Rheology of fibrin clots. II. Linear viscoelastic behavior in shear creep.

Abstract

Abstract Creep of human fibrin clots in small shearing deformations has been studied over a time scale from 20 to 10 4 s. Most clots were prepared by the action of thrombin on purified fibrinogen, under various conditions of pH and ionic strength to produce networks with either coarse or line structure, and with or without ligation by calcium ion and fibrinoligase. Combination of data with previous results on dynamic viscoelastic properties provided a description of viscoelastic behavior over seven logarithmic decades of time or frequency scale. Fine, unligated clots showed very little creep over most of this range. Coarse, unligated clots showed substantial creep but this was suppressed by ligation. It is postulated that the basic fibril element formed by polymerization of fibrin units remains intact under stress but that, in coarse clots, some slippage of these elements within aggregated bundles occurs unless they are secured by α-α ligation. The elastic modulus (in a time scale of seconds) is proportional to the 1.5 power of fibrin concentration in unligated clots (the magnitude for fine clots being somewhat smaller than for coarse) and to the 2.3 power for coarse, ligated clots. Creep behavior with different shear strain magnitudes (up to 0.2) and tests of creep recovery by the Boltzmann superposition principle show strict adherence to linear viscoelastic behavior, indicating no basic structural changes during the experiments, provided the structure is fully developed before strain is imposed. If progressive ligation continues in the strained state, subsequent creep recovery is incomplete. Clots formed with ancrod instead of thrombin show a smaller modulus and much more extensive creep.