Biomaterials Exam 2
Identify important properties and common uses of alumina and partially-stabilized zirconia.
Alumina: - Bioinert, hard, stong, wear resistant, no implant rejections, good blood compatibility, no cytotoxicity - Bone-replacement implants PSZ: - Higher flexural strength, fracture toughness, better reliability, lower Young's modulus, bioinert, noncytotoxic, variable wear properties, - Total hip replacement femoral heads
Identify the issues that determine the success of a material combination used at the head-liner face of a hip implant.
Wear - Releases particulate debris, results in rough surface and loosening Corrosion - Release of metal ions that can have local and system toxicity Mechanical fracture
Compare the benefits of cast and wrought cobalt chromium alloys.
Wrought: Good for fatigue strength (heated and worked with tools) Casting is less expensive and offers processing flexibility and good wear resistance. (heated and poured into a mold, cooled)
Describe what modifications can be made of biological apatites by making elemental substitutions (you do not have to identify specific elemental substitutions).
- Substituting metal ions for Ca results in decreased crystallinity and increased solubility - Substitution of CO3 for PO4 decreases crystallinity and increases solubility - Substitutions for OH increase crystallinity and decrease solubility (explains effects of fluoride on teeth)
Identify calcium phosphates commonly used as bioactive ceramics, and identify applications of such materials
- Tricalcium phosphate (Ca3(PO4)2): Widely investigated, tissue compatibility superior - Hydroxyapatite (HA) (Ca10(PO4)6(OH)2): Naturally occurs in bone and teeth High elastic modulus, hard, very biocompatible (bonds with bone) (see study guide #19) Applications: - Bone filler to heal critical-sized defect in skull - Bone cement to heal a critical-sized defect in spine - Coating on total hip replacement - Tissue engineering scaffold made from beta-tricalcium phosphate
Provide an example of a carbon-based material with potential use as a biomaterial. Describe its properties and potential applications.
1. Pyrolytic Carbon 2. Carbon Nanotubes 3. Graphene: Emerging Biomedical Applications (see study guide #24)
The elemental differences between 316, 316L, and Orthinox stainless steels and the significance of those elemental differences (i.e., what do the elements do? For corrosion-resistance, explain how each element enhances corrosion resistance)
316 and 316L have molybdenum to enhance resistance to pitting type of corrosion. 316L is low in carbon and less brittle than martensitic steel. Nitrogen can be used in 316 to stabilize FCC structure and prevent pitting corrosion (Orthinox) 316L: Low cost, but has corrosion issue that leads to limitations in fatigue strength and biocompatibility. Commonly used for short term implants with lower stress requirements. Orthinox: Higher cost, better corrosion properties so used for high stress permanent implants. Some corrosion/biocompatibility and bending fatigue concerns.
The difference in properties between 316L and Orthinox stainless steels and how these relate to subsequent different common implant uses of the two metals.
316L has limitations in fatigue strength and biocompatibility, so it is used for short-term implants and longer-term implants with lower stress requirements, also stress shielding concerns. More common for temporary implants. Orthinox is more expensive but has better corrosion properties and can be used for high stress implants. Still corrosion, biocompatibility and bending fatigue concerns, also stress shielding still concerns. More common for permanent implants.
Explain the difference between bioactive and bioresorbable ceramics.
Bioactive: Surface forms direct chemical bonds with bone or other tissue Bioresorabable: Bulk of material (not just the surface) reacts with tissue (increase in crystallization decreases bioresorbable)
Identify which molecules compose bioactive glasses and describe the general importance of the relative amounts of these molecules in a material.
Bioactivity is dependent on 3 oxides, CaO, Na2O and SiO2! (see study guide #21)
Given commonly used implantable ceramics, identify whether each is either bioinert or bioactive/bioresorbable.
Bioinert: Typically alumina and zirconia (fibrous capsule forms around it to isolate) Alumina: Bioinert Partially Stabilized Zirconia (PSZ) ZrO2: Bioinert Biological near‐inert ceramics: tissue typically attempts to reject the material by forming a fibrous capsule around it. Surface bioactive ceramics: bond forms at the interface of the material and the tissue.(surface bonds) Biodegradable ceramics: material dissolves and is absorbed by the tissue
Describe the body response to implantation of bioinert ceramics.
Biological near‐inert ceramics: tissue typically attempts to reject the material by forming a fibrous capsule around it
Describe the structural differences between ceramics, glasses and glass-ceramics.
Ceramics: Crystalline inorganic, nonmetal materials Glasses: Amorphous/non-crystalline inorganic, nonmetal materials Glass-ceramics: Partially crystalline inorganic, nonmetal materials
The primary elements of cobalt-chromium alloys used for implants and significance of Cr, Mo and Ni as elements.
Co with Cr and Mo (casting or wroughting). Co with Ni, Cr and Mo for wroughting/hot forging. Mo results in finer grains. 1. Cr forms passivating layer 2. Mo and Ni further enhance corrosion-resistance (pitting type corrosion)
What element makes a steel a stainless steel.
Cr (chromium)
Toxicity concerns of stainless steels
Due to released Ni and Cr Ni and Cr not biocompatible
Describe how setting type polymers and thermoplastics can exhibit elastomeric behavior.
Elastomers (rubbers) are characterized by their ability to undergo stretching and return to their original shape. Thermoset elastomers have sparsely cross-linked networks, cross-links serve as anchor points during stretching (e.g. latex gloves and medical tubing) Thermoplastic elastomers include some form of physical (or mechanical) cross-linking, rather than chemical cross-linking. In semi-crystalline materials, crystalline regions can serve as anchor points to stretchy amorphous regions, could be multiphase materials
Between addition polymerization and condensation polymerization, identify which typically involves unsaturated monomers and which typically releases a byproduct.
Free radical addition is initiated by a monomer becoming a free radical. The free radical attacks double bond backbone of another monomer. It breaks the bond and becomes another free radical (involves unsaturated monomers) Condensation polymerization releases water as a byproduct.
The properties, limitations and resulting medical uses of martensitic stainless steels.
Harder than austenitic steel and easier to keep sharp (often used for surgical instruments) The more loosely-packed BCC structure leads to stress corrosion cracking (not used for orthopedic implants)
List three dental applications of bioinert ceramics.
Implants: - Replace root - Anchor tooth in jawbone Porcelains: - Used to replace the exposed tooth
Describe the biocompatibility and mechanical characteristics of bioactive glasses, as well as common applications of these materials.
Made up entirely of macroelements of the body, so few concerns about elemental biocompatibility - Silica intake has actually been shown to be beneficial - Some concerns about larger grains (particularly if inhaled) - In rapidly bioresorbable materials, element levels may become high enough to be toxic Mechanical Properties - Good in compression, but brittle with low tensile strength and toughness - Often used in composite form (bone filler, etc.) - Sintering improves mechanical properties - Sintered 45S5 Bioglass is widely used Applications - Treatment of periodontal disease with PerioGlas - Filling a bone void with bone cement after tumor removal - Bone tissue engineering scaffolds.
The structural and elemental differences between martensitic and austenitic stainless steels.
Martensitic: Cr-steels (with little or no Ni) and BCC structure Austenitic: Cr-Ni steels and closely packed FCC structure (stabilized by Ni)
Describe the general molecular form of a polymer.
Means "many monomers". Linking one monomer to another with covalent bonds. There can be interactions between one polymer molecule and another (van der Waals forces and covalent bonds) Polymers can have backbones made of either a chain of carbon, oxygen or nitrogen atoms at regular intervals or inorganic molecules usually silicon and oxygen.
Identify the most common combination of materials used at the head-liner interface of a hip implant.
Metal-on-Polymer: Fracture resistant, few toxicity issues; wear of polymer causes aseptic loosening Ceramic-on-Polymer: Fracture resistant, few toxicity issues; wear of polymer causes aseptic loosening Metal-on-Metal: Fracture resistant; metal debris causes toxicity and possible loosening Ceramic-on-ceramic: Superior wear resistance due to very low surface roughness from polishing and possible lubrication effects; fracture problems with ceramic liner, but active area of research Ceramic-on-metal: Fracture resistant; metal debris causes toxicity although not as bad as MoM—relatively new
The benefits of Mo in stainless steels.
Mo (molybdenum) enhances resistance to pitting type of corrosion by trapping residual carbon preventing the formation of chromium carbide at grain boundaries. Mn (manganese) improves hot working properties and increases strength, toughness and hardness
Identify how each of the following factors affects polymer crystallization, flexibility/rigidity, and melting temperature (except if not discussed in the class presentations): Molecular weight Substitution of backbone C with S or O Isotactic versus atactic side group arrangements Plasticizers Branching of the polymer backbone Length of side groups Degree of cross-linking Temperature
Molecular weight - Decrease in molecular weight measure more easily to organize crystals - lower melting temperature if not crystallized - Increase the molecular weight you have a harder time organizing crystals, higher melting temperature • Substitution of backbone C with S or O - Changing C in a backbone to S or O can make a molecule even more flexible (flexible covalent bonds) - Lower melting temperature • Isotactic versus atactic side group arrangement - The short the side groups, the easier to crystallize and pack in - Isotactic = increase in crystallinity - Atactic = decrease crystallinity • Plasticizers - Short chains or other molecules introduced - Interfere with packing, so make chains more mobile - Lower melting and glass transition temperature - Reduce rigidity - Polymers that are toxic and dangerous has to do with what plasticizers they were introduced to - Lower melting temperature • Branching of the polymer backbone - Considered "branched" if there are one or more polymeric side chains - Branches generally impede both flexibility and crystallization - Increase branches, decrease crystallinity (can't close pack), higher melting temp due to branches - Increase branches decrease flexibility • Length of side groups - Large atactic side groups decrease the crystallinity, higher melting temp due to large side groups - Short Isotactic side groups increase the crystallinity, higher melting temp due to crystallization • Degree of cross-linking Dense crosslinks: - Tons of crosslinks - Decrease flexibility - Increase the rigidity of the material - Decrease crystallinity - Doesn't allow water do get in and is good for implants - Higher melting temperature Sparse Crosslinks: - Increase flexibility - Increases crystallization, but crosslinks still cause it to be LOWER CRYSTALLIZATION!!! - Decrease rigidity - Lower melting temperature Temperature - If temperature goes above room temp, it is a hard, somewhat brittle material - If temperature is lower than room temp then it is rubbery and sometimes liquid behavior - The more quickly it is cooled, the harder it is to form crystals
Define morphological fixation, biological fixation and bioactive fixation, and identify which of these would likely apply given a material and application.
Morphological Fixation: Dense, nonporous, nearly inert ceramics attach by bone growth into surface irregularities by cementing the device into the tissues or by press-fitting into a defect Ex: Al2O3 (Single crystal and polycrystalline) Biological Fixation: For porous inert implants, bone ingrowth occurs that mechanically attaches the bone to the material Ex: Al2O3 (polycrystalline) hydroxyapatite-coated porous materials Bioactive Fixation: Dense, nonporous surface-reactive, ceramics, glasses and glass-ceramics attach directly by chemical bonding with the bone Ex: Bioactive glasses, bioactive glass-ceramics, hydroxyapatite Dense, nonporous (or porous) resorbable ceramics are designed to be slowly replaced by bone Ex: Calcium sulfate (plaster of Paris), Tricalcium phosphate, calcium-phosphate salts
Explain what is meant by a copolymer.
Polymers made up of more than one monomer in the backbone. Regular alteration: ABABABABABA or AAABBBAAABBBAAABBB Organized Blocks: AAAAAAAAAABBBBBBBBBBAAAAAAAAAA (insert 1 polymer into another)
Mechanical limitations of titanium alloys and how they limit circumstances in which titanium is used for orthopedic implants.
Poor bending properties, not used in implants with wear or bending properties. This is why titanium can be used for the stem of hip implants but not the head
Identify the favorable and unfavorable mechanical tendencies of ceramics.
Processing ceramics introduces imperfections, often at surface, during cooling Leads to stress concentration resulting in brittleness in tension or bending Unpredictable strength (depends on flaws) Compressive strength is usually greater than 10X tensile strength
Explain what is meant by a semicrystalline structure.
Semicrystalline is when some ordered chain arrangements, some uncoordinated (think of proteins) - some crystalline and some amorphous
Briefly describe each of the following polymer manufacturing methods and identify whether each mainly applies to thermoplastic or setting-type polymers:
Simple casting - Thermal or chemical setting polymer (not thermoplastic) "resin" is poured into a mold. The mold can be made of an inexpensive non-heat resistant material. The process is slow but the equipment cost is low. (not good for mass production) (SETTING POLYMER) • Compression molding - Used for materials that are too viscous to be cast. Gets gel-like materials into all the nooks and crannies of detailed molds. Often used for thermal or chemical setting silicone elastomer medical items. (SETTING POLYMER) • Transfer molding - A quicker version of compression molding. Pre-heat temperature to just below the crosslinking temperature and put it into mold and heat to cross-link temperature. The plunger pushes the hot resin into the mold for final heating and curing. (SETTING POLYMER) • Reaction injection molding - Two chemicals, 1 induces crosslinking to the other. Plunge both into the mixing chamber. Used for chemical setting resins. Generally lower productivity than injection molding, but better mechanical properties. (SETTING POLYMER) • Injection molding - Melt and pressurize polymer chips or beads. Inject melted polymer into the mold (fill and pack). Cool the mold and solidify polymer. Open mold and remove the part. (THERMOPLASTIC POLYMER)
Identify the four parts of a hip implant and the biomaterials that are commonly used for each.
Stem: Titanium alloy or CoCr alloy Head (ball): Ceramic (Al2O3) or CoCr alloy Liner (socket): Polymer (UHMWPE), CoCr alloy or Ceramic (Al2O3) Cup: Titanium alloy
Explain the difference between a thermoplastic and a setting-type polymer
Thermoplastic polymers you heat them up, they become soft and pliable (sometimes liquid), put them in a mold, and cool down and they retain the mold shape (by molding or excursion), include linear and branched polymers Setting type polymers aka thermosets start as a viscous liquid, you put them into a mold, and they begin to solidity by introducing crosslinking. (induced by heating, UV light, mixing in a "hardener", include cross-linked (network) polymers
Explain how setting type polymers can be either elastomeric or rigid materials.
Thermoset resins are crosslinked rigid materials, very hard, can be brittle (e.g. epoxy glue starts off as soft material but when it dries it turns into a hard and brittle material due to cross-linking) Depends on the number of crosslinks in the material to determine if it will be an elastomeric or rigid material
Identify how 316L, CoCrMo alloy and titanium alloy would rank in terms of corrosion resistance.
Titanium alloy would have the most corrosion resistance, CoCrMo would be middle and 316L would have the least corrosion resistance.
How and why Nb (Niobium) is used as an alloying element with titanium.
Vanadium (V) is toxic Nb has higher alpha - beta ratio and stabilizes beta phase making it tougher
Disadvantages of wrought CoCrMo for orthopedic implant applications.
i. Occasional corrosion issues ii. Aseptic loosening due to wear iii. Release of Co, Cr and Ni iv. Stress Shielding: heated and pounded to shape could have resulted in microcracks
The microstructural difference between pure titanium and Ti-6Al-4V and subsequent differences in mechanical properties and implant applications.
i. Pure titanium has alpha (HCP - hexagonal closest packed) Lower strength, lower fatigue strength, lower bending properties: Ex: Pacemaker cases ii. Ti-6AI-4V has alpha (HCP) - beta (BCC - body centered cubic) Higher fatigue strength: Ex: Orthopedic applications
The advantages of titanium and titanium alloys.
i. Very corrosive resistant ii. High temperature resistance iii. High strength-to-weight ratio iv. Lower density v. Lower Young's modulus vi. Biocompatibility vii. Good UTS and fatigue strength
Explain the advantages and disadvantages of using mercury amalgam for dental fillings.
· Advantages: Hard, strong, adheres well to teeth, similar thermal expansion properties, good corrosion resistance, sets in minutes (fully in a day), minimal shrinkage, good compressive strength and wear resistance, adheres to teeth well, inexpensive · Disadvantages: Not aesthetically pleasing, Hg ions are toxic, controversial if harmful
Identify the distinguishing property of Nitinol and name two implant uses that exploit this property.
· Shape memory: A special type of transformation that is reversible, heating or unloading of stress can cause material to return to shape · Self-expanding stents, cardiovascular filters, aneurysm clips, orthopedic devices.
Describe the concerns with metal-on-metal hip implants, and describe the FDA's actions taken in response to dangers associated with these implants.
• Beginning in 2011, the FDA has issued statements regarding MoM implants - Require manufacturers to conduct post‐market surveillance studies - Require manufacturers submit PMA (rather than 510(k) exemption as in the past) - Identifies patients who are high‐risk for MoM implants - Recommend more rigorous tracking of patients for symptoms of failure or toxicity - If symptoms are present, recommend diagnostics for soft tissue damage and/or metal ion testing • Five‐year failure rates of MoM implants reported between 6%‐13% • Stryker and DePuy have issued major recalls