## 2 DNA-DNA Interactions: Strand Displacement, Base Stacking and Transient Binding
The movement of the robot was solely based on random walk, and thus, it did not require any additional energy to operate. The robot performed on average 300 steps during the cargo sorting, which is a huge improvement (one to two magnitudes) on previously reported DNA walkers that performed tasks while walking. In 2016, [PERSON] et al. [70] presented a nanoactuator design consisting of four DNA origami beams linked into a rhombus shape via flexible ssDNA joints (Figure 1d). The opening angle of the device was controlled through ssDNA lock strands of different lengths. Attaching two halves of enhanced GFP (eGFP) to the device and closing the device with short locking strands was shown to bring the halves together and restore fluorescence of the protein. A similar working principle was amplified into a large-scale reconfiguration of a 2D origami lattice by [PERSON] et al. [71]. They built a DNA accordion rack from long DNA beams connected via multiple flexible joints. The aspect ratio of the whole lattice could be controlled by adding DNA lock strands at selected positions in the structure. By applyingtoehold-mediated strand-displacement processes, multiple rounds of conformational switching could be demonstrated. In addition to using strand displacement, the needed DNA trigger can also be grown using DNA polymerase, as described by [PERSON] et al. [72]. They demonstrated the straightening and rigidifying of a deformed wireframe DNA origami having ssDNA gaps by growing a complementary gap-filling strand on site by DNA polymerase. Base stacking of blunt-ended dsDNA segments can form strong attractive interactions between different DNA nanostructures, or within a single device [73; 74]. [PERSON] et al. [74] showed that transitions between different conformational states in various DNA origami designs could be controlled by adjusting the strength of base-stacking interactions between shape-complementary parts with cation concentration or temperature. A tweezer-type structure based on the design from Reference [74], and its rapid dynamics triggered by cation concentration change were recently characterized using high-speed atomic force microscopy (AFM) [75] and small-angle X-ray scattering (SAXS) [76]. Base-stacking interactions were also used as a driving force in conveying information in large 2D DNA origami arrays [77]. The system constructed by [PERSON] et al. [77] consisted of multiple interconnected trapezoidal \"antijunction units\". Selected units were firstly locked into a defined conformation through the addition of trigger strands complementary to ssDNA regions at the edges of the units. Neighboring units would then switch into the same conformation, since new base-stacking interactions formed in the process would lower the energy of the system. This was seen to generate a cascade of conformational switching, so that all the units in the system would eventually be found in the same conformational state. DNA-DNA interactions, including base stacking, can also be used to assemble complex nanomachines from multiple DNA origami elements, as demonstrated by [PERSON] et al. [78]. They manufactured a miniature rotary apparatus analogous to F1F0-adenosine triphosphate (ATP) synthase (Figure 1e). The device was constructed from multiple tight-fitting DNA origami components by guiding the self-assembly with specific base-stacking interactions or DNA hybridization events. Based on single-particle fluorescence microscopy recordings, the devices were shown to exhibit random Brownian rotary motion. Controlling the movement of such devices with external triggers could lead to the realization of intricate DNA-based nanomachines. Recently, dynamic DNA devices were combined with plasmonic systems, whose optical responses are extremely sensitive to the relative positions and orientations of the components. [PERSON] et al. [79] assembled a metamolecule from two gold nanorods (AuNRs) and two interconnected DNA origami beams.