"Nano-Scale Armor," Feature Article, September 2006

Focus on Nanotechnology

nano-scale armor

Molecular hinges, electrorheological fluids—will they protect tomorrow's infantry?

by Michael Abrams, Contributing Editor

it's more than an understatement to say that today's soldiers have it rough. In addition to the usual hazards of bullets, shrapnel, and chemical and biological weapons, infantry soldiers typically haul between 100 and 150 pounds. But there may well come a day when a soldier in a battlesuit can take a few bullets in the chest, wipe a few shards of shrapnel off his person, walk through the fumes of a freshly exploded chemical bomb, calmly continue on his way through any raining pestilence, and do it all with maybe 500 extra pounds on his back.

Researchers at the Institute for Soldier Nanotechnologies are making this near-superhero a reality sooner than you might think. Founded at the Massachusetts Institute of Technology in 2002, ISN is dedicated to achieving such objectives through nanotechnology.

It might seem at first glance that today's soldier hasn't changed much from the infantryman of the last world war. If you omit night-vision goggles and other sundry gadgets, boots, helmets, uniform, and loads of gear look more or less the same.

Molecular models show how artificial muscles can work on the nanoscale. Though each is very small, in networks they can achieve large dimensional changes, perhaps sufficient to deliver CPR.

In fact, helmets are no longer metal; they're Kevlar. Similarly, today's uniform includes rigid body armor. But tomorrow's soldier, although he may continue to look fairly similar, will be clad in far lighter materials capable of protecting, assisting, and even delivering first aid—making soldiers stronger, faster, and safer. If the general appearance of the gear seems unchanged to the untrained eye, it will be only because the difference is nanoscopic.

Of course, the world already has Kevlar, biohazard suits, and even hydraulic robotic exoskeletons, like those made by the Berkeley Robotics Laboratory. Nanotechnology is expected to make better versions of them, but its real promise is the reduction of weight.

"There is a real concern to develop lighter-weight stuff for the soldiers on the ground," said Neville Hogan, a professor of mechanical engineering as well as of brain and cognitive science at the institute. "I've visited some of the training camps and seen what some of these guys are carrying around. It's staggering—no pun intended. They're carrying astonishing amounts of weight. Honestly, in a combat situation they'll chuck most of that stuff, except for what they absolutely need."

To lighten the future soldier's physical burden, research at the ISN revolves around making fibers and coatings for fabrics, and single layers are often measured by the molecule.

Pluses and Minuses

Paula T. Hammond, an associate professor of chemical engineering at the institute, leads a research team that hopes to discover materials that can both detect and resist chemical weapons or biological attacks. Eventually, molecules in a soldier's uniform will be able to neutralize specific chemicals and literally pop the cells of less-than-friendly biological agents.

"Right now we're incorporating materials into these polymer films that are reactive," Hammond said. "For them to be effective, the film itself has to allow some permeation of the toxic agent."

The difficulty in using multiple polymers—and other materials—has long been that polymers tend to separate from each other. Hammond's solution is a novel use of polyelectrolytes.

First, she takes a positively charged polymer chain and dissolves it in water. Then, another polymer, first charged separately, can be applied to this substrate. The polymer will adsorb onto the surface, thanks to the attracting positive and negative charges, and then generate the same surface charge of the substrate. Once it gets highly charged, no more polymer will deposit on the substrate because a new, light charge on the surface creates repulsion, keeping the layer nano-thin. A simple rinse removes anything that's not fully adsorbed onto the surface. This "self-limiting adsorption" is repeated layer after layer "plus, minus, plus, minus, all the way through," Hammond said.

Soldiering is a heavier job than ever. Infantrymen routinely carry upwards of 150 pounds. Soon, though, their very uniforms may help lift that weight.

Polymers are not the only material that can be incorporated into the multilayer. "The nice thing is you can change the ingredients as long as you keep that plus-minus cycle going," Hammond said. Right now, the team is working on incorporating titanium, which is able to break down a number of chemical agents, including nerve and mustard agents, in the presence of sunlight. Hammond plans to also include a layer of nano-size molecules called dendrimers, which can react with mustard gas and deactivate it. Many of the coatings they've come up with have the added benefit of being waterproof—protecting soldiers from the elements as well as from things like E. coli.

To detect biological threats whether or not they're subsequently disarmed, Hammond and her team attempted to use non-infectious viruses genetically engineered by their colleague Angela Belcher. Six to eight nanometers in diameter and a micrometer in length, these viruses look like spaghetti noodles and can detect various chemical and biological agents.

Much to their surprise, when the researchers went to incorporate them into one multilayer, they found that the viruses had congregated on the top of each layer. As the polymer they were using happened to be "amenable to the transport of ions," the viruses were making tiny wires, giving Hammond and her colleagues hope that they could eventually develop tiny lithium-ion batteries. Although the result wasn't exactly what they had planned to develop, it certainly fits in with the goals of the ISN. Power is one of the heaviest things a soldier has to carry.

Pumping Iron

The only way to lighten a soldier's load—other than actually lightening his load—is to give him extra strength to lift it. And a soldier in combat could use something lighter than, say, a forklift. Ian Hunter hopes to make an artificial muscle that when not in use is neither heavy nor bulky and activates instantly to aid a soldier only when needed.

"What we are competing with is what nature has produced," said Hunter, the Hatsopoulos Professor of Mechanical Engineering, as well as a professor of biological engineering and director of the Bioinstrumentation Laboratory at the institute. Nature's long and somewhat haphazard way of engineering things is proving difficult to beat, and Hunter and his team have identified five characteristics of flesh and blood muscles they hope to emulate to make a material that's light, mobile, and full of power to be built into a battle suit.

The first characteristic is the simple ability to contract. Mammalian skeletal muscles produce about 0.35 mega- newtons per square meter—a level of force that Hunter's polymers can already achieve. Mammalian muscles can easily contract over 20 percent of their length. Hunter's artificial muscles can do that also to provide movement, but not without losing the edge they have when it comes to force.

"The best we have achieved would be equivalent to a slow-contracting muscle," Hunter said, "but what we want is fast-twitch mammalian muscle."

In addition to the ability to contract quickly and with force, an artificial muscle should be able to produce large forces while retaining its lightness. ISN polymers are nearly at the human mark of 50 watts per kilogram, but the hope is to come closer to what many insects can achieve: 500 watts per kilogram. "Our efficiency is somewhat less, but we're slowly bringing it up," Hunter said.

Nanocoatings keep minesweepers in the water longer.

A third characteristic is something you don't tend to find in humans—the ability of a muscle to expand with force. The next time you're at the gym admiring the watts per kilogram that your biceps exert while performing a curl, note that if you want your forearm to go the other way, it takes an entirely different muscle—the triceps. "If you've got an artificial muscle that can actively expand, you don't need that other muscle to pull it out again," Hunter said.

Humans fail, too, when it comes to something called a "latch state." Mollusks, on the other hand, are way ahead of us, and so is the ISN. After a startled clam has closed its shell, it remains contracted, and in doing so it continues to generate force without expending energy.

Actually, humans can achieve a latch state, too, but it's called rigor mortis. What the Institute for Soldier Nanotechnologies has achieved is what they call a "reversible rigor state" in an artificial muscle.

"So the interesting thing is mollusk muscle is more sophisticated, in a sense, than mammalian muscle," Hunter said. "The army could use that to stabilize a soldier's arm, to lock it up."

The final challenge is to make a material that can stand up to as many repetitions as a human muscle goes through, if not in a lifetime, then at least in the time it takes to carry out an extended mission.

Currently, Hunter's team is trying to model new materials at the molecular level and at the same time is trying as many new things as possible—in what Hunter calls "an Edison-style dragnet approach." The researchers have made some headway with polymers that work as nano-hinges. When hit with a charge, they collapse into a closed state called a "dimmer formation." The ability of these materials to expand and collapse in specific areas may mean that a soldier's artificial muscles may one day be able to perform CPR.

Potentially Viscous Sandwich

While some researchers at the ISN are trying to create strength through flexing materials, others are looking for strength through rigidity. If a fabric could go from loose to taut in an instant, soldiers could have lighter, more comfortable body armor and clothing could act as sudden splints. "We want to have materials that can change, that can give you rigidity on demand," said Neville Hogan. "You could essentially have something that is just like a blanket— flexible—but when you activate it, it becomes semi-rigid like a stretcher."

Such snap-to-it-ness is made possible by a fabric that is layered like a deck of cards—very, very thin cards. In between each card is fluid, and when it's hit with an electric charge, the layers become stiff relative to one another. "That geometry solves the problem of how to get human-scale forces transmitted down to micro- and nano-scale levels," Hogan said. The difficulty is how to do that without having the layers short out: The fabric will see no end of bending and twisting once it's in an actual battle suit.

From flexible to bulletproof: Electrorheological materials may one day go from free-form (above) to rigid in an instant, or fast enough to stop a bullet.

The first step is making materials that are actuators, but keeping them as simple as possible. The materials Hogan and his team are using have a Poisson's ratio very close to 0.5—meaning it's next to impossible to compress them. When chemicals or electric charges are applied, the material may change shape, but it won't change volume.

The fluid between layers is the key to creating sudden change in viscosity with as little power as possible. Hogan has focused on using an electrorheological fluid, which resembles liquid crystals. "Their polarities have longish molecules," he said. When they are hit with electricity, these fine anisotropic particles suddenly become rigid as their polarities line up. "The big plus is they allow you to go down, in principle, to the molecular scale," Hogan said.

Hogan used MEMS techniques to place conduction plates as close together as possible and still maintain some separation. "If you have wrinkles or dents in the surfaces, that's where your fields will concentrate. You get a breakdown, and the whole thing falls apart," Hogan said. The spacing on his current deck of cards is on the order of 10 to 20 micrometers. "We're in the process of doing experiments to see how well it works."

Although we are still some way from lightening the load of today's soldier or creating new generations of body armor with nano-materials, researchers at the ISN have made strides toward those goals in just a few years.

"The agency gave us concepts they'd like to see, not in technical terms, but general concepts," Paula Hammond said. "You see the need and the science coming together, meeting specific needs. It's all about the protection of the soldier."

nano in the navy

the promise of nano research often points to the future—an exciting one, to be sure, but one that sometimes seems distant. "When I talk to audiences, there's a perception that nano has been around a while but what's come of it?" said Richard Colton, director of the Institute for Nanoscience at the Naval Research Laboratory.

Although perhaps not as glamorous as body armor capable of giving CPR, nano-coatings have already saved the military millions of dollars.

Until recently, mine countermeasures warships were taken out of service and put into dry dock for maintenance every year. Because they had to use nonmagnetic materials to avoid setting off mines, the propeller driveshafts would become quickly—and severely—corroded while the ships patrolled the sea lanes. Now, as the ships come in for service, the driveshafts are coated with a nonmagnetic nano-ceramic composite using a thermal spray. Parts that have received the nano coating show no visible signs of damage, even after as much as four years of duty.

"When you're dealing with a nano-scale material, the particles are smaller and packed more densely," Colton said. "There are a limited number of defects and a limited failure rate." —Michael Abrams