GITAM DENTAL COLLEGE & HOSPITAL VISAKHAPATNAM - 45
DEPARTMENT OF ORAL & MAXILLOFACIAL SURGERY
SEMINAR ON BIOMATERIALS IN ORAL AND MAXILLOFACIAL SURGERY
Presented by
Dr. M.PRUDHVI RAJ III MDS TABLE OF CONTENTS
INTRODUCTION
AIM & OBJECTIVE
MATERIALS
APPLICATIONS
CONCLUSION
BIBLIOGRAPHY
INTRODUCTION Biomaterial is used to make devices to replace a part or a function of the body in a safe, reliable, economic, and physiologically acceptable manner. A variety of devices and materials presently used in the treatment of disease or injury include such common place items as sutures, needles, catheters, plates, tooth fillings, etc. The use of biomaterials did not become practical until the advent of an aseptic surgical technique developed by Dr. J. Lister in the 1860s. Earlier surgical procedures, whether they involved biomaterials or not, were generally unsuccessful as a result of infection. Problems of infection tend to be exacerbated in the presence of biomaterials, since the implant can provide a region inaccessible to the body’s immunologically competent cells. The earliest successful implants, as well as a large fraction of modern ones, were in the skeletal system.
Definitions of biomaterials : 1)
A Biomaterial can be simply defined as a synthetic material used to replace part of a living system or to function in intimate contact with living tissue.
2)
The Clemson University Advisory Board for Biomaterials has formally defined a biomaterial to be “a systemically and pharmacologically inert substance designed for implantation within or incorporation with living systems.”
3)
Black (1992) defined biomaterials as “a nonviable material used in a medical device, intended to interact with biological systems”.
4)
Bruck (1980) defined biomaterials as “materials of synthetic as well as of natural origin in contact with tissue, blood, and biological fluids, and intended for use for prosthetic, diagnostic, therapeutic, and storage applications without adversely affecting the living organism and its components”.
5)
Williams (1987) defined the biomaterials as “any substance (other than drugs) or combination of substances, synthetic or natural in origin, which can be used for any period of time, as a whole or as a part of a system which treats, augments, or replaces any tissue, organ, or function of the body�.
Selection of Biomedical Materials The process of material selection should ideally be for a logical sequence involving: 1.
Analysis of the problem;
2.
Consideration of requirement;
3.
Consideration of available material and their properties leading to:
4.
Choice of material.
The choice of a specific biomedical material is now determined by consideration of the following: 1.
A proper specification of the desired function for the material;
2.
An accurate characterization of the environment in which it must function, and the effects that environment will have on the properties of the material;
3.
A delineation of the length of time the material must function;
4.
A clear understanding of what is meant by safe for human use.
As the number of available materials increases, it becomes more and more important to be protected from unsuitable products or materials, which haven't been thoroughly evaluated. Most manufacturers of materials operate an extensive quality assurance program and materials are thoroughly tested before being released to the general practitioner.
1.
Standard Specifications: Many standard specification tests of both national and international standards organizations (ISO) are now available, which effectively maintain quality levels. Such specifications normally give details for: (a) the testing of certain products, (b) the method of calculating the results (c) the minimum permissible result, which is acceptable.
2.
Laboratory Evaluation: Laboratory tests, some of which are used in standard specification, can be used to indicate the suitability of certain materials. It is important that methods used to evaluate materials in laboratory give results, which can be correlated with clinical experience.
3.
Clinical Trials: Although laboratory tests can provide many important and useful data on materials, the ultimate test is the controlled clinical trial and verdict of practitioners after a period of use in general practice. Many materials produce good results in the laboratory, only to be found lacking when subjected to clinical use. The majority of manufacturers carry out extensive clinical trials of new materials, normally in cooperation with a university or hospital department, prior to releasing a product for use by general practitioners. TYPES OF BIOMATERIALS
The most common classes of materials used as biomedical materials are: 1.
polymers,
2.
metals
3.
ceramics.
These three classes are used singly and in combination to form most of the implantation devices available today.
I)
Metallic biomaterials
Metals are used as biomaterials due to their excellent electrical and thermal conductivity and mechanical properties. Since some electrons are independent in metals, they can quickly transfer an electric charge and thermal energy. The mobile free electrons act as the binding force to hold the positive metal ions together. This attraction is strong, as evidenced by the closely packed atomic arrangement resulting in high specific gravity and high melting points of most metals. Since the metallic bond is essentially nondirectional, the position of the metal ions can be altered without destroying the crystal structure resulting in a plastically deformable solid. The first metal alloy developed specifically for human use was the “vanadium steel� which was used to manufacture bone fracture plates (Sherman plates) and screws. Most metals such as iron (Fe), chromium (Cr), cobalt (Co), nickel (Ni), titanium (Ti), tantalum (Ta), niobium (Nb), molybdenum (Mo), and tungsten (W) that were used to make alloys for manufacturing implants can only be tolerated by the body in minute amounts. Sometimes those metallic elements, in naturally occurring forms, are essential in red blood cell functions (Fe) or synthesis of a vitamin B 12 (Co), but cannot be tolerated in large amounts in the body [Black, 1992].The metallic systems most frequently used in the body are:
(a) Iron-base alloys of the 316L stainless steel (b) Titanium and titanium-base alloys, such as (i)Ti-6% Al-4%V, and commercially pure ≼ 98.9% (ii) Ti-Ni (55% Ni and 45% Ti) (c) Cobalt base alloys of four types (i) Cr (27-30%), Mo (5-7%), Ni (2-5%) (ii) Cr (19-21%), Ni (9-11%), W (14-16%) (iii) Cr (18-22%), Fe (4-6%), Ni (15-25%), W (3-4%) (iv)Cr (19-20%), Mo (9-10%), Ni (33-37%)
1)
Stainless steel :
The first stainless steel utilized for implant fabrication was the 18-8 (type 302 in modern classification), which is stronger and more resistant to corrosion than the vanadium steel. Later 18-8s Mo stainless steel was introduced which contains a small percentage of molybdenum to improve the corrosion resistance in chloride solution (salt water). This alloy became known as type 316 stainless steel. In the 1950s the carbon content of 316 stainless steel was reduced from 0.08 to a maximum amount of 0.03% (all are weight percent unless specified) for better
corrosion resistance to chloride solution and to minimize the sensitization, and hence became known as type 316L stainless steel. The minimum effective concentration of chromium is 11% to impart corrosion resistance in stainless steels. The chromium is a reactive element, but it and its alloys can be passivated by 30% nitric acid to give excellent corrosion resistance. The austenitic stainless steels, especially types 316 and 316L , are most widely used for implant fabrication. These cannot be hardened by heat treatment but can be hardened by cold-working. This group of stainless steels is nonmagnetic and possesses better corrosion resistance than any others. The inclusion of molybdenum enhances resistance to pitting corrosion in salt water. Compositions of 316L Stainless Steel (American Society for Testing and Materials) Element
Composition (%)
Carbon
0.03 max
Manganese
2.00 max
Phosphorus
0.03 max
Sulfur
0.03 max
Silicon
0.75 max
Chromium
17.00–20.00
Nickel
12.00–14.00
Molybdenum
2)
2.00–4.00
Cobalt chromium ( Co Cr) alloys :
There are basically two types of cobalt-chromium alloys: 1)
The castable CoCrMo alloy
2)
The CoNiCrMo alloy which is usually Wrought by (hot) forging .
The castable CoCrMo alloy has been used for many decades in dentistry and, relatively recently, in making artificial joints. The wrought CoNiCrMo alloy is relatively new, now used for making the stems of prosthesis for heavily loaded joints such as the knee and hip. The ASTM lists four types of CoCr alloys which are recommended for surgical implant applications: 1) cast CoCrMo alloy (F75) 2) wrought CoCrWNi alloy (F90) 3) wrought CoNiCrMo alloy (F562), 4) wrought CoNiCrMoWFe alloy (F563).
At the present time only two of the four alloys are used extensively in implant fabrications, the castable CoCrMo and the wrought CoNiCrMo alloy. The two basic elements of the CoCr alloys form a solid solution of up to 65% Co. The molybdenum is added to produce finer grains which results in higher strengths after casting or forging. The chromium enhances corrosion resistance as well as solid solution strengthening of the alloy. The CoNiCrMo alloy originally called MP35N (Standard Pressed Steel Co.) contains approximately 35% Co and Ni each. The alloy is highly corrosion resistant to seawater (containing chloride ions) under stress. However, there is a considerable difficulty of cold working on this alloy, especially when making large devices such as hip joint stems. Only hot-forging can be used to fabricate a large implant with the alloy. 3)
Titanium Alloys : A)
Pure Titanium and Ti6Al4V :
Titanium used for implant fabrication since 1930s. Titanium’s lightness (4.5 g/cm3) and good mechanochemical properties are salient features for implant application. There are four grades of unalloyed commercially pure (cp) titanium for surgical implant applications. The impurity contents separate them; oxygen, iron, and nitrogen should be controlled carefully. Oxygen in particular has a great influence on the ductility and strength.
The main alloying elements of the alloy are aluminum (5.5~6.5%) and vanadium (3.5~4.5%). Titanium is an allotropic material, which exists as a hexagonal close packed structure up to 882째C and body-centered cubic structure above that temperature. Titanium alloys can be strengthened and mechanical properties varied by controlled composition and thermomechanical processing techniques. The addition of alloying elements to titanium enables it to have a wide range of properties: (1) Aluminum tends to stabilize the alpha-phase, that is increase the transformation temperature from alpha to beta -phase (2) vanadium stabilizes the beta-phase by lowering the temperature of the transformation from alpha to beta phase. The alpha-alloy has a single-phase microstructure which promotes good weldability. The stabilizing effect of the high aluminum content of these groups of alloys makes excellent strength characteristics and oxidation resistance at high temperature (300~600째C). These alloys cannot be heat treated for precipitation hardening since they are single-phased. The addition of controlled amounts of beta-stabilizers causes the higher strength beta-phase to persist below the transformation temperature which results in the two-phase system. The precipitates of beta-phase will appear by heat treatment in the solid solution temperature and subsequent quenching, followed by aging at a somewhat lower temperature.
The aging cycle causes the coherent precipitation of some fine alpha particles from the metastable beta particles; Imparting alpha structure may produce local strain field capable of absorbing deformation energy. Cracks are stopped or deterred at the alpha particles, so that the hardness is higher than for the solid solution. The higher percentage of beta-stabilizing elements (13%V in Ti13V11Cr3Al alloy) results in a microstructure that is substantially beta which can be strengthened by heat treatment. Another Ti alloy (Ti13Nb13Zr) with 13% Nb and 13% Zr showed martensite structure after being water quenched and aged, which showed high corrosion resistance with low modulus (E = 79 MPa). Formation of plates of martensite induces considerable elastic distortion in the parent crystal structure and increases strength. B)
TiNi Alloys
The titanium–nickel alloys show unusual properties i.e., after it is deformed the material can snap back to its previous shape following heating of the material. This phenomenon is called shape memory effect (SME). The SME of TiNi alloy was first observed by Buehler and Wiley at the U.S. Naval Ordnance Laboratory [Buehler et al., 1963]. The equiatomic TiNi or NiTi alloy (Nitinol) exhibits an exceptional SME near room temperature: if it is plastically deformed below the transformation temperature, it reverts back to its original shape as the temperature is raised. The SME can be generally related to a diffusionless martensitic phase transformation which is also
thermoelastic in nature, the thermoelasticity being attributed to the ordering in the parent and martensitic phases [Wayman and Shimizu, 1972]. Another unusual property is the superelasticity. As can be seen, the stress does not increase with increased strain after the initial elastic stress region and upon release of the stress or strain the metal springs back to its original shape in contrast to other metals such as stainless steel. The superlastic property is utilized in orthodontic archwires since the conventional stainless steel wires are too stiff and harsh for the tooth. In addition, the shape memory effect can also be utilized. Some possible applications of shape memory alloys are orthodontic dental archwire, intracranial aneurysm clip, vena cava filters , contractile artificial muscles for an artificial heart, vascular stent, catheter guide wire, and orthopedic staple [Duerig et al., 1990] 4)
5)
Dental metals : A.
Dental amalgam
B.
Gold and gold alloys
Other metals : A.
Tantalum
B.
Platinum group metals (PGM) such as Pt, Pd, Rh, Ir, Ru, and Os
CERAMIC BIOMATERIALS
II)
The most frequently used ceramic implant materials include aluminum oxides, calcium phosphates, and apatites and graphite. Glasses have also been developed for medical applications. The use of ceramics was motivated by: (i) their inertness in the body, (ii) their formability into a variety of shapes and porosities, (iii) their high compressive strength, and (iv) some cases their excellent wear characteristics. Applications of ceramics are in some cases limited by their generally poor mechanical properties: (a) in tension; (b) load bearing, implant devices that are to be subjected to significant tensile stresses must be designed and manufactured with great care if ceramics are to be safely used. Ceramics are used for the repair and restoration of diseased or damaged parts of the musculo-skeletal system.
1)
Nonabsorbable or Relatively Bioinert Bioceramics A.
Pyrolitic carbon-coated devices
2)
3)
B.
Dense and nonporous aluminum oxides
C.
Porous aluminum oxides
D.
Zirconia ceramics
E.
Dense hydroxyapatites
Biodegradable or Resorbable Ceramics A.
Aluminum–calcium–phosphorous oxides
B.
Glass fibers and their composites
C.
Corals
D.
Calcium sulfates, including plaster of Paris
E.
Ferric–calcium–phosphorous oxides
F.
Hydroxyapatites
G.
Tricalcium phosphate
H.
Zinc–calcium–phosphorous oxides
I.
Zinc–sulfate–calcium–phosphorous oxides
Bioactive or Surface-Reactive Ceramics A.
Bioglass and Ceravital
III)
B.
Dense and nonporous glasses
C.
Hydroxyapatite
Polymeric Biomaterials
Polymers have assumed an important role in medical applications. In most of these applications, polymers have little or no competition from other types of materials. Their unique properties are: 1- Flexibility; 2- Resistance to biochemical attack; 3- Good biocompatibility; 4- Light weight; 5- Available in a wide variety of compositions with adequate physical and mechanical properties; 6- Can be easily manufactured into products with the desired shape. There are a large number of polymeric materials that have been used as implants or part of implant systems. The polymeric systems include acrylics, polyamides, polyesters, polyethylene, polysiloxanes, polyurethane, and a number of reprocessed biological materials. Some of the applications include the use of membranes of ethylene-vinyl-acetate (EVA) copolymer for controlled release and the use of poly-glycolic acid for use as a resorbable suture material.. As bioengineers search for designs of ever increasing capabilities to meet the needs of medical practice, polymeric materials alone and in combination with metals and ceramics are becoming increasingly incorporated into devices used in the body.
1)
Polyvinylchloride (PVC)
2)
Polyethylene (PE)
3)
Polypropylene (PP)
4)
Polymethylmetacrylate (PMMA)
5)
Polystyrene (PS)
6)
Polyethylenterephthalate (PET)
7)
Polytetrafluoroethylene (PTFE)
8)
Polyurethane (PU)
9)
Polyamide (nylon)
(PTFE) Polytetrafluoroethylene is a fluorocarbon–based polymer. Commercially, the material is best known as Teflon. It is made by free-radical polymerization of tetrafluoroethylene and has a carbon backbone chain, where each carbon has two fluorine atoms attached to it. Properties of PTFE 1-Hydrophobic (Water hating) 2- Biologically inert* 3- Non-biodegradable 4- Has low friction characteristics 5- Excellent "Slipperiness" 6- Relatively lower wear resistance. 7- Highly crystalline (94%) 8- Very high density (2.2 kg.m-3)
9- Low modulus of elasticity (0.5MPa) 10- Low tensile strength (14MPa) PTFE has many medical uses, including: 1- Arterial grafts (artificial vascular graft); 2- Catheters; 3- Sutures; 4- Uses in reconstructive and cosmetic facial surgery. PTFE can be fabricated in many forms, such as: 1- Can be woven into a porous fabric like mesh. When implanted in the body, this mesh allows tissue to grow into its pores, making it ideal for medical devices, such as vascular grafts; 2- Pastes; 3- Tubes; 4- Strands; 5- Sheets. Disadvantages of PTFE PTFE has relatively low wear resistance. Under compression or in solutions where rubbing or abrasion can occur, it can produce wear particles. These can result in a chronic inflammatory reaction, an undesirable outcome. Polyethylene, (PE) It is chemically the simplest of all polymers and as a homochain polymer. It is essentially: 1- Stable and suitable for long-time implantation under many circumstances;
2- Relatively inexpensive; 3- Has good general mechanical properties. So that it has become a versatile biomedical polymer with applications ranging from catheters to joint-replacement. Polypropylene, (PP) Polypropylene is widely used in medical devices ranging from sutures to finger joints and oxygenerators. Poly (methyl methacrylate), PMMA It is a hard brittle polymer that appears to be unsuitable for most clinical applications, but it does have several important characteristics. (a) It can be prepared under ambient conditions so that it can be manipulated in the operating theater or dental clinic, explaining its use in dentures and bone cement. (b) The relative success of many joint prostheses is dependent on the performance of the PMMA cement, which is prepared intraoperatively by mixing powdered polymer with monomeric methylmethacrylate, which forms a dough that can be placed in the bone, where it then sets. The disadvantages of PMMA (a) The exotherm of polymerization; (b) The toxicity of the volatile methylmethacrylate; (c) The poor fracture toughness. (But no better material has been developed to date)
Polyesters
Polyurathanes
MATERIALS IN MAXILLOFACIAL PROSTHETICS Despite improvements in surgical and restorative techniques, the materials used in maxillofacial prosthetics are far from ideal. An ideal material should be inexpensive, biocompatible, strong, and stable. In addition, the material should be skin-like in color and texture. Maxillofacial materials must exhibit resistance to tearing and should be able to withstand moderate thermal and chemical challenges. Currently, no material fulfills all of these requirements. A brief description of maxillofacial materials is included in the following paragraphs: LATEXES Latexes are soft, inexpensive materials that may be used to create lifelike prostheses. Unfortunately, these materials are weak, degenerate rapidly, exhibit color instability and can cause allergic reactions. A recently developed synthetic latex is a tripolymer of butylacrylate, methyl methacrylate, and methyl metharylamide. This material is nearly transparent, but has limited applications. VINYL PLASTISOLS They are plasticized vinyl resin sometimes are used in maxillofacial applications. Plastsols are thick liquids comprising small vinyl particles dispersed in a plasticizer.
Colorants are added to these materials to match individual skin tones. Unfortunately, vinyl plastisols harden with age because plasticizer loss. Ultraviolet light also has an adverse effect on these materials. For these reasons, the use of vinyl is limited. SILICONE RUBBERS Both heat-vulcanizing and room temperature vulcanizing silicones are in use today and both exhibit advantages and disadvantages. Room temperature vulcanizing silicones are supplied as single- paste systems. These silicones are not as strong as the heat-vulcanized silicones and generally are monochromatic. Heat-vulcanizing silicone is supplied as a semi-solid material that requires milling, packing under pressure, and 30-minute heat treatment application cycle at 180oC. Heat vulcanizing silicone displays better strength and color than room temperature vulcanizing silicone. POLYURETHANE POLYMERS Polyurethane is the most recent of the materials used in maxillofacial prosthetics. Fabrication of a polyurethane prosthesis requires accurate proportioning of three materials. The material is placed in a stone or metal mold and allowed to polymerize at room temperature. Although a polyurethane prosthesis has a natural feel and appearance, it is susceptible to rapid deterioration. The loss of natural teeth, through disease or trauma, has for many years been compensated by the provision of artificial teeth in the form of bridges and dentures. These essentially provide an aesthetic replacement of crown of the tooth but do nothing to replace the root and its attachment to the bone of the jaw.
NATURAL POLYMERS Natural polymers, or polymers, derived from living creatures, are of great interest in the biomaterials field. In the area of tissue-engineering, for example, scientists and engineers look for scaffold on which one may successfully grow cells to replace damaged tissue. Typically, it is desirable for these scaffolds to be: (1) Biodegradable; (2) Non-toxic/ non-inflammatory; (3) Mechanically similar to the tissue to be replaced; (4) Highly porous; (5) Encouraging of cell attachments and growth; (6) Easy and cheap to manufacture; (7) Capable of attachment with other molecules ( to potentially increase scaffold interaction with normal tissue) Normal polymers often easily fulfill these expectations, as they are naturally engineered to work well within the living beings from which they come. Three examples of natural polymers that have been previously studied for use as biomaterials are: collagen, chitosan, and alginate.
IV)
COMPOSITE BIOMATERIALS
Composite materials have been extensively used in dentistry and prosthesis designers are now incorporating these materials into other applications. Typically, a matrix of ultrahigh-molecular-weight polyethylene (UHMWPE) is reinforced
with carbon fibers. These carbon fibers are made by pyrolizing acrylic fibers to obtain oriented graphitic structure of high tensile strength and high modulus of elasticity. The carbon fibers are 6-15mm in diameter, and they are randomly oriented in the matrix. In order for the high modulus property of the reinforcing fibers to strengthen the matrix, a sufficient interfacial bond between the fiber and matrix must be achieved during the manufacturing process. Since the mechanical properties of these composites with the proportion of carbon fibers in the composites, it is possible to modify the material design flexibility to suit the ultimate design of prostheses. Composites have unique properties and are usually stronger than any of the single materials from which they are made. Workers in this field have taken advantages of this fact and applied it to some difficult problems where tissue in-growth is necessary. -
Deposited Al2O3 onto carbon;
-
Carbon / PTFE;
-
Al2O3 / PTFE;
-
PLA-coated Carbon fibers.
Some applications of composites in biomaterial applications are: (1) dental filling composites (2) reinforced methyl methacrylate bone cement and ultra-high-molecularweight polyethylene (3) orthopedic implants with porous surfaces.
BIODEGRADABLE POLYMERIC BIOMATERIALS
V)
Another class of materials that is receiving increased attention is biodegradable materials. Generally, when a material degrades in the body its properties change from their original values leading to altered and less desirable performance. It is possible, however, to design into an implant's performance the controlled degradation of a material, such that natural tissue replaces the prosthesis and its function. Suture material that hold a wound together but resorb in the body as the wound heals and gains strength. Another application of these materials occurs when they are used to encourage natural tissue to grow. Certain wound dressings and ceramic bone augmentation materials encourage tissue to grow into them by providing a "scaffold". The scaffold material may or may not resorb over a period of time but in each case, natural tissue has grown into the space, then by restoring natural function. One final application of biodegradable materials is in drug therapy, where it is possible to chemically bond certain drugs to the biodegradable material, when these materials are placed within the body the drug is released as the material degrades, thereby providing a localized, sustained release of drugs over a predictable period of time.
1)
Glycolide/Lactide-Based Biodegradable Linear Aliphatic polyesters A.
Glycolide-Based Biodegradable Homopolymer Polyesters
B.
Glycolide-Based Biodegradable Copolyesters Having Aliphatic
Polyester-Based Co-Monomers C.
Glycolide-Based Biodegradable Copolyesters with Non-Aliphatic PolyesterBased Co-Monomers
D.
Glycolide-Derived Biodegradable Polymers Having Ether Linkage
E.
Lactide Biodegradable Homopolymers and Copolymers
2)
Non-Glycolide/Lactide-Based Linear Aliphatic Polyesters
3)
Non-Aliphatic Polyesters Type Biodegradable Polymers A.
Aliphatic and Aromatic Polycarbonates
B.
Poly(alkylene oxalates) and Copolymers
BIOLOGIC BIOMATERIALS
VI) 1)
Tissue-Derived Biomaterials (Collagen)
Collagen is the most widely found protein in mammals (25% of our protein mass) and is the major provider of strength to tissue. A typical collagen molecule consists of three interwined protein chains that form a helical structure similar to a typical staircase). These molecules polymerize together to form collagen fibers of varying length, thickness and interweaving pattern (some collagen molecules will form ropelike structures, while others will form meshes
or networks). There are actually at least 15 different types of collagen, differing in their structure, function, location, and other characteristics. The predominant form used in biomedical applications, however, is type I collagen, which is a "rope-forming" collagen and can be found almost everywhere in the body, including skin and bone. Collagen can be resorbed into the body, is non-toxic produces only a minimal immune response, and is excellent for attachment and biological interaction with cell. Collagen may also be processed into a variety of formats, including porous sponges, gels and sheets, and can be cross-linked with chemicals to make it stronger or to alter its degradation rate. The number of biomedical applications in which collagen has been utilized is too high to count here, it not only explored for use in various types of surgery, cosmetics, and drug delivery, but in bio-prosthetic implants and tissue-engineering of multiple organs as well. Cells grown in collagen often come close to behaving as they do within the body, which is why collagen is so promising when one is trying to duplicate natural tissue function and healing. However, some disadvantages to using collagen as a cell substrate do exist. Depending on how it is processed, collagen can potentially cause alteration of cell behavior (e.g. changes in growth or movement), have inappropriate mechanical properties, or undergo contraction (shrinkage). Because cells interact so easily with collagen, cells can actually pull and reorganize collagen fibers, causing scaffolds to lose their shape if they are not properly stabilized by cross-linking or mixing with another less "vulnerable material". Fortunately, collagen can be easily combined with other biological or synthetic materials, to improve its mechanical properties or change the way cells
behave when grown upon it. CHITOSAN It is derived from chitin, a type of polysaccharide (sugar) that is present in the hard exoskeletons of shellfish like shrimp and crab. Chitin has sparked interest in the tissue-engineering field due to several desirable properties: 1- Minimal foreign body reaction; 2- Mild processing conditions (synthetic polymers often need to be dissolved in harsh chemicals; chitosan will dissolve in water based on pH); 3- Controllable mechanical/biodegradation properties (such as scaffold porosity); 4- Availability of chemical side groups for attachment to other molecules. Chitosan has already been investigated for use in the engineering of cartilage, nerve and liver tissues. Chitosan has also been studied for use in wound healing and drug delivery. Current difficulties with using chitosan as a polymer scaffold in tissue-engineering, however, include low strength and inconsistent behavior with seeded cells. Fortunately, chitosan may be easily combined with other materials in order to increase its strength and cellattachment potential. Mixtures with synthetic polymers such as poly (vinyl alcohol) and poly (ethylene glycol) or natural polymers such as collagen have already been produced.
ALGINATE It is a polysaccharide derived from brown seaweed. Like chitosan,
alginate can be processed easily in water and has been found to be fairly nontoxic and non-inflammatory enough, so that it has been approved in some countries for wound dressing and for use in food products. Alginate is biodegradable, has controllable porosity, and may be linked to other biologically active molecules. Interestingly, encapsulation of certain cell types into alginate beads may actually enhance cell survival and growth. In addition, alginate has been explored for use in liver, nerve, heart, and cartilage tissue-engineering. Unfortunately, some drawbacks of alginate include mechanical weakness and poor cell adhesion. Again, to overcome these limitations, the strength and cell behavior of alginate have been enhanced by mixing with other materials, including the natural polymers agarose and chitosan.
CONCLUSION In the future, the practice of oral and maxillofacial surgery will be shaped by these tools. The emerging fields of tissue engineering and biomaterials are truly exciting. Products of the future, including those from genetically engineered proteins all the way to tissue-engineered organs, are and will be in clinical trials in the very near future. Our specialty will benefit from the development of many of these biomaterials, and our future will be shaped by their use.