Jilin University Develops Nanoliter-Scale Optical Fiber Probe for Biological Fluids
Researchers at Jilin University have developed an optical fiber probe capable of measuring the electrical conductivity of biological fluids in volumes as small as 50 nanoliters.
This innovation addresses a long-standing challenge in analyzing fluids like tears, cerebrospinal fluid, and prostate fluid, which appear in tiny amounts yet carry crucial health signals.
Probe Design and Functionality
The device, detailed in the International Journal of Extreme Manufacturing, is an optical fiber probe approximately the thickness of a human hair. It is engineered for stable, real-time measurements and exhibits resilience to temperature and pH variations, factors that commonly distort readings in biological settings.
Traditional conductivity sensors rely on metal electrodes, which are difficult to miniaturize and are susceptible to signal drift and interference with small samples. The Jilin team circumvented this by translating electrical conductivity into an optical signal.
Using a laser-based 3D printing method called two-photon polymerization, a microscopic Fabry-Perot cavity was created at the fiber's tip. This cavity reflects light in a manner highly sensitive to the surrounding liquid's refractive index. Minor changes in ion concentration, which dictate conductivity, induce a measurable shift in the reflected wavelength.
To facilitate fluid sampling, the probe incorporates a microcapillary and a thin filtration membrane. Capillary forces automatically draw fluid into the cavity. The membrane filters out large molecules like proteins and cells, allowing only small ions to enter, ensuring the optical signal primarily reflects ionic conductivity.
Performance and Potential Applications
Laboratory tests demonstrated the probe's stable performance with only tens of nanoliters of liquid, a volume significantly smaller than required by most existing sensors. Its optical sensing mechanism avoids issues associated with electrode-based probes, such as polarization effects and chemical degradation.
Prof. Qi-Dai Chen, a corresponding author of the study, noted that many clinically important fluids are only available in trace amounts, necessitating sensors that can operate at this scale and maintain stability in complex biological environments.
The probe's compact size and high aspect ratio make it suitable for invasive measurements, such as monitoring cerebrospinal fluid or conditions within the gastrointestinal tract. The platform is also adaptable; modifying the materials or structures at the fiber tip could enable the detection of temperature, pH, or specific biomolecules.
This research highlights the increasing influence of precision micro-fabrication in medical sensing, adapting photonics and advanced materials techniques to create instruments for internal body use where space is constrained and conditions are challenging. While the study has not yet demonstrated use in living systems, it provides a pathway for continuous physiological signal tracking using probes smaller than a needle, potentially enhancing early detection and real-time monitoring capabilities.