The present invention provides a method for producing articles with a gradient of density, porosity and/or concentration by subjecting a viscous material to centrifugation during production of the article. The viscous material may be a composite material comprising a hydrogel. The viscous material can be molded or cast into the article. In certain embodiments, the viscous material is used to create an articulating surface implant such as a replacement plug, a knee spacer, or a spinal disc. The article may also be an implant such as a shoulder implant or other socket type implant that is produced by centrifuging in two axes which produces a gradient relative to both axes of rotation.