Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
3.052 Nanomechanics of Materials and Biomaterials Thursday 04/19/07 Prof. C. Ortiz, MIT-DMSE I LECTURE 17: NANOMECHANICS AND BIOCOMPATIBILITY : PROTEIN-BIOMATERIAL INTERACTIONS Outline : REVIEW LECTURE #16 : NANOMECHANICS OF CARTILAGE ............................................................. 2 BIOCOMPATIBILITY OF MATERIALS IMPLANTED IN VIVO: DEFINITIONS ......................................... 3 TEMPORAL BIOLOGICAL RESPONSES TO IMPLANTED BIOMATERIALS ......................................... 4 BLOOD-BIOMATERIAL INTERACTIONS ................................................................................................ 5 KINETICS OF PROTEIN ADSORPTION ................................................................................................. 6 USE OF STERIC REPULSION TO INHIBIT PROTEIN ADSORPTION ................................................... 7 THERMAL MOTION OF POLYMER BRUSHES : MOVIE ........................................................................ 8 POLY(ETHYLENE GLYCOL) AS A BIOINERT COATING ....................................................................... 9 Objectives: To establish a fundamental qualitative and quantitative scientific foundation in understanding the biocompatibility of biomaterials implanted in vivo Readings: Course Reader Documents 29, 30 Multimedia : Polymer Brush Demos (posted on stellar) 1 3.052 Nanomechanics of Materials and Biomaterials Thursday 04/19/07 Prof. C. Ortiz, MIT-DMSE REVIEW : LECTURE 16 NANOMECHANICS OF CARTILAGE 0 m) Stiffness (MPa) Force (nN) Substrate -Definitions; articular cartilage function and structure, proteoglycan, aggrecan, hyaluronan, link protein, collagen, chondrocyte, glycosaminoglycan (GAG), chondroitin sulfate - Loading Conditions : withstands ~3 MPa compressive stress and 50% compressive strain (static conditions), equilibrium compressive moduli ~0.1-1MPa - Composition : 80% HOH, collagen (50-60% solid content, mostly type II), aggrecan (30-35% solid content), hyaluronan, ~3-5% cartilage cells (chondrocytes) (Seog+ J. Biomech 2005 in press, online & Aggrecan Tip vs. Aggrecan Substrate (Macro) Dean, Han+ unpublished data 2005) 1.5 10 compressive moduli : human M ankle cartilage 8 1 + (approximate) 50% prestrain 6 (Nano) 0.5 Ag-Ag colloid 4 2 1M 1200 0.01M 0.1M 0.001M 0 0 0 0 400 800 1200 Distance (nm) - Force vs. Distance converted into Stress, 0.2 0.4 0.6 Strain 0.8 1 ● stiffens nonlinearly with increasing strain at the molecular level→ mechanism to prevent large strains that could result in permanent deformation, fracture, or tearing. Lf F D 1 1 , where As= surface interaction area vs. Strain As Lo ho (aggrecan) d where ho(aggrecan)= initial uncompressed height of aggrecan (zero force), Stiffness d 0 2 3.052 Nanomechanics of Materials and Biomaterials Thursday 04/19/07 Prof. C. Ortiz, MIT-DMSE Lecture 16 Nanomechanics of Cartilage: Definitions articular cartilage : connective avascular (contains no blood vessels) tissue covering the ends of the bones in synovial joints that allow smooth, low friction, painless motion proteoglycan: A molecule that contains both a protein core and glycosaminoglycans, which are a type of polysaccharide. Proteoglycans are found in cartilage and many other connective tissues. aggrecan: the largest aggregating proteoglycan, found in cartilage tissue and the intervertebral disc, has a bottle-brush configuration composed of a protein core backbone and densely spaces glycosaminoglycans glycosaminoglycan (GAGs) : Polysaccharides containing repeating disaccharide units that contain either of two amino sugar compounds -- Nacetylgalactosamine or N-acetylglucosamine, and a uronic acid such as glucuronate (glucose where carbon six forms a carboxyl group). Also called mucopolysaccharides in the older literature. GAGs are found in the lubricating fluid of the joints and as components of cartilage, synovial fluid, vitreous humor, bone, and heart valves. chondroitin sulfate (CS) : One of several classes of sulfated glycosaminoglycans that is a major constituent in various connective tissues, especially in the ground substance of blood vessels, bone, and cartilage. Chondroitin sulfate is a sulfated glycosaminoglycan (GAG) composed of a chain of alternating sugars (N-acetylgalactosamine and glucuronic acid). It is usually found attached to proteins as part of a proteoglycan. CS→ ← HA hyaluronan (HA) : Hyaluronan (HA also called hyaluronic acid or hyaluronate) is an anionic polysaccharide composed of repeating disaccharides of beta-1-4-glucuronate-beta-1-3-N-acetylglucosamine distributed widely throughout connective, epithelial, and neural tissues. The polysaccharide appears to be unique amongst glycosaminoglycans as it is synthesized, and exists in vivo, without attachment to any protein. (As such, it is not synthesized via the usual intracellular organelles that involve protein synthesis; rather, it is extruded from the cell membrane, catalyzed by the enzyme hyaluronan synthetase.) It can be synthesized with a very large molecular weight (1,000 - 5,000 kDa). In cartilage, a globular domain at the N-terminus of aggrecan, termed G1 or the hyaluronic acid binding region (HABR). binds to HA in an interaction that is stabilized by link protein. link protein : A ~45 kDa globular protein that stabilizes the interaction between aggrecan and HA. chondrocyte : cartilage cells responsible for the synthesis and maintenance of the extracellular matrix; chondrocyte-like cells are also found in the intervertebral disc if the spine collagen: The fibrous protein constituent of bone, cartilage, tendon, and other connective. Type II is the major fibril-forming collagen in cartilage. (There are currently 28 known, distinct, collagen types.) 3 3.052 Nanomechanics of Materials and Biomaterials Thursday 04/19/07 Prof. C. Ortiz, MIT-DMSE BIOCOMPATIBILITY OF MATERIALS IMPLANTED IN VIVO: DEFINITIONS Biocompatibility : the ability of a material to perform with an appropriate host response in a specific application, a material that does not producing a toxic, injurious, or immunological response in living tissue; no irritation, inflammation, thrombosis, allergic reactions, coagulation, cancer contact lenses intraocular lenses dentures • tissue engineering scaffolds • artificial organs Bioinert - Biomaterials that elicit little or no host response, in terms of nanomechanics a net zero (or close to zero) surface potential with physiological environment, sometimes called "non-fouling" Interactive or Bioactive- Biomaterials designed to elicit specific, beneficial responses, e.g. ingrowth, bioadhesion. Bioadhesion : may be defined as the state in which two materials, at least one of which is biological in nature, are held together for extended periods of time by interfacial forces. The biological substrate may be cells, bone, dentine, or the mucus coating the surface of a tissue. If adhesive attachment is to a mucus coating, the phenomenon is sometimes referred to as mucoadhesion. e.g. cell-to-cell adhesion within a living tissue, wound dressing, and bacteria binding to tooth enamel. cochlear implants hip implant heart pump/ valves bone substitute materials drug delivery systems knee implant blood-contacting devices (vascular grafts, stents) Viable - incorporating live cells at implantation, treated by the host as normal tissue matrices and actively resorbed and/or remodeled Biomaterial, Biomedical Materials : nonliving (artificial) material intended to interact with a living (biological) system, replacement for “broken” anatomical parts or physiological systems Biofilm : When a biomaterial is exposed a physiological environment the resulting a complex aggregation that may contain biomacromolecules, cells, bacteria, microorganisms Examples of Biomaterials; medical implants, heart valves, vascular grafts, contact lenses, drug delivery systems, scaffolding for tissue regeneration, breast implant, hip joint 4 3.052 Nanomechanics of Materials and Biomaterials Thursday 04/19/07 Prof. C. Ortiz, MIT-DMSE TEMPORAL BIOLOGICAL RESPONSES TO IMPLANTED BIOMATERIALS • living materials respond rapidly to foreign materials (<1 s) • new layer of protein coats (isolates) biomaterial surface (minutes) • attachment of platelets, bacteria, yeasts, and additional proteins to surface (minutes-hours) • alteration in cell and tissue behavior (minutes-years) Host Effects : Blood Clots and Thrombosis Inflammatory Response Immune Rejection Fungal Infections and Diseases Irritation and Inflammation Fresh out of box Biomaterial Degradation Abrasive Wear Fatigue Stress-Corrosion Cracking Absorption from Biofluids (*chemical attack) Mechanical Failure after a few minutes 15 m TappingMode AFM images in saline of the convex face of commercial PHEMA soft contact lenses. Fresh "out of the box" contact lens (left) displays scratches on the surface that originate from the mold during the manufacturing process. The scratches are 5 to 10nm in depth and 150 to 850nm in width. Several large isolated features (130 to 250nm in height) are also observed. The RMS roughness on the surface is 14nm. Used contact lens (right) of the same exact type and brand in the left image. The lens surface is coated with particulate adsorbates. A scratch-like feature is visible running top to bottom in the image, and appears decorated with contaminants. The RMS roughness on the surface is 30nm. Scan size for both images is 15µm and z range is 160nm (Veeco, Inc) 5 3.052 Nanomechanics of Materials and Biomaterials Thursday 04/19/07 Prof. C. Ortiz, MIT-DMSE BLOOD-BIOMATERIAL INTERACTIONS Blood Compositions and Solution Conditions : pH7.15 - 7.35 IS=0.15 M Temperature ~37°C -cells and (~45 wt.%) BLOOD FLOW blood plasma proteins PLATELETS! D. Gregory http://medphoto.wellcome.ac.uk http://www.rinshoken.or.jp/org/CR/photo-e.htm platelets The liquid portion of the blood, the plasma or serum (55 wt. %), is a complex solution containing more than 90% wate - 6-8 wt.% proteins in plasma (over 3,000 different types), including : -58% albumins -38% globulins -4% fibrinogens BLOOD PRESSURE+ ATTRACTIVE FORCES BLOOD CLOT! -acute occlusive thrombosis mobility, denaturation, - infection / inflammation exchange, desorption - neointimal hyperplasia adsorbs BIOMATERIAL SURFACE solid-liquid Interface bound water, ions, small molecules 2-10Å -The majority of blood plasma proteins are net negatively charged. Each has its' own heterogeneous surface chemistry and unique intermolecular potential with biomaterial surface that changes and evolves with time in vivo. → want bioinert surface 6 3.052 Nanomechanics of Materials and Biomaterials Thursday 04/19/07 Prof. C. Ortiz, MIT-DMSE KINETICS OF PROTEIN ADSORPTION Molecules can be brought to the surface by diffusion; (I. Szleifer, Purdue University) z U(D) BIOMATERIAL SURFACE C 2C D 2 ; C = concentration, D = diffusion coefficient, z = distance, t = time t z 2 protein (z,t) protein (z,t) U mf (z,t) D ( z,t) protein 2 t z z z Ideal diffusion-controlled Kinetically activated regime, "driven" diffusion, controlled only by regime motion that arises from surface-protein interactions density gradient ρ protein (z,t)= local density of protein molecules U mf (z,t)= net "potential of mean force" including protein - surface potential more complex theories take into account protein - protein interactions and protein conformational changes U mf (z,t) - can have many different components, both attractive (e.g. hydrogen, ionic, van der Waals, hydrophobic, electrostatic) and repulsive (e.g. configurational entropy, excluded volume, osmotic, enthalpic, electrostatic, hydration), can lead to complex interaction profiles, will change if conformation of protein changes - Initial protein adsorption will be determined by longer range, larger spatial length scale of averaged surface properties (e.g. average surface charge per unit area→EDL) - Secondary stages of protein adsorption depend on shorter range biomolecular adhesive binding processes that take place when the protein is in close contact with the surface (e.g. the conformation, orientation, and mobility of the adsorbed proteins, the time scale of conformational changes, protein exchange and desorption, and interactions of adsorbed proteins with each other). 7 3.052 Nanomechanics of Materials and Biomaterials Thursday 04/19/07 Prof. C. Ortiz, MIT-DMSE USE OF STERIC REPULSION TO INHIBIT PROTEIN ADSORPTION → generally can't use charged surface EDL repulsion as a mechanism to inhibit protein adsorption (I. Szleifer, Purdue University) → one method: use steric repulsion of surface functionalized (chemisorption, physisorption) with polymers Charged surface: Hydrophobic surface: proteins expose charged groups. proteins expose hydrophobic patches. e.g. human serum albumin7 RP adsorbed proteins (II.) (I.) Modes of protein adsorption: (I.) adsorption of proteins to the top boundary of the polymer brush (II.) local compression of the polymer brush by a strongly adsorbed protein (III.) protein interpenetration into the brush followed by the non-covalent complexation of the protein and polymer chain (IV.) adsorption of proteins to the underlying biomaterial surface via interpenetration with little disturbance of the polymer brush RP (III.) (I.) end-grafted polymer brush Lo s solid substrate (IV.) (Halperin, Langmuir 1999) 8 3.052 Nanomechanics of Materials and Biomaterials Thursday 04/19/07 Prof. C. Ortiz, MIT-DMSE THERMAL MOTION OF POLYMER BRUSHES : MOVIES (left) : (J. Marko (Cornell U) : http://www.lassp.cornell.edu/marko/thinlayer.html) (right) : (*J. Hoh (John Hopkins U) : http://162.129.38.210/Hoh_Lab_Pages/Gallery.html) 9 3.052 Nanomechanics of Materials and Biomaterials Thursday 04/19/07 Prof. C. Ortiz, MIT-DMSE POLY(ETHYLENE OXIDE) AS A BIOINERT COATING The most extensively used polymer for biomaterial surface coatings: (tgt) - hydrophilic and water-soluble at RT t O H H -forms an extensive H-bonding network; intramolecular H- bond bridges between -Ogroups and HOH→ large excluded volume -• locally (7/2) helical supramolecular structure (tgt axial repeat = 0.278 nm) O O H O t g tO t t O t t t t H g 0.278 nm - - -- - - -- • steric (large excluded volume) -high flexibility, molecular mobility -low van der Waals attraction • electrostatic double layer forces • neutral However: -poor mechanical stability -protein adhesion reported under certain conditions (long implant times) -maintains some hydrophobic character • hydrophilic/ water soluble : hydration enthalpic penalties for disruption of supramolecular structure H-bonding with water O O H H H O O H O • high flexibility & mobility : no local steric or charge O • neutrality : won’t attract oppositely charged species 10