Tough hydrogel could replace damaged cartilage in joints

Harvard University experts have created what is claimed to be an extremely stretchy and tough gel that may pave the way to replacing damaged cartilage in human joints.

According to a statement, the new hydrogel material is a hybrid of two weak gels that combine to create something much stronger.

Not only can this new gel stretch to 21 times its original length, but it is also exceptionally tough, self-healing, and biocompatible — attributes that open up new opportunities in medicine and tissue engineering.

The material, its properties, and a simple method of synthesis are described in the 6 September issue of Nature.

‘Conventional hydrogels are very weak and brittle — imagine a spoon breaking through jelly,’ said lead author Jeong-Yun Sun, a postdoctoral fellow at the Harvard School of Engineering and Applied Sciences (SEAS). ‘But because they are water-based and biocompatible, people would like to use them for some very challenging applications such as artificial cartilage or spinal disks. For a gel to work in those settings, it has to be able to stretch and expand under compression and tension without breaking.’

To create the new hydrogel, they combined two common polymers. The primary component is polyacrylamide, known for its use in soft contact lenses and as the electrophoresis gel that separates DNA fragments in biology labs; the second component is alginate, a seaweed extract.

The alginate portion of the gel consists of polymer chains that form weak ionic bonds with one another, capturing calcium ions (added to the water) in the process. When the gel is stretched, some of these bonds between chains break releasing the calcium. Consequently, the gel expands slightly, but the polymer chains themselves remain intact. Meanwhile, the polyacrylamide chains form a grid-like structure that bonds covalently with the alginate chains.

If the gel acquires a tiny crack as it stretches, the polyacrylamide grid helps to spread the pulling force over a large area, tugging on the alginate’s ionic bonds and breaking them in places. The research team showed that even with a critically large hole the hybrid gel can still stretch to 17 times its initial length.

Importantly, the new hydrogel is capable of maintaining its elasticity and toughness over multiple stretches. Provided the gel has some time to relax between stretches, the ionic bonds between the alginate and the calcium can heal and the researchers have shown that this process can be accelerated by raising the ambient temperature.

The group’s combined expertise in mechanics, materials science, and bioengineering enabled it to apply two concepts from mechanics — crack bridging and energy dissipation — to a new problem.

Sun and his co-authors were led by three faculty members: Zhigang Suo, Allen E and Marilyn M Puckett Professor of Mechanics and Materials at SEAS and a Kavli Scholar at the Kavli Institute for Bionano Science and Technology at Harvard; Joost J Vlassak, Gordon McKay Professor of Materials Engineering and an Area Dean at SEAS; and David J Mooney, Robert P Pinkas Family Professor of Bioengineering at SEAS and a Core Faculty Member at the Wyss Institute for Biologically Inspired Engineering at Harvard.