The Science Behind Period Underwear

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There are two types of period underwear out there right now. One type is more of a physical aid: panties that help hold pads in place. The other relies on special materials to help absorb and wick away leakage, or even replace tampons or pads for your lighter days.

Image: Dear Kate

The underwear that help hold in pads are easiest to understand, so let’s start there. The most recent of these designs comes from a company called PantyProp, and was invented by Crystal Etienne. Etienne told me that she came up with the idea one night at home. "I was laying watching TV and I looked down and I saw my maxi pad sticking out," she says. "And that day it just really bothered me. There is no way that in 2015 we should still be dealing with even seeing it at all." So Etienne said her first step was to simply try to buy a pair of underwear that would help her keep her pad in place. "I went online just to get one," she says, and to her surprise, she couldn’t find anything.

Etienne said she couldn’t believe this wasn’t already on the market, and decided that she’d be the one to make it a reality. She had owned lingerie boutiques before, so she was familiar with the world of intimates, but she had never designed her own product before. "I basically called 300 factories, a million places to finally get to what I needed," she says. "I just started going to them, showing them what I had. I just had a picture on a piece of paper," she laughs, "and I’m not a drawer."

But the idea for PantyProp is incredibly simple. It’s a pair of underwear that has a slot for a pad to slip into. And yet even with this simple idea, Etienne said she had some trouble convincing the (mostly male) designers and manufacturers she met with. "I already expected that. Because just imagine, my patent is on a hole, that’s what it really is." But eventually Etienne had a few prototypes, and enlisted her friends and 18-year-old daughter to test them out and give her feedback.

PantyProp’s first two designs — a bikini and a hipster — went on sale on November 15th. In the first three weeks, Etienne made $15,000. When I got in touch with her recently to check in on how things were going she was in Atlanta, meeting with investors and finalizing their next product: the bodysuit, which went on presale in December, and will ship on January 17th. Etienne says the bodysuits are already half sold out. "Women really have proved they needed a undergarment solution for sanitary pad fails," she says.

When I asked Etienne why she focused on pads and not tampons or other forms of period issues, she said she simply wanted to develop something that she could use. "There’s a million period companies fighting for that spot for the free bleed, but there was no company big or small that addressed the issue of your pad shifting," she says. Etienne felt that so many of the products she found skipped over people like her who wear pads. And in fact, according to a 2015 study from the CDC, 62% of American women surveyed use pads, compared with 42% who use tampons. "The industry has become so sophisticated and competitive that they just skipped this whole group of women."

This brings us to the second type of period technology that has boomed in the last few years: underwear to help wick away or absorb blood. There’s Dear Kate, which aims to help people deal with leakage, incontinence, and light days. And there’s THINX, which aims to help people replace tampons altogether. Both of these companies rely on slightly more sophisticated materials and technologies to create their wicking, leak-proof undies.

Julie Sygiel started working on period panties back in 2008, as a chemical engineering student at Brown University. She then turned that project into a business, first calling it Sexy Period and then shifting to a more general name, Dear Kate. THINX was founded by Miki Agrawal, a self-described "serial entrepreneur," and was inspired by an incident at a family picnic in 2005. "I was at my family BBQ and my twin sister and I were defending our 3 legged race championship, and in the middle of the race my sister got her period and we had to race to the finish line and head to the bathroom still tied together so she could change out of her bathing suit bottoms. As we were washing out the blood the idea came to me."

Both Dear Kate and THINX work using similar principles: a layered combination of special fabrics designed to pull liquid away from the body, and trap it inside the underwear so it doesn’t leak out. Dear Kate undies are made of three layers: two that wick away moisture and one that keeps that moisture from leaking through onto your clothes. The wicking layers are made of microfiber polyester, which helps to pull moisture away from the body and into the material. Microfibers are, as their name implies, full of very tiny fibers. What that does is help trap liquid, which then has to navigate through a giant maze of super small filaments as it moves through the material. So instead of shooting right through giant channels present in other kinds of fibers, liquid moves through microfibers slowly, keeping it from leaking out. The outer layer on Dear Kate is made of a combination of nylon and lycra, and topped off with a liquid repellent finish. This all combines to an underwear that can hold up to three teaspoons of liquid at a time, without leaking out onto your pants.

Image: THINX

The THINX design is similar in that it employs a system of layers. Agrawal wouldn’t go into details with me on the materials they use, but the patents for THINX products describe a multilayer system including a "moisture-impermeable polymer layer," a "moisture-absorbent layer," and a "moisture-wicking layer." They have a variety of designs, the most absorbent of which can hold up to five teaspoons of liquid.

Both Sygiel and Agrawal spent years testing out various combinations of fibers and layers to get their designs. For Sygiel, the big challenge was making something that didn’t have a plastic lining. She looked at what was out there already, and noticed that every design featured a plastic polyurethane layer. "When we were developing this, I got in touch with a lot of people who worked at fabric mills" she says. "And when I told all of them we wanted to do a leak resistant product they told us that if you want it to be stretchy, you have to put a plastic polyurethane laminate in between the fabric layers." But when they got samples of the design featuring the plastic laminate, Sygiel says she wasn’t happy. "It felt like a diaper," she says. "Nobody wants to feel like they have a yellow rain slicker in your underwear." So she spent years experimenting with fabrics to avoid that plastic coating.

The patents for THINX do include a "moisture-impermeable polymer layer," and describe a "breathable urethane or wax" but when we spoke Agrawal wouldn’t go into details. In a follow up email, she told me that the layer was "what differentiates us and makes the garment leak-proof" and said that they had done a lot of testing on the layer to make sure it was comfortable and thin. The panties do feature a form of polyurethane lining, but Agrawal said it was "breathable and safe for down under. We can't quite say what it is."

The idea of using specially-designed materials to help people during their periods isn’t a new one. When I showed the patents for Dear Kate and THINX to a textiles expert, he simply wrote back, "at first glance, it appears to me that this is simply a re-application of the technology that has been used since the 90’s or earlier for disposable diapers." Sygiel says that it’s less about developing a brand new type of material, and more about combining existing materials in a new way. "Our 'invention' is more that we figured out the right configuration of fabric types with performance finishes that works best in underwear and allows the underwear to be leak resistant without the use of a plastic lining." Agrawal said something similar. Yes, these materials existed before, but she told me "the technology we use has never been used for the purpose of underwear."

The specific materials that Dear Kate and THINX use are new, but idea of specially designed period underwear goes back nearly 40 years. In 1967, a patent for a "protective petticoat" was issued to a woman named Gladys Ruppel Williams. The undergarment was a half-slip, "constructed with a moisture-proof material" to protect the outer clothing from being stained. In 1988, a Chinese company was issued a patent for "woman menstruation underpants" that included two layers of cloth sewn into the crotch of the panties, each lined with a "non-toxic, flexible plastic film." In 1995 another Chinese company patented a "clean-keeping women undergarment," which included a leakproof liner.

This is all to say that Dear Kate and THINX aren’t doing anything that hasn’t been tried before. But their companies have taken off far better than those that have come before. And part of this is that they’ve done a good job of combining different fabrics to create a good product. But part of it is also that they’ve come at a time when advertising for period underwear isn’t quite as taboo as it once was. "When we first launched in 2011 under the brand name Sexy Period, a features writer at a newspaper interviewed me, wrote a story, and then it was never published because the top editors felt it would make readers too uncomfortable," Sygiel told me.

Of course, there are still hurdles. When THINX put up advertisements in the NYC subways for their underwear, featuring images like raw eggs and fruit, they were told theads had to be changed before they could run on the subway. The head of the MTA said that the ads offended him. THINX pushed back, pointing out that there were plenty of advertisements for breast implants on the subway, and that their ads followed the MTA advertising guidelines. Eventually they came to a truce.

Image: THINX

Despite being competitors, Sygiel says she’s glad that other companies are around and pushing for more awareness surrounding periods. "Each of our successes helps the other one in terms of helping people understand that period underwear exists," she told me. She also points to the rise in conversations around feminism more generally as a reason companies like hers can succeed where past attempts may have failed. "These changes in our culture have allowed the current brands on the market to gain traction and finally break through the barrier of silence that has surrounded menstruation in the past." So while the textiles and technologies involved in these panties might not be groundbreaking, the fact that they exist and are advertising themselves so forcefully is.