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The Photonic pH Sensor Which Could Regenerate Tissue

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Today’s surgeons can graft skin, bone, and cartilage using stem cells and a scaffold in order to generate new tissue ready for transplanting onto areas of the body affected by injury or illness. However, grafts using scaffolds have their limitations such as auto-immune rejection, additional scarring, and the risk of infection. 

This has led some researchers, scientists and doctors to delve deeper into research and development to come up with new, innovative techniques that aid the regeneration of complex tissues including cartilage, bone, and even entire organs

Now, a team of scientists at the US National Institute of Standards and Technology (NIST) has developed a new photonic pH sensor that could further support and enhance the research and development of such tissue growth techniques. The concept of their work, published in the journal Sensors and Actuators B, highlights the abilities of a small photonic sensor that harnesses a light-based signal in order to measure pH, a key property in cell-growth. 

More advanced than conventional sensors, the photonic pH sensor could be implanted into a cell culture long term with minimal disturbance. What’s more, the sensors would enable continuous observation. “We want to make sensors that can be put inside growing tissue to give researchers quantitative information,” said Zeeshan Ahmed, co-author of the paper and a chemist at NIST. The team also describes how the same sensor design could be modified to measure other important properties such as cell-growth factor and calcium.

Continuous Information in Real Time

Thus, having the ability to continually observe changes in the properties of tissue in real time could provide a significant boost to studies concerned with the engineering and growth of new tissue. NIST guest researcher and co-author, Matthew Hartings stated, “What these sensors could give people is real-time information about tissue growth and disease progression.” With this continuous real time information and fresh insights into the growth of tissue and spread of disease, researchers and scientists would be equipped with a system comparable to a navigation app or GPS system. 

We want to provide researchers with a detailed map of the incremental changes that happen as tissue either grows in a healthy way or becomes diseased. Once researchers know the ‘streets’ a disease is taking, then they can better prevent or support the changes that are happening in a patient’s body.

Matthew Hartings, Guest Researcher and Co-author, NIST

Conventional pH instruments and sensors require frequent maintenance and calibration in order to remain accurate. They can lose 0.1 pH units of accuracy every day. This prevents them from being able to provide continuous information and means that only a snapshot of cell growth and disease progression is taken. Therefore, if a culture of stem cells requires a prolonged period to grow then having a sensor that must be adjusted every day is an inefficient method. 

An increment of 0.1 pH is significant. If your pH value changes by 1, you kill the cells. If after a few days I can’t trust anything about my pH measurement, then I’m not going to use that measurement method.

Zeeshan Ahmed, Co-author of the paper and a Chemist, NIST

Modification and Demonstration 

A measurement system that can remain in an incubator with the cell culture for weeks at a time while dispensing of the need to remove or calibrate the sensors during that period would give researchers the boost they need. Thus, Ahmed and his team have been tirelessly working on advancing their photonic pH sensors. These sensors are miniature, lightweight devices that utilize optical signals to record various important qualities such as pressure, temperature and humidity. 

To modify their photonic devices to record an accurate pH measurement, Ahmed and Hartings relied on one of science’s well-known adages: When an object absorbs light, the energy absorbed, “has to go somewhere,” said Ahmed. “For every individual photon, the heat produced is a very small amount of energy,” continued Ahmed. “But if you have lots of photons coming in, and you have lots of molecules, it becomes an appreciable change in heat.”

To demonstrate their method, Ahmed, Hartings, and the team used red cabbage juice powder that responds to pH variance with changes in color. Depending on the acidity levels, the cabbage juice color ranges from shades of dark purple to light pink. Such changes can then be detected by the modified photonic temperature sensors when the solution is subject to different frequencies of light. “We were able to show that it works over a wide range, from a pH of 4 to a pH of 9 or 10,” said Ahmed. 

Next Steps 

The NIST team has already stated that they believe the sensors can be modified to measure properties beyond pH. This would simply require using a solution that would react to the property researchers wanted to observe and measure. Accordingly, by diversifying the range of study the photonic sensors could be particularly useful for the study of bone and organ cell growth. 

With the first round of tests already underway the team will continue analysis on real stem cell growth cultures in collaboration with colleagues from NIST that have expertise in tissue engineering. Ahmed dreams that one day the system they have developed can be implanted directly into a human body to monitor the growth of tissue or spread of disease.

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. That's the ultimate dream. But baby steps first.

Zeeshan Ahmed, Co-author of the paper and a Chemist, NIST

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David J. Cross

Written by

David J. Cross

David is an academic researcher and interdisciplinary artist. David's current research explores how science and technology, particularly the internet and artificial intelligence, can be put into practice to influence a new shift towards utopianism and the reemergent theory of the commons.


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