A team of researchers at the University of Virginia has found a way to improve the performance of American football helmets by combining nanofoam with special liquid. According to the researchers, the innovation could be used in other protective sports equipment and in hospital settings.
This new design integrates nanofoam with ‘non-wetting ionised liquid’, a form of water that blends perfectly with nanofoam to create a liquid cushion. This versatile and responsive material will give better protection to athletes and is promising for use in protecting car occupants and aiding hospital patients using wearable medical devices.
For maximum safety, the protective foam sandwiched between the inner and outer layers of a helmet should not only be able to take one hit but multiple hits, game after game. The material needs to be cushiony enough to create a soft place for a head to land, but resilient enough to bounce back and be ready for the next blow. And the material needs to be resilient but not hard, because ‘hard’ hurts heads, too. Asking a single material to do all of these things is a tall order.
The team started exploring the use of liquids in nanofoam some time ago, aiming to create a material that meets the complex safety demands of high-contact sports. ‘We found out that creating a liquid nanofoam cushion with ionised water instead of regular water made a significant difference in the way the material performed,’ said Baoxing Xu (pictured above), an associate professor of mechanical and aerospace engineering at the University of Virginia. ‘Using ionised water in the design is a breakthrough because we uncovered an unusual liquid–ion coordination network that made it possible to create a more sophisticated material.’
The liquid nanofoam cushion allows the inside of the helmet to compress and disperse the impact force, minimising the force transmitted to the head and reducing the risk of injury. It also regains its original shape after impact, allowing for multiple hits and ensuring the helmet’s continued effectiveness in protecting the athlete’s head during the game.
‘An added bonus is that the enhanced material is more flexible and much more comfortable to wear,’ Xu continued. ‘The material dynamically responds to external jolts because of the way the ion clusters and networks are fabricated in the material.’
‘The liquid cushion can be designed as lighter, smaller and safer protective devices,’ said associate professor Weiyi Lu, a collaborator from civil engineering at Michigan State University. ‘Also, the reduced weight and size of the liquid nanofoam liners will revolutionise the design of the hard shell of future helmets. You could be watching a football game one day and wonder how the smaller helmets protect the players’ heads. It could be because of our new material.’
In traditional nanofoam, the protection mechanism relies on material properties that react when it gets crunched, or mechanically deformed, such as ‘collapse’ and ‘densification’. Collapse is what it sounds like and densification is the severe deformation of foam on strong impact. After the collapse and densification, traditional nanofoam doesn’t recover very well because of the permanent deformation of materials, making the protection a one-time deal. When compared to the liquid nanofoam, these properties are very slow (a few milliseconds) and can’t accommodate the ‘high-force-reduction requirement’, which means it can’t effectively absorb and dissipate high-force blows in the short time window associated with collisions and impacts.
Another downside of traditional nanofoam is that, when subjected to multiple small impacts that don’t deform the material, the foam becomes hard and behaves as a rigid body that can’t provide protection. The rigidity could potentially lead to injuries and damage to soft tissues, such as traumatic brain injury.
By manipulating the mechanical properties of materials – integrating nanoporous materials with ‘non-wetting liquid’ or ionised water – the team developed a way to make a material that could respond to impacts in a few microseconds because the combination allows for superfast liquid transport in a nanoconfined environment. Due to its non-wetting nature, upon unloading – that is, after impacts – the liquid nanofoam cushion can return to its original form because the liquid is ejected out of the pores, thereby withstanding repeated blows. This dynamic conforming and reforming ability also remedies the problem of the material becoming rigid from micro-impacts.
The same liquid properties that make this new nanofoam safer for athletic gear also offer a potential use in other places where collisions happen, such as cars, whose safety and material protective systems are being reconsidered to embrace the emerging era of electric propulsion and automated vehicles. It can be used to create protective cushions that absorb impacts during accidents or help reduce vibrations and noise.
Liquid nanofoam could also play in a hospital setting – in wearable medical devices such as a smartwatch that monitors your heart rate and other vital signs. By incorporating liquid nanofoam technology, the watch can have a soft and flexible foam-like material on its underside and help improve the accuracy of the sensors by ensuring proper contact with the skin. It can conform to the wrist’s shape, making it comfortable to wear all day.
The research has been published in Advanced Materials.