Fig. 4.

Reversible hierarchical assembly. (A) Two monovalent droplets (T₁ and T₃) and one divalent droplet (T₂) with patches bearing preprogrammed binding specificity (α on T₁, α′ and β on T₂, β′ on T₃) first assemble into a linear trimer. The linker strands are then added to bridge the handles on T₁ (red) and T₃ (green), resulting in the folding of the linear trimer into a triangle. The displacing strands are introduced to unfold the triangle via toehold strand displacement (see insets). Representative confocal images of (B) the linear trimers formed in the first step (trimer yield 59% after 142 h, SI Appendix, Table S6), (C) the triangles formed after adding the linkers with an ∼95% yield (SI Appendix, Fig. S9), (D) the unfolding of a triangle after adding the displacing strands with an ∼80% yield (SI Appendix, Fig. S10). (Scale bars, 10 μm.)