Muscle tissue is flexible, can impart large actuating force and is damage-tolerant, shock-dampening and generally resilient. It also functions with a low actuation voltage. Synthetic muscle tissue could have many uses in biotechnology and medicine, particularly for improving the quality of life for patients suffering muscle damage, but synthetic implants must be both biocompatible and operate at low actuation voltage. Luhua Lu and Wei Chen from the Suzhou Institute of Nano-Tech and Nano-Bionics in China1 have now reported an ionic actuator that fulfils these requirements. The device, built entirely from biocompatible components, displays impressive bending actuation at low voltages.

The actuators consisted of two nanotube-based electrodes sandwiching a layer of ionic liquid. Chitosan — a biopolymer derived from the exoskeleton of crustaceans — was used as a biocompatible support for the electrolyte (ionic liquid) layer and also to wrap the nanotubes forming the two electrodes to form a chitosan-supported supramolecular structure. As the chitosan fills the gaps between nanotubes, the electrodes themselves could be prepared with almost any density of nanotubes and hence a wide range of conductivity. These flexible electrodes formed a sticky and uniform contact with the electrolyte layer, and the nanotube structure provided high ‘electrochemical capacitance’ — the ability to absorb and store ions.

Fig. 1: Scanning electron microscopy images (upper) of a cross-section at the boundary between the electrode and electrolyte layers of the ‘biomorph’ actuator. The actuator bends responsively (lower) with low actuation voltage.Adapted from Ref. 1. Reproduced with permission. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA

When a low voltage was applied between the two electrodes, anions and cations in the electrolyte migrated to opposite electrodes. The uptake of ions by the electrodes caused the electrodes to expand, but since the cations are much larger than the anions, one electrode expanded more than the other, causing the membrane to bend. This effect was fully reversible and highly responsive, with the actuator bending by as much as 15 mm at a rate of 2 mm per second over hundreds of cycles (Fig. 1).

“One of our goals is to develop artificial muscles that are both biocompatible and intelligent,” says Chen. Although the biocompatibility in vivo of the overall composites has yet to be investigated, the researchers are hopeful that their materials can in the future find use as surgical tools, ocular muscles or even heart compression devices.