Biomaterials for Medical Devices

Biomaterial Fundamentals

• Biocompatibility and interaction with biological elements, material, and tissue changes are becoming more important in device design

Surface and Bulk Properties

• Surface and bulk properties influence a device's biocompatibility

Intermolecular vs. Intramolecular Forces

• Intermolecular forces occur BETWEEN molecules forming liquids and solids, and are weaker
• Intramolecular forces are WITHIN a molecule, holding it together, and are stronger

Common Metals in Medical Devices

• Titanium, Stainless Steel, and Chromium-Cobalt
  • Pro: Easy to machine, strong, easy to sterilize
  • Con: Corrosion, toxicity

Atomic Structure of Metals

• Atomic structure of metals determines mechanical, chemical, and electrical properties via plastic deformation, conductivity, metallic bonds, and stress shielding

Common Ceramics in Medical Devices

• HA,
  • Pro: Not specified
  • Con: Not specified
• High compressive strength, low tensile strength, hard to machine, degrade

Atomic Structure of Ceramics

• Metallic and nonmetallic elements held together by ionic/covalent bonds determine mechanical, chemical, and electrical properties

Common Polymers in Medical Devices

• PMMA, PLGA, PTFE
  • Pro: Flexible, largest range of mechanical properties, smooth
  • Con: Degrade

Atomic Structure of Polymers

• Solid due to intermolecular interactions, weak secondary bonds between chains, crosslinked with covalent bonds

Protein Adsorption

• Gene expression involves transcription and translation

Structure of a Protein

• Primary: amino acids undergo condensation reactions forming peptide bonds that hold together the polypeptide chain; backbone is covalently bonded carbon and nitrogen molecules
• Secondary: the amine group and carboxyl group of different amino acids interact with each other with dispersion forces or van der Waals forces
• Tertiary: ionic/covalent/hydrogen/hydrophobic interactions/bonds create larger 3D shapes
• Quaternary: Not specified

Protein Function and Conformation

• Protein conformation changes can cause the protein to break and form new bonds
• Shape changes can lead to loss of function due to new binding sites being exposed or taken away

Extracellular Matrix (ECM) Function

• ECM provides structure and strength
• Proteins and sugars control cell behavior by binding to integrin receptors

Collagen Composition and Structure

• Made of 3 amino acid chains forming a triple helix with covalent bonds and cross-links, creating a hierarchical structure

Elastin Composition and Structure

• Hydrophobic interactions allow return to original shape once stretched

Fibronectin Composition and Structure

• RGD binding regions, identical amino acid chains

Fibrinogen Composition and Structure

• RGD site binds to cell integrins to clot and becomes fibrin

Surface Characterization Methods

• Surface Characterization Methods are useful for assessing biocompatibility

Contact Angle Analysis

• The mechanism, benefits, and limitations of are not specified

X-Ray Photoelectron Spectroscopy (XPS)/Electron Spectroscopy for Chemical Analysis (ESCA)

• Uses X-rays at sample to emit electrons, collected to create a spectrum
• Allows to see atoms on surface, measured in a vacuum

Fourier Transform Infrared (FTIR) Spectroscopy

• Vibration of bonds to see orientation of molecules and protein 2nd structure

Secondary Ion Mass Spectroscopy (SIMS)

• The mechanism, benefits, and limitations of are not specified

Scanning Electron Microscopy (SEM)

• The mechanism, benefits, and limitations of are not specified

Atomic Force Microscopy (aFM) and scanning probe microscopies (SpM)

• Uses a photodiode, laser, and cantilever tip to see surface roughness

Ellipsometry

• Uses polarized light to determine protein layer thickness

Physiological Components

• The information on which physiological components accumulate on the material surface is not specified

Importance of Protein Adsorption

• Protein adsorption on a biomaterial is important for cell attachment

Ways Proteins Damage Materials

• Release of molecules, specifically H2O2 from WBC

Protein and Surface Properties

• Which protein and surface properties influence protein adsorption is not specified

Protein Adsorption and Desorption

• What is the process of protein adsorption and desorption is not specified

Vroman Effect

• What is the Vroman effect and how does it apply to blood serum proteins is not specified

Protein Adsorption Studies

• How to study protein adsorption on biomaterials is not specified

Carbon Nanotube Properties

• Carbon nanotubes properties link to possible applications in biomedical engineering.

Measuring Protein Shape Change

• Use of enzymes to measure protein shape change with adsorption.

How Proteins Alter Shape

• Proteins unfold to change shape with adsorption

Nanoscale Supports

• Using nanoscale supports on carbon nanotubes alters enzyme activity to reduce protein-protein interactions

Surface Geometry Influence

• Surface geometry influence adsorbed protein activity

Carbon Nanotubes with Proteases

• Design a carbon nanotube with attached proteases for selective protein destruction

Carbon Nanotubes and Protein Deactivation

• To deactivate proteins used of targeting ligands

Types of Carbon Nanotube Functionalization

• Covalent: strong, designed, limited loading, alters molecules
• Non-covalent: weak, natural affinity, higher loading, no molecules affected

Risks of Using Carbon Nanotubes

• The method by which to mitigate risk of using carbon nanotubes for cancer treatments is not specified

Circulatory System Components

• Veins, arteries, capillaries, endothelial cells, smooth muscle, basal lamina, endothelium

Blood Components and Functions

• RBC, WBC, Platelets, Serum, Plasma

Red Blood Cell Structure

• Red blood cells come from hematopoietic stem cells in the bone marrow
• Have no nucleus, so they cannot proliferate or make proteins, but carry hemoglobin

Red Blood Cell Shape Change

• Red blood cells change shape due to shear stresses
• They are small donut-like shaped cells normally

Causes of Red Blood Cell Shape Change

• High shear stress/rates, osmolarity, pH, biomaterial interaction

Platelet Initiation of Blood Clots

• Platelets initiate blood clots when there is an injury to the tissue
  • Become activated if they bind to biomaterial, collagen in the vessel (collagen usually in basal lamina but if there is vascular injury then collagen leaks into the vessel), fibrin, or other platelets
  • Activated platelets release granules i.e. clotting factors, increases surface area, gets sticky
  • Clotting factors activate more platelets, causes fibrinogen (soluble) to activate thrombin to form fibrin (thrombin cuts fibrinogen)
  • Platelets and fibrin catch rbc and stick together

Control Blood Coagulation

• Use of non-fouling surfaces, and hydrophilic surfaces

Vascular Grafts

• The endothelium, platelets, RBCs, and WBCs are all altered by vascular grafts
• The endothelium is cut causing the platelets to clot and signal to the WBCs
• Intimal hyperplasia can affect the shape of the RBCs leading to blood clots

Blood Pumps

• Blood pumps can damage blood cells by causing too high of shear forces
• Can cause the blood to be stagnant in some areas which can cause clots

Inflammation and Infection

• Inflammation goes away after a few days, and infection has other symptoms such as pus

Functions of Inflammation Cells

• Neutrophils and monocytes are part of immune response, made from hematopoietic stem cells in bone marrow
• Monocytes in blood differentiate into macrophages in tissue
• All clear debris and fight pathogens via phagocytosis
• Failed phagocytosis causes macrophages to fuse together and form a FBGC

Cell Processes of WBCs

• Chemotaxis
• Phagocytosis
  • Opsonization
  • Phagosome formed
  • Make ROI and hydrogen peroxide via metabolic burst
  • Kill microbes
  • Expel phagosome
• If phagocytosis fails, phagosome released early and spills out material/ROI/enzymes, cell lysis, another macrophage tries again, macrophages fuse to form FBGC that releases ROI, acids, and enzymes, and signals to fibroblasts to secrete collagen

Order of Arrival at Wound

• Macrophages in tissue migrate to injury, neutrophils in blood get into tissue (margination and diapedesis), monocytes in blood become macrophages in tissue

Physiological Inflammation Signs

• Redness: sticky endothelial cells, slow blood
• Heat: WBC performing phagocytosis
• Pain: pushing on nerves due to swelling
• Swelling: leaky blood vessels

Components of Pus and Abscesses

• Bacteria, fungi, and viruses colonize a wound; bacteria release toxins creating necrotic tissue
• WBC start phagocytosis and digesting bacteria; large infection means there is too much bacteria and EPS for WBC to digest and they die, other WBC release phagosomes
• Pus made of bacteria, dead WBC, and necrotic tissue.

Biomaterial Based Infections

• Superficial immediate infections: topical ointments, replace sutures
• Deep immediate infection: systematic antibiotics
• Deep late infection: systematic antibiotics don't work well can only remove top layer of EPS, implant removal and replace

Minimizing Infection in Biomaterial

• Porous material to increase surface area, release antibiotics via diffusion or dissolution, coat materials (silver), remote release of antibiotics, antibiotic pellets during implantation

Key Terms

• In vivo: in the body
• In vitro: in the lab
• Ex vivo: outside the body
• Biocompatibility: ability of a material to perform with an appropriate host response in a specific application
• Biomaterial Bulk: size, shape, density, mechanical properties, match tissue being replaced
• Biomaterial Surface: texture, surface roughness, protein interaction, exposed to environment, determine biocompatibility
• Van der Waals Forces: intermolecular bonds, weak forces, pos and neg charges within molecules
• Hydrogen Bonds: intermolecular bonds, H+ in molecule interaction with neg atom in another molecule
• Metallic Bonds: intramolecular bonds, strong, groups of positive metal atoms surrounded by a sea of electrons
• Covalent Bonds: intramolecular bonds, shared electrons between atoms (C–C)
• Ionic Bonds: intramolecular bonds, between positive and negative atoms, crystalline, highly ordered, NaCl
• Stress shielding: implants carry more loads and cells lose mechanical stimulation
• Corrosion: chemical process that occurs at high temp, aqueous solutions, acids, and electrolytes i.e. environment of the body
• Passivation: oxide surface film to prevent rust and corrosion
• Transcription: DNA to mRNA
• Translation: mRNA to amino acids
• Primary protein structure: sequence of amino acids that from a polypeptide chain, carbon and nitrogen backbone with strong covalent bonds that are hard to break
• Secondary protein structure: interactions between amino group and carboxylic group of different amino acids, alpha helix and beta sheet
• Tertiary protein structure: larger 3D shapes, conformation,covalent/ionic/hydrogen/hydrophobic interactions/bonds
• Quaternary protein structure: subunits, monomer/dimer/trimer/tetramer
• Amino Acid: 20 types with different properties, made up of amino group, side chain, carboxylic acid group, and central carbon atom
• Side chain: R-group that determines the higher order structure of proteins i.e. shape and function
• Polar: has a charge
• Non-polar: does not have a charge
• Hydrophilic: water loving
• Hydrophobic: water fearing
• Condensation reaction: negative and positive side of two amino acids react producing a peptide bonded polymer and water, sequential amino acids react to form a polypeptide chain
• Peptide bonds: bond formed by condensation reaction between two amino acids
• Alpha helix and Beta pleated sheet: 2nd structure, thermodynamically stable, easily broken bonds
• Disulfide bonds: covalent, strong bonds between cysteines leads to harder proteins
• Hydrophobic Interactions: nonpolar amino acids move to the center of protein to hide from water
• Protein subunit: collection of alpha helix and beta sheets interacting
• Protein conformation: protein rearranges for changing shape and function with the breaking of bonds and new bonds forming
• Proteases: enxyme that cuts prorteins
• Denaturation: changes in temp, pH, ionic strength alter conformation and bioactivity (proteins changing shape)
• Extracellular matrix (ECM): interconnected network of proteins and polysaccharides, provides structure and strength
• Integrin: like cytoskeleton to ECM (anchor cells)
• Cell differentiation: changing of cell type
• Cell proliferation: cells multiplying
• Cell apoptosis: programmed cell death
• Collagen: most common in all tissue, structural protein, 3 amino acid chains in triple helix via covalent bonds, hierarchical i.e. lots of interactions between chains, cross-linked covalent bonds
• Elastin: elastic, skin/blood vessels/ligaments/lungs, strong cross-links, short chains, hydrophobic interactions allow it to be stretched and return to original shape
• Fibronectin: "golden boy", adhesive, attach cells and ECM, promising for tissue integration, binding regions for fibrin/collagen (ECM)/cells (RGD), identical amino acid chains
• RGD: 3 Amino acids that are in a lot of proteins and focus on binding to the cell integrins
• Fibrinogen: blood clots, RGD site binds to platelets, becomes fibrin in blood coagulation
• Adsorption: proteins adhering to the biomaterial surface
• Molecular weight: size of protein
• Protein Softness: how rigid protein is/likely to unfold
• Contact angle: cheap and simple way to compare materials and assess biocompatibility, not very specific information, measures surface energy (react to environment), drop of liquid on surface to measure angle
• High Surface energy: hydrophilic, spreading, low angle
• Low Surface energy: hydrophobic, water more attracted to itself than to surface, large angle
• Zisman plot: method to approximate materials surface energy, y-axis = theta, x-axis = liquid vapor surface tension, critical surface tension energy (perfect wetting, extrapolate line of best fit to angle=0)
• X-Ray Photoelectron Spectroscopy (XPS): e- emitted from x-rays penetrating into sample, releasing atoms on surface, collected by sensor, output spectrum of chemical composition and chemical bonding state, expensive, vacuum (no hydrated samples or delicate tissue); similar to ESCA
• Electron Spectroscopy for Chemical Analysis (ESCA): e- emitted from x-rays penetrating into sample, releasing atoms on surface, collected by sensor, output spectrum of chemical composition and chemical bonding state, expensive, vacuum (no hydrated samples or delicate tissue); similar to XPS
• Fourier Transform Infrared (FTIR) Spectroscopy: analyze molecular bonds from vibrations induced by infrared light (add energy, sense wavelength of vibration of bonds with detector), shows chemical bonds and functional groups that vibrate at different frequencies, shows orientation of molecules, no vacuum, give info about protein 2ndary structures
• Secondary Ion Mass Spectroscopy (SIMS): similar to XPS but bigger fragments, see chemical functional groups and molecular fragments, bombard material with ions, secondary particles are released from the surface and analyzed with mass spec, vacuum setting
• Scanning Electron Microscopy (SEM): beam of high energy electrons scanned across surface, secondary electrons are collected to determine the topography, vacuum setting, high sensitivity but not quantitative
• Atomic Force Microscopy (aFM): sharp tip attached to cantilever scanned across surface, as surface topography changes, tip moves due to attractive and repulsive forces on the surface, deflection of tip measured via laser beam reflected onto photosensitive detector,
• Scanning probe microscopies (SpM): sharp tip attached to cantilever scanned across surface, as surface topography changes, tip moves due to attractive and repulsive forces on the surface, deflection of tip measured via laser beam reflected onto photosensitive detector, no vacuum, different tips to get chemical info, angstrom or nanometer level resolution, could destroy surface; similar to aFM
• Ellipsometry: nondestructive way to measure protein and surface layer thickness using polarized light directed at surface and reflected to detector, cheaper, no vacuum, all samples,
• Blood plasma: cell free portion of the blood that contains nutrients and clotting factors, centrifuge to get separation of plasma, WBC/platelets, and RBC
• Blood serum: cell free portion of the blood that contains water, salts, ions, and proteins, but no clotting factors i.e. what remains after blood has clot, centrifuge 2nd time to get serum and clot
• Vroman effect: large/soft proteins will likely replace smaller/hard proteins, shows protein layer is dynamic, relies on diffusion and protein conformation, neighboring proteins will have repulsive forces making competition between proteins, proteins with high MW, balanced charges, and softness more likely to stay
• Soft proteins: bioactivity, likelihood of proteins to unfold on biomaterial surface, exposing more binding sites
• Hard proteins: unlikely to stay long on surface, less binding sites, rigid, form monolayer on biomaterial before they are replaced
• Albumin: blood serum protein that is small and has highest concentration, maintains blood concentration to similar level as cell concentration, reaches biomaterial first (after water), does not attach to cell integrins, replaced
• Fibrinogen: big, low concentration blood serum protein that replace the smaller proteins that have formed a monolayer on biomaterial surface
• Carbon Nanotube (CNT): much smaller than cell/size of proteins, interact with proteins/cells easily, can be a composite, light, high hardness/strength/stiffness, electrically/thermally conductive, photoluminescent, hydrophobic, functionalized with functional groups
• Quantom dots: semiconductor nanoparticle
• Nanoshells: spherical nanoparticle consisting of a dielectric core covered by thin gold shell, heat conductive
• Single walled nanotubes (SWNTs)
• Multiwalled nanotubes (MWNTs)
• Graphene
• Soybean Peroxidase (SPB): enzyme that creates peroxide and oxidizes molecules
• a -chymotrypsin (CT): digestive enzyme for milk proteins
• Protein denaturation: change in protein structure causing it to lose function
• Nanoscale supports: strong covalent bonds between proteins and CNT using functional groups making proteins active and stable at high protein loading even under harsh conditions, leaves more space limiting protein-protein interaction and maintaining protein shape and function
• Functional groups: non-covalent and covalent functionalization by adding monomer units to CNT
• Subtilisin Carlsberg (SC): hydrolyzes proteins
• Trypsin (TRY): digestion protein
• Near-Infrared (NIR) Light: pass through tissue, but heat CNT to damage and kill proteins/cell in area, uses thermal conductivity of CNT, deactivate proteins
• Covalent functionalization: strong/longer binding/supports, maintain activity, works for molecules without affinity for CNT, but loading limited to # of available functional sites on CNT and structure of attached molecules are altered, ex) sidewall and defect
• Non-covalent functionalization: weak bonds, temporary, easily desorbed, only works for molecules with affinity for surface, but higher loading, molecular chemistry unaffected, ex) pi-stacking, polymer wrapping, and endohedral
• PEG: hydrophilic polymer chain added to cancer therapy SWNT-DOX to decrease clearance by macrophages and increase time for tumor uptake of the drug
• Liposomes: vesicle with plasma membrane, increase drug delivery, fill with drug and add to CNT to get more drug per CNT and use less CNT
• Vein: carry deoxygenated blood to heart, less structure, floppy, loose, low pressure
• Artery: carry oxygenated blood away from heart, high pressure, stiffer, thick connective tissue
• Capillary: brings oxygen to tissue, blood exposure on devices
• Endothelium: endothelial cells, inner lining of all vessels
• Basal lamina: full of ECM proteins (collagen, laminin, and elastin), holds onto epithelial cells
• Connective tissue: surround blood vessels, more around arteries
• Endothelial cells: make up capillaries, inner lining of any vasculature, functional, let things in and out
• Pericytes: fibroblast like cells sometimes called stromal cells, maintain ECM, structural cells
• Fibroblasts: maintain ECM, in connective tissue
• Plasma: water, salts, proteins, nutrients, made up of clotting factors (fibrinogen-fibrin) and blood serum
• Serum: Newtonian fluid, everything in plasma but clotting factors, used for growing cells in vitro
• Erythrocytes: RBC, provide oxygen to cells, large size and amount (hematocrit), no nucleus, cant proliferate or make proteins, contain protein hemoglobin, come from stem cells in bone marrow, production can be controlled
• Leukocytes: WBC, smaller amount, fight pathogens, clear debris
• Platelets: clot blood, not many of them
• Erythropoietin: hormone that tells hematopoietic stem cells in bone marrow to differentiate into RBCs
• Hemoglobin: protein found in RBC
• Newtonian: fluid with the same viscosity
• Viscosity:
• Shear rate: the change in the velocity of blood flow from the outermost wall to the center of the vessel where there is less resistance, gradient
• Rouleaux: aggregated stacks of RBC
• Hematocrit: percentage of the blood that is RBC, number changes blood flow
• Osmolarity: water diffuses across a semipermeable membrane to higher concentrations of solutes via ion channels
• Hypertonic: water leaves cells, crenate (shriveled)
• Hypotonic: water enters cells, lyse (burst)
• Isotonic: equilibrium, use PBS in vitro
• Thrombin: enzyme that makes fibrin
• Fibrin: sticky fiber that helps platelets clot
• Inflammation: FBR, tissue integration, infection, 7-10 days, scab, redness, swelling, heat, pain, WBC active (immune response)
• Infection:
• Suppurate: pus
• Neutrophils: high amount, come from hematopoietic stem cells in bone marrow, part of immune response, clear debris and fight pathogens via phagocytosis
• Monocytes: part of immune response, clear debris and fight pathogens, found in blood, differentiate into macrophages
• Macrophages: differentiated monocytes, in tissue, die after it does its job, performs phagocytosis, clear debris
• Foreign Body Giant Cells: fused macrophages (multinucleated) as a result of FBR/frustrated phagocytosis, release ROI/acids/enzymes as feedback loop causing chronic inflammation, and signals to fibroblasts to secrete collagen and from a fibrous capsule
• Chemotaxis: cells migrating toward chemoattractant to get WBC to wound
• Chemoattractants: stimulate migration with GF (cytokines) and soluble proteins
• Phagocytosis: process of neutrophils and macrophages clearing debris and fighting pathogens; 1) chemoattractant causes chemotaxis 2) initiated by receptors on macrophages binding to markers (opsonization) via pseudopodia 3) pseudopodia move around object and fust to form a phagosome 4) neutrophil/macrophage make hydrogen peroxide and ROIs via metabolic burst 4) inside the phagosome hash molecules inactivate proteins and kill bacteria/fungi/yeast/viruses 5) phagosome expelled
• Opsonization: coat object with proteins and other immune cells marks as foreign objects
• Phagosome: pseudopodia around an object fuse to form a vacuole (plasma membrane surrounded object) where harsh molecules created by neutrophil/macrophage will enter to inactivate proteins and kill bacteria, fungi, viruses, and yeast
• Margination: WBC attach to sticky endothelial cells at sight of injury, strong receptors, WBC projects pseudopod out and increase receptors
• Diapedesis: WBC squeeze through endothelial cells to leave the blood stream and get into the tissue i.e. sight of wound, endothelial cells retract (via chemical signal that decreases cell-cell adhesion receptors), increased receptors on lumen and WBC
• Necrosis: dead cells, created by bacteria releasing toxins
• Abscess: long term infection, bacteria isolated by fibrous tissue, grows in size, moves towards the surface of tissue because it is mechanically weaker, drain naturally or with surgery
• Superficial immediate infections: 1-2 weeks, pus, on/near skin, topical ointments, replace sutures
• Deep immediate infection: shortly after surgery, systematic antibiotics
• Deep late infection: months/years after surgery, large colony of bacteria (redness, swelling, pain, heat, pus you don't see), large colony of bacteria in blood stream, systematic antibiotics don't work well can only remove top layer of EPS, implant removal and replace, higher risk of infection