Every now and again, Cornell University Professor Dan Luo gets a surprise. His research team has discovered a new variety of hydrogel – like Jello, except made with DNA instead of gelatin. When full of water, it is a soft, elastic solid. But when the water is removed, the hydrogel collapses, losing its shape. The resulting material pours like a liquid, and conforms to the shape of its container. The most interesting part, however, is that the liquid hydrogel remembers its shape. Add water and you get back the original Jello-like shape. Terminator T-1000, anyone?

DNA has a wide range of potential applications based solely on its properties as an unusual polymer. These include controlled delivery of pharmaceuticals, 3D tissue scaffolding and engineering, and a range of other biomedical applications. Among these DNA-based materials are self-assembling hydrogels, in which standard cross-linked DNA polymers form large, loose polymer networks that can adsorb huge amounts of water. As they do so, their mechanical properties change dramatically.

Micron-sized spheres of DNA polymer form the basis of the DNA hydrogels

The polymer networks that make up the DNA hydrogels form spontaneously under certain conditions, and in the process take the shape of micron-sized spheres which bond weakly to each other. It is this bonding that allows a hydrogel to be formed in a particular shape – if the tiny polymer spheres did not bond together in some manner, the "wet" (hydrated) form of the hydrogel would be a thick soup rather than, say, the letters DNA.

When most of the water surrounding the DNA hydrogel letters is removed, they collapse into a pool of what to all intents seems to be a fluid. The collapsed material flows, pours, fills molds of other shapes, and appears to have lost all trace of the original shape. Despite this, when water is reintroduced, some memory of the bonding between the spheres remains – enough to completely reproduce the original shape of the DNA letters.

This is a new behavior, and the ultimate mechanism that preserves shape information is not yet known, nor is the strength of the mechanism. For example, will stirring the collapsed hydrogels destroy the shape memory? Prof. Luo's group is still investigating such questions.

As a simple example of a potential application, one might imagine an injectable stent. Such a stent would be produced with the desired shape in the presence of large amounts of environmental water, then collapsed into a pseudo-liquid to be injected into the proper place in the body, whereupon it would reproduce the original shape. While it is too early to guess what other applications may appear, there are very few unusual mechanical properties which remain laboratory curiosities for long.

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