Image: FBGs turn an optical fiber into a sensing element by reflecting a specific wavelength (Photo courtesy of Maria Konstantaki).
A new study describes how bioresorbable sensors could be used for in-body monitoring of bone fracture healing and for safer exploration of sensitive organs such as the brain.
Developed at the Polytechnic University of Turin (Italy), Consiglio Nazionale delle Ricerche (CNR; Trento, Italy), and other institutions, the new sensing element is a modified fiber Bragg grating (FBG) inscribed into a bioresorbable calcium-phosphate glass optical fiber using a 193 nm excimer laser. The laser inscription creates a pattern that causes the fiber to reflect a specific wavelength back in the direction from which it came. A modification that tilts the FBG allows some of the reflected light to escape from the fiber core to the cylindrical surface; the back-reflected light can then be monitored.
The researchers created both tilted and standard optical FBGs to understand how the parameters used for inscription affect the grating sensing characteristics. They found that exposing the bioresorbable fiber to ultraviolet (UV) laser light with a given spatial intensity distribution created a corresponding surface relief pattern in the optical fiber volume after dissolution. According to the researchers, the phosphate glass FBG could be used in soluble photonic sensing probes for efficient in-vivo monitoring of vital mechanical or chemical parameters.
The researchers are now performing systematic experiments to better understand how the fiber composition and UV laser irradiation conditions affect the speed at which the fiber Bragg grating dissolves. This information could be used to create fiber Bragg gratings that dissolve within a specific time period; before being used in people, the dissolving and sensing properties of the FBG will need to be studied in animals. The study was published on February 15, 2018, in Optics Letters.
“This glass combines excellent optical properties with biocompatibility and water solubility, thus providing a reliable platform from which to make optical fibers that dissolve in water or biological fluids,” said study co-author Daniel Milanese, PhD, of the Polytechnic University of Turin. “The glass is made of phosphorous oxide combined with oxides of calcium, magnesium, sodium, and silicon. The properties of the optical fibers can be tuned by properly changing the glass composition.”
A FBG is an invisible reflector inside the core of an optical fiber that is set to a specific wavelength of light. When the fiber where the FBG is located is exposed to strain or temperature, the FBG’s “center wavelength” shifts to a higher or lower wavelength. The direction and magnitude of the shift is proportional to the change in strain or temperature. Using different wavelengths allows multiplexing of dozens of FBGs on a single fiber. FBGs are commonly used for applications such as real-time monitoring of the structural health of bridges or tracking the integrity of airplane wings.
Polytechnic University of Turin
Consiglio Nazionale delle Ricerche