Polyethylene glycol-modified vesicles (liposomes) encapsulating hemoglobin (HbV) are artificial oxygen carriers that have been developed as a transfusion alternative. The HbV suspension in an albumin solution is nearly Newtonian; other biopolymers, hydroxyethyl starch (HES), dextran (DEX), and modified fluid gelatin, induce flocculation of HbVs through depletion interaction and render the suspensions as non-Newtonian. The flocculation level increased with hydrodynamic radius (Rh) or radius of gyration (Rg) of series of HES or DEX with different molecular weights at a constant polymer concentration (4 wt %). However, the flocculation level differed markedly among the polymers. A crowding index (Ci) representing the crowding level of a polymer solution is defined as (excluded volume of one polymer) x (molar concentration) x Avogadro's number, using Rh or Rg. All polymers' flocculation level increases when Ci approaches 1: when the theoretical total excluded volumes approach the entire solution volume, the excluded HbV particles are forced to flocculate.
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