Many of the soft, hydrated gel-like materials found in natural living systems such as muscle tissue have ordered molecular structures tailored to ensure that they have the desired mechanical properties to perform their necessary mechanical and physiological function. However, most synthetic gels lack ordered structures and thus exhibit poor mechanical properties. Now, Jian Ping Gong and colleagues from Hokkaido University1 have developed synthetic gels which can be reversibly stretched up to 22 times their original length as a result of their ordered molecular structures.

Fig. 1: Schematic of the high-strength double network gel.

Gong and colleagues used a semi-rigid polyelectrolyte to form a nematic liquid crystal state which was crosslinked into a network structure by the controlled diffusion of calcium ions in a specific direction into the liquid crystal. The rod like molecules formed a grid-like network. The liquid crystalline gel was then immersed into a solution containing a monomer and a chemical crosslinker for a second polymer (gel) system—polyacrylamide (gel)—thereby generating a second network around the original ordered liquid crystal structure—a “double-network gel”. While double network gels have been reported previously, none have had ordered architectures such as these generated by Gong and colleagues. The researchers believe that the anisotropic liquid crystalline network is maintained and strengthened by the second polymer system which tangles up around it (Fig. 1).

The liquid crystalline phase on its own was brittle and lacked elasticity—although this depended on the concentration. On the other hand, the polyacrylamide gel was very soft and lacked the reversibility on stretching. Combining the two gels into one system resulted in a soft but strong material which could be reversibly stretched to huge limits even when it contained large concentrations of water of up to 97%.

The mechanical properties of the gel strongly depended on the concentration of the liquid crystal polymer phase, but weakly on the direction in which the gel was stretched.

The relationship of the mechanical properties between the ordered molecular structures in the gels is still not well understood. “We will continue to investigate the mechanism operating in the ordered hydrogels,” say the authors.