Top-down and bottom-up design

Top-down and bottom-up are strategies of information processing and knowledge ordering, mostly involving software, and by extension other humanistic and scientific system theories (see systemics).

In a top-down approach an overview of the system is first formulated, specifying but not detailing any first-level subsystems. Each subsystem is then refined in yet greater detail, sometimes in many additional subsystem levels, until the entire specification is reduced to base elements. A top-down model is often specified with the assistance of "black boxes" that make it easier to manipulate. However, black boxes may fail to elucidate elementary mechanisms or be detailed enough to realistically validate the model.

In a bottom-up approach the individual base elements of the system are first specified in great detail. These elements are then linked together to form larger subsystems, which then in turn are linked, sometimes in many levels, until a complete top-level system is formed. This strategy often resembles a "seed" model, whereby the beginnings are small, but eventually grow in complexity and completeness. However, "organic strategies", may result in a tangle of elements and subsystems, developed in isolation, and subject to local optimization as opposed to meeting a global purpose.

Nanotechnology

Top-down and bottom-up are used as two approaches for assembling nanoscale materials and devices. Bottom-up approaches seek to have smaller (usually molecular) components arrange themselves into more complex assemblies, while top-down approaches seek to create nanoscale devices by using larger, externally-controlled ones to direct their assembly.

The top-down approach often uses the traditional workshop or microfabrication methods where externally-controlled tools are used to cut, mill and shape materials into the desired shape and order. Bottom-up approaches, in contrast, use the chemical properites of single molecules to cause single-molecule components to automatically arrange themselves into some useful conformation. These approaches utilize the concepts of molecular self-assembly and/or molecular recognition. See also Supramolecular chemistry.

Such bottom-up approaches should, broadly speaking, be able to produce devices in parallel and much cheaper than top-down methods, but could potentially be overwhelmed as the size and complexity of the desired assembly increases.