Hard tissue substitute

Wan ACA, Khor E, Hastings GW (1997) Hydroxyapatite modified chitin as potential hard tissue substitute material. J Biomed Mater Res 38 235-241... 

Polyelectrolytes have been widely investigated as components of biocompatible materials. Biomaterials come into contact with blood when used as components in invasive instruments, implant devices, extracorporeal devices in contact with blood flow, implanted parts of hard structural elements, implanted parts of organs, implanted soft tissue substitutes and drug delivery devices. Approaches to the development of blood compatible materials include surface modification to give blood compatibility, polyelectrolyte-based systems which adsorb and/or release heparin as well as polyelectrolytes which mimic the biological activity of heparin. 

They are used in hard tissue engineering applications [75]. A successful tissue engineering implant mainly depends on the role played by porous scaffolds. The ideal scaffolds should be biodegradable to support the substitution of new tissues. Besides, the scaffolds must be biocompatible without inflammation or immune reactions and possess proper mechanical properties to support the growth of new tissues. 

Hydroxyapatite (HAp) [Caj (PO )g(OH2)] is the main inorganic mineral constituent close to human bone chemistry, and is also an outstanding synthetic bone substitute because of its osteoconductive properties. HAp ceramics can be manufactured synthetically from its constituents via a range of production methods. In addition, they have been manufactured by demineralizing bovine or human hard tissues. 

Hydroxyapatite (Hench, 1993), /3-tricalcium phosphate (Wilson, 1993), Bioglass (Hench, 1991) and Glass Ceramics A-W (Yamamuro, 1993 Kokubo, 1991) are typical inorganic materials that can directly bond to bone tissues when embedded in human bodies. They have already been used as bone-substitutes in clinics. Such tissue-bonding property is denoted as bioactivity and is exceptional because it is only found for a limited kind of materials (Ohtsuki, 1991, 1992) and the rest of all materials, i.e., metals, ceramics, and polymers are not bioactive. However, those ceramic materials are far from ideal tissue substitutes since their fracture toughness is lower than that ofhuman cortical bone, and too hard to be applied to soft tissue replacement. Hence, they find a limited range of use. 

Some metals are used as passive substitutes for hard tissue replacement such as total hip and knee joints, for fracture healing aids as bone plates and screws, spinal fixation devices, and dental implants because of their excellent mechanical properties and corrosion resistance. Some metallic alloys are used for more active roles in devices such as vascular stents, catheter guide wires, orthodontic archwires, and cochlea implants. 

Calcium phosphate bioceramics have drawn worldwide attention as the important substitute and scaffolding materials in hard tissue engineering due to their biocompatibility and bioactivity. This is due to the chemical similarity of these materials especially carbonated hydroxyapatite to the mineral constituent of bone [1-6]. 

Hydroxyapatite is a bioactive ceramic that is commonly used in particulate form in bone repairs, as well as coatings for metaUic prostheses to improve their in vivo biological response. Due to the chemical similarity between HA and mineralized bone, synthetic HA exhibits a strong affinity to host hard tissues. The formation of chemical bonds with the host tissue offers HA a greater advantage in cUnical applications over other bone substitutes such as allografts or xenografts. 

Sarkar, S., Lee, B., 2015. Hard tissue regeneration using bone substitutes anupdateon innovations in materials. Korean J. Intern. Med. 30(3), 279-293. Available at http //www.pubmedcen tral. nih.gov/articlerender.fcgi artid=4438282 tool=pmcentrez rendertype=abstract. 

Ceramics have numerous uses in the field of biomaterials, mainly becairse of their physicochemical properties. Their chemical inertness helps to minimize organic reactions of the host organism and their hardness and resistance to abrasion makes them suitable for substitution of hard tissues (bones and teeth). Some ceramics also have excellent tribological properties and are utilized in friction couples intended to replace malfunctioning joints. Finally, other properties (appearance, electrical insulation) also determine certain biomedical applications. 

Hard bone tissue contains Ca, P04, OH, small amounts of carbonate, magnesium and sodium and trace elements of fluorine, chlorine and sulphur. That is why the Ca/P value in bone is not 1.67. Substitution of strange ions results in a change in the crystal structure. Consequently it is impossible to imitate the mineral part of bone exactly. 

Calcium phosphate A family of calcium phosphate ceramics including aluminum calcium phosphate, ferric calcium phosphate, hydroxyapatite and tricalcium phosphate (TCP), and zinc calcium phosphate which are used to substitute or augment bony structures and deliver drugs. Glass-ceramics A glass crystallized by heat treatment. Some of those have the ability to form chemical bonds with hard and soft tissues. Bioglass and Ceravital are well known examples. 

To mimic the mechanical behavior of the native tissue is a basic assumption to facilitate the biointegration and function of the substitute. Incorporation of fibers to the biomaterial matrix opens the way to inhomogeneity, anisotropy, nonlinearity, and viscoelasticity features. Provided that interfacial binding between matrix and fibers in the composite goes on, mechanical properties could be tailored by varying the amount and orientation of fibers embedded, to suit the purpose applications, whether it s related to hard or soft tissue substimtes. Individual fibers, no matter how small and isolated compared to fibrous structure, could affect mechanical properties. To be effective, fibers principal direction should be considered and controlled. 

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