|Shiny Verghese in her lab at UCSD
A smart material, first created in a laboratory in Pune years ago, is set to change the way we deal with cracks and leaks. A team of researchers from the University of California San Diego (UCSD), the US, has tweaked this self-repairing hydrogel in such a way that it binds in seconds. The scientists hope to harness this property for an array of applications in medicine and industry as well as everyday life.
The work, reported online in the early edition of the Proceedings of the National Academy of Sciences last week, is also important for the medical field because the hydrogel is capable of giving rise to a new class of medical sutures — tissue adhesives that can mend stomach perforations — and a novel drug delivery system.
The hydrogel was first developed at the National Chemical Laboratory (NCL) in Pune by Shyni Varghese — the UCSD bioengineer who led the research — when she was a doctoral student under former Council of Scientific and Industrial Research (CSIR) director general R.A. Mashelkar and polymer scientist Ashish Lele in the 2000s. She continued to work with the hydrogel, named A6ACA, after she moved to UCSD in 2006. The hydrogel, made of linked chains of polymer molecules, looks like jelly.
Self healing is one of the most fundamental properties of living tissues that allows them to sustain repeated damage, says Varghese. Being bioengineers, one question that repeatedly appeared before us was whether one could mimic self healing in synthetic, tissue-like materials such as hydrogels. The benefits of creating an aqueous self-healing material would be far-reaching in medicine and engineering, she says.
This is a significant piece of work. The self-healing gels exhibit excellent mechanical properties that find applications in fields as diverse as self-healing coatings to drug delivery to robotic devices, says Arun Kumar Nandi, a polymer scientist with the Indian Association for the Cultivation of Science, Calcutta.
Mashelkar, currently a CSIR Bhatnagar Fellow at NCL, says this is a remarkable feat. Self-healing hydrogels have been created in the past. But they take hours to seal a crack. The new material does it in a jiffy — just like zipping it up, says Mashelkar, who has done pioneering work in this field. Mashelkar and his colleague Lele are co-authors of the paper as they contributed to analysing the data.
Achieving self healing in chemically crosslinked hydrogels has been a daunting task, says Varghese. Their computer simulations showed that the length of the side-chain molecules is a critically important factor for optimal self-healing. One can imagine these dangling chains like fingers on the hand. The dangling side chains have to have an optimal length to mediate hydrogen bonds across two hydrogel pieces like interlocking fingers in a clasped hand, Varghese told KnowHow.
If the side chains are too short, they will become inaccessible for forming a bond across two hydrogel surfaces facing each other. It the chains are too long, they collapse back into the hydrogel through a phenomenon called hydrophobicity, making them unavailable for mediating bonds across the interface, she explains.
The scientists achieved another important feat. They have shown that it is possible to control the hydrogels ability to bind or separate by adjusting the pH levels of the solution in which it is kept. The pH value of a liquid determines whether it is acidic or alkaline in nature. The pH value of water is seven, which is considered neutral. Anything below seven is considered acidic and above alkaline.
This property makes the smart hydrogel suitable for biomedical applications. The human stomach, for instance, is acidic in nature. So an adhesive made of the hydrogel can be ideal for healing stomach perforations or as a device that delivers drugs in a controlled manner in stomach ulcers. Similarly, a paint prepared with the hydrogel can prevent surfaces painted with it from developing cracks or peeling off. It can also plug leaks if applied on the surface of plastic containers that hold higly corrosive solutions.
The UCSD scientists intend to engineer other varieties of smart hydrogels that self heal in different pH environments so that their applications can be extended.