Biomaterials Science: Ceramics PDF

Ratner et.al 4th ed. Biomaterials Science; An Introduction to Materials in Medicine Chapter 1.3.4 Chapter 1.3.5 1 Ceramics Ceramics are inorganic, non-metallic compounds formed between metall...

Ratner et.al 4th ed. Biomaterials Science; An Introduction to Materials in Medicine Chapter 1.3.4 Chapter 1.3.5 1 Ceramics Ceramics are inorganic, non-metallic compounds formed between metallic and nonmetallic elements They are usually crystalline in structure (except for glass which are amorphous or lack organized structure) Their bonding is non-metallic They are and are held together by ionic or covalent bonds. 2  Ceramics The main characteristics of ceramic materials are Hardness Brittleness great strength stiffness resistance to corrosion and wear low density. They work mainly on compression forces On tension forces, their behavior is poor. 3  Ceramics Ceramics are typically electrical and thermal insulators. Ceramics are used in several different fields such as dentistry, orthopedics, and as medical sensors. Typically fail with little, if any, plastic deformation, and they are sensitive to the presence of cracks or other defects Actual fracture strengths of ceramics tend to be one to two orders of magnitude lower than their theoretical strengths Because it will contain flaws and cracks. These then act as sites of stress concentrations and initiate failure 4 Ceramics Ceramic can be classified into three groups: Bioinert (Alumina (Al2O3), Zirconia (ZrO2) and Pyrolytic carbon) Bioactive (Bioglass and glass ceramics) Biodegradable (Calcium phosphate ceramics) 5  Bioinert refers to a material that retains its structure in the body after implantation and does not induce any immunologic host reactions Remember--No material implanted in living tissue is really inert because all materials elicit a response from tissue Biomaterials science : an introduction to materials in medicine edited by Buddy D. Ratner... [et al 6 Ceramics Nearly inert ceramics e.g., alumina and carbons show little chemical reactivity even after thousands of hours or exposure to the physiological pH and therefore show minimal interfacial bonds with living tissues. The fibrous tissue at the interface of implants is very thin. Bioinert Characteristics 1. Maintain their physical and mechanical properties while in host 2. Corrosion and wear resistant 3. Have a reasonable fracture toughness Bioactive material exhibits an intermediate behavior. Bioactive ceramic is one that elicits a specific biological response at the interface of the material, resulting in the formation of a bond between the tissues and the material. Bioactive Characteristics 1. Direct and strong chemical bond with tissue 2. Low mechanical strength 3. Low fracture toughness 7  Degradable (Resorbable) biomaterials are designed to degrade gradually over a period of time and be replaced by the natural host tissue. This leads to a very thin or nonexistent interfacial thickness. This is the optimal biomaterial solution, if the requirements of strength and short-term performance can be met, since natural tissues can repair and replace themselves throughout life. Optimal solution!!!!! Limitation: maintenance of strength and the stability of the interface during the degradation period matching resorption rates to the repair rates of body tissues Ex :calcium phosphate, release ions from the surface over a period of time as well as provide protein bond sites. The ions released, aid in promoting hydroxyapatite nucleation, yielding mineralized bone, growing from the implant surface.  Sintering (Firing) is the process by which a powder compact is transformed to a strong, dense ceramic body upon heating. Usually sintering temperature is lower than melting temperature Sintering could eliminate most pores and compress residual pores, and bring grain growth and improve binding among grain The smaller the particle size the smaller the pores and the better the mechanical properties will be. 9  Alumina is well known for its good electrical and thermal insulation Alumina, Al2O3, or aluminum oxide, also known as corundum The substitution of small quantities of chromium or iron in the crystal structure results in the gemstones ruby and sapphire respectively. The implant devices are prepared from purified alumina powder by isostatic pressing and subsequent firing at 1500-17000C. 10 Alumina (Al2O3): Medical grade alumina is a single phase material comprising alpha-alumina or corundum (Single crystal). It is a chemically pure material with less than 0.1% grain boundary impurities The α–alumina has Hexagonal Close Packed (HCP) structure. (polycrystalline) Natural single crystal alumina known as sapphire has been successfully used to make implants. (dental implant) α–alumina is used in load-bearing hip prostheses and dental implants, because of its combination of excellent corrosion resistance, good biocompatibility, and high wear resistance, and high strength. 11 Alumina Strength, fatigue resistance and fracture toughness of polycrystalline alumina are function of grain size and purity. Alumina with an average grain size of