The fluorescent signals were generated as an effect of enzyme inhibitors. Roliparm (Fig. 4B)/roflumilast (Fig. 5B) spotted per area shows that the decrease in FRET signals is directly related to the increase in PDE4B inhibitors spotted on same spot surface area. Individual acquired images clearly verify the variance for roliparm (Fig. 4B)/roflumilast (Fig. 5B) printed per spot with respect to the printing-output settings (C value). In order to classify FRET signals and their corresponding C values; fluorescent images were categorized as S1 (2, 12, 22, 32, 42, 52, 62, 72, 82), S2 (4, 14, 24, 34, 44, 54, 64, 72, 84), S3 (6, 16, 26, 36, 46, 56, 66, 76, 86), and S4 (8, 18, 28, 38, 48, 58, 68, 78, 88). Figure 4C and Fig. 5C represent the fluorescent signal generated on paper, where the X-axis represents the C value and the Y-axis represents the FRET signal. The graph clearly shows a linear reduction in the FRET signal, confirming the PDE4B enzyme inhibitory activity on paper. As previously mentioned, to perform the PDE4B inhibitory assay, 2.79 × 10−16 mole of cAMP and 2.81 × 10−13 mole of PDE4B were printed on parchment paper. It was found that 0.29 × 10−13 mole to 2.72 × 10−13 mole of roliparm and 0.04 × 10−13 mole to 2.5 × 10−13 mole of roflumilast were printed on the surface. Reproducibility of FRET signal on the parchment paper was confirmed by measuring the FRET signals of 3 different sets using three different inkjet-printed parchment papers with same parameters. A graph was plotted to corroborate the volume of roliparm/roflumilast, mole printed per spot area, and their corresponding FRET signals generated on paper. As shown in Fig. 4D and Fig. 5D, the X-axis represents mole of roliparm/roflumilast printed per spot area, and the primary Y-axis represents the volume of roliparm/roflumilast printed per spot area and the secondary Y-axis represents the resultant FRET signals produced on parchment paper. Error bars in the plot represent the standard deviation of three independent measurements. The inhibitory mole 50 (IM50) of roliparm and roflumilast, which is required to reduce the PDE4B activity by half, was determined as 2.46 × 10−13 mole and 1.86 × 10−13 mole, respectively (supplementary Table S3).