A reactor 10 for forming fully coated particles having a solid core comprises a reactor vessel 12, a gas phase coating mechanism 21, 23 for introducing pulses of gas phase materials, a sieve 16 located within the reactor vessel and a forcing means 19 for forcing the particles through the sieve to deagglomerate any particle aggregates. The forcing means may include shaking, tumbling, centrifugal force, sonic vibration, ultrasonic vibration, vacuum, air column, gas flow or gravitational. A forcing aid may include a brush, balls, scraper, spatula, paddle or air jet. First and second shutters 15a, 15b may be located above and below the sieve. The reactor vessel may include two reactor chambers that are configured to switch places, possibly by rotation about an axle 26. The gas phase coating mechanism is preferably an atomic layer deposition (ALD) mechanism. Forcing the particles through the sieve between pulses ensures that the particles are fully exposed to the gas phase materials and contact spots or pinholes in the coating are obviated. A pharmaceutical composition comprising a plurality of coated particles is also disclosed.
