research highlights

nanotechnology Robotics on the nanoscale

Nanoscale robots can be programmed to the feet (Gu et al., 2010). Additionally, as the walk a DNA origami track. walker progresses, its three hands can pick Tiny, programmable robots: it sounds like up nanoparticle cargo from three different something right out of science fiction. Yet DNA machines flanking the track. “You can scientists are beginning to engineer nano- think of the walker as the chassis of a car scale machines that can manipulate the running through an automobile assembly environment at the molecular level. One plant; it picks up nanoparticles in the same day, complex molecular assemblies may be way that workers standing by the car would made in nanofactories, and in vivo nanode- add a windshield wiper, a steering wheel, a vices that kill cancer cells or repair tissues radiator,” explains Seeman. Depending on may be a mainstay of medicine. how the DNA machines are programmed, But first, a solid foundation of nanotech- eight different products are possible. nology tools and methods must be devel- One challenge both groups had to oped, upon which these now fantastical address was how to actually detect the applications can grow. Currently, DNA is the movement of the walker; Stojanovic and darling of a segment of the nanotechnology colleagues achieved this with super-res- community owing to its well-defined speci- olution fluorescence imaging and atomic ficity rules and the ease of working with it. force microscopy, and Seeman’s team used The field has rapidly progressed as methods atomic force microscopy to track the walk- have been developed to create DNA origami er and transmission electron microscopy (two- and three-dimensional folded DNA to verify assembly of the products. Both nanostructures), DNA walkers (mobile mol- groups have immediate goals to extend the ecules powered by DNA hybridization) and distances the DNA walkers can travel. In various nanomechanical devices made from the future, Seeman hopes to scale up pro- DNA. Two independent groups recently duction of the assembly line and make it integrated these technologies to construct repeatable, so that it can be used to make simple nanoscale robots. useful products. Stojanovic is keen to A collaborative team led by Milan investigate whether nanoscale robots can Stojanovic of Columbia University, Erik be programmed to interact with natural Winfree of the California Institute of environments. However, “before we move Technology, Hao Yan of Arizona State to higher complexity,” notes Stojanovic, © 2010 Nature America, Inc. All rights reserved. All rights Inc. America, Nature © 2010 University and Nils Walter of the University “we have to improve the individual steps of Michigan, showed that a DNA walker of assembly and how we observe it.” can move along a programmed DNA ori- Another long-term goal is to even further gami track while following instructions reduce the scale on which these robots act. to turn and stop by reading the landscape “It’s hard to imagine anything more con- (Lund et al., 2010). The walker consists of venient [than DNA],” notes Seeman. But a streptavidin molecule with three deoxy- whereas the large size of DNA allows it to ribozyme, or catalytic DNA, legs, which encrypt a lot of information, alternative catalyze strand shortening of a DNA sub- chemistries will likely need to be developed strate on the origami surface. After the leg to do work on the molecular scale. dissociates from the product it looks for a What nanoscale robotics technology will fresh substrate; as such, the walker motors be able to do in the future is limited only by down the origami track, stopping when it researchers’ imaginations. “The key ques- binds to uncleavable DNA strands. The tion,” sums up Stojanovic, “is how to learn walker could take up to 50 steps, covering how to program them to do things and see a distance of about 100 nanometers. where we could really go.” Nadrian Seeman of New York University Allison Doerr and his team showed that a four-footed RESEARCH PAPERS walker can move along an origami track as Gu, H. et al. A proximity-based programmable DNA they add ‘anchor’ strands that join the walk- nanoscale assembly line. Nature 465, 202–205 er’s single-stranded DNA feet to the origami (2010). Lund, K. et al. Molecular robots guided by surface and ‘fuel’ strands that preferentially prescriptive landscapes. Nature 465, 206–210 bind to the anchor strands, thus releasing (2010).

490 | VOL.7 NO.7 | JULY 2010 | nature methods