The injured soldiers had been treated well since their return from fighting in Afghanistan. At the San Antonio Military Medical Center in Texas, surgeons had carefully grafted healthy tissue over their burns and wounds, using microsurgery to connect their blood vessels to the new skin. But the patients still faced an uncertain recovery. The vessels might not supply enough oxygen for the transplants to thrive.
When Conor Evans visited San Antonio in 2010 and saw these soldiers, he realized that conventional techniques for monitoring oxygen levels did not work very well, and they often failed to give enough warning if the graft was failing. “What these physicians do is nothing short of amazing,” says Evans, a chemist at Harvard Medical School and the Wellman Center for Photomedicine at Massachusetts General Hospital. “But the sensors they had just weren't cutting it.”
So Evans built a better bandage. He and his colleagues started with dyes that react to different oxygen levels, added nano-sized molecules that control the dye activity, and used them to create a liquid bandage that indicates the health of the wound it covers. “The bandage changes color, just like a traffic light, from green through yellow and orange to red,” depending on the amount of oxygen present, Evans says. After success in laboratory animals in 2014, human trials are set to begin this year.
By taking advantage of newfound abilities to manipulate materials as small as a few billionths of a meter, scientists such as Evans can not only improve rapid health assessments, they can also turn wound dressings into precise drug-delivery systems “Nanotechnology plays a large role in being able to control the amounts released and how well formulations get to the area of a wound that we need them to reach,” says Paula Hammond, a chemist at the Massachusetts Institute of Technology. That precision has a major advantage over flooding body parts with drugs, only some of which find their targets.
Coming Up for Air
Poor wound healing caused by a lack of oxygen affects more than six million people in the U.S. every year, and the medical costs are estimated to reach $25 billion. Typically physicians stick needle electrodes into injured tissue to measure tissue oxygenation, but the needles can be painful and give readings from only a single point in a large wound. Evans's bandage, in contrast, can provide an instant oxygen map of the entire injury.
It relies on two dyes mixed into a quick-drying liquid bandage that can be painted onto wounds. A brief burst of blue light energizes and illuminates both dyes: one glows bright red, the other green. Then oxygen molecules switch off the red dye's phosphorescence, so the bandage will appear green if the adjacent tissue is bathed in oxygen and is healthy. But if areas of the wound are oxygen-starved, patches of yellow, orange and, finally, an alarming red shine through.
The key to the alert is a nanoscale addition to the red dye molecules. Evans coupled each of these molecules to a dendrimer, a treelike molecule with a branching structure up to two nanometers across. This molecular thicket prevents neighboring molecules from overlapping and quenching one another's phosphorescence. They also physically block some—but not all—of the oxygen molecules from reaching the dye; starting with lower levels makes any changes more obvious.
In a hospital, the warning red would prompt a nurse to photograph the bandage, and doctors would to try to improve the blood and oxygen circulation in the trouble spots. In principle, the bandage could work at home, Evans says: patients could take their own bandage snapshots and send them to a doctor for assessment.
Evans's team has also created alternative dyes that are much more efficient at converting blue light into red. “Our new bandage is so bright that it can be seen with very low dye loading, in a sunlit room,” Evans says. In the future, the bandage might even be engineered to dispense therapeutic drugs into wounds, he adds.
In Hammond's lab, researchers have already loaded bandages with nanoengineered therapeutic substances. They have developed coatings that slowly release RNA or proteins, molecules that can shut down certain cell activities that might hamper wound recovery. Some RNA molecules, called small interfering RNAs, can hobble the ability of genes that give rise to problem-causing proteins, for example.
Her team encapsulated some of these RNAs within calcium phosphate shells, each about 200 nanometers wide, sandwiched the shells between two layers of a positively charged polymer made of biological molecules and then “buttered” one side of this sandwich with a negatively charged clay. (The opposite charges stick the layers to each other.) Stacking up 25 of these sandwiches formed a coating roughly half a micron thick, which Hammond placed on a conventional nylon bandage.
As natural enzymes in the body break down the layers, the dressing discharges the RNA molecules into the wound over the course of a week. The slow, steady release could reduce side effects caused by a single, large dose of a conventional drug; this release method could also ensure that the wound is constantly treated.
Hammond has also used this so-called layer-by-layer coating to supply a therapeutic protein that aids wound healing in diabetic mice. The protein is already available as an ointment, but she says that the formulation is not very effective—after initially delivering a huge burst of protein, its activity fades away within 24 hours. Hammond's bandage, in contrast, sustains a steady flow over five to seven days to maintain the optimum dose of protein.
The layer-by-layer strategy could improve treatments for another ailment: coronary artery disease, which is caused by a buildup of plaque in vessels that carry blood through heart muscle. Treatment usually involves widening the artery with an inflatable balloon and keeping it open by inserting a small tube of stainless-steel mesh known as a stent. Some stents come loaded with therapeutic molecules to prevent the artery from narrowing again, but patients must then take more drugs to reduce the associated risks of blood clots that could break free from the area.
Treating the artery with doses of DNA, carefully delivered by devices with nanoscale coatings, could offer a better solution, according to David Lynn, a chemist at the University of Wisconsin–Madison. Inside the body, the DNA could make cells produce a protein that helps to stabilize and reconstruct blood vessel walls. To deliver such genetic therapies exactly when and where they are needed, Lynn has coated stents with successive layers of DNA and a biodegradable polymer, each several nanometers thick. By varying the number of layers, researchers can control the amount of DNA released into blood vessel walls. Experiments on pigs showed that the DNA gradually penetrated the surrounding tissue during the days after the stent was implanted. Fine-tuning the design of the coating, other tests show, can change the rate of release. “We now have reasonable control that allows us to time the release from seconds to months by modifying the structure of the polymer or how we put the film together,” Lynn says.
The basic nanoengineering behind these inventions could be adapted for a wide range of other applications. Lynn is using polymer coatings to deliver biological molecules called peptides that interrupt the chemical conversations among bacteria. Cut off from one another, the bacteria cannot team up to form tough biofilms that resist breakup by antibiotics. Evans, for his part, is using his phosphorescent dyes in tissue samples to identify oxygen-poor tumor cells, which can be particularly resistant to chemotherapy, and he plans to test the technique in animals later this year. The same dye approach could also be used to detect the presence of infectious bacteria in wound tissue or reveal other kinds of molecules. “Really, the sky's the limit,” Evans says.