## 3 DNA Origami Devices with Molecular Interactions  
In this section, DNA devices with dynamic properties mediated by molecular interactions (other than DNA-DNA) between the device and other molecules in the solution are discussed. The scale of the dynamic devices ranges from tools designed for measuring or detecting a specific molecular interaction to aptamer-functionalized objects for nanorobotics, computing, and drug delivery. An early example of dynamic DNA origami devices used in molecular detection is the single-molecule beacons presented by [PERSON] et al. in 2011 [84] (Figure 2a). These \"DNA origami pliers\" or \"DNA origami forceps\" consist of two rigid beams connected via a flexible DNA crossover region, which allows the arms to rotate relative to each other. When a target such as protein, metal ion, or human microRNA (miRNA) binds to both arms of the device, the arms are locked into a parallel orientation. Thus, single-molecule binding events are amplified into a major conformational change that can be detected using transmission electron microscopy (TEM) or AFM. Other neat examples showing the potential of DNA origami-based measurement tools are the various studies carried out with DNA origami devices and nucleosomes [85; 86; 87]. [PERSON] et al. [85] and [PERSON] et al. [86] both introduced measurement devices with similar hinge-like designs, where two DNA origami arms were joined together at one end via a flexible ssDNA hinge (Figure 2b). The system presented by [PERSON] et al. was initially introduced as a static tool for placing molecules at set distances with extreme precision [88]. In the nucleosome studies, the molecular interaction under interest changed the opening angle of the device, which could then be used as a measure of the interaction  
Figure 1: DNA origami mechanics via DNA–DNA interaction. (**a**) A DNA nanovault that displays cargo when opened via strand displacement [61]; (**b**) DNA origami nanomechanics [67]; (**c**) A robot that picks up cargo and delivers it to a goal on top of a DNA origami [42]; (**d**) A DNA origami actuator; movement on the left (driver) side is mirrored to the right side [70] (**e**) A DNA origami rotary apparatus constructed from tight-fitting components [78]; (**f**) Super-resolution imaging with DNA origami by taking advantage of transient DNA binding [81] (**a**) is reproduced with permission from the authors of [61], published by Nature Publishing Group, 2017; (**b**) is reproduced with permission from the authors of [67], copyright National Academy of Sciences 2015; (**c**) is reproduced with permission from the authors of [42], copyright The American Association for the Advancement of Science, 2017; (**d**) is reproduced with permission from the authors of [70], published by Nature Publishing Group, 2016; (**e**) is reproduced with permission from the authors of [78], published by The American Association for the Advancement of Science, 2016; (**f**) is reproduced with permission from the authors of [81], copyright Nature Publishing Group, 2014.  
strength. The device was used to study both nucleosome unwrapping at different ionic strengths [86] and the strength of attractive interactions between two nucleosomes [85]. [PERSON] et al. [87] used their device for probing various properties of nucleosome-DNA interaction, such as nucleosomal end-to-end distance, nucleosome conformation, and nucleosome stability. The previous examples of molecular measurement devices all share a relatively similar working principle: molecular binding or interaction under interest converts the device into a discrete, relatively immobile orientation, which is then characterized. In contrast to this, [PERSON] et al. [89] measured compressive depletion forces in a solution with a dynamic device that constantly fluctuated between an open (uncompressed) and a closed (compressed) state. Increased depletion forces caused by molecular crowding agents, particularly by poly(ethylene-glycol) (PEG), were observed to shift the dynamics of the device more toward the closed state.