Biomaterials and Their Classification

Biomaterials

• Definition: A material that can function as a whole or part of a system which augments, repairs, or replaces any tissue or organ system.
• Examples: knee/hip implants, contact lenses, dental implants, 3D printed tissues, heart valves, skin grafts, hearing aids, hair implants, stents, eyeballs, and prosthetics.

Classification of Biomaterials

• Metals: Metallic bonding (sea of electrons), sharing of free electrons. Examples: iron, gold, silver.
• Ceramics: Ionic bonding, non-directional, electrons are taken from the outer level. Examples: salt, silica/sand, alumina, hydroxyapatite.
• Polymers: Covalent bonding, directional, electrons are shared. Examples: PEG, hyaluronic acid.

Properties of Biomaterials

• Mechanical: includes stiffness, soft, hard, viscoelasticity.
• Chemical: NMR, Raman spectroscopy, Fourier-transform infrared spectroscopy.

Equations

• Stress: σ = F/A
• Strain: ε = Δl/lo

Polymers

• Definition: many units, larger macromolecule.
• Polymerization: combining monomers (repeat unit) into a polymer.
• Types of polymerization:
  • Addition: chain polymerization, no by-products.
  • Condensation: step polymerization, water is a byproduct.
• Examples of common polymers:
  • Polyester
  • Polyether
  • Polyanhydride
  • Polyamide
  • Polyurethane
  • Polyurea

Polymer Weight

• Molecular weight
• Polydispersity: shows how uniform the population of polymers are.
  • Monodisperse: ideal polymer, small polydispersity.
  • Polydisperse: large polydispersity.

Classification of Polymers

• Thermoplastic: polymers that flow more easily when squeezed, pushed, or stretched.
• Thermoset: polymers that flow and can be molded initially, but their shape becomes set upon curing.
• Elastomer: polymers that are elastic like rubbers.

Copolymers

• Random: uncontrolled order, randomly positioned.
• Alternating: one after the other in a pattern.
• Block: large blocks alternate.
• Graft: chains of one polymer grafted onto the backbone of another polymer.

Polymer Structures

• Linear: in terms of packing, linear can be more densely packed.
• Branched: has a variety of branching.
• Cross-linked: still has points of connection.
• Network: has many points of connection.
• Order of strength: branched < linear < cross-linked < network.

Polymer Crystallinity

• Chain folded structure.
• Linear polymers can form ordered repeating regions of crystal.

Natural vs Synthetic Polymers

• Natural: examples are gelatin, proteins, zein, hyaluronic acid, mucin.
• Protein synthesis: an example of condensation polymerization.

Protein Classification

• Globular, membrane bound, fibrous.

Levels of Protein Structure

• Primary: sequence of amino acids.
• Secondary: includes alpha helix and beta sheets.

Degradation Types

• Surface erosion: poly(ortho)esters and polyanhydrides, most water sensitive.
• Bulk degradation: PLA, PGA, PLGA, or PCL, faster than surface erosion.

Biodegradable Polymers

• Broken down by hydrolysis.
• Rate of erosion is determined by chemical bond stability, hydrophobicity, and morphology.

PEGylation

• Increases drug size, reduces hydrophobicity, and increases stability.
• Widely used in drug modification and protein therapeutics.

Bioconjugation

• Attachment of one molecule to another, usually through a covalent bond.

Immunocytochemistry

• Detection and visualization of proteins, including immunohistochemistry, immunofluorescence, and immunocytochemistry.

Amino Acids

• Sites for potential conjugation.
• Functional groups: amine, carboxyl, thiol, alcohol.

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Orthopedics

• Fixture vs replacement: fixture stops movement, replacement replaces function/movement.
• Osteoconductive: enables integration of new bone with the host bone.
• Osteoinductive: promotes new bone growth.

Properties of Orthopedic Implants

• No toxicity.
• Suitable for mechanical strength.
• High wear resistance.
• Minimize stress shielding.
• Osseointegration abilities.

Corrosion

• Process that transfers electrons from one substance to another.
• Mechanism: in acidic and neutral environments.
• Prevention: avoid using combinations of metals, use metals with similar nobility, and use passive methods.

Suitable Mechanics

• Cyclic loading: material needs to withstand years of repeated loading.

Wear Resistance

• Aseptic loosening: joints and orthopedics damage over time.

Stress Shielding

• Creates osteopenia: decrease in bone density.

Osseointegration

• Formation of a direct interface between an implant and bone without intervening soft tissue.

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Metals

• Uses: bone and joint replacement, dental implants, facial reconstruction, cardiovascular devices, external prostheses, surgical instruments.

Properties of Metals

• Physical: luster, good conductors of heat and electricity, high density, high melting point, ductile, malleable.
• Chemical: easily lose electrons, surface reactive, loss of mass, change in mechanical properties.

Crystal Structures

• Unit cell: the smallest repeating unit within a crystal lattice.
• BCC, FCC, and HCP structures.

Metal Processing

• Casting: contained nucleation starts at the edge.
• Formation of crystals: nucleation, growth, and grain boundaries.

Crystal Defects

• Point defects: one or two atoms.
• Solid solution: normal crystal structure is maintained with impurity.

Rules for Solid Solution

• Size difference should be within 15%.
• Electronegativities should be similar.
• Valence charges should be similar.
• Crystal structures should be the same.

Solid State Diffusion

• Movement of atoms due to thermal energy.
• Through diffusion, atoms can move to select locations.

Strengthening Mechanisms

• Grain size reduction: improves toughness.
• Solid solution strengthening: increases strength.
• Strain hardening: ductile metals become stronger when deformed plastically.
• Recovery-annealing: relieves internal strains, reduces dislocations, and makes metal weaker.

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Ceramics

• Uses: orthopedic implant, coatings and thin films, bone cements, scaffolds and bone grafts, dental screws, porous scaffolds.

Properties of Ceramics

• Generally inert.
• Resists chemical reactivity.
• Strong.
• Sensitive to deformation.
• Advantages: inert, wear-resistant, high modulus, and compressive strength.
• Disadvantages: brittle, low fracture resistance, poor fatigue resistance.

Structures

• Crystalline ceramic: long-range order, organized structure of grains.
• Glassy ceramic: short-range order, typically do not form grains.
• Glass-ceramics: combination of crystalline grains surrounded by amorphous material.

Pauling's Rules

• Magnitude of charge: ceramic crystals are neutral.
• Relative size of the ions: cations typically smaller.

Crystal Structure

• Larger the cation is relative to the anion, the more anions that surround it.
• Coordination number and ratio of cation radius to anion radius.

Linear Defects

• Edge dislocation: occurs when there is termination of a plane of atoms in a crystal.

Bio ceramic-tissue Interaction

• Morphological fixation: dense, inert, nonporous ceramics attach to bone.
• Biological fixation: porous inert ceramic attaches by bone growth.
• Bioactive fixation: dense, non-porous surface-reactive ceramics attach directly to bone.

Types of Bio ceramics

• Bioinert: stable and non-reactive.
• Bioactive: direct bone implant bond.
• Bioerodible: gradual degradation.

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Material Characterization

• Types of forces: tension, compression, shear, torsion.
• Stress and strain.
• Elastic deformation: linear, reversible strain.
• Plastic deformation: non-linear, irreversible strain.
• Viscoelastic: viscous liquid + elastic solid-like.

Time-dependent Properties

• Creep: plastic deformation of material under constant load over time.
• Stress relaxation: decrease in stress over time under constant strain.

Viscoelastic Behaviors

• Why stiffness and viscoelasticity matter: important for biomaterials because they mimic tissue.### Cellular Morphology and Development

• Viscoelasticity drives cellular morphology and development, including stem cell division and differentiation.

Surface Analysis Techniques

• Contact angle analysis:
  • Hydrophilic: θ < 90°
  • Hydrophobic: θ > 90°
  • Superhydrophobic: θ > 150°, typically requires texture
• Single point: equilibrium between liquid droplet and solid surface has been reached
• Dynamic contact angle:
  • Advancing angle: contact angle of the liquid with dry surface
  • Receding angle: contact angle with the water-absorbed surface

Microscopy Techniques

• Optical microscopy:
  • Advantages: low magnification, quick, non-destructive, color differentiation available
  • Disadvantages: limited resolution, smaller depth of field, light reflections can mask certain features
• Scanning Electron Microscopy (SEM):
  • Advantages: greater depth of field, high magnification, sub-nanometer resolution available
  • Disadvantages: slower inspection time, grayscale only
• Transmission Electron Microscopy (TEM):
  • Advantages: create higher resolution images, allow users to examine more characteristics of a sample, higher magnification
  • Disadvantages: more expensive, slower speed, more complicated operation
• Scanning probe microscopy → atomic force microscopy (AFM):
  • AFM: uses Van der Waals forces to promote interaction of a cantilever tip with the surface, scanning across the surface gives atomic detail of the topography

Spectroscopy Techniques

• X-ray photoelectron spectroscopy (XPS)
• Fourier transform infrared spectroscopy (FTIR):
  • Measures vibrational modes in molecules with infrared light (stretching, twisting, scissoring, rocking, wagging)
• Matrix assisted laser desorption ionization (MALDI) mass spectrometry:
  • Laser desorption of material on a surface complexed with ionizable small molecules (the matrix)
• Secondary ion mass spectrometry (SIMS):
  • Useful technique for measuring the composition of a material
  • Ions are shot at a sample, stripping off the surface layer of material, and these secondary ions are then analyzed using MS principles

Biological Characterization

• Analytical techniques:
  • Types of assays:
    • Scratch assay (wound healing assay): measures a cell's ability to close the wound
    • MTT Assay: measures metabolism, reduction potential
    • PicoGreen assay: measures cell quantity by quantifying DNA amount
    • Alamar Blue Assay: measures cell viability and proliferation
    • Live/Dead Assay: simultaneous determination of live and dead cells
    • BrdU Assay: shows cell division, incorporates BrdU into the cell's DNA
    • Generic cell division assay: measures cell division

Cell Death and Inflammation

• Anoikis: death due to lack of cell-matrix interactions
• Apoptosis: programmed cell death
• Necrosis: chaotic cell death
• Autophagy: recycle components
• Inflammation:
  • 4 main components of the innate immune system:
    • Anatomic barriers: skin, mucus membranes
    • Physiological barriers: temperature (37°C)
    • Phagocytic cells: engulf, granulocytes → neutrophils, monocytes → macrophages
    • Acute inflammation
  • 4 main cardinal signs of inflammation:
    • Rubor (redness)
    • Tumor (swelling)
    • Calore (heat)
    • Dolore (pain)
  • Common cells involved in inflammation:
    • Granulocytes: family of cells that can phagocytose foreign invaders
    • Monocytes: phagocytic capability
    • Macrophages: monocytes that have migrated or been born within a tissue
    • Megakaryocytes: break apart to form platelets

Blood-Material Interactions

• When a biomaterial is put into the body:
  • Injury
  • Blood-biomaterial interaction
  • Provisional matrix formation
  • Acute inflammation
  • Chronic inflammation
  • Granulation tissue
  • Foreign body response
  • Fibrous capsule formation
• Sequence of events:
  • Biomaterial implantation
  • Protein adsorption
  • Macrophage adhesion
  • Encapsulation
  • Capsular contracture
• What regulates protein-material interactions and adsorption:
  • Properties of the protein:
    • Size
    • Charge
    • Hydrophobicity
    • Stability
  • Properties of the material:
    • Surface hydrophobicity
    • Surface charge
    • Topology
    • Composition
    • Heterogeneity
    • Potential
• Vroman effect: order of protein binding
  • Small, abundant molecules bind first, such as Albumin

Coagulation

• Thrombosis: blood coagulation
  • 4 steps to coagulation:
    • Injury
    • Activation of platelets
    • Formation of platelet plug
    • Blood coagulation
  • Intrinsic and extrinsic pathways → common pathway (fibrin polymer network)
  • Calcium dependence for both pathways
  • How to design material to regulate coagulation: capture calcium, decrease protein binding

Innate Immune System

• 4 main defense components:
  • Anatomic barriers
  • Physiological barriers
  • Phagocytic cells
  • Acute inflammation
• Common cells involved in inflammation:
  • Granulocytes
  • Monocytes
  • Macrophages
  • Megakaryocytes
• Order of cell activation:
  • Neutrophils
  • Macrophages
  • Foreign body giant cells (FBGCs)

Wound Healing

• Granulation: formation of many new blood vessels, making tissue look granular
• Neovascularization: formation of new vasculature (blood vessels, vasculogenesis, angiogenesis)
• Fibroblasts: responsible for synthesis of ECM (collagen)
• Foreign Body reaction:
  • Continuation of granulation process, attempting to phagocytose the biomaterial
  • Monocytes/macrophages fuse into a big multinuclear cell called Foreign Body Giant Cells (FBGCs)
• Fibrous encapsulation: final stage of healing, considered acceptable for biomaterial implants

Case Study on Breast Implants

• Different surface textures (smooth and textured) and can quantify these through SEM and profilometry
• After some time, a fibrous encapsulation forms around the implant

ECM and Mechanotransduction

• Extracellular Matrix (ECM):
  • A large network of proteins and other molecules that surround, support, and give structure to cells and tissues in the body
  • Transport is regulated by ECM
  • Cell + ECM = Tissue
• Emergent behavior: collective property of individual components, commonly used to describe both material and biological properties
• ECM enables emergent behavior from a single cell to an organized structure with functionality
• Primary components of the ECM:
  • Fiber-forming elements: collagen
  • Link forming elements: proteoglycans, hyaluronan, glycoproteins
  • Space-filling elements: same as above
  • Free and sequestered factors: growth factors, ions
• Functions of ECM:
  • Cell adhesion
  • Cell-cell communication
  • Cell-matrix communication
  • Differentiation
• Why do we care about ECM and biomaterials?:
  • Overall design new biomaterial to include aspects of ECM to direct the tissue response