Disclosed are systems and methods that can be utilized to define and control the delivery rate of a biological agent from a carrier matrix such as a biocompatible hydrogel. The carrier matrices of the present invention can include ligands incorporated within the matrix at a predetermined concentration level (CLT). In addition, the ligands within the matrix can display a particular, predetermined affinity for the biologically active agents to be delivered by the system. In particular, the affinity between the ligand and the biologically active agent can have a known predetermined dissociation constant (KD). When utilizing the system, the agent can be incorporated within the matrix due to association of the agent with the ligand. In addition, the agent can be protected from side reactions due to the association of the agent with the ligand. Through particular selection of the parameters CLT and KD, the rate of release of the biologically active agent from the matrix can be controlled. The disclosed methods and systems can be advantageously used in both in vivo clinical settings and ex vivo settings, such as tissue engineering applications.
