Skin-Inspired Hydrogel Hybrid Won’t Dry Out

Hydrogels can be embedded with electrical sensors for applications such as smart bandages. (Image courtesy of MIT.)

Hydrogels are strong and stretchy water-based polymer materials used in a variety of applications, including contact lenses and smart bandages. But one of the persistent problems of these gel-like materials has been that they eventually dehydrate, as you might expect of something that’s 90 percent water.

Now, taking inspiration from human skin, engineers from MIT have developed a new hydrogel hybrid that won’t dry out.

Mimicking Skin

Human skin is composed of a thin outer epidermis layer bonded to the dermis layer underneath. The epidermis protects the dermis and its blood vessels, and everything else in the body like muscles and organs, from drying out. The MIT engineers looked to mimic the bond between the epidermis and dermis as a way to achieve the same protection for hydrogels.

But what material should take on the role of the epidermis? The engineers decided on elastomers, which are rubbery and elastic, yet have one significant challenge.

“Most elastomers are hydrophobic, meaning they do not like water,” said engineer Hyunwoo Yuk. “But hydrogels are a modified version of water. So these materials don’t like each other much and usually can’t form good adhesion.”

Thus the crux of the team’s research involved figuring out a practical and suitable method for bonding the elastomers to the hydrogels. Ultimately, they discovered that an organic compound called benzophenome is up to the task.

Creating a Hybrid Hydrogel

The technique is quite straightforward. You start with a thin sheet of elastomer (the team used a variety of common elastomers including latex and polyurethane). Then, you submerge it in a benzophenome solution. After treating the elastomer, you wrap it around a sheet of hydrogel and expose the hybrid to ultraviolet (UV) light. After 48 hours, you get a robust bond.

In fact, according to engineer Xuanhe Zhao, the elastomer-hydrogel bond proved to be even stronger than that between the epidermis and dermis. The bonded materials required over 1000 joules per square meter to separate, although the equivalent data for the epidermis-dermis bond is difficult (and probably immoral) to ascertain due to the complexity of human skin (see this paper, this paper, or this paper for possible insights). Regardless, the hydrogel hybrid is resilient enough to stretch to seven times its original length and retain its bond.

The engineers have also developed a method to etch tiny channels into the hybrid as a way to simulate blood vessels. This opens the door to applications such as microfluidic bandages that can deliver drugs directly into the skin.

The team has gone even further and explored the potential of the hydrogel hybrid as an ionic circuit, by submerging it in a concentrated solution of sodium chloride and using it to switch on an LED light.

“We show very beautiful circuits not made of metal, but of hydrogels, simulating the function of neurons,” said Yuk. “We can stretch them, and they still maintain connectivity and function.”

From wearable electronics to circuit-embedded contact lenses, and even to artificial skin, the new hybrid opens the door to exciting new hydrogel applications. You can read the team’s paper in Nature to learn more.

For more about hydrogels, read Newly Developed Hydrogel Improves Soft Tissue Healing or Conductive Adhesive Is Over 90 Percent Water.