Sustainable, non-toxic polymers that harden in seconds could be used to close wounds in the human body during surgery, researchers in the US believe. The materials, which are made from renewable resources and can be easily recycled, could potentially find wider use as adhesives in everything from sticky notes to load-bearing structural materials.

α-Lipoic acid is produced by many plants and animals and has been documented to undergo ring-opening polymerization reactions. In theory, this could provide a renewable source of nontoxic polymers. Unfortunately, the resulting polymers are prone to spontaneous depolymerization. Researchers have attempted to stabilize poly(α-lipoic acid) sufficiently to form useful materials, but success has been limited by an active radical at the end of the chain.

Phillip Messersmith and colleagues at the University of California, Berkeley, concluded that incorporating an electrophile would reduce the reactivity of this radical and stabilize the polymer. In the new work, they presented two N-Hydroxysuccinimide esters of α-lipoic acid and dissolved them together with the α-lipoic acid itself in ethanol. When added to water, a stable, elastic, self-healing adhesive polymer formed within seconds.

The researchers tested this material as a tissue glue. The most commonly used medical glue today is cyanoacrylate, better known as superglue, but it is not suitable for sealing internal surgical incisions because it is highly cytotoxic, brittle, and slowly degradable. The researchers tested their material for its ability to prevent mice’s amniotic sacs from rupturing when punctured, which is sometimes necessary to repair deformities in fetuses. The researchers found that when they punctured the amniotic sacs using patches made from their polymer, the patches around the puncture site healed on their own and all fetuses survived to birth. Fetuses whose bladders were simply punctured all died. “We are actively pursuing other uses for this polymer as a surgical glue in my academic lab and in a startup company we founded to move it toward commercial use,” Messersmith says.

The researchers also showed that their material, once cured, formed a pressure-sensitive viscoelastic polymer that could be used in adhesive tapes and sticky notes. Unlike other adhesives used for this purpose, the poly(α-lipoic acid)-based adhesive worked almost as well when wet as it did when dry. When the researchers also added a divalent salt of α-lipoic acid to the monomer, the polymer became a rigid, cross-linked thermoset that was at least as strong as industrial epoxies. However, the material could be depolymerized easily by heating it in an alkaline solution.

Messersmith acknowledges that large-scale industrial use will require a more readily available and cheaper renewable α-lipoic acid, but points out that “styrene was considered an exotic monomer many decades ago.”

Biomedical engineer Mark Grinstaff of Boston University in the US is impressed. “What’s really unique about this paper is that it’s a system that has been known to polymerise for some time, but hasn’t been very useful,” he says. “Messersmith was able to identify critical aspects of its chemical structure and make changes to it so that it can now be polymerised, processed and used as a functional material, but still retains its degradation properties.” He predicts that the biggest impact will be in medicine. “The recycling aspects will be important when this is really deployed on a large scale and becomes a widely used consumer product,” he says.