Since ancient times, humans have been inventing ways to keep themselves safe from harm using a range of materials.
Personal protective equipment (PPE) has evolved over time, from the animal skins worn to protect from cold, wind and rain, to the heavy, restrictive armor worn in medieval times to protect the wearer in battle. We are now more used to seeing people whose professions require safety gear wearing impact-proof vests made of a Kevlar. This bright yellow fiber is relatively light and can be woven into a material that has a tensile strength high enough to protect the wearer from the impact of bullets and sharp weapons. While these safety vests retain a certain degree of movement that allows the wearer to remain relatively agile, their inflexibility still restricts the movement of the wearer. Recent research has focused on alternatives to Kevlar that are lighter and more flexible. Non-Newtonian materials, or materials whose viscosity drastically increases when a sudden force is applied to it, are being studied to utilize their impact-dissipating properties. Over at the Worcester Polytechnic Institute, however, one researcher may be close to finding a novel material to finally replace present-day PPE.
Nikhil Karanjgaokar, assistant professor of aerospace engineering, is working on creating materials that can both detect an incoming impact and respond appropriately to said impact. His work is in some ways similar to how cornflour particles in water lock together on impact to briefly turn the mixture into a solid and dissipate impact across the mixture rather than propagating the force through the mixture in the direction of the impact. Karanjgaokar’s work takes this a step further. He looks at the interactions between granules in materials to see how they manage to transfer energy through the material to reduce the force of impact. By studying these tiny interactions in fine detail, Karanjgaokar can select materials whose granules can be selectively modified in areas of impact to enhance the energy-dissipating properties of the material exactly where they are needed.
“I want to design materials that can absorb impact,” said Karanjgaokar, whose previous work at CalTech looked at wave motion in granular materials. “People trying to protect themselves from bullets or shrapnel have used sandbags since before the Second World War to absorb impacts. I’m working from the same basic principle. How can we create a versatile material to create a barrier against any impact?”
In impact protective PPE, the force of an impact needs to be diverted away from the wearer rather than travel through a material, so the wearer does not feel the full force of the impact. On studying materials whose granules behave more like those in the cornflour mixture, Karanjgaokar realized that he could further enhance this energy-disrupting effect by carefully selecting his granular material. If he used granules whose material properties can also be altered to further favor the dissipation of energy away from the wearer, the effect would be greatly enhanced. When a sensor detects an incoming impact, technology within the protective vest can modify the granular material in a specific area. If the granules are made of an electroactive polymer, for example, an electrical impulse can be delivered in a specific area that would cause the polymer granules to change their size and shape, giving rise to an area of higher density where the impact is expected. This would allow the material to disrupt the propagation of the impact through the material, blocking it and dissipating the energy away from the wearer. When the electric field is turned off, the granules return to their regular size, shape and density.
Karanjgaokar has also experimented with magnetically responsive granules in a fluid such as an oil. When a magnetic field is applied to the area required, the particles lock together and become more elastic and rubbery, again disrupting the direction and magnitude of the force of the impact, protecting the wearer. To create a shape-shifting vest requires a lot of experimentation, much of which will be done computationally to better understand the theoretical impact different materials could have on the dissipation of energy within the material, and how effectively different patterns of induced enhanced density affect the safety features of the material. The latter investigation would also allow such materials to be better tailored to manage multiple impacts from different directions and at different speeds.
“You can’t use one pattern to protect against all types of incoming projectiles,” Karanjgaokar said. “Depending on the size, direction, and velocity of the projectile, you would need a different pattern to absorb or disperse an impact.”
Technology such as this has obvious potential applications in PPE for high-risk professionals, such as the police, however, there are some other situations in which materials with high resistance to impact would also be very useful. Take the International Space Station, or ISS, for example. As more and more space junk litters Earth’s orbit, the chances of tiny flecks of high-speed space debris ripping through components of the ISS greatly increase. These tiny dust particles can move at speeds greater than 10 km per second, and so can easily rip through solar panels and more. By cladding certain parts of the ISS with a material such as this, there is less risk of damage from space dust hitting the spacecraft and causing significant damage.
Karanjgaokar has a lot of experiments to do, and a wealth of information to analyze before he can implement his new findings, but by looking at a natural phenomenon such as non-Newtonian materials and applying some modern-day materials science to it, it is possible that we are on the cusp of the next stage of PPE evolution.
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