Aimed at eliminating the need for surgical procedures to remove them, the devices could be used inside the body as stents, staples or for drug delivery.
To create the devices, the researchers took advantage of a phenomenon that leads to fractures in metal. They showed that biomedical devices made from aluminium can be disintegrated by exposing them to a liquid metal known as eutectic gallium-indium (EGaIn). In practice, this might work by painting the liquid onto staples used to hold skin together, or by administering EGaIn microparticles to patients.
“What this enables, potentially, is the ability to have systems that don’t require an intervention such as an endoscopy or surgical procedure for removal of devices,” said mechanical engineer, gastroenterologist and senior author of a study on the work, Giovanni Traverso.
Traverso’s lab has previously worked on ingestible devices that could remain in the digestive tract for days or weeks, releasing drugs on a specific schedule. Most of those devices were made from polymers, but recently the researchers started exploring the possibility of using metals, which are stronger and more durable. One of the challenges of using metal devices is finding a way to remove them once they are no longer needed, however.
The MIT team drew inspiration from a phenomenon known as liquid metal embrittlement, which has been well-studied as a source of failure in metal structures, including those made from zinc and stainless steel.
“It’s known that certain combinations of liquid metals can actually get into the grain boundaries of solid metals and cause them to dramatically weaken and fail,” said lead author Vivian Feig. “We wanted to see if we could harness that known failure mechanism in a productive way to build these biomedical devices.”
One type of liquid metal that can induce embrittlement is gallium. The researchers used EGaIn, an alloy of gallium that scientists have explored for a variety of applications in biomedicine, as well as energy and flexible electronics.
For the devices themselves, the researchers chose to use aluminium, which is known to be susceptible to embrittlement when exposed to gallium. The liquid metal can diffuse through the grain boundaries of the metal – ‘border lines’ between the crystals that make up the metal – causing pieces of the metal to break off. The MIT team showed they could design metals with different types of grain structures, allowing the metals to break into small pieces or to fracture at a given point.
Gallium also prevents aluminium from forming a protective oxide layer on its surface, increasing the metal’s exposure to water and enhancing its degradation.
The team showed that after they painted gallium-indium onto aluminium devices, the metals would disintegrate within minutes. They also created nanoparticles and microparticles of gallium-indium and showed that these particles, suspended in fluid, could also break down aluminium structures.
To demonstrate potential applications, the researchers designed a star-shaped device, with arms attached to a central elastomer by a hollow aluminium tube. Drugs could be stored in the arms, and the shape of the device could keep it in the gastrointestinal tract for an extended period. In a study in animals, the researchers showed that it could be broken down after treatment with gallium-indium.
The team then created aluminium staples and showed that they could be used to hold tissue together, before dissolving them with a coating of gallium-indium.
“Right now, removing the staples can actually induce more tissue damage,” Feig said. “We showed that with our gallium formulation we can just paint it on the staples and get them to disintegrate on demand instead.”
The researchers also showed that an aluminium stent they designed could be implanted in oesophageal tissue, then broken down by gallium-indium. Currently, oesophageal stents are either left in the body permanently or endoscopically removed when no longer needed. Such stents are often made from metals such as nitinol, an alloy of nickel and titanium. The researchers are now working to see if they could create dissolvable devices from nitinol and other metals.
“An exciting thing to explore from a materials science perspective is: Can we take other metals that are more commonly used in the clinic and modify them so that they can become actively triggerable as well?” Feig said.
The team conducted initial toxicity studies in rodents and found that gallium-indium was non-toxic even at high doses. More work would be needed to ensure it would be safe to administer to patients.
The study was published in Advanced Materials.
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