  Surface assays, such as ELISA and immunofluorescence, are nothing short of
ubiquitous in biotechnology and medical diagnostics today. The development and
optimization of these assays generally focuses on three aspects: immobilization
chemistry, ligand-receptor interaction and concentrations of ligands, buffers
and sample. A fourth aspect, the transport of the analyte to the surface, is
more rarely delved into during assay design and analysis. Improving transport
is generally limited to the agitation of reagents, a mode of flow generation
inherently difficult to control, often resulting in inconsistent reaction
kinetics. However, with assay optimization reaching theoretical limits, the
role of transport becomes decisive. This perspective develops an intuitive and
practical understanding of transport in conventional agitation systems and in
microfluidics, the latter underpinning many new life science technologies. We
give rules of thumb to guide the user on system behavior, such as advection
regimes and shear stress, and derive estimates for relevant quantities that
delimit assay parameters. Illustrative cases with examples of experimental
results are used to clarify the role of fundamental concepts such as boundary
and depletion layers, mass diffusivity or surface tension.
