Extrusion freeform fabrication is a 3-D layerwise writing technique for forming objects directly under the control of a CAD program. This method is one of a family of rapid prototyping methods which include stereolithography, selective laser sintering and fused deposition modeling. This system can be used to build shapes, layer by layer, from hydrogels of agarose, polyacrylamide or other cross-linked water-soluble acrylic polymers.
Mineralization can be induced in these gels by building a part with alternating layers of gels containing calcium and carbonate or phosphate that can be formed into stacks which then mineralize by cross-diffusion. The write head can be conceived of as a cell which delivers the appropriate minerals to a site within a swollen gel matrix. This diffusive mineralization process can thus be compared with bone mineralization. The gel structure controls the morphology of the mineral. The site of mineralization is controlled by osmotic forces which localize most of the mineral in whichever zone has the higher ionic strength. The mineral content, expressed as a fraction of the polymer content, is similar to that of bone but the water content is much higher than in bone. This raises the question of what process drives the water exclusion during bone mineralization.