Porous materials, tissue response

The end-stage healing response to biomaterials is generally fibrosis of fibrous encapsulation. However, there may be exceptions to this general statement (e.g., porous materials inoculated with parenchymal cells or porous materials implanted into bone). As previously stated, the tissue response to implants is in part dependent upon the extent of injury or defect created in the implantation procedure. 

The development of biomechanical models derived from continuum formulations for transport of water and charged species in porous media has been carried out for various soft tissues [1-3] and implemented using finite element models (FEMs) [4-8], Such models provide quantitative views of the response of these complex structures that is especially useful in the study of orthopedic, vascular, ocular, and soft tissue substitutes developed by tissue engineering. In this paper a formulation and FEM are described that incorporate and extend these works in a very general model that identifies physical material properties and allows transient analyses of both natural and artificial soft tissue structures. 

Soft biological structures exhibit finite strains and nonlinear anisotropic material response. The hydrated tissue can be viewed as a fluid-saturated porous medium or a continuum mixture of incompressible solid (s), mobile incompressible fluid (f), and three (or an arbitrary number) mobile charged species a, (3 = p,m, b). A mixed Electro-Mechano-Chemical-Porous-Media-Transport or EMCPMT theory (previously denoted as the LMPHETS theory) is presented with (a) primary fields (continuous at material interfaces) displacements, Ui and generalized potentials, ifi ( , r/ = /, e, to, b) and (b) secondary fields (discontinuous) pore fluid pressure, pf electrical potential, /7e and species concentration (molarity), ca = dna/dVf or apparent concentration, ca = nca and c = Jnca = dna/dVo. The porosity, n = 1 — J-1(l — no) and no = no(Xi) = dVj/dVo for a fluid-saturated solid. Fixed charge density (FCD) in the solid is defined as cF = dnF/dV , cF = ncF, and cF = cF (Xf = JncF = dnF/d o. 

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,... 

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