Hydrogels are squishy, bio-friendly materials that are made mostly of water and a bit of polymer. The Jell-O-like substance is available in the form of medical patches, sprays, and glues, and can be stuck to the skin or implanted in the body to dress wounds, affix implants, and encapsulate and release medicine over time.
For all their sticky, stretchy, and protective properties, hydrogels lack one key trait: breathability. If worn for too long, a bandage or patch can trap moisture and sweat, which can irritate tissues and reduce the effectiveness of any device that a hydrogel adheres.
Now MIT engineers have come up with a recipe for a hydrogel that is both hydrated and aerated, or permeable to air. The new material is just as soft, stretchy, and robust as conventional hydrogels, but a network of tiny tunnels running through the gel allows air to pass through.
The aerated hydrogel can be worn for longer periods of time compared to conventional hydrogels, without causing skin irritation. It can also reduce sweat buildup, even during exercise. In experiments, volunteers wore wireless heart monitors that were attached to their chest with the new breathable hydrogel. After working out regularly for 10 days, the volunteers showed no signs of skin irritation, and the heart monitors maintained clear readings.
The results, which are reported today in the journal Nature, may enable longer-lasting hydrogel products, such as breathable bandages and dressings, cosmetic face masks, and contact lenses, along with better-performing health monitors and implants.
"Water and oxygen are both essential for life," says Xuanhe Zhao, the Uncas (1923) and Helen Whitaker Professor of Mechanical Engineering, and a professor of civil and environmental engineering, and medical engineering and science. "Now that we've added air to hydrogels, people can find broad applications."
Zhao's MIT co-authors on the study include Xiao-Yun Yan, Shucong Li, Won Jun Song, Runze Li, Bastien Aymon, Jingjing Wu, Gengxi Lu, Jiayi Liu, Shu Wang, Eric Lu, Hyunhee Lee, James Zhang, Casey O'Brien, and Zachary Smith, along with collaborators from multiple other institutions.
Breathing through Jello
Water makes up about 90 percent of a typical hydrogel. The rest of the material consists of polymers. When mixed with water in a chemical process known as "cross-linking," the polymers settle into a sort of scaffold that holds the water in place, forming a gel that's both squishy and stretchy. But because hydrogel's composition is mainly water, it's inherently challenging for any air to make its way through the material effectively.
"In general, water is not breathable," co-lead author Xiao-Yun Yan says. "Hydrogel is 80 to 90 percent water, similar to Jell-O. And you cannot breathe through Jell-O."
Other groups have tried to design air-permeable hydrogels, mainly taking one of two approaches. The first has been to essentially puncture microscopic holes throughout the gel. Such designs are breathable, but only in air. When they are placed in liquid, the holes quickly clog up.
Researchers have also tried mixing hydrogel with certain polymers, such as silicone, that naturally allow air through. But this approach requires adding a large amount of polymers to the hydrogel in order to create enough permeable space for air to move through the entire gel. These hydrogels end up having a greater balance of polymer to water, making them less hydrated in general.
Zhao, who has been a leader in the development and application of hydrogels, looked to make a hydrogel that lets air through without losing its water-heavy makeup.
"We want to have lots of tiny channels to let air through, while also maintaining lots of water in the gel," Zhao says. "This was a significant challenge, and something that people thought was impossible to do."
Highways for air
After several years of investigation, the team hit on an ideal recipe for a breathable hydrogel that minimizes the non-water ingredients needed to let air through. In their new study, they report that the key to the recipe is "phase separation." A common example of this process is the interaction between oil and water. The difference in the two liquids' phases cause them to instantly separate. When the two are mixed, oil and water glom to their own kind, while avoiding the other.
Zhao and his colleagues took advantage of viscoelastic phase separation in concocting a breathable hydrogel. For their new design, they mixed their conventional hydrogel recipe with a very small amount of silica aerogel particles, which are essentially "solid-form" air bubbles.
"They are like boba beads," Yan offers. "The particles are made of silica, which is hydrophobic, meaning that water does not want to leak through them, so they are very stable in water."
And as it turns out, the particles are similar to oil when mixed with water. The researchers found that when they mixed just a small amount of the particles with a solution of the water-heavy hydrogel, the water molecules glommed together, essentially finding each other faster than the less abundant silica particles. This effect of viscoelastic phase separation created large pockets of water and squeezed the silica particles into skinny, interconnected tunnels. The team observed that after a few hours, this effect formed a network of thin and sturdy, silica-skinned tunnels through which air could flow.
Credit: Melanie Gonick, MIT
"It's as if the particles formed a network of connected tunnels, like air-permeable highways within the hydrated hydrogel," says co-lead author Shucong Li.
Once they confirmed that the network had formed, the team cross-linked the mixture, essentially freezing the gel, and its breathable network, in place. They then tested the gel's breathability and mechanical performance over multiple experiments, including one in which they asked several volunteers to wear the gel, attached to a wireless electrocardiogram (ECG) monitor, while exercising for 20 minutes. The volunteers also wore monitors with conventional, commercial hydrogel adhesives.
Throughout the workouts, the researchers observed that the breathable hydrogel maintained a strong ECG signal, in contrast to the conventional gel which exhibited significant signal fluctuations.The researchers observed similar results in an experiment with several volunteers who wore the breathable hydrogel and ECG monitor over 10 days.
"We reliably saw that after 10 days, the quality of the ECG signal is still pretty good, and after you take off the monitor, there were no noticeable blisters or redness on the skin," Li says. "This indicates healthy skin conditions."
The team also exercised the gel itself, putting it through 10,000 cycles of stretching and compression. After these tests, they found the gel still retained the network of air channels, maintaining its breathability.
"After 10,000 cycles, there was less than a 5 percent drop in oxygen permeability," Li says. "That matters, because even with your heartbeat, your chest continuously undergoes small strains. So we have to make sure this gel is durable for such daily activity."
Zhao says the new study provides a novel approach for others to fabricate breathable and multifunctional hydrogels, using the concept of visoelastic phase separation as a guide.
"We've discovered that this process can create these air-permeable hydrogels, and we demonstrate one application," he says. "But we think there can be very broad applications. This is a technology platform."
This work was carried out in part through the use of MIT.nano's facilities. This work was supported in part by the MIT Hatsopoulos Faculty Fellowship, the Uncas and Helen Whitaker Professorship, a HEALS seed grant, the National Institutes of Health, the National Science Foundation, and the Department of Defense Congressionally Directed Medical Research Programs.
