Biomaterials

Biomaterials can be classified with respect to their chemical composition and their use. Generally, the following criteria have to be fulfilled by biomaterials:

• Functionality: The biomaterial must replace at least in the important parts natural body functions. This means that, for example, for bone substitution a suitable mechanical stability has to be achieved.

• Biocompatibility: The biomaterial must not cause negative body reactions resulting, for example, in inflammation. Furthermore, a good integration into the body environment is favorable.

• Aesthetical aspects: Biomaterials used for external implants should not deface the patient. Preferably, they should look like the replaced tissue.

Surface properties influence strongly the mechanical function, the biochemical interaction, and the optical appearance of the biomaterial. Therefore the interface of the materials deserves special interest in investigation, control, and modification of properties to reach the criteria mentioned above.

Mechanical aspects are relevant for applications which require force transfer (e.g., bone substitution), which impose dynamic loads on, for example, artificial joints, and which, for example, in the case of substitutes for blood vessels, meet certain hydrodynamic respectively fluid-mechanical properties. The interaction of the biomaterial with the body environment concerns the tissue contact (cell adhesion and fixation), the tissue organization (cell-to-cell linkage and communication), and the exchange of substances across the interface. Optical appearance is determined by light transmission (e.g., of contact lenses) or light reflection (e.g., of natural looking tooth replacements). Mechanical, biochemical, and optical properties depend mainly on the topography and the chemistry of the surface.

Topography may be defined as size, shape, distribution, and hierarchy of surface features. These features are either discrete (holes and peaks) or continuous (furrows and ridges) in random (statistical), fractal (self-similar on different length scales), or periodic distribution across the surface. Rough surfaces exhibit a larger surface area and more contact points for biological molecules and cells. The reason for larger entities like cells to adhere to the surface and how they arrange their layer growth is dependent on the size and distribution of the surface features: Cavities which are smaller than single cells cannot be colonized, while larger structures regulate the direction, connectivity, and differentiation of the growing cell layer. A hierarchical surface consists of structure elements with increasing complexity, for example, two-dimensional planes scrolled up to three-dimensional fibers which are organized in three-dimensional fiber composites. Examples of those materials were synthesized with carbon nanotubes with increased mechanical stability and electrical conductivity and decreased thickness compared to common wires [9-11]. In principle such materials are also interesting for biomaterials as on one hand they show a very large surface area and on the other hand they enable combinations of surface sections with different properties and therefore different cell types within a single superstructure.

The chemical properties of the interface between biomaterial and body environment determine the interaction of the surface with water molecules, ions, biological macro-molecules, and cells. The surface reactivity depends on the chemical composition, the production process, and the pre-treatment before use of the materials and is essential for fixation, growth, and proliferation of tissue and bone cells. Cell adherence via membrane receptors and adhesion proteins is influenced by active surface groups (generated, e.g., by oxidation of metals or coating of materials with specially designed polymer layers) which regulate the adsorption of the anchoring protein layer. The functionalization of the substrate additionally controls the wetting behavior where particularly hydrophilic surfaces show enhanced cell adhesion. The latter is due to the fact that systems like cells and hydrophilic surfaces reach a thermodynamically favorable state by combining their high-energy surfaces. In addition, even hydrophobic interfaces like gold can get more hydrophilic by protein adsorption [12] which may help to increase biocompatibility. Polarizability and charging of the interface affect electrostatic interactions between charged species and the surface. Charge screening and complexa-tion by multivalent ions present in all types of body fluids allow attraction between molecular and surface groups of like charges and stabilize the biological layers. At conductive interfaces electrochemical reactions with charge transfer between the electrolyte solution and the substrate may occur, which interfere with the cellular metabolism and the conformation of adsorbed adhesion proteins. This can set free toxic substances and cause allergic and inflammatory body reactions.

Topographical as well as chemical effects play a role for tribological properties of biomaterials. Tribology describes the behavior of interfaces in motion and gets important when implants are designed to support body movements. In this case friction, wear, and lubrication of implant and body respectively different moving implant elements influence function and longevity of the applied biomaterial. Interfacial friction depends on kind and strength of interaction forces, the clasping of surface uprisings and troughs, and force transfer properties of intermediate fluids. Hardness, cohesion, and adhesion of the individual surfaces in contact determine to what extent the materials are worn off by abrasion (e.g., scratch and particle formation), adhesion (e.g., welding processes), and surface fatigue (e.g., crack formation). Liquid films in the crevice between two hard surfaces can reduce friction and wear. The effect of such lubricants is influenced by surface chemistry and separation as well as by the viscoelastic and hence force transferring properties of the fluids themselves.

This comprehensive discussion of biomaterials in view of surface properties shows that the interplay between implant and body environment can be very complex and includes a large variety of parameters from materials science, biology, and medicine explained in more detail in [7]. The need of surface analysis with nanometer resolution arises on one hand from the importance of the interface between natural and artificial material and on the other hand from the high dependence of the macroscopic behavior on the microscopic appearance of the biomaterial.

In the following the main different chemical substance classes for biomaterials will be described shortly.

Biomaterials can be divided into native materials like teeth and bone, and artificial biomaterials which are in contact with biological systems like implants in medicine. Within these fields the materials in their actual state of occurrence and model surfaces like thin layers on supports are under investigation.

The three metal systems mainly utilized in implantol-ogy are stainless steel, cobalt-chromium alloys, and titanium. Such metals are mainly used in bone replacement and here titanium is the most important metal because of its good biocompatibility and nearly bonelike mechanical properties. That is why titanium and its alloys TiAl6V4 and TiAl6Nb7 are frequently applied as biomaterials for hard tissue replacement such as dental and orthopedic (e.g., hip and knee joint prosthesis) implants, in the audiological field, and for cardiovascular applications such as mechanical heart valves and as material for surgical instruments such as vascular clamps, needle holders, and forceps [13]. Titanium surfaces are covered by a 2- to 6-nm-thick oxide layer that is formed spontaneously in the presence of oxygen. This layer contributes to the biocompatibility of titanium by preventing corrosion and the release of ions from the metal surface [7]. The oxide layer thickness and surface morphology can be altered by according treatment or coatings applied to change the physicochemical properties and the biological response [13].

Stainless steel is also used for orthopedic implants. Its lower biocompatibility and its mechanical properties, which are less bonelike than titanium, reduce the use as human implant material, but it is cheaper than other metals and therefore is still used in animal medicine and for surgical instruments. Other applications of steel are cardiovascular stents.

Cobalt-chromium-molybdenum alloys are very hard and in combination with other smooth surfaces very abrasion-proof. They are therefore an ideal material for joint implants. In many patients Co-Cr-Mo, that is sintered from Co-Cr-Mo beads, is used as a femoral hip implant.

Gold as one of the oldest implant materials is used not only as dental restorative material but also as electrode material for implantable biosensors or to increase the contrast in electron microscopy. An overview on studies concerning metals as biomaterials is given in Section 4.1.

Bone, tooth enamel, and dentin have a composition which is very similar to that of special ceramic materials like hydroxyapatite, calcium phosphate, and calcium carbonate. These materials in their natural or manmade origin are used as coatings to increase the biocompatibility of implant materials. On the other hand they can replace parts of bones; for example, porous hydroxyapatite is a good basis for new bone growth. Biologically active glass or bioglass is similar to ceramics and assists actively new bone formation. It is therefore used in every place where bone has to be built up. This is necessary after tumor surgery or if natural bone is destroyed by an inflammation. Ceramic and glassy materials are further considered in Section 4.2.

Another kind of biomaterials are the polymers. They can be tailor-made for many different applications as their properties and their composition are variable in a large range. Polymers can be used as manufactured or treated with different techniques to yield special functional groups at the surface for specific interaction, increased surface energy, hydrophilicity respectively hydrophobicity, or chemical inertness. Furthermore, such surface layers may induce cross-linking to increase hardness, remove weak boundary layers or contaminants, modify the surface morphology (increase or decrease of surface crystallinity and roughness), or increase surface electrical conductivity or lubricity. A review of these modifications, examples, and characterization is given in [14]. Polymers like high molecular weight polyethylene (HMWPE) are used as counterpart for the titanium ball head in artificial hip joints. Poly(tetrafluoro ethylene) (PTFE) and other fluorinated polymers are used for blood vessel replacements and catheters, poly(methyl methacry-late) (PMMA) for intraocular lenses, silicone acrylate for contact lenses, polyurethane for pacemakers and left ventricular assist devices, cellulose for renal dialyzers, and sili-cone for catheters and breast implants [15]. Biodegradable polymers like poly(sebacic anhydride) (PSA) and poly(DL-lactic acid) (PLA) are naturally removed from the body after a definitive time span and can be used for controlled drug delivery [16]. Generally, carbon in different forms (e.g., diamond, graphite, or activated carbon) is an important biomaterial particularly because of its high blood compatibility. Glassy polymeric carbon (GPC) made from phenolic resins by pyrolization, is mostly used as coating, for example, in prosthetic heart valves and other prosthetic devices [17,18]. Some selected studies on polymers are discussed in Section 4.3.

A newer group of materials used in implantology are composites, especially composites of inorganic material and polymers which combine the advantages of both. Examples can be found in Section 4.4.

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