Biomaterials requirements surface properties

The surface is a crucially important factor of biomaterial, and without an appropriate biocompatibility the biomaterial could not function. On the other hand, the bulk properties of materials are equally important in the use of biomaterials. An opaque material cannot be used in vision correction, and soft flexible materials cannot be used in bone reinforcement. The probability of finding a material that fulfills all requirements in physical and chemical bulk properties for a biomaterial application and whose surface properties are just right for a specific application is very close to zero, if not absolutely zero. From this point of view, all biomaterials should be surface treated to cope with the biocompatibility. However, if the surface treatment alters the bulk properties, it defeats the purpose. In this sense, tunable LCVD nanofilm coating that causes the minimal effect on the bulk material is the best tool available in the domain of biomaterials. 

Biomaterials have played a vital role in the treatment of cardiovascular diseases, examples of applications including heart valve prostheses, vascular grafts, stents, indwelling catheters, ventricular assist devices, total implantable artificial heart, pacemakers, automatic internal cardioverter defibrillator, intraaortic balloon pump, and more. A key requirement for materials in cardiovascular applications, particularly blood-contacting devices, is blood compatibility, that is, nonthrombogenic. Additional requirements include mechanical and surface properties that are application specific. Surveying the field of polymers used in cardiovascular applications reveals that PUs, polyethylene terephthalate (PET), and expanded PTFE (ePTFE) are the most commonly used. This section will review each of the three polymers followed by a brief introduction of other emerging polymers for use in the cardiovascular area. 

The third topic in polyphosphazene biomaterials that will be described in this article concerns surface implications. One of the major problems in the utilization of polyphosphazenes in solid state is their exploitation in the construction of implantable devices, for which good physical properties, minimum biological response, and good resistance to fungal or bacterial colonization may be required. 

Because of the instability of the anhydride bond in the presence of water, special properties are required for stable polyanhydride devices. A critical element in the development of polyanhydride biomaterials is controlling hydrolysis within a polymeric device. To obtain implants where hydrolysis is confined to the surface of the polymer, hydrophobic monomers can be polymerized via anhydride linkages to produce a polymer that resists water penetration, yet degrades into low molecular weight oligomers at the poly-mer/water interface. By modulating the relative hydrophobicity of the matrix, which can be achieved by appropriate selection of monomers, the rate of degradation can then be adjusted. For example, copolymers of sebacic acid, a hydrophilic monomer, with carboxyphenoxypropane, a hydrophobic monomer, yield ... 

The ability to modulate the composition and physicochemical properties of polymeric biomaterials to meet release and surface requirements of different immunotherapeutic agents makes them quite valuable as carriers of these agents. 

Research in the area of nanotechnology has been rapidly advancing that we find that the tools either lack sensitivity or resolution that is required to effectively characterize very low signals. Characterization of the various properties such as topography, morphology, mechanical, and porosity of biomaterials before their use is very critical and important so that we can predict their behavior in vivo. However, characterization tools have improved over the past couple of decades, and collectively, we have been able to image and understand not only the surface of materials but also their properties. For example, the atomic force microscope provides atomic-scale surface data, while the scanning electron microscope provides micro- and nanoscale surface data, and the nanoscale data about the internal structure of materials can be obtained from transmission electron microscope. With this information and with data on the mechanical properties, wettability, porosity, etc., we would be able to understand the surfaces of materials and how they would behave in vitro and in vivo. 

An original universal technique based on layer-by-layer (LbL) adsorption of oppositely charged macromolecules onto a surface of inorganic colloid particles has been recently elaborated [30,31]. Hollow nano- or microcapsules with entrapped biomaterial can be easily prepared by decomposing an inorganic core, although the microcapsule wall can provide desired release properties. The use of various polymer materials allows a proper shell design, in order to adjust required stability, biocompatibility, and affinity properties of the microcapsules. 

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