Someday, doctors would like to grow limbs and other body tissue for soldiers who have lost arms in battle, children who need a new heart or liver, and many other people with critical needs. Today, medical professionals can graft cells from a patient, deposit them onto a tissue scaffold, and insert the scaffold into the body to encourage the growth of bone, cartilage, and other specialized tissue. But researchers are still working toward building complex organs that can be implanted into patients.
Scientists have developed a promising new kind of light-based sensor to study tissue growth in the lab. The small sensor uses a light-based signal to measure pH — the measurement unit for acidity and 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.
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. 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. The work could have benefits beyond tissue engineering, into studying the progression of diseases such as cancer. Conventional sensors give researchers a series of snapshots without showing them the path between those points; however, photonic sensors could provide scientists with continuous information — the equivalent of a GPS navigation app for disease.
Measurements of pH are a vital part of tissue engineering studies. As cells grow, their environment naturally becomes more acidic. If the environment becomes too acidic — or too basic — the cells will die. Scientists measure pH on a scale from 0 (very acidic) to 14 (very basic), with an ideal environment for most cells in a narrow range around a pH of 7.
Commercial pH instruments are highly accurate but unstable, meaning they require frequent calibrations to ensure accurate readings day to day. Without calibration, these conventional pH meters lose up to 0.1 pH units of accuracy daily. But tissue engineering studies take place on the order of weeks. If researchers disturb the growing cells every time they have to measure the cell culture’s pH, then they introduce another kind of uncertainty to their measurements, since they are altering the cells’ environment. What’s needed is a measurement system that can stay inside an incubator with the cells in their culture medium and not need to be removed or calibrated for weeks at a time.
Photonic sensors are small, lightweight devices that use optical signals to measure a range of qualities. Some use commercially available, flexible, optical fibers etched with a Bragg grating, a kind of filter for light that reflects certain wavelengths and allows others to pass. Changes in temperature or pressure alter the wavelengths of light that can pass through the grating. To adapt the photonic devices to a pH measurement, the researchers used a scientific principle that when an object absorbs light, the energy absorbed turns into heat.
To demonstrate, the scientists used a substance that changes color in response to changes in pH — red cabbage juice powder. Cabbage juice changes its color from shades of dark purple to light pink, depending on the acidity of a solution. That change in color can be picked up by the photonic temperature sensors. Another demonstration used a pH-sensitive dye called phenol red. The dye is encapsulated in a plastic coating around the fiber itself so that it does not interact with the cell medium.