Fluorescent sensors are typically used to label and image a variety of molecules to give a unique glimpse inside living cells. However, the method has been limited to cells grown in a lab dish or in tissues closer to the surface of the body because the signal from the sensors are lost when implanted too deeply in the body.
The team of MIT engineers’ photonic technique “dramatically improved” the fluorescent signal, according to a news release. The researchers showed that sensors could be implanted as deep as 5.5 cm in the tissue and still provide a strong signal. Improved signaling could help fluorescent sensors to track specific molecules inside the brain or other tissues deep within the body for medical diagnosis or monitoring drug effects.
“If you have a fluorescent sensor that can probe biochemical information in cell culture, or in thin tissue layers, this technology allows you to translate all of those fluorescent dyes and probes into thick tissue,” said lead author on the study Volodymyr Koman.
Traditionally, scientists use different kinds of fluorescent sensors, including quantum dots, carbon nanotubes and fluorescent proteins, to label molecules inside the cells. The fluorescence of the sensors can be seen by shining a laser light on them. However, the method does not work in thick, dense tissue or deep within tissue because tissue emits some fluorescent light called autofluorescence, which makes implant signals weak.
“All tissues autofluoresce, and this becomes a limiting factor,” Koman said. “As the signal from the sensor becomes weaker and weaker, it becomes overtaken by the tissue autofluorescence.”
The MIT researchers modulated the frequency of the fluorescent light emitted by the sensor so it could become more easily distinguishable from tissue autofluorescence. Called wavelength-induced frequency filtering (WIFF), the method uses three lasers to create a laser beam with an oscillating wavelength.
Oscillating beams are shined on the sensors and cause fluorescence emitted by the sensor to double its frequency, according to the researchers. The signal can then be easily picked out from the background’s autofluorescence. The researchers reported they enhanced the sensor signal-to-noise ratio more than 50-fold.
The researchers suggest the method could be used to monitor the effectiveness of chemotherapy drugs. To demonstrate its usability, the team focused on glioblastoma. Patients with this aggressive form of brain cancer typically undergo surgery to remove as much as the tumor as possible and then receive chemotherapy to eliminate the remaining cancer cells.
“We are working on technology to make small sensors that could be implanted near the tumor itself, which can give an indication of how much drug is arriving at the tumor and whether it’s being metabolized. You could place a sensor near the tumor and verify from outside the body the efficacy of the drug in the actual tumor environment,” said Michael Strano, senior author of the study and Carbon P. Dubbs professor of chemical engineering at MIT.
When the cancer drug temozolomide enters the body, it is broken down into smaller compounds, and the MIT team designed a sensor to detect the compound known as AIC. They found that the implant could be placed as deep as 5.5 cm within an animal brain and could read the signal from the sensor even through the animal’s skull.
The researchers suggest the sensors could also be used to detect molecular signatures of tumor cell death. The WIFF method could be used to enhance the signal from other types of sensors, including carbon-nanotube-based sensors that detect hydrogen peroxide, riboflavin and ascorbic acid.
“The technique works at any wavelength, and it can be used for any fluorescent sensor,” Strano said. “Because you have so much more signal now, you can implant a sensor at depths into tissue that were not possible before.”
The MIT team hopes to continue the research using a tunable laser to create the signal and improve the technique even further. They are also working on sensors that are biologically resorbable and do not need to be surgically removed.
The research was funded by the Koch Institute for Integrative Cancer Research and Dana-Farber/Harvard Cancer Center Bridge Project with additional funding from the Swiss National Science Foundation, the Japan Society for the Promotion of Science, the King Abdullah University of Science and Technology, the Zuckerman STEM Leadership Program, the Israeli Science Foundation and the Arnold and Mabel Beckman Foundation. It was published in the journal Nature Nanotechnology.