For example, U.S. Pat. No. 4,221,778 to Raghunathan discloses ion exchange resin-drug complexes treated with an impregnating agent such as polyethylene glycol or propylene glycol prior to coating with a water-permeable diffusion barrier such as ethylcellulose; the impregnating treatment is said to prevent swelling and fracturing of the resin particle in solutions and to improve coatability.