'Chain mail' fabric can stiffen on demand

Scientists from Nanyang Technological University, Singapore (NTU Singapore) and the California Institute of Technology (Caltech) have developed a chain mail fabric that is flexible like cloth but can stiffen on demand.

Described in the journal Nature, the lightweight fabric is 3D-printed from nylon plastic polymers and comprises hollow octahedrons (a shape with eight equal triangular faces) that interlock with each other. When the soft fabric is wrapped within a flexible plastic envelope and vacuum packed, it turns into a rigid structure that is 25 times stiffer or harder to bend than when relaxed.

The scientific concept behind the variable-stiffness fabric is called ‘jamming transition’. This is a transition in which aggregates of solid particles switch from a fluid-like soft state to a solid-like rigid state, with a slight increase in packing density — similar to the stiffening behaviour in vacuum-packed bags of rice or beans. However, typical solid particles are usually too heavy and do not provide enough tensile resistance for wearable applications.

In their research, the authors designed structured particles — where each particle is made of hollow frames — in the shape of rings, ovals, squares, cubes, pyramids and different shapes of octahedrons that are then interlocked together. These structures, known as topologically interlocked structures, can then be formed into chain mail fabric that has a low density and yet high tensile stiffness, using state-of-the-art 3D printing technology to print them as a single piece.

They then modelled the number of average contact points per particle and how much each structure will bend in response to the amount of stress applied. The team discovered that by customising the particle shape, there was a trade-off between how much weight the particles will have versus how much the fabric can bend, and how to balance the two factors.

To add a way of controlling the stiffness of the fabric, the team encapsulated the chain mail fabric in a flexible plastic envelope and compacted the fabrics using a vacuum, which applies pressure from the outside. The vacuum pressure increases the packing density of the fabric, causing each particle to have more contact with its neighbours, resulting, for the octahedron-based fabric, in a structure that is 25 times more rigid. When formed into a flat, table-shaped structure and vacuum locked in place, the fabric could hold a load of 1.5 kg — more than 50 times the fabric’s own weight.

In another experiment, the team dropped a small steel ball (30 g, measuring 1.27 cm in diameter) onto the chain mail at 3 m/s. The impact deformed the fabric by up to 26 mm when it was relaxed, but by only 3 mm when it was stiffened, a six times reduction in penetration depth.

The lead author of the paper, NTU Assistant Professor Wang Yifan, said the research has fundamental significance as well as industrial relevance and that it could lead to a new platform technology with applications in medical and robotic systems that can benefit society.

“With an engineered fabric that is lightweight and tuneable — easily changeable from soft to rigid — we can use it to address the needs of patients and the ageing population, for instance, to create exoskeletons that can help them stand, carry loads and assist them with their daily tasks,” said Asst Prof Wang, who started the research when he was a postdoc researcher at Caltech.

To show the possibilities of their fabric concept using different source material, the team 3D-printed the chain mail using aluminium and demonstrated that it has the same flexibility and ‘soft’ performance as nylon when relaxed and yet it could also be ‘jammed’ into structures that are much stiffer compared to nylon due to aluminium’s higher stiffness and strength.

These metallic chain mails could be used in applications such as body armour, where they must protect against hard and high-speed impacts from sharp objects. In such a case, the encapsulation or envelope material could be made from aramid fibres, commonly known as Kevlar, used as a fabric in bulletproof vests. Metallic variants could also be used for larger-scale industrial applications requiring higher load capacity, such as bridges or buildings.

Moving forward, the team is looking to improve the material and fabric performance of their chain mail and to explore more methods of stiffening it, such as through magnetism, electricity or temperature.

“We wanted to make materials that can change stiffness on command,” said Caltech Professor Chiara Daraio, corresponding author on the study. “We’d like to create a fabric that goes from soft and foldable to rigid and load-bearing in a controllable way.”

Image caption: When stiffened, the new chain mail fabric can withstand up to 50 times its own weight.

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