(A) Microscope image of a 3-μm-thick layer of low-permeability Parylene C (PC) membrane deposited on the surface of the LFA ruler. Scale bar, 50 μm. (B) The ink advancement distances in different LFA rulers with/without PC membrane, pushed by oxygen generated as a result of different numbers of PtNPs (0, 5.6 × 104; 0, 5.6 × 104, respectively) reacting with 30% H2O2. Illustrations on both sides are the enlarged views of the black dotted rectangles. Under a certain angle of illumination, the label on the device without PC membrane is gray; the label on the device with PC membrane is colored.