by Chris Woodford. Last updated: July 28, 2016.
The human body's remarkable design can protect against heat and cold, cushion blows, and even repair its own injuries. But it has not evolved fast enough to keep pace with the threats posed by humankind's more recent inventions. Our bodies are not fireproof or bullet-proof, they cannot protect us against the extremes of temperature and pressure found deep in the ocean or way out in space, and they are poorly suited to defend us against the types of injuries sustained during high-speed accidents. Fortunately, evolution of a different kind—technological progress—has developed a range of protective materials that offer better defense against the trials of modern living. Let's take a closer look at how they work!
Photo: Ed White, the first American spacewalking astronaut, relied heavily on protective materials in his suit and helmet. There's no way to explore space without the help of protective synthetics such as Kevlar® and Teflon®. Photo courtesy of NASA on the Commons
Fire and flames
Humans are believed to have used fire more than a million years ago, but it was not until the 20th century that people perfected the art of protecting themselves against the destruction that fire could bring. The widespread use of electricity and petroleum products in the last century or so has made fire protection all the more important.
Fire means intense heat and temperatures, so fire-protective materials must be capable of resisting heat for some time without themselves catching fire. Some plastics and composites are naturally heat- and fire-resistant. Polyvinyl chloride (PVC), for example, is used as an insulator in domestic electrical wiring, because it is difficult to set on fire and prevents flames from spreading. A composite of silica and alumina, two naturally nonflammable rock materials, is used in industrial buildings such as power plants to offer heat protection up to 2200°F (1200°C). Another promising use for this composite is in airplanes, whose combustible plastic interiors give off pound-per-pound as much energy as petroleum when they catch fire. In 1998, Geopolymer, a lightweight, nonflammable fabric laminate based on the alumina-silica composite, became the first material to withstand arduous Federal Aviation Authority (FAA) tests based on an airplane fuel fire. Materials such as this can be expensive, and an alternative approach is to wrap flammable materials in flame-retardant ones. For example, airplane seat cushions are typically made from flammable polyurethane, but can be made safer by coating them in fire-resistant Kevlar®, a tough composite made from aramid fibers.
Photo: A gasoline fire can quickly create temperatures in the range 500–1000°C (900–1800°F)—far too hot for humans to endure without protective aluminum clothing. Photo by Adrian Cadiz courtesy of US Air Force.
People who work close to intense heat, such as firefighters, welders, and vulcanologists (scientists who study volcanoes), wear protective clothing made from lightweight, fireproof materials. Aluminum-based fabrics are used in heavy-duty, silver-colored fire-fighting suits, while waterproof fire clothing may be made from a tightly woven plastic or composite material coated with a flash-fire and chemical proof outer layer made from neoprene (a synthetic rubber used in wetsuits). Firefighting visors and airplane smoke hoods are made from Kapton® coated with Teflon®, two tough fireproof composites used in the outer layers of Apollo space suits.
You might think firefighters swathed in aluminum are superbly protected and at no risk whatsoever from the leaping flames all around them, but unfortunately it's not quite that simple. Although fireproof clothes do a great job stopping heat from getting in, they stop it getting out as well. Since firefighting clothes are also designed to resist attack from liquids and chemicals, they tend to stop moisture getting out as well, which cuts off one of the body's essential cooling mechanisms: sweating. In short, even if firefighters can be completely protected from burns, they're still at high risk from overheating. A firefighter working in a burning building may experience a rapid rise in core body temperature. Normal body temperature hovers around 37°C (98.6°F) and doesn't need to rise or fall very much to endanger our health. If your core temperature rises to 39°C (102°F), your body and mind are likely to stop functioning quite quickly (in a medical condition known as hyperthermia); if it hits 43°C (109°F) or so, you're very likely to die. So it's critically important that fireproof clothes help keep the body as cool as possible. One recently developed solution to the problem is a vest that firefighters can wear that has built in cooling tubes. They constantly circulate cool carbon dioxide gas from a small portable tank, helping sweat to evaporate and keeping the firefighter's body cool.
Artwork: Firefighting clothes have to protect against heat as well as flames. This jacket uses removable blocks of dry ice (cold, solid carbon dioxide; shown in blue) held in a mesh textile (red). These fit into an outer insulating sleeve (gray), made of bubblewrap-like material, which helps to spread the cooling effect over a wider area of the wearer's body. In one version of the design, shown in the picture on the right, the cooling blocks are removable and fixed into deep pockets (green) by Velcro. From US Patent 3,950,789: Dry ice cooling jacket by Stephan Konz and Jerry Duncan, assignees to Kansas State University, April 20, 1976, courtesy of US Patent and Trademark Office.
Bullets and blows
If the human body is ill-designed to withstand fire, it has even less chance where bullets, knives, and bombs are involved. But revolutionary materials that act like a protective outer skin can turn a fatal gunshot into little more than a bruise.
Five times stronger than steel (by weight) and much lighter, the composite Kevlar® has revolutionized body protection. It consists of fibers made from long molecular chains of a polymer (plastic material) called polyparaphenylene terephthalamide. Woven tightly together, the fibers effectively knit into a tough protective grid that can withstand knife blows and cuts and some types of gunfire. Effectively, the grid of fibers absorbs and dissipates the energy of a knife blow or bullet before it can damage the body beneath it. Kevlar is also chemical and flame resistant. All this makes it the material of choice in flak jackets, bulletproof vests, anti-mine boots, and chainsaw-protective clothing. An even tougher material called Spectra® Fiber is made from a polyethylene composite, in which the fibers are woven at right angles to one another in a flexible resin and coated with a laminate film. This material is ten times stronger than steel, yet extremely light, and provides better protection than woven composites such as Kevlar against automatic weapons.
Photo: This is a piece of Kevlar after being hit by a projectile. You can see a dent (coming up toward the camera)—but you can't see a hole. Picture courtesy of US Army.
It is not always practical for people to wear bulletproof clothing; sometimes it is more convenient to protect them by armoring the vehicles in which they travel. Armor-plated cars have long been used by heads of state and celebrities and used to feature heavy duty steel reinforcement built between the vehicle's bodyshell and chassis. Today, composite materials provide the same protection as anti-ballistic steel but add only a third of the weight to the vehicle. Armored vehicles are typically also fitted with bulletproof "glass" (really a tough sandwich of glass and polycarbonate, a transparent thermoplastic).
From bicycle helmets to space suits
Not everyone has to defend themselves against petroleum fires or automatic weapons, but everyone is exposed to some risk, every day. Urban traffic makes using a bicycle an increasingly dangerous undertaking, but using a bicycle helmet can reduce the risk of injury by around 85 percent. Most helmets consist of a "styrofoam" (expanded polystyrene (EPS) foam) liner and an outer shell made of plastic or composite. In an impact, the liner crushes gradually, reducing and dissipating the energy of the blow and increasing the time it takes for the wearer's head to come to a stop. Meanwhile, the rounded outer shell helps to spread the impact over a larger area, reducing the damage it does in any one place. (Find out more in our main article on how bicycle helmets work.)
Some people have no choice but to wear protective materials, all the time. Sufferers of a rare genetic condition called xeroderma pigmentosum have skin that lacks natural protection against ultraviolet (UV) light. Without protective clothing, sunlight readily burns their skin and causes skin cancer. To reduce the risk, they typically wear hats and gloves impregnated with a UV-absorbing chemical called benzotriazole, plus sunglasses or masks with UV-resistant visors similar to those worn by astronauts. They also have to apply sun cream to their bodies regularly throughout the day. Sun creams contain chemicals such as paraaminobenzoic acid (PABA), zinc oxide, and titanium dioxide, which absorb or reflect different wavelengths of UV light and prevent them from reaching the skin. Increasing damage to Earth's ozone layer, which reduces the Sun's harmful ultraviolet rays, means protective measures such as this will become more commonplace for everyone in future. Find out more in our main article on sunscreen.
Most at risk from ultraviolet radiation are space-walking astronauts, closer to the Sun and far outside Earth's ozone layer. Astronauts on the Apollo program in the 1960s wore polycarbonate helmets with gold-plated visors, specially designed to protect against ultraviolet and infrared radiation. Many of today's most important protective materials started life on the Apollo program. Indeed, early specifications for Apollo space suits read like a catalog of today's most advanced materials. Those suits contained some 24 different layers designed to protect against extreme temperatures ranging from -250°F to +250°F (-157°C to +120°C), the lack of atmospheric pressure, and attacks from tiny meteorites.
The innermost garment featured a lightweight nylon suit, surrounded by water-cooling vinyl tubes, and a comfortable nylon outer layer. Next came the outer suit, comprising a layer of comfortable Nomex® cloth touching the skin, a knitted jersey laminate, a neoprene rubber joint material, a neoprene oxygen-retaining bladder to maintain artificial air pressure inside the suit, a nylon outer layer for the bladder, five layers of aluminized Mylar (polyester film) interleaved with four layers of DACRON® (polyester fiber) to protect against extreme heat and cold, two layers of Teflon-coated silica bonded to Kapton, two more layers of Kapton for heat insulation, an inner layer of fire-protective beta cloth (Teflon coated with fiberglass), and an outer layer of white Teflon resistant to flames and micro-meteorites.