Microreactor and method of determining a microreactor suitable for a predetermined mammal

A method of determining a suitable microreactor for a predetermined host including identifying a suitable microreactor diameter, optionally identifying a minimum second matrix thickness, and optionally identifying a microreactor having a centrally located microcapsule. A microreactor having a diameter such that, when implanted in a higher order mammal, the microreactor exhibits no greater than 25% fibrosis for at least 14 days after implantation. A microreactor that includes a semipermeable membrane capable of eliciting a fibrotic response from a higher order mammal, and a second matrix surrounding the semipermeable membrane, the second matrix being of a thickness sufficient to prevent nucleation of fibrosis by the semipermeable membrane when the microreactor is implanted in a higher order mammal for at least 14 days.