What Are Aerated Hydrogels? MIT's Breathable Material

“We want to have lots of tiny channels to let air through, while also maintaining lots of water in the gel,” Zhao says. The new design of the hydrogel, right, is compared to a previous hydrogel (clear).
 Photo Credit: Melanie Gonick, MIT
(CC BY-NC-ND 3.0)

Scientific Frontline: Extended "At a Glance" Summary: Aerated Hydrogels

The Core Concept: An aerated hydrogel is a soft, highly hydrated, and bio-friendly polymeric material engineered with interconnected microscopic tunnels that freely permit airflow.

Key Distinction/Mechanism: Unlike conventional hydrogels that trap sweat, or prior permeable designs that sacrifice hydration by utilizing large volumes of silicone, this material relies on viscoelastic phase separation. Mixing a minimal amount of hydrophobic silica aerogel particles into a water-heavy polymer solution causes the water molecules to cluster, naturally forcing the silica into stable, interconnected, and air-permeable pathways.

Major Frameworks/Components:

• Viscoelastic Phase Separation: A physical dynamic akin to the interaction between oil and water, which forces differing liquid phases to rapidly separate and form distinct structural networks.
• Silica Aerogel Particles: Hydrophobic, solid-form air bubbles that resist water infiltration and establish the structural foundation of the air channels.
• Polymer Cross-Linking: The chemical mechanism utilized to solidify the polymer scaffold, locking the breathable tunnel network permanently into place.

Branch of Science: Materials Science, Mechanical Engineering, and Biomedical Engineering.

Future Application: This technology is positioned to yield longer-lasting wearable sensors (such as continuous ECG monitors), highly breathable bandages, wound dressings, cosmetic face masks, contact lenses, and advanced internal health implants.

Why It Matters: Prolonged application of conventional hydrogels typically causes skin irritation and disrupts biometric readings due to moisture accumulation. By remaining durable and breathable through 10,000 compression cycles and up to 10 days of continuous wear, this new hydrogel maintains clear biological signals and healthy tissue, significantly expanding the viability of long-term medical monitoring and wearable health technology.

“Now that we’ve added air to hydrogels, people can find broad applications,” says Xuanhe Zhao. Here, a rectangular hydrogel floats on top of water.
Photo Credit: Felice Frankel
(CC BY-NC-ND 3.0)

Hydrogels are squishy, bio-friendly materials made mostly of water and a small amount of polymer. The Jell-O-like substance is available as medical patches, sprays, and glues, and can be adhered to the skin or implanted in the body to dress wounds, affix implants, and encapsulate and release medicine over time.

Despite their adhesive, elastic, 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 to which a hydrogel is adhered.

Now, MIT engineers have developed 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 microscopic tunnels running through the gel allows air to pass through.

The aerated hydrogel can be worn for longer periods than conventional hydrogels without causing skin irritation. It can also reduce sweat buildup, even during exercise. In experiments, volunteers wore wireless heart monitors attached to their chests with the new breathable hydrogel. After working out regularly for ten days, the volunteers showed no signs of skin irritation, and the heart monitors maintained clear readings.

The results, 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, as well as a professor of civil and environmental engineering and of medical engineering and science. "Now that we’ve added air to hydrogels, people can find broad applications."

The new design of the breathable hydrogel (white) floats on water because of the air tunnels. The old design (clear) drops to the bottom of the cup.
Image Credit: Melanie Gonick, MIT
(CC BY-NC-ND 3.0)

Breathing through Jell-O

Water makes up about 90 percent of a typical hydrogel. The remainder of the material consists of polymers. When mixed with water in a chemical process known as "cross-linking," the polymers settle into a scaffold that holds the water in place, forming a gel that is both squishy and stretchy. However, because a hydrogel’s composition is mainly water, it is inherently challenging for air to permeate 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 attempted to design air-permeable hydrogels, primarily taking one of two approaches. The first essentially involves puncturing microscopic holes throughout the gel. Such designs are breathable, but only in air; when placed in liquid, the holes quickly clog.

Researchers have also attempted to mix hydrogels with certain polymers, such as silicone, that naturally allow air to pass through. This approach, however, requires adding a large amount of polymers to the hydrogel to create enough permeable space for air to move through the entire structure. These hydrogels ultimately have a higher ratio of polymer to water, making them less hydrated overall.

Zhao, a leader in the development and application of hydrogels, sought to create a hydrogel that permits air to pass through without losing its water-heavy composition.

"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 discovered an ideal recipe for a breathable hydrogel that minimizes the non-water ingredients needed to permit airflow. 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 causes them to separate instantly. When the two are mixed, oil and water aggregate with their own kind while avoiding the other.

Zhao and his colleagues took advantage of viscoelastic phase separation to concoct a breathable hydrogel. For their new design, they mixed their conventional hydrogel recipe with a minimal 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."

As it turns out, the particles behave similarly 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 aggregated, 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 narrow, interconnected tunnels. The team observed that after a few hours, this effect formed a network of thin, sturdy, silica-skinned tunnels through which air could flow.

The new design of the breathable hydrogel (white) floats on water because of the air tunnels. The old design (clear) drops to the bottom of the cup. 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 twenty 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 ten days.

"We reliably saw that after ten 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 viscoelastic 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."

Funding: 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.

Published in journal: Nature

Title: Air-permeable hydrogels through viscoelastic phase separation of aerogels

Authors: Xiao-Yun Yan, Shucong Li, Won Jun Song, Runze Li, Aarosh Dahal, Bastien F. G. Aymon, Haodong Hu, Deep K. Malu, Gabriella E. Carreira, Jingjing Wu, Gengxi Lu, Bolei Deng, Jiayi Liu, Siqin Yu, Shu Wang, Eric Lu, Hyunhee Lee, Hui Xu, Anqi Chen, Yuxing Yao, James H. Zhang, Chen Gong, Yiyuan Sun, Jeong-Yun Sun, David A. Weitz, Casey O’Brien, Yuhang Hu, Zachary P. Smith, Aditya Kumar, and Xuanhe Zhao

Source/Credit: Massachusetts Institute of Technology | Jennifer Chu

Edited by: Scientific Frontline

Reference Number: ms070826_02

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