Fig. 6. Application of pathway control to a molecular comparator for miRNA concentrations.

a Concept of a molecular comparator of miRNA concentrations. The triangle is a symbol for a comparator element. miRNAs miR-6875-5p and miR-4634 were used for Input 1 for the comparator; miR-1246 and miR-1307-3p were used for the Input 2. The Output is the selection of the droplet division pathway, which changes depending on the difference between two initial total concentrations of miRNAs (working as inhibitor RNAs), $c_{\mathrm{AB}}$ and $c_{\mathrm{AC}}$. This concentration comparison is achieved by the two time-delay circuits as shown in (b) and (c). b Pathway 1 is selected: if the Input 1 concentration is larger than the Input 2 concentration ($c_{\mathrm{AB}}>c_{\mathrm{AC}}$), the L^†_AB cleavage delays longer than the L^†_AC because more R^†_ABi causes a longer time delay of the L^†_AB cleavage. Thus, C-droplet is divided first, and B-droplet is divided subsequently. c Pathway 2 is selected: if $c_{\mathrm{AB}}<c_{\mathrm{AC}}$, the L^†_AC cleavage delays longer. Thus, B-droplet is divided first, and C-droplet is divided subsequently.