Engineers at the Massachusetts Institute of Technology (MIT) have developed a method to make synthetic hydrogel comprising over 90% water that can adhere to surfaces such as glass, silicon, ceramics, aluminum and titanium with a toughness comparable to the bond between tendon and cartilage on bone.

In experiments to demonstrate its robustness, the researchers applied a small square of their hydrogel between two plates of glass, from which they then suspended a 55-lb. weight. They also glued the hydrogel to a silicon wafer, which they then hit with a hammer. Although the silicon shattered, its pieces remained stuck in place.

Engineered hydrogel being pulled away from a glass surface. Image credit: Felice Frankel.Engineered hydrogel being pulled away from a glass surface. Image credit: Felice Frankel.Such durability makes the hydrogel a candidate for protective coatings on underwater surfaces such as boats and submarines. As the hydrogel is biocompatible, it may also be suitable for health-related applications, such as biomedical coatings for catheters and sensors implanted in the body.

“Since the hydrogel contains over 90% water, the bonding may be regarded as a water adhesive, which is tougher than natural glues, such as in barnacles and mussels and bio-inspired underwater glues,” says Xuanhe Zhao, associate professor in MIT’s Department of Mechanical Engineering.

In developing the hydrogel, Hyunwoo Yuk, a graduate student in mechanical engineering, mixed a solution of water with a dissipative ingredient to create a stretchy, rubbery material. He then placed the hydrogel atop various surfaces, such as aluminum, ceramic, glass and titanium, each modified with functional silanes, which are molecules that create chemical links between each surface and its hydrogel.

The researchers then tested the hydrogel’s bond using a standard peeling test, in which they measured the force required to peel the hydrogel from a surface. On average, they found the hydrogel’s bond was as tough as 1,000 joules per square meter, about the same level as tendon and cartilage on bone.

Zhao's group compared these results with existing hydrogels, as well as elastomers, tissue adhesives and nanoparticle gels, and found that their hydrogel adhesive has both higher water content and a stronger bonding ability.

In addition to testing the hydrogel’s toughness with a hammer and a weight, Zhao and his colleagues explored its use in robotic joints, using small spheres of hydrogel to connect short pipes to simulate robotic limbs.

“Hydrogels can act as actuators,” Zhao says. “Instead of using conventional hinges, you can use this soft material with strong bonding to rigid materials, and it can give a robot many more degrees of freedom.”

The researchers also looked into its application as an electrical conductor. Yuk and other students added salts to a hydrogel sample and attached the hydrogel to two metal plates connected via electrodes to an LED light. They found that the hydrogel enabled the flow of salt ions within the electrical loop, ultimately lighting up the LED.

According to Zhao, the work has implications in understanding bio-adhesion, as well as practical applications such as in hydrogel coatings, biomedical devices, tissue engineering, water treatment and underwater glues.

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