Breaking the Specificity Barrier in Microwave Sensing: Highly Specific Lactate Microwave Biosensor for Fitness and Exercise Optimization

Noninvasive biomarker sensing plays a vital role in health monitoring and sports physiology, particularly for tracking sweat lactate in real time to gauge exercise intensity without disrupting activity. This work introduces a high-specificity microwave biosensor for lactate detection, addressing the challenge of specificity seen in current microwave biosensors, which limits their practical applications. Our approach leverages a cost-effective complementary split-ring resonator (CSRR) combined with lactate oxidase (LO_<i>x</i>) immobilized on spherical glass beads that act as mini-reactors within a microfluidic reservoir, enabling highly specific lactate sensing. The sensor was tested in phosphate buffer saline (PBS) and artificial sweat, achieving a high linear sensitivity of 10.9 and 11.3 MHz/mM, respectively, across lactate concentrations up to 150 mM and limit-of-detection (LOD) of 8.76 mM, with validation using the gold-standard HPLC method. It demonstrated excellent specificity against common interferences, including glucose, uric acid, and several ions. Testing with a diverse group of adult volunteers confirmed the sensor's capability to detect dynamic lactate changes during exercise and reliably identify the lactate threshold (LT), underscoring its promise for applications in sports physiology. This innovative method not only offers a powerful tool for lactate monitoring but also paves the way for enzyme-specific microwave biosensors adaptable to detect a range of biomarkers by simply exchanging the target enzyme.