What are nanobots?

Nanobots ŕ la Michael Crichton's Prey and other science fiction contraptions of nanoscale robots don't exist (yet).
Not to be confused with these fictional nanorobots, for medical nanotechnology researchers a nanorobot, or nanobot, is a popular term for molecules with a unique property that enables them to be programmed to carry out a specific task. These nanobots are a reality and are being actively researched and developed.
An ever-increasing number of research groups are exploiting programmable self-assembly properties of nucleic acids in creating rationally designed nanoshapes and nanomachines for many different uses. That's why one of the most actively researched areas of nanorobotics today involves DNA assembly, particularly a technique called DNA origami (see below for more details).

Nanorobotics

Let's start with some basics: Robotics can be defined as the theory and application of robots, a completely self-contained electronic, electric, or mechanical device, to such activities as manufacturing.
Scale that robot down to a few billionth of a meter and you are talking nanotechnology robotics; nanorobotics in short. The field of nanorobotics brings together several disciplines, including nanofabrication processes used for producing nanomotors, nanoactuators, nanosensors, and physical modeling at nanoscales.
Nanorobotic manipulation technologies, including the assembly of nanometer-sized parts, the manipulation of biological cells or molecules, and the types of robots used to perform these tasks also form a component of nanorobotics.
For instance, researchers have translated the autonomous movement trajectories of nanomotors into controlled surface features that brings a twist to conventional static optical fabrication systems, which establishes an early stage approach for a nanorobotics platform for nanomanufacturing.
Schematic of nanomotor lithography by using a Janus sphere motor as a self-propelled nanolens
Schematic of nanomotor lithography by using a Janus sphere motor as a self-propelled nanolens. (Image: Jinxing Li, UC San Diego)
Nanotechnology robots are quintessential NEMS (nanoelectromechanical systems) and raise all the important issues that must be addressed in NEMS design: sensing, actuation, control, communications, power, and interfacing across spatial scales and between organic and inorganic materials. Due to their size, comparable to biological cells, nanorobots have a vast array of potential applications in fields such as environmental monitoring or medicine.
However, researchers today mostly use the term nanorobot to describe molecules with a unique property that enables them to be programmed to carry out a specific task.
Which brings us to:

Nanobots in medicine

When we talk about nanobots today the reference mostly is to self-propelled nanomotors and other biodegradable nanodevices made of bio-nano components, which carry cargo to the target sites, i.e. deliver drugs to diseased cells. For instance, these nanorobots can be programmed to transport molecular payloads and cause on-site tumor blood supply blockages, which can lead to tissue death and shrink the tumor.
an enzyme-propelled nanorobot
An enzyme-propelled nanorobot: urease-coated nanotubes turn into a propulsion system in a urea-containing liquid because the enzyme breaks down the urea into gaseous products. Since the tubes always have small asymmetries, the reaction products generate a current in the fluid which propels them out of the tube like a jet. (Image: MPI for Intelligent Systems)
This active motor based drug delivery approach promises an effective and improved drug delivery compared to conventional methods. Gold nanoparticle loaded PEDOT/zinc-based artificial micromotors are tested in mouse models via oral administration. They showed excellent acid-driven, self propulsive properties with high cargo-loading capacities.
Another example are cell-like nanorobots that clear bacteria and toxins from blood. Researchers built these nanorobots by coating gold nanowires with a hybrid of platelet and red blood cell membranes. This hybrid cell membrane coating allows the nanorobots to perform the tasks of two different cells at once—platelets, which bind pathogens like MRSA bacteria (an antibiotic-resistant strain of Staphylococcus aureus), and red blood cells, which absorb and neutralize the toxins produced by these bacteria.
Unimolecular submersible nanomachines that are activated by UV light, DNA-origami based nanorobots, light-induced actuating nanotransducers, magnetic multilink nanoswimmers, etc., are some of the other technological developments that are anticipating the application of nanorobots in drug delivery.

DNA nanotechnology

There is a rich body of work on DNA use in nanorobotics ("Nanorobotic arm to operate within DNA sequence") and nanofabrication such as Ned Seeman's work at NYU (see for instance: "Researchers send DNA on sequential, and consequential, building mission" or "Scientists use DNA origami to create 2D structures" or "RNA used to control a DNA rotary nanomachine").
DNA origami and beyond
DNA origami and beyond. A) DNA octahedron that inspired development of DNA origami. B) 2D DNA origami (smiley face serves as an example). C) Hollow 3D DNA origami shapes that are folded from 2D origami sheets. D) Lattice-based 3D origami (square and honeycomb lattice). E) 3D origami with twists and bends. F) Single-stranded tile-based assembly in 2D and in 3D. G) Wireframe-based DNA structures. (© Wiley-VCH Verlag) (click on image to enlarge)

DNA origami nanorobot

Structural DNA nanotechnology, specifically the molecular self-assembly process known as DNA origami, has emerged as a versatile approach to fabricate nanodevices with complex nanoscale geometry, defined placement of molecular functionalities, and programed mechanical and dynamic properties.
Scientists already are using DNA origami technology to design and build structures on the scale of viruses and cell organelles.
DNA-Cylinder
In a first step, scientists form V-shaped building blocks using DNA-origami techniques. Determined by the opening angle a defined number of building blocks self-assemble into a gear-wheel. In a third step these gear-wheels form tubes with sizes of virus-capsids. (Image: Hendrik Dietz, Technical University of Munich) (click on image to enlarge)
DNA origami is a design technique – similar to the traditional Japanese art or technique of folding paper into decorative or representational forms – that is used by nanotechnology researchers to fold DNA strands into something resembling a programmable pegboard on which different nanocomponents can be attached.
These DNA assemblies allow the bottom-up fabrication of complex nanostructures with arbitrary shapes and patterns on a <100 nm scale.

Conclusion

With reference to fiction stories and movies: there are no nanobots yet, and won't be for a while. One of the fundamental problems to be solved for possible future molecular machinery is the challenge of controlling many molecule-sized machines simultaneously to perform a desired task.
Simple nanoscale motors have been realized over the past few years but these are systems that do nothing more than generate physical motion of their components at a nanoscale level.
To build a true nanorobot – a completely self-contained electronic, electric, or mechanical device to do such activities as manufacturing at the nanoscale – many breakthrough advances will need to be achieved (background read: Mind the gap - nanotechnology robotics vision versus lab reality).
One of them is the issue of controlling large numbers of devices, i.e. how to build and program the 'brains' of these machines. Another issue is to separate the concept of science fiction style 'thinking' robots (artificial intelligence) from a more realistic (yet still distant) concept of machines that can be programmed to perform a limited task in a more or less autonomous way for a period of time.
These tasks could range from fabricating nanoscale components to performing medical procedures inside the body. For nanoscale machinery this would require the availability of nanoscale control units, i.e. computers. Researchers in Japan already demonstrated a self organizing 16-bit parallel processing molecular assembly that brings us a step closer to building such a nanoscale processor.
By Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny. Copyright © Nanowerk