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Biomaterials Information

The development of biomaterials, as a science, is about fifty years old. The study of biomaterials is called biomaterials science. It has experienced steady and strong growth over its history, with many companies investing large amounts of money into the development of new products. Biomaterials science encompasses elements of medicine, biology, chemistry, tissue engineering and materials science.

The iridescent nacre inside a Nautilus shell.

Contents

Introduction

Biomaterials can generally be produced either in nature or synthesized in the laboratory using a variety of chemical approaches utilizing metallic components or ceramics. They are often used and/or adapted for a medical application, and thus comprises whole or part of a living structure or biomedical device which performs, augments, or replaces a natural function. Such functions may be benign, like being used for a heart valve, or may be bioactive with a more interactive functionality such as hydroxy-apatite coated hip implants. Biomaterials are also used every day in dental applications, surgery, and drug delivery. E.G. A construct with impregnated pharmaceutical products can be placed into the body, which permits the prolonged release of a drug over an extended period of time. A biomaterial may also be an autograft, allograft or xenograft used as a transplant material.

Materials scientists are currently paying more and more attention to the process inorganic crystallization within a largely organic matrix of naturally occurring compounds. This process typically generally occurs at ambient temperature and pressure. Interestingly, the vital organisms through which these crystalline minerals form are capable of consistently producing intricately complex structures. Understanding the processes in which living organisms are capable of regulating the growth of crystalline minerals such as silica could lead to significant scientific advances and novel synthesis techniques for nanoscale composite materials -- or nanocomposites.

Biomineralization

Main article: Biomineralization

Biomineralization (e.g. silicification) is quite common in the biological world and occurs in bacteria, single-celled organisms, plants (e.g. petrified wood), and animals (invertebrates and vertebrates). Crystalline minerals formed in this type of environment often show exceptional mechanical properties (e.g. strength, hardness, fracture toughness) and tend to form hierarchical structures that exhibit microstructural order over a range of length or spatial scales. The minerals are typically crystallized from an environment that is undersaturated with respect to certain metallic elements such as silicon, calcium and phosphorus, which are readily oxidized under conditions of neutral pH and low temperature (0 - 40 degrees C). Formation of the mineral may occur either within or outside of the cell wall of an organism, and specific biochemical reactions for mineral deposition exist that include lipids, proteins and carbohydrates. The significance of the cellular machinery cannot be overemphasized, and it is with advances in experimental techniques in cellular biology and the capacity to mimic the biological environment that significant progress is currently being reported. [1] [2] [3][4] [5] [6]

Sand from Pismo Beach, California including quartz, shell and rock fragments.

Examples include silicates in algae and diatoms, carbonates in invertebrates, and calcium phosphates and carbonates in vertebrates. These minerals often form structural features such as sea shells and the bone mineral in mammals and birds. Organisms have been producing mineralized skeletons for nearly 600 million years. The most common biominerals are the phosphate and carbonate salts of calcium that are used in conjunction with organic polymers such as collagen and chitin to give mechanical strength to bones and shells. Other examples include copper, iron and gold deposits involving bacteria. [7]

Thus, most natural (or biological) materials are complex composites whose mechanical properties are often outstanding, considering the weak constituents from which they are assembled. These complex structures, which have risen from hundreds of million years of evolution, are inspiring materials scientists interested primarily in the design of novel materials with exceptional physical properties for high performance in adverse conditions. Their defining characteristics such as hierarchy, multifunctionality, and the capacity for self-healing, are currently being investigated. [8] [9] [10] [11]

Collagen fibers of woven bone SEM 10,000x magnification of crystalline bone mineral.

The basic building blocks begin with the 20 amino acids and proceed to polypeptides and polysaccharides. These, in turn, compose the basic proteins, which are the primary constituents of the ‘soft tissues’ common to most biominerals. With well over 1000 proteins possible, current research emphasizes the use of collagen, chitin, keratin, and elastin. The ‘hard’ phases are often strengthened by crystalline minerals, which nucleate and grow in a biomediated environment that determines the size, shape and distribution of individual crystals. The most important silicate phases have been identified as hydroxyapatite, silica, and aragonite. Using the classification of Wegst and Ashby, the principal mechanical characteristics and structures of a number of biological ceramics, polymer composites, elastomers, and cellular materials have been recently characterized. Selected systems in each class are being investigated with emphasis on the relationship between their microstructure over a range of length scales and their mechanical response (esp. fracture toughness). [12] [13] [14] [15]

Thus, the crystallization of inorganic materials in nature generally occurs at ambient temperature and pressure. Yet the vital organisms through which these minerals form are capable of consistently producing extremely precise and complex structures. Understanding the processes in which living organisms control the growth of crystalline minerals such as silica could lead to significant advances in the field materials science, and open the door to novel synthesis techniques for nanoscale composite materials, or nanocomposites.

Self-assembly

Self-assembly is the most common term in use in the modern scientific community to describe the spontaneous aggregation of particles (atoms, molecules, colloids, micelles, etc.) without the influence of any external forces. Large groups of such particles are known to assemble themselves into thermodynamically stable, structurally well-defined arrays, quite reminiscent of one of the 7 crystal systems found in metallurgy and mineralogy (e.g. face-centered cubic, body-centered cubic, etc.). The fundamental difference in equilibrium structure is in the spatial scale of the unit cell (or lattice parameter) in each particular case.

Molecular self-assembly is found widely in biological systems and provides the basis of a wide variety of complex biological structures. This includes an emerging class of mechanically superior biomaterials based on microstructural features and designs found in nature. Thus, self-assembly is also emerging as a new strategy in chemical synthesis and nanotechnology. Molecular crystals, liquid crystals, colloids, micelles, emulsions, phase-separated polymers, thin films and self-assembled monolayers all represent examples of the types of highly ordered structures which are obtained using these techniques. The distinguishing feature of these methods is self-organization. [16] [17] [18]

Structural hierarchy

Nearly all materials could be seen as hierarchically structured, especially since the changes in spatial scale bring about different mechanisms of deformation and damage. However, in biological materials this hierarchical organization is inherent to the microstructure. One of the first examples of this, in the history of structural biology, is the early X-Ray scattering work on the hierarchical structure of hair and wool by Astbury and Woods.[19] In bone, for example, collagen is the building block of the organic matrix—a triple helix with diameter of 1.5 nm. These tropocollagen molecules are intercalated with the mineral phase (hydroxyapatite, a calcium phosphate) forming fibrils that curl into helicoids of alternating directions. These "osteons" are the basic building blocks of bones, with the volume fraction distribution between organic and mineral phase being about 60/40. In another level of complexity, the hydroxyapatite crystals are platelets that have a diameter of approximately 70–100 nm and thickness of 1 nm. They originally nucleate at the gaps between collagen fibrils.

Similarly, the hierarchy of abalone shell begins at the nanolevel, with an organic layer having a thickness of 20–30 nm. This layer proceeds with single crystals of aragonite (a polymorph of CaCO3) consisting of "bricks" with dimensions of 0.5 and finishing with layers approximately 0.3 mm (mesostructure).

Crabs are arthropods whose carapace is made of a mineralized hard component (which exhibits brittle fracture) and a softer organic component composed primarily of chitin. The brittle component is arranged in a helical pattern. Each of these mineral ‘rods’ ( 1 μm diameter) contains chitin–protein fibrils with approximately 60 nm diameter. These fibrils are made of 3 nm diameter canals which link the interior and exterior of the shell.

Applications

Biomaterials are used in:

Biomaterials must be compatible with the body, and there are often issues of biocompatibility which must be resolved before a product can be placed on the market and used in a clinical setting. Because of this, biomaterials are usually subjected to the same requirements of those undergone by new drug therapies.[9] All manufacturing companies are also required to ensure traceability of all of their products so that if a defective product is discovered, others in the same batch may be traced.

Heart valves

In the United States, 45% of the 250,000 valve replacement procedures performed annually involve a mechanical valve implant. The most widely used valve is a bileaflet disc heart valve, or St. Jude valve. The mechanics involve two semicircular discs moving back and forth, with both allowing the flow of blood as well as the ability to form a seal against backflow. The valve is coated with pyrolytic carbon, and secured to the surrounding tissue with a mesh of woven fabric called DacronTM (du Pont's trade name for polyethylene terephthalate). The mesh allows for the body's tissue to grow while incorporating the valve.[20]

Skin repair

main article Tissue Engineering

Most of the time "artificial" tissue is grown from the patients own cells. However, when the damage is so extreme that it is impossible to use the patient's own cells, artificial tissue cells are grown. The difficulty is in finding a scaffold that the cells can grow and organize on. The characteristics of the scaffold must be that it is biocompatible, cells can adhere to the scaffold, mechanically strong and biodegradable. One successful scaffold is a copolymer of lactic acid and glycolic acid.[20]

Compatibility

Biocompatibility is related to the behavior of biomaterials in various environments under various chemical and physical conditions. The term may refer to specific properties of a material without specifying where or how the material is to be used. For example, a material may elicit little or no immune response in a given organism, and may or may not able to integrate with a particular cell type or tissue). The ambiguity of the term reflects the ongoing development of insights into how biomaterials interact with the human body and eventually how those interactions determine the clinical success of a medical device (such as pacemaker or hip replacement. Modern medical devices and prostheses are often made of more than one material—so it might not always be sufficient to talk about the biocompatibility of a specific material. [21]

Also, a material should not be toxic unless specifically engineered to be so—like "smart" drug delivery systems that target cancer cells and destroy them. Understanding of the anatomy and physiology of the action site is essential for a biomaterial to be effective. An additional factor is the dependence on specific anatomical sites of implantation. It is thus important, during design, to ensure that the implement will fit complementarily and have a beneficial effect with the specific anatomical area of action.

Biopolymers

Main article: Biopolymer

Biopolymers are polymers produced by living organisms. Cellulose and starch, proteins and peptides, and DNA and RNA are all examples of biopolymers, in which the monomeric units, respectively, are sugars, amino acids, and nucleotides. Cellulose is both the most common biopolymer and the most common organic compound on Earth. About 33% of all plant matter is cellulose. [22] [23]

Some biopolymers are biodegradable. That is, they are broken down into CO2 and water by microorganisms. In addition, some of these biodegradable biopolymers are compostable. That is, they can be put into an industrial composting process and will break down by 90% within 6 months. Biopolymers that do this can be marked with a 'compostable' symbol, under European Standard EN 13432 (2000). Packaging marked with this symbol can be put into industrial composting processes and will break down within 6 months (or less). An example of a compostable polymer is PLA film under 20 μm thick: films which are thicker than that do not qualify as compostable, even though they are biodegradable. A home composting logo may soon be established: this will enable consumers to dispose of packaging directly onto their own compost heap. [24] [25] [26]

See also

References

  1. ^ Berg, J.M. et al. (2002). Biochemistry (5 ed.). W.H. Freeman & Co.. ISBN 0716747383.
  2. ^ Omori, M. and Watabe, N., Eds., Mechanisms of biomineralization in animals and plants, Tokai University Press, Tokyo (1980)
  3. ^ Perry, C. C. (2003). "Silicification: The Processes by Which Organisms Capture and Mineralize Silica". Reviews in Mineralogy and Geochemistry 54: 291. doi:10.2113/0540291.
  4. ^ Weiner, S., and Lowenstam, H.A. (1989). On Biomineralization. Oxford University Press. ISBN 0-19-504977-2.
  5. ^ Mann, S. (2005). Biomineralization. Oxford University Press. ISBN 0895736721.
  6. ^ Edited by Astrid Sigel, Helmut Sigel, Roland K. O. Sigel (2008). Astrid Sigel, Helmut Sigel and Roland K.O. Sigel. ed. Biomineralization: From Nature to Application. Metal Ions in Life Sciences. 4. Wiley. ISBN 978-0-470-03525-2.
  7. ^ Sarikaya M (1999). "Biomimetics: materials fabrication through biology". Proc. Natl. Acad. Sci. U.S.A. 96 (25): 14183–5. doi:10.1073/pnas.96.25.14183. PMID 10588672.
  8. ^ Heuer, A.H., et al. (1992). "Innovative Materials Processing Strategies: A Biomimetic Approach". Science 255: 1098. doi:10.1126/science.1546311.
  9. ^ a b Lin, A., Meyers, M.A., et al., Biological Materials: Structure & Mechanical Properties, Prog. Mat. Sci., Vol. 53 (2008)
  10. ^ Currey, J. D. (1977). "Mechanical Properties of Mother of Pearl in Tension". Proceedings of the Royal Society of London. Series B, Biological Sciences (1934-1990) 196: 443. doi:10.1098/rspb.1977.0050.
  11. ^ Currey, JD (1999). "The design of mineralised hard tissues for their mechanical functions.". The Journal of experimental biology 202 (Pt 23): 3285–94. PMID 10562511.
  12. ^ Heuer, A.; Fink, D.; Laraia, V.; Arias, J.; Calvert, P.; Kendall, K; Messing, G.; Blackwell, J et al. (1992). "Innovative materials processing strategies: a biomimetic approach". Science 255: 1098. doi:10.1126/science.1546311.
  13. ^ Whitesides, G.; Mathias, J.; Seto, C. (1991). "Molecular self-assembly and nanochemistry: a chemical strategy for the synthesis of nanostructures". Science 254: 1312. doi:10.1126/science.1962191.
  14. ^ Aksay, I.A., et al. (2000). "Self-Assembled Ceramics". Ann. Rev. Phys. Chem. 51: 601. doi:10.1146/annurev.physchem.51.1.601. PMID 11031294.
  15. ^ Sarikaya, M., K.E. Gunnison, M. Yasrebi and I.A. Aksay, Mechanical property–microstructural relationships in abalone shell; Mater. Res. Soc., Vol. 174, p. 109 (1990) (conference presenation)
  16. ^ Whitesides, G.M., et al. (1991). "Molecular Self-Assembly and Nanochemistry: A Chemical Strategy for the Synthesis of Nanostructures". Science 254: 1312. doi:10.1126/science.1962191.
  17. ^ Dabbs, D. M and Aksay, I.A. (2000). "Self-Assembled Ceramics". Ann. Rev. Phys. Chem. 51: 601. doi:10.1146/annurev.physchem.51.1.601. PMID 11031294.
  18. ^ Ariga, K., et al., Challenges and breakthroughs in recent research on self-assembly, Sci. Technol. Adv. Mater., Vol. 9, p. 14109 (2008)
  19. ^ Thoru Pederson, Present at the Flood: How Structural Molecular Biology Came About, FASEB J. 20: 809-810.
  20. ^ a b Brown, Theodore L.; LeMay, H. Eugene; Bursten, Bruce E. (2000). Chemistry The Central Science. Prentice-Hall, Inc. pp. 451–452. ISBN 0-13-084090-4.
  21. ^ Considerations for the Biocompatibility Evaluation of Medical Devices, Kammula and Morris, Medical Device & Diagnostic Industry, May 2001
  22. ^ Stupp, S.I and Braun, P.V., "Role of Proteins in Microstructural Control: Biomaterials, Ceramics & Semiconductors", Science, Vol. 277, p. 1242 (1997)
  23. ^ Klemm, D., Heublein, B., Fink, H., and Bohn, A., "Cellulose: Fascinating Biopolymer / Sustainable Raw Material", Ang. Chemie (Intl. Edn.) Vol. 44, p. 3358 (2004)
  24. ^ Chandra, R., and Rustgi, R., "Biodegradable Polymers", Progress in Polymer Science, Vol. 23, p. 1273 (1998)
  25. ^ Meyers, M.A., et al., "Biological Materials: Structure & Mechanical Properties", Progress in Materials Science, Vol. 53, p. 1 (2008)
  26. ^ Kumar, A., et al., "Smart Polymers: Physical Forms & Bioengineering Applications", Progress in Polymer Science, Vol. 32, p.1205 (2007)

Further reading

External links

Categories: Biomaterials

 

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