Lecture 3: Biomaterials Surfaces: Chemistry

1 3.051J/20.340J Lecture 3: Biomaterials Surfaces: Chemistry Surfaces are high-energy regions of materials and thereby facilitate chemical reactions that influence performance of biomaterials. This lecture will focus on 2 classes of surface chemistry relevant to biomaterials: • Chemisorption on metals and oxides • Aqueous corrosion of metals 1. Chemisorption Strong modifications to electronic structure/ electron density of adsorbate molecule (> 0.5 eV/surface site) Important Examples: a) Metal Oxide Formation on Metals “metals just wanna be oxides” xM + ½yO2 → MxOy ∆G0 of oxide formation is negative for all but a few metals (e.g., Au) Reaction 2Cr + 3/2 O2 = Cr2O3 ∆G0 (joules) -1,120,300 + 260T T range (K) 298-2100 Fe + ½ O2 = FeO -259,600 + 62.55T 298-1642 2Fe + 3/2 O2 = Fe2O3 Ti + O2 = TiO2 -810,520 + 254.0T -910,000 + 173T 298-1460 298-2080 from D.R. Gaskell, Intro. To Metallurgical Thermodynamics, McGraw-Hill, 1981 2 3.051J/20.340J How does metal oxidation happen? One scenario is… step 1: physisorption of O2; ~20-25 kJ/mol 1 eV/molec = 96.5 kJ/mol kT293 ≈ 0.025 eV step 2: molecular oxygen dissociates and reduces by chemisorption; ~600 kJ/mol step 3: bond rearrangement; crystallization of oxide layer Resultant reduction in surface energy Compare: at 1400°C: γδ-Fe = 1900 dyn/cm γFeO = 580 dyn/cm Consider metal oxidation as 2 half reactions: M → Mz+ + ze- O2 + ze- → ½z O2- M Electrons and ions must traverse the oxide layer for rxn to proceed. Mz+ Across the oxide film, an oxidation potential, E0 ~ 1V generates an electric field: ∆G 0 = − E 0 zF - F= 96,480 C/mol e 1 J = 1 V-C x~1 nm E-Field ≈ 1 V/nm = 10 MV/cm Ionic species are “pulled” through oxide film! WHAT HAPPENS AS THE OXIDE CONTINUES TO GROW? 3 3.051J/20.340J The E-field decreases. Subsequent oxide growth occurs by thermal diffusion of Mz+ to oxide surface or O2- to metal/oxide interface under the concentration gradient ∆c: l 2 = k pt l oxide thickness Requirements for Passivation: i) small kp (rate const) time k p = const D ∆c ii) adherent oxide Oxide layer must not scale or spall ⇒ minimize ∆Vmolar & stress build-up xM + ½yO2 → MxOy ex., Ti (TiO2), Cr (Cr2O3), Al (Al2O3) (Al metal not used in biomaterials applications due to toxicity) Voxide ( formed ) ρM M M O x y PB = = Pilling-Bedworth ratio: Vmetal (consumed ) xM M ρ M O x y Want PB ~ 1 (PB > 1) for adherance of oxide to underlying metal—in practice, this rule is marginally predictive, however. 4 3.051J/20.340J Other Metal Oxidation Rxns by Chemisorption: Reaction with water: xM + yH2O → MxOy + yH2 Reaction with CO2: xM + yCO2 → MxOy + yCO IS THE FORMED METAL OXIDE SURFACE STABLE? b) Acid/Base (Acceptor/Donor) Rxns on Oxides i) **H2Oadsorbed + O2-lattice→ OH-lattice + OH-surface Ubiquitous! e.g., oxides of Co, Ti, Cr, Fe, etc. H H2O cleavage with H+ transfer to surface basic O2- site & OHcoordination with M2+ H H O • • • • O2­ M2+ O2­ M2+ M2+ acts as Lewis acid (e- pair acceptor) for oxygen lone pairs O2­ + O- H M2+ O2­ M2+ 5 3.051J/20.340J ii) CO2,adsorbed + O2-lattice→ CO32­ (carbonate formation) Experimentally seen, e.g., on TiO2 (110) iii) w/ Hydrocarbons: Alcohols (similar to HOH): ROHadsorbed + O2-lattice → RO- + OH­ Carboxylic Acids: RCOOHadsorbed + O2-lattice → RCOO- + OH- c) Redox (Oxidation/Reduction) Reactions on Oxides Example Alcohol dehydrogenation to aldehyde: O RCH2OH + 2O2-→ RC H + 2OH- + 2ealcohol is oxidized reference: V.E. Henrich and P.A. Cox, The Surface Science of Metal Oxides, Cambridge Univ. Press: 1994 electrons reduce Mz+ at surface 6 3.051J/20.340J 2. Aqueous Corrosion of Metals In water or in vivo, even a “passive” oxide layer (terminated by bound water) becomes susceptible to corrosion. Why? • Mz+ diffusion will always occur • oxide may dissolve • damage to oxide layer Corrosion: the destructive result of chemical rxn between a metal or metal alloy and its environment. Aqueous corrosion: involves electronic charge transfer i.e., an electrochemical rxn Typically, metal surface acts as both anode (oxidation=loss of e-) & cathode (reduction=consumption of e-) in different regions anodic rxn: M → Mz+ + ze- cathodic rxns: O2 (dissolved) + 4H+ + 4e- → 2H2O 2H+ + 2e- → H2 (g) O2 (dissolved) + 2H2O + 4e- → 4OH­ 2H2O + 2e- → H2 (g) + 2OH­ in acidic in neutral or basic Locally, a biological environment can be neutral, acidic or basic. 7 3.051J/20.340J Metals in aqueous solution Metal ions leave surface; surface becomes negatively charged. Metal M Mz+ are attracted back toward surface, establishing a dynamic equilibrium. Mz+ 2e- H+ H2 The resulting charged double layer exhibits a characteristic E. H+ Values of Ε measured relative to a reference electrode (e.g., std. H2 electrode = SHE), give metric of reactivity in aqueous soln. V on M electrode: Inert Pt reference electrode M → Mz+ + zeaM=1 on reference electrode: H2 → 2H + 2e + ­ aMz+=1 aH+=1 Semi-permeable membrane PH2 1atm 8 3.051J/20.340J emf series (Table 5, p. 432 of text) Metal Au Pt Ag H Sn Mo Co Fe Cr Al Ti Li Standard electrode potentials E0 measured at unit activity (~1 N) anodic of Mz+ ions in soln. with SHE ref. Potential (V) 1.43 1.2 0.79 0.0 -0.14 -0.20 -0.28 -0.44 -0.73 -1.33 -1.63 -3.05 If the activity of Mz+ is less than unity, the electrical potential across the metal/solution interface is modified according to the Nernst equation: RT aoxidized species E=E + ln zF areduced species 0 Equating activity to concentration: aoxidized species = [M z+ ] E (V ) = E 0 + areduced species = [ M ] = 1 0.059 log[ M z+ ] z At this potential, the system is in dynamic equilibrium, i.e., equal metal dissolution and deposition rates (the exchange current density). 9 3.051J/20.340J A Simple Corrosion Rule: anything that upsets the dynamic equilibrium of the charged double layer can accelerate corrosion. Galvanic Corrosion: if 2 metals in contact where -EA > -EB (i.e., A more neg./anodic), B becomes an e- “sink” ⇒ accelerating corrosion of A + B e- + The ↓ in e- from A surface allows ↑ release of Az+ + A Can be macroscopic scale or microscopic scale effect! Examples: • Plate & screw of different alloys • Cr-depleted region at grain boundary (due to carbide formation) • Formation of surface oxides or sulfides that conduct e­ • Alloys exhibiting 2 phases • Metal grains of different orientation Fretting Corrosion: accelerated corrosion from two surfaces rubbing together ¾ metal/metal, metal/bone, metal/oxide contacts ¾ wear of passive oxide film localized stress ⇒ ↑ Mz+ dissolution defects (terraces, steps): less bonds ⇒ ↑ Mz+ dissolution ¾ repetitive movement ⇒ continual loss of corrosion products 10 3.051J/20.340J Pitting and Crevice Corrosion: localized attacks resulting in rapid Mz+ release at discrete sites ¾ initiated at local failure of passive film pitting: surface defect; crevice: region of restricted transport ¾ remaining surface acts as cathode → rapid penetration at defect (oxidation = reduction) ¾ catalyzed by presence of Cl¾ autocatalyzed by ↑ acidity from rxn in confined volume Photo removed for copyright reasons. Corrosion on neck region of a femoral stem of a CoCr hip prosthesis. from J.J. Jacobs et al, “Corrosion of Metal Orthopaedic Implants”, J. Bone and Joint Surgery 80-A, 268 (1998). Ex. Stainless steels in Aqueous Soln: For Cr contents below 12.5 at%: FeOOH resides at surface in hydrated form: FeOx(OH)y•nH2O (iron oxyhydroxide hydrate) In absence of Cl-, slow dissolution: FeOOH + H2O → Fe3+ + 3OH­ 11 3.051J/20.340J With Cl- present: FeOOH + Cl- → FeOCl + OH­ FeOCl + 2H2O → Fe3+ + Cl- + 2OH­ Low solubility Fe3+ precipitates as Fe(OH)3 (rust) restricted transport oxide Fe2O3 S.S. (Fe,Cr)2O3 Fe-9Cr Breakdown of passivation layer until reaching metal: Fe → Fe2+ + 2e­ accelerated corrosion! attracts Cl­ Fe2+ + 2Cl- + 2H2O → Fe(OH)2 + 2HCl local acidity ↑ in pit or crevice Outside pit or crevice: Fe2+ → Fe3+ + eO2 + 2H2O + 4e- → 4OH­ Fe3+ + 3(OH)- → Fe(OH)3 precipitates, further restricting transport 12 3.051J/20.340J Steel composition effects: Cr: For Cr > 12.5 at%: surface is hydrated CrOOH (chromium oxyhydroxide), i.e., CrOx(OH)3-2x•nH2O ¾ renders S.S. surface passive ¾ increases susceptibility to pitting/crevice corrosion (CrCl3 solutions have low/negative pH!) Mo: ↓ dissolution rate in pits/crevices (mechanism?) In the corrosion of metallic implants, further considerations arise with the complexity of biological fluids. In vivo environment ¾ pH 7.4 ¾ T = 37°C ¾ anions: Cl , PO3 , HCO3 (bicarbonate) ¾ cations: Na+, K+, Ca2+, Mg2+ ¾ proteins & other biomolecules Influence of other ions found in vivo: Ca2+ and Mg2+: decrease corrosion rate ¾ form precipitates on metal surfaces Ca2+ + HCO3- + OH- → CaCO3 + H2O Mg2+ + 2OH- → Mg(OH)2 precipitates ¾ O2 influx restricted → inhibits cathodic reduction 3.051J/20.340J 13 Biological Factors Influencing Corrosivity: • O2 levels (↑ O2 ⇒ ↑ corrosion; anodic≡cathodic) • pH variations (inflammation/infection ⇒ ↓ pH) • Cellular activity (e.g., phagocytic cells ⇒ local ↑ in oxidant conc.) • Protein-Mz+ binding (Mz+ carried away ⇒ ↑ Mz+ dissolution) e.g., cytochrome oxidase, peroxidase, catalase bind Fe2+ • Protein adsorption (↓ O2 access to surface) • Bacterial oxidation of Fe2+ to Fe3+ (↑ anodic rxn/Fe2+ dissolution) Corrosion’s Influence on in vivo Performance: ¾ Immunological sensitization; e.g., Ni & Cr allergic response ¾ Wear debris – initiates biological cascade (foreign body response, bone loss) ¾ Loss of implant structural integrity (mechanical failure) ¾ Metal ion toxicity: Al, Ni, Cr, V, Co, Ti; ↑ levels in blood/organs ¾ Carcinogenicity (tumors at implant sites, leukemia/lymphoma correlation with CoCr implants)