Directed self-assembly of microcomponents enabled by laser-activated bubble latching.

Directed assembly methods, such as robotic pick-and-place, have been the essential manufacturing process for constructing complicated multicomponent systems on large scales. These include methods that require some form of energy input into the system, such as the robotic or manual assembly of buildings and cars. Once the sizes of the components reach the submillimeter scale, these serial assembly methods begin to become prohibitively slow and expensive because of the need for high positioning accuracy and the presence of obstructive adhesion forces. Self-assembly techniques that are driven by a system’s tendency to reach an equilibrium state, termed static self-assembly, represent an alternative that overcomes the limitations of serial assembly on the microand nanoscales. 4 Surface tension forces are frequently used as a latchingmethod for self-assembly on the microscale. 6 In such systems, a liquid gas interface will tend to a low-energy configuration by minimizing its surface area. In early work, components were placed in a large water reservoir and assembled on the basis of the geometric properties or wettability characteristics of their faces. 9 Subsequently, droplet latching techniques were developed as a more targeted assembly approach. Small liquid droplets of solders, 17 resin, and water 21 were used to make sitespecific connections between components. These two-phase methods have achieved highly accurate alignment, alignment in specific orientations, 3D structures, 12 and the formation of electrical networks. Using large substrates as receptor platforms, assembly through surface tension techniques can be parallel and therefore potentially faster and cheaper than directed assembly approaches. Thus far, surface tension has primarily been employed to achieve self-assembly, but it has several drawbacks. Assembly is achieved probabilistically; therefore, the assembly yield is not yet as high as that of deterministic approaches. Additionally, in most cases the final structure’s shape and size are critically constrained by the initial design and the number of subelements used in the experiment, making arbitrary structures difficult to generate. Recent directed fluidic assembly techniques such as railedmicrofluidics and dynamically programmablefluidic assembly, 26 in which the fluidics is themain force for manipulating components, exploit microscale flows to circumvent adhesion problems of other microscale directed assembly methods and provide the user with more control than self assembly techniques in the generation of structures. Unfortunately, the latches used in these cases, along with the surface tension-based methods mentioned previously, all share characteristics that restrict the level of control given to the user. For example, the latches must be predefined during the fabrication process and are permanently on during use. This restricts the assembly of individual pieces to linking at specific locations. In addition, the quasi-permanent nature of the latches restricts the ability to reconfigure structures once they are assembled. Some work has been done to add a switching capability to soldering sites using microheaters. In that work, the switching mechanism was integrated on a large substrate, and there was no switching capability on a componentto-component level.