An implantable device for providing a therapeutic agent includes a container configured to contain multiple cells capable of producing the therapeutic agent within an interior region of the container. The container defines first pores defined by an interior wall portion of the container and second pores defined by an exterior wall portion of the container. The first pores have a first average size that (i) allows passage of the therapeutic agent and oxygen through the first pores and (ii) prevents passage of immune cells through the first pores. The second pores have a second average size that is larger than the first average size, and the second pores are sized to promote vascularization and oxygen transfer along the exterior wall portion.