Abstract: The materials and methods are particularly useful in media for cell culturing but have other applications as well, such as in tissue engineering. The materials comprise at least one growth factor or other cell growth facilitating substance contained within a polymeric structure which can incorporate the substance and allow controlled release of the substance. Alginate materials are particularly useful as the polymeric structure. Another feature described herein is the above-described materials provided in a lyophilized (freeze-dried) form which greatly enhances their storage life.