that can be subsequently removed or altered chemically thermally electrically or mechanically or that can be rendered porous by any method to allow fluid flow or vacuum through the lumen of the resulting hollow fiber the second material or coating material 21 can be any material that can be processed in such a way that will allow for depositing a layer of the second material over the first material fiber 32 the coating can be achieved by any suitable process such as but not limited to extrusion and dipping the second material may be selected such that the mass and heat transfer properties are suitable for the application where the resulting hollow fibers are used for example for a heat transfer application the coating material used may have a high heat conducting coefficient as one example fluoropolymers may be used one method of continuously coating the first material fiber 32 with a coating 21 of the second material is illustrated in fig4 referring to fig4 the fiber 32 is coated using a coating die 40 also called a centering crosshead die of the kind commonly used for coating electric wire the fiber 32 is threaded through the fiber guide 42 of the coating die 40 and the second material is injected into the die as designated by arrow 43 and flows toward the coating head 44 as designated by arrow 43 u2032 and coats the fiber 32 at.