candidates for further investigation,” the researchers write.
A Promising Candidate
Andreas H. Gomoll, MD, assistant professor of orthopedics at Harvard Medical School, notes that one of the issues surrounding surgical treatment of cartilage damage is the vulnerability of the grafts used to repair the tissue. There has been an ongoing search for carrier substances that are mechanically more resilient and that have mechanical stability close to normal cartilage but still can be absorbed over time.
The ligament repair procedure can be likened to putting an asphalt patch on a pothole. If it isn’t protected until the asphalt has cured, damage may reoccur. This hydrogel may work the same way as a steel plate in the road to protect the repair until it has settled.
“We currently don’t have that type of material, although this hydrogel is a very promising candidate,” he says. “For the field of cartilage care and transplantation, I think this is very exciting.”
Kurt Ullman is a freelance writer based in Indiana.
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