The invention provides a blood substitute product 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.