Finite element models of tissue engineering scaffolds are powerful tools to understand scaffold function, including how external mechanical signals deform the scaffold at the meso- and microscales. Fiber geometry is needed to inform finite element models of fiber-based tissue engineering scaffolds; however, the accuracy and utility of these models may be limited if they are informed by non-hydrated geometries. Scanning electron microscopy and confocal microscopy, coupled with Fourier analysis of the resulting images, were used to quantify how hydration alters fiber geometry in electrospun collagen and polycaprolactone (PCL) scaffolds. The results also quantify how image size affects fiber geometry. Hydration is demonstrated to increase fiber tortuosity, defined as the ratio of actual fiber length:end-to-end fiber length. For collagen scaffolds, hydration increased the mean tortuosity from 1.05 to 1.21, primarily from large similar to 2- to 10-fold) increases in smaller (<40 mu m) wavelength amplitudes. For PCL fibers, the mean tortuosity increased from 1.01 to only 1.04, primarily from modest similar to 2-fold) increases in larger (>100 mu m) wavelength amplitudes. The results demonstrate that mechanical simulations of electrospun scaffolds should be informed with hydrated scaffold geometries of at least 200 mu m scale, in order to capture geometrical effects associated with fiber straightening. Published by Elsevier Ltd. on behalf of Acta Materialia Inc.