## 4 DNA Origami Devices Triggered by External Stimuli or Multiple Interactions
[107] demonstrated that a folded bracket-shape DNA origami functioned as a nanoscopic force clamp for probing multiple different interactions. In this nanodevice, the system of interest (red rectangle, Figure 3d) was linked between two immobile attachment points via entropic ssDNA springs that exert a constant force on a piconewton (pN) scale over time. The force can be tuned by controlling ssDNA strand length, i.e., the entropic force can be increased with reducing nucleotide amount. This leaves ssDNA reservoirs on both sides of the clamp (as seen in Figure 3d). Conformational transitions of the system of interest can be monitored via single-molecule FRET. With this device, switching between two different states of a FRET-pair-equipped four-way Holliday junction was resolved. The authors also demonstrated the sensitivity of the system by characterizing protein-induced DNA bending. Very recently, related to abovementioned force spectroscopy, [PERSON] and co-workers [108] reported a DNA origami tension probe (DOTP) capable of depicting the traction forces generated by living cells. Various DOTP combinations were employed to map the forces applied by human blood platelets during initial adhesion and activation. Traction forces with piconewton (pN) resolution were measured utilizing tension-to-fluorescence transduction upon unfolding the DNA hairpin that was incorporated into the system. In addition to the examples above, external electromagnetic fields can be used to manipulate DNA origami movement. Recently, [PERSON] and co-workers [43] displayed a truly dynamic DNA origami platform for a nanoscale (up to 400 nm) robotic arm controlled by electric fields. The system was composed of a rigid DNA origami plate equipped with a 25-nm-long six-helix-bundle (6 HB) arm attached via ssDNA scaffold crossovers (see the close up in Figure 3e). The flexible joint allowed stochastic switching of the arm due to transient binding, which was detected from FRET signals generated by the donor fluorophore on the tip of the arm and two acceptor dyes mounted on the rectangular base plate. Electrically controlled movement of the robot arm (angular movement up to 25 Hz) was measured when the system was mounted at the center of a cross-shaped electrophoretic chamber with two perpendicular fluid channels. The electric field was applied by two pairs of electrodes inserted in the reservoirs at the ends of the channels. Furthermore, the ability to move inorganic nanoparticles, e.g., AuNRs, using the robotic arm was demonstrated. DNA origami polarizability can also be used to direct and trap DNA origami via dielectrophoresis in a non-uniform electric field, as shown by [PERSON] et al. [109] and [PERSON] et al. [110]. [PERSON] et al. [111] introduced yet another way of controlling DNA origami movement by employing external magnetic fields. Three quasi-analogous nanostructures, i.e., lever, rotor, and hinge systems (Figure 3f), having diverse angular movement paths, were demonstrated. All constructs were assembled from three components: a base platform, a stiff 56-helix-bundle rotor arm equipped with a micromagnetic bead on the free rotating end, and a ductile pivot anchoring the rotor to the base platform via biotin-streptavidin affinity. The concept allowed sustained rotational motion (up to 2 Hz), the capability of operating up to 80 pNnm of torque, and a definite control (\(\pm 8^{\circ}\) resolution) over the angular conformation.