Lateral flow assays (LFAs) utilize the capillary flow of liquids for simple detection of analytes. While useful for spontaneously wicking samples, the capillary flow inherently limits performing complex reactions that require the timely application of multiple solutions. In this thesis, we introduce a new technique to control capillary flow on paper by simply imprinting roadblocks on the flow path with water-insoluble ink and utilizing the gradual formation of a void between a wetted paper and a sheath polymer tape to create timers. Timers are drawn at strategic nodes to hold the capillary flow for the desired period and thereby enable multiple liquids to be introduced into multi-step chemical reactions following a programmed sequence. By tuning the timer either geometry or material property of the water-insoluble ink and polymer tape, the capillary flow delay is tunable on paper in a scalable manner from tens of seconds to minutes and even hours as desired. The ability to sequentially deliver different reagents into a reaction via programmable timers imprinted on paper makes it possible to automate multi-step assays, that otherwise could only be performed in laboratories or with manual intervention. Using our technique, we have developed multiple point-of-care (POC) diagnostic devices to convert the current COVID-19 assays into POC form factors, such as (1) chemically amplified multiplex antigen test of severe acute respiratory syndrome–coronavirus 2 (SARS-CoV-2), influenza A, and B viruses in saliva, (2) paper-based dipstick enzyme-linked immunosorbent assay (ELISA) that reports SARS-CoV-2 antibody titers (IgM/IgG antibodies to nucleocapsid and spike protein) in a digital manner without the need for an external instrument or operator, and (3) POC toolkit for multiplex molecular diagnosis of SARS-CoV-2 and influenza A & B viruses from saliva.