A Microcontroller-Integrated Multichannel Time Detector for Paper-Based Analytical Devices — Applications to Viscosity Measurements in Saliva Analysis and Protease-Activity Assays

We designed and developed a microcontroller-integrated multichannel time detection system that uses a microfluidic paper-based analytical device (µPAD) to measure liquid viscosity. This detection system is equipped with ten detectors to enhance the throughput of the measurements. The µPAD utilizes capillary action to determine viscosity based on the flow time of a liquid sample. A conductivity detection system begins counting the flow time when a sample is introduced and stops the count when the sample reaches the detection electrode. Sodium chloride (NaCl) was pre-deposited in the detection channel of the µPAD to enhance the conductivity of non-conductive samples, and Grade 1 CHR chromatography paper was selected as the optimal vehicle for substrates after comparing various channel widths, paper types, and channel lengths. The device demonstrated a linear correlation between flow time and viscosity for bovine serum albumin (BSA) and glucose solutions, which validates the theoretical model. The time readout measured protease activity when gelatin was used as a substrate and revealed an activity order of bromelain > papain > trypsin. The practical applicability of this system was further confirmed by testing real saliva samples, which demonstrated that the viscosity of saliva decreases rapidly after collection. Moreover, the results indicate that saliva viscosity was increased during extended durations of exercise. Overall, this µPAD system provides a simple, low-cost, and portable solution for viscosity measurement, with potential applications in clinical diagnostics and field measurements.