A porous silicon implant impregnated with a beneficial substance, such as a micromineral required for healthy physiology, is implanted subcutaneously and is entirely corroded away over the following months/year to release the micromineral in a controlled manner. In a second embodiment the implant may have a large number of holes which contain beneficial substance and which are closed by bio-erodible doors of different thickness so as to stagger the release of the beneficial substance over time as the doors are breached.