4D-Printed Redox-Responsive Needle-Flow Reactors Enabling Online Quantitative Profiling of Living Rat Brain Extracellular Lactate and Glucose

Various assays are performed based on the use of oxidase-based derivatization schemes coupled with chromogenic or fluorogenic reagents and spectrometric determination. To enable online quantitative profiling of living rat brain extracellular glucose and lactate without requiring a spectrophotometer or fluorometer, a flow reactor with a redox-responsive needle is fabricated using the digital light processing four-dimensional printing (4DP) technique and photocurable resins incorporating 2,2'-(ethylenedioxy)diethanethiol (EDT). Upon exposure to hydrogen peroxide (H₂O₂), oxidation of the thioether groups of the copolymer increases the polarity and swelling of the EDT-incorporated layer, inducing [H₂O₂]-dependent bending of the needle. Coupling this device with glucose- and lactate-oxidase-mediated reactions facilitates the reliable determination of glucose and lactate. Furthermore, the needle returns to its original state after treatment with a sodium borohydride solution, demonstrating the reversible, redox-responsive shape programming for repeated measurements. Integration of the microdialysis sampling apparatus, oxidase-based derivatization schemes, and 4D-printed redox-responsive needle-flow reactors into an online automatic analytical system achieves the method's detection limits of 0.3 μM for glucose and 0.4 μM for lactate. To verify the reliability and applicability of this method, we perform analyses of human urine and sweat, fetal bovine serum, and rat plasma with their spike analyses, compare the results with those obtained from assay kits, and profile living rat brain extracellular glucose and lactate levels with triggered neuronal depolarization. The findings demonstrate that 4DP technologies enable the efficient fabrication of redox-responsive analytical devices and enhance the applicability of conventional enzymatic assays for online quantitative chemical analysis.