Characterisation of halloysite for use as a microtubular drug delivery system

Halloysite supplied from New Zealand was shown by electron microscopy to be composed mainly of hollow microtubules having typical dimensions of 2-3 microm long and 0.3/0.1 microm outer/inner diameter. Aggregates of microtubules, double tubules and occasional split or partially unrolled tubules were observed. Energy dispersive analysis showed the mineral to be composed mainly of aluminium, oxygen and silicon, with a low content of iron. The dehydrated state of the mineral was confirmed by XRD analysis, which was partially reversible using a rehydration procedure with subsequent exchange of the intercalated water gained by glycerol, but larger molecules including the drug, diltiazem HCl, failed to exchange. The surface charge was predominantly negative over most of the physiologically relevant pH range (> 2) and the specific surface area of the material was very large (approximately 57 m(2)/g), indicating that the material has significant potential for extensive binding of cationic drugs. Removal of allophanc present by a hot alkali treatment had little effect on luminal porosity, which prior to treatment was estimated to be approximately 0.25 ml/g by a mercury intrusion technique and consequently should be the major site for drug loading. Halloysite also extruded and spheronised well to form smooth round pellets as an aid to further formulation development, which rapidly disintegrated in water unless prevented by sintering at 200 degrees C.