The present invention provides novel, biocompatible matrices for cell encapsulation and transplantation. It further provides methods for delivering agents to encapsulated cells and to the local environment of a host system. The invention also provides methods for targeting and manipulating particular cells and/or proteins of the host system. In a composition aspect of the invention, a composition including a collection of capsules is provided. The capsules comprise an inner core, and the inner core is covered by an outer shell composed of a positive polyelectrolyte and a negative polyelectrolyte. The inner core of the capsules contains at least one cell.
