Guillermo Ameer

Citric acid a key in Guillermo Ameer's search for new biomaterials

What does a vascular graft have in common with an orange? Or a ligament replacement with a lime? According to Guillermo Ameer, assistant professor of biomedical engineering, they could have quite a bit in common — namely, citric acid, which produces that tart citrus taste and which Ameer uses to produce revolutionary new biomaterials.

“Although citric acid is very common, it’s an interesting molecule,” Ameer says. “Chemically, it’s very convenient because it has three or four reactive groups that can form a basis for biodegradable biomaterials. It’s a platform technology that can produce a wide range of materials.”

Ameer uses citric acid as the base for new polyester biomaterials that hold the promise of helping the body accept medical implants, develop replacement ligaments for injuries, and even deliver pharmaceuticals within the body. Citric acid occurs as a natural metabolic product in the body, so the materials Ameer creates are expected to be inherently biocompatible.

Ameer points to the low cost of citric acid in addition to its compatibility with the body. It is widely available and already used in consumer products such as shampoo, toothpaste, and soda. Ameer produces his materials at relatively low temperatures when compared with the synthesis conditions of other polyesters and is exploring ways to cure the materials using light, with the goal of low-cost production. And the materials are flexible and have a wide range of degradation properties, a vast improvement over other prosthetic materials. “We focus on materials that match the physical and chemical properties of tissues, something that was ignored for many years,” he says.

One potential application for Ameer’s new biomaterials might provide the ability to produce scaffolds to facilitate the development of replacement ligaments. A flexible scaffold to replace torn ligaments — such as the commonly injured anterior cruciate ligament (ACL) in the knee — would allow patients to recover from the injury without relying on replacements from cadavers or a second surgery to remove another tendon in the body.

“You can develop our materials into strong scaffolds to implant into the knee,” Ameer explains. Cells then invade the scaffold and begin to form tissue. As the scaffold degrades, the body is left with a replacement ligament.

Ameer also focuses on using his materials to improve vascular grafts. In patients with peripheral artery disease, blocked arteries are replaced with veins from other parts of the body or with prosthetic grafts. Using his biomaterials, Ameer hopes to improve the functionality of those grafts and even create a new class of tissue-engineered grafts.

Ameer received a highly competitive grant from the Illinois Regenerative Medicine Institute — which was organized by the state of Illinois to fund stem-cell research — to study the use of adult stem cells to improve prosthetic grafts. He uses the stem cells as progenitor cells in combination with his biomaterials to develop thin layers of endothelial cells on prosthetic grafts. The layers of endothelial cells — which naturally line blood vessels — assist the body in accepting a prosthetic graft. “These cells do two things: They secrete factors that inhibit cell growth, preventing the overproliferation of smooth muscle cells in the area, and they secrete factors that signal to the body that everything is okay and working properly,” he says.

In addition to improving prosthetic grafts, Ameer’s research team is also exploring methods to create its own grafts. “The idea is to develop a graft from scratch, using our material as a biodegradable scaffold for the cells,” Ameer says. “Prosthetic grafts don’t work as replacement small-diameter arteries. This idea could provide a living graft with improved results.”

Applications for Ameer’s biomaterials are still in development, ranging from new types of bone screws and pins to possible patches for in vivo drug delivery. Ameer collaborates extensively with faculty at Northwestern’s Feinberg School of Medicine to develop clinically appropriate uses for the technology. So far his techniques have been successful in gaining a wide variety of funding and recognition. Over the past year he has received an NSF Early Career Development Award, an Established Investigator Award from the American Heart Associa­tion, and the Illinois Stem Cell Award, to name only a few. With continued success, he hopes to develop small-scale clinical trials in 5 to 10 years.

So the next time that you enjoy a lemon or a lime, just think: The citric acid that produces that tart flavor could also be instrumental in treating your future health conditions.

—Kyle Delaney