CHAPTER 8 Design Parameters 8.1 Biomaterials: Relative Properties 8.2 Bulk (Mechanical) and Surface Properties 8.3 Reactivity: Molecular Interactions 8.4 Bioadhesion (Tissue Bonding): Physical and Chemical Mechanisms 8.5 Factors Affectin g Biomaterials 8.1 BIOMATERIALS: RELATIVE PROPERTIES ADVANTAGES METALS Stainless Steel Strength Ease of manufacturing Availability Cobalt- Chromium Strength Corrosion resistance Relative wear resistance Titanium (6Al- 4V) Strength Low modulus Corrosion resistance CERAMICS Alumina Resistance to chemical degradation Wettability Resistance to wear Calcium Phosphates (Slightly Soluble and Resorbable) Hydroxyapatite Bone- bonding Slight solubility Whitlockite Bone- bonding Solubility Natural (Resorbable) Bone Apatite Bone- bonding Resorbability DISADVANTAGES Potential for corrosion High modulus of elasticity Unknown long- term effects of Co and Cr ions High modulus Low wear resistance Low te nsile and flexural strength Low tensile and flexural strength Slight solubility Low tensil e and flexural strength Solubility Low strength POLYMERS Synthetic Thermoplastics PTFE (Teflon) Resistance to chemical degradation Low wear resistance Hydrophobicity Hyd rophobic Low friction Does not display typical thermoplastic flow behavior UHMWPE Relatively high wear resistance Subject to oxidation PET (Dacron) PMMA Polymerization in vivo PSF High strength thermoplastic PEEK High strength (> PSF) Low water absorption C/PSF; C/PEEK Very high strength Relatively low modulus Elastomers PDMS High flex life Ease of manufacture Subject to hydrolysis Low MW contaminants Low fatigue st rength (for load- bearing applications) Water absorption (dec. strength in water) Unproven Unproven Low wear resistance Release of low MW PDMS Range of mechanical properties Immunogenicity? Polyurethane High flex life Uncertain molecular structure- Range of mechanical properties property relationships Surface radically different from bulk (high mobility of "soft segments") Low MW contaminants Subject to hydrolysis, oxidation, and calcification Hydrogel P - HEMA Low reactivity Transparent Absorbable PLA/PGA Programmable absorption Metabolizable degradation products Natural Collagen Replicates ECM components Hyaluronan Replicates ECM com ponent Low strength Uncertain biological response to bolus- release of metabolites Low strength Immunogenicity? Unproven Chitosan Substitutes for GAG (e.g., hyaluronan) Unproven PTFE polytetrafluoroethylene, UHMWPE, ultra high molecular weight polyethylene PET polyethylene terephthalate; PMMA, polymethyl methacrylate; PSF, polysulfone; PEEK, polyetheretherketone; PDMS, polydimethyl siloxane; P- HEMA, poly hydroxyethyl methacrylate; PLA, polylactic acid; PGA, polyglycolic acid. 8.2 BULK (MECHANICAL) AND SURFACE PROPERTIES 8.2.1 Properties Dependent on Atomic Bonding in the Bulk and Surface of Ma terials BULK Mechanical - Strength - Elasticity/Plasticity/Viscoelasticity - Wear (Abrasive and Fatigue) SURFACE Mechanical - Wear (Adhesive) - Friction/Lubrication Chemical - Corrosion - Oxidation - Hydrolysis - Enzymolysis - Dissolution Bioadhesion - Mechanical - Chemical 8.2.2 Bulk (Mechanical) and Surface Properties METALS Stainless Steel Cobalt- Chromium Titanium (6Al- 4V) CERAMICS Alumina BULK Mechanical Strength Modulus (MPa) (GPa) 500- 1000 200 700 240 900 110 4000 380 259 Calcium Phosphates Hydroxyapatite <900 <100 Whitlockite Natur al Bone Apatite 140 18 POLYMERS Synthetic PTFE (Teflon) 14- 34 0.4 UHMWPE 21 1 PET (Dacron) <40 PMMA 55 3 PSF 70 2.5 PEEK 90 3.6 C/PSF; C/PEEK 500 60 PDMS 2.4- 7 <.01 Pol yurethane 1- 69 .07- 6.9 P - HEMA PLA PGA Natural Collagen Hyaluronan NA NA Chitosan NA NA SURFACE Mechanical Chemical Wear Reactivity (0- ++++) (0- ++++) Comment + + Tension + + ++ + 0 0 Compress. Tension NA ++* Compress. NA +++* NA +++* Compress. ++++ 0 ++ + +++ + +++ + Tension Tension Composites ++++ 0 NA 0 NA 0 NA ++++** NA ++++** NA ++++** NA ++++** NA ++++** PTFE polytetrafluoroethylene, UHMWPE, ultra high molecular weight polyethylene PET polyethyl ene terephthalate; PMMA, polymethyl methacrylate; PSF, polysulfone; PEEK, polyetheretherketone; PDMS, polydimethyl siloxane; P- HEMA, poly hydroxyethyl methacrylate; PLA, polylactic acid; PGA, polyglycolic acid. * Soluble **Absorbable 8.3 REACTIVITY: MOL ECULAR INTERACTIONS 8.3.1 Surface Modifying/Degradative Interactions: Effects of the Body on the Biomaterial 8.3.1.1 Water 8.3.1.1.1 Absorption (e.g., high water absorption by hydrogels is desired but even low water absorption by thermoplastics po lymers can adversely affect mechanical properties) 8.3.1.1.2 Hydrolysis (e.g., of ester linkage of polymers) 8.3.1.1.3 Water as electrolyte solution facilitates corrosion of metal 8.3.1.1.4 Dissolution of certain substances (e.g., calcium phosphates) 8.3 .1.2 Oxygen 8.3.1.2.1 Oxide formation (e.g., on metals) 8.3.1.2.2 Oxidative degradation of polymers 8.3.1.2.3 Corrosion of metal (e.g., sites of depleted oxygen undergo anodic, reduction, reaction) 8.3.1.3 Cations and Anions Contributing to Corrosion, Dissolution, and Precipitation (e.g., mineralization/calcification) 8.3.1.4 Enzymes (e.g., enzymolysis of natural polymers such as collagen) Macromolecule Absorption (e.g., lipid absorption) 8.3.2 Molecular Interactions with Biological Molecules: Effects of the Biomaterial on the Body 8.3.2.1 Water 8.3.2.1.1 Hydrophobic interactions 8.3.2.2 Charge Interactions 8.3.2.2.1 Ionic (primary bonding) 8.3.2.2.2 Secondary 8.3.2.2.2.1 Hydrogen bonding 8.3.2.2.2.2 Van der Waals interactions 8.4 BIOADHESION (TISSUE BONDING): PHYSICAL AND CHEMICAL MECHANISMS 8.4.1 Physical/Mechanical 8.4.1.1 Entanglement of macromolecules (nm scale) 8.4.1.2 Interdigitation of ECM with surface irregularities/porosity ( mm scale) 8.4.2 Chemical 8.4.2.1 Primary 8.4.2.2.1 Ionic 8.4.2.2 Secondary 8.4.2.2.1 Hydrogen bonding 8.4.2.2.2 Van der Waals 8.4.2.3 Hydrophobic Interactions 8.4.3 Size and Time Scales for Bioadhesion Size Tissue Mechanism Scale Level of Bonding mm- cm Organ Interference Fit Grouting Agent Tissue (Bone) Ingrowth Chemical Bonding mm Tissue Same mm Cell Integrin nm Protein Secondary Bonding GAG Hydrophobic Interactions nm Mi neral Epitaxy crystallites Ionic Bonding Time Constant Weeks- Months- Years Wee ks Days- Weeks Seconds- Minutes- Hours- Days Seconds- Minutes- Hours- Days Measurement(s) Radiographic (qualitative) Mechanical Testing (quantitative) Mechanical Testing Light Microscopy/Histology (qualitative) Scanning Electron Microscopy (qualitative and quantitative) Histology Transmission Electron Microscopy (qual.) Immunohistochemisty (qual.) Adsorption Isotherm (quan.) Transmission Electron Microscopy In vitro Precipitation (quan.) m 8.4.4 Characteristics of Porous Materials for Selected Applications Device Function/ Purpose Tissue Cell Cell Process(es) Pore Size (m m) Pore Geometry/ Orientation Facilitate dermal regeneration/ Prevent contraction Dermis Fibroblast Contraction 20- 120 (3- D) Isotopic or planar isotropic (?) Facilitate nerve regeneration/Axon elongation Nerve Nerve Migration 1- 10 U niaxial Attachment of prosthesis to bone/Bone ingrowth Bone Osteoblast Mitosis Synthesis 100- 600 Isotropic 8.4.5 Types of Bonding and Biomaterials for Implants in Bone Types of Bonding Press Fit (Interference Fit) Grouting Agent Bone Ingrowth Bone Bonding Screw Fixation Bone Interdigitation Materials Titanium Alloy Cobalt- Chromium Alloy Polymethylmethacrylate Cement (Bone Cement) Porous Cobalt- Chromium Alloy Porous Titanium Alloy Porous Commercially Pure Titanium Plasma Sprayed Hydroxyapatite Stainless Steel Cobalt- Chromium Alloy Titanium Alloy Cobalt- Chromium Alloy Titanium Alloy 8.5 FACTORS AFFECTING BIOMATERIALS 8.5.1 Exposure to Air (e.g., hydrocarbon contaminants). 8.5.2 Handling (e.g., contamination with particles and alteration of topography). 8.5.3 Storage Time (e.g ., residual stresses can result in dimensional changes). 8.5.4 Sterilization 8.5.4.1 Autoclave (steam) Effects of temperature and absorbed water in altering mechanical properties of certain thermoplastics. 8.5.4.2 Dry heat (prolonged high temperatures) 8.5.4.3 Gas (ethylene oxide) Prolonged period of aeration required for certain polymers. 8.5.4.4 Gamma radiation Scission, crosslinking, and oxidation (when performed in air) of polymers.