For millions of patients managing chronic conditions like kidney disease, heart failure, or neurological disorders, monitoring blood sodium levels is a routine but invasive necessity. Blood draws, while effective, are uncomfortable, time-consuming, and often impractical for continuous tracking. But a new technology developed by researchers at Tianjin University in China could soon make the needle obsolete—replacing it with a whisper of light and sound.
In a study published in Optica, the team unveiled a novel system that combines terahertz spectroscopy with optoacoustic detection to measure blood sodium levels noninvasively and in real time. This hybrid approach overcomes two of the biggest hurdles in terahertz-based biomedical sensing: water interference and limited tissue penetration. Terahertz radiation, which lies between microwaves and infrared light on the electromagnetic spectrum, is ideal for biological applications because it’s low-energy and non-ionizing. However, it’s also strongly absorbed by water, making it difficult to isolate signals from specific biomolecules in the body. That’s where optoacoustics comes in.
The researchers’ system works by directing terahertz waves at a target area—such as a blood vessel under the skin. As sodium ions in the blood absorb the terahertz energy, they vibrate and generate ultrasonic waves. These sound waves are then picked up by an ultrasonic transducer, effectively translating the terahertz signal into a clearer, more measurable acoustic one. This clever workaround allows the system to detect sodium concentrations even in the presence of water, which would otherwise drown out the signal.
Initial tests in live mice showed that the system could track sodium fluctuations on a millisecond timescale for over 30 minutes. Measurements were taken from the ear, with the skin cooled to 8°C to reduce background noise from water. The team also tested the system on human blood samples and healthy volunteers, demonstrating that it could distinguish between high and low sodium levels without the need for skin cooling in some cases. The optoacoustic signal was found to correlate with blood flow, suggesting that the system could be tuned for different anatomical sites.
The implications of this technology are profound. Real-time, noninvasive sodium monitoring could revolutionize how clinicians manage fluid balance in critical care, dialysis, and endocrine disorders. It could also enable wearable devices that continuously track electrolyte levels, alerting patients and providers to dangerous imbalances before symptoms arise. For example, in patients with hyponatremia or hypernatremia—conditions where sodium levels fall dangerously low or high—timely intervention is crucial. A noninvasive monitor could provide that window of opportunity without the need for repeated blood draws.
Beyond sodium, the researchers believe the system could be adapted to detect other biomolecules like glucose, proteins, and enzymes, each of which has a unique terahertz absorption signature. This opens the door to a new class of diagnostic tools that are not only needle-free but also label-free—requiring no dyes or contrast agents to function.
Of course, challenges remain. The system currently requires some degree of skin cooling to suppress water interference, which may not be practical in all clinical settings. The team is exploring alternative signal processing techniques to eliminate this requirement and is also working to identify optimal detection sites on the human body, such as the inside of the mouth, where cooling is easier and signals are stronger.
