Infections resulting from joint-replacement surgeries are costly and potentially deadly. Now researchers at MIT are developing coatings for medical implants that can be loaded with multiple drugs, including antibiotics that are released over time. The process involves layering antibiotic films, which are released over the short term, onto a permanently antibacterial polymer designed to prevent infection over the long term.
About one percent of knee and hip replacement surgeries result in infection; the number rises to three to five percent for second surgeries. “It’s a low rate, but if you are the one out of one hundred who gets an infection, the complications are catastrophic,” says Lloyd Miller, assistant professor of orthopedic surgery at the University of California, Los Angeles. All the infected tissue and hardware must be surgically removed and replaced with an antibiotic block; the patient cannot walk for six to eight weeks while being treated with intravenous antibiotics to eliminate all traces of infection; and then a revision surgery is done. Complications due to infection are also enormously expensive. A joint replacement costs about $30,000 in the United States, but dealing with infections can raise the tab to nearly $150,000.
Most infections happen when bacteria enter the body with an implant. But artificial joints can become infected years later when bacteria are introduced into the bloodstream during dental work, colonoscopies, and other procedures, says Miller, who is not affiliated with the MIT group. Orthopedic coatings that have permanent antibacterial properties in addition to a transient coating of antibiotics could keep patients protected.
Although antibiotic coatings for many other medical devices have already been developed, coatings for joints pose particular challenges. Unlike stents and other devices that are static, joints have to be able to move. So the coating can’t be too thick, and it mustn’t interfere with joint articulation.
Researchers led by Paula Hammond, professor of chemical engineering at MIT, are using a polymer-coating technique called layer-by-layer assembly to load large concentrations of drugs into implant coatings without making them too thick. Hammond’s group builds up these films by dipping an implant alternately in solutions of negatively and positively charged molecules such as polymers and drugs. The difference in surface charge holds each layer tightly to those above and below it. This process leads to very thin layers of materials, on the order of tens of nanometers thick. The drugs will be released when the polymers biodegrade inside the body.