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
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3.052 Nanomechanics of Materials and Biomaterials Thursday 04/19/07 Prof. C. Ortiz, MIT-DMSE
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LECTURE 17: NANOMECHANICS AND BIOCOMPATIBILITY : PROTEIN-BIOMATERIAL INTERACTIONS
Outline : REVIEW LECTURE #16 : NANOMECHANICS OF CARTILAGE.............................................................. 2BIOCOMPATIBILITY OF MATERIALS IMPLANTED IN VIVO: DEFINITIONS.......................................... 3TEMPORAL 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)
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3.052 Nanomechanics of Materials and Biomaterials Thursday 04/19/07 Prof. C. Ortiz, MIT-DMSE
(Seog+ J. Biomech 2005 in press, online &Dean, Han+ unpublished data 2005)
● stiffens nonlinearly with increasing strain at the molecular level→mechanism to prevent large strains that could result in permanent
deformation, fracture, or tearing.
(Macro)compressive
moduli : human ankle cartilage + (approximate) 50% prestrain(Nano)
Ag-Ag colloid
0
0.5
1
1.5
0 0.2 0.4 0.6 0.8 1Strain
Stiff
ness
(MPa
) M
(Seog+ J. Biomech 2005 in press, online &Dean, Han+ unpublished data 2005) (Macro)
compressive moduli : human ankle cartilage + (approximate) 50% prestrain(Nano)
Ag-Ag colloid
● stiffens nonlinearly with increasing strain at the molecular level→mechanism to prevent large strains that could result in permanent
deformation, fracture, or tearing.
- Force vs. Distance converted into Stress, , where As= surface interaction area vs. Strain 1 1(aggrecan)
ε = − = −f
o o
L DL h
Fσ =
sA
where ho(aggrecan)= initial uncompressed height of aggrecan (zero force),0
dd →
⎛ ⎞= ⎜ ⎟⎝ ⎠ε
σε
Stiffness
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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 -- N-acetylgalactosamine 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.)
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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
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.
Viable - incorporating live cells at implantation, treated by the host as normal tissue matrices and actively resorbed and/or remodeled Biofilm : When a biomaterial is exposed a physiological environment the resulting a complex aggregation that may contain biomacromolecules, cells, bacteria, microorganisms
Biomaterial, Biomedical Materials : nonliving (artificial) material intended to interact with a living (biological) system, replacement for “broken” anatomical parts or physiological systems Examples of Biomaterials; medical implants, heart valves, vascular grafts, contact lenses, drug delivery systems, scaffolding for tissue regeneration, breast implant, hip joint
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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
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)
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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 platelets (~45 wt.%) 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
-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
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3.052 Nanomechanics of Materials and Biomaterials Thursday 04/19/07 Prof. C. Ortiz, MIT-DMSE
KINETICS OF PROTEIN ADSORPTION
BIOMATERIAL SURFACE
U(D)
BIOMATERIAL SURFACE
U(D)z
Molecules can be brought to the surface by diffusion; (I. Szleifer, Purdue University)
2
2protein protein
protein
;∂ ∂=
∂ ∂
∂ ∂ ∂= +
∂ ∂ ∂
ρ ρρ
2
2
C CD C = concentration, D = diffusion coefficient, z = distance, t = timet z
(z,t) (z,t)D (
t z zIdeal diffusion-controlledregime controlled only by density gradient
⎡ ⎤⎢ ⎥⎢ ⎥∂⎛ ⎞⎢ ⎥⎜ ⎟⎢ ⎥∂⎝ ⎠⎢ ⎥⎢ ⎥⎣ ⎦
mf
protein
U (z,t)z,t)
z
ρ (z,t)= local density of protein molecules
U
Kinetically activated regime, "driven" diffusion, motion that arises from surface-protein interactions
→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
mfU (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).
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3.052 Nanomechanics of Materials and Biomaterials Thursday 04/19/07 Prof. C. Ortiz, MIT-DMSE
USE OF STERIC REPULSION TO INHIBIT PROTEIN ADSORPTION (I. Szleifer, Purdue University)
Charged surface:
proteins expose charged groups.
Hydrophobic surface:
proteins expose hydrophobic patches.
solid substrate
e.g. human serum albumin7
(IV.)
(I.)
Loend-grafted
polymer brush
s
RP
RP(III.)(II.)(I.)
adsorbed proteins
→ generally can't use charged surface EDL repulsion as a mechanism to inhibit protein adsorption → one method: use steric repulsion of surface functionalized (chemisorption, physisorption) with polymers 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
(Halperin, Langmuir 1999)
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3.052 Nanomechanics of Materials and Biomaterials Thursday 04/19/07 Prof. C. Ortiz, MIT-DMSE
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: - hydrophilic and water-soluble at RT -forms an extensive H-bonding network; intramolecular H- bond bridges between -O- groups and HOH→ large excluded volume -• locally (7/2) helical supramolecular structure (tgt axial repeat = 0.278 nm) -high flexibility, molecular mobility -low van der Waals attraction • neutral However: -poor mechanical stability -protein adhesion reported under certain conditions (long implant times) -maintains some hydrophobic character
OO
OO
HH
OH
H O
t 0.278 nmgt
t
t tt
gt t t
(tgt)
• neutrality : won’t attract oppositely charged species
OO
HHO
O
O
H
H
O
• hydrophilic/ water soluble :hydration enthalpic penalties for
disruption of supramolecularstructure H-bonding with water
