Smart hazmat suits based on cactus skin

Materials World magazine
,
4 Jan 2019

Ageing cactus skin is informing the development of high-tech hazmat suits. Ceri Jones reports. 

The wettability of cactus skin has been helping scientists develop a new smart membrane for military-grade hazard protection clothing.

Prickly pear cacti, prevalent in the Sonora Desert in Arizona, USA, have adapted to the adverse and extreme weather fluctuations, ranging from 11-57oC with 8-41cm of rainfall over two rainy seasons. To cope, the cacti use sophisticated water vapour regulation capabilities, which develop over their lifetime.

The unusual characteristics of the cacti pads were initially noticed by Konrad Rykaczewski, Professor of Engineering at Arizona State University (ASU), USA, who observed that when sprayed with a water bottle, younger cacti pads repel the water droplets while older ones grasp them, allowing water ingress into the plant.

‘What I noticed about prickly pear cacti is that the new pads are superhydrophobic, but the ones a season older, right below them,
are superhydrophilic,’ Rykaczewski told Materials World. He wondered why this would happen, and set out to test the wettability of the cacti, as well as examining desert-dwelling creatures that absorb water through their skin, such as rattlesnakes.

Under the skin

With a seed grant from ASU, Rykaczewski set up a team to analyse the microscopic 3D epidermal structure of the cactus skin to observe its nature during the wet and dry seasons. This revealed that the young cactus pads have a seamless waxy surface that repels liquid, with high-speed video showing liquid drops literally bouncing off the surface.

‘After the pad grows and is around for one season, it can expand up to 60% and then lose up to 60% of its volume in the dry season,’ Rykaczewski explained.

The xeric climate of Arizona and its bimodal seasons cause cycles of extreme wetting and drying, which continuously hydrates and shrinks the young pads, causing a network of multilevel creases to develop.

‘The outside wax gets so stretched that it cracks,’ Rykaczewski says. When these microscale changes occur, the emerging cracks reveal the inner hydrophilic layer that holds on to any liquid it encounters, transporting it from the surface straight to the cells.

‘From then on these microcracks open and close, so that when the cacti are hydrated, there’s a secondary pathway besides the stomata for water vapour to escape. They get so saturated that they almost fall over. And as they start dehydrating, they need to slow the water release, so these cracks close up.’

While this sounds like a mechanical process, it is actually due to changes in water pressure inside the cacti. ‘We’ve been looking at the geometrical patterns they’ve been forming in this hydration-dehydration process, as it’s opening up and closing up for smart vapour transport,’ Rykaczewski continued.

‘While dramatically altering how droplets spread on the cacti overall, our results suggest this seasonal epidermis microstructure is predominantly helping to regulate water vapour transport.’

Making smart membranes

While this discovery did not directly lead to the development of a new material, it coincided with the pursuit of a smart membrane for hazmat suits, and both projects have managed to inform and advance the other.

High-grade hazmat suits must protect the wearer from exposure to dangerous substances, so conventionally, they prevent any form of vapour transport through their surface – making them very hot and uncomfortable to wear for long periods. So the US Navy enlisted Rykaczewski and co-workers to develop a solution.

‘We made this fabric that’s a new polymer that doesn’t respond to water, but when exposed to things that are close to the target chemicals, then it swells up to 40 times,’ he said. ‘It’s an open, porous fabric and we’re trying to create a geometrical design to match different properties, so it would swell up fast in response.

Using information gained from the cacti project, Rykaczewski is now examining how and why the microcracks form specific geometrical patterns, with the thinking that, if there’s an order to it, this could be replicated and applied to the polymer design.

‘We’re making little beads of this polymer and are in the process of attaching them to fabrics and the question is, “what size of bead should they have?” In a way, we’re making a network of pores and cracks that close up through swelling, so it’s a similar principle.’

Research was led by Rykaczewski, working alongside Kenneth Manning, a PhD student at the Biomimicry Centre of ASU, who is also working on the hazmat smart membrane project. The team presented their findings at the 71st Annual Meeting of the American Physics Society’s Division of Fluid Dynamics in Georgia, USA, in November 2018, but results have not yet been published.