Gel point and fractal microstructure of blood clots. (Left) A 3-dimensional reconstruction of a hydrated fibrin network obtained using fluorescent confocal microscopy. Fractal analysis has long been used to characterize the growth of branched networks like this. (Image from A. Stout, K. Gersh, and J. Weisel, Department of Cell and Developmental Biology, School of Medicine, University of Pennsylvania, Philadelphia.) (Right) Fourier Transform mechanical spectroscopy of clotting blood. Oscillatory shear measurements of the ratio of viscous and elastic responses, or loss tangent (tan δ) of coagulating blood. The initial, preincipient clot response is characteristic of a viscoelastic fluid, with increasing frequency of oscillation causing tan δ to decrease. Tan δ becomes frequency independent as the incipient clot is established at the gel point, and thereafter, the frequency dependence is characteristic of a viscoelastic solid, with increasing frequency causing tan δ to increase. (Professional illustration by Paulette Dennis).