New implantable sensor paves way to long-term monitoring

Carbon nanotubes are encapsulated within a hydrogel, allowing them to be implanted under the skin. PHOTO: BRYCE VICKMARK

Carbon nanotubes that detect nitric oxide can be implanted under the skin for more than a year.

Nitric oxide (NO) is one of the most important signaling molecules in living cells, carrying messages within the brain and coordinating immune system functions. In many cancerous cells, levels are perturbed, but very little is known about how NO behaves in both healthy and cancerous cells.

“Nitric oxide has contradictory roles in cancer progression, and we need new tools in order to better understand it,” says Michael Strano, the Carbon P. Dubbs Professor of Chemical Engineering at MIT. “Our work provides a new tool for measuring this important molecule, and potentially others, in the body itself and in real time.”

Until now, that is: Led by postdoc Nicole Iverson, Strano’s lab has built a sensor that can monitor NO in living animals for more than a year. The sensors, described in the Nov. 3 issue of Nature Nanotechnology, can be implanted under the skin and used to monitor inflammation — a process that produces NO. This is the first demonstration that nanosensors could be used within the body for this extended period of time.

Such sensors, made of carbon nanotubes, could also be adapted to detect other molecules, including glucose. Strano’s team is now working on sensors that could be implanted under the skin of diabetic patients to monitor their glucose or insulin levels, eliminating the need to take blood samples.

Sensors for short and long term

Carbon nanotubes — hollow, one-nanometer-thick cylinders made of pure carbon — have drawn great interest as sensors. Strano’s lab has recently developed carbon nanotube sensors for a variety of molecules, including hydrogen peroxide and toxic agents such as the nerve gas sarin. Such sensors take advantage of carbon nanotubes’ natural fluorescence, by coupling them to a molecule that binds to a specific target. When the target is bound, the tubes’ fluorescence brightens or dims.

Strano’s lab has previously shown that carbon nanotubes can detect NO if the tubes are wrapped in DNA with a particular sequence. In the new paper, the researchers modified the nanotubes to create two different types of sensors: one that can be injected into the bloodstream for short-term monitoring, and another that is embedded in a gel so it can be implanted long-term under the skin.

To make the particles injectable, Iverson attached PEG, a biocompatible polymer that inhibits particle-clumping in the bloodstream. She found that when injected into mice, the particles can flow through the lungs and heart without causing any damage. Most of the particles accumulate in the liver, where they can be used to monitor NO associated with inflammation.

“So far we have only looked at the liver, but we do see that it stays in the bloodstream and goes to kidneys. Potentially we could study all different areas of the body with this injectable nanoparticle,” Iverson says.

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