Toward achieving painless injections and other microfluidic applications, we micro-fabricated conically tapered needles of micron dimensions. The relationship between pressure drop and flow rate through microneedles was experimentally quantified as a function of fluid viscosity and microneedle length, diameter, and cone half angle. At Reynolds numbers <= 100, dimensionless pressure drop (2 Delta P/pv(2)) sharply decreased with increasing Reynolds number, indicating the importance of viscous forces. At larger Reynolds numbers, the flow was almost inviscid, as indicated by a weak dependency of dimensionless pressure drop on Reynolds number. Numerical simulations showed good agreement with experimental data and predicted that flow through conically tapered microneedles is primarily controlled by the diameter and taper angle at the microneedle tip. A characteristic feature of flow through conically tapered microneedles is a favorable a-vial pressure gradient that accelerates fluid through the microneedle, thus inhibiting growth of the viscous boundary layer on the microneedle wall. (c) 2005 American Institute of Chemical Engineers.