American chemists have made molecular 'daisy-chains' containing threaded rings that can be pulled taut or slackened by chemical stimuli. The polymers are a step towards making materials that stretch or contract on demand, and show great potential for applications such as actuators in nanomachinery or designing artificial muscles. 'Artificial muscle tissue is still a long way off, but we are starting to demonstrate the kind of systems where it could be possible,' says Robert Grubbs, who led the work at the California Institute of Technology, US. Grubbs won the Nobel Prize in Chemistry in 2005 for his work on olefin metathesis reactions, which allow carbon-carbon...