US researchers are developing a nanofibre they believe could revolutionise safety gear for soldiers, emergency service personnel and astronauts.
It offers protection from both ballistic threats and extreme temperatures while being light enough not to hamper mobility, they say, thanks to a mix of order and disorder in its molecular structure and orientation.
The work by Harvard University, in collaboration with the US Army Combat Capabilities Development Command Soldier Centre and West Point, is described in a paper in the journal Matter.
Materials that offer mechanical protection have a highly ordered and aligned molecular structure, the researchers say, allowing them to withstand and distribute the energy of a direct blow. Insulating materials, on the other hand, are much less ordered, to prevent the transmission of heat.
And this can apply to the same material. For example, woven Kevlar, a synthetic fibre, has a highly aligned crystalline structure and is used in bulletproof vests, while porous Kevlar aerogels have been shown to have high thermal insulation.
“Our idea was to use this Kevlar polymer to combine the woven, ordered structure of fibres with the porosity of aerogels to make long, continuous fibres with porous spacing in between,” says first author Grant M Gonzalez. “In this system, the long fibres could resist a mechanical impact while the pores would limit heat diffusion.”
Gonzalez and colleagues use a technique called Rotary Jet-Spinning to manufacture the fibres.
A liquid polymer solution is loaded into a reservoir and pushed out through a tiny opening by centrifugal force as the device spins. As it leaves the reservoir, the solution first passes through an area of open air where the polymers elongate, and the chains align.
The solution then hits a liquid bath that removes the solvent and precipitates the polymers to form solid fibres. Since the bath is also spinning, the nanofibres follow the stream of the vortex and wrap around a rotating collector at the base of the device.
By tuning the viscosity of the liquid polymer solution, the researchers are able to spin long, aligned nanofibers into porous sheets, providing enough order to protect against projectiles but enough disorder to protect against heat.
The prototype performed well in ballistic tests, they say, and in heat tests provided 20 times the insulation capability of existing synthetic fibres.
“While there are improvements that could be made, we have pushed the boundaries of what’s possible and started moving the field towards this kind of multifunctional material,” says Gonzalez.