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 nanosized 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.
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.
