Particulate porous absorptive material such as carbon is coated with a biocompatible polymer to give a thin (e.g. 1-10 micron) smooth permeable coating without plugging the pores. The resulting material is free from fines and surface debris and useful for haemoperfusion. Coating is best effected by spraying hot washed carbon granules, free from fines and in rapid movement with a solution of polymer (e.g. polyHEMA) so as to bring about rapid evaporation and a coating weight preferably of up to 20% usually 0.25% to 5%.
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