This diagram illustrates how a liquid nanofoam cushion responds to an impact. Image contributed by B. Xu.
This diagram illustrates how a liquid nanofoam cushion responds to an impact. Image contributed by B. Xu.

The discovery that players of American football were unknowingly acquiring permanent brain damage as they racked up head hits throughout their professional careers created a rush to design better head protection. One of these inventions is nanofoam, the material on the inside of current American football helmets.

Thanks to mechanical and aerospace engineering associate professor Baoxing Xu at the University of Virginia and his research team, nanofoam has just received a big upgrade. Their newly invented design integrates nanofoam with a ‘non-wetting ionized liquid’, a form of water that Xu and his research team now know blends perfectly with nanofoam to create a liquid cushion. This versatile and responsive material will give better protection to athletes and is also promising for use in protecting car occupants and aiding hospital patients using wearable medical devices. The team reports its work in a paper in Advanced Materials.

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. Having one material that can fulfil all these requirements is a pretty tall order.

This study advances previous work that the team reported in a paper in the Proceedings of the National Academy of Sciences, in which the researchers started exploring the use of liquids in nanofoam to create a material that meets the complex safety demands of high-contact sports.

"We found out that creating a liquid nanofoam cushion with ionized water instead of regular water made a significant difference in the way the material performed," Xu said. "Using ionized water in the design is a breakthrough because we uncovered an unusual liquid-ion coordination network which made it possible to create a more sophisticated material."

The liquid nanofoam cushion allows the inside of the helmet to compress and disperse an impact force, minimizing the force transmitted to the head and reducing the risk of injury. This cushion can also regain its original shape after impact, allowing for multiple hits and ensuring the helmet's continued effectiveness in protecting the athlete's head during a game.

"An added bonus," Xu continued, "is that the enhanced material is more flexible and much more comfortable to wear. 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 civil engineering collaborator from Michigan State University. “Also, the reduced weight and size of the liquid nanofoam liners will revolutionize 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.”

The protective effect of traditional nanofoam relies on mechanisms such as ‘collapse’ and ‘densification’, which come into play when the nanofoam gets crunched, or mechanically deformed. Collapse is what it sounds like, and densification is the severe deformation of foam on strong impact.

After collapse and densification, traditional nanofoam doesn’t recover very well because of its permanent deformation – making the protection a one-time deal. When compared to the liquid nanofoam, these protection mechanisms are very slow (a few milliseconds) and cannot accommodate the ‘high-force reduction requirement’. This means traditional nanofoam 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 completely ‘hard’ and behaves as a rigid body that cannot provide protection. This rigidness could potentially lead to injuries and damage to soft tissues, such as traumatic brain injury (TBI).

By integrating nanoporous materials with a ‘non-wetting liquid’ or ionized water, the team was able to produce a material that could respond to impacts in a few microseconds via superfast liquid transport in a nanoconfined environment. This liquid nanofoam cushion can also return to its original form upon unloading, i.e., after impacts, because the liquid is ejected out of the pores, thereby allowing it to withstand repeated blows. This dynamic conforming and reforming ability also resolves the problem of the material becoming rigid from micro-impacts.

The same liquid properties that make this new nanofoam safer for athletic gear could also prove of use in other places where collisions happen, like cars, where safety and material protective systems are being reconsidered for the emerging era of electric propulsion and automated vehicles. The nanofoam could be used to create protective cushions that absorb impacts during accidents or help reduce vibrations and noise.

It could also be used in wearable medical devices like a smartwatch, which 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 that helps improve the accuracy of the sensors by ensuring proper contact with the skin. It can conform to the shape of your wrist, making it comfortable to wear all day. Additionally, the foam can provide extra protection by acting as a shock absorber. If you accidentally bump your wrist against a hard surface, the foam can help cushion the impact and prevent any harm to the sensors or your skin.

This story is adapted from material from the University of Virginia School of Engineering and Applied Science, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.