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 at the National Institute of Standards and Technology (NIST) are supporting this
field of research by developing a promising new kind of light-based sensor to study tissue
growth in the lab.
The NIST team’s proof-of-concept work, published in Sensors and Actuators B, demonstrates a
small sensor that uses a light-based signal to measure pH, the measurement unit for acidity,
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 too, 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.
But 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.
A culture of stem cells might need to be grown for almost a month before they turn into bone.
On the other hand, if researchers disturb the growing cells every time they have to
measure the cell culture’s pH, then the scientists are introducing another kind of
uncertainty to their measurements, since they are altering the cells’ environment.
What’s needed for this kind of research, 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.
For years, the NIST research team have been developing photonic sensors, small lightweight
devices that use optical signals to measure a range of qualities including temperature,
pressure and humidity.
Some of these novel devices 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 their photonic devices to a pH measurement, the team relied on a well-known concept in
science: When an object absorbs light, the energy absorbed “has to go somewhere,”
and in many cases that energy turns into heat.
For their demonstration, the scientists used a substance that changes color in response
to changes in pH, a material that many people may remember from biology classes: 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.
Researchers filled a petri dish with the cabbage juice solution.
One optical fiber was positioned above the dish.
It was connected to a laser pointer and shined light into the sample.
A second optical fiber was physically embedded in the liquid.
This second fiber contained the Bragg grating and acted as the temperature sensor.
The team controlled the solution’s pH manually.
To make a measurement, the researchers shone one color of light — such as red — into
the sample from above.
The cabbage juice absorbed the red light to varying degrees based on its color, which
depended on the pH of the solution at that time.
The photonic thermometer fiber picked up these slight changes in the juice’s heat.
A change in temperature changes the wavelengths of light that can pass through the fiber’s
Bragg grating.
Next, the researchers shone a second color of light — such as green — into the liquid,
and repeated the process.
By comparing how much heat was generated by each color of light, researchers could determine
the exact color of the cabbage juice at that moment, and that told them the pH.
The researchers were able to show that it works over a wide range, from a pH of 4 to
a pH of 9 or 10.
Ongoing work shows the photonic pH measurements are accurate to plus or minus 0.13 pH units
and are stable for at least three weeks, much longer than conventional measurements.
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.
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.