Voids prevention techniques

A possible technique adopted to prevent fibrous capsule formation around the implant is the addition of a tissue intermediary [203,204]. Indeed, if this material has a continuous, interconnected, porous structure (pore diameter >8-10 p-m), macrophages are capable of invading structure voids. Consequently, vascularized tissue can grow in the implant and the foreign body response is avoided as this porous stmcture is able to mimic extracellular matrix. The first example of intermediary tissue use concerns the coating of an implanted catheter by means of a silicone mbber cage [205]. Typically,... 

To density a bulk material by expressing air from the voids, it is essential that the bulk material is not in a fluidized condition, otherwise the material will leak backwards through the screw form. When the supply material is in a highly dilated state, the feed hopper design should be of mass flow form with sufficient residence time for the material to de-aerate to a stable flow condition. Note that a mass flow hopper will not prevent the development of a preferential flow path for a bulk material in a fluidized condition. If necessary, techniques to accelerate de-aeration should be employed to secure a bulk condition that is amenable to screw compaction, e.g. vibrating rod frames, as described by the author in Bulk Solids Handling, 1986, Vol. 6, No. 1, p. 77. 

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