An optical fiber made from agar has been produced at the University of Campinas (UNICAMP) in the state of São Paulo, Brazil. This device is edible, biocompatible and biodegradable. It can be used in vivo for imaging body structure, local delivery of light in phototherapy or optogenetics (eg stimulation of neurons with light to study nerve circuits in a living brain) and localized drug delivery.
Another possible application is the detection of microorganisms in specific organs, in which case the probe would be completely absorbed by the body after performing its function.
The research project, supported by the São Paulo Research Foundation - FAPESP, was led by Eric Fujiwara, Professor at UNICAMP’s School of Mechanical Engineering, and Cristiano Cordeiro, Professor at UNICAMP’s Gleb Wataghin Institute of Physics, in collaboration with Hiromasa Oku, Professor at Gunma University in Japan .
An article about the study is published in Scientific reports.
Agar, also called agar-agar, is a natural gelatin obtained from marine algae. Its composition consists of a mixture of two polysaccharides, agarose and agaropectin. “Our optical fiber is an agar cylinder with an outer diameter of 2.5 millimeters [mm] and a regular internal arrangement of six 0.5 mm cylindrical air holes around a solid core. Light is limited due to the difference between the refractive index of the agar core and the air holes, Fujiwara said.
“To produce the fiber, we poured food grade agar into a mold with six inner rods placed longitudinally around the main axis,” he continued. “The gel is distributed to fill the available space. After cooling, the rods are removed to form air holes, and the solidified waveguide is released from the mold. The refractive index and geometry of the fiber can be adjusted by varying the agar composition solution and mold design.”
The researchers tested the fiber in various media, from air and water to ethanol and acetone, and concluded that it is context sensitive. “The fact that the gel undergoes structural changes in response to variations in temperature, humidity and pH makes the fiber suitable for optical sensing,” Fujiwara said.
Another promising application is its simultaneous use as an optical sensor and a growth medium for microorganisms. “In this case, the waveguide could be designed as a disposable sample unit containing the necessary nutrients. The immobilized cells in the unit would be optically sensed, and the signal would be analyzed with a camera or spectrometer,” he said.