You have been hired by a firm to assist in the development of permanent metal me
ID: 164012 • Letter: Y
Question
You have been hired by a firm to assist in the development of permanent metal mesh porous coatings to be applied to joint replacement prostheses to facilitate the fixation of the implant to bone. Your boss has suggested that you might consider applying an adhesion material (protein) to the surface of the porous material to facilitate the migration of the cells, which will form the blood vessels and bone, on the surface of the implant. She would prefer that only one adhesion protein be used. What is your viewpoint? Explain. Why do ceramics break and metals bend? What makes rubber elastic? Why does Cr make steel stainless?Explanation / Answer
Answer 1)
An orthopaedic implant is a medical device used to replace a missing joint or bone or to support a damaged bone. The medical implant is mainly made up of stainless steel or titanium alloys (for strength and durability) and plastic coating is done on it that acts as an artificial cartilage. This coating of the implants is termed as surface modification. If, instead of plastic, protein molecules are used for surface modification, then the biocompatibility of the implant can be improved greatly. The rate of rejection of the implants by the body's immune system can also be greatly lowered.
Extra-cellular matrix (ECM) proteins greatly influence the process of bone formation—the attachment and proliferation of osteogenitor cells, differentiation to osteoblasts, matrix formation, and mineralization. Designing biomaterials for bone-contacting devices with bone matrix proteins can prove to be beneficial and help to promote bone growth. It is also possible to covalently and directionally immobilize osteoinductive peptides in the surface of the ceramic materials such as hydroxyapatite/-tricalcium phosphate to stimulate osteoblast differentiation and better bone regeneration. The heparin binding domain in fibronectin is actively involved in specific interaction with osteogenic cells. Modification with heparin binding domains have the potential to enhance the binding of osteoblasts without affecting the attachment of endothelial cells and fibroblasts.Additionally, growth factors likethose in the bone morphogenic protein family are important polypeptides to induce bone formation. These growth factors can be covalently bound to materials to enhance the osteointegration of implants.
Thus, ECM proteins, together with the growth factors, can serve as a great option for surface modification of orthopaedic implants. This kind of modification will help in the growth and regeneration of bones and also protect the implant from the actions of the body's immune system.
Answer 2)
a) Ductile or brittle behaviour is dependent on the bonding of the material. The basic difference between metals and ceramics is their bonding. Metals possess non directional bonds in contrast to the directional bonds in ceramics. Directional bonds means that they will have to be oriented in a particular way or they will break. Ceramics possess directional bonds and thus, do not exhibit ductile behaviour. Metals, on the other hand, possess non-directional bonds and are, hence, ductile.
Another factor which plays a key role in the determination of the ductility of materials is dislocation.Dislocations are a type of defect in crystal structures. Metals form crystalline structures where the atoms ar not very tightly packed and hence, the dislocations can easily move. When dislocations move, the material bends. In ceramics, the atoms are tightly packed and hence, the dislocations in the structures cannot move easily. Thus, the ceramics are very brittle and when atoms try to move, the material usually breaks first.
In case of metals, the atoms are loosely packed and there are provisions for the atoms to move.
b) As per the dictionary, elasticity is the ability of an object or material to acquire its normal shape after being stretched or compressed. Rubber is a polymer (made up of achain of repeating units called monomers) of a carbon compound named isoprene. Isoprene has two caron-carbon double bonds. During the formation of rubber, as the latex from the rubber tree dries and the isoprene molecules comes in close vicinity of one another. They then attack the carbon-carbon double bond of the neighbouring isoprene molecules, leading to the breaking up of one of the double bonds. This leads to a rearrangement of the electrons and a bond formation occurs between neighbouring isoprene molecules which, in turn, forms a chain called the polyisoprene polymer. The polyisoprene strands sticks together by forming electrostatic bonds and the attraction between the strands (owing to the electrostatic bonds between them) holds the rubber fibre together and allows them to stretch and recover (the property of elasticity).
Another reason is the entropy. All matter in this universe tends to exist in a state of disorder called entropy (as per second law of thermodynamics). When a rubber is stretched then the polymer chains become aligned in a definite direction. The state of order in the molecule leads to lower entropy. Then the entropic forces come into play and favour the retraction to the higher entropy or equilibrium state (unstretched state).
Thus, these are the reasons behind the elasticity of rubber.
c) Stainless Steel is a Steel alloy with a minium of 10.5% Chromium content in it. Stainless steel is popularly used for its property of corrosion resistance. Chromium forms a thin film of rough, adherent, corrosion resistant chromium oxide, in the presence of oxygen. This process is called passivation. The film is too thin to be visible and has the property of self healing (i.e. the layer quickly reforms if it is scratched). This layer is impervious to water and air.and blocks the process of oxygen diffusion on the steel surface. In this way, chromium content helps stop the corrosion from spreading into the metal surface and thus, protects it.
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