A therapeutic device for extended release drug delivery including a refillable reservoir configured to receive a therapeutic agent and having an outlet for delivery of the therapeutic agent to a patient from the reservoir over an extended period. A porous structure is coupled near the outlet of the reservoir, the porous structure formed of sintered material. A barrier layer is coupled to the reservoir on or adjacent a surface of the porous structure such that the therapeutic agent passes through both the porous structure and the barrier layer upon delivery from the reservoir through the outlet. The porous structure is tuned to deliver the therapeutic agent at a diffusion rate and the barrier layer is adapted to block passage of particles having an average particle size within an average particle size range that is outside an average particle size range blocked by the porous structure. Related methods and systems are provided.