Photograph courtesy NASA
Published September 13, 2010
If you're headed for space, you might rethink that manicure: Astronauts with wider hands are more likely to have their fingernails fall off after working or training in space suit gloves, according to a new study.
In fact, fingernail trauma and other hand injuries—no matter your hand size—are collectively the number one nuisance for spacewalkers, said study co-author Dava Newman, a professor of aeronautics and astronautics at the Massachusetts Institute of Technology.
"The glove in general is just absolutely one of the main engineering challenges," Newman said. "After all, you have almost as many degrees of freedom in your hand as in the rest of your whole body." (See a space exploration time line.)
The trouble is that the gloves, like the entire space suit, need to simulate the pressure of Earth's atmosphere in the chilly, airless environment of space. The rigid, balloonlike nature of gas-pressurized gloves makes fine motor control a challenge during extravehicular activities (EVAs), aka spacewalks. (See pictures of early U.S. space exploration.)
A previous study of astronaut injuries sustained during spacewalks had found that about 47 percent of 352 reported symptoms between 2002 and 2004 were hand related. More than half of these hand injuries were due to fingertips and nails making contact with the hard "thimbles" inside the glove fingertips.
In several cases, sustained pressure on the fingertips during EVAs caused intense pain and led to the astronauts' nails detaching from their nailbeds, a condition called fingernail delamination.
While this condition doesn't prevent astronauts from getting their work done, it can become a nuisance if the loose nails gets snagged inside the glove. Also, moisture inside the glove can lead to secondary bacterial or yeast infections in the exposed nailbeds, the study authors say.
If the nail falls off completely, it will eventually grow back, although it might be deformed.
For now, the only solutions are to apply protective dressings, keep nails trimmed short—or do some extreme preventative maintenance.
"I have heard of a couple people who've removed their fingernails in advance of an EVA," Newman said.
Astronaut Gloves Make Gripping a Pain
In the current glove design, astronauts wear a pressurized inner layer under a thick outer layer that offers protection from the cold and any passing micrometeorites. On Earth, wearing such space suit gloves might feel like donning a thick set of gardening gloves—a bit restrictive but not too uncomfortable.
"When the glove pressurizes, that nice, flexible fabric surface becomes stiff, like putting air into bicycle tires," said Peter Homer, founder of commercial space suit design company Flagsuit LLC in Maine and two-time winner of NASA's Astronaut Glove Challenge.
"What you find is, depending on the design of the glove, there's pressure on the hard points the hand presses against, and that can give you blisters or cuts," said Homer, who was not involved in the new study.
"Also, the materials tend to be rubberized to make the gloves airtight, but that creates a lot of friction against the skin, and that can again create blisters."
During EVAs, astronauts have to work in these gloves for six to eight hours at a stretch, Homer said: "It amazes me that astronauts push through all that pain and get stuff done." (Related: Find out what it takes to be an astronaut.)
To help design more comfy space suit gloves, MIT's Newman and colleagues initially tested whether fingernail trauma is related to the length of astronauts' fingers.
The team first collected data from the Injury Tracking System, a database of astronaut medical logs at NASA's Johnson Space Center in Houston, Texas. Of the 232 crewmembers with complete injury records and body measurements, 22 reported at least one case of fingernail delamination.
Surprisingly, an analysis of hand measurements among injured astronauts and a noninjured control group showed no statistical relationship between finger length and the instances of nails falling off, according to the study.
Instead, the team found that fingernail trauma was a bigger problem for people with a wider hand circumference, or the size of the hand around the metacarpophalangeal, or metacarpal, joint, where the fingers meet the palm.
"If you take a pencil and grip it, you're using your metacarpal joint," Newman said. "That's a really difficult thing to repeat when you have a pressurized glove on. A hard palm bar in the soft fabric glove ... helps make that crease," but the bar also puts pressure on the joint.
The team's analysis, to be published in the October issue of the journal Aviation, Space, and Environmental Medicine, showed that astronauts with hand circumferences greater than about 9 inches (22.8 centimeters)—what Newman called the "large to extra-large range"—had a 19.6 percent chance of fingernail injuries during an EVA.
By contrast, astronauts with smaller hand circumferences had just a 5.6 percent chance of losing their nails on the job.
Future Space Suit Ideas: Robotic Gloves and Shrink-wrap?
"What surprised me is that conventional wisdom is that fingernail problems are caused by repetitive tapping on the fingernail ... and you'd think that if you had longer fingers, you'd be banging on the end of the glove more," said Flagsuit's Homer.
But the hand-width hypothesis "is good, too," he said. "The bigger the hand is, the more the glove squeezes on [the metacarpophalangeal] joint and cuts off blood flow."
Circulation getting repeatedly shut off then restored at the knuckle joint would damage the tissue underneath the fingernail, leading to delamination. It could also explain why so many astronauts have reported that their fingertips get cold during EVAs despite their thermal gloves, Homer said.
Overall, he added, the new paper "shines light on a whole new direction on how to address this issue."
According to Homer, the key is to make all parts of a glove custom fitted for each astronaut.
For anyone selected for an EVA, the airtight inner layer for the current glove design is custom made via hand casts, laser scanning, computer modeling, and special machining techniques. But the outer layer is built in discrete sizes—more like a "small, medium, large" situation, he said.
"It costs around a hundred thousand dollars up front to custom fit the airtight bladder," Homer said. "In my opinion that also needs to apply to the outer layer, which really gives the glove its shape."
Customization may not always solve the issue, though, MIT's Newman said: "Some may like a tighter or looser fit—there's variability in subjective desires. And if you have a really tight fit, you're going to have a lot more pressure" on the metacarpal joint.
Newman thinks another option worth looking into is robotic amplification inside the glove. (Related: "Robot Arm to Grab Robotic Ship—A Space Station First.")
"Say I'm grabbing on to something. I'm using muscles to act against gas-pressurized gloves," she said. "But what if I had little actuators in there? My fingers can do less work—that'd be great!
"There are design trade-offs" to robotic gloves, she added. "But we have some big dreams here: small-mass systems close to the skin that work in concert with muscles and bones, not big clunky exoskeletons."
Newman and colleagues have also been experimenting with entire skintight space suits that rely on mechanical counterpressure: Rather than working in a gas-pressurized bubble, astronauts would effectively get shrink-wrapped in a suit made of flexible material.
No matter what the approach, Newman said, "the bottom line is we want people to be working in a space suit glove that's working with them, not against them."
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