Stephanie Kwolek, the chemist who was credited with developing the Kevlar fiber used in bullet-resistant gear, conducts an experiment at her DuPont laboratory. (Courtesy of DuPont/Via AP)

Stephanie L. Kwolek, a chemist credited with developing the ultra-strong Kevlar fiber used in bullet-resistant gear, an innovation that is estimated to have saved hundreds of lives since she created the polymer five decades ago, died June 18 at a hospice in Wilmington, Del. She was 90.

Rita Vasta, her executor and a former colleague at the DuPont chemical company, confirmed her death but declined to disclose the cause.

Miss Kwolek described her innovation as “a case of serendipity.” In the mid-1960s, halfway through her career as a DuPont chemist, she was tasked with developing a synthetic material that might offer a lighter, more fuel-efficient alternative to metal reinforcements in automobile tires.

In the course of her research, Miss Kwolek stumbled on a surprising compound — a liquid crystalline solution — that could be transformed into astoundingly strong fibers. Of them, the best known became Kevlar.

Five times as strong as steel, it proved immensely useful in a range of applications. Kevlar is used to make boats, airplanes and sporting equipment more sturdy. Woven into fabrics, it makes butcher’s gloves and lumberjack gear more protective. Ropes produced with Kevlar are strong enough to hold hulking ships in place and are easy for sailors to handle.

Miss Kwolek holds a spool of Kevlar, the ultra-strong fiber she helped develop, in this 2007 photo. (Jennifer Corbet/The News Journal via AP)

Most notably, it is used in the bullet- and stab-resistant vests and helmets worn by police officers, military personnel and other professionals working in dangerous environments.

“Not in a thousand years did I think the discovery of this liquid solution would save thousands of lives,” Miss Kwolek told USA Today in 2003, amid the wars in Afghanistan and Iraq. “When I watch the war on TV, I take great pride in saying, ‘We at DuPont invented that.’ ”

Miss Kwolek was said to have encountered resistance during her research, in part because the polymer she created was so unusual.

“Ordinarily, when you have a polymer solution of a flexible polymer chain, it sort of reminds you of molasses,” Miss Kwolek told an interviewer for the Smithsonian Institution. “It may not be as thick but is generally of a transparent or translucent nature. With the polymer solution that I had, it was almost like water — and it was cloudy.”

To make fibers, polymer solutions were processed through the pores of a tool called a spinneret. The machine operator noted the cloudiness of Miss Kwolek’s substance and concluded that it indicated the presence of solid particles, which would have clogged the equipment.

In fact, the liquid crystalline solution represented a state of matter between conventional liquids and solids, with some of the properties of each, and it could be safely processed through the machine. In subsequent testing, the fibers proved exceptionally strong.

“The stiffness was absolutely spectacular,” Miss Kwolek told the New York Times. “That’s when I said, ‘Aha.’ I knew then and there it was an important discovery.”

Body armor has saved the lives or prevented the serious injury of more than 3,100 law enforcement officers, according to records maintained by the Kevlar Survivors’ Club, a joint initiative of DuPont and the International Association of Chiefs of Police.

Stephanie Louise Kwolek was born July 31, 1923, in New Kensington, Pa. She told interviewers that her father, who died when she was 10, had introduced her to the natural sciences by taking her foraging in the woods for insects and plants. Her mother introduced her to sewing, and in her youth Miss Kwolek hoped to become a fashion designer.

In 1946, she received a bachelor’s degree in chemistry from the Margaret Morrison Carnegie College, a women’s school associated with what is now Carnegie Mellon University in Pittsburgh. Lacking funds for medical school, she pursued employment at DuPont and developed an expertise in polymers.

She said she encountered challenges at a time when relatively few women worked in the advanced sciences.

“Although I’ve made many strides in my field, those were not enlightened times for the recognition and advancement of women in scientific research,” Miss Kwolek said in 1999, when she received a lifetime achievement award from the Lemelson-MIT Program.

“While I was doing work that was acknowledged to be on an equal level to that done by men, it took 15 years for me to get my first promotion, and that was far too long to wait,” she said.

Miss Kwolek credited her colleagues with making important contributions to the development of Kevlar and retired from DuPont in 1986. She was inducted into the National Inventors Hall of Fame in 1995 and received the National Medal of Technology in 1996, among other honors. She had no immediate survivors.

Miss Kwolek sometimes met or received phone calls from people who had worn her bullet-resistant fiber.

“Not long ago, I got to meet some troopers whose lives had been saved,” she once told the Philadelphia Inquirer. “They came with their wives, their children, their parents. It was a very moving occasion.”