Hydrogels: Changing Health Care in Weird Ways

On May 23, 2023, a world where a simple injection can heal a broken bone, ingestible devices can track our health, and brain and heart implants can mesh with flesh seamlessly may not be far off. For decades, materials scientists have been working to mimic the complex architecture of the human body in hopes of replacing broken parts or treating disease. However, 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. This is where hydrogels come in.

Hydrogels are three-dimensional networks of molecules swollen with water. They were first described in 1960 by creators of soft contact lenses and are able to morph from liquid to solid to a squishy in-between. Hydrogels are like a mesh bag of water made of polymers, or spaghetti-like strands of molecules, stitched together in a repeating pattern and swollen with H2O. 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, including cells or drugs that help mimic some of those functions.

Hydrogels are also soft and pliable like flesh. So, if used in implants, they may be less likely to damage surrounding tissue. 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.

Hydrogels are ideal for merging humans and machines. Eric Appel, PhD, an associate professor of materials science and engineering at Stanford University, likens hydrogels to a soccer net, with all of these long fibers woven together to create the net. While the broader category of "gels" could be filled with anything, including chemical solvents, water is the key ingredient that sets hydrogels apart.

Hydrogels have the potential to transform the way we take medication and treat worn-out joints. Appel has been tinkering with gel formulas for years in hopes that these high-tech globs could someday ferry timed-release drugs to just the right spot in the body. His new hydrogels start as fully formed gels inside a syringe. But once the plunger is pushed, they magically shape-shift to a liquid thin enough to flow easily through a standard needle. Upon exit, they immediately reform into gels, protecting the inherent cargo from degrading. Such slow-release hydrogels could make vaccines last longer and deliver tumor-busting therapies more precisely.

Meanwhile, another team at the Massachusetts Institute of Technology has taken a different approach, developing a standard-sized ingestible hydrogel pill that swells up like a puffer fish in the stomach, lasting a month and slowly releasing drugs all the while. To remove the pill, a patient simply drinks a salt-based solution that shrivels the ping-pong ball-sized device so it can be passed out of the body. The puffer fish pill could also be loaded with tiny cameras or monitors to track conditions like ulcers or cancer.

Hydrogels have also been mulled for replacing human cartilage. Wiley and his colleagues at Duke recently reported that they’d developed the first gel-based cartilage substitute even stronger and more durable than the real thing. By attaching their hydrogel to a titanium backing to help stick it in place, they hope to repair damaged cartilage "much like a dentist fills a cavity" long before surgery is necessary.

At the University of Toronto, chemist Karina Carneiro, PhD, and dentist Christopher McCulloch, DDS, have developed a hydrogel made of DNA that can be injected, migrate to a defect in bone, and fill in the gap like putty. But not only does it patch the hole, it prompts the bone to regenerate.

The wildest potential applications of hydrogels come in the realm of human-machine interaction. Numerous companies are already dabbling in neural prosthetic or brain computer interfaces that might someday let someone who is paralyzed and can't speak write on a laptop using their thoughts. The spoon-in-the-Jell-O problem has been a major stumbling block. But Christina Tringides, PhD, a materials scientist who studies neural engineering, and her team have developed a seaweed-based hydrogel loaded with tiny flecks of nanomaterials that can not only meld nicely into squishy brain tissue but also conduct electricity. Within a decade, this could replace the clunky platinum metal discs used for electrocorticography.

In 30 to 50 years, the possibilities are endless. Hydrogels have the potential to transform the way we take medication and treat worn-out joints and pave the way for a seemingly sci-fi future in which organs, including brains, can interact directly with machines.