Progress in the Synthesis of Colloidal Machines.

For the past decade, the field of colloidal science has expanded the collection of colloidal particles to include an entire library of subunits that can be isotropic or anisotropic in terms of structural morphology or chemical composition. Using anisotropic subunits, the field has assembled a variety of static and dynamic structures. For this Account, we use the umbrella term "dynamic colloids" to describe subunits capable of movement, shape-shifting, or any other type of action in response to a stimulus and "static colloids" to describe those that are unresponsive to such stimuli. We view dynamic colloids as an access point to colloidal machines, a unique and emerging subfield of machines, and colloidal science. The assembly of dynamic subunits into colloidal machines differs from traditional self-assembly only in the final structures assembled, not the methods used. Dynamic assemblies have the capacity to interact with their environment in ways that traditional anisotropic self-assemblies do not. Here, we present the current state of the field of colloidal science toward the introduction of the next wave of colloidal machines. Machines are ubiquitous in nature and synthetic systems, governing every aspect of life. In mechanics, a machine is a device that transmits or modifies force or motion. In biology, nature's machines such as kinesin or ATP synthetase are essential to life. In the synthetic realm, molecular machines and nanomachines, recognized with the Nobel prize, include diverse systems, such as molecular rotors and elevators fabricated using bottom-up synthetic methods. On the microscale, microscopic motors based on microelectromechanical systems (MEMs) have been achieved via top-down methods such as micromachining. On the colloidal scale, machines are conspicuously absent due, in part, to the difficulty in navigating combinatory design spaces. We view colloidal machines (100 nm to 10 μm) as the next line of miniaturization in machines. Due to the bottom-up fabrication methods generally used in creating dynamic colloids, one can achieve complexity at a smaller scale than possible with top-down approaches. The introduction of colloidal scale machines would bridge the gap between the microscopic world with its macroscopic counterparts, the nanoworld with its molecular machines, and the biological world with nature's machinery. Reported colloidal machines to date are apparatuses that consist of multiple components of a single composition of dynamic subunits that come together to perform some work. The next step toward complex colloidal machines is systems containing