BME 220_Unit 2.pdf

BME – 220 Biomaterials Dr. Abeer Syed Department of Biomedical Engineering King Faisal University Physical Properties of Biomaterials – Bulk properties and surface properties Unit 2  “A nonviable material used in a medical device, intended to interact with biological sys...

BME – 220 Biomaterials Dr. Abeer Syed Department of Biomedical Engineering King Faisal University Physical Properties of Biomaterials – Bulk properties and surface properties Unit 2  “A nonviable material used in a medical device, intended to interact with biological systems.”* Comparable mechanical and chemicals properties No undesirable biological effects carcinogenic, toxic, allergenic or immunogenic Possible to process, fabricate and sterilize with a good reproducibility Acceptable cost/benefit ratio Biomaterials - Definition * Biomaterials Science: An Introduction to Materials in Medicine, 2nd ed., B.D. Ratner et al., eds., Elsevier, NY 2004 Material Properties Deterioration Bonding properties Corrosion Compressive strength Tensile strength Degradation Bending strength Calcification Elastic Modulus Hardness and density Mechanical loading Creep Combined Fatigue resistance Surface tension Hydrophobic/philic Water sorption/solubility Surface friction General Criteria for Biomaterial Selection  Bulk Properties Physical Properties of Materials Biomaterials Science: An Introduction to Materials in Medicine, 4th ed., Wagner et al., eds., Elsevier, NY 2020 Sec 1.2.3  Different materials have different bulk properties. Bulk properties are a result of different types of interatomic or intermolecular forces that hold atoms and molecules together. Metals and alloys - metallic bonds, Ceramics - ionic bonds, and Polymers - covalent bonds The differences in these bonding mechanisms and energies dictate the different bulk properties of these materials. Bulk Properties Biomaterials Science: An Introduction to Materials in Medicine, 4th ed., Wagner et al., eds., Elsevier, NY 2020 Sec 1.2.3 Bulk Properties  The mechanical properties of a material refer to the characteristic values of a material under various mechanical loading conditions. Stress and strain are two mechanical variables that are commonly referred to for a material, but they are not properties of the material. Mechanical Variables and Mechanical Properties Biomaterials Science: An Introduction to Materials in Medicine, 4th ed., Wagner et al., eds., Elsevier, NY 2020 Sec 1.2.3  Stress (σ) is defined as force per unit area, obtainable by dividing the applied force (F) by the cross-section area (A) of the material. σ=F/A (N/m2) or pascal (Pa) Strain (ε) is deformation per unit length, obtainable by dividing the deformation (δ) by the length of the material (L) ε=δ/L Dimensionless Stress and Strain Biomaterials Science: An Introduction to Materials in Medicine, 4th ed., Wagner et al., eds., Elsevier, NY 2020 Sec 1.2.3  F = Force V = Shear forces T = Torque M = Moments Newton (N) Newton x meter (Nm) Types of Mechanical Loading Biomaterials Science: An Introduction to Materials in Medicine, 4th ed., Wagner et al., eds., Elsevier, NY 2020 Sec 1.2.3  Measure of stress a material can withstand Strength of a Material Elastic and Plastic Behaviour Biomaterials Science: An Introduction to Materials in Medicine, 4th ed., Wagner et al., eds., Elsevier, NY 2020 Sec 1.2.3 Tensile Strength Ridwan, Ridwan & Prabowo, Aditya & Muhayat, Nurul & Putranto, Teguh & Sohn, Jung-Min. (2020). Tensile analysis and assessment of carbon and alloy steels using FE approach as an idealization of material fractures under collision and grounding. Curved and Layered Structures. 7. 188-198. 10.1515/cls-2020-0016.  Ultimate tensile strength - maximum load that a material can support without fracture when being stretched. Tensile strength measures the force required to pull something such as rope, wire, metal plate, or a structural beam to the point where it breaks. Tensile Strength  Yield strength is the maximum stress that can be applied before it begins to change shape permanently. Yield Strength  Resilience is a measure of the elastic energy that can be stored in a unit volume of stressed material (ability of a material to absorb energy when deforming elastically). Toughness is a measure of the energy required to deform a unit volume of material to its breaking point (ability of a material to absorb energy up to facture). Resilience and Toughness Biomaterials Science: An Introduction to Materials in Medicine, 4th ed., Wagner et al., eds., Elsevier, NY 2020 Sec 1.2.3  The value of plastic strain required to break the material defines the ductility Ductility is the measure of the ability of the material to deform plastically before fracture Ductility Biomaterials Science: An Introduction to Materials in Medicine, 4th ed., Wagner et al., eds., Elsevier, NY 2020 (Sec 1.2.3) https://www.e-education.psu.edu/matse81/node/2107 https://www.aboutcivil.org/imajes/bitumen-ductility-test.png Ductility https://www.e-education.psu.edu/matse81/node/2107 Summary https://www.youtube.com/watch?v=WSRqJdT2COE  The Young's Modulus (or Elastic Modulus) describes the stiffness of a material. In other words, it is how easily it is bended or stretched. 𝑆𝑡𝑟𝑒𝑠𝑠 Young’s Modulus = 𝑆𝑡𝑟𝑎𝑖𝑛 where: Stress = force / cross sectional area Strain = change in length / original length Young’s Modulus Young’s Modulus Examples https://material-properties.org/mild-steel-density-strength-hardness- melting-point/  For materials that must not undergo in situ permanent deformation, failure is synonymous with the yield stress being exceeded, and so the yield stress represents an estimate of the strength of the material in those cases. Failure of a Material Biomaterials Science: An Introduction to Materials in Medicine, 4th ed., Wagner et al., eds., Elsevier, NY 2020 Sec 1.2.3  Material fatigue is a Fatigue Limit -" The maximum phenomenon where structures stress that a metal will withstand fail when subjected to a cyclic without failure for a specified large load. number of cycles.” Fatigue is the most common Often more important than tensile source behind failures of or yield strength mechanical structures. The process of fracture may result from the initiation and propagation of a crack. Fatigue strength is often assessed by subjecting a sample to a cyclic loading test, counting the number of cycles applied until failure. Fatigue  It is a time-dependent deformation under a certain applied load. Generally occurs at high temperature (thermal creep), but can also happen at room temperature in certain materials (e.g. lead or glass), albeit much slower. As a result, the material undergoes a time dependent increase in length, which could The rate of deformation is called the creep be dangerous while in service. rate. It is the slope of the line in a Creep Strain vs. Time curve. Creep  Biomaterials Science: An Introduction to Materials in Medicine, 4th ed., Wagner et al., eds., Elsevier, NY 2020 Sec 1.2.3 Textbook reference  Surface Properties Physical Properties of Materials  Surfaces have unique reactivity. The surface is inevitably different from the bulk. The mass of material that makes up the surface zone is very small. Surfaces readily contaminate. Surface molecules can exhibit considerable mobility. Surface Properties Biomaterials Science: An Introduction to Materials in Medicine, 4th ed., Wagner et al., eds., Elsevier, NY 2020 Sec 1.2.4  The physical properties of the biomaterial are fundamental for its interaction with a biological tissue, e.g. the response of cell adhesion. When cells adhere to the biomaterial surface physical chemical reactions between cell and biomaterial occur. These reactions are influenced by factors such as cell behavior, biomaterial surface properties, and environmental factors. Surface Properties Biomaterials Science: An Introduction to Materials in Medicine, 4th ed., Wagner et al., eds., Elsevier, NY 2020 Sec 1.2.4  Wettability Roughness Surface free energy Surface Properties  Wettability(hydrophobicity/hydrophilicity) affects the biological response of the biomaterial and describes the balance between the solid surface intermolecular interactions with a liquid. Among the affected properties are protein adsorption, platelet adhesion/activation, blood coagulation, and cellular and bacterial adhesion. Hydrophobic surfaces are, generally, considered as more protein-adsorbent than hydrophilic surfaces due to the hydrophobic interactions occurring at the surface. Wettability Hydrophillic/Hydrophobic Hydrophillic/Hydrophobic  The more hydrophobic cells adhere more strongly to hydrophobic surfaces, while hydrophilic cells strongly adhere to hydrophilic surfaces Medical implants such as catheters, mechanical heart valves or pacemakers are constructed from hydrophobic materials (silicon, stainless steel, teflon, etc.), so hydrophobic microorganisms easily adhere to them. One strategy of preventing surfaces from bacterial colonization is the modification of surfaces by coating them with less hyrdophobic materials e.g noble metals, i.e., silver nanoparticles. Hydrophillic/Hydrophobic Krasowka and Sigler, Front. Cell. Infect. Microbiol., 2014 Superhydrophobicity Surface Tension https://gifer.com/en/HUPD  Surface tension is defined as the energy required to increase the surface area of a liquid by a specific amount. Surface tension is measured as energy per unit area, such as joules per square meter (J/m2) Surface Tension  Surface tension results from the net inward force experienced by the molecules on the surface of a liquid. It causes water to “bead up” when in contact with nonpolar surfaces. Those forces are called the surface tension. Surface Tension  Cohesion is the intermolecular attraction between like molecules. Example H-bonding in water. Adhesion is the intermolecular attraction between unlike molecules. It binds a substance to a surface. Liquids with high surface tensions have strong cohesion forces, and they are poor wetting liquid due to low adhesion forces. A detergent or wetting agent is a substance that increases the adhesion force between two different materials. Cohesive and Adhesive Forces Cohesive and Adhesive Forces https://studiousguy.com/adhesive-force-examples/  The rise of liquids up narrow tubes is called capillary action. Adhesive forces attract the liquid to the wall of the tube. Cohesive forces attract the liquid to itself. Water has stronger adhesive forces with glass; mercury has stronger cohesive forces with itself. Capillary Forces  The work expended in order to increase the size of the surface of a phase is referred to as the surface free energy. As energy per unit area, the surface free energy has the unit J/m2 or N/m. The symbol used in formula is σ (lower case sigma). The term surface free energy is normally used for solid surfaces. When a liquid phase is concerned, surface tension is used. Surface Free Energy  Molecules in the bulk of a material (e.g. crystal lattice) have a low relative energy state due to nearest neighbor interactions (e.g. bonding). Performing sufficient work on the system to create an interface can disrupt this harmony. Surface Free Energy  Molecules at a surface are in a state of higher free energy than those in the bulk. This is in large part due to the lack of nearest neighbor interactions at a surface. Surface Free Energy  Systems move toward lowering their free energy Surfaces do so by: Geometric changes (if possible) Protein Bonding (strong and weak interactions) Dynamic rearrangement OH OH CH3 CH3 CH3 CH3 OH OH Surface Free Energy Surface Free Energy Tam et al., J. Mater. Chem. A, 2018,6, 18384-18388  The roughness of the biomaterial also plays an important role in the adhesion and cellular behavior and exerts direct influence both in vitro and in vivo. Smooth surface and rough surface have different contact areas with molecules and cells and this difference in contact influences the kind of biological units’ links and therefore, conformation and function. Roughness  In most of the cases cells prefer rough surface to smooth ones, due to the fact that rough surfaces favor proliferation For low friction applications, such as in implants of orthopedic joints, biomaterials with mirrored finishes are preferred. And when tissue- implant integration is desired, as is the case of endobone implants, high roughness is preferred Roughness Krishna, Lekshmi & Dhamodaran, Kamesh & Jayadev, Chaitra & Chatterjee, Kaushik & Shetty, Rohit & Khora, Samanta & Das, Debashish. (2016). Nanostructured scaffold as a determinant of stem cell fate. Stem Cell Research & Therapy. 7. 10.1186/s13287-016-0440-y.  Crystallinity Composition Charge Surface or Bulk Properties?  Crystallinity: Repeating or periodic array over large atomic distances. 3-D pattern in which each atom is bonded to its nearest neighbors Crystal structure: the manner in which atoms, ions, or molecules are spatially arranged. Crystallinity Crystalinity  The physical properties, composition, and chemical properties influence the kind of cell bond and determine the biomaterial chemical stability and reactivity. The corporeal ambience is harsh and may cause corrosion of biomaterials. Thus, the biomaterials’ chemical stability becomes a relevant factor as regards biocompatibility. Corrosion products may cause adverse reactions to the implant neighborhood. Body fluids are in balance with specific ions under normal physiological conditions. When a biomaterial is implanted the concentration of these ions increases significantly around it and may cause swelling and pain, besides the fact that the corrosion wastes may migrate to other parts of the body and cause undesirable reactions, both for the tissues and the implant. Chemical Composition  Corrosion of biomaterials alters not only chemical stability, but also affects the mechanical integrity, with possible premature failure of the material. As with corrosion, the corporeal ambience may cause and/or accelerate the biomaterial degradation. Degradation can also be influenced by sterilization processes to which materials are submitted. When the biomaterial is degraded, modifications occur at the material structure and, consequently, modifications in its properties. Surface functional groups can also influence the biomaterial response, since the surface chemical functionality affects adsorbed protein and cell/protein interactions. Commonly investigated functionalities as relates biomaterials are carboxyl (–COOH), hydroxyl (–OH), amino (–NH2), and methyl (–CH3) groups. Chemical Composition Effect of surface functionality on protein adsorption, cell behavior and tissue responses Chemical Composition/Charge Thevenot et al., (2008), Curr Top Med Chem., 8(4); 270-280 Chemical Composition/Charge Thevenot et al., (2008), Curr Top Med Chem., 8(4); 270-280