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A controllable molecular system operated by specific DNA strands has been realized for the construction of DNA‐based nanomachines. DNA molecular machines are operated by exchanging specific DNA strands to create complex movements. For this purpose, an additional sequence called a “toehold” is attached to the end of the DNA strand. When a DNA strand that is fully complementary to a toehold‐containing strand is added, the initial toehold‐containing strand is selectively removed by strand displacement. The thermodynamic stabilization energy works as “fuel” during hybridization to provide the mechanical motion of DNA machines. Using this strategy, DNA tweezers that perform open–close motions were constructed (Figure 1.2) [8]. Two examples of a DNA walking device have been created: a DNA walker with two legs that can control its direction of motion and a DNA motor that can move forward autonomously by cleavage of a DNA‐nicking enzyme [16].

Seeman and coworkers developed molecular machines that are capable of rotating 180° at the ends of two adjacent dsDNAs, termed PX‐JX₂ devices, by hybridization and removal of DNA strands [9]. They also successfully captured triangular DNA nanostructures using the sequence specificity of four single‐stranded ends by introducing two devices onto DNA origami and rotating each triangle [85]. Using this method, four types of PX‐JX₂ patterns could be operated by specific DNA strands, and four different types of nanostructures were selectively captured.