metals A variety of minimally bioreactive metals are routinely implanted. The most commonly implanted form of stainless steel is 316L. Cobalt-chromium and titanium-based implant alloys are also permanently implanted. All of these are made passive by a thin layer of oxide on their surface. A consideration, however, is that metal ions diffuse outward through the oxide and end up in the surrounding tissue. Bioreaction to metal implants includes the formation of a small envelope of fibrous tissue. The thickness of this layer is determined by the products being dissolved, and the extent to which the implant moves around within the enclosing tissue. Pure titanium may have only a minimal fibrous encapsulation. Stainless steel, on the other hand, may elicit encapsulation of as much as 2 mm. List of implantable metal alloys Stainless Steel ASTM F138/F139 316L ASTM F1314 22Cr-13Ni–5Mn Titanium Alloy ASTM F67 Unalloyed (Commercially Pure) Titanium ASTM F136 Ti-6Al-4V-ELI ASTM F1295 Ti-6Al-7Nb ASTM F1472 Ti-6Al-4V Cobalt Chrome Alloy ASTM F90 Co-20Cr-15W-10Ni ASTM F562 Co-35Ni-20Cr-10Mo ASTM F1537 Co-28Cr-6Mo Tantalum ASTM F560 Unalloyed Tantalum Porosity in Implants Porous implants are characterized by the presence of voids in the metallic or ceramic matrix. Voids can be regular, such as in additively manufactured (AM) lattices, or stochastic, such as in gas-infiltrated production processes. The reduction in the modulus of the implant follows a complex nonlinear relationship dependent on the volume fraction of base material and morphology of the pores. Experimental models exist to predict the range of modulus that stochastic porous material may take. Above 10% vol. fraction porosity, models begin to deviate significantly. Different models, such as the rule of mixtures for low porosity, two-material matrices have been developed to describe mechanical properties. AM lattices have more predictable mechanical properties compared to stochastic porous materials and can be tuned such that they have favorable directional mechanical properties. Variables such as strut diameter, strut shape, and number of cross-beams can have a dramatic effect on loading characteristics of the lattice. AM has the ability to fine-tune the lattice spacing to within a much smaller range than stochastically porous structures, enabling the future cell-development of specific cultures in tissue engineering. Porosity in implants serves two primary purposes 1) The elastic modulus of the implant is decreased, allowing the implant to better match the elastic modulus of the bone. The elastic modulus of cortical bone (~18 MPa) is significantly lower than typical solid titanium or