A material system including a plurality of expandable pellets or a monolithic flexible carrier is disclosed, each of which may include a polymer matrix configured to hold an expandable core, and a flexible skin configured to encapsulate the polymer matrix, and where the flexible skin is at least partially permeable with respect to the expandable core or a gas released by the expandable core. Implementations of the material system may include where the polymer matrix may include a thermoplastic polymer. The material system may include a charging source configured to introduce blowing agent. A method of curing a composite part is also described.