Have you ever tried to pick up Jell-O in your two hands? How many pieces did it break into? Despite what your mother told you at the table, it’s okay to play with your Jell-O in the name of science.
A very basic technique in biology labs is separating different sized molecules by means of gel electrophoresis. Proteins or nucleic acids (DNA or RNA) will travel in the ‘gel’ when an electric current is passed through it. The smaller the molecule, the further it will travel in the gel, and vice-versa. The gels are made out of various materials such as polyacrylamide or agarose, a carbohydrate purified from seaweed, and are about the consistency of Jell-O but colorless. New graduate students often learn the hard way to not drop, pull, push, bend, or otherwise be careless with their gels, because they will break very easily. The fact that they are slippery doesn’t help matters, but one learns how to handle them with care.
Now along comes a new kind of gel from researchers at Harvard that can be stretched up to twenty-one times its original length without breaking – and it’s not made out of rubber! In fact, it is a composite of two individual gel materials -- polyacrylamide and alginate, another seaweed derivative -- each of which alone is no stronger than Jell-O. When combined, however, they synergize wonderfully to make a tough, elastic-like gel that can be stretched and then allowed to relax to its original size. Whether it’s usable for gel electrophoresis remains to be seen, but it does hold promise as replacement cartilage -- such as one can feel in the front half of the nose, or as present in the disks of the spine that separate one vertebra from another.
Acrylamide and alginate form two different kinds of polymers, acrylamide having strong covalent crosslinking bonds and the alginate with weaker ionic bonds (opposite charges attracting). The final concentration of alginate in the gels was about 1.5% and that of acrylamide approximately 12.5% - the gel is 86% water and is hence quite appropriately called a hydrogel. The paper’s authors speculate the “load sharing” of the two networks is due to entanglements of the polymers as well as possible formation of new covalent bonds between the acrylamide and the alginate.
The researchers, led by Dr. Zhigang Suo and first author Dr. Jeong-Yun Sun, believe that the ionic bonds of alginate progressively “unzip” when the gel is stretched, while the polyacrylamide network remains intact. The ionic bonds are then reformed when the gel is relaxed again (see molecular schematic picture). After clamping to a loading device -- a small, high-tech ‘torture rack’ -- the hydrogels have working dimensions of 75 millimeters wide, 5 mm long (the rest of this dimension under clamps on each end) and 3 mm thick.
The 5 mm dimension of the gel has load applied to it -- stretching, in this case -- at a set rate, doubling length every minute. The gels could be easily stretched out by hand, so the programmable load device is for uniformity and accuracy, not brute force. An intact gel like this stretches from 5 mm to 105 mm before breaking. Remarkably, the gel can even suffer a small cut in the middle before being pulled and it is still able to extend up to 17 times its original width, to 85 mm, before snapping.
The gels were not able to stretch as well a second time unless, like athletes, they were allowed to rest for a day and “heal” themselves. The researchers went on to show that this healing was helped by progressively higher temperatures, up to 80o C (176o F). In this case, the gel recovered 74% of its original strength by the next day. Alas no athlete could survive such a sauna.
“Why Didn’tI Think of That?”
Gels with more than one polymerizing component have been used before, both in biomedicine and in pure and applied materials research, but the previous such gels were not very strong. Or perhaps they were never tested for their strength. As Dr. Suo said in an email, “Hydrogels are made by many people for many applications, but their fracture energy is usually not measured. What is not measured cannot be optimized!”
Dr. Suo was not aware of his lab’s particular gel recipe having been tried in an engineering context before. In an accompanying commentary, however, Dr. Kenneth Shull of Northwestern University noted that previously reported ‘tough’ hydrogels had also been hard to make, whereas the Harvard group’s gel is very simple in nature. Indeed, no new technologies or substances were involved in the discovery of this super-stretching material. Like many naturally occurring materials in biology, the new gel type is thin but very tough, as well as biocompatible -- perfect for artificial cartilage and possibly other applications, such as artificial heart valves and long-lasting contact lenses.
No doubt there somewhere sits at least one former biology graduate student, remembering a gel s/he dropped that didn’t break -- and wishing they had looked into that a bit further. But as the nineteenth century French scientist Louis Pasteur once remarked, “Chance favors the prepared mind.” In other words, hindsight may be 20/20 but it won’t get you a paper in Nature!