Jun 29 2016
A cube of Jell-O left on the kitchen counter would ultimately turn into a shrunken, hardened mass once all of the water within it has evaporated. The same occurs with hydrogels that are made mostly of water. These gelatin-like polymer materials are flexible and absorbent until they unavoidably dry out.
Engineers at MIT have devised a method to bind two stretchy materials: gelatin-like polymer materials called hydrogels, and elastomers, which are impervious to water and can thus seal in the hydrogel’s water. (Credit- Melanie Gonick/MIT)
MIT engineers have discovered a way to prevent hydrogels from dehydrating, using a technique that could result in flexible microfluidic devices, more durable contact lenses, stretchy bioelectronics, and artificial skin.
The engineers, headed by Xuanhe Zhao, the Robert N. Noyce Career Development Associate Professor in MIT’s Department of Mechanical Engineering, formulated a technique to strongly attach hydrogels to elastomers, which are elastic polymers like silicone and rubber that are stretchy like hydrogels but impermeable to water. They discovered that coating hydrogels using a thin elastomer layer offered a water-trapping barrier that kept the hydrogel moist, stretchy, and strong. Their results are reported in the Nature Communications journal.
Zhao states that the research group was inspired by human skin for it's design, which contains an outer epidermis layer connected to an underneath dermis layer. The epidermis is similar to a shield, which protects the dermis and its nerves and capillaries networks, and the remaining body’s organs and muscles from drying.
The hydrogel-elastomer hybrid is alike in design and multiple times tougher than the bond between the dermis and epidermis. The research group created a physical model to guide the design of different hydrogel-elastomer bonds quantitatively. Additionally, the scientists are investigating different applications of the hybrid material, like artificial skin. They have reported the invention of a technique to pattern small channels into the hybrid material, just like blood vessels in the same research paper. They have also implanted complex ionic circuits in the material to imitate nerve networks.
We hope this work will pave the way to synthetic skin, or even robots with very soft, flexible skin with biological functions.
Xuanhe Zhao, Department of Mechanical Engineering, MIT
Hyunwoo Yuk, MIT graduate student is the paper’s main author. MIT graduate students Xinyue Liu and German Alberto Parada and previous Zhao group postdoc Teng Zhang, currently an Assistant Professor at Syracuse University are the other authors.
Getting Under the Skin
In December 2015 Zhao and his colleagues reported that they had devised a method to achieve highly strong bonding of hydrogels to solid surfaces, like ceramic, metal, and glass. The research group used the method to insert electronic sensors into hydrogels to produce a “smart” bandage. However, they discovered that the hydrogel would ultimately dry out, losing its flexibility.
Other researchers have attempted to treat hydrogels with salt in order to prevent dehydration, but according to Zhao this method could render a hydrogel incompatible with biological tissues, and make it toxic. The scientists instead were inspired by skin, and reasoned that coating hydrogels using a material that was stretchy, but at the same time water-resistant, would be an ideal strategy to prevent dehydration. They soon ended up with elastomers as the perfect coating, although the rubbery material had one major challenge - the material was naturally resistant against bonding with hydrogels.
Most elastomers are hydrophobic, meaning they do not like water, but hydrogels are a modified version of water. So these materials don’t like each other much and usually can’t form good adhesion.
Hyunwoo Yuk, Graduate Student, MIT
The research group linked the materials together using the method they formulated for solid surfaces, but using elastomers, the hydrogel bonding was “horribly weak” stated Yuk. The scientists discovered a potent compound that might bring hydrogels and elastomers together after scanning the literature for chemical bonding agents. They found that benzophenone, which could be activated by UV light was a potent compound.
After immersing a thin elastomer sheet into a benzophenone solution, the scientists wrapped the treated elastomer around a hydrogel sheet and exposed the hybrid material to UV light. They discovered that after 48 hours in a dry lab atmosphere, the hybrid material’s weight was not altered, signifying that the hydrogel maintained its moisture. The researchers measured the force needed to peel the two materials away, and found that to separate them required 1,000 J/m2, much more than the force required to peel the epidermis of the skin away from the dermis.
This is tougher even than skin. We can also stretch the material to seven times its original length, and the bond still holds.
Xuanhe Zhao, Department of Mechanical Engineering, MIT
Expanding the Hydrogel Toolset
Taking the comparison with skin a step further, the researchers formulated a method to engrave small channels within the hydrogel-elastomer hybrid to create a simple network of blood vessels. The researchers initially cured a common elastomer onto a silicon wafer mold with a simple three-channel pattern, engraving the pattern onto the elastomer making use of soft lithography.
The researchers then immersed the patterned elastomer into benzophenone, placed a hydrogel sheet over the elastomer, and exposed the two layers to UV light. The scientists could flow green, red, and blue food coloring through each channel of the hybrid material in their experiments.
According to Yuk, the hybrid-elastomer material can be used in the future, as a stretchy microfluidic bandage, to distribute drugs directly via the skin.
We showed that we can use this as a stretchable microfluidic circuit. In the human body, things are moving, bending, and deforming. Here, we can perhaps do microfluidics and see how [the device] behaves in a moving part of the body.
Hyunwoo Yuk, Graduate Student, MIT
The scientists investigated the potential of the hybrid material as a complex ionic circuit. A neural network is a similar circuit where nerves in the skin drive ions back and forth to signal sensations like pain and heat. According to Zhao, hydrogels are composed of water mostly, and are natural conductors where ions flow. The introduction of an elastomer layer acts as an insulator, preventing the escape of ions in a vital arrangement for any circuit.
The scientists inserted the hybrid material in a concentrated sodium chloride solution to make it conductive to ions. They then coupled this hybrid material to an LED light. On placing electrodes at the two ends of the material, the researchers were able to produce an ionic current that turned on the light.
“We show very beautiful circuits not made of metal, but of hydrogels, simulating the function of neurons,” says Yuk. “We can stretch them, and they still maintain connectivity and function.”
Syun-Hyun Yun, Associate Professor at Harvard Medical School and Massachusetts General Hospital, elucidates that elastomers and hydrogels have separate physical and chemical properties which when combined, may result in novel applications.
It is a thought-provoking work. Among many [applications], I can imagine smart artificial skins that are implanted and provide a window to interact with the body for monitoring health, sensing pathogens, and delivering drugs.
Syun-Hyun Yun, Associate Professor, Harvard Medical School
The researchers hope to test the potential of the hybrid material further for numerous applications, including wearable electronics and on-demand drug-delivering bandages, along with nondrying, circuit-embedded contact lenses.
“Ultimately, we’re trying to expand the argument of using hydrogels as an advanced engineering toolset,” says Zhao.
The Office of Naval Research, Draper Laboratory, MIT Institute for Soldier Nanotechnologies, and National Science Foundation funded this research.