Optical fiber probe detects nanoliter-scale biological fluid conductivity

Some of the most revealing signals about human health are carried in fluids that are almost impossible to measure. Tears, cerebrospinal fluid, and prostate fluid appear only in tiny volumes, yet their chemical composition can reflect inflammation, hydration, or disease. For decades, engineers have struggled to analyze these fluids in real time, because most sensors require far more liquid than the body can easily provide.

Researchers at Jilin University now report a way to measure the electrical conductivity of such fluids using volumes as small as 50 nanoliters. The device, reported in the International Journal of Extreme Manufacturing, is an optical fiber probe no thicker than a human hair. It is designed for stable and real-time measurements and is largely unaffected by temperature or pH, two factors that often distort readings in biological environments.

Electrical conductivity is a basic but powerful indicator in physiology. It reflects the concentration of dissolved ions, which are tightly regulated in the body. Changes in conductivity can signal dehydration, electrolyte imbalance, inflammation, or disease. But conventional conductivity sensors rely on metal electrodes, which are difficult to miniaturize and prone to signal drift, fouling, and interference especially when working with extremely small samples.

The Jilin team took a different approach. Instead of measuring electrical signals directly, they translated conductivity into an optical one.

Using a laser-based 3D printing method known as two-photon polymerization, the researchers fabricated a microscopic Fabry-Perot cavity at the tip of an optical fiber. This cavity reflects light in a way that is highly sensitive to the refractive index of the surrounding liquid. Even slight changes in ion concentration, which determine conductivity, cause a measurable shift in the reflected wavelength.

To bring fluid into the sensing region, the probe integrates a microcapillary and a thin filtration membrane. Capillary forces draw the fluid into the cavity automatically. The membrane blocks large molecules such as proteins and cells, allowing only small ions to enter. This ensures that the optical signal is dominated by the ions responsible for conductivity, rather than by biological debris that could destabilize the measurement.

In laboratory tests, the probe maintained stable performance using just tens of nanoliters of liquid, a volume far below what most existing sensors require. Because the sensing mechanism is optical rather than electrical, the device avoids many of the problems associated with electrode-based probes, including polarization effects and chemical degradation.

Many clinically important fluids are available only in trace amounts. If we want to monitor them in real time, we need sensors that can work at that scale and remain stable in complex environments."

Prof. Qi-Dai Chen, professor at Jilin University and corresponding author of the study

The probe's small size and high aspect ratio make it suitable for invasive measurements, such as sensing cerebrospinal fluid through narrow biological pathways or monitoring conditions inside the gastrointestinal tract. The authors also note that the platform is adaptable: by changing the materials or structures at the fiber tip, similar probes could be engineered to detect temperature, pH, or specific biomolecules.

The work highlights the growing role of precision micro-fabrication in medical sensing. Techniques once developed for photonics and advanced materials are now being used to build instruments that operate inside the body, where space is limited and conditions are difficult to control.

The study does not yet demonstrate use in living systems, but it outlines a path toward sensors that can continuously track physiological signals using probes smaller than a needle. As diagnostic tools move toward earlier detection and real-time monitoring, the ability to measure the chemistry of a single drop of fluid may prove increasingly valuable.