Researchers in the US have taken inspiration from nature to create materials that combine stiff and elastic parts. Their multimaterial was created from cis-cyclooctene, a commodity monomer, with stiff sections generated by shining light on them (Science, 2022, 378, 211).
‘What makes our material unique is the fact we can change the material properties, not by changing the chemical composition locally, but by changing how the molecules are arranged in 3D space,’ says Zachariah Page, materials chemist at The University of Texas at Austin.
The mix of stretchability and rigidity gives the material potential in applications for flexible electronics and soft robotics. In nature, hard and soft materials are often combined, for example in spider silk, in bone, in shells and in wood. The researchers started with liquid monomer, cis-cyclooctene. In the presence of ruthenium catalysts, these ring-shaped monomers form long polymer chains - a reaction known as a ring-opening metathesis polymerisation (ROMP). However, when a blue LED light was focused on parts of the material during polymerisation, trans carbon-carbon double bonds formed that were able to pack neatly. This yielded a rigid semi-crystalline structure – strong and stiff.
Meanwhile, those areas where light was not directed polymerise at a slower rate, with cis double bonds, introducing kinks that prevented the polymer chains form packing together, generating a soft and elastic polyoctenamer rubber. ‘We’re converting the monomer essentially into two different materials, even though chemically they are the same,’ says Page. ‘It is only the shape of the molecules which is changing.’
This means a rigid polymer could be created with soft areas within it. Once the material is stretched, the soft parts allow flexibility, but hard patches introduce limits.
Ordinarily, two different materials are combined to produce a multimaterial, and the risk of failure occurs at the interface. This is not such an issue with the new material, says Page. ‘Our interfaces are completely tied together, so we don’t have an adhesive failure between two dissimilar structures,’ he adds.
He plans now to study the biocompatibility of the material and look with collaborators at potential applications. The process is almost entirely free of solvents and can be carried out at room temperature. The ruthenium catalysts are somewhat sensitive to oxygen and moisture, but industrial processes of this type are known. ‘They’re combining a system of hard and softer materials using essentially the same compound,’ says Roland Kroeger, physicist at the University of York, ‘and that’s quite unusual.’
Kroeger’s interest is in nanomaterials for medical applications. He is collaborating with clinicians to develop materials suitable for preventing incisional hernias, a common side effect after abdominal surgery. This new type of material might inspire novel approaches for tailormade mesh implants, he notes, to support incisions after surgery.
Material chemist Craig Hawker at the University of California, Santa Barbara, describes the concept as ‘exciting and impactful,’ noting that ‘the use of a single starting material simplifies the overall process and lowers the barriers to implementation.’ He adds that ‘the process itself is quite straightforward, with few components required to access sophisticated multi-material structures.’ The most expensive agents are the ruthenium-based metathesis catalysts, ‘but these are used in ppm quantities, which makes it potentially viable for scale-up.’
It suits ‘applications that would benefit from mechanically robust substrates where stretching can be localised – something that could be transformative in the wearable electronics community,’ Hawker concludes.