Biosensors Lecture Notes PDF

BME328: Principles of Biomaterials Lecture 24: Biosensors Dr Andrew Daly Associate Professor in Biomedical Engineering College of Science and Engineering, BMS 1012 Biomedical Sciences Building University of Galway Email: [email protected] Bioelectronics B...

BME328: Principles of Biomaterials Lecture 24: Biosensors Dr Andrew Daly Associate Professor in Biomedical Engineering College of Science and Engineering, BMS 1012 Biomedical Sciences Building University of Galway Email: [email protected] Bioelectronics Biosensor overview Electrochemical biosensors can be used to detect concentrations of biological compounds (e.g., sodium, lactate, glucose) These devices transduce biochemical events into electrical signals. Bioelectronics Biosensor overview a, Microneedle-based implantable electrochemical biosensors for the monitoring of analytes in interstitial fluid. The working electrode is modified with multiple functional layers, including an inner sensing layer consisting of a redox polymer and an enzyme, a mass transport-limiting layer to improve stability, and an outer biocompatible layer to prevent fouling of the sensor. b, Implantable electrochemical biosensors allow continuous glucose monitoring and in vivo detection of neurochemicals in the brain. Device integration of electrochemical biosensors. Nat Rev Bioeng 1, 346–360 (2023). https://doi.org/10.1038/s44222-023-00032-w Bioelectronics Biosensor overview a) Wearable sensors can be applied to monitor health-related or disease-related analytes in different body fluids, including tears, saliva and sweat. b) Health management can be based on continuous monitoring using wearable devices, including electrochemical biosensors, power supply and wireless communication modules. BC, biocapacitor; BFC, biofuel cell; PENG, piezoelectric nanogenerator; TENG, triboelectric nanogenerator. Device integration of electrochemical biosensors. Nat Rev Bioeng 1, 346–360 (2023). https://doi.org/10.1038/s44222-023-00032-w Bioelectronics Biosensor overview a, Portable blood glucose meter consisting of a handheld electrochemical detector and disposable test strips. The test strip c ontains a bottom electrode layer, an adhesive spacer layer and a hydrophilic cover layer. The blood sample is introduced to the reaction chamber by capillary force. b, A paper-based microfluidic electrochemical biosensor for the detection of adenosine through aptamer-based affinity sensing. In one channel, adenosine is recognized by aptamer-functionalized microbeads (blue), resulting in the release of glucose oxidase-labelled DNA to catalyze the oxidation of glucose, which leads to the conversion of [Fe(CN)6]3− to [Fe(CN)6]4−. In the other channel, the microbeads are not functionalized (purple), allowing quantification of adenosine concentration. The current signal from the discharging of the capacitor is collected by a portable digital multimeter. ox, oxidation; red, reduction. Device integration of electrochemical biosensors. Nat Rev Bioeng 1, 346–360 (2023). https://doi.org/10.1038/s44222-023-00032-w Bioelectronics Biosensor overview The electrochemical sensor can use various types of detection probes depending on the analyte that is being sensed. Detection probes are used to specifically interact with or bind to the analyte of interest. DNA: A single strand of DNA could bind to a complementary sequence Antibody: To bind to a specific antigen Device integration of electrochemical biosensors. Nat Rev Bioeng 1, 346–360 (2023). https://doi.org/10.1038/s44222-023-00032-w Bioelectronics Biosensor overview Potentiometric sensors: Measure the equilibrium E (no current flow) Amperometric sensors: Control E, and measure the subsequent I Bioelectronics Biosensor overview Amperometric Biosensors Measure the current generated by an electrochemical reaction (typically redox reactions) at a fixed applied potential. The current is directly proportional to the analyte concentration Amperometric biosensing of metabolite targets based on an enzyme electrode, including the current–time (i–t) curve and the i signal for quantification. Device integration of electrochemical biosensors. Nat Rev Bioeng 1, 346–360 (2023). https://doi.org/10.1038/s44222-023-00032-w Bioelectronics Biosensor overview Potentiometric Biosensors Potentiometric biosensors measure the potential difference between a working electrode and a reference electrode to detect the presence and concentration of a target analyte (no current flowing in this case) Bioelectronics Biosensor overview 1. Biosensor overview 2. Electrochemical principles and electrode reactions Nernst Equation Gibbs Free Energy 3. Ion-selective electrodes 4. Glucose sensors Bioelectronics Electrochemical principles and electrode reactions Biosensors leverage electrochemical reactions involving electron charge transfer at an electrode- solution interface. The electron transfer generates a current proportional to the concentration of biological compounds. *Iron ions in two oxidation states: Fe²⁺ (ferrous ion) and Fe³⁺ (ferric ion). Pethig, Ronald R.. Introductory Bioelectronics : For Engineers and Physical Scientists, John Wiley & Sons, Incorporated, 2012. Bioelectronics Electrochemical principles and electrode reactions An example system that can generate a current flow is the Daniell cell. Zinc electrode immersed into a zinc sulphate solution copper electrode immersed in a copper sulphate solution. Solutions are ionically connected via a salt bridge glass (tube containing a gel saturated with a potassium chloride solution) - prevents charge from building up in each half-cell during the electron transfer Copper ions (Cu²⁺) Zinc ions (Zn²⁺) Pethig, Ronald R.. Introductory Bioelectronics : For Engineers and Physical Scientists, John Wiley & Sons, Incorporated, 2012. Bioelectronics Electrochemical principles and electrode reactions The electrical current is generated due to a charge difference between the copper (cathode) and zinc electrodes (anode) Reduction at the cathode - Cu²⁺ gain electrons and deposit as copper atoms Oxidation at the anode - Zn atoms lose electrons becoming Zn²⁺, releasing electrons Copper ions (Cu²⁺) Zinc ions (Zn²⁺) Pethig, Ronald R.. Introductory Bioelectronics : For Engineers and Physical Scientists, John Wiley & Sons, Incorporated, 2012. Bioelectronics Electrochemical principles and electrode reactions We have a thermodynamically favourable redox (reduction-oxidation) reaction that generates an electrical current between the electrodes Reduction Oxidation 𝐶𝑢 2+ + 2𝑒 − → 𝐶𝑢 𝑍𝑛 → 𝑍𝑛2+ + 2𝑒 − Bioelectronics Electrochemical principles and electrode reactions 𝑜 Standard cell potential/voltage 𝜀𝑐𝑒𝑙𝑙 is the voltage of the electrochemical cell when the reactants and products are in their standard states (1 molar) at 25° 1𝑀 concentration of 𝑍𝑛2+ 1𝑀 concentration of 𝐶𝑢 2+ 𝑍𝑛 + 𝐶𝑢 2+ → 𝐶𝑢 + 𝑍𝑛2+ 𝜀 𝑜 𝑐𝑒𝑙𝑙 = 1.12𝑉 𝑎𝑡 25°𝐶 Reduction Oxidation 𝐶𝑢 2+ + 2𝑒 − → 𝐶𝑢 𝑍𝑛 → 𝑍𝑛2+ + 2𝑒 − Bioelectronics Electrochemical principles and electrode reactions The standard potential is the sum of the half- cell potentials that develop at the electrode solution interfaces. In the case of the Daniell cell (Zinc-copper) 𝐶𝑢 2+ + 2𝑒 − → 𝐶𝑢 𝜀 𝑜 𝑟𝑒𝑑 = + 0.34𝑉 𝑍𝑛2+ + 2𝑒 − → 𝑍𝑛 𝜀 𝑜 𝑟𝑒𝑑 = − 0.76𝑉 Hydrogen reference 𝑍𝑛 → 𝑍𝑛2+ + 2𝑒 − 𝜀 𝑜 𝑜𝑥 = + 0.76𝑉 𝑍𝑛 + 𝐶𝑢 2+ → 𝐶𝑢 + 𝑍𝑛2+ 𝜀 𝑜 𝑐𝑒𝑙𝑙 = 𝜀 𝑜 𝑟𝑒𝑑 + 𝜀 𝑜 𝑜𝑥 𝜀 𝑜 𝑐𝑒𝑙𝑙 = 0.34 + 0.76 = +1.1𝑉 Pethig, Ronald R.. Introductory Bioelectronics : For Engineers and Physical Scientists, John Wiley & Sons, Incorporated, 2012. Bioelectronics Electrochemical principles and electrode reactions Question 1: Calculate the standard cell potential 𝜀 𝑜 𝑐𝑒𝑙𝑙 for the silver-chromium electrode combination 3𝐴𝑔+ + 𝐶𝑟 → 3𝐴𝑔 + 𝐶𝑟 3+ Solution: 𝐴𝑔+ + 𝑒 − → 𝐴𝑔 𝜀 𝑜 𝑟𝑒𝑑 = + 0.80𝑉 Hydrogen reference 𝐶𝑟 → 𝐶𝑟 3+ + 3𝑒 − 𝜀 𝑜 𝑜𝑥 = + 0.74𝑉 𝜀 𝑜 𝑐𝑒𝑙𝑙 = 𝜀 𝑜 𝑟𝑒𝑑 + 𝜀 𝑜 𝑜𝑥 𝜀 𝑜 𝑐𝑒𝑙𝑙 = 0.80 + 0.74 = +1.54𝑉 Pethig, Ronald R.. Introductory Bioelectronics : For Engineers and Physical Scientists, John Wiley & Sons, Incorporated, 2012. Bioelectronics Electrochemical principles and electrode reactions Hydrogen reference Pethig, Ronald R.. Introductory Bioelectronics : For Engineers and Physical Scientists, John Wiley & Sons, Incorporated, 2012. Bioelectronics Biosensor overview 1. Biosensor overview 2. Electrochemical principles and electrode reactions Nernst Equation Gibbs Free Energy 3. Ion-selective electrodes 4. Glucose sensors Bioelectronics Nernst equation – Cell potential under non-standard conditions Previously, we calculated cell potential when the reactants and products were in standard states (1M concentrations at 25℃) The Nernst equation allows us to calculate the cell potential for non-standard states. Ideal Gas constant Temperature 𝑜 𝑅𝑇 𝜀𝑐𝑒𝑙𝑙 = 𝜀𝑐𝑒𝑙𝑙 − ln 𝑄 𝑛𝐹 Instantaneous cell potential Reaction quotient Standard cell potential Faraday’s constant No. electrons transferred Bioelectronics Nernst equation – Cell potential under non-standard conditions The Nernst equation allows us to calculate the cell potential for non-standard states. 𝑜 𝑅𝑇 𝜀𝑐𝑒𝑙𝑙 = 𝜀𝑐𝑒𝑙𝑙 − ln 𝑄 𝑛𝐹 𝑜 0.0592𝑉 𝜀𝑐𝑒𝑙𝑙 = 𝜀𝑐𝑒𝑙𝑙 − log 𝑄 at 25℃ 𝑛 The Nernst equation can be used to calculate the voltage at a specific timepoint. 𝑃𝑟𝑜𝑑𝑢𝑐𝑡 𝑐𝑜𝑛𝑐 𝑄= 𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡 𝑐𝑜𝑛𝑐 Bioelectronics Nernst equation – Cell potential under non-standard conditions 𝑍𝑛 + 𝐶𝑢 2+ → 𝐶𝑢 + 𝑍𝑛2+ Question 2: Calculate the instantaneous cell potential when the 𝐶𝑢 2+ concentration is 5.0 M 𝐶𝑢 2+ + 2𝑒 − → 𝐶𝑢 and the 𝑍𝑛2+ concentration is 10.0M. You can assume the reaction is taking place at 25℃ 𝑍𝑛 → 𝑍𝑛2+ + 2𝑒 − 𝑜 𝑅𝑇 𝜀𝑐𝑒𝑙𝑙 = 𝜀𝑐𝑒𝑙𝑙 − ln 𝑄 𝑛𝐹 𝑜 0.0592𝑉 𝜀𝑐𝑒𝑙𝑙 = 𝜀𝑐𝑒𝑙𝑙 − log 𝑄 at 25℃ 𝑛 5.0 M 10.0M Bioelectronics Nernst equation – Cell potential under non-standard conditions 𝑍𝑛 + 𝐶𝑢 2+ → 𝐶𝑢 + 𝑍𝑛2+ Question 2: Calculate the instantaneous cell potential when the 𝐶𝑢 2+ concentration is 5.0 M and the 𝑍𝑛2+ concentration is 10.0M. 𝐶𝑢 2+ + 2𝑒 − → 𝐶𝑢 𝑍𝑛 → 𝑍𝑛2+ + 2𝑒 − Solution: 0.0592𝑉 𝑍𝑛2+ 𝜀𝑐𝑒𝑙𝑙 = 𝑜 𝜀𝑐𝑒𝑙𝑙 − log 2+ at 25℃ 2 𝐶𝑢 0.0592𝑉 10 𝜀𝑐𝑒𝑙𝑙 = 1.1𝑉 − log = 1.0793 𝑉 2 5 The cell potential will continue to decrease as the 𝑍𝑛2+ concentration increases and the 𝐶𝑢 2+ decreases during the reaction. 5.0 M 10.0M Bioelectronics Biosensor overview 1. Biosensor overview 2. Electrochemical principles and electrode reactions Nernst Equation Gibbs Free Energy 3. Ion-selective electrodes 4. Glucose sensors Bioelectronics Gibbs free energy in electrode reactions The Gibbs free energy ∆𝐺 of an 𝑅𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠 ⇋ 𝑃𝑟𝑜𝑑𝑢𝑐𝑡𝑠 electrochemical cell is related to the potential 𝑅⇋𝑃 for the redox reaction. difference in free energy between the ∆𝐺 = −𝑛𝐹𝜀 reactants and the products or number of electrons transferred in the redox reaction 𝑛 = number of moles electrons transferred per mole reactant & product 𝐹 = Faraday's constant 96,845 C per mole Gibbs free energy is a thermodynamic potential of electrons that can be used to predict the spontaneity of a 𝜀= potential/voltage of the redox reaction process. Negative ΔG = spontaneous reaction Positive ΔG = non-spontaneous reaction. Bioelectronics Gibbs free energy in electrode reactions If the reactants and products are both in their standard state, then ∆𝐺 0 = −𝑛𝐹𝜀 0 𝑛 = number of electrons transferred 𝐶 𝐹 = Faraday's constant 96,845 𝑚𝑜𝑙.𝑒 − 𝜀= potential/voltage of the redox reaction Reduction Oxidation 𝐶𝑢 2+ + 2𝑒 − → 𝐶𝑢 𝑍𝑛 → 𝑍𝑛2+ + 2𝑒 − 1𝑀 𝐶𝑢 2+ 1𝑀 𝑍𝑛2+ Bioelectronics Gibbs free energy in electrode reactions The Gibbs free energy is related to the equilibrium constant (K) for the reaction 𝑅𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠 ⇋ 𝑃𝑟𝑜𝑑𝑢𝑐𝑡𝑠 ∆𝐺 0 = −𝑅𝑇 ln 𝐾 𝑅 = Ideal gas constant (J. mol-1. K-1) 𝑇 = Temperature (K) 𝐾= Equilibrium constant or ratio of products to reactants at equilibrium K is the equilibrium constant of a reversible chemical reaction. ∆𝑮𝟎 is negative: This means K>1, and the reaction favors the formation of products (spontaneous) K is the ratio of products to reactants at equilibrium. ∆𝑮𝟎 is zero: This means K=1, and the reaction is at equilibrium (neither reactants/products are favored) ∆𝑮𝟎 is positive: This means K80), as they are less likely to need revision surgeries - removing cemented implants for a revision surgery is difficult. Joint replacements The interface challenge – Bone cements PMMA-based bone cements are double- phase systems, with a polymer powder and monomer liquid Powder phase - The solid powder phase contains spherical PMMA beads with barium sulfate (radiographic contrast agent). Benzoyl peroxide (BPO) is included in the powder phase as a polymerisation catalyst. The liquid phase contains MMA monomer, along with N,N-dimethyl-p-toluidene (DmpT; which increases polymerisation rate) Joint replacements The interface challenge – Bone cements When the two phases are mixed the initiator (BPO) reacts with the accelerator (DmpT) to create free radicals (benzoyl radical and benzoyl anion) This starts a polymerisation reaction of MMA into PMMA by adding to the polymerisable double-bond of the monomer molecule. (a) Schematic showing decomposition of BPO leaving a benzoyl radical and a benzoyl anion. (b) How benzoyl radicals initiate polymerization of MMA. (c) Formation of polymer chain.84 https://onlinelibrary.wiley.com/doi/10.1002/pi.6136 https://books.google.ie/books?hl=en&lr=&id=R9CPDwAAQBAJ&oi=fnd&pg=PA337&ots=VJoxyegF VW&sig=vP0S6uJ4-tbOrhFMpOKailh-znc&redir_esc=y#v=onepage&q&f=false Joint replacements The interface challenge – Bone cements The polymerization reaction is exothermic which can affect local cell viability within the marrow cavity. Joint replacements The interface challenge – Bone cements PMMA has a high stiffness with a Young's modulus of ~3 GPa (1/10 of cortical bone and much softer than typical stem materials such as titanium or cobalt chromium (100–200 GPa). The fracture toughness of PMMA is however relatively low (1.0 MPa m1/2) and PMMA is considered a brittle material. Joint replacements The interface challenge The tensile strength of PMMA is ~65MPa (without any defects or flaws). The fracture toughness of PMMA is 1.0 MPa m1/2 𝑆𝑖𝑛𝑔𝑙𝑒 𝑒𝑑𝑔𝑒 𝑐𝑟𝑎𝑐𝑘, 𝑡ℎ𝑖𝑛 𝑝𝑙𝑎𝑡𝑒 Tensile stress will cause fracture in PMMA cement when 𝜎 under tension—localised stress concentrations at cracks in the bone cement. We can calculate how much this will reduce failure stress. Assume the PMMA is a simple thin plate with a maximum defect size of 200µm on one side of the material specimen. We can use the single-edge notch plate model (Y=1.12). 𝐾𝑐 = 𝑌𝜎𝑐 𝜋𝑎 𝜎 𝑌 = 1.12 Joint replacements The interface challenge – Class example 𝐾𝑐 𝐾𝑐 = 𝑌𝜎𝑐 𝜋𝑎 𝜎𝑐 = 𝑌 𝜋𝑎 1 MPa m1/2 𝜎𝑐 = = = 35𝑀𝑃𝑎 1.12 𝜋0.0002 m 1/2 Even though the tensile strength is 65MPa, failure will occur at 35MPa when a defect of 200µm emerges in the material Joint replacements Materials for joint prosthesis – Biological fixation Class question: Alternative fixation strategies to bone cement? Porous hip stems can also be used to promote integration with the surrounding bone. If the pores are of sufficient size, bone cells can invade the material and begin synthesising new bone material. Joint replacements Materials for joint prosthesis – Biological fixation A representative set of BSE micrographs showing the ingrowth and interdigitation of new bone tissue into the porous, coated region at 6 months postsurgery (B) compared with time 0 (at surgery), when the implant was placed in close apposition with the host bone (A). The image shows porous coating (white), bone (gray), and marrow cellular components (black) BME328: Principles of Biomaterials Lecture 7: Cardiovascular devices II – Stents Dr Andrew Daly Associate Professor in Biomedical Engineering College of Science and Engineering, BMS 1012 Biomedical Sciences Building University of Galway Email: [email protected] Module outline Lectures 1. Introduction 2. Host response to biomaterials III (Soft and hard tissue implants) 3. Host response to biomaterials IV (Blood- Section 1 - Host response to contacting implants) biomaterials 4. Biomaterial infections 5. Sterilisation methods 6. Cardiovascular devices I – Heart Valves 7. Cardiovascular devices II – Stents Section 2 - Biomaterial products (Failures and case studies in 8. Orthopaedic devices I – Joint replacements optimizing design) 9. Orthopaedic devices II – Joint replacements 10. 3D printing overview 11. Lab preparation lecture Module outline Lectures 12. Contact lens biomaterials Section 2 - Biomaterial products 13. Adhesive biomaterials (Failures and case studies in 14. Neural implants optimizing design) 15. Advanced Drug Delivery I Section 3 – Drug Delivery 16. Advanced Drug Delivery II 17. Advanced chemical characterization techniques Section 4 – Characterization 18. Advanced biological characterisation techniques technologies 19. Regulatory affairs for biomaterials I Section 5 – Regulatory affairs 20. Regulatory affairs for biomaterials II 21. 3D Bioprinting 22. Electrospinning Section 6 – Future directions in 23. Frontiers in biomaterials I biomaterials 24. Frontiers in biomaterials II 25. Exam overview Module outline Learning outcomes 1. Describe the structure and composition of advanced polymer, ceramic, and metal biomaterials. 2. Evaluate and select an appropriate biomaterial for a given implant design. 3. Assess optimal sterilization methods for a biomaterial implant to limit implant associated infections. 4. Develop an understanding of biocompatibility and how it can be optimised through biomaterial design 5. Design an optimal manufacturing method for a given biomaterial implant. 6. Perform experiments to characterize the physical and biological properties of biomaterials. 7. Leverage emerging technologies/materials to design medical implants. Cardiovascular devices – Stents Lecture outline 1. Atherosclerosis and vascular biology 2. Biomaterial stents i. Bare Metal Stents ii. Drug Eluting stents iii. Biodegradable stents Cardiovascular devices – Stents Atherosclerosis Caused by fatty deposits inside the arterial walls narrowing the arteries and restricting blood flow Eventually leads to ischemia (lack of oxygen and tissue death) Heart – myocardial infarction Brain – stroke Lower limb ischemia Cardiovascular devices – Stents Atherosclerosis Treatment options for atherosclerosis include Bypass with a tissue graft Percutaneous transluminal coronary angioplasty (PTCA) Balloon angioplasty Limitation is that 40% of patients develop restenosis after 4-6 months following balloon angioplasty Cardiovascular devices – Stents Atherosclerosis Endothelial cells ❑ Form inner lining of blood vessels ❑ Tightly packed to prevent blood leaking into surrounding tissue ❑ Regulates exchanges between the bloodstream and the surrounding tissues ❑ Anticoagulation properties Cardiovascular devices – Stents Atherosclerosis Vascular smooth muscle cells ❑ Primary function is to maintain vascular homeostasis through active contraction and relaxation. ❑ Excessive proliferation of vascular smooth muscle cells contributes to the progression of pathological conditions such as restenosis Cardiovascular devices – Stents Atherosclerosis Vascular smooth muscle cells ❑ Vascular smooth muscle cells use multiple sensing mechanisms to detect the mechanical stimulus resulting from pulsatile stretch ❑ Transduce it into intracellular signals that lead to modulations of gene expression and cellular functions, e.g., proliferation, apoptosis, migration, and remodelling. Mechanical stretching for 48 h induced a significant increase in proliferating VSMCs Cardiovascular devices – Stents Atherosclerosis – restenosis The three major pathogenic mechanisms that underlie restenosis are: Early elastic return (recoil) Vascular remodelling Neointimal hyperplasia Cardiovascular devices – Stents Lecture outline 1. Atherosclerosis and vascular biology 2. Biomaterial stents i. Bare Metal Stents ii. Drug Eluting stents iii. Biodegradable stents Cardiovascular devices – Stents Biomaterial stents Stent can be delivered to keep the vessel lumen open. Stenting procedure includes - Inserting catheter/stent - Positioning of stent - Balloon inflation (expanding stent) - Balloon deflation - Removing catheter (leaving stent) Cardiovascular devices – Stents Biomaterial stents WALLSTENT® (Schneider AG), a self- expanding, stainless-steel wire-mesh structure, was the first coronary stent implanted in a human coronary artery by Sigwart et al. in 1986 WALLSTENT® (Schneider AG) The first stent was FDA-approved in 1994 – produced by J&J, based on the invention of Dr Julio Palmaz at the University of Texas Health centre Used to treat occluded coronary arteries and was based on an expandable metal mesh formed using stainless steel, was Palmaz-Schatz® (Johnson & Johnson) stent balloon expandable Cardiovascular devices – Stents Biomaterial stents Many other stents were subsequently developed in early 1990s and included: Flexstent® (Cook), Wiktor® (Medtronic), Micro® (Applied Vascular Engineering), Cordis® (Cordis) and Multi-link® (Advanced Cardiovascular Systems). There are three main categories of stents Bare metal Drug eluting stents Biodegradable/bioresorbable stents Cardiovascular devices – Stents Lecture outline 1. Atherosclerosis and vascular biology 2. Biomaterial stents i. Bare Metal Stents ii. Drug Eluting stents iii. Biodegradable stents Cardiovascular devices – Stents Biomaterial stents Class Question I: What material properties should be considered when designing balloon- expandable metal stents? Ideal stent properties Crimping ability Expandability (controlled expansion to conform to vessel wall) Suitable mechanical strength to prevent collapse under arterial wall forces (radial hoop stress) Flexibility - travelling into small vessel diameters and tortuous anatomies Fatigue resistant – pulsatile environment (around 40x10^6 beats/y) Radiopacity for (x-ray)/magnetic resonance imaging (MRI) imaging – allow clinicians to see stent during minimally invasive surgery Thromboresistant – prevent platelet adhesions and clot formation Cardiovascular devices – Stents Bare metal stents Mechanical property considerations for metal stents. 1. Elastic modulus/Yield strength/tensile strength – should be high enough to prevent vessel recoil and withstand vessel hoop stresses. 2. However, stiffer stents materials will make it harder for balloon expansion. To balance these design considerations, you ideally want a metal stent with the following properties 1. Low yield strength to enable crimping and collapse within the ballon catheter 2. Sufficient stiffness at relevant strains to withstand forces in the vessel once delivered 3. Small strut thickness (good for crimping, and minimally invasive delivery) Cardiovascular devices – Stents Bare metal stents Cardiovascular devices – Stents Bare metal stents - Nitinol Nitinol stents are now widely used due to their ❑ Superelasticity ❑ Shape memory properties ❑ Biocompatibility ❑ Fatigue resistance Cardiovascular devices – Stents Bare metal stents - Nitinol Superelasticity: The property that certain alloys have that allows them to return to their original shape after a substantial deformation Cardiovascular devices – Stents Bare metal stents - Nitinol Superelastic properties enables the design of self-expanding stent delivery mechanisms. Advantages over ballon based delivery Cardiovascular devices – Stents Bare metal stents - Nitinol Nickel 50 -56 wt. % - For medical grade applications (Ni ≈55%) Balance is Titanium Tensile properties of Ti and Ni individually Titanium (ASTM grade 4): Nickel commercially pure (99.6% Ni) E = 104GPa E = 170 - 200GPa Tensile strength 550MPa Tensile strength 380 - 450MPa Yield strength 485MPa Yield strength 100-200MPa Density 4.5 g.cm-3 Density 8.9 g.cm-3 Cardiovascular devices – Stents Bare metal stents - Nitinol The superelastic and shape-memory properties of nitinol are due to its unique atomic arrangements. Elastoplastic materials Normally metals deform due to permanent rearrangements of grains Ease of deformation is related to atomic structure - planes of atoms/crystals must slip over each other Sliding and gliding along grain boundaries is the second level resistance to deformation Cardiovascular devices – Stents Bare metal stents - Nitinol Nitinol displays very different behaviour under loading, which gives rise to superelasticity Even though the atoms move under loading, the overall atomic organisation is mostly maintained no permanent slippage of planes of atoms Large amounts of deformation can be induced in a material with these conformational changes before permanent plastic deformation occurs https://www.youtube.com/watch?v=wI-qAxKJoSU&ab_channel=engineerguy Cardiovascular devices – Stents Bare metal stents - Nitinol Shape memory: The ability of certain metallic alloys to be deformed at a low temperature and then return to their original shape upon heating Cardiovascular devices – Stents Bare metal stents - Nitinol NiTi superelasticity is temperature dependent https://www.youtube.com/watch?v=wI-qAxKJoSU&ab_channel=engineerguy Cardiovascular devices – Stents Bare metal stents Limitations of bare metal stents - Balloon angioplasty and stent placement procedures can induce endothelial and smooth muscle cell damage, which leads to restenosis. - Restenosis occurs in 30% of patients after 6 months. - Mechanical properties and architecture of stent have been optimized to reduce restenosis rates but this doesn’t fully alleviate the problem Cardiovascular devices – Stents Lecture outline 1. Atherosclerosis and vascular biology 2. Biomaterial stents i. Bare Metal Stents ii. Drug Eluting stents iii. Biodegradable stents Cardiovascular devices – Stents Drug eluting stents Stents can be loaded with therapeutic agents (drugs) that prevent restenosis. Drug targets include inhibiting the proliferation of vascular smooth muscle cells (while not affecting the restoration of endothelial cells) Can also include antithrombotic agents to prevent clotting following implantation. Target is to deliver these drugs within first 30 days (initial biological response) Cardiovascular devices – Stents Drug eluting stents Drugs for treatment of restenosis Heparin - reduce thrombosis Sirolimus & Paclitaxel – reduce VSMC proliferation The drug can be loaded directly onto the metal surface, but it is more common to coat the metal using a polymer with the drug Cardiovascular devices – Stents Drug eluting stents Siromilus-eluting Cypher stent (Cordis) Stainless steel 316L coated with PEVA (polyethylene vinyl acetate) and PBMA (Poly(butyl methacrylate) - contains 140 microgram/cm2 sirolimus “A key trial showing the efficacy of the Cypher stent found that up to five years after receiving the stent, the risk of restenosis of the artery is reduced by 60 to 70 percent compared to an uncoated stent” https://www.dicardiology.com/content/cypher-stent-10-year-follow-results- announced#:~:text=A%20key%20trial%20showing%20the,compared%20to%20an%2 0uncoated%20stent. Cardiovascular devices – Stents Lecture outline 1. Atherosclerosis and vascular biology 2. Biomaterial stents i. Bare Metal Stents ii. Drug Eluting stents iii. Biodegradable stents Cardiovascular devices – Stents Biodegradable stents Biodegradable polymer stents are being explored to replace metal stents Do not require a second surgery for removal Metal stent problems 1. CT scans can't be performed 2. Prevention of vessel remodeling and adaption Biodegradable stent will reduce time of interference with vascular wall (avoiding endothelial dysfunction or inflammation long-term) Cardiovascular devices – Stents Biodegradable stents Design considerations for biodegradable stents Materials that are being explored for biodegradable stents Biodegradation Sufficient mechanical strength for 3-6 months Metals - Pure Iron (Fe): >99.5 wt% Fe Full degradation within 12-24 months - Magnesium alloys - Zinc Mechanical properties Comparable to regular metals used for stents Polymers - poly(glycolic acid) - PGA Resistance to cyclic fatigue forces during - poly(glycolic acid) - PGA degradation is challenging - poly(L-lactic acid) - PLLA - poly(DL-lactic acid) - PDLLA - poly(ε-caprolactone) - PCL Biocompatibility - polydioxanone - PDS Non-toxic and non-inflammatory degradation - poly(DL-lactic-co-glycolic acid) - products PDLGA No release of large particles Cardiovascular devices – Stents Biodegradable stents – Abbot absorb Abbot absorb - made from poly(L- lactide) (PLLA), and covered with a layer of poly(D,L-lactide) (PDLLA) containing everolimus Full resorbs in 3 years, provides mechanical support for 6–12 months strut thickness was 158 μm, crossing profile 1.4 mm, stent-to-artery coverage 25% Cardiovascular devices – Stents Biodegradable stents – degradation rates PLLA undergoes hydrolysis into lactic acid, which is further metabolized into carbon dioxide and water within the body Degradation varies with degree of crystallinity and MW, but formulations display 20% mass loss over a year, fully resorbs within 3 years Cardiovascular devices – Stents Biodegradable stents – degradation mechanisms PLLA has non-uniform degradation profiles within the material Amorphous regions degraded faster than crystalline regions Cardiovascular devices – Stents Biodegradable stents – Crystallinity and degradation The crystallinity of manufactured PLLA stents was found to be lower on internal core regions of the polymer. Which may influence structural degradation mechanisms. Cardiovascular devices – Stents Biodegradable stents – degradation mechanisms Surface erosion PLLA predominantly undergoes bulk erosion. Water penetrates into the bulk of the material, causing hydrolysis of ester bonds throughout its structure. Bulk erosion Leads to decreases in molecular weight and mechanical properties occurring throughout the entire volume. Cardiovascular devices – Stents Biodegradable stents – predicting degradation A new PLLA stent formulation is implanted in a patient's artery. The initial mass of the stent is 50 milligrams. The degradation of the PLLA formulation follows an exponential decay model, with a half-life of approx. 3 years. i. Calculate the mass of the stent remaining after 5 years. ii. Determine how long it will take for this PLLA stent formulation to degrade to 20% of its original mass. Part i Part ii Half-life formula 20% 𝑜𝑓 𝑚𝑎𝑠𝑠 𝑖𝑠 10𝑔𝑟𝑎𝑚𝑠 1 5 𝑡 𝑀 5 = 50( ) 1.5 = 5𝑚𝑔 1 𝑡 1 1 2 𝑀 𝑡 = 𝑀0 ( ) 𝑡 2 10 = 50( ) 1.5 2 2 𝑡 ≈ 3.48 𝑦𝑒𝑎𝑟𝑠 Cardiovascular devices – Stents Biodegradable stents – predicting degradation Half-life degradation curves can be a useful approximation for modelling the degradation of polymers. However, they don’t account for changes in crystallinity, molecular weight distribution, and surface area Actual degradation kinetics may deviate from a simple exponential decay. Surface erosion 𝑡 1 1 𝑀 𝑡 = 𝑀0 ( ) 𝑡2 2 Bulk erosion Cardiovascular devices – Stents Biodegradable stents – mechanics and degradation Key question for stents, for how long is it mechanically functional? Cardiovascular devices – Stents Biodegradable stents – mechanics and degradation Key question for stents, for how long is it mechanically functional? Cardiovascular devices – Stents Biodegradable stents failures Mar 18, 2017 “The FDA has issued a safety alert for the Absorb GT1 Bioresorbable PLLA Vascular Scaffold (BVS) by Abbott Vascular due to an increased rate of major adverse cardiac events observed in patients receiving the device. The alert comes after the FDA’s initial review of two-year data from the ABSORB III trial that showed an 11 percent rate of major cardiac events (cardiac death, myocardial infarction or an additional procedure to re-open the treated heart vessel) in patients treated with BVS at two years, compared with 7.9 percent in patients treated with the already approved metallic XIENCE drug-eluting stent, also by Abbott Vascular” https://www.acc.org/latest-in-cardiology/articles/2017/03/18/16/32/fda- issues-safety-alert-for-absorb-gt1-bioresorbable-vascular-scaffold Sep 08, 2017 “Abbott Vascular is calling a halt to sales of the Absorb bioresorbable vascular scaffold as of September 14, 2017, attributing the decision to “low commercial sales.” https://www.tctmd.com/news/no-more-absorb-bvs-abbott-puts-stop-sales Cardiovascular devices – Stents Biodegradable stents failures “In human patients the stents appeared stable for the first year, but then problems began to arise.” “After three years, over 11 percent of patients had experienced a heart attack, including fatal heart attacks, or had to go through another medical intervention. That is higher than the 8% rate seen in patients with metal stents.” What was happening? https://news.mit.edu/2018/study-reveals-why-polymer-stents-failed-0226 Cardiovascular devices – Stents Biodegradable stents failures https://news.mit.edu/2018/study-reveals-why-polymer-stents-failed-0226 Cardiovascular devices – Stents Biodegradable stents failures 1. Because of the nonuniform degradation the core of the structs will degrade faster 2. This can lead to large deformation and collapse of the stent when it’s still under radial compression. 3. Displaced stent material in the vascular lumen will lead to flow disruption and vessel clotting https://news.mit.edu/2018/study-reveals-why-polymer-stents-failed-0226 Cardiovascular devices – Stents Biodegradable stents failures 1. Because of the nonuniform degradation the core of the structs will degrade faster 2. This can lead to large deformation and collapse of the stent when it’s still under radial compression. 3. Displaced stent material in the vascular lumen will lead to flow disruption and vessel clotting https://news.mit.edu/2018/study-reveals-why-polymer-stents-failed-0226 BME328: Principles of Biomaterials Lecture 6: Cardiovascular devices - Heart valves Dr Andrew Daly Associate Professor in Biomedical Engineering College of Science and Engineering, BMS 1012 Biomedical Sciences Building University of Galway Email: [email protected] Module outline Lectures 1. Introduction 2. Host response to biomaterials III (Soft and hard tissue implants) 3. Host response to biomaterials IV (Blood- Section 1 - Host response to contacting implants) biomaterials 4. Biomaterial infections 5. Sterilisation methods 6. Cardiovascular devices I – Heart Valves 7. Cardiovascular devices II – Stents Section 2 - Biomaterial products (Failures and case studies in 8. Orthopaedic devices I – Joint replacements optimizing design) 9. Orthopaedic devices II – Joint replacements 10. 3D printing overview 11. Lab preparation lecture Module outline Lectures 12. Contact lens biomaterials Section 2 - Biomaterial products 13. Adhesive biomaterials (Failures and case studies in 14. Neural implants optimizing design) 15. Advanced Drug Delivery I Section 3 – Drug Delivery 16. Advanced Drug Delivery II 17. Advanced chemical characterization techniques Section 4 – Characterization 18. Advanced biological characterisation techniques technologies 19. Regulatory affairs for biomaterials I Section 5 – Regulatory affairs 20. Regulatory affairs for biomaterials II 21. 3D Bioprinting 22. Electrospinning Section 6 – Future directions in 23. Frontiers in biomaterials I biomaterials 24. Frontiers in biomaterials II 25. Exam overview Module outline Learning outcomes 1. Describe the structure and composition of advanced polymer, ceramic, and metal biomaterials. 2. Evaluate and select an appropriate biomaterial for a given implant design. 3. Assess optimal sterilization methods for a biomaterial implant to limit implant associated infections. 4. Develop an understanding of biocompatibility, and how it can be optimized through biomaterial design 5. Design an optimal manufacturing method for a given biomaterial implant. 6. Perform experiments to characterize the physical and biological properties of biomaterials. 7. Leverage emerging technologies/materials to design medical implants. Heart valve replacements Introduction Heart valves ensure unidirectional forward blood flow through the heart. Open and close with each cardiac cycle (once per second - 40 million times per year - 3 billion times in a 75-year lifetime) Disorders of heart valves can cause stenosis (i.e., obstruction to flow) or regurgitation (i.e., reverse flow across the valve) Heart valve replacements Introduction Heart valve replacements Lecture outline 1. Heart valve disease 2. Artificial heart valve design 3. Heart valve complications 4. Class problems Heart valve replacements Heart valve disease Disorders of heart valves can cause stenosis (i.e., obstruction to flow) or regurgitation (i.e., reverse flow across the valve) Calcific aortic stenosis can cause Infective endocarditis (infection of a stiffening of the valve, which heart valve) can lead to chronic prevents proper closer inflammation that also affects valve function (A) Severe degenerative calcification of a previously anatomically normal tricuspid aortic valve, the predominant cause of aortic stenosis, and the leading form of valvular heart disease. Heart valve replacements Heart valve disease Calcific aortic stenosis Age-related calcification of the cusps of a valve where calcific nodules in the valve cusps prevent full opening This can lead to pressure overload of the left ventricle (induces hypertrophy, thickening of the heart muscle) Heart valve replacements Heart valve disease Aortic regurgitation, caused by dilation of the aortic root, is another cause of valve dysfunction Loss of elastin Dilation of the aortic root prevents effective closure of the cusps, allowing backflow Dilatation of the aortic root is due to medial degeneration and destruction of the elastic and collagen fibres Transverse sections of ascending aorta specimens from organ donors (nondilated) and patients undergoing aneurysm repair (aneurysm) were stained with H&E, VVG, or Movat and graded. Heart valve replacements Heart valve disease The mortality of aortic stenosis is ~50% at 2–3 years following the onset of symptoms (without aortic valve replacement) Survival curves for patients with untreated aortic valve stenosis (natural history of valve disease) and aortic valve stenosis corrected by valve replacement, as compared with an age-matched control population without a history of aortic valve stenosis. The numbers presented in this figure for survival following valve replacement nearly four decades ago remain accurate today. This reflects the fact that improvements in valve substitutes and patient management have been balanced by a progressive trend toward operations on older and sicker patients with associated medical illnesses. (Reproduced by permission from Roberts, L. et al. (1976). Long- term survival following aortic valve replacement. Am. Heart J. 91: 311–317.) Heart valve replacements Lecture outline 1. Heart valve disease 2. Heart valve prosthesis 3. Heart valve complications 4. Class problems  Heart valve replacements Heart valve prosthesis Class Question I: What material properties should be considered when designing an artificial heart valve prosthesis? Heart valve prosthesis should ideally be 1. mechanically functional o durable (repeated cycles) o able to rapidly open and close 2. non-thrombogenic 3. infection resistant 4. able to integrate into surrounding tissue (immune response) Heart valve replacements Heart valve prosthesis The earliest heart valve design was the Hufnagel ball valve - 1940’s Designed for implantation into the descending thoracic aorta using proximal and distal fixation rings ‘The interior ball was made of hollow methylmethacrylate. The ball made so much noise the wearer could be heard walking down the hall. Hufnagel later replaced the noisy ball with ones coated with silicone. They proved to be considerably quieter’ Heart valve replacements Heart valve prosthesis Heart valve replacements Heart valve prosthesis Heart valves designs can be categorized as I. Mechanical valves (plastic, metal components) II. Biological tissue valves (or bioprosthesis) - porcine tissue, decellularized tissues Heart valve replacements Heart valve prosthesis – Mechanical valves Mechanical prosthetic heart valves generally use an occluder housed in a metal cage Biomaterials used for metal cage Bjork-Shiley, Medtronic-Hall, and OmniScience valves (cobalt-chrome or titanium alloy) St. Jude Medical, CarboMedics CPHV, and the Medical Carbon Research Institute or On-X prostheses (two carbon hemidisks in a carbon housing) Occluder biomaterials The majority of valve occluders are fabricated from pyrolytic carbon. Heart valve replacements Heart valve prosthesis – Mechanical valves Pyrolytic carbon is a material with similar properties to graphite (remember DLC coatings from previous lectures) graphite pyrolytic carbon. Carbon atoms covalently bonded together in hexagonal arrays – arrays are held together by weaker interlayer binding In pyrolytic carbon the layers are more disordered (distortions within layers) which improves durability. Pyrolytic carbon is formed from the thermal decomposition of hydrocarbons (propane, propylene - in the absence of oxygen) which results in a “polymerization” of carbon atoms Fig. 1. The microstructures of the carbon fibers (a–b) as-received carbon fiber, (c–d) PyC coated carbon fiber. https://www.sciencedirect.com/science/article/pii/B9780080877808000231 The term “pyrolytic” is derived from “pyrolysis,” which is thermal decomposition. Heart valve replacements Heart valve prosthesis – Mechanical valves Pyrolytic carbon has high strength, fatigue and wear resistance, and relatively high thromboresistence (compared to other synthetic biomaterials) As a result, it is now widely used as a coating material for mechanical heart valves Although pyrolytic carbon is highly thromboresistant, patients still need lifelong anticoagulation drugs to prevent thrombosis. CarboMedics (29 mm) mitral valve prosthesis, with large shallow thrombus (arrow) entirely covering inflow aspect of one flap, completely immobilizing it, but without obstructive fibrous ingrowth (pannus). Heart valve replacements Heart valve prosthesis – Biological tissue valves Biological tissue valves (or bioprosthesis) are becoming an increasingly popular alternative to mechanical valves. Formed using animal derived tissue – bovine pericardium treated with glutaraldehyde is the most common source material Often combined with a polymer or metal to Medtronics Mosaic™ bioprosthesis provide structure stability and geometric fidelity. Heart valve replacements Heart valve prosthesis – Biological tissue valves The pericardium is a thin fibrous tissue that surrounds the heart. Not crucial to cardiac function, but it reduces friction between the heart and surrounding tissues. Bovine pericardium Glutaraldehyde fixation 1. Preserve the tissue from degradation 2. Kills cells present within the tissue to reduce immunological reactivity upon implantation 3. Enhances mechanical properties Heart valve replacements Heart valve prosthesis – Biological tissue valves Glutaraldehyde fixations acts by crosslinking amine groups together. Increases mechanical properties of the tissue. BG - Before glutaraldehyde fixations AG - After glutaraldehyde fixations Heart valve replacements Heart valve prosthesis – Biological tissue valves The main advantage of tissue valves is their relative non- thrombogenicity It has not been possible to engineer biomaterial surfaces that match the non-thrombogenicity of the pericardium Immunosuppression is generally not required as the cells Patients with tissue valves usually are no longer viable, and glutaraldehyde crosslinking can do not require anticoagulant mask immune recognition of animal proteins. Major therapy advantage compared to mechanical valves However, glutaraldehyde crosslinking prevents host cell invasion and remodelling of the heart valve with the surrounding tissue Heart valve replacements Heart valve prosthesis – Biological tissue valves Several variations bioprosthesis valves have evolved over the years. Heart valve replacements Heart valve prosthesis Outcome following cardiac valve replacement. (A) Survival curves for patients with untreated aortic valve stenosis (natural history of valve disease) and aortic valve stenosis corrected by valve replacement, as compared with an age-matched control population without a history of aortic valve stenosis. The numbers presented in this figure for survival following valve replacement nearly four decades ago remain accurate today. This reflects the fact that improvements in valve substitutes and patient management have been balanced by a progressive trend toward operations on older and sicker patients with associated medical illnesses. (Reproduced by permission from Roberts, L. et al. (1976). Long-term survival following aortic valve replacement. Am. Heart J. 91: 311–317.) (B) Frequency of valve related complications for mechanical and tissue valves following mitral valve replacement (MVR) and aortic valve replacement (AVR). (Reproduced by permission from Hammermeister, K. et al. (2000). Outcomes 15 years after valve replacement with a mechanical versus a bioprosthetic valve: Final report of the Veterans Affairs Randomized Trial. J. Am. Coll. Cardiol. 36: 1152–1158.) ❑ Thrombogenic complications more common early (mechanical valves) ❑ Mechanical failures more common later (Bioprosthetic) Heart valve replacements Heart valve prosthesis Trend in bioprosthetic or mechanical valve usage for aortic valve replacement in patients aged 18 to 50 years. https://www.sciencedirect.com/s cience/article/pii/S00225223173 18962 Heart valve replacements Heart valve prosthesis https://europe.medtronic.com/xd-en/healthcare- professionals/products/cardiovascular/heart-valves-surgical/hancock-ii- hancock-ii-ultra-bioprostheses.html Types of all implanted prostheses from 2006–2016 https://jtd.amegroups.com/article/view/29312/pdf Heart valve replacements Heart valve prosthesis “There was no significant survival difference in 15- year follow-up in patients who underwent bioprosthetic aortic valve replacement compared with those who underwent mechanical aortic valve replacement in the propensity score-matched cohort“ Figure 1. Kaplan-Meier survival curves in patients aged 18 to 50 years after aortic valve replacement according to the prosthesis type in propensity score-matched patients. CI, Confidence interval; HR, hazard ratio. https://www.sciencedirect.com/science/article/pii/S0022522317318962 Heart valve replacements Heart valve prosthesis “In patients aged 18 to 50 years undergoing aortic valve replacement in California and New York State, there was no significant difference in survival at 15 years with bioprosthetic versus mechanical valve replacement. The long-term risks of stroke and major bleeding events were greater with mechanical compared with bioprosthetic valves, whereas mechanical valve replacement had improved freedom from reoperation compared with bioprostheses. These findings suggest that in patients aged 18 to 50 years, bioprosthetic aortic valves represent a reasonable alternative to mechanical valve replacement.” https://www.sciencedirect.com/science/article/pii/S0022522317318962 Heart valve replacements Lecture outline 1. Heart valve disease 2. Artificial heart valve design 3. Heart valve complications 4. Class problems Heart valve replacements Heart valve complications There are three main categories of valve- related complications 1. Thrombosis and thromboembolism 2. Infection 3. Structural or mechanical dysfunction Complications of prosthetic heart valves. (A) Thrombosis on a Bjork-Shiley tilting disk aortic valve prosthesis, localized to outflow strut near minor orifice, a point of flow stasis. (B) Thrombosis of Hancock porcine bioprosthetic valve. (C) Thromboembolic infarct of the spleen (light area at left) secondary to embolus from valve prosthesis. (D) Prosthetic valve endocarditis with large ring abscess (arrow), viewed from the ventricular aspect of an aortic Bjork-Shiley tilting disk aortic valve. (E) Strut fracture of Bjork-Shiley valve, showing valve housing with single remaining strut and adjacent disk. Sites of prior attachment of missing fractured strut designated by arrows. (F) Structural valve dysfunction (manifest as calcific degeneration with tear) of porcine valve. ((D): Reproduced by permission from Schoen, F. J. (1987). Cardiac valve prostheses: Pathological and bioengineering considerations. J. Card. Surg. 2: 65; (A) and (E): Reproduced by permission from Schoen, F. J., Levy, R. J., Piehler, H. R. (1992). Pathological considerations in replacement cardiac valves. Cardiovasc. Pathol. 1: 29.) Heart valve replacements Heart valve complications – Mechanical valves The Björk–Shiley valve was a mechanical artificial heart valve developed in the 1970s based on a single cusp tilted design Following implantation into patients, fractures at the struts were observed. Failure was due to disk closure occurring at an unanticipated velocity/force, which caused crack propagation at the metal strut (fatigue fracture) Heart valve replacements Heart valve complications – Mechanical valves Over 80,000 valves of this model were implanted, and ~600 fractured with patients dying in two-thirds of those cases In 1992, Pfizer (Shiley's parent company) and patients with defective valves agreed to a settlement of $215 million https://www.nytimes.com/1992/01/25/us/lawsuit-settled- over-heart-valve-implicated-in-about-300-deaths.html Heart valve replacements Heart valve complications – Bioprosthesis Between 30–50% of bioprosthesis require replacement after 15 years of implantation Mechanical failure and calcification are major causes of failure Pannus = excessive tissue ingrowth Endocarditis = infection Thrombus = clotting Heart valve replacements Class problem I The bovine pericardium has been tested for its mechanical properties. The stress–strain curve is shown in in the following illustration. Answer the following questions. a) Calculate the initial modulus. b) Calculate the secondary modulus. c) What is the toughness? Stress–strain curve of bovine pericardium. Heart valve replacements Class problem I - Solution a. The initial modulus is, from the slope of the graph, 0.8 − 0 𝐸𝑖 = = 4𝑀𝑃𝑎 0.2 − 0 b. The secondary modulus is, from the slope of the graph 12 − 2.5 𝐸𝑠 = = 29𝑀𝑃𝑎 0.6 − 0.3 c. The toughness is the area under the curve up to the failure strain. It can be approximated by a triangle: 0.6 − 0.2 𝑚 𝑁𝑚 (𝐽) 𝑇𝑜𝑢𝑔ℎ𝑛𝑒𝑠𝑠 = 12𝑀𝑃𝑎 𝑥 = 2.4 𝑀𝑃𝑎. = 2.4 𝑥106 2 𝑚 𝑚3 Heart valve replacements Class problem II i. Stress-strain curves for healthy human aortic valve tissue in radial and circumferential directions are provided below. ii. Compare these mechanical properties to the bovine pericardium (previous question) iii. Comment on the suitability of the bovine pericardium as an implant material for heart valve leaflets. Circumferential Radial Heart valve replacements Class problem II – Low strain values (initial modulus) a. In the circumferential direction, the modulus is Bovine pericardium Initial modulus 4𝑀𝑃𝑎 0.15 − 0 𝐸𝑖 = = 3𝑀𝑃𝑎 Secondary modulus 29𝑀𝑃𝑎 0.05 − 0 b. In the radial direction, the modulus is 0.05 − 0 𝐸𝑠 = = 1𝑀𝑃𝑎 0.05 − 0 Heart valve replacements Class problem II – High strain values (secondary modulus) Bovine pericardium a. In the circumferential direction, the modulus is Initial modulus 4𝑀𝑃𝑎 Secondary modulus 29𝑀𝑃𝑎 1.8 − 0.5 𝐸𝑖 = = 16𝑀𝑃𝑎 0.18 − 0.10 b. In the radial direction, the modulus is 0.25 − 0.1 𝐸𝑠 = = 1.8𝑀𝑃𝑎 0.18 − 0.10 Heart valve replacements Class problem II – High strain values (secondary modulus) Secondary modulus is much higher for bovine pericardium (in radial direction in particular), may impact valve function at higher strain levels - more stress/force required to open value to a given strain point etc. Circumferential Radial Heart valve replacements Pressure differentials across mitral valves and blood flow velocities How can we calculate pressures and blood flow velocities acting across heart valves? Heart valve replacements Pressure differentials across mitral valves and blood flow velocities Point 1 = left Bernoulli equation and heart valves atrium (la) 1 1 𝑃𝑙𝑎 + 𝜌𝑣𝑙𝑎 2 + 𝜌𝑔ℎ𝑙𝑎 = 𝑃𝑙𝑣 + 𝜌𝑣𝑣𝑎𝑙𝑣𝑒 2 + 𝜌𝑔ℎ𝑣𝑎𝑙𝑣𝑒 2 2 1 1 𝑃𝑙𝑎 + 𝜌𝑣𝑙𝑎 + 𝜌𝑔ℎ𝑙𝑎 = 𝑃𝑙𝑣 + 𝜌𝑣𝑣𝑎𝑙𝑣𝑒 2 + 𝜌𝑔ℎ𝑣𝑎𝑙𝑣𝑒 2 Point 2 = valve 2 2 1 1 𝑃𝑙𝑎 − 𝑃𝑣𝑎𝑙𝑣𝑒 = 𝜌𝑣𝑣𝑎𝑙𝑣𝑒 − 𝜌𝑣𝑙𝑎 2 2 2 2 1 𝑃𝑙𝑎 − 𝑃𝑣𝑎𝑙𝑣𝑒 ≈ 𝜌𝑣𝑣𝑎𝑙𝑣𝑒 2 𝑣𝑣𝑎𝑙𝑣𝑒 2 ≫ 𝑣𝑙𝑎 2 2 1 ∆𝑃𝑚𝑡𝑖𝑡𝑟𝑎𝑙 𝑣𝑎𝑙𝑣𝑒 ≈ 𝜌𝑣𝑣𝑎𝑙𝑣𝑒 2 2 Heart valve replacements Pressure differentials across mitral valves and blood flow velocities Bernoulli equation and heart valves 1 Point 1 ∆𝑃𝑚𝑡𝑖𝑡𝑟𝑎𝑙 𝑣𝑎𝑙𝑣𝑒 ≈ 𝜌𝑣𝑣𝑎𝑙𝑣𝑒 2 1𝑚𝑚𝐻𝑔 = 133.3 𝑃𝑎 2 𝑘𝑔 𝜌 = 1060 𝑚3 Point 2 1 (1060) ∆𝑃𝑚𝑣 ≈ 2 𝑣𝑣𝑎𝑙𝑣𝑒 2 𝑚 𝑣𝑙𝑣 𝑖𝑛 ( ) 133.33 𝑠 𝑚 ∆𝑃𝑚𝑣 ≈ 4 𝑣𝑣𝑎𝑙𝑣𝑒 2 𝑣𝑙𝑣 𝑖𝑛 ( ) 𝑠 Left atrium = la Left ventricle = lv Heart valve replacements Pressure differentials across mitral valves and blood flow velocities Heart valve replacements Pressure differentials across mitral valves and blood flow velocities BME328: Principles of Biomaterials Lecture 5: Sterilization methods Dr. Andrew Daly Associate Professor in Biomedical Engineering College of Science and Engineering, BMS 1012 Biomedical Sciences Building University of Galway Email: [email protected] Module outline Lectures 1. Introduction 2. Host response to biomaterials III (Soft and hard tissue implants) 3. Host response to biomaterials IV (Blood- Section 1 - Host response to contacting implants) biomaterials 4. Biomaterial infections 5. Sterilisation methods 6. Cardiovascular devices I – Heart Valves 7. Cardiovascular devices II – Stents Section 2 - Biomaterial products (Failures and case studies in 8. Orthopaedic devices I – Joint replacements optimizing design) 9. Orthopaedic devices II – Joint replacements 10. 3D printing overview 11. Lab preparation lecture Module outline Lectures 12. Contact lens biomaterials Section 2 - Biomaterial products 13. Adhesive biomaterials (Failures and case studies in 14. Neural implants optimizing design) 15. Advanced Drug Delivery I Section 3 – Drug Delivery 16. Advanced Drug Delivery II 17. Advanced chemical characterization techniques Section 4 – Characterization 18. Advanced biological characterisation techniques technologies 19. Regulatory affairs for biomaterials I Section 5 – Regulatory affairs 20. Regulatory affairs for biomaterials II 21. 3D Bioprinting 22. Electrospinning Section 6 – Future directions in 23. Frontiers in biomaterials I biomaterials 24. Frontiers in biomaterials II 25. Exam overview Module outline Learning outcomes 1. Describe the structure and composition of advanced polymer, ceramic, and metal biomaterials. 2. Evaluate and select an appropriate biomaterial for a given implant design. 3. Assess optimal sterilization methods for a biomaterial implant to limit implant associated infections. 4. Develop an understanding of biocompatibility, and how it can be optimized through biomaterial design 5. Design an optimal manufacturing method for a given biomaterial implant. 6. Perform experiments to characterize the physical and biological properties of biomaterials. 7. Leverage emerging technologies/materials to design medical implants. Implant-associated infection and sterilization methods. Sterilization methods Sterilization elimination of bacterial contamination through a mechanism of DNA disablement. Several sterilization techniques available: 1. Autoclaving 2. Gamma irradiation 3. Ethylene oxide gas 4. Gas plasma Implant-associated infection and sterilization methods. Sterilization methods All medical implants must be sterilized to ensure no bacterial contamination to the patient. 1. How would you sterilize a total hip replacement comprising a titanium stem, a cobalt chromium alloy head, and an ultra-high molecular weight polyethylene acetabular shell? 2. Could the same method be employed for all three materials? 3. How do you ensure that there is no degradation to the material or its structural properties? 4. What factors would you need to consider in the optimization of this problem? Implant-associated infection and sterilization methods. Sterilization methods Design and development of a new implant Implant-associated infection and sterilization methods. Lecture outline 1. Biomaterial sterilization methods 2. Biomaterial compatibility with sterilization methods 3. Terminal sterilization validation principles Implant-associated infection and sterilization methods. Sterilization methods - Autoclaving Autoclaving works by subjecting devices or materials to high-pressure steam at temperatures of the order of 121°C in order to destroy bacterial contamination. Autoclaving is highly accessible and is often available in hospitals and surgical units. Commonly employed to sterilize surgical tools. Because of the temperature of the steam, autoclaving is generally not used with polymeric systems. Implant-associated infection and sterilization methods. Sterilization methods - Autoclaving Autoclaving works by subjecting devices or materials to high-pressure steam at temperatures of the order of 121°C in order to destroy bacterial contamination. Implant-associated infection and sterilization methods. Sterilization methods - Autoclaving Materials require different sterilization times depending on their texture, porosity, and other characteristics of each material. Rubber gloves 15 minutes Probes (latex) 15 minutes Glass Bottles 20 minutes Clothes 30 minutes Surgical pack 45 minutes Stainless steel instruments 45 minutes Implant-associated infection and sterilization methods. Sterilization methods - Gamma irradiation Gamma sterilization process uses high-energy photons emitted from a Cobalt 60 isotope source to produce ionization (electron disruptions) throughout the medical device to kill bacteria. Gamma process is deeply penetrating and has been employed in many devices and materials Alpha, beta, and gamma radiation Terminal sterilization doses typically range from 8 to 35 kGy (0.01 kGy lethal to a human) Implant-associated infection and sterilization methods. Sterilization methods - Gamma irradiation Implant-associated infection and sterilization methods. Sterilization methods - Gamma irradiation The microbial sterilization involves radiation-induced scission of DNA chains which prevents microbial reproduction. In living cells, electron disruptions result in damage to the DNA and other cellular structures. These photon-induced changes at the molecular level cause the death of the organism or render the organism incapable of reproduction. Implant-associated infection and sterilization methods. Sterilization methods – E-beam irradiation Electron beam (e-beam) sterilization accounts for approximately 20% of the radiation sterilization market High energy electrons for e-beam sterilization are generated by accelerating electrons from power grid (200 volt) to 0.2 million to 10 million electron volts (0.2–10 MeV). Implant-associated infection and sterilization methods. Sterilization methods – E-beam irradiation Electrons from accelerators do not penetrate nearly as far as photons from gamma sources. The maximum penetration (in centimeters) from a two-sided e-beam process is 0.8 times the beam energy (in MeV) divided by the density of product (in g/cm3). Maximum penetration distances are therefore relatively small, with higher beam energies resulting in higher penetration. 0.8 (𝐵𝑒𝑎𝑚 𝐸𝑛𝑒𝑟𝑔𝑦) 𝑃𝑒𝑛𝑒𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑑𝑒𝑝𝑡ℎ ≈ 𝜌 Implant-associated infection and sterilization methods. Sterilization methods – E-beam irradiation Disadvantage of e-beams versus gamma irradiation is lower penetration depths - negatively charged electrons do not penetrate as deeply into a material compared with neutral gamma rays Advantage of e-beams versus gamma irradiation ❑ Dose is delivered very quickly ❑ Gamma sterilization processes often take several hours ❑ e-beam process typically minutes. Implant-associated infection and sterilization methods. Sterilization methods - Ethylene oxide (ETO) Ethylene oxide (EtO) gas sterilization is a chemical process that utilizes a combination of gas concentration, humidity, temperature, and time to render materials sterile. Ethylene oxide is an alkylating agent that disrupts the DNA of microorganisms and prevents them from reproducing. Ethylene oxide sterilization is considered a low temperature method and is commonly employed in a variety of materials including many polymers such as orthopedic-grade polyethylene Implant-associated infection and sterilization methods. Sterilization methods - Ethylene oxide (ETO) Ethylene oxide is a highly reactive cyclic ether with two carbons and one oxygen, CH2CH2O. Ethylene oxide has a boiling point of 11°C (gas at room temp). Needs to be pressurized to be stored as a liquid. The mechanism of bacterial killing is alkylation of the amine groups of DNA. Implant-associated infection and sterilization methods. Sterilization methods - Ethylene oxide (ETO) EO kill rate is a function of temperature and concentration of EO gas. Inactivation of Bacillus atrophaeus at different temperatures at 500 mg/L ethylene oxide and 40% relative humidity. Implant-associated infection and sterilization methods. Sterilization methods - Gas plasma Gas plasma is a low-temperature, sterilization method that relies upon ionized gas for deactivation of biological organisms on surfaces of devices or implants. Low-temperature hydrogen peroxide gas is produced at temperatures lower than 50°C. Implant-associated infection and sterilization methods. Sterilization methods - Summary Implant-associated infection and sterilization methods. Class problem 1. How would you sterilize a total hip replacement comprising a titanium stem, a cobalt chromium alloy head, and an ultra-high molecular weight polyethylene acetabular shell? 2. Could the same method be employed for all three materials? 3. What factors would you need to consider in the optimization of this problem? Implant-associated infection and sterilization methods. Lecture outline 1. Biomaterial sterilization methods 2. Biomaterial compatibility with sterilization methods 3. Terminal sterilization validation principles Implant-associated infection and sterilization methods. Material compatibility – Irradiation Sterilization technologies can impact the molecular structure of biomaterials, and therefore can alter physical, chemical, biological, and mechanical properties of the material. Knowledge of a sterilization technologies key processing parameters and how they differentially interact with materials is essential for a biomaterial engineer Implant-associated infection and sterilization methods. Material compatibility – Irradiation Radiation sterilization leads to generation of free radicals within the material, which in the case of polymers, can cause polymer chain scission or cross-linking The general mechanism of cross-linking via irradiation for all polymers is the scission of C-H bond on one polymer chain to form a hydrogen atom leaving a free radical on this chain, and abstraction* of another hydrogen atom by cleavage of adjacent chain to form a hydrogen molecule. Then two radical-generated polymer chains combine to form a new bond called intermolecular cross- linking. abstraction = Removal of an atom or group from a molecule by a radical Implant-associated infection and sterilization methods. Material compatibility – Irradiation Irradiation crosslinking can increase certain mechanical properties (ultimate strength), while decreasing others (elasticity, stiffness) These effects are highly dose and polymer dependant. Mechanical response of high density polyethylene to gamma radiation from a Cobalt-60 irradiator Implant-associated infection and sterilization methods. Material compatibility – Irradiation Irradiation can also lead to polymer chain scission and degradation (polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), polyacetals, and natural polypropylene (PP) The two mechanisms of chain scission and crosslinking occur simultaneously and competitively, with the more prominent effect depending on the polymer type and irradiation dose Implant-associated infection and sterilization methods. Material compatibility – Irradiation Gamma irradiation will lead to a reduction in polymer molecular weight, which can significantly affect implant performance. Mechanical properties are diminished at higher irradiation doses. MW versus gamma irradiation dose for poly(L-lactide-co-ε-caprolactone) (PLCL) Implant-associated infection and sterilization methods. Case study – Gamma irradiation sterilization of polymers Initially, gamma irradiation was a common method for sterilizing polymers. Gamma radiation can result in oxidation of polymers that can affect mechanical properties. Until about 1995, UHMWPE was sterilized with a nominal dose of 25–40 kGy of gamma radiation Figure Effect of gamma radiation dose on the maximum strength of ultrahigh molecular weight polyethylene in the presence of air. By 1998, all of the major manufacturers had shifted their sterilization method away from gamma irradiation. Today most UHMWPE components are sterilized with Ethylene oxide Implant-associated infection and sterilization methods. Material compatibility – Ethylene oxide (ETO) Ethylene oxide (ETO) which uses alkylation chemistry to kill microbes can produce material effect, but in general these effects are less damaging than irradiation. Main consideration is that the material must be able to withstand conditions used in an EO cycle. This includes high humidity and temperature cycles of 40–65°C; for 6–24 hours ETO can have deleterious effects on some polymers (see table) Implant-associated infection and sterilization methods. Material compatibility – Ethylene oxide (ETO) One of the main drawbacks of ETO sterilization is that EO residues can remain in some polymers post-processing. To reduce this, mechanical aeration for 8 to 12 hours at 50 to 60°C allows toxic ETO residuals to deadsorb from the sterilized material. Implant-associated infection and sterilization methods. Sterilization methods - Summary Implant-associated infection and sterilization methods. Lecture outline 1. Biomaterial sterilization methods 2. Biomaterial compatibility with sterilization methods 3. Terminal sterilization validation principles Implant-associated infection and sterilization methods. Terminal sterilization validation principles Sterilization of a biomaterial is defined as a “validated process used to render product free from viable microorganisms” (ISO/TS 11139, 2006). Sterility is expressed in terms of probability. This probability can be reduced to a very small number but can never be reduced to 0. Implant-associated infection and sterilization methods. Terminal sterilization validation principles The probability of a non-sterile unit is defined by the term Sterility Assurance Level (SAL). The SAL required for regulatory purposes is 10−3 or 10−6, the probability that 1 in 1,000 or 1,000,000 is non-sterile Implant-associated infection and sterilization methods. Terminal sterilization validation principles To demonstrate an SAL of 10-6 would require sterility testing of one million products with only one positive (neither possible nor practical). Therefore, sterilization processes are validated. EN 556-1 2001 - Sterilization of medical devices indicates the following: Evidence that a medical device is sterile comes from: (1) the initial validation of the sterilization process and subsequent revalidations that demonstrate the acceptability of the process; and (2) information gathered during routine control and monitoring which demonstrates that the validated process has been delivered in practice. The achievement of sterility is predicted from the bioburden level on products, the resistance of the micro-organisms comprising that bioburden, and the extent of treatment imposed during sterilization. Implant-associated infection and sterilization methods. Terminal sterilization validation principles The pharmaceutical and medical device industries, along with regulatory authorities, have worked together to establish sterilization standards. Contract sterilization providers, biomaterials processing, device manufacturers, regulatory authorities, and academic resources have worked together to develop these standards. The standards provide guidance and requirements for sterilization performance and validation. Biomaterials and standards Unlicensed implants made with non-medical parts used in Temple Street spinal operations https://www.ontheditch.com/unlicensed-implants-temple-street/ Biomaterials and standards Unlicensed implants made with non-medical parts used in Temple Street spinal operations “European Springs & Pressings Ltd, headquartered in Kent, have confirmed to The Ditch that it fulfilled an order to Temple Street in January 2020 for a consignment of 10 compression springs (at a cost of £4.58 per spring) that were not intended for use as implants in surgery. The company’s business development manager in Ireland says they had no idea of the end use for the springs and that, if they had known they were to be used in surgery, the company would have advised against it. The springs are made from low-grade steel, rather than the titanium alloy required to prevent corrosion.” “In Temple Street, someone saw springs, bought them and just used them, had them sterilised and used them without any form of thought towards the type of metal. The regulations are so important: you have to follow them because otherwise it will fail,” they said.” https://www.ontheditch.com/unlicensed-implants-temple-street/ Biomaterials and standards Unlicensed implants made with non-medical parts used in Temple Street spinal operations https://www.cresco-spine.com/ Biomaterials and standards Unlicensed implants made with non-medical parts used in Temple Street spinal operations https://www.cresco-spine.com/ BME328: Principles of Biomaterials Lecture 4: Implant and biomaterial associated infections Dr. Andrew Daly Associate Professor in Biomedical Engineering College of Science and Engineering BMS1012 Biomedical Sciences Building University of Galway Email: [email protected] Module outline Lab groups and schedule Module outline Lab groups and schedule Please bring your lab coats Module outline Lectures 1. Introduction 2. Host response to biomaterials III (Soft and hard tissue implants) 3. Host response to biomaterials IV (Blood- Section 1 - Host response to contacting implants) biomaterials 4. Biomaterial infections 5. Sterilisation methods 6. Cardiovascular devices I – Heart Valves 7. Cardiovascular devices II – Stents Section 2 - Biomaterial products (Failures and case studies in 8. Orthopaedic devices I – Joint replacements optimizing design) 9. Orthopaedic devices II – Joint replacements 10. 3D printing overview 11. Lab preparation lecture Module outline Lectures 12. Contact lens biomaterials Section 2 - Biomaterial products 13. Adhesive biomaterials (Failures and case studies in 14. Neural implants optimizing design) 15. Advanced Drug Delivery I Section 3 – Drug Delivery 16. Advanced Drug Delivery II 17. Advanced chemical characterization techniques Section 4 – Characterization 18. Advanced biological characterisation techniques technologies 19. Regulatory affairs for biomaterials I Section 5 – Regulatory affairs 20. Regulatory affairs for biomaterials II 21. 3D Bioprinting 22. Electrospinning Section 6 – Future directions in 23. Frontiers in biomaterials I biomaterials 24. Frontiers in biomaterials II 25. Exam overview Module outline Learning outcomes 1. Describe the structure and composition of advanced polymer, ceramic, and metal biomaterials. 2. Evaluate and select an appropriate biomaterial for a given implant design. 3. Assess optimal sterilization methods for a biomaterial implant to limit implant associated infections. 4. Develop an understanding of biocompatibility, and how it can be optimized through biomaterial design 5. Design an optimal manufacturing method for a given biomaterial implant. 6. Perform experiments to characterize the physical and biological properties of biomaterials. 7. Leverage emerging technologies/materials to design medical implants. Implant-associated infections Introduction Implants such as joint prostheses, intravenous catheters, prosthetic heart valves, dialysis catheters, and cardiac pacemakers all carry the risk of infection. Biomaterial associated infections are a major cause of implant failures, and sterilization procedures are an essential aspect of medical device design. Implant-associated infections Introduction Implant infections rates vary depending on the medical device and implantation site. For example, rates are between 1– 3% for orthopaedic implants and 1- 7% for cardiac pacemakers. Can be much higher for specific implants/surgical procedures (e.g., bladder catheters) Implant-associated infections Introduction Chronic infection following implantation can be major challenge to reverse or alleviate. Oral or intravenous antibiotic therapy frequently fails to resolve the infection – location dependent. Following chronic infection, in some circumstances, the only course of action is surgical debridement, or Lateral radiograph of elbow following total elbow arthroplasty with partial or total revision placement of an intercalary tibial allograft. C: Lateral radiograph of elbow thirteen months after the total elbow arthroplasty. An area of lucency (arrowheads) surrounds the ulnar component (infection, local loss of bone) Direct demonstration of viable Staphylococcus aureus biofilms in an infected total joint arthroplasty: a case report. Implant-associated infections Introduction Advances in surgical practice have reduced the prevalence of implant associated infections (laminar flow air, sterilization procedures). However, bacteria cannot be completely removed from the “sterile field” – and during surgery, a wound/device is typically exposed to ~270 bacterial-carrying particles per cm2 per hour. Once the medical device is removed from sterilized packing there is potential for implant contamination Implant-associated infections Introduction Infection probability is related to the relative time taken to colonize the biomaterial surface by host cells (immune, vascular, fibroblasts) versus bacteria This has been termed “The race to the surface” For example, if the biomaterial surface is colonized with host cells before bacterial colonies become established, the infection probability decreases significantly. Implant-associated infections Introduction Before implantation Bacterial contaminations are very difficult to observe by eye prior to implantation Bacteria can form extensive biofilms on the surface of implants within just 2-3 days, leading to a chronic infection around the implant. Biofilm aggregated attached to a braided permanent surgical suture associated with a knee arthroplasty revision. Left panels: Outgrowth of biofilm (white arrow) after 2 days from a section of the suture that was placed in nutrient agar immediately after explantation. No visual indication of biofilm presence was on the suture before incubation (top left, white arrow). Right panel: Confocal microscopy of biofilm on a suture from the same case. Bacterial cocci stained red with the nucleic acid probe Syto59 and surrounding EPS stained green with a lectin that labels poly-N-acetyl glucosamine, an EPS polymer of staphylococcal biofilms. PCR confirmed Staphylococcus aureus. Scale bar = 5 μm. Implant-associated infections Lecture outline 1. Bacterial biofilms 2. Biomaterial surface properties and biofilms 3. Detecting device-related infections Implant-associated infections Bacterial biofilms Bacterial biofilms are communities of bacteria that attach and subsequently grow on material surfaces. Once attached, bacteria embed themselves in a highly hydrated and protective “biofilm” The biofilm consists of bacterially produced polymers, such as extracellular DNA (eDNA), polysaccharides, lipids, proteins Implant-associated infections Bacterial biofilms Key processes in biofilm development. a) Initial attachment of single cells and cell aggregates in a biofilm fluid. b) Initiation of microcolony formation where production of extracellular polymeric substances more firmly adheres bacterial cells to the surface. c) Early development of biofilm clusters d) Mature biofilm e) Dispersion of bacteria and biofilm growth Implant-associated infections Bacterial biofilms Once formed, biofilms are difficult to eradicate by host immune cells Biofilm defense mechanisms include matrix production, which protects bacteria within the biofilm from phagocytosis as immune cells cannot invade the biofilm easily Bacteria in biofilms are also more resistant to environmental stresses such as ultraviolet (UV) light exposure than free- floating bacteria. Implant-associated infections Bacterial biofilms Biofilms are extremely difficult to treat with conventional topical or systemic antibiotic therapy. Treatment of biofilms with antibiotics often results in incomplete killing, allowing unaffected bacteria to act as a nucleus for the spread of infection following the withdrawal of antibiotic therapy. Implant-associated infections Lecture outline 1. Bacterial biofilms 2. Biomaterial surface properties and biofilms 3. Detecting Device-Related Infections Implant-associated infections Class question What physicochemical properties of a biomaterial implant could impact potential for biofilm formation? Implant-associated infections Biomaterial surface properties and biofilms The physicochemical properties of the biomaterial surface can affect bacterial adhesion Surface charge, roughness, wettability (hydrophobicity/hydrophilicity), and mechanical properties of the biomaterial surface may either encourage or discourage bacterial adhesion Some biomaterial are more prone to infection than others (see table). Schematic illustration of the main material properties (wettability, surface charge, roughness, and topography) affecting bacterial adhesion. Implant-associated infections Biomaterial surface properties and biofilms - Wettability Material surfaces are classified as hydrophilic if the contact angle (θ) is less than 90 degrees and hydrophobic when higher than 90 degrees. The hydrophobic/hydrophilic nature of the material depends on the surface chemical compositions. Implant-associated infections Biomaterial surface properties and biofilms - Wettability Bacterial adhesion is typically higher on hydrophobic surfaces. Larger biofilms form on agar hydrogels with increased hydrophobicity. Implant-associated infections Biomaterial surface properties and biofilms - Surface roughness Biomaterials with higher surface roughness values have been shown to promote bacterial biofilm formation It is speculated that this occurs as bacteria will have a larger surface available for attachment and will also be more protected from shear forces (from fluid or surrounding tissues). Implant-associated infections Biomaterial surface properties and biofilms - Surface roughness 3D roughness representation of