design a biomaterial exam

Midterm Take Home Component (Part 2) Please answer the following question and turn in your response for next class, November 5, 2015 You have been hired by a Large Educational Testing and Assessment firm to prepare an appropriate test for evaluating what a student should learn in the field of Biomaterials. Your job is to look over the materials that have been covered in BE 501 and prepare an outline for what you feel are the most important concepts that a student should learn from the course (from the beginning of class till today). Based on this outline, design a midterm exam to assess student’s knowledge and prepare an answer key that fully answers your exam questions. Extra points may be awarded based on creativity, insight, alignment with Next Generation Science Standards, and a thorough coverage of the most important concepts in the course so far. Alignment with the NGSS can be accomplished by developing an Elementary, Middle, or High School version of your midterm exam. Metallic Implant Materials Notes Distinctive Features of Metals - Passivation Corrosion Metallic Bonding Slip Planes, thermal, and electrical properties Role in biological function Biocompatibility of implant metals - very crucial due to corrosion Corrosion products released into surrounding tissue with potentially adverse effects. But not all metal interactions are bad Bioinorganic Chemistry - Fe, Cr, Co, Ni, Ti, Ta, Mo, W (used in manufacturing metal alloy inplants) can be tolerated in the body in small amounts Some M+ are integral in bodily system function Fe - essential for red blood cell function dioxygen transport via proteins for mammalian vertebrates (hemoglobin) mollusks and arthropods (hemocyanin) marine invertebrates (hemerythrin) toxic versus non toxic effects on cell function Co - synthesis of vitamin B12 (coenzyme B12) ferritin and biomineralization (iron storage protein, 24 protein subunits [approx. 175 amino acids/subunit] surrounding a hydrated ferric oxide core containing different amounts of phosphate - each molecule can accommodate up to 4500 Fe atoms), Magnetite metal ligand complexes organometallic (compounds defined by the presence of metal to C (M-C) bonds vs metal organic M-OR or M-O-C bonds) Major Classes of Metallic Implants Stainless Steel Co based Alloys Ti and Ti based Alloys other metals (Ni based (dentistry), Ta, Au and Ag, Pt and noble metals Phase Diagrams (Quick Intro) Lever Rule for solid solution (complete solid solution) and eutectic diagrams Less popular metals (not widely used) before covering the more widely used metallic implants Ticonium - high nickel content alloy used in dentistry to make bridges (modeled after Inconel, a high temperature corrosion resistant alloy) approximate composition - 70% Ni, 15% Cr, 5% Mo, <0.5% Be (toxic) Dental bridge work (see ingots and partial bridge) Tantalum (named after Tantalus [Greek Mythology] due to its characteristic feature of being hard to produce metal ion solution and once in solution, difficult to bring out of solution) advantages - very biocompatible in animal implant studies disavantages - very poor mechanical properties, high density (16.6 g/cc) restricted use wire sutures for plastic and neurosurgery radioisotope for bladder tumors Platinum group - extremely corrosion resistant, poor mechanical properties mostly used as alloys for electrodes pacemaker tips due to high corrosion resistance low threshold potential Au and Ag - resistant to environmental attack (corrosion resistance), poor mechanical properties current interest is small (dental fillings?) Sherman Vanadium Steel - first metal developed specifically for implant (1912) fracture plates and screws First Stainless Steel (ss) used for implantation 18-8 - 18% Cr, 8% Ni steel alloy (302 in modern classification) stronger than Vanadium steel and more corrosion resistant 18-8sMo - 18% Cr, 8% Ni, 2-3% Mo for increased corrosion resistance in saltwater (saline) became known as 316 316L - C content reduced from 0.08% to 0.03% (1950) for better corrosion resistance (less chance of metal carbide formation) Stainless Steels require a minimum of 11% Cr to resist corrosion (Cr is reactive, but it passivates to give excellent corrosion resistance) Key to Corrosion Resistance Passivation - Formation of tightly adherent metal oxide layer that forms a protective barrier against corrosion Cr plays a number of roles in ss (beneficial and detrimental) form a strongly adherent surface oxide - Cr2O3 (desireable) stabilizes ?Fe ferritic phase (BCC) with Mo and Si (undesireable) Ni added to stabilize the stronger ?Fe austenitic phase (FCC) Formation of Cr-C (Cr23C6) - tendency for formation of carbides increases when C% increases above 0.03% (316 ss) carbides tend to precipitate at grain boundaries (gb) under right conditions and deplete adjacent gb region of Cr (limits passivation) “sensitized” steels (steels with carbide formation) prone to failure via corrosion assisted fracture at sensitized (weakened gb) Manufacturing of Metallic Implants metal containing ore to raw metal - (see fig 1) Ti metal production- rutile (TiO2) or ilmenite (FeTiO3) processed treat oxide with Cl to make TiCl4 treat TiCl4 w/ Mg and Na Ti metal sponge and MgCl, NaCl (Kroll process) Purify Ti metal (refining, vacuum furnace, etc) O content very important in determining final mechanical prop. Tenths of a percent of oxygen content differentiate commercially pure (CP) Ti Production of Fe - hematite (Fe2O3, 4th most abundant compound in earth’s crust) reduction with C Alloying - melt and mix component metals in proper ratios Stock Shapes - bar, wire, rods, plate, sheet, tube, powder formed by remelting and casting, hot rolling, forging, cold drawing through dies heating and cooling cycles mechanical properties control and development Cold Working - inc. strength, dec. ductility due to formation and interaction of slip planes (Park, 90), slip systems, CRSS, increased shear strength prop. to square root of dislocation density, Frank-Read Source for dislocation production Solute Hardening - add solute (impurity) atoms that interact with dislocations. If solute atom bigger than solute (matrix) atom, compressive field and vice versa Precipitation Hardening - form a solid solution, then quench and age below phase separation T (Al-Cu phase Diagram). Supersaturated with respect to ? phase (room T, ppt small stable particles, hardness increase due to increase in lattice strain energy (particles act to “pin” dislocations), can increase resistance to shear by reducing the effective distance between ppt particles (make finer ppt by control of ?T). microstructural control and development (phase diagrams, processing methodology) smaller grain sizes tend to increase strength Hall-Petch relationship ?y = ?f + kd-m ?y is the yield stress, ?f is the friction stress, k is a constant (associated with deformation propagation across grain boundaries), and m is about 0.5. anneal out stresses (metals tend to be very forgiving) heat and remove stresses, heal dislocations, change microstructure and phase distribution Fabrication of metallic implants investment casting (see diagram) control of mold T used to control the microstructure - coarser grains at higher T for Co-Cr alloys Casting defects machining (conventional, CAD/CAM) developments in computer technology, cutting tools, Computer Numerical Control (CNC) Systems, stereolithography, cutting fluids greatly impact innovations in CAD/CAM forging powder metallurgy (sintering, HIP) bulk device fabrication Surface Modification Porous surface coating - apply metal powder to implant surface and sinter min. 100-200 ?m interconnected porosity for healthy bone tissue ingrowth Plasma/Flame Spraying - add metal powder to a hot, high velocity gas plasma and “shoot” at implant surface. Particles partially melt and fuse with surface during rapid cooling. Ion implantation - in vacuum, high energy beam of N atoms directed at implant surface. Become imbedded in implant surface at lattice sites and can enhance surface hardness and wear properties. final finishing (grinding, polishing, chemical cleaning, and passivation) Fabrication technique is based on factors such as machinability of alloy, shape of final implant, desired properties in final product Co based alloys difficult to machine - use investment casting or powder metallurgy Ti difficult to cast (probably due to combination with O at red heat) - machine Sterilization Four Major Classes of stainless steels (grouped according to their microstructures) Group I - Martensitic Chromium Steels Hardened by heat treatment - used to make surgical instruments (420 ss) Group II - Ferritic, Non Hardenable steels ferrite (aFe) structure good deep drawing characteriastics good corrosion resistance (especially stress corrosion) mostly used as chemical and food storage containers Group III - Austenitic Stainless Steels Non-magnetic hardenable by cold working (dislocation entaglement) best corrosion resistance (Mo enhances resistance to pitting corrosion) lower C in 316L enhances corrosion resistance in Cl- solution (physiological saline) mostly used for implants Ni helps to stabilize the austenitic phase Table 1 Composition of 316 and 316L ss (from Park) Grade 1 (316) Grade 2 (316L) C 0.08 max 0.03 max Mn 2.00 max P 0.030 max S 0.030 max Si 0.75 max Cr 17.00-20.00 Ni 12.00-14.00 Mo 2.00-4.00 Group IV - Age Hardenable Steels Age hardenable at 450-550 C Properties of ss Mechanical Properties of ss (tables from Park) Corrosion Issues 1.) incorrect composition or processing history 2.) intermixing of components improper fit (crevice corrosion) different processing history (galvanic corrosion) different implant components in close proximity (plates and screws with compositional range (fretting, crevice, or galvanic corrosion Co-based Alloys - Co-Cr alloys first used in dentistry Four types (compositions) recommended by ASTM for implants F76, F90, F562, and F563 (see viewgraph) two types mainly used - 1.) cast Co-Cr-Mo alloy - (dentistry, articial joints) 2.) wrought (hot forged) Co-Ni-Cr-Mo alloy - (heavily loaded joints (femoral stem) use due to superior fatigue and ultimate tensile strength - need long service life without fracture or stress fatigue) Microstructure and strength control - pure Co is FCC above and HCP below 419 C transformation occurs via martensitic type shear reaction (transformation becomes easier with alloying additions and plastic deformation) Alloying Co in F562 - 50% cold work (increases driving force for transfromation of retained FCC to HCP (fine platelets of HCP within FCC grains). HCP platelets impede dislocation motion for improved strength. Further strength can be obtained by aging at 430-650 C for ppt of Co3Mo on HCP platelets (hence older designation of MP35N (multi-phasic) derives strength from cold working, solid solution and ppt hardening Ti based alloys - Low density, 4.5 g/cc (compare to 316 ss (7.9), cast CoCrMo (8.3) or wrought CoNiCrMo (9.2), While its Elastic Modulus and strength are much lower than ss or Co-Cr alloys, its specific strength (strength/density) is much greater than other matallic implants Impurity content (Fe, N, especially O) distinguish 4 commercial grades of Ti alloys Manufacturing issues - Ti very reactive at high T and burns readily in presence of O (melt in inert atmosphere or vacuum) Ti forms thin, adherent passivation layer (TiO2) for corrosion resistance O2 diffuses in Ti dissolved O2 embrittles Ti (requires hot working below 925 C) Machining difficulties (galling) seizes cutting tool need to use very sharp tools with slow speeds and large feed rates to minimize galling, or use electrochemical machining Topic Outline Polymers Structure (repeat units, MW determination (number, weight average), polymer configuration, crosslinked and branched structures (fig 2.27), polymer synthesis, percent crystallinity, chain fold model, polymer defects (linear and volume)) Characterization Techniques Defects (chapter 3) Point, Linear, planar, and volume defects Volume – grain boundaries and twin planes Thermal Transitions in crystalline and non-crystalline materials Viscous flow Melt T (metals, ceramics, polymers) Glass transition Thermal analysis techniques TGA DTA DSC Polymers for Biomaterial Implants Polymers (poly - many, mer - unit) are widely used as biomaterial implants (also drug delivery) Key Benefits of polymers Ability to tailor surface and bulk properties via chemistry (swell, degrade, in situ reaction, mechanical support) design - biomimetics synthesis of analogues for natural polymers spider silk, collagen surface modification Blood-Materials Interactions - heparin coating to prevent clotting bioactivity - controlled dissolution for release of agents interfacial bonding - composites wound healing adhesives orthopedic and dental luting agents fabrication (manufacture, processing) Polymerization Reactions Monomer ? POLYMERIZATION ? Polymer single molecular unit long chain molecule elementary building block for polymer synthesis Many ways to form polymers Focus on two types - Condensation and Addition Condensation - monomer units react and a small molecule (water, alcohol) is condensed out methyl terephthalate + ethylene glycol ? Polyethylene terephthalate + MeOH DACRON Most natural polymers made by condensation reactions - cellulose (polysaccharide), protein Glucose Cellulose Polysaccharides are present in connective tissue to lubricate joints and fibrous tissue layers like collagen and elastin Amino acids polymerize into poly peptides via condensation reaction Addition or Free Radical Polymerization Example for PE Three reaction stages - Initiation ROOR + energy ? RO + OR Propagation Activated monomer reacts with neighboring ethylenes Termination Activated radical polymer meets a free radical or two activated Various classes of Natural and Synthetic Polymers Natural - Proteins, Polysaccharides, Polynucleotides, Rubber (Table from Ratner et al.) Aspects of Natural Polymers similarities with macromolecular substances that the biological environment deals with routinely Suppression of toxic or inflammatory response Immunogenic response activation (production of antibodies) potential to design for biological activity at the molecular level Degradation by naturally occurring enzymes Structural complexity complicates technological manipulation Examples - Collagen, Chitin Synthetic - Homopolymers (made from a single monomer type) poly (methyl methacrylate) (PMMA) - lucite, plexiglas hyrophobic, linear chain polymer, glassy at RT, optical properties, mechanical properties (bullet proof windows), good stability, used in intraocular lenses and hard contact lenses HEMA (2-hydroxyethyl methacrylate) addition of methylol (-CH2OH) imparts hydrophilicity Swollen hydrogel when fully hydrated (can dissolve) used as soft contact lenses (partially crosslink with ethylene glycol dimethacrylate (EGDM) to prevent dissolution) Polyolefins (PE and PP) polyethylene (PE) - 3 types low density, high density, ultra-high MW (UHMWPE) Low Density - 1930’s research on the effects of high pressure on gas molecules - ethylene in a rigid container at high P (100-300 Mpa) and T (200 C) in presence of initiator (O, peroxide). Low density form results because side chains branches can develop. 50-70% crystallinity, not used in biomedical applications because can’t withstand sterilization conditions. High Density - polymerization of ethylene gas using Zeigler catalyst (TiCl4, AlCl3) at lower P (10 Mpa) and lower T (100-175 C). 70-80% crystallinity due to absence of side chain branches, fairly easy to process (soluble in xylene, melts at about 125 C for molding), used as tubing for drains and catheters UHMWPE - widely used in orthopedic implants (acetabular cup for hip joints, knee joints) no known solvent at RT, process by hi T, P sintering polypropylene (PP) Synthesized with stereospecific Zeigler type catalysts (atactic-random, synditactic-alternating, isotactic-same) (see view graph) used for similar applications as PE, superior stress cracking resistance compared to PE, high rigidity, good chemical resistance, good tensile strength, exceptionally high flex life (used in making integrally molded finger joint prosthesis), properties depend on the percentage of isotactic materials present, increase in isotacticity increases density, crystallinity, softening T, and chemical resistance polytetraflouroethylene (PTFE) - teflon H in PE replaced by F, synthesized from tetrafluoroethylene under P w/ peroxide catalyst and water used to transfer heat, highly crystalline (>94%), avg MW 5x105-5x106, very high density (2.15-2.2 g/cc), low surface tension (18.5 ergs/cm2, thermally and chemically very stable (difficult to process), very hydrophobic, excellent lubricity (low friction coefficient of 0.1), loe tensile strength (17-28 Mpa) and low elastic modulus (0.5 GPa), processed via sintering at T above 327 C under pressure (due to high melt viscosity and inability to plasticize) microporous teflon - Gore-Tex - used for vascular grafts poly(vinyl chloride) (PVC) - Biomedical tubing applications (blood transfusion, feeding, dialysis), plasticizers added to improve flexibility and reduce brittleness, in vitro interactions can lead to removal of plasticizing agents (low toxicity, but effects properties of PVC) poly(dimethyl siloxane) (PDMS) - Si-O backbone as compared to C in most polymers, used in catheter and drainage tubes, pacemaker lead insulation, component in some vascular grafts, high O2 permeability allows use in membrane oxygenators, excellent flexibility and stability allows use in many prostheses such as finger joints, blood vessels, heart valves, breast implants, outer ears, chin and nose implants polycarbonate (PC) - bisphenol A + phosgene ? polycarbonates, clear and tough material with high impact strength, used for eyeglass and safety lenses, housings for devices (oxygenators, heart-lung bypass) polamides (Nylon) - surgical sutures Copolymers poly (glycolide lactide) - random copolymer used in resorbable sutures, maintains strength longer than natural suture material (catgut-PGA) PTFE + hexafluoropropylene ? fluorinated ethylene propylene copolymer (FEP) similar applications as teflon, only lower crystalline melting point of 265 C for easier processing while maintaining the properties of teflon. Polyurethanes - block copolymer with hard and soft blocks, tough elastomer with good fatigue and blood containing properties, used in pacemaker led insulation, vascular grafts, heart assist ballon pumps, artificial heart bladders Degradable Polymers valuable for short term applications, no need for surgical removal 4 main types of degradable polymers temporary scaffold - temporary mechanical support for broken bones, damaged blood vessel, or wounds (fixation devices, vascular grafts, and sutures), engineering criteria involve tailoring the degradation rate of the scaffold for proper stress transfer to healing tissue (major challenge), first synthetic degradable sutures were PGA, then copolymers of PLA and PGA were used (vicryl - 90:10 mix of PGA/PLA temporary barrier - prevention of adhesion formation between neighboring tissue types (caused by blood clotting in extravascular tissue space and subsequent inflammation and fibrosis), part of natural healing process, but causes problems if it occurs where it shouldn’t. Prevent by putting a thin polymer film to prevent contact drug delivery device - ideally suited for this application, PLA, PGA often used first because already approved, undergoing advanced clinical trials - intracranial polyanhydride for treatment of glioblastoma multiformae (severe form of brain cancer (administration of chemotherapeutic agents at proper site) multifunctional implant - biodegradable bone fixation devices (nails, screws, plates) incorporated with bone morphogenetic proteins that can be released during degradation for enhanced bone remodeling and healing (mechanical support with site specific drug delivery) - ultra high strength PLA biodegradable stents for implant in coronary arteries (prevention of collapse and restenosis (reblocking) of arteries after reopening by ballon angioplasty (potential delivery of anti-thrombogenic or anti-inflammatory agents directly to injury site