Figure 1. Overview of DNA droplets as a hybrid from DNA nanotechnology and liquid-liquid phase separation (LLPS) studies. A) DNA droplets, micro-scale condensates of DNA nanostructures (DNA motifs) self-assembled via sticky ends (SEs). A Y-shaped DNA motif (Y-motif) is a branched nanostructure of three single-stranded DNAs (ssDNAs) hybridized to form the branching stems. At the end of each branch, a single-stranded SE protrudes. Two basic features are highlighted: (i) Programmable interactions. DNA droplets favor sequence-specifically selective interactions as encoded in the SE sequences. (ii) Programmability in mechanical properties.The number of the SEs in a single DNA motif, serving as the valency, can be designed to determine the macroscopic rheological properties, including diffusion coefficient and viscoelastic behavior. Adapted with permission. [29]

We demonstrate remote-controlled microflow using photoresponsive DNA fluid. The DNA fluid was fabricated through the self-assembly of branched DNA motifs, nanostructures of base-paired single-stranded DNAs (ssDNAs): Three ssDNAs hybridized in the stem region to form a branched motif possessing sticky ends (SEs); via the hybridization of SEs, the branched DNA motifs self-assembled into DNA fluid, which was micrometer-scale condensates. The motifs were equipped with a photoresponsive capability by introducing azobenzene (Azo), well-studied photoisomerizable compound44, in the SEs. The photoswitchable Azo isomerization enabled the reversible association/dissociation between branched DNA motifs, leading to photoreversible fluidity regulation via gel–liquid–dispersed state transition. To design an energy-transducing system, we exploited the following sequence-specific programmability of DNA. By cross-linking the photoresponsive DNA motif with a branched DNA motif possessing nonphotoresponsive orthogonal SEs, we achieved photocontrollable “transporter” DNA and “cargo” DNA fluids; the photoresponsive DNA fluid hydrodynamically performed mechanical actions upon the nonphotoresponsive DNA fluid by transducing light energy. Notably, we discovered multiple modes in the generated fluid’s mechanical action as a function of the applied temperature and the Azo insertion site in the SE. Each flow mode was characterized as a single-peak-shaped profile in the temperature-dependent flow mobilities. The highest and lowest mobilities were obtainable in moderate and lower and higher temperature ranges, respectively. This anomalous profile was analogous to the reentrant phase behavior of DNA microstructures41,50,51, which behave as solid-like only in a moderate temperature range and as liquid-like in the lower and higher temperature ranges. Following thermodynamic arguments on the observed reentrancy, we describe the observed flow-mobility behavior as a manifestation of a transient non-equilibrium state in the SEs binding stability. We also explain the mode-changeability of the mechanical actions with experimental support by explaining the role of the Azo insertion site in the SE in determining the degree of the nonequilibrium in the binding stability.