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  1. 1. Biomaterials <ul><li>The objective of these lectures is to review the fundamental requirements for biomaterials used in biomechanical engineering applications. </li></ul>
  2. 2. Material Attributes for Medical Applications <ul><li>Biocompatibilty </li></ul><ul><ul><li>Non-carinogenic, non-pyrogenic, non-toxic, non-allergenic, blood compatible, non-inflammatory </li></ul></ul><ul><li>Sterilizability </li></ul><ul><ul><li>Not destroyed or severely altered by sterilizing techniques such as autoclaving, dry heat, radiation, ethylene oxide </li></ul></ul><ul><li>Physical Characteristics </li></ul><ul><ul><li>Strength, toughness, elasticity, corrosion-resistance, wear-resistance, long-term stability </li></ul></ul><ul><li>Manufacturability </li></ul><ul><ul><li>Machinable, moldable, extrudable </li></ul></ul>
  3. 3. Biocompatibility <ul><li>Early Definition: </li></ul><ul><li>“ Lack of interaction between material and tissue” </li></ul><ul><ul><li>Implies inert, non-toxic, non-carcinogenic, non-allergenic, non-inflammatory, non-degradable </li></ul></ul><ul><ul><li>Thus, material has zero influence… </li></ul></ul>
  4. 4. Biocompatibility <ul><li>Contemporary Definition: </li></ul><ul><li>“ Ability of a material to perform with an appropriate host response, in a specific application” </li></ul><ul><ul><li>Refers to a collection of processes and interdependent mechanisms of interaction between material and tissue </li></ul></ul><ul><ul><li>“ Ability of material to perform ” and not just reside in the body </li></ul></ul><ul><ul><li>“ Appropriate host response ” must be acceptable given the desired function </li></ul></ul><ul><ul><li>“ Specific application ” must be defined </li></ul></ul>
  5. 5. Biocompatibility <ul><li>Specific application must also consider the time scale over which the host is exposed to the material : </li></ul>
  6. 6. Host Response <ul><li>Types of Reactions: </li></ul><ul><ul><li>Normal wound healing response </li></ul></ul><ul><li>Protein adsorption  Acute Inflammation  Resolution </li></ul><ul><ul><li>Persistent Inflammation </li></ul></ul><ul><li>Acute  Chronic </li></ul><ul><ul><li>Effect of relatively reactive tissue environment on material (i.e. corrosion, degradation products) </li></ul></ul><ul><li>Possibility of remote or systemic effects (transient or chronic) if reaction products are transported away from implant site </li></ul>
  7. 7. Host Response <ul><li>Types of Reactions: </li></ul><ul><ul><li>Infection (early or late onset) </li></ul></ul><ul><ul><li>Osteolysis </li></ul></ul><ul><ul><li>Neoplasia (cancer) </li></ul></ul>
  8. 8. Biocompatibility Testing <ul><li>Considerations: </li></ul><ul><ul><li>Type of device, principle tissue(s) in contact, period of implantation </li></ul></ul><ul><li>Tests for Chronically Implanted Devices: </li></ul><ul><ul><li>In Vitro : cytotoxicity, carcinogenicity, mutagenicity </li></ul></ul><ul><ul><li>In Vivo : pyrogenicity, systemic/acute toxicity </li></ul></ul><ul><ul><li>Chronic Animal Implantation Studies </li></ul></ul><ul><ul><li>(3 species for 6, 12 and 24 months) </li></ul></ul><ul><ul><li>Human Clinical Trials </li></ul></ul>
  9. 9. Implantable Materials <ul><ul><li>Metals </li></ul></ul><ul><ul><li>Polymers </li></ul></ul><ul><ul><li>Ceramics </li></ul></ul><ul><ul><li>Composites </li></ul></ul>
  10. 10. Biomaterials – Metals
  11. 11. Biomaterials – Metals Heart Valves Gold Alloys Bone and Joint Replacements Dental Implants Heart Valves CoCr Alloys Bone and Joint Replacements Dental Implants Pure Titanium Ti-6Al-4V Ti-13Nb-13Zr Fracture fixation Joint Replacement Spinal Instruments Surgical Instruments 316, 316L Stainless Steel Applications Material
  12. 12. Biomaterials – Polymers
  13. 13. Biomaterials – Polymers Implant Fixation Polymethylmethacrylate (PMMA) Ophthalmology Silicones Ophthalmology Hydrogels Vascular Prosthetics, Drug Delivery, Sutures, Ligament Grafts Polyesters Vascular Prosthetics, Heart Valves, Catheters Polyurethanes Catheters Polyvinylchloride (PVC) Resorbable Devices, Drug Delivery Polylactic and Polyglycolic Acid Vascular Prosthetics Polytetrafluoroethylene (Teflon) Sutures, MCP Joints Polypropylene Joint Replacement Bearings Polyethylene (UHMWPE) Applications Material
  14. 14. Biomaterials – Ceramics
  15. 15. Biomaterials – Ceramics Bone Grafting, Surface Coatings for Fixation Bioactive Glasses Dental Implants Porcelain Bone Grafting, Surface Coatings for Fixation Calcium Phosphates Joint Replacements Zirconia Joint Replacements Alumina Applications Material
  16. 16. Biomaterial Properties – 0 500 – 650 – 200 Zirconia – 0 270 – 350 Alumina 6 0.25 35 – 3 PMMA 16 390 30 20 1 UHMWPE – 5 – 7 10 – 20 5 – 30 0.2 – 0.5 Cancellous Bone 30 1 – 3 80 – 150 80 18 Cortical Bone 245 – 300 35 – 75 520 – 620 250 – 330 200 316 SS 400 – 440 10 – 15 930 825 100 Ti-6Al-4V 250 – 280 15 – 20 550 – 620 480 – 510 100 Titanium 400 – 600 35 – 55 896 650 – 1000 210 Co-Cr-Mo (forged) 235 – 275 8 650 – 750 440 – 570 200 Co-Cr-Mo (cast) Endurance Limit (MPa) Elongation at Break (%) UTS (MPa) Yield Strength (MPa) Tensile Modulus (GPa)
  17. 17. Corrosion <ul><li>Galvanic </li></ul>Crevice Stress-Corrosion Cracking Fretting
  18. 18. Galvanic Corrosion <ul><ul><li>Electrochemical circuit between two dissimilar metals </li></ul></ul><ul><ul><li>Anodic material is more basic and oxidizes (corrodes) </li></ul></ul><ul><ul><li>Cathodic material is more noble and is protected </li></ul></ul>
  19. 19. Implant Fixation Methods <ul><li>No such thing as absolute rigidity since both the implant and the underlying bone are deformable. </li></ul><ul><li>Some deformation will occur at the bone/implant interface and is only acceptable if: </li></ul><ul><ul><li>Magnitude does not progressively increase </li></ul></ul><ul><ul><li>Does not give rise to pain </li></ul></ul><ul><ul><li>Does give rise to unacceptable quantities of debris </li></ul></ul><ul><li>Biological restrictions: </li></ul><ul><ul><li>Cortical and cancellous bone are significantly weaker in tension and compression </li></ul></ul><ul><ul><li>Fibrous tissue layer that is laid down at the bone/implant interface during initial healing phase is also weak in tension and shear </li></ul></ul><ul><ul><li>Therefore, try to avoid tension and shear when condidering fixation method </li></ul></ul>
  20. 20. Implant Fixation Methods <ul><li>Interference Fits </li></ul><ul><ul><li>Can provide good fixation </li></ul></ul><ul><ul><li>Bone remodeling can remove interference on which fixation depends and can lead to loosening </li></ul></ul>
  21. 21. Implant Fixation Methods <ul><li>Screws </li></ul><ul><ul><li>Do not ensure tightness regardless of how many screws are present and can result in loosening </li></ul></ul><ul><ul><li>Crevice corrosion is a common problem under screw heads (observed in fracture fixation plates) and can lead to loosening </li></ul></ul><ul><ul><li>Locally high contact stresses at bone/screw interface </li></ul></ul><ul><ul><li>Only suitable for temporary fixation (e.g. fracture fixation) </li></ul></ul>
  22. 22. Implant Fixation Methods <ul><li>Bone Cement </li></ul><ul><ul><li>Gap filling agent </li></ul></ul><ul><ul><li>Polymethylmethacrylate (PMMA) which is polymerized in situ </li></ul></ul><ul><ul><li>Distributes load over largest possible area (low contact stresses) </li></ul></ul><ul><ul><li>Provides mechanically interlocking between implant and cancellous bone </li></ul></ul><ul><ul><li>Problems : monomers are toxic, polymerization process is exothermic (>50 ° C) and cement is generally brittle </li></ul></ul>
  23. 23. Implant Fixation Methods <ul><li>Bone Ingrowth </li></ul><ul><ul><li>Porous coats, grooves and/or meshes </li></ul></ul><ul><ul><li>Good for long-term fixation </li></ul></ul><ul><ul><li>Relative motion must be restricted to ensure bone ingrowth </li></ul></ul><ul><ul><li>Pore size has a distinct effect on the amount of ingrowth </li></ul></ul><ul><ul><li>Common approach is to create a layer of partially sintered beads on the surface of the implant </li></ul></ul>
  24. 24. Wear <ul><li>In any system if there is contact and relative motion between two materials, then wear will occur. </li></ul><ul><li>The extent of wear is the key issue in biomaterials: </li></ul><ul><ul><li>Biological response to wear debris </li></ul></ul><ul><ul><li>Degradation of implant  premature failure </li></ul></ul><ul><li>Wear is still the major unsolved problem of joint replacements: </li></ul><ul><ul><li>Early failures (< 7 years for TKRs) </li></ul></ul><ul><ul><li>Requires revision surgery (typically less effective than primary surgery) </li></ul></ul>
  25. 25. Wear <ul><li>Factors to consider: </li></ul><ul><ul><li>Material Selection </li></ul></ul><ul><ul><ul><li>Select more wear resistant materials (e.g. Co-Cr >> Ti) </li></ul></ul></ul><ul><ul><ul><li>Develop surface modifications (e.g. TiN) </li></ul></ul></ul><ul><ul><li>Materials Combinations </li></ul></ul><ul><ul><ul><li>Same (metal-on-metal) </li></ul></ul></ul><ul><ul><ul><li>Mixed (metal-on-plastic) </li></ul></ul></ul><ul><ul><li>Contact Mechanics </li></ul></ul><ul><ul><ul><li>Loads (magnitude, static, dynamic) </li></ul></ul></ul><ul><ul><ul><li>Mechanical properties of materials </li></ul></ul></ul><ul><ul><ul><li>Geometry of contacting bodies (e.g. congruency) </li></ul></ul></ul>
  26. 26. Wear <ul><li>Factors to consider: </li></ul><ul><ul><li>Lubrication </li></ul></ul><ul><ul><ul><li>Lubricant properties </li></ul></ul></ul><ul><ul><ul><li>Mechanism of lubrication (e.g. elastohydrodynamic) </li></ul></ul></ul><ul><ul><li>Surface Finish </li></ul></ul><ul><ul><ul><li>2 nd body wear, 3 rd body wear </li></ul></ul></ul><ul><ul><li>Kinematics of Articulation </li></ul></ul><ul><ul><ul><li>Velocity, rolling/sliding </li></ul></ul></ul><ul><ul><li>Biological Response </li></ul></ul><ul><ul><ul><li>Bulk versus particulate debris </li></ul></ul></ul>
  27. 27. Material Combinations (THRs) Limited experience UHMWPE Zirconia Severe wear of UHMPE and cartilage Cartilage UHMWPE Wear of femoral head Co-Cr-Mo UHMWPE Low rate of wear Alumina Alumina Low rate of wear UHMWPE Alumina High rate of cup wear UHMWPE Ti-6Al-4V Low rate of wear UHMWPE Co-Cr-Mo Satisfactory Cartilage Co-Cr-Mo High friction, high levels of metal ions in tissue Co-Cr-Mo Co-Cr-Mo Abrasion of femoral head and wear of cup Silica-filled PTFE Stainless Steel Wearing out, tissue reaction to wear products PTFE Stainless Steel Result Acetabular Material Femoral Head Material
  28. 28. Mechanisms of Wear <ul><li>Flat surfaces, even those polished to a mirror finish, are not truly flat on an atomic scale. They are rough, with sharp, rough or rugged outgrowth peaks, termed asperities . </li></ul><ul><li>Under compression, the asperities deform, leading to increased contact area (lower stresses) with higher coefficients of friction ( µ s , µ d ). </li></ul><ul><li>Depending on how the asperities interact under relative motion, different wear mechanisms can occur. </li></ul>
  29. 29. Mechanisms of Wear <ul><li>Fatigue </li></ul><ul><ul><li>Primarily related to one material (UHMWPE) </li></ul></ul><ul><ul><li>Cyclic subsurface tension and compression </li></ul></ul><ul><li>Adhesive </li></ul><ul><ul><li>Related to two materials (metal & UHMWPE) </li></ul></ul><ul><ul><li>Surface energy between materials in contact </li></ul></ul><ul><li>Abrasive </li></ul><ul><ul><li>Related to three materials (metal, UHMWPE and debris) </li></ul></ul><ul><ul><li>Hard, rough material removes soft material </li></ul></ul><ul><li>Combinations of above can occur </li></ul>
  30. 30. Wear Testing <ul><li>Screening tests are typically used to reproduce the mechanisms of wear observed in retrieved implants in a controlled environment. </li></ul><ul><li>Stimulators </li></ul><ul><ul><li>Pros : actual implants used </li></ul></ul><ul><ul><li>Cons : difficult to model actual biomechanics </li></ul></ul><ul><li>Rotating Pin-on-Flat </li></ul><ul><ul><li>Pros : simpler model than simulator </li></ul></ul><ul><ul><li>Cons : does not actually model kinematics/dynamics </li></ul></ul><ul><li>Reciprocating Pin-on-Flat </li></ul><ul><ul><li>Pros : sliding motion modeled well (good for THRs) </li></ul></ul><ul><ul><li>Cons : does not actually model knee kinematics/dynamics </li></ul></ul>
  31. 31. Consequences of Wear <ul><li>Excessive wear can lead to premature failure of the component; however, there can also be a biological response to the generated wear debris, such as inflammation and/or osteolysis. </li></ul><ul><li>Osteolysis refers to the active resorption of bone tissue as part of a biological reaction to wear particles generated from artificial joint replacements. This process ultimately results in implant loosening and eventually requiring revision surgery. </li></ul><ul><li>The magnitude of the osteolytic response is dependent on the nature of the wear particles generated: </li></ul><ul><ul><li>chemical composition </li></ul></ul><ul><ul><li>size (smaller particles have greater effect) </li></ul></ul><ul><ul><li>shape (shaper particles have greater effect) </li></ul></ul>
  32. 32. Osteolysis
  33. 33. Sterilization Methods <ul><li>Autoclave (Steam): </li></ul><ul><ul><li>High temperature process (121 – 134 °C) </li></ul></ul><ul><ul><li>Commonly for repeat sterilization (e.g. instruments) </li></ul></ul><ul><ul><li>Cheap </li></ul></ul><ul><li>Ethylene Oxide (EO): </li></ul><ul><ul><li>Low temperature process (for heat sensitive materials, e.g. UHMWPE) </li></ul></ul><ul><ul><li>Residual gas can linger </li></ul></ul><ul><ul><li>Environmental impact and occupational hazard </li></ul></ul><ul><li>Gamma Radiation: </li></ul><ul><ul><li>Very effective </li></ul></ul><ul><ul><li>Can cause polymer oxidation and crosslinking </li></ul></ul>