Lecture 4: Mechanical Properties of Biomaterials PDF

# Lecture - 4: Mechanical Properties of Biomaterials ## Role of Implant Biomaterials - Biomaterials as medical implants are primarily aimed at recovering physical function, specifically mechanical function, of injured or diseased organs or tissues. - Biomaterials provide physical support; they do...

# Lecture - 4: Mechanical Properties of Biomaterials ## Role of Implant Biomaterials - Biomaterials as medical implants are primarily aimed at recovering physical function, specifically mechanical function, of injured or diseased organs or tissues. - Biomaterials provide physical support; they do not have other biological functions. ## Mechanical Properties of General Importance - The mechanical working conditions of implant biomaterials are complex in the body. - Special mechanical properties (e.g., fatigue) are always closely correlated to each other and can be predicted by their mechanical properties of general importance (Young's modulus E, yield strength σy, ultimate tensile strength UTS, fracture toughness, and elongation at break). ### Young's Modulus - Young's modulus is the only property that is nonsensitive to microstructures. - It is primarily determined by the chemical bonding between atoms and/or molecules. - It is considered the most important of the general mechanical properties. ### Other Mechanical Properties - Yield Strength: The transition from elastic to plastic is a gradual one for most metals. - Ultimate Tensile Strength: The point of rupture. - Fracture Toughness: The material’s resistance to crack propagation. - Elongation at Break: The amount that a material stretches before breaking. - These four properties are all sensitively influenced by microstructures. ### Elasticity and Resilience - Elasticity is the ability of a material to return to its original shape after being deformed. - Resilience is the amount of energy that a material can store elastically. ### Hardness - Hardness is a measure of a material’s resistance to wear and friction. - It is often measured using the Rockwell or Brinnell hardness tests. ### Failure - Understanding the reasons behind failure is important. - The fracture mechanism of a material varies with its working conditions. - These fractures appear complicated and vastly different, but the fundamental mechanism is the same: the breakage of chemical bonding between atoms or molecules. - Fatigue: Fatigue, or progressive structural damage, is caused by cyclic loading. - Stress Corrosion Cracking: Stress corrosion cracking (SCC) is an unexpected sudden brittle failure of normally ductile or tough metals subjected to a tensile stress in a mild corrosive environment. ## Essential Mechanical Properties of Orthopedic Implant Biomaterials In order to serve safely and appropriately at load-bearing sites for a long period of time without rejection, a metallic implant should possess the following essential characteristics: 1. No toxicity (equivalent to excellent corrosion resistance) 2. Suitable mechanical strength 3. High wear resistance 4. Osseo-integration ability (including bone bonding) ## Mechanical Working Environments of Implants in the Body ### Fatigue - Fatigue is a common problem for orthopedic implants due to the cyclic loading that occurs during walking, running, and other activities. - High loads on the hip joint, due to the balance of weight and muscle strength, can lead to fatigue. ### Fretting Fatigue and Corrosion Fretting Fatigue - The mechanical working conditions of orthopedic implants are frequently complicated by concurrent cyclic stress and friction. - Fretting fatigue occurs when a foreign body is statically pressed to the surface of the specimen to which cyclic stress is applied. - This results in the production of oxide debris and the development of fresh metal surfaces. - Cracks initiate at the contact site, and the implant can suffer from fretting fatigue. - This type of fatigue is common in hip joints, bone plates, and wires. - Corrosion fatigue is a combination of fatigue and corrosion. ## Wear of Joints - Wear is an inevitable problem in any joint replacement. - Aseptic loosening is a frequent incident caused by wearing damage. - This occurs when a large amount of tiny microscale particles are generated around a joint replacement. - These particles attract macrophages, which engulf the particles as foreign bodies. - Macrophages break down and release enzymes and metabolites that damage the implant. ## Osseo-Integration - Osseo-integration is the ability of an implant to bond with host bone. - This is crucial for the longevity of permanent orthopedic implants. - The incapability of an implant surface to join with the adjacent bone and other tissues will cause the formation of fibrous tissue around the implant and promote loosening of the prosthesis. - Materials need to be designed to integrate well with surrounding bone to prevent osteolysis. - Surface chemistry, surface roughness, and surface topography are all factors that need to be considered for good osseo-integration. ## Conclusion - The interior of the living body is a mechanically and chemically harsh environment for metallic materials.