# 3D DNA Nanostructures: The Nanoscale Architect
## 3 Dynamic Mechanical Control and Motion
### Configuring Physical States of DNA Origami-Scale Systems  
An important design feature for implementing and controlling actuation in DNA nanostructures will be to relate the degrees of motion to some easily quantifiable, direct stimulus. A most straightforward solution is for mechanical deformations to be determined by individual ssDNA within the structure, so that simple metrics, such as its presence or length, induce predictable mechanical changes for the entire structure.  
One strategy to do so as shown by the [PERSON] and [PERSON] groups has been to tether points on two distant regions of the structure with the tension of a suspended ssDNA \"spring\", so the length of the string modulates the bending of the structure [61; 62] (Figure 3A). An extension of this concept also by the [PERSON] group demonstrated more complex mechanisms that could slide and pivot in a fashion very similar to human-scale machines such as crank-sliders and Bennett linkages [54]. Mechanically, the elasticity of DNA [63] plays a major role in coercing the energy landscape of DNA origami assemblies into constrained physical configurations. The [PERSON] group demonstrated a more modular strategy for implementing curved DNA origami bundles by consolidating regions for inducing curvature such that its mechanical deformation can be isolated to specific strands. A bundle is designed with segmentation such that it becomes a series of extruding _piers_ and gaps, while continuous only through a thin backbone. Each pair of adjacent peers can then be tethered by a ssDNA linker of selected length corresponding to the desired curvature [64] (Figure 3B). A DNA nanostructure with cascading transformations was also demonstrated by the [PERSON] group. A small structural DNA unit called an _anti-junction_ was designed thatcould transform from a horizontally elongated form to a vertically elongated form in response to a ssDNA trigger. These units could be chained in a lattice, where subsequent units could be triggered by previous units in their series. A single trigger to the first unit would then produce a domino-like transformation of the structure observable by AFM [65] (Figure 3D). In work by the same group, modular units of DNA origami contained hairpin loops built into their strand sequences. Invading ssDNA could bind to the loop as a toehold and displace an existing strand of the structure, which stretched the helix using the excess bases originally hidden in the loop. Tailored placement of this operation could induce programmable transformation of the same structure into elongated, twisted, and curved variants [66].  
In demonstrating more DNA analogues of simple machines, the [PERSON] group showed the design of both rotating and sliding gear mechanisms [53] (Figure 3C) by designing DNA nanostructures that \"walked\" with respect to a central gold nanorod acting as a cyclic track. The operation was akin to traditional DNA walkers. Two DNA origami bundles were placed in parallel with a gold nanorod via distinct columns of sequences protruding from each side of each hexagonal DNA bundle. By consuming fuel, the extended sequences that encircle the DNA bundle could sequentially bind to complementary sequences extending from the central gold nanorod to induce rotation of the DNA nanostructure in place or with respect to the gold nanorod.