Researchers at Jilin University have developed a new optical fiber probe capable of measuring the electrical conductivity of biological fluids in volumes as small as 50 nanoliters. The device, described in the International Journal of Extreme Manufacturing, is about as thin as a human hair and provides stable, real-time readings that are largely unaffected by temperature or pH changes.
Electrical conductivity in body fluids reflects ion concentration, which can indicate hydration status, inflammation, electrolyte imbalance, or disease. Conventional sensors for this purpose use metal electrodes that are difficult to miniaturize and often suffer from signal drift and interference when used with very small fluid samples.
The team at Jilin University approached the problem differently by converting conductivity changes into optical signals instead of relying on direct electrical measurements. Using two-photon polymerization—a laser-based 3D printing technique—they built a microscopic Fabry-Perot cavity at the tip of an optical fiber. This structure is highly sensitive to shifts in the refractive index caused by variations in ion concentration within the surrounding fluid.
To ensure only relevant ions enter the sensing region, the probe incorporates a microcapillary and a filtration membrane. Capillary forces automatically draw fluid into the cavity while blocking larger molecules like proteins and cells. This design ensures that only small ions affect the optical signal.
Laboratory tests showed that the probe maintained stable performance with just tens of nanoliters of liquid—much less than what most existing sensors require. Because it relies on optical rather than electrical detection, it avoids issues such as polarization effects and chemical degradation common with electrode-based probes.
The probe’s dimensions make it suitable for invasive applications where access is limited, such as sampling cerebrospinal fluid or monitoring conditions inside narrow parts of the gastrointestinal tract. According to its developers, modifying materials or structures at the fiber tip could allow similar probes to detect other physiological parameters like temperature or specific biomolecules.
This research demonstrates how precision micro-fabrication techniques originally developed for photonics are being adapted for medical sensing applications inside challenging biological environments. While this study did not test use in living systems, it suggests a path toward future diagnostic tools capable of tracking physiological signals continuously with minimally invasive probes.
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