invention applies also to other porous tini such as that described in s u 1381764 in preferred embodiments employing the preferred porous tini referred to above the porosity is defined by a network of interconnected passageways extending throughout the alloy the network exhibits a permeability for fluid material effective to permit complete migration of the fluid material throughout the network this alloy is elastically deformable preferably the porosity is at least 30 and preferably not more than 70 preferably the permeability is derived from capilliarity in the network of passageways which define the porosity this capilliarity may be produced in the alloy by inclusion therein of a large number of pores of fine size which interconnect to produce capilliary passages capilliarity is advantageous in that it promotes migration of spinal biological fluids into the network of passageways and retention of the fluid material in the network without the need to apply external hydraulic forces in general the network has a coefficient of permeability of 2 u00d7 10 u2212 13 to 2 u00d7 10 u2212 5 and the permeability is isotropic the capilliarity and the isotropic character are in particular achieved when the network defining the porosity comprises pores of different pore size the pore size distribution preferably being as follows suitable tini alloys for use in the invention are described in wo 99 34845 published jul 15 1999 the teachings of which are incorporated herein by reference pore size is an important factor in tissue or biological aggregate growth at least some of.