A blood substitute product is disclosed, comprising haemoglobin and a self-assembled microparticle having an acid having two or more acid groups and an organic base in a solvent. The particle is of micron scale. The microparticle may be obtained by contacting a bis-acid and organic base in a hydrophilic solvent, wherein the acid is insoluble or sparingly soluble in the hydrophilic solvent and the organic base is soluble in a hydrophilic solvent. Preferably the acid is a bis-carboxylic fatty acid with a carbon chain length of between 5 and 40 and the base is alkylated amine or polyamine. In a preferred embodiment the acid is brassylic acid, the base is polydimethylammonium chloride and the solvent is an aqueous solution of sodium hydroxide. Preferably the haemoglobin is mammalian haelmoglobin. The microparticles are 1-10 micrometres in diameter.
