Carbon Nanotubes Reveal Cancer Deep Inside Tissue

New method exploits the luminescence of carbon nanotubes to detect tumors deep inside tissue

2 min read
Carbon Nanotubes Reveal Cancer Deep Inside Tissue
Illustration: Weisman Lab

Researchers at Rice University in Houston, Texas, have developed a medical imaging technique that combines carbon nanotubes, LED light, and a photodiode detector to pinpoint the location of tumors buried 20 millimeters deep in simulated tissue. The researchers believe that this is the deepest that carbon nanotubes have been detected inside of tissue.

In research described in the journal Nanoscale, the Rice team exploited the ability of single-walled carbon nanotubes (SWNTs) to luminesce in the short-wave infrared (SWIR) region of the spectrum. While this SWNT luminescence has previously proven effective in illuminating internal organs, researchers were still unable to reliably detect and localize the source of the SWIR emission from inside tissues. The answer they came up with is a method called spectral triangulation.

This technique involves exciting the SWNTs embedded in the tissue by shining LED lights on the tissue. The light causes the nanotubes to luminesce; the infrared emissions are picked up by a scanning fiber optic probe that is connected to an indium gallium arsenide avalanche photodiode detector.

“We’re using an unusually sensitive detector that hasn’t been applied to this sort of work before,” said Bruce Weisman, the Rice chemistry, materials science, and nanoengineering professor who led the research, in a press release. Says Weisman:

This avalanche photodiode can count photons in the short-wave infrared, which is a challenging spectral range for light sensors. The main goal is to see how well we can detect and localize emission from very small concentrations of nanotubes inside biological tissues. This has potential applications in medical diagnosis.

The method the Rice team came up with departs from previous approaches in some innovative ways. First is the use of LEDs to excite the SWNTs. Typically, lasers are used to do this, but the laser beams can’t be focused inside of the tissues because of scattering. “We bathe the surface of the specimen in unfocused LED light, which diffuses through the tissues and excites nanotubes inside,” Weismann explains.

Another interesting variation from earlier work is the technique used to determine just how deeply embedded the SWNTs are inside the tissue. The fiber optic probe touches the surface of the tissue and takes readings along grid points that make it possible to determine the X and Y coordinates of the nanotubes. But to home in on the depth, or the Z coordinate, they take advantage of the way water in the tissue absorbs some of the light emitted by the nanotubes.

Weisman explained:

We make use of the fact that different wavelengths of nanotube emission are absorbed differently going through tissue. Water (in the surrounding tissue) absorbs the longer wavelengths coming from nanotubes much more strongly than it does the shorter wavelengths.

This means that, for nanotubes close to the surface, the long and the short wavelength emissions are similar in intensity because there is less tissue between the nanotubes and the detector to absorb the longer wavelengths. Accordingly, the farther the nanotubes are from the surface, the lower the intensity of long wavelengths reaching the detector. “So the balance between the intensities of the short and long wavelengths is a yardstick to measure how deep the source is,” says Weisman. “That’s how we get the Z coordinate.”

Researchers at the University of Texas MD Anderson Cancer Center, also in Houston, are testing the detector.

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Restoring Hearing With Beams of Light

Gene therapy and optoelectronics could radically upgrade hearing for millions of people

13 min read
A computer graphic shows a gray structure that’s curled like a snail’s shell. A big purple line runs through it. Many clusters of smaller red lines are scattered throughout the curled structure.

Human hearing depends on the cochlea, a snail-shaped structure in the inner ear. A new kind of cochlear implant for people with disabling hearing loss would use beams of light to stimulate the cochlear nerve.

Lakshay Khurana and Daniel Keppeler

There’s a popular misconception that cochlear implants restore natural hearing. In fact, these marvels of engineering give people a new kind of “electric hearing” that they must learn how to use.

Natural hearing results from vibrations hitting tiny structures called hair cells within the cochlea in the inner ear. A cochlear implant bypasses the damaged or dysfunctional parts of the ear and uses electrodes to directly stimulate the cochlear nerve, which sends signals to the brain. When my hearing-impaired patients have their cochlear implants turned on for the first time, they often report that voices sound flat and robotic and that background noises blur together and drown out voices. Although users can have many sessions with technicians to “tune” and adjust their implants’ settings to make sounds more pleasant and helpful, there’s a limit to what can be achieved with today’s technology.

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