Distribution of biotin at a depth of 3-10 nm from the surface of electrospun polylactic acid (PLA) fibers has been assessed by X-ray photoelectron spectroscopy (XPS) and compared to the distribution predicted by bulk calculations. Biotin concentration in the outer 3-10 nm of the fibers is greater than the predicted if biotin was distributed uniformly within the fiber. Availability of biotin for streptavidin binding at the surface of the fibers has been determined via a competitive colorimetric assay. Availability of biotin at the fiber surface was also determined to be greater than predicted by calculations assuming uniform biotin distribution. Additionally, the segregation of biotin to the exterior of the fiber increases disproportionately with increasing overall biotin concentration in the fibers. Confocal microscopy has been used to confirm capture of streptavidin, primary antibodies and fluorescence labeled secondary antibodies on PLA/biotm fibers. (c) 2007 Elsevier Ltd. All rights reserved.