Development toward a novel integrated tear lactate sensor using Schirmer test strip and engineered lactate oxidase

L-Lactate is an important biomarker for clinical diagnostics and fitness monitoring that shows oxygen deficiency or elevated salt concentrations due to pathophysiological conditions or intensive exercise. To avoid current painful and inconvenient blood testing techniques of measuring lactate levels, tears represent a non-invasive and potential sampling medium. However, lactate oxidase (LOx) is susceptible to the fluctuation and deficiency of oxygen, leading to inaccurate lactate measurements in tears. By utilizing a recently published, protein engineered LOx that eliminates the oxygen interference, a novel tear lactate (TL) sensor was assembled and tested. Screen-printed sensors were prepared with a redox solution containing the engineered LOx, and a novel tear sampling component made of Schirmer's test strip was attached to absorb the simulated tear fluid samples. The dynamic range of the TL sensor was found to be 0.39-16.60 mM in simulated tear fluid, satisfying the clinically relevant range of TL. In addition, the proposed TL sensor was found to be insensitive to ascorbic acid, acetaminophen, and uric acid, which are common interfering compounds in tears, and showed no sign of degradation after 8 weeks of shelf life study. The proposed sensor exhibited potential usefulness in providing an alternative noninvasive method of measuring lactate and in calibrating the continuous lactate contact lens.