MIT hydrogel with air-filled channels could make wearable sensors more breathable

Wearable health sensors that can track bodily changes continuously have become a major advance in health technology over the past few years.

Many of these devices - as well as medical patches and wound dressings - must sit against the skin for hours or even days at a time. The snag is that the water-rich hydrogels typically used for this purpose can hold in heat and sweat, which may irritate the skin and interfere with sensor accuracy.

The problem with conventional hydrogels for wearable health sensors

A 2024 paper in Science Advances demonstrated eight days of uninterrupted skin monitoring with a gas-permeable hydrogel sensor about 10 micrometres thick, strengthened using a polyurethane nanomesh.

A newly published study in Nature describes a different solution. Engineers led by a team at the Massachusetts Institute of Technology (MIT) report a hydrogel built around a stable, three-dimensional network of tiny, air-filled channels.

MIT’s lung-inspired hydrogel with air-filled channels

Rather than making an ultra-thin film, the MIT group created an enduring set of air pathways throughout a bulk hydrogel while still keeping its water content at 70 percent.

In laboratory measurements, the material - despite being 70 percent water - reached an oxygen permeability of up to 185 barrer (that's not a typo, that's the unit of measurement for gas permeability!).

That level is roughly 10 times higher than what you would expect from a standard hydrogel.

Taking inspiration from the way human lungs move air, the researchers started with a conventional hydrogel formulation and mixed in a small quantity of silica aerogel particles - effectively 'solid-form' air bubbles. Because these particles repel water, they hold onto air and stop the minuscule air pockets inside them from collapsing or becoming waterlogged.

As the material was made, the particles aggregated within the hydrogel into a fine, connected web of air-filled channels. This internal network let oxygen and water vapour pass through the hydrogel while it still maintained a high level of hydration.

The researchers also found the hydrogels allowed water vapour through at rates 10 to 100 times higher than silicone and polyurethane patches, helping moisture leave the skin.

Alongside breathability, the hydrogel remained supple and hard-wearing, keeping about 95 percent of its air permeability after 10,000 stretch cycles.

Comfort and ECG testing on skin

To show the material could be worn comfortably, the team compared patches made from the new hydrogel with common silicone patches.

Infrared images taken two minutes after patch removal indicated that skin temperature under a commercial silicone patch had increased by 6.5 degrees Celsius after a 20-minute workout.

Under the new hydrogel patch, in contrast, skin temperature dropped by about 1 degree, which the researchers suggest could be because the material releases heat more effectively.

Sweat also built up substantially beneath the silicone patch, whereas the skin under the breathable hydrogel looked much like uncovered skin.

In a separate wear test, 10 volunteers placed the new patches on their chests during an hour of moderate exercise, and none reported itching, irritation, or other unwanted skin reactions.

The team then modified the hydrogel to work as an electrode for recording the heart’s electrical activity.

In cycling trials, standard hydrogel electrodes delivered increasingly unstable electrocardiogram (ECG) traces as sweat accumulated. By comparison, electrodes made from the air-permeable hydrogel produced cleaner signals during exercise and afterwards.

For longer-term monitoring, participants wore the hydrogel electrodes continuously for 10 days, and the electrodes continued to capture usable ECG signals during activities such as sleeping, working, walking, and exercising.

What must happen before clinical and implantable use

Xuanhe Zhao, the study’s senior author and a mechanical engineer at MIT, told ScienceAlert that the nearest-term uses may include wearable medical devices, wound dressings, and skin-mounted health monitors.

"These technologies often require prolonged contact with the skin, but conventional hydrogels trap heat and moisture because they do not allow sufficient oxygen and water vapor to pass through," Zhao said.

"Our material overcomes this limitation while maintaining the high water content and softness that make hydrogels comfortable and biocompatible."

Even so, the hydrogel is not yet ready for clinical deployment. Zhao says additional work is needed to evaluate long-term biocompatibility, performance in large animals, sterilisation, scaled manufacturing, shelf life, and regulatory safety.

The human studies were limited and primarily intended to show feasibility.

The exercise and comfort work involved up to 10 volunteers, while measurements of skin physiology included only two participants, and the exercise ECG comparison involved three. To verify the results, larger studies with more diverse participants will therefore be required.

The hydrogel will also have to be tailored for specific applications, because a wound dressing, a wearable sensor, and an implant each come with different demands.

At present, the material is not naturally adhesive, so it still needs a separate backing or attachment method to stay in place on the skin.

Further ahead, the same design concept may be useful in tissue engineering and implantable devices, where delivering sufficient oxygen to cells remains a significant obstacle.

"Many engineered tissues and implants require efficient oxygen transport to maintain cell viability, and our material offers a way to improve gas exchange while preserving the hydrated environment that cells need," Zhao said.

However, he cautions that implantable uses would require far more extensive testing, making skin-based applications the more realistic starting point.

The results are reported in Nature.

This article was fact-checked by Rachel Garner and edited by Fiona MacDonald. We take care with our process, but we are only human - if you notice an error, please tell us.

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