Solid-like behaviour of unsheared sickle haemoglobin gels and the effects of shear

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Abstract
Pathogenesis in [human] sickle cell disease depends on the polymerization of deoxyHb S into long fibers followed by gel formation. The gelation renders the affected erythrocytes less deformable than normal so that they obstruct the microvasculature and the gelation process has been one of the targets for the development of therapeutic treatments of sickle cell disease. Gelation acts through the rheological properties it induces and rheological abnormalities are the immediate bases of pathogenesis. Although there has been very little study of the rheology of Hb S hemolysates, the gels are generally considered to be highly viscous and thixotropic on the basis of gross observation. Limited and generally qualitative observations show that high viscosity depends on deoxygenation and exhibits hysteresis in gelling/ungelling cyles; also that shearing accelerates gelation and can induce formation of fiber aggregates and crystals in suspension which, in contrast to the gel, are fluid. Using transient and steady state methods, it was shown that unsheared sickle deoxyHb preparations are solid-like, consistent with a gel-like nature, whereas shearing converts them to thixotropic viscous systems. The marked variation and thixotropy the system can undergo is emphasized. Relevance to the pathogenesis, clinical course and therapy of sickle cell disease is discussed.
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