BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 Lecture 8: Physical Hydrogels Last Day Overview of biomedical applications of hydrogels Structure of covalent hydrogels Thermodynamics of hydrogel swelling Today Bonding in physical hydrogels Structure and thermodynamics of block copolymer hydrog Reading L.E. Bromberg and E.S. Ron, Temperature-responsive gels and thermogelling polymer matrices for protein and peptide delivery, Adv. Drug Deliv. Rev., 31, 197(1998) Associative forces in physical hydrogels Cross-link structure in physical hydrogels ·Drⅳ ing associative forces 1. Hydrophobic associations/Van der Waals forces i. LCST polymers, hydrophobic-hydrophilic block copolymers 2. Micellar packing 3. Hydrogen bonding(Rubner) 4. lonic bonding(later lecture) 5. crystallizing segments 6. Combinations of the above interactions o Peptide interactions(e.g. coiled coils)(1) Stability requires cooperative bonding interactions(2)(Guenet, Thermoreversible gelation of polymers and o Individual non-covalent bonds are relatively weak o Strength of covalent bond Hydrophobic association o lonic bond o Hydrogen bond in water o Cooperativity: lowered energy barrier for second and subsequent bonds after first has formed Used in biological associations Lecture 8 sical gels 1of12
BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Lecture 8 – Physical gels 1 of 12 Lecture 8: Physical Hydrogels Last Day: Overview of biomedical applications of hydrogels Structure of covalent hydrogels Thermodynamics of hydrogel swelling Today: Bonding in physical hydrogels Structure and thermodynamics of block copolymer hydrogels Reading: L.E. Bromberg and E.S. Ron, ‘Temperature-responsive gels and thermogelling polymer matrices for protein and peptide delivery,’ Adv. Drug Deliv. Rev., 31, 197 (1998) Associative forces in physical hydrogels Cross-link structure in physical hydrogels • Driving associative forces: 1. Hydrophobic associations/ Van der Waals forces i. LCST polymers, hydrophobic-hydrophilic block copolymers 2. Micellar packing 3. Hydrogen bonding (Rubner) 4. Ionic bonding (later lecture) 5. crystallizing segments 6. Combinations of the above interactions o Peptide interactions (e.g. coiled coils)(1) • Stability requires cooperative bonding interactions(2) (Guenet, Thermoreversible gelation of polymers and biopolymers) o Individual non-covalent bonds are relatively weak: o Strength of covalent bond: o Hydrophobic association: o Ionic bond: o Hydrogen bond in water: o o Cooperativity: lowered energy barrier for second and subsequent bonds after first has formed o Used in biological associations
BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 · Alpha helix,beta non- cooperative interactions Unstable, no gelation cooperative interactions sheet Stable interactions, gel forms General characteristics of physical gel biomaterials o Dehydration of hydrophobes/hydrophobic association PEO-b-PPo-b-PEO, PPo-b-PEO-b-PPo(commercially known as Pluronics(BASF)) o Similar associative properties from PLGA-PEG-PLGA copolymers and PE PLGA-PEG copolymers 粥 Example blocks: Poly(ethylene glycol)(PEG) CHCH Hydrophilic B blocks Hydrophobic A blocks 。 CoacH R Poly(propylene oxide)(PPO Poly(butylene oxide)(PBO) Lecture 8 sical gels 20f12
BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Lecture 8 – Physical gels 2 of 12 • Alpha helix, beta sheet non-cooperative interactions: cooperative interactions: Unstable, no gelation Stable interactions, gel forms General characteristics of physical gel biomaterials o Dehydration of hydrophobes/hydrophobic association o Examples: • PEO-b-PPO-b-PEO, PPO-b-PEO-b-PPO (commercially known as Pluronics (BASF)) 2 o Similar associative properties from PLGA-PEG-PLGA copolymers and PEGPLGA-PEG copolymers water Hydrophilic B blocks Hydrophobic A blocks Poly(propylene oxide) (PPO) Poly(butylene oxide) (PBO) Poly(ethylene glycol) (PEG) Example blocks: CH3O CH3 O O O HO-(CH-C-O-CH-C-O-)x-(CH2-C-O-CH2-C-O)y-(CH2-CH2-O)zPLGA CH3O CH3 O O O (CH-C-O-CH-C-O-)x-(CH2-C-O-CH2-C-O)y-H PEG PLGA PEO PPO PEO
BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 Poly(N-isopropylacrylamide) CHe-CH ordered water molecules minimize water-hydrophobe contacts) Hydroxypropylmethyl cellulose(natural biopolymer) Hydroxypropyl groups dehydrate to associate and form a gel o Micellar packing Pluronics PEO-PPo-PEo block copolymers Cubic lipid gel phases(3) 8小 FIG 1. Schematic model of a bicontinuous cubic phase composed monoolein wate a membrane protein, The matrix consists of Fig.2. Structure of glyceryimonoleate-water cubic phase in three and w conr ydrophobic proteins dillie latelein or mono dimensions with inset showing the lipid bilayer. (Adapted with cating aqueous channel system(see texi modifications from Refs. 14, 39) Sara bu straat 25, 1055 KV Amsterdam, The Ne (Landau and Rosenbusch 1996(4)) Lecture 8 sical gels 3of12
BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Lecture 8 – Physical gels 3 of 12 • Poly(N-isopropylacrylamide) ordered water molecules (minimize water-hydrophobe contacts) Dehydration allows water to disorder (entropically-driven) ∆S = Sdehydrated - Shydrated > 0 • Hydroxypropylmethyl cellulose (natural biopolymer) o Hydroxypropyl groups dehydrate to associate and form a gel o Micellar packing o Examples: Pluronics PEO-PPO-PEO block copolymers Cubic lipid gel phases(3) (Landau and Rosenbusch 1996(4))
BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 micelle 崇 5 mm http://www.ecs.umass.edw/hamilton/matthewlle(micellarcrystalfigurefromwebsiteMatthewBhatia,andRoberts o 5mm hydrogel shown above is a composite box formed by Sciperio printer from Pluronic F-127 and PPF-CO-PEG o Hydrogen bonding o Hydrogen bonds can form between H and C, N, o, and F o Examples Poly (vinyl alcohol) Poly(vinyl alcohol)/PEO blends N一H tcm-H- 一N~o…H-N o Polymers that can form hydrogen bonded gels(5) Poly(vinyl alcohol) Gelatin(natural biopolymer) From Sigma pro o Gelatin is a heterogeneous mixture of water-soluble proteins of high average molecular weights, present in collagen. The proteins are extracted by boiling skin, tendons, ligaments, bones, etc in water. Type A used as a stabilizer adhesives, cements, lithographic and printing inks, plastic compounds s thickener and texturizer in foods in the manufacture of rubber substitutes artificial silk, photographic plates and films, matches, and light filters for Lecture 8 sical gels 4of12
BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Lecture 8 – Physical gels 4 of 12 5 mm PEO PPO PEO (micellar crystal figure from website, Matthew, Bhatia, and Roberts; http://www.ecs.umass.edu/hamilton/matthew_julie.htm) o 5mm hydrogel shown above is a composite box formed by Sciperio printer from Pluronic F-127 and PPF-co-PEG o Hydrogen bonding o Hydrogen bonds can form between H and C, N, O, and F o Examples: Poly(vinyl alcohol) Poly(vinyl alcohol)/PEO blends o Polymers that can form hydrogen bonded gels(5): Poly(vinyl alcohol) Gelatin (natural biopolymer) o From Sigma product sheet: o Gelatin is a heterogeneous mixture of water-soluble proteins of high average molecular weights, present in collagen. The proteins are extracted by boiling skin, tendons, ligaments, bones, etc. in water. Type A used as a stabilizer, thickener and texturizer in foods; in the manufacture of rubber substitutes, adhesives, cements, lithographic and printing inks, plastic compounds, artificial silk, photographic plates and films, matches, and light filters for
BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 mercury lamps; in textiles; to inhibit crystallization in bacteriology and prepare cultures; in PCR hybridization in molecular biology; in the pharmaceutical industry as a suspending agent, encapsulating agent and tablet binder; and in veterinary applications as a plasma expander and o Percec: form hydrogels by H-bonding between water-insoluble short chains and long water-soluble end 4 cowned on moose noea epon lonic bondi o Sodium alginate (7)(Grant, Morris, FEBS Lett. 32, 195 (1973)) Crosslinked by divalent cations, forming salt bridges o Sensitive to salt concentration in physiological locations rosslinked by blending with cationic polymer o E.g. chitosan, polylysine Used extensively for gentle cell encapsulation Lecture 8 sical gels 5of12
BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Lecture 8 – Physical gels 5 of 12 mercury lamps; in textiles; to inhibit crystallization in bacteriology and prepare cultures; in PCR hybridization in molecular biology; in the pharmaceutical industry as a suspending agent, encapsulating agent and tablet binder; and in veterinary applications as a plasma expander and hemostatic sponge. o Percec: form hydrogels by H-bonding between water-insoluble short chains and long water-soluble chains (Percec and Bera(6)) o Ionic bonding o Examples: o Sodium alginate(7) (Grant, Morris, FEBS Lett. 32, 195 (1973)) Crosslinked by divalent cations, forming salt bridges o Sensitive to salt concentration in physiological locations Crosslinked by blending with cationic polymer o E.g. chitosan, polylysine Used extensively for gentle cell encapsulation
BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 cationic polymer divalent cations o Crystallizing segments o Examples Isotactic Polyvinyl alcohol Merrill/Bray(8)) Isotactic Poly(methacrylic acid) ON BOARD o Protein interactions Avidin cross-linked particle networks( 9) Associating alpha helices(Wang et al. coiled-coil peptide cross-links Lecture 8 sical gels 6of12
BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Lecture 8 – Physical gels 6 of 12 n +- Ca++ - - Salt bridge Divalent cations Alginate (polysaccharide) + cationic polymer + divalent cations e.g. chitosan (cationic polysaccharide), polylysine o Crystallizing segments o Examples: Isotactic Polyvinyl alcohol (Merrill/Bray(8)) Isotactic Poly(methacrylic acid) ON BOARD: o Protein interactions o Examples: Avidin cross-linked particle networks(9) Associating alpha helices (Wang et al.): coiled-coil peptide cross-links
BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 Batr LA-PEG micmparticle Roure 1 stuctural representIon of the hybrid hydrog primary chains and the wth transtionmetal ions, such asNtowhch the termind histidine residues nhe coled co are tached. A wewramene coled col (not dawn to scale. isting of wwo paa mers assoc sting in an ant paralel fashion, sshowm Fig. 1. a schematic representation of scaffold sel hes a an example of many of he poss ible aonbormions Lecture 8 sical gels 7of12
BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Lecture 8 – Physical gels 7 of 12
BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 Structure of Associating Block Copolymer Hydrogels Pluronics( trade name, BASF)are an important class of hydrophobically-associating block copolymers: FDA approved for use in vivo Starting at relatively low concentrations, Hydrophilic-hydrophobic block copolymers form micelles upon passing a critical concentration(cmc)or temperature(cmt) O ON BOARD unimers micelles doweromicelle ncreasing c, T Core-shell micelle s Hydrophilic block o At higher concentrations, micelles overlap and gelation can occur o Micelles pack together Interactions between micelles depend on structure of block copolymer 路 Intermicelle physical cross links Entanglement of packed micelle coronas Lecture 8 sical gels 8of12
BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Lecture 8 – Physical gels 8 of 12 Structure of Associating Block Copolymer Hydrogels4 Pluronics (trade name, BASF) are an important class of hydrophobically-associating block copolymers: FDA approved for use in vivo Starting at relatively low concentrations, Hydrophilic-hydrophobic block copolymers form micelles upon passing a critical concentration (cmc) or temperature (cmt)2 o ON BOARD: Increasing c, T Hydrophobic block Hydrophilic block unimers micelles ŌflowerÕ micelle Core-shell micelle o At higher concentrations, micelles overlap and gelation can occur: o Micelles pack together Interactions between micelles depend on structure of block copolymer: Intermicelle physical cross-links Entanglement of packed micelle coronas
BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 UQUID POLYMER CONCENTRATION .100000.100,100.000.100100000.10 FIG 20 Temperature-concentration phase diagram showing the characteristics of EasPanEzs fully dissolved in D O. In the low-concentration, low temperature range. the /temperature, a liquid of micelles is formed with micellar volume f10m时钟是 shear plane of 25% solution Ens PaoE25. The three columns 25C, T-27 C, and T=68C, (Source: from (391. with permission.) Transition range: micelles in equlibrium with unimers Experiments by Hatton group at MIT PEO-PPO-PEO micellization at 3 different temperatures measured a by adding a hydrophobic dye that absorbs UV light when bound in a hydrophobic environment(e.g micelle core) but not free in solution logC %o w/v) Figure 3. Effect of concentration, with temperature eter, on the absorption intensity of DPh at 356 nm in solutions of pluronic p104. The critical micellization trations can be estimated from the first break in the Block length determines gel structure H-+OCH2 CH2 .C P 122 FIG 14 Phase diagrams of the systems of (EO)( PO)(EO) block copolymers with y 69 and x increasing from 5 to 106. (Source: from [13], with permis Lecture 8 sical gels 9of12
BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Lecture 8 – Physical gels 9 of 12 Experiments by Hatton group at MIT: PEO-PPO-PEO micellization at different temperatures measured by adding a hydrophobic dye that absorbs UV light when bound in a hydrphobic environment (e.g. micelle core) but not free in solution Transition range: micelles in equilibrium with unimers o Block length determines gel structure (Chu 1996)
BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 Relation between structure and applications in bioengineering o Cubic phase gels studied most often(pluronic F-127 at concentrations circa 20% w/vol) Hydrogel scaffolds for tissue engineering of cartilage (10) Cubic phase gels erode by surface dissolution Provides zero-order drug release(11) Cubic phase gel drug depots E;:〓 micelle drug nanocarriers 1050nm Drugs can be incorporated into micelles as nano-carriers o E.g. Kim et al. PEO-PLGA-PEo block copolymers for drug delivery(Nature Thermodynamics of Hydrophobic Association vs H-Bonding Gels2 CsT polymers(5 o Amphiphilic copolymers like PEO/PPO block copolymers associate on increasing temperature o They belong to a class of materials exhibitin LCST(lower critical solution temperature)behavior o LCST materials phase separate from their solvent with increasing temperature, in contrast to the more common UCST materials, that phase separate at low temperatures ON BOARD UCST CST PS P+ S PS 0 Mole %b100 0 Mole %b 100 s= polymer solution P+s=two-phase region: polymer-rich, polymer-poor Lecture 8 sical gels 10of12
BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Lecture 8 – Physical gels 10 of 12 • Relation between structure and applications in bioengineering o Cubic phase gels studied most often (pluronic F-127 at concentrations circa 20% w/vol) o Applied to: Hydrogel scaffolds for tissue engineering of cartilage(10) Drug delivery • Cubic phase gels erode by surface dissolution o Provides zero-order drug release(11) 10-50 nm Micelle drug nanocarriers Cubic phase gel drug depots • Drugs can be incorporated into micelles as nano-carriers o E.g. Kim et al. PEO-PLGA-PEO block copolymers for drug delivery (Nature paper)(12) Thermodynamics of Hydrophobic Association vs. H-Bonding Gels2,5 LCST polymers(5) o Amphiphilic copolymers like PEO/PPO block copolymers associate on increasing temperature o They belong to a class of materials exhibitin LCST (lower critical solution temperature) behavior o LCST materials phase separate from their solvent with increasing temperature, in contrast to the more common UCST materials, that phase separate at low temperatures: ON BOARD: T 0 Mole % B 100 T 0 Mole % B 100 UCST LCST P + S PS PS = polymer solution P + S = two-phase region: polymer-rich, polymer-poor PS P + S
