Researchers at the National Institute of Standards and Technology (NIST) report the development of a new kind of sensor to study tissue growth in the lab. Their paper (“A photonic pH sensor based on photothermal spectroscopy“), published in Sensors and Actuators B, describes a small sensor that uses a light-based signal to measure pH, an important property in cell-growth studies.

The same basic design could be used to measure other qualities such as the presence of calcium, cell growth factor, and certain antibodies, according to team members who say that unlike conventional sensors, this measurement method could be used to monitor the environment in a cell culture long-term—for weeks at a time—without having to disturb the cells regularly to calibrate the sensing instruments.

“Although the determination of pH is a standard laboratory measurement, new techniques capable of measuring pH are being developed to facilitate modern technological advances. Bio-industrial processing, tissue engineering, and intracellular environments impose unique measurement requirements on probes of pH. We describe a fiber optic-based platform, which measures the heat released by chromophores upon absorption of light. The optical fibers feature fiber Bragg gratings (FBG) whose Bragg peak redshifts with increasing temperature,” the investigators wrote.

petri dish filled with a solution of phenol red, which changes color depending on acidity
Go Ahead and Cross the Streams: In this version of the experiment, a petri dish is filled with a solution of phenol red, which changes color depending on acidity. The lower pH of the solution on the left gives the substance a deep yellow color. The higher pH of the solution on the right gives the substance a reddish color. Depending on its color, the liquid absorbs more or less of the green laser light versus the blue laser light. Notice that the green light is much more visible inside the yellow liquid (top left) than it is in the red liquid (top right). In contrast, the blue light is much more visible in the red liquid (bottom right) than it is in the yellow liquid (bottom left). The more light the liquid absorbs, the more its temperature increases. By measuring these temperature changes with the optical fiber on the left (shining red light in this illustration), researchers can assess the exact color of the liquid, which tells them its pH. [Jennifer Lauren Lee/NIST]

“Using anthocyanins (pH-sensitive chromophores found in many plants), we are able to correlate visible light absorption by a solution of anthocyanins to heat released and changes in FBG signal over a pH range of 2.5–10. We tested the ability of this platform to act as a sensor coating the fiber within a layer of crosslinked polyethylene glycol diacrylate (PEG-DA). Incorporating the anthocyanins into the PEG, we find that the signal magnitude increases over the observed signal at the same pH in solution. Our results indicate that this platform is viable for assessing pH in biological samples and point at ways to optimize performance.”

Watching properties of the tissue in real-time as they slowly change, over days or weeks, could greatly benefit tissue engineering studies to grow teeth, heart tissue, bone tissue, and more, noted NIST chemist Zeeshan Ahmed, PhD.

“We want to make sensors that can be put inside growing tissue to give researchers quantitative information,” Ahmed said. “Is the tissue actually growing? Is it healthy? If you grow a bone, does it have the right mechanical properties or is it too weak to support a body?”

The work could have benefits beyond tissue engineering, into studying the progression of diseases such as cancer. “What these sensors could give people is real-time information about tissue growth and disease progression,” pointed out American University chemist and NIST guest researcher Matthew Hartings, PhD.

Conventional sensors give researchers a series of snapshots without showing them the path between those points, Hartings explained. But photonic sensors could provide scientists with continuous information, the equivalent of a GPS navigation app for disease.

The researchers say that according to their tissue engineering collaborators, the new photonic sensors could provide useful information for a range of biological systems being studied, particularly the growth of heart and bone cells.

For their next round of experiments, already underway, the NIST researchers are using another pH-sensitive dye called phenol red. In addition, they are working to encapsulate the dye in a plastic coating around the fiber itself so that it does not interact with the cell medium. The team is also conducting its first test of the system in a real cell culture, with help from NIST colleagues who specialize in tissue engineering.

Future plans include measuring quantities beyond pH, which would simply require swapping out phenol red for a different dye sensitive to whatever property researchers want to measure. And much further in the future, Ahmed hopes the measurement scheme could potentially be used to monitor the growth of tissue in a real person’s body.

“The long-term goal is being able to put implantable devices into people where you’re trying to grow bones and muscles, and then hopefully over time the sensors could be designed to dissolve away and you wouldn’t even have to go back in and remove them,” said Ahmed. “That’s the ultimate dream. But baby steps first.”

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