Closing the Loop
A microscopic device stocked with drugs and a keen aim would be clever indeed, but the ultimate intelligent drug-delivery apparatus would also boast one more piece of equipment: a biosensor, which would enable it to respond to changes in the body’s chemistry and behavior. Coupled with a drug-delivery device, a biosensor could sense when concentrations of a drug were too high or too low, for example, and tell the device to respond accordingly-making the entire system not just smart but also autonomous.Although almost all researchers in the field of smart drug delivery have an eye on biosensors, sometimes key advances come from outside the field-for example, from someone working directly on the sensors themselves. Take University of Michigan electrical engineer Kensall Wise, who is developing a tiny implantable neural probe, designed to measure electrical activity in the brains of patients with diseases like epilepsy or Parkinson’s disease. Wise quickly realized such a device could do double duty, delivering drugs to combat the very diseases it was monitoring, right where they were needed most.
An area where biosensors might have an even greater impact than they would in brain diseases is diabetes. Approximately 16 million people in the United States suffer from diabetes, making it the country’s seventh leading cause of death and one of its most costly chronic illnesses. Studies have shown that patients who diligently control their blood glucose levels can prevent or delay the secondary complications of diabetes-kidney and eye disease, for example-by up to 60 percent.
Finger-prick blood tests can tell patients what their sugar levels are at specific moments during the day, but they don’t say anything about the fluctuations between tests. Patients risk taking too much or too little insulin, both of which can cause serious and even life-threatening side effects. A glucose sensor implanted directly in the body and connected to an insulin-delivery device could solve those problems.
But nobody has come up with a glucose sensor that can survive an extended tour of duty, because the body’s response-scar formation-prevents delicate biosensors from making accurate readings. “The sensors are really the rate-limiting step in this whole thing,” says Texas A&M University engineer Michael Pishko. “Once we’ve got the sensor down, which is probably still five years away, fixing it to an insulin-delivery device will happen very quickly.”
Francis Moussy, a biomedical engineer at the University of Connecticut Health Center, believes he’s already found a solution. Moussy is developing a sensor smaller than a grain of rice that uses an enzymatic reaction to measure blood glucose levels. To evade scar tissue, Moussy plans to cover the sensor with tiny “microspheres,” beads made of a biodegradable polymer that he can fill with different chemicals. In the body, the microspheres would slowly degrade, releasing anti-inflammatory agents that block scar formation. Other microspheres would release a chemical to promote the growth of blood vessels near the sensor’s surface, giving the device greater access to glucose in the blood.
Confident that his system will work based on preliminary tests, Moussy is working on a design suitable for mass production. And since many smart drug-delivery devices are being designed to take advantage of manufacturing techniques already proven in the computer industry, there’s reason to think that scaling up production of the devices could go smoothly when the time comes. That time may be as little as a decade away. And it could mark the beginning of an era in medicine that is not only “smarter” but a whole lot more humane.