Researchers from The Hong Kong Polytechnic University developed a miniaturized, highly sensitive optical fiber sensor that could be used to measure small pressure changes within the body — changes as minute as 2 kilopascals (kPa). The sensor is made from a polymer called Zeonex, and its basis for operation is a fiber Bragg grating (FBG) — periodic microstructures that can be inscribed onto a fiber. The FBG sensor is biocompatible and chemically inert. It is also not sensitive to moisture. Those qualities, said research team leader Hwa-Yaw Tam, make the device usable in various medical applications. “Our ultimate objective is to use these types of sensors to monitor various parameters — including pressure, temperature, and strain — inside animals and people,” Tam said. The optical fiber used to make the new sensor is made completely of the advanced polymer Zeonex. The diagram shows how the researchers made the preform that was then heated and pulled to make the fiber. Courtesy of Xin Cheng, The Hong Kong Polytechnic University. When pressure rises, an FBG-based sensor stretches, increasing the grating period and changing its refractive index. That change shifts the light output toward the red end of the spectrum. A decrease in pressure would cause a blue shift. Silica, a traditional material for optical fibers, is not ideal for medical applications due to its tendency to be brittle and stiff; FBGs embedded in silica fibers also tend to have limited sensitivity to small pressure changes because the material doesn’t stretch or contract easily. Existing and previous versions of polymer optical fibers often absorb water, which can alter measurements. Those optical fibers are not easily inscribed with an FBG. Zeonex exhibits a higher shift in response to a pressure change compared with silica fibers. Rather than using dopants, which are typically used to create materials with varied refractive indexes for the inner core and outside cladding of fibers, the researchers instead used different grades of Zeonex to make a single-material fiber. “Eliminating the use of dopants allows the optical fibers to be fabricated with good reproducibility,” Tam said. “We were able to use an excimer laser to easily inscribe the FBG and to add a side hole that runs parallel to the core. The side hole enhanced the pressure measurement sensitivity and significantly reduced lag, therefore providing better measurement accuracy.” The researchers compared their design side by side with a traditional polymer-based sensor of a similar physical design. They placed the sensors inside a chamber where the pressure was manually increased and decreased step by step above and below atmospheric pressure. The corresponding light shift was monitored in real time for both sensors. Tests determined that the Zeonex-based sensors with the side-hole design produced a linear response that was repeatable, and with negligible or lag errors. The sensor, the tests determined, could be used for low-pressure measurement up to 50 kPa above or below atmospheric pressure with a resolution of 2 kPa. The sensitivity of the sensor was ultimately demonstrated to be 80% higher than the traditional polymer-based sensor. “The pressure sensor is most useful in conditions where the change in pressure is in the order of few kilopascals above and below the atmospheric pressure,” Tam said. “It could be useful for low-pressure sensing in medical and high-altitude environments as well as for detecting pressure changes in gaseous containers.” To improve on their design, the researchers aim to further reduce the sensor’s response time, currently a few tens of seconds. They also want to expand the sensor to measure other physical and chemical parameters such as pH and to functionalize the probe so that it can detect the pressure of a particular gas. The research was published in Optics Letters (www.doi.org/10.1364/OL.418096).