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September 09, 2012

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“Supergel” could replace damaged joint cartilage

Sept. 9, 2012
Courtesy of Harvard University
and World Science staff

Re­search­ers have cre­at­ed what they say is an ex­tremely stretchy, tough ge­l that has po­ten­tial as a re­place­ment for dam­aged car­ti­lage in hu­man joints.

The wa­ter-containing gel, or hydro­gel, is a strong hy­brid of two weak ge­ls. Sci­en­tists say it’s self-heal­ing; “bio­com­pat­ible,” or com­pat­ible with liv­ing tis­sues; and can stretch to 21 times its orig­i­nal length. Har­vard Uni­vers­ity re­search­ers de­scribe the ma­te­ri­al and a meth­od of pro­duc­ing it in the Sept. 6 is­sue of the jour­nal Na­ture.

The researchers pinned both ends of the new gel in clamps and stretched it to 21 times its initial length before it broke. (Photo courtesy Jeong-Yun Sun)

“Con­ven­tional hydro­gels are very weak and brit­tle,” like jel­ly, said lead au­thor Jeong-Yun Sun, a post­doc­tor­al fel­low at the Har­vard School of En­gi­neer­ing and Ap­plied Sci­ences. 

“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 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 ma­te­ri­al, the re­search­ers com­bined two com­mon poly­mers, sub­stances with long mo­le­cules made of re­peat­ing un­its. The main com­po­nent is poly­acry­lamide, known for use in soft con­tact lens­es and as a ge­l that sep­a­rates DNA frag­ments in labs. The sec­ond com­po­nent is al­gi­nate, a sea­weed ex­tract com­monly used to thick­en food.

Sep­a­rate­ly, these gels are weak — al­gi­nate, for in­stance, can stretch to only 1.2 times its length be­fore it breaks. Com­bined in a mix­ture of eight parts to one, though, the two poly­mers were found to form a com­plex net­work of cross-linked chains that re­in­force one an­oth­er. The chem­i­cal struc­ture of the net­work al­lows the mo­le­cules to pull apart very slightly over a large ar­ea in­stead of let­ting the ge­l crack.

The researchers used a razor blade to cut a 2-cm notch across the gel. In the image above (left), the gel has been stretched very slightly so that the notch is visible. This damaged gel was still able to stretch to 17 times its initial length without breaking. (Photo courtesy Jeong-Yun Sun)

The al­gi­nate por­tion of the gel con­sists of pol­y­mer chains that form weak “ion­ic” bonds with one an­oth­er. In the pro­cess they cap­ture charged atoms of cal­ci­um, or ions, in the wa­ter. 

When the ge­l is stretched, some of the bonds be­tween chains break — or “un­zip,” as the re­search­ers put it — re­leas­ing the cal­ci­um. As a re­sult, the gel ex­pands slight­ly, but the pol­y­mer chains them­selves re­main in­tact. Mean­while, the poly­acry­lamide chains form a grid­like struc­ture that bonds tightly 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 un­zip­ping them here and the­re. The re­search team showed that even with a huge crack, the hy­brid gel can still stretch to 17 times its in­i­tial length.

The re­search­ers pinned both ends of the gel in clamps and stretched it to 21 times its in­i­tial length be­fore it broke. It can main­tain its elas­ti­city and tough­ness over mul­ti­ple stretches: pro­vid­ed it has time to re­lax be­tween stretches, the bonds be­tween the al­gi­nate and the cal­ci­um can “re-zip,” the sci­en­tists said. Be­yond ar­ti­fi­cial car­ti­lage, they see po­ten­tial for the new gel in soft robotics, in op­tics, in ar­ti­fi­cial mus­cle, and as a pro­tec­tive co­vering for wounds.

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Researchers have created what they say is an extremely stretchy, tough gel that has potential as a replacement for damaged cartilage in human joints. The water-containing gel, or hydrogel, is a strong hybrid of two weak gels. Scientists say it’s self-healing; “biocompatible,” or compatible with living tissues; and can stretch to 21 times its original length. Harvard University researchers describe the material and a method of producing it in the Sept. 6 issue of the journal Nature. “Conventional hydrogels are very weak and brittle,” like jelly, said lead author Jeong-Yun Sun, a postdoctoral fellow at the Harvard School of Engineering and Applied Sciences. “But because [these gels] are water-based and biocompatible, people would like to use them for some very challenging applications like 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 material, the researchers combined two common polymers, substances with large molecules made of repeating units. The main component is polyacrylamide, known for use in soft contact lenses and as a gel that separates DNA fragments in labs. The second component is alginate, a seaweed extract commonly used to thicken food. Separately, these gels are weak — alginate, for instance, can stretch to only 1.2 times its length before it breaks. Combined in a mixture of eight parts to one, though, the two polymers were found to form a complex network of cross-linked chains that reinforce one another. The chemical structure of the network allows the molecules to pull apart very slightly over a large area instead of letting the gel crack. The alginate portion of the gel consists of polymer chains that form weak “ionic” bonds with one another. In the process they capture charged atoms of calcium, or ions, in the water. When the gel is stretched, some of the bonds between chains break — or “unzip,” as the researchers put it — releasing the calcium. As a result, the gel expands slightly, but the polymer chains themselves remain intact. Meanwhile, the polyacrylamide chains form a gridlike structure that bonds tightly 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 unzipping them here and there. The research team showed that even with a huge crack, the hybrid gel can still stretch to 17 times its initial length. The researchers pinned both ends of the new gel in clamps and stretched it to 21 times its initial length before it broke. It can maintain its elasticity and toughness over multiple stretches: provided it has time to relax between stretches, the bonds between the alginate and the calcium can “re-zip,” the scientists said. Beyond artificial cartilage, the researchers see potential for the new hydrogel in soft robotics, in optics, in artificial muscle, and as a protective covering for wounds.