Interposer circuitry (130) is formed on a possibly sacrificial substrate (210) from a porous core (130′) covered by a conductive coating (130″) which increases electrical conductance. The core is printed from nanoparticle ink. Then a support (120S) is formed, e.g. by molding, to mechanically stabilize the circuitry. A magnetic field can be used to stabilize the circuitry while the circuitry or the support are being formed. Other features are also provided.