Channeled articles having very small diameter channels spaced very closely can be made by packing elongated cores in a fixture, clamping them, and then introducing matrix material around the cores. The matrix material is formed into a unitary body and solidified. The cores are pulled out, leaving open channels where they had been. Some core and matrix combinations will permit the cores to be pulled out. Others require a core release coating to be applied to the cores. The cores can be metal or ceramic or polymer, and the matrix can be metal or ceramic or polymer. The cores can be solid, or hollow. Rather than pulling the cores out, if they are polymer, they can be burned out. The matrix can be formed by liquid state, solid state, or hybrid liquid/solid state techniques. A related technique uses hollow cores, which are not pulled out, but which remain in the body after unification. For such tube-walled articles, the matrix can be formed similarly. Rather than insuring core release, core retention is required. Such may occur due to the nature of the materials, or a specific core retention coating may be provided. Articles made of such material include heat sinks for semiconductor devices, light-weight structural components, thermally activated actuators, etc. Very small channel diameters and very large length to opening aspect ratios can be achieved. Heat exchange fluid can be compressed and pumped through such an article at very high efficiencies, to cool semiconductor devices.