The realization that football players were unknowingly causing permanent brain damage from repeated head hits in their professional careers sparked a race to develop better head protection. One invention that arose from this pursuit is nanofoam, a material found inside football helmets.
Thanks to the work of mechanical and aerospace engineering associate professor Baoxing Xu and his research team at the University of Virginia, nanofoam has recently undergone a major upgrade, paving the way for potential enhancements in protective sports equipment. The team has successfully integrated nanofoam with “non-wetting ionized liquid,” a type of water that blends perfectly with nanofoam to create a liquid cushion. This versatile and responsive material holds promise for providing improved protection to athletes and may also have applications in safeguarding car occupants and assisting hospital patients who use wearable medical devices.
The team’s findings were recently published in Advanced Materials.
In order to ensure maximum safety, the foam used to cushion impacts between the inner and outer layers of a helmet must be capable of withstanding not just one hit, but multiple hits sustained throughout a game or even over multiple games. The material needs to have enough cushioning to create a soft landing spot for the head, while also being resilient enough to quickly bounce back and be ready for the next impact. Furthermore, it must be resilient without being too hard, as excessive hardness can also cause damage to the head. Finding a material that can fulfill all these requirements is a significant challenge.
The team built upon their previous research, published in the Proceedings of the National Academy of Sciences, which explored the use of liquids in nanofoam to create a material that meets the complex safety demands of high-contact sports.
Xu explained, “We discovered that using ionized water instead of regular water in the design of a liquid nanofoam cushion made a significant difference in how the material performed. Incorporating ionized water is a groundbreaking development because we uncovered an unusual liquid-ion coordination network, enabling the creation of a more sophisticated material.”
The liquid nanofoam cushion inside the helmet compresses upon impact, effectively dispersing the force and minimizing its transmission to the head, thus reducing the risk of injury. After impact, the cushion regains its original shape, allowing for repeated hits and maintaining the helmet’s effectiveness in protecting the athlete’s head during the game.
Xu added, “An added bonus is that the improved material is more flexible and much more comfortable to wear. It dynamically responds to external shocks due to the fabrication of ion clusters and networks within the material.”
Collaborator and associate professor Weiyi Lu from the civil engineering department at Michigan State University also noted, “The liquid cushion can be used to design lighter, smaller, and safer protective devices. Additionally, the reduced weight and size of the liquid nanofoam liners will transform the design of future helmets’ hard shells. You may one day watch a football game and wonder how the smaller helmets are still able to protect the players’ heads. It could be due to our new material.”
Traditional nanofoam relies on material properties that react when mechanically deformed, such as “collapse” and “densification,” to provide protection. Collapse occurs when the foam is crushed, and densification is the severe deformation of the foam upon a strong impact. However, traditional nanofoam does not recover well after collapse and densification due to its permanent material deformation, rendering the protection as a one-time occurrence. Additionally, compared to liquid nanofoam, these properties are slower (taking a few milliseconds) and cannot effectively absorb and dissipate high-force impacts within the short time frame associated with collisions and impacts.
Another drawback of traditional nanofoam is that, when subjected to multiple small impacts that do not deform the material, the foam becomes completely rigid and loses its ability to provide protection. This rigidity can potentially lead to injuries and harm soft tissues, such as traumatic brain injury (TBI).
By manipulating the mechanical properties of materials and combining nanoporous materials with non-wetting liquid or ionized water, the team has developed a material that can respond to impacts in a few microseconds. This combination facilitates rapid liquid movement in a confined nanoenvironment. Furthermore, the liquid nanofoam cushion can return to its original shape after unloading from an impact because its non-wetting nature allows the liquid to be expelled from the pores. This dynamic conforming and reforming ability also addresses the issue of the material becoming rigid due to micro-impacts.
The liquid properties that make this new nanofoam safer for athletic gear also present potential uses in other collision-prone environments, such as cars. With the emergence of electric propulsion and automated vehicles, the safety and material protective systems of cars are being reevaluated. Liquid nanofoam can be utilized to create protective cushions that absorb impacts during accidents or help reduce vibrations and noise.
The application of liquid nanofoam in the field of healthcare may not be immediately apparent. However, it can be utilized in wearable medical devices such as smartwatches that monitor vital signs. Incorporating liquid nanofoam technology into these devices allows for a soft and flexible foam-like material to be placed on the underside, improving the accuracy of the sensors by ensuring proper contact with the skin. This foam can conform to the wrist’s shape, making it comfortable to wear throughout the day. Additionally, it serves as an extra layer of protection by acting as a shock absorber. If the wrist accidentally bumps against a hard surface, the foam cushion mitigates the impact and prevents any harm to the sensors or the skin.