Muscling in on the real thing

Polymer chemists from Sheffield University claim they have created a system which mimics the flexing of natural muscle.




Based around a self-assembled long-chain molecule, the system could be used as the ‘engine’ for nanoscale devices designed to ‘swim’ through fluids.



The crucial property for a synthetic muscle is that it must expand and contract in response to some stimulus from its surroundings.



Bulk materials can expand and contract, but the repetitive cycles can lead to cracking, so the researchers, led by Richard Jones, a ‘soft machines’ specialist from the university’s polymer centre, and Jon Howse, focused their efforts on gel systems.



‘The simplest basis for a macro-molecular shape change we knew about was the collapse and expansion of a weak polyacid when the pH was changed,’ said Jones. The polyacid they settled upon was polymethacrylic acid, a fairly simple polymer which contains acid groups at regular intervals.



The team incorporated the acid into a molecule known as a ‘triblock copolymer’, where each end of the polyacid chain was capped with a glassy substance called polymethyl methacrylate, or acrylic.



The second crucial part of system is the environment. Howse immersed the polymer in a solution containing potassium bromate, sodium sulphite, potassium ferro-cyanide and sulphuric acid.



This mixture undergoes what is known as a clock reaction, where the pH oscillates between 3 (acidic) and 7 (neutral) in a regular cycle. When the pH is low, the acid groups on the polymer are capped with hydrogen atoms. But as the pH rises, the hydrogen detaches and the acid groups become charged. As the charges are all the same, they repel each other, and the polyacid stretches out to three times its original length. As the pH falls again, the hydrogen reattaches and the chain shrinks back.



The team tested the ‘strength’ of the ‘muscle’ by using the expansion to bend a soft cantilever, and deter-mined that it can produce a power per unit mass of at least 20MW/kg, making it about 10,000 times weaker than the muscle fibres that move the human body. However, Jones believes that even this small force could act as the propulsion system for nanoscale machines, or could find uses in drug delivery, with the pH-triggered expansion and contraction being used to control the release of active ingredients from a reservoir.



The next phase of the research is likely to focus on increasing the power that the system can deliver.



There are three ways this could be done, according to the team. The active part of the polymer could be made stiffer, so it develops larger forces; the cycle rate of the oscillating reaction could be reduced; or the length of the active portion could be increased by making a device from microfibres of a reactive gel.



‘A dimensional change of the gel by a factor of 100 could yield an improvement in the rate of length change of 10,000, and we are currently exploring routes to do this,’ said the researchers. This could make block copolymers for delivering similar specific power to striated muscle fibres, although the efficiency would be much lower.



‘We’ll also be looking for different driving chemical reactions to allow us to power devices with higher efficiencies in a wider range of environments,’ said Jones.