Safety comparison of insecticide microencapsulation and investigation of its mechanism.

Four kinds of microencapsulated insecticides were prepared with fenthion, mevinphos, acetofenate and methylparathion. From observations made after the experiment it was found that type II microcapsules with polyurea capsule walls formed by interfacial polymerization had a special advantage: they had greatly enhanced safety in mice when administered via mouth, respiratory tract and skin. Type I microcapsules, with capsular walls of gelatin and Arabic gum formed through complex condensation, had no safety protection with oral administration. The mechanism of safety enhancement associated with type II microcapsules depends mainly on maintenance of microcapsule integrity within the digestive tract; type II microcapsules rarely disintegrated in the gastrointestinal tract and only about 5 per cent were ruptured. Of the small quantity of insecticide that leaked through the ruptured capsule walls, the amount absorbed by the digestive tract was not sufficient to kill the mice before the microcapsules were discharged in faeces--The amount of discharge within 48 hours exceeded 80 per cent. Administration of large numbers of microcapsules was necessary before sufficient insecticide could be absorbed by the digestive tract to cause lethal dosage; on post-mortem examination some microcapsules were found intact in the stomach and small intestine. Type I microcapsules, however, were totally disintegrated in the digestive tract and no traces could be found in the faeces; therefore their drug safety could not be increased.