Bioceramics Materials Lecture 5-6 PDF

Materials. Eng. Dep. -------- Advanced Ceramic --------- Lecture 5-6 Bioceramics Materials A biomaterial is any substance, other than a drug, or combination of substances, synthetic or natural in origin, which can be used for any period, as a whole or as a part of a system, which tr...

Materials. Eng. Dep. -------- Advanced Ceramic --------- Lecture 5-6 Bioceramics Materials A biomaterial is any substance, other than a drug, or combination of substances, synthetic or natural in origin, which can be used for any period, as a whole or as a part of a system, which treats, augments, or replaces any tissue, organ, or function of the human body. It is also defined as a non-viable material that used in a medical device intended to interact with biological systems. Bioceramics materials are defined as ceramic used as a biomaterial for the repair and reconstruction of human body parts. Bioceramics are typically classified into subgroups based on their chemical reactivity in the body. Inert bioceramics like Al2O3, vitreous carbon; and ZrO2. If a nearly inert material is implanted into the human body it initiates a protective response that leads to encapsulation by a nonadherent fibrous coating about (1 μm) thick. Over time this leads to the complete isolation of the implant. A similar response occurs when metals and polymers are implanted. In the case of bioactive ceramics, a bond forms across the implant tissue interface that mimics the body’s natural repair process. These 1 Asst. Prof. Dr. Hamza M. Kamal 2022-2023 Postgraduate Studies Materials. Eng. Dep. -------- Advanced Ceramic --------- Lecture 5-6 bioactive ceramics such as (HA) can be used in bulk form or as part of a composite or as a coating. Resorbable bioceramics like tricalcium phosphate (TCP) dissolve in the body and are replaced by the surrounding tissue. It is an important requirement that dissolution products are not toxic. As in the case of (HA), TCP is often used as a coating rather than in bulk form. It is also used in powder form, e.g., for filling space between bones. Osteoconductive bioceramics are the ability of a ceramics material to support the in-growth of the bone cells, blood capillaries, and the perivascular tissue into the gap between the ceramics implant and the existing bone. Efficient in-growth is supported by interconnected pores of (150 – 450 μm) size. Hence, the development of such a pore system in plasma-sprayed hydroxyapatite coatings is of the utmost importance, as nonporous coatings may act like bioinert ceramics materials, and their eventual substitution by bone is not guaranteed. Fig. (1) shows a number of clinical uses of bioceramics. The uses go from head to toe and include repairs to bones, joints, and teeth. These 2 Asst. Prof. Dr. Hamza M. Kamal 2022-2023 Postgraduate Studies Materials. Eng. Dep. -------- Advanced Ceramic --------- Lecture 5-6 repairs become necessary when the existing part becomes diseased, damaged, or just simply wears out. Fig (1): Head-to-toe clinical uses for bioceramics. 3 Asst. Prof. Dr. Hamza M. Kamal 2022-2023 Postgraduate Studies Materials. Eng. Dep. -------- Advanced Ceramic --------- Lecture 5-6 There are many other applications of bioceramics including alumina hip prostheses pyrolytic carbon coatings for heart valves and special radioactive glass for the treatment of certain tumors. Alumina is classified as an inert bioceramic because it has very low reactivity in the human body. However, bioactive materials have the ability to bond directly with bone. The advantages are: Earlier stabilization of the implant. Longer functional life. Bioactive ceramics are relatively weak compared with the common implant metals and high-strength ceramics such as alumina and zirconia. As a result, they are often used as coatings, relying on the mechanical strength and toughness of the substrate. An important bioactive ceramic is a hydroxyapatite (HA). Natural bone is a composite in which an assembly of (HA) particles is reinforced by organic collagen fibers. Hydroxyapatite-reinforced polyethylene composites have been developed in an attempt to replicate the mechanical behavior of bone. The main advantage of ceramics over other implants is their biocompatibility: some are inert in the physiological environment 4 Asst. Prof. Dr. Hamza M. Kamal 2022-2023 Postgraduate Studies Materials. Eng. Dep. -------- Advanced Ceramic --------- Lecture 5-6 while others have a controlled reaction in the human body. The main disadvantages of most bioceramics are the low toughness and the high modulus of elasticity (E). One of the main ways of increasing the toughness of ceramics is to form a composite. The ceramic may be the reinforcement phase, the matrix, or both. An example of a polymer–matrix composite (PMC) reinforced with a bioceramic is the polyethylene (PE( reinforced with the (HA) particles. The toughness of the composite is greater than that of the HA. (E) is more closely matched to that of bone. Bioceramics are also used as coatings on the metal substrates. An example is a bioactive glass coating on the stainless steel, which utilizes the strength and toughness of the steel and the surface-active properties of the glass material. Ceramic Implants The requirements for a ceramic implant depend on its role in the body. For example, the requirements for a total hip prosthesis (THP) will be different from those for a middle ear implant. However, there are two basic criteria: 5 Asst. Prof. Dr. Hamza M. Kamal 2022-2023 Postgraduate Studies Materials. Eng. Dep. -------- Advanced Ceramic --------- Lecture 5-6 The ceramic should be compatible with the physiological environment. Its mechanical properties should match those of the tissue being replaced. Most bioceramic implants are in contact with bone. Bone is a living material composed of cells and a blood supply encased in a strong composite structure. The composite consists of collagen, which is flexible and very tough, and crystals of an apatite of calcium and phosphate, resembling calcium hydroxyapatite; we will proceed as if it is HA. It is the HA component that gives bone its hardness. The acicular apatite crystals are (20-40nm) in length and (1.5-3nm) wide in the collagen fiber matrix. Two of the various types of bone that are of the most concern in the use of the bioceramics in the human body organs are: Cancellous (spongy bone). Cortical (compact bone). Cancellous bone is less dense than cortical bone. Every bone of the skeleton has a dense outer layer of the compact bone covering the 6 Asst. Prof. Dr. Hamza M. Kamal 2022-2023 Postgraduate Studies Materials. Eng. Dep. -------- Advanced Ceramic --------- Lecture 5-6 spongy bone, which is in form of a honeycomb structure. Because of its lower density, cancellous bone has a lower (E) and higher strain- to-failure ratio than cortical bone. Both types of bone have higher (E) than the soft connective tissues, such as tendons and ligaments. The difference in (E) between various types of connective tissues ensures a smooth gradient in mechanical stress across a bone, between bones, and between muscles and bones. The mechanical properties of implant are clearly very important, (E) of cortical bone is (10-50) times less than that of Al2O3. (E) of cancellous bone is several hundred times less than that of Al2O3. If the implant has a much higher (E) than the bone it is replacing then a problem called stress shielding can occur. Stress shielding weakens bone in the region at which load is applied causing biological change that leads to resorption. Alumina and Zirconia Bioceramics Al2O3 and ZrO2 are two nearly inert bioceramics. They undergo little or no chemical change during long-term exposure to body fluids. High-density, high-purity (>99.5%) alumina is used in a number of implants, particularly as load-bearing hip prostheses and dental implants. 7 Asst. Prof. Dr. Hamza M. Kamal 2022-2023 Postgraduate Studies Materials. Eng. Dep. -------- Advanced Ceramic --------- Lecture 5-6 Although some alumina dental implants are made from single crystals, most alumina implants are very fine grained polycrystalline Al2O3. These are usually made by pressing followed by sintering at temperatures in the range of (1600–1800°C). A small amount of MgO (900°C. There are many applications for dense HA in both block form and as particles; one important application is replacement for tooth roots following extraction. The particular advantage of porous (HA) is that it permits ingrowth of tissue into pores, providing biological fixation of the implant. The minimum pore size necessary is about 100 μm. Several methods and techniques have been used to produce porous (HA) ceramics. One of these techniques involves sintering of (HA) powders with the mixture of suitable reactant powders, uses naphthalene particles that volatilize during the heating to create an interconnected porous network. Bioceramics Composites The main reason for forming biocomposites is to improve the mechanical properties, most often the toughness, above that of the stand-alone ceramic. For bioceramic composites we often are trying to increase KIC and decrease E. 13 Asst. Prof. Dr. Hamza M. Kamal 2022-2023 Postgraduate Studies Materials. Eng. Dep. -------- Advanced Ceramic --------- Lecture 5-6 The first bioceramic composite was a stainless-steel fiber/bioactive glass composite. The composite was made by first forming a preform of discontinuous metal fibers, then impregnating it with the molten glass, and finally heat-treating the composite to develop the desired mechanical properties. For effective stress transfer between glass matrix and reinforcing metal fibers when composite is under load, there must be a strong glass– metal bond. This requires that the glass wet metal surface during processing. Wetting is achieved by oxidizing the metal fibers before they are immersed in the glass matrix phase. One of the potential problems associated with forming composites is that of mismatch in the (α) between the two components, which is significant for glass and steel. For reinforcing fibers for which the difference in (α) with glass phase is even greater than that with steel, e.g,. Ti, it is necessary to change the composition of the glass to lower its (α). Other recent bioceramic composites are Ti-fiber-reinforced bioactive glass, ZrO2-reinforced (A-W) glass ceramics, TCP-reinforced PE, and HA-reinforced PE. Hydroxyapatite-reinforced PE biocomposite is a good illustration of a composite that can have properties that are not available in a single material. These composites were developed as a 14 Asst. Prof. Dr. Hamza M. Kamal 2022-2023 Postgraduate Studies Materials. Eng. Dep. -------- Advanced Ceramic --------- Lecture 5-6 bone replacement that would have a matched modulus, be ductile, and bioactive. Bioceramics Coatings Applying a glass or ceramic coating onto the surface of a substrate allows us to have the bulk properties of substrate and the surface properties of the coating. There are three main reasons for applying a coating: Protect the substrate against corrosion. Make the implant biocompatible. Turn a nonbioactive surface into a bioactive one. There are four substrate-coating combinations: Polycrystalline ceramic on ceramic. Glass on ceramic. Polycrystalline ceramic on metal. Glass on metal. Bioceramic coatings are often used on metallic substrates in which the fracture toughness of metal is combined with the ability of the coating to present a bioactive surface to the surrounding tissue. The use of a bioceramic coating on a metal implant can lead to earlier stabilization 15 Asst. Prof. Dr. Hamza M. Kamal 2022-2023 Postgraduate Studies Materials. Eng. Dep. -------- Advanced Ceramic --------- Lecture 5-6 of the implant in the surrounding bone and extend the functional life of the prosthesis. The important ceramic coatings are HA and TCP. HA is resorbable bioceramic and TCP when implanted into the body, it will eventually dissolve and be replaced by tissue. Because bulk TCP is mechanically weak, it cannot be used in load-bearing applications. Therefore, TCP is often used as a coating on metal substrates. The most widely used method for applying coatings of HA and TCP is plasma spraying, which is one of methods used to produce thermal barrier layers. Plasma spraying uses a plasma, an ionized gas, that partially melts the HA particles and carries them to the surface of the substrate. One of the advantages of plasma spraying is that the substrate remains at a relatively low temperature (generally less than 300°C; the plasma temperature usually exceeds 10,000°C) so that the mechanical properties of the metal are not compromised. The coating thickness is typically in the range of (40–60) μm with a residual porosity of