Engineered hydrogel being pulled away from a glass surface. The material shows a property called “tough wet adhesion” comparable to tendon and bone interface. The wavy edge instability at the interface is a hallmark of strongly adhered soft material on a rigid surface. Credit: Felice Frankel
New ‘water adhesive’ is tougher than natural adhesives employed by mussels and barnacles. The hydrogel, a transparent, rubber-like material, 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.
APPS: Protective coatings on underwater surfaces eg boats and submarines. As the hydrogel is biocompatible, it may also be suitable for a range of health applications, eg biomedical coatings for catheters and sensors implanted in the body. Zhao’s group is currently exploring uses for the hydrogel in soft robotics, where the material may serve as synthetic tendon and cartilage, or in flexible joints.
MOA: A tough, flexible hydrogel that bonds strongly requires 2 characteristics: ~energy dissipation and chemical anchorage. A hydrogel that dissipates energy is essentially able to stretch significantly without retaining all the energy used to stretch it. A chemically anchored hydrogel adheres to a surface by covalently bonding its polymer network to that surface. “Chemical anchorage plus bulk dissipation leads to tough bonding,” Zhao says. “Tendons and cartilage harness these, so we’re really learning this principle from nature.”
METHOD: Yuk mixed a solution of water with a dissipative ingredient to create a stretchy, ubbery material. He then placed the hydrogel atop various surfaces, such as aluminum, ceramic, glass, and titanium, each modified with functional silanes – molecules that created chemical links between each surface and its hydrogel.
They 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 it was as tough as 1,000 j /sq meter ~ same as tendon and cartilage on bone. Zhao group compared results with existing hydrogels, as well as elastomers, tissue adhesives, and nanoparticle gels, and found the new hydrogel adhesive has both higher water content and a much stronger bonding ability.
In addition to testing hydrogel’s toughness with a hammer and a weight, Zhao et al 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.”
They also looked at electrical conductor. They added salts to a hydrogel sample, and attached the hydrogel to 2 metal plates connected via electrodes to an LED light. They found hydrogel enabled flow of salt ions within the electrical loop, lighting up the LED. “We create extremely robust interfaces for hydrogel-metal hybrid conductors,” Yuk adds. http://news.mit.edu/2015/hydrogel-superglue-water-adhesive-1109
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