CHAPTER 10 Characteristics of the Surfaces of Biomaterials 10.1 Surface Characteristics Related to Chemical Bonding 10.2 Surface Chemistry Related to Bonding of Biological Molecules 10.3 Porosity 10.4 Factors Affecting the Biomaterial Surface 10.5 Surface Characteristics and Methods of Analysis 10.6 Bioadhesion (Tissue Bonding): Physical and Chemical Mechanisms 10.7 Size and Time Scales for Bioadhesion 10.8 Chemical and Physical* Bonding (Nanometer Scale)10.1 SURFACE CHARACTERISTICS RELATED TO CHEMICAL BONDING The surface of a material has different chemical bond characteristics from the bulk and, therefore, should be considered to be different. 10.1.1 Polar Groups Molecules in which one atom has an excess of negative charge, i.e., unequal sharing of bonding electrons. One atom is more electronegative than the others. 1) Can become ionized and interact by ionic forces (10-20 kcal/mol.). Like ionic charges repel. 2) Form permanent dipoles and interact by: a. Hydrogen bonding (approx. 3-7 kcal/mol) b. Molecular orientation c. Induction interactions 3) Intensity of dipole/dipole interactions are dependent on distance of separation and orientation of individual dipoles. Parallel dipoles can repel each other. 4) Polar/ionic groups can have acidic (proton donating or electron accepting) or base (proton accepting, or OH or electron donating) character. Acid-base interactions are likely as opposed to acid-acid or base-base which are repulsive. 5) Biomaterials with ionic groups or strong dipoles on the solid surface will tend to bind water molecules (hydrophilic). 6) The presence of specific polar groups is inferred from determination of elements, molecules, and bonding in the surface of biomaterials. 10.1.2 Nonpolar (Apolar) Molecules Nonpolar molecules might be available to undergo van der Waals interactions (nonspecific) with biological molecules. The "bond energy" of approximately 1-2 kcal/mol. is due to the correlation of the electronic motion of the molecules. 10.1.3 Surface Free Energy (Critical Surface Tension) 1) An atom in the free surface has no neighbors on one side. Since bond energies are negative its energy is higher than interior atoms by the missing share of bond energy. 2) Chemical bonds of surface atoms are asymmetrically directed toward the interior of the material, attracting the surface atoms inward and causing surface tension. 3) Energy required to create a free surface (e.g., by fracture of the material) is reflected in the surface energy. Thermodynamic forces act to minimize surface energy. a. Low energy molecules near the surface are translated and rotated toward the surface. Low energy components in the bulk migrate (diffuse) to the surface. These include: 1. Low molecular weight additives (e.g., antioxidants, processing aids, etc.) or degradation products 2. Contaminants b. Exposed surface groups attempt to lower their energy (i.e., unsatisfied bonding) by adsorbing or reacting with ambient molecules. 1. Adsorption of hydrocarbon (contamination) by all materials2. Oxide formation on metals 4) Surfaces with low critical surface tension are hydrophobic. Hydrophobic surfaces interrupt the hydrogen bonded structure of water (i.e., force water molecules to structure in an ice-like conformation on or near the surface). Water droplets do not spread on hydrophobic surfaces. 5) "Bond energy" of hydrophobic interactions (dispersion force) is approx. 1-2 kcal/mol. 6) Critical surface tension of solids can be determined from contact angle measurements and the Zisman plot.10.2 SURFACE CHEMISTRY RELATED TO BONDING OF BIOLOGICAL MOLECULES 10.2.1 Molecules 10.2.1.1 Type (with respect to polarity) 1) Magnitude of the dipole moment. Molecules with no dipole moment are nonpolar and thus provide a very hydrophobic surface (e.g., PTFE). 2) Potential to become ionized to form ionic bond. 10.2.1.2 Distribution Could affect chemical specificity by providing a certain template of bonds. 10.2.1.3 Density For example, hydrogels have a very low density of molecules at the surface to accommodate a considerable amount of water. 10.2.1.4 Mobility Surface of certain polymers (e.g., polyurethanes) are dynamic (i.e., always changing because of mobility of the molecules). 10.2.2 Surface Characteristics Resulting from Chemistry 10.2.2.1 Hydrophobicity 10.2.2.2 Charge10.3 POROSITY (Pore characteristics and what features that they affect) Void Fraction; Percentage Porosity Strength of the material Amount of tissue that can form in the material Surface area Pore Diameter; Interconnecting Pore Diameter Surface area Size of the tissue elements (e.g., cells) that can infiltrate the material Pore Orientation Direction of cell migration and architecture of the tissue that forms10.4 FACTORS AFFECTING THE BIOMATERIAL SURFACE 1) Exposure to air (e.g., hydrocarbon contaminants). 2) Handling (e.g., contamination with particles and alteration of topography). 3) Storage time (e.g., residual stresses can result in dimensional changes). 4) Sterilization a. Autoclave (steam) Effects of temperature and absorbed water in altering mechanical properties of certain thermoplastics. b. Dry heat (prolonged high temperatures) c. Gas (ethylene oxide) Prolonged period of aeration required for certain polymers. d. Gamma radiation Chain scission followed by oxidation. Crosslinking of certain polymers.10.5 SURFACE CHARACTERISTICS AND METHODS OF ANALYSIS Scale of Features (Not Detection Depth of Penetration) Macroscopic (>10 mm) CharacteristicsHydrophobicity Charge Topography Porosity Water content Surface area Mechanical compliance Microstructure (>0.2mm) Particles on surface Topography Profilometry (stylus pulled Crystallite Structure/Size Method Contact angle (Critical surface tension from Zisman plot) Electrophoresis of particles (zeta potential) Light microscopy (LM) Scanning electron microscopy (SEM) LM, SEM, Mercury intrusion porosimetry Drying/weighing Gas adsorption methods Mechanical testing (modulus of elasticity) Light microscopy/SEM Light microscopy, SEM, over surface) X-ray diffractionNanostructure >0.01mm Particles SEM (>10 nm) Topography SEM, Profilometry 1-10 nm Elemental composition Energy dispersive x-ray analysis (EDX) Wavelength dispersive x-ray analysis (WDX) Electron spectroscopy for chemical analysis (ESCA, also referred to as x-ray photoelectron spectroscopy, XPS) Auger electron spectroscopy (AES) Secondary ion mass