Natural bone

Similarly, a composite of hydroxyapatite and a network formed via cross-linking of chitosan and gelatin with glutaraldehyde was developed by Yin et al. [ 169]. A porous material, with similar organic-inorganic constituents to that of natural bone, was made by the sol-gel method. The presence of hydroxyapatite did not retard the formation of the chitosan-gelatin network. On the other hand, the polymer matrix had hardly any influence on the high crystallinity of hydroxyapatite. 

The so-called bioactive ceramics have been attractive because they spontaneously bond to living bone, however, they are much more brittle and much less flexible than natural bone. Previous studies reported that the essential condition for ceramics to show bioactivity is formation of a biologically active carbonate-containing apatite on their surfaces after exposure to the body fluid [337]. Calciiun sulfate was also used [338]. 

PHB has been claimed to have piezo-electric properties similar to those of natural bone, giving it potential as biodegradable fixative plates that could actually stimulate bone formation and consequently promote the healing of the patient [117]. Furthermore, PHB has been used to produce non-woven patches for pericardium repair following open-heart surgery. 

Most of the bio-nanocomposites tested as implants for bone regeneration are based on the assembly of HAP nanoparticles with collagen, trying to reproduce the composition, biocompatibility and suitable mechanical properties of natural bone. 

Natural bone grafts would appear to provide the ideal material. However, autografts are necessarily limited in volume and xenografts or allografts should be considered with caution due to the potential risk for transmission of viruses or other non-conventional agents [6]. For these reasons, there has been a growing... 

Composites provide an atPactive alternative to the various metal-, polymer- and ceramic-based biomaterials, which all have some mismatch with natural bone properties. A comparison of modulus and fracture toughness values for natural bone provide a basis for the approximate mechanical compatibility required for arUficial bone in an exact structural replacement, or to stabilize a bone-implant interface. A precise matching requires a comparison of all the elastic stiffness coefficients (see the generalized Hooke s Law in Section 5.4.3.1). From Table 5.15 it can be seen that a possible approach to the development of a mechanically compatible artificial bone material... 

A group of materials technologists and surgeons led by Brent Constanz of the Norian Corporation in Cupertino, California, USA developed a bone paste to be injected into bone fractures. The paste, trade name Norian SRS, hardens in minutes and thus braces broken bones. In 12 hours the material has a compression strength which equals that of natural bone. So far the material has been tested in fractures of the hip, knee, shoulder and wrist. The operations proceed faster than their traditional equivalents fewer plates and screws are necessary and the patients are able to resume their normal activities much more quickly. 

Norian SRS bone mineral substitute is injected directly into osteoporotic and other traumatic fracture sites, filling the bone void. Hardening in minutes, Norian SRS is a patented formulation that forms carbonaated apatite - the main contituent of natural bone - in the patient s body (Illustration courtesy of Norian Corporation, Cupertino, USA). 

Schlenker RA, Keane AT, Unni KK. 1989. Comparison of radium-induced and natural bone sarcomas by histologic type, subject age and site of occurrence. Br J Radiol 21 55-62. 

These polymeric materials are fabricated to mimic the shape of natural bone, in a form referred to as a scaffolding. The scaffolding provides open spaces in which the body s osteoblasts can begin the regeneration of new hone. When regeneration is complete, the new bone can take over the structural chores temporarily performed by the bone implant. 

Researchers are also exploring ways of using inorganic materials, especially those closely related to natural hone material, for artificial bone filling. Interpore International of Irvine, California, for example, received approval in 1992 for its hydroxyapatite-based hone substitute called Pro Osteon. The material is made from coral that has been heated to temperatures of about 2000°C to obtain hydroxyapatite (a primary component of coral) of 95 percent purity. The material is then formed into a scaffolding resembling natural bone, and this final product is irradiated with gamma rays to sterilize it. 

In the diffusion model presented here, the value of the diffusion constant D remains constant, which is not the case for natural bone systems. The problems arising from bone diagenesis will be discussed in Section 3.2.7 and by Reiche in this compendium. 

The biocompatible CBPC development has occurred only in the last few years, and the recent trend has been to evaluate them as biocompatible ceramics. After all, biological systems form bone and dentine at room temperature, and it is natural to expect that biocompatible ceramics should also be formed at ambient temperature, preferably in a biological environment when placed in a body as a paste. CBPCs allow such placement. We have discussed such calcium phosphate-based cements in Chapter 13. Calcium-based CBPCs, especially those constituting hydroxyapatite (HAP), are a natural choice. HAP is a primary mineral in bone [3], and hence calcium phosphate cements can mimic natural bone. Some of these ceramics with tailored composition and microstructure are already in use, yet there is ample room for improvement. This Chapter focuses on the most recent biocompatible CBPCs and their testing in a biological environment. To understand biocompatible material and its biological environment, it is first necessary to understand the structure of bone and how it is formed. 

Osteoblasts become trapped in lacunae, in the process of forming the bone and are then termed osteocytes . Osteocytes are considered to be important in the mineralization process. Thus, if an artificial bone is to be fabricated that should eventually be mineralized to a natural bone, then the artificial bone should have a porous stmcture with canaliculi and lacunae. The porous stmcture will enable new osteocytes to enter the artificial bone and cause mineralization. 

This brief description of processes in bone indicates that it is chemically very active. For this reason, the objective of research in bioceramic is to mimic the internal processes and the structure of bone with man-made materials. Once placed, there should be little distinction between the natural bone and man-made ceramic implant or an artificial graft. 

Materials that are compatible with biomtneralized phosphates (see Section 7.2) may be useful for implants A variety of synthetic calcium phosphates have been investigated. Ca3(P04)2 can be used as a biodegradable bone implant, which is gradually replaced by autogenous bone, and less degradable apatite implants can attach to natural bone without inflammation. Bioinert zinc phosphate cements are used in prosthetic dental applications. 

Techniques to produce multiscale biomaterial scaffolds with designer geometries are the need of the hour to provide improved biomimetic properties for functional tissue replacements. While micrometer fibers generate an open pore stnicture, nanofibers support cell adhesion and facilitate cell-cell interactions. This was further proven by cell penetration studies, which showed superior ingrowth of cells into hierarchical structures. Mixed bimodal scaffolds of two different polymers are another promising approach, because they exhibit hierarchical pore/ surface systems and combine the beneficial properties of both polymers at two different scales. Vaiious 3D micro- and nanoscale multiscale scaffolds have been fabricated through various techniques and were found to have the potential to essentially recreate natural bone, cardiac, neural, and vascular tissues. 

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