Imagine a day when a simple injection prompts a broken bone to heal. When tiny, ingestible devices linger in the body, unnoticed, tracking our health or delivering life-saving medications. When brain and heart implants mesh with flesh so seamlessly that the body thinks they’ve been there all along.
These are the dreams of materials scientists who have toiled for decades to mimic the complex architecture of the human body in hopes of replacing broken parts or treating disease.
The problem, say bioengineers, is that most replacement and corrective parts – from prosthetics to pacemakers – are made of hard, dry, lifeless materials, like metal or plastic, while biological tissue is soft, wet, and living.
The body knows the difference and tends to reject imitations.
Enter hydrogels, three-dimensional networks of molecules swollen with – by definition – water.
First described in 1960 by creators of soft contact lenses, these weird, shape-shifting substances are able to morph from liquid to solid to a squishy in-between. (Early, simple uses include hair gel or Jell-O.). Slow to gain attention, growing to just 1,000 studies published by 1982, they’ve become the subject of intense study recently, with 100,000 papers published by 2020, and 3,800 already this year alone.
As chemists, biologists, and engineers begin to work more with one another and with medical doctors,
“We are, essentially, hydrogels,” said Benjamin Wiley, PhD, a chemistry professor at Duke University in Durham, N.C. “As people develop new hydrogels that more closely match the tissues in our body, we’ll be able to treat a whole host of ailments we couldn’t treat before.”
From contact lenses to brain implants
Put simply, a hydrogel is like a mesh bag of water.
The mesh is made of polymers, or spaghetti-like strands of molecules, stitched together in a repeating pattern and swollen with H2O, much like the way 3D matrixes in our body surround, support, and give structure to our cells and tissues.
“Imagine a soccer net, with all of these long fibers woven together to create the net,” said Eric Appel, PhD, associate professor of materials science and engineering at Stanford (Calif.) University.
While the broader category of “gels” could be filled with anything, including chemical solvents, water is the key ingredient that sets hydrogels apart, making them ideal for, as some scientists put it, “merging humans and machines.”
Human bones are about 25% water, while muscles hover around 70% and the brain is 85%. The precious liquid plays a host of critical roles, from shuttling nutrients in and waste out to helping cells talk to each other.
Lab-made hydrogels can be loaded with cargo (like a ball in the net), including cells or drugs that help mimic some of those functions.
Hydrogels are soft and pliable like flesh. So, if used in implants, they may be less likely to damage surrounding tissue.
“Think about a metal spoon in your bowl of pudding. As you’re shaking the bowl, the spoon doesn’t stay in place, and you get scarring around the spoon,” said Christina Tringides, PhD, a materials scientist who studies neural engineering. That, she says, is exactly what happens to brain implants when patients breathe or move. “It’s a mechanical mismatch. But with hydrogels, you could get perfect mechanical matching.”
Hydrogels also tend to be nontoxic, so the immune system may be less likely to attack them as foreign bodies.
All this has made hydrogels the new darling of the bioengineering world.
“There has been an absolute explosion of interest in these materials,” Dr. Appel said.