At the start of the coronavirus pandemic, Patrick Guerra’s wife, a doctor, would strap on an N95 respirator mask, then cover it with a disposable surgical mask to prolong its use. The Centers for Disease Control and Prevention (CDC) recommends healthcare workers put on a new N95 respirator after each patient, but a national shortage of N95s and other equipment made that impossible.
“My wife’s a radiology medical resident and was doing these procedures that are aerosol generating, where she’s within a few inches of the person’s face,” says Guerra, a biological sciences professor at the University of Cincinnati. “I thought, I’ve got to figure something out.”
A Natural Fit
Guerra, who studies the complex architecture of silk moth cocoons, wondered if a silk mask might serve as a better protective barrier over the N95 because he’d observed that the cocoons are naturally water repellent. “The caterpillars basically build these hydrophobic layers,” he says, “so they're all cozy, like in their own little sleeping bag, and then if it rains, it takes a lot of saturation to waterlog it.”
He and his graduate students tested different types of fabrics such as cotton, polypropylene (the material in many disposable surgical masks) and silk to see which would be most likely to repel water, particularly aerosolized water droplets from someone coughing or sneezing — an important transmission avenue for SARS-CoV-2, the virus that causes COVID-19.
Their research, published in Plos One, showed that silk was by far the best material, not just because it is extremely water repellent, but also due to the fact that silk is highly breathable. And it retains both of these properties after repeated washings. “Another thing is that silk is supposedly anti-microbial,” says Guerra, noting that this is possibly due to the silk worms’s diet of mulberry leaves, which contain copper. “That’s something we’re going to look at next.”
More Than Masks and Stitches
Silk has long been recognized as a medically useful material. David Kaplan, an engineering professor at Tufts University who is an expert on silk biomaterials, says silk is strong, yet pliable. Also, it does not cause an antibody reaction when it is placed inside the body and biodegrades over time. For centuries, silk has been used for sutures. More recently, Kaplan says, scientists have shown silk is an outstanding material for tissue reconstruction.
Typically surgeons take fat from other parts of the body or use synthetic materials for reconstruction when patients lose a large amount of soft tissue due to something like an automobile accident or because they have a cancerous tumor removed. Yet, Kaplan notes, neither of those solutions work well — the body often reabsorbs the fat and patients find that the synthetic material rarely feels natural.
“But with silk, we can make a sponge that fits the size and shape [of the affected area], implant that, and it will take maybe two years to fully degrade because of the structure,” he says. “So, during those two years, the soft tissue regrows itself, with vascularization, innervation. Everything's normal when you're done and the silk is gone.”
Kaplan’s lab has helped launch a number of biomedical devices that use silk such as Silk Voice, an FDA-approved injectable silk protein that helps bulk up tissue in the vocal cord folds; among other uses, this can aid patients in retaining their voices after they have had polyps or a cancerous growth removed. His lab also helped develop silk microneedles, which aid in controlled-release drug delivery. “You can control that delivery for up to six months and the silk tips will just slowly degrade away,” Kaplan says.
As with Guerra, some of Kaplan’s motivation for experimentation with silk biomaterials came from concern for a family member. He’s been working on the development of silk ear tubes for children who have chronic ear infections. A common complication of the plastic ear tubes that are currently in use is that they may pop out of the ear prematurely, requiring additional surgical procedures to replace them. Kaplan’s silk ear tubes would never have to be removed because they would degrade after a period of time.
“As someone with a daughter who's had six ear tube operations,” Kaplan says, “that was the motivation.”