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Tough, Stretchable Hydrogel Cartilage Replacement Developed

Metal ball dropped in a hydrogel film (Harvard University, SEAS)

Metal ball dropped in a hydrogel film (Harvard University, SEAS)

Biomedical engineers at Harvard University created a tough, stretchable, and biocompatible synthetic material with the capacity to replace damaged cartilage in human joints. The findings from Harvard’s School of Engineering and Applied Sciences appears in this week’s issue of the journal Nature (paid subscription required).

The hydrogel developed by lead author and postdoctoral researcher Jeong-Yun Sun, with faculty colleagues in engineering, is a water-based material that combines two other weaker gels to make a much stronger product. Conventional hydrogels tend to be weak and brittle, says Sun, “But because they are water-based and biocompatible, people would like to use them for some very challenging applications like artificial cartilage or spinal disks.”

For these applications, a gel product needs to be able to stretch and expand under compression and tension without breaking. To create the new hydrogel, Sun and colleagues combined two common polymers: polyacrylamide, a material found in soft contact lenses, and alginate, a seaweed extract that is often used as a food thickener. Neither material is particularly strong or has much stretching ability, but when combined, the two polymers form a complex chemical network structure that allows the molecules to pull apart very slightly over a large area instead of allowing the gel to crack.

The researchers added calcium ions to the water that bind to the alginate part of the hydrogel. These calcium ions are then released when the material is slightly stretched, an action that keeps the polymer chains intact while the material is stretched further.

At the same time, the polymer chains from the two types of hydrogels bind tightly with each other. As a result, the material may experience tiny cracks, but it still holds together as it is stretched. The team’s tests showed that even with a large crack in the material, it could still be stretched to 17 times its initial length. Undamaged gel could be stretched more than 20 times its original length.

The findings also show the new hydrogel can maintain its elasticity and toughness when stretched multiple times. If the gel can relax between stretches, the ionic bonds between the alginate and the calcium can reform, a process that can be accelerated by increasing ambient temperatures.

The following brief video shows how a thin film of the hydrogel stretches when a metal ball is dropped into it.

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