Metals as biomaterials

Fig. 1 Representative a Nyquist and b Bode plots from electrochemical impedance spectroscopy measurements (HubrechtJ (1998) Metals as Biomaterials, Helsen J, Breme H (eds) John WUey Sons Limited. Reproduced with permission)...

Hubrecht J (1998) In Helsen JA, Breme HJ (eds) Metals as Biomaterials. Wiley, Chichester... 

From Helsen, J. A., and H. J. Breme (Editors), Metals as Biomaterials, John Wiley Sons, Chichester, UK, 1998. Reprinted with permission of John Wiley Sons Inc.]... 

Polymers, metals, ceramics, and glasses may be utilized as biomaterials. Polymers (see Ppolymerprocessing), an important class of biomaterials, vary gready in stmcture and properties. The fundamental stmcture may be one of a carbon chain, eg, in polyethylene or Tedon, or one having ester, ether, sulfide, or amide bond linkages. PolysiHcones, having a —Si—O—Si— backbone, may contain no carbon. 

The low density of carbon fibre composites compared with metals has resulted in their application as biomaterials. Carbon fibre/epoxy composites are used as plates in bone surgery, replacing the titanium plates previously employed. The combination of high strength and flexibility exhibited by carbon fibre composites has had a huge impact in the performance of sports equipment Fishing rods, golf clubs, bicycle frames, rackets and skis are examples of applications for carbon fibre composites. 

A wide variety of natural and synthetic materials have been used for biomedical applications. These include polymers, ceramics, metals, carbons, natural tissues, and composite materials (1). Of these materials, polymers remain the most widely used biomaterials. Polymeric materials have several advantages which make them very attractive as biomaterials (2). They include their versatility, physical properties, ability to be fabricated into various shapes and structures, and ease in surface modification. The long-term use of polymeric biomaterials in blood is limited by surface-induced thrombosis and biomaterial-associated infections (3,4). Thrombus formation on biomaterial surface is initiated by plasma protein adsorption followed by adhesion and activation of platelets (5,6). Biomaterial-associated infections occur as a result of the adhesion of bacteria onto the surface (7). The biomaterial surface provides a site for bacterial attachment and proliferation. Adherent bacteria are covered by a biofilm which supports bacterial growth while protecting them from antibodies, phagocytes, and antibiotics (8). Infections of vascular grafts, for instance, are usually associated with Pseudomonas aeruginosa Escherichia coli. Staphylococcus aureus, and Staphyloccocus epidermidis (9). 

Many of the properties of the synthetic materials have been available for some time, for example those of the various metallic alloys used in clinical practice have been specified in various International, European and National Standards and can be found by searching. In the case of polymeric materials, while the information is in commercial product literature and various proprietary handbooks, it is diverse by the nature of the wide range of materials commercially available and the search for it can be time consuming. The situation is much the same for ceramic and composite materials there the challenge is finding the appropriate properties for the specific compositions and grades in use as biomaterials. 

Polymers used as biomaterials can be natural, synthetic or hybrid. With the growing field of regenerative medicine and medical devices, polymers dominate the soft tissue engineering and drug delivery industry and are gradually replacing metals and ceramics in the hard tissue engineering field as well. 

There are a wide range of substrates and films which can be produced as biomaterials. Metals such as titanium alloys are often used where high strength or toughness is required, such as hip implants. Their mechanical properties can be somewhat similar to bone, which makes them ideal candidates as structural bioimplants. However, as outlined by Skinner and Kay (2011), metal erosion can be a potentially dangerous problem. Ceramic films such as Ti02, Si02 or hydroxyapatite Caio(P04)6(OH)2 are often added to the surface to reduce wear of the implant and improve biocompatibUity. 

Materials discussed in this chapter are metals. Some types of corrosion behavior discussed also apply to combinations of metals with ceramics or pol5rners. Information given in this chapter may serve as a guide for future assessment of metals and alloys considered for use as biomaterials. 

Glass-ceramics of the P203-Al203-Ca0-type are produced as monolithic bulk glass-ceramics and as composites of glass-ceramics and metals. Both types are used as biomaterials for bone substitution in human medicine. 

Another new field consists of gloss-ceramic materials used as biomaterials in restorative dentistry or in human medicine. New high-strength, metal-free glass-ceramics will be presented for dental restoration. These are examples that demonstrate the versatility of material development in the field of glass-ceramics. At the same time, however, they clearly indicate how complicated it is to develop such materials and what kind of simultaneous, controlled solid-state processes are required for material development to be beneficial. 

Metals and polymers are the two most important classes of materials used as biomaterials. In many papers, XPS is used to check or to evaluate the chemical composition of the surface layer of these materials, with the aim to correlate this chemical composition with properties such as resistance to or easiness of degradation, inhibition or promotion of protein adsorption, increase or decrease of cell adhesion. The properties that are searched for largely depend on the application that is envisioned. 

Choosing a metal for implantation should take into account the corrosion properties discussed above. Metals which are in current use as biomaterials include gold, cobalt chromium alloys, type 316 stainless steel, cp-titanium, titanium alloys, nickel-titanium alloys, and silver-tin-mercury amalgam. 

The interest of the HA as biomaterial comes clearly by its similarity with the mineral phase of the bone tissue. In principle, HA would be the suitable material as much as for restoration as for bone substitution. However, the relatively low strength and toughness of HA, produced little interest among researchers when the focus of attention is on dense structural samples. Therefore, its use is restricted to all those applications where mechanical efforts are not required, finding its application concentrated as coating on metallic substrate with the object to accelerate and to increase the fixation of the prosthesis to the bone [54,55],... 

Conducting polymers have been explored as biomaterials to replace metals or modify metal surfaces in tissue engineering. Localized electrical stimulation has been shown to promote tissue repair, cell growth, bone regrowth, and wound healing [52, 58]. 

With the rich chemical properties of alkyne monomers, polymers with rare and complicated stmctures can be designed and synthesized via alkyne-based polymerizations. For instance, polyamidines are a series of important polymers with potential roles as biomaterials, metallic conductors, and photosensitive semiconductors their syntheses are limited to two-component, step-growth polymerization reactions [37, 38]. An efficient Cu-catalyzed MCR of alkynes, sulfonyl azides, and amines was recently reported by Chang and coworkers to afford a library of small-molecule amidines [3, 5], The reaction undergoes a Cu-catalyzed azide-alkyne... 

The use of metal and metalloid-containing macromolecules is widespread. For example, polysiloxanes are used as biomaterials rather than drugs. Polysiloxanes are widely used as contact lens and in the reconstruction of finger, hip, toe, and wrist joints and in the manufacture of artificial lungs, skin, dialysis imits, orbital floors, tracheal stents, brain membranes, ear frames, and hearts. 

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