Researchers from the University of Connecticut have fabricated a new biodegradable composite from strands of silk fibroin, the foundational element of spider and moth silk, to replace the metal plates and screws currently used by orthopedists to help repair broken load-bearing bones.
Spider-Man, Marvel’s resident wall crawler, used his strong, tensile webs to eagerly swing from building to building, quickly retrieve falling objects and catching the bad guys. But that super-strong material is no longer just the stuff of comic books. Researchers from the University of Connecticut have fabricated a new biodegradable composite from strands of silk fibroin, the foundational element of spider and moth silk, to replace the metal plates and screws currently used by orthopedists to help repair broken load-bearing bones.
When someone breaks one of those bones, a leg or hip for example, doctors may fix it in place with plates made from stainless steel, titanium, or other titanium alloys.
But the metal plate can pose a few problems. To start, it can irritate the bone and surrounding joints, leading to inflammation and pain. Some patients can’t tolerate the metal inserts even during the initial period of fixation. The metals are also very stiff, with a flexural modulus of around 200 Gigapascals, and may bear too much of the load for the recovering body, resulting in a healed bone that is more susceptible to future fractures.
“In many cases, you are looking at a surgery to put in these plates and then a second surgery to remove them because they cause irritation,” says Mei Wei, a UConn materials scientist who specializes in creating novel scaffolding materials for bone and osteochondral repair. “Also, if the metal plate takes most of the weight during healing, you have a bone that can be very weak. Once you remove the fixation device, there’s a high probability of refracture, so then another surgery may be needed.”
The ideal fixation device should be easily tolerated by the human body, Wei says. It should be tough and strong enough to bear the required load, yet have elasticity like natural bone, somewhere around 9 to 25 Gigapascals. And it should be biodegradable, something that can be left in the body to decompose over time, no removal required. With that criteria in mind, Wei and her colleagues looked to silk fibroin to design a load-bearing composite to replace the metal plates used in load-bearing fracture healing today.
“Silk fibroin has been used as sutures, for scaffolding materials, and also for some types of bone repair in the past,” Wei says. “But no one has tried to use it for a bone fixation device. We used the silk, with its good mechanical strength, together with hydroxyapatite, or ceramic material that we call bone minerals. Basically, bone is a composite of collagen and hydroxyapatite. We wanted to create something similar with the fibroin.”
Using the core components of hydroxyapatite, long silk fibers, and polylactic acid, a binding polymer, the researchers had to go through quite a bit of trial and error to come up with the right mix.
“We had to work to find the right ratio of components to make sure it was strong enough for our needs,” Wei says. “We needed to make sure the binding polymer was sufficient to wet all the components so they could bind in the right way.”
They ultimately found a solution where the silk fibers and polylactic acid were coated with fine hydroxyapatite crystals. Using a hot compression mold, they could pack several layers of the fibers together to create a dense material that has a flexural modulus of 13.7 Gigapascals, as well as notable strength. The results were published in the May 2018 issue of Journal of the Mechanical Behavior of Biomedical Materials.
“For biodegradable composites, the elastic modulus we reported is the highest ever reported in the field,” Wei says. “But we are still refining the process and are now reaching a modulus of more than 18 Gigapascals. But we want to really maximize the mechanical properties of this composite. We want to eventually get to 20-22 Gigapascals. We are very, very close.”
Once the research team gets to that range, they plan to start animal testing, followed quickly by clinical trials. Wei is hopeful that such biodegradable composites will reach clinical use within 5 to 10 years. “We feel this biodegradable bone fixation device bears huge potential,” Wei says. “It can really help the patients and can save on medical costs by avoiding a lot of unnecessary secondary surgeries.”
And that’s something that would be sure to get Peter Parker’s spidey senses tingling.
Kayt Sukel is an independent technology writer based in Houston, TX.
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