Enhanced sample preconcentration through fast capillary-driven flow in microfluidic paper-based analytical devices.*

Microfluidic paper-based analytic devices (µPADs) for early diagnosis have gained significant attentions in the last decade. Due to low sensitivity of µPADs, several preconcentration mechanisms for high enrichment factor have been developed, but still have critical nuisances such as complex fabrication and long process times. While conventional µPADs based ion concentration polarization (ICP) preconcentrator has used only passive capillary imbibition, in this work, we introduced air gap between the stacked paper layers for fast flow generation to demonstrate fast and enhanced preconcentration by ICP in µPADs. First, we developed air-gap forming doubly stacked µPADs and demonstrated an enhanced preconcentration factor of up to 135-folds in 10 minutes. Second, we analyzed theoretically why the fast flow can be formed and the preconcentration time can be shortened in the air-gap forming doubly stacked µPADs. Thus, this air-gap forming doubly stacked µPADs with reduced sample loading time and increased sample volume would be utilized as point-of-care testing devices to detect low abundant targets in shorter operation time.