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 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 PEG- PLGA-PEG copolymers water Hydrophilic B blocks Hydrophobic A blocks Poly(propylene oxide) (PPO) Poly(butylene oxide) (PBO) Poly(ethylene glycol) (PEG) Example blocks: CH 3 O CH 3 O O O HO-(CH-C-O-CH-C-O-) x -(CH 2 -C-O-CH 2 -C-O) y -(CH 2 -CH 2 -O) z - PLGA CH 3 O CH 3 O O O (CH-C-O-CH-C-O-) x -(CH 2 -C-O-CH 2 -C-O) y -H PEG PLGA PEO PPO PEO 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 = S dehydrated - S hydrated > 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 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 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 Lecture 8 – Physical gels 6 of 12 n + - Ca ++ - - Salt bridge Divalent cations Alginate (polysaccharide) + divalent cations+ cationic polymer 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 Lecture 8 – Physical gels 7 of 12 BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Lecture 8 – Physical gels 8 of 12 Structure of Associating Block Copolymer Hydrogels 4 ? 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 ōflower? 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 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 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 Gels 2,5 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 BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Lecture 8 – Physical gels 11 of 12 o Homopolymers can also exhibit LCST behavior o E.g. poly(N-isopropylacrylamide) ? Soluble in water at low temperatures, but collapses at 32 deg. C o Thermodynamically, any polymer that phase separates from water on increasing temperature must be driven by entropy o Go into theory? Associating block copolymers(13, 14) o The association of hydrophobic blocks in aqueous solution is described by the closed association model o Micelles form in a transition from unimers directly to multimolecular structures, with no intermediate bi-mer/tri- mer/etc. o Examining the driving force for copolymer self-assembly o What is the free energy change for formation of micelle structures? Eqn 1 ?G 0 =G micelle ?G unimer cmc /cmt =?H 0 ?T?S 0 = RTlnc cmc Foundations of colloid Science R.J. Hunter Eqn 2 ?H 0 = R ?lnc cmc ?(1/T) ? ? ? ? ? ? T,P o Association of hydrophobic blocks driven by increasing concentration is exactly equivalent to association driven by increasing temperature, thus: () () ( ) () cmt cmc T c T c /1 ln /1 ln ? ? = ? ? (determined experimentally) o From X, a plot of ln c vs 1/T cmt allows ?H 0 to be determined:(15) o Positive slope of 1/T cmt plot indicates ?H 0 is positive: but ?G 0 is negative (micellization is a spontaneous process at the cmc/cmt) ? Thus, association is enthalpically unfavorable ? ? BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Lecture 8 – Physical gels 12 of 12 ? Driven by favorable entropy change ? Ordered water around hydrophobic groups is released to disorder if hydrophobic blocks associate, and this entropy gain outweighs the entropy penalty for confining the blocks to the micelle core o Consistent with direction of transition temperature change: ? Any entropically-driven process will occur with increasing temperature, since G = H - TS o Entropy/hydrophobic effect-driven gelation contrasts with hydrogen-bonding association of gels ? Hydrogen bonded gels break cross-links with increasing temperature (gel formed driven by enthalpy gain on H-bonds) ? H-bonds weak; thermal energy can become greater than bonding energy ? H-bonding transition gels (gels dissociate with increasing temperature): Nature 349, 400 (1991) ? Opposite temperature behavior- gel is dissociated/swells with increasing temperature ? E.g. gelatin/PVA are fluid at elevated temperatures and gelled at lower temperatures, while pluronics are fluid at low temperature and gel at elevated temperatures References 1. Wang, C., Stewart, R. J. & Kopecek, J. (1999) Nature 397, 417-20. 2. Guenet Thermoreversible Gelation of Polymers and Biopolymers, New York). 3. Shah, J. C., Sadhale, Y. & Chilukuri, D. M. (2001) Adv Drug Deliv Rev 47, 229-50. 4. Landau, E. M. & Rosenbusch, J. P. (1996) Proc Natl Acad Sci U S A 93, 14532-5. 5. Ron, E. S. & Bromberg, L. E. (1998) Adv Drug Deliv Rev 31, 197-221. 6. Percec, V., Bera, T. K. & Butera, R. J. (2002) Biomacromolecules 3, 272-9. 7. Kuo, C. K. & Ma, P. X. (2001) Biomaterials 22, 511-21. 8. Bray, J. C. & Merrill, E. W. (1973) Journal of Applied Polymer Science 17, 3779-3794. 9. Salem, A. K., Rose, F. R. A. J., Oreffo, R. O. C., Yang, X., Davies, M. C., Mitchell, J. R., Roberts, C. J., Stolnik- Trenkic, S., Tendler, S. J. B., Williams, P. M. & Shakesheff, K. M. (2003) Advanced Materials 15, 210-213. 10. Cao, Y., Rodriguez, A., Vacanti, M., Ibarra, C., Arevalo, C. & Vacanti, C. A. (1998) J Biomater Sci Polym Ed 9, 475-87. 11. Zhang, L., Parsons, D. L., Navarre, C. & Kompella, U. B. (2002) J Control Release 85, 73-81. 12. Jeong, B., Bae, Y. H., Lee, D. S. & Kim, S. W. (1997) Nature 388, 860-2. 13. Chu, B. & Zhou, Z. (1996) in Nonionic Surfactants: Polyoxyalkylene Block Copolymers, ed. Nace, V. M. (Marcel Dekker, New York), pp. 67-143. 14. Chu, B. (1995) Langmuir 11, 414-421. 15. Alexandridis, P., Holzwarth, J. F. & Hatton, T. A. (1994) Macromolecules 27, 2414-2425.