BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 Lecture 7: Hydrogel Biomaterials: Structure and Physical Chemistry Last Day: programmed/regulated/multifactor controlled release for drug delivery and tissue engineering Toda Applications of hydrogels in bioengineering Covalent hydrogels structure and chemistry of biomedical gels Thermodynamics of hydrogel swelling Readin N.A. Peppas et aL., "Physicochemical foundations and structural design of hydrogels in medicine and biology, Annu. Rev. Biomed Eng, 2, 9-29(2000) Supplementary Reading: P.J. Flory, Principles of Polymer Chemistry, Cornell University Press, Ithaca, pp 464 469, pp. 576-581 Statistical thermodynamics of networks and network swelling) Applications of hydrogels in bioengineering Hydrogels: insoluble network of polymer chains that swell in aqueous solutions Gels can be classified by the type of crosslinker Covalent covalent junctions Physical non-covalent junctions Lecture 7-Hydrogels 1 1of15
BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Lecture 7: Hydrogel Biomaterials: Structure and Physical Chemistry Last Day: programmed/regulated/multifactor controlled release for drug delivery and tissue engineering Today: Applications of hydrogels in bioengineering Covalent hydrogels structure and chemistry of biomedical gels Thermodynamics of hydrogel swelling Reading: N.A. Peppas et al., ‘Physicochemical foundations and structural design of hydrogels in medicine and biology,’ Annu. Rev. Biomed. Eng., 2, 9-29 (2000). Supplementary Reading: P.J. Flory, ‘Principles of Polymer Chemistry,’ Cornell University Press, Ithaca, pp. 464- 469, pp. 576-581 (Statistical thermodynamics of networks and network swelling) Applications of hydrogels in bioengineering • Hydrogels: insoluble network of polymer chains that swell in aqueous solutions • Gels can be classified by the type of crosslinker:1 • Covalent - covalent junctions • Physical - non-covalent junctions Lecture 7 – Hydrogels 1 1 of 15
BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 Physical gels: example-Hydrophobic interactions in physical gels Physical gels are formed by noncovalent cross-links Example blocks: Poly(ethylene glycol)(PEG) Hydrophilic B blocks 3} Hydrophobic A blocks Poly(propylene oxide)(PPO) Poly(butylene oxide)(PBO) Key properties of gels for bioengineering applications 1. in situ formability 2. degradability 3. responsⅳ e swelling 4. tissue-like structure/properties In situ formability Gelation of liquid solutions by Irradiation with light Temperature change(e.g. 4.C to 37C) Cross-linking enzymes Presence of divalent salts ON BOARD In situ formation Heat ACrosslinking by enzymes ntroduction of divalent cations(e.g. Ca*, Mg*) Lecture 7-Hydrogels 1 20f15
BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Physical gels: example- Hydrophobic water interactions in physical gels Physical gels are formed by noncovalent Example blocks: cross-links Poly(ethylene glycol) (PEG) Hydrophilic B blocks Hydrophobic A blocks Poly(propylene oxide) (PPO) Poly(butylene oxide) (PBO) • Key properties of gels for bioengineering applications: 1. in situ formability 2. degradability 3. responsive swelling 4. tissue-like structure/properties • In situ formability Gelation of liquid solutions by: • Irradiation with light • Temperature change (e.g. 4°C to 37°C) • Cross-linking enzymes • Presence of divalent salts ON BOARD: In situ formation ¥hν ¥Heat ¥Crosslinking by enzymes ¥Introduction of divalent cations (e.g. Ca++, Mg++) Lecture 7 – Hydrogels 1 2 of 15
BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 Key properties of hydrogels for bioengineering applications: example: rintableggels printing heads provide470°C dispensing Temperature-controlled stage (Irvine lab) ON BOARD gra Gel with degradable cross links or network chains Eliminable/metabolizable Basis of sensors and 'smart materials (to be covered later) Lecture 7-Hydrogels 1 3of15
BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Key properties of hydrogels for bioengineering applications: example: ÔprintableÕgels ∆T Chilled/heated (Landers et al. 2002) printing heads provide 4-70°C dispensing Temperature-controlled stage (Irvine lab) • Degradability ON BOARD: Degradability ¥Hydrolysis ¥Enzymatic attack Gel with degradable cross- Eliminible/metabolizable links or network chains Water-soluble fragments • Responsive swelling Temperature-, pH-, and molecule-responsive swelling Basis of sensors and ‘smart’ materials (to be covered later) Lecture 7 – Hydrogels 1 3 of 15
BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 ON BOARD esponsive swell ￥ADH ￥△T ￥△c( change in concentration of a molecule Tissue-like structure/properties Form swollen networks similar to collagen, elastin, proteoglycans General areas of application in bioengineering Controlled release ON BOARD Controlled rele Tissue barriers(Hubbell? Prevent thrombosis(vessel blocked by coagulating platelets)and restenosis(re-narrowing of blood vessel after operation) in vessels after vascular injurylangioplastyletc. Prevent tissue-tissue adhesion after an operation Tissue barriers and conformal coatings Adsorbed layer of Blood vessel vessel Photoinitiator solution Two layers of Green laser formed in situ 2=H-c-0(H)c-c=o2 late solutio (An and Hubbell 2000) Lecture 7-Hydrogels 1 40f15
BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Lecture 7 – Hydrogels 1 4 of 15 ON BOARD: Responsive swelling ¥ ∆pH ¥ ∆T ¥ ∆c (change in concentration of a molecule • Tissue-like structure/properties Form swollen networks similar to collagen, elastin, proteoglycans • General areas of application in bioengineering: • Controlled release ON BOARD: Controlled release • Tissue barriers (Hubbell2,3) Prevent thrombosis (vessel blocked by coagulating platelets) and restenosis (re-narrowing of blood vessel after operation) in vessels after vascular injury/angioplasty/etc. Prevent tissue-tissue adhesion after an operation Tissue barriers and conformal coatings (An and Hubbell 2000) Adsorbed layer of Blood photoinitiator vessel Photoinitiator solution PEG-diacrylate solution Green laser 3) 2) 1) Two layers of hydrogel formed in situ vessel
BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 TE scaffolds/cell encapsulation/immunoisolation. 5 Colloidal crystal template Poly(methyl methacrylate) microspheres 1. Perfect connectivity for Filling of the interstices cell migration atrix or wi 2. Improved nutrient nanoparticles Hydrogel 3. No dead volume precursor polymerize Ordered Dissolve porous microspheres 签签 structure Structured porous replica A M. Lenhoff. CuT. Hydrogel @verse opals Optical micrograph/20 um pores Fluorescence micrograph/60 um pores Biosensors( to be covered later) Contact lenses Lecture 7-Hydrogels 1 5of15
BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 • TE scaffolds/cell encapsulation/immunoisolation4,5 Poly(methyl methacrylate) microspheres Perfect connectivity for cell migration Improved nutrient transport No Ôdead volumeÕ O.D. Velev and A.M. Lenhoff, Curr. Opin. Coll. Interf. Sci. 5, 56 (2000) Advantages: 1. 2. 3. Hydrogel precursor polymerize Dissolve microspheres Ordered porous structure Hydrogel Ôinverse opalsÕ Optical micrograph/20 µm pores 60 µm Fluorescence micrograph/60 µm pores • Biosensors (to be covered later) • Contact lenses Lecture 7 – Hydrogels 1 5 of 15
BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 Structure of covalent hydrogel biomaterials Chemical and physical structure Structure and swelling of hydrogel materials network chain polymerization solution hydrogel dilution swelling Networks formed by stitching together monomers in aqueous solutions via cross-linkers that are multifunctional units o Draw an example of a crosslinker: bisacrylamide networks from hydrophilic vinyl monomers hydroxyethyl methacrylate poly(ethylene glycol)methacrylate acrylic acid acrylamide, N-isopropylacrylamide Common crosslink PEGDMA EGDMA Hydrogels undergo swelling in analogy to dilution of free polymer chains in solution o Difference lies in limit to'dilution when chains are cross-linked together(ENTROPIC Poly (2-hydroxyethyl methacrylate)hydrogels One of the first biomedical hydrogels; applied to contact lenses in late 1950s Lecture 7-Hydrogels 1 6of15
BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Structure of covalent hydrogel biomaterials Chemical and physical structure Structure and swelling of hydrogel materials Ôineffective network = = = x = x x x x x x chainÕ Ô effective network chainÕ = x = = = x = polymerization solution hydrogel dilution swelling • Networks formed by stitching together monomers in aqueous solutions via cross-linkers that are multifunctional units o Draw an example of a crosslinker: bisacrylamide • networks from hydrophilic vinyl monomers • hydroxyethyl methacrylate • poly(ethylene glycol) methacrylate • acrylic acid • acrylamide, N-isopropylacrylamide • Common crosslinkers: PEGDMA, EGDMA bis-acrylamide • Hydrogels undergo swelling in analogy to dilution of free polymer chains in solution o Difference lies in limit to ‘dilution’ when chains are cross-linked together (ENTROPIC) • Poly(2-hydroxyethyl methacrylate) hydrogels 6 • One of the first biomedical hydrogels; applied to contact lenses in late 1950s Lecture 7 – Hydrogels 1 6 of 15
BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 PEGDMA-CO-PHEMA -CH2-CH-CH2-C-CH2-CH-CH =0c=0C=0 CH2 CH2 CH2 CH2 CH2CH2 CH2 CH OH O 。 -CH2-CH-CH2-C-CH2-CH-CH2-CH- C=O C=O C=O o Free water o Interfacial water o Bound water -Polymer chains (Chiellini et al. Interpenetrating networks Useful for obtaining gels with properties in between two different materials o E.g. mix a swelling polymer with a temperature- or pH-responsive polymer to obtain networks that have a defined amount of swelling in response to changes in temperature or pH nitrating network o Sem-interpenetrating networks: second component is entangled with first network but not cross-linked Biological recognition of hydrogels Inclusion of peptide- functionalized co-monomers allows hydrogels to have tailored biological recognition roperties similar to solid degradable polymers o Promoting cell adhesion Incorporating biological recognition: dhesion NR6 fibro blast adhesion on PEg-rGD PEG WGRGD (no cell adhesion on ligand-free hydrogels) Lecture 7-Hydrogels 1 7of15
BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 PEGDMA-co-PHEMA CH3 CH3 CH3 CH3 -CH2-CH-CH2-C-CH2-CH-CH2-CHC=O O CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 OH CH2 CH2 O C=O -CH2-CH-CH2-C-CH2-CH-CH2-CHC=O C=O O O CH2 CH2 CH2 CH2 CH2 CH2 OH OH (Chielline et al.7 ) Interpenetrating networks • Useful for obtaining gels with properties in between two different materials o E.g. mix a swelling polymer with a temperature- or pH-responsive polymer to obtain networks that have a defined amount of swelling in response to changes in temperature or pH Interpenetrating networks x x x x x o Sem-interpenetrating networks: second component is entangled with first network but not cross-linked Biological recognition of hydrogels • Inclusion of peptide-functionalized co-monomers allows hydrogels to have tailored biological recognition properties similar to solid degradable polymers o Promoting cell adhesion: Incorporating biological recognition: peptides photopolymerization = PEG -WGRGDSP adhesion sequence NR6 fibroblast adhesion on PEG-RGD hydrogel (no cell adhesion on ligand-free hydrogels) C=O C=O C=O O O O O OH OH C=O O OH Lecture 7 – Hydrogels 1 7 of 15
BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 o Promoting remodeling/cell migration through synthetic networks PEG PEG HcHo. 定长 B K. Mann. AS. Gobin. A.T. Tsai Schmedlen. J. L West. Biomaterials 22 Example synthesis strategy: photoencapsulation of live cells Photoencapsulation: expose solution of cells, prepolymer/cross-linkermonomer, and photoinitiator to light to initiate free radical polymerization In sterile culture media Cyclohexyl phenyl ketone Provides very rapid polymerization(2-20 seconds typical), at neutral pH and room temp. -37C soft UV photoinitiators are common and non-toxic(illuminate at 365 nm) Lecture 7-Hydrogels 1 8of15
BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 o Promoting remodeling/cell migration through synthetic networks: PEG photopolymerization = = PEG peptides -GWGLGPAGK- -CH2CH2Ocollagenase sequence collagenase B.K. Mann, A.S. Gobin, A.T. Tsai, R.H. Schmedlen, J.L. West, Biomaterials 22, 3045 (2001) Example synthesis strategy: photoencapsulation of live cells5 • Photoencapsulation: expose solution of cells, prepolymer/cross-linker/monomer, and photoinitiator to light to initiate free radical polymerization = = = = = = = = = = = = = = = = In sterile culture media: hν Cyclohexyl phenyl ketone: UV hν • Provides very rapid polymerization (2-20 seconds typical), at neutral pH and room temp. – 37°C • ‘soft’ UV photoinitiators are common and non-toxic (illuminate at 365 nm) Lecture 7 – Hydrogels 1 8 of 15
BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 Cells can be entrapped with high lability 4. 8. UV lamp Collagen 3 gAG %ww)0.2 (W ww liquid H山 ⊥LL 810121416 lel Figure 3. Biochemical analysis. Evolution of GAG and total collagen contents (%o wet weight) over 14 days of photon- capsulated bovine chondrocytes Example Biomedical Hydrogel Materials Formed from hydrophilic biocompatible polymers, often polymers that can be safely eliminated by the body if the breaks down medicine CH 小(tk内c[G wl(ethylene lysol) monomethyl ether[PEGME) R Contact Lenses General methacrylates PHEMAlpolyfetby lene temphhalateIPTPEI Artifcial Tendons Membranes for plasmapheresis HEMA-b-aloxane) xual epan reconstraction PVA poly(ar b acid [PAAL, poly Ophthal appliation PA HEMA MMA Articular Cartilage Cantreard Drug Deliery Poly(glycolic aod)[ HGAL Poly( lacte arid (PLAL TAHL. 1 IComtieundH polycyanoucn, fumane acid- PEGi, seba acid 1,3-bopcarboxypbraaxyl propane[P(CFP-SA) Complexing hydrogels ALAA-g-EGL polYacrylic acid-grafted-pol PHEMA, P\A PNVE polylethylene-oa-vinyl acetate) ly(- isopropyl ary lami)IPNIPAAm mperalun-sensatIve PAAl Poly (actic acid [PAAl PNIPAAm PAA. PNPAAPALAA dniy甲中 bn tud tm t endk naan dd wed than Lecture 7-Hydrogels 1 9of15
BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 • Cells can be entrapped with high viability4,8: UV lamp liquid gel Example Biomedical Hydrogel Materials6 Formed from hydrophilic biocompatible polymers, often polymers that can be safely eliminated by the body if the gel breaks down. CH3 CH=C C=O O R General methacrylates Lecture 7 – Hydrogels 1 9 of 15
BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 Chemical structure of biodegradable hydrogels Mechanism A (non-degradable water-soluble polymers with degradable cross-links) Degradable cross-links o e.g. dextran hydrogels bacterial exo-polysaccharide branched polymer composed of a-1, 6-linked D-glucopyranose residues with a low of a-1, 2 and 1, 3 side chains Dextran with polylactide crosslinks: hydrolyzable crosslinks Figure 1, The chemical dextran dextran can be functionalized with methacrylate and then crosslinked in the presence of a small amount of vinyl monomer 2 EMA. I Lacto.2 o'o m Rootor .C EMA-lactvedl. s Figure 8. Reaction scheme for the synthesis of dex-lactateHEMA Figure 3. Schematic representation of the formation of dextran hydrogels degradable gels show first swelling then dissolution as cross-links are hydrolyzed Time(days) Figure 12. Swelling behavior of dex HEMA(O), dex-lactate-HEMA()and dex- lactate- hydrogels was 80%, the degree of methacryloyl substitution was approximate/y6 ent of the HEMA(▲) hydrogels queous solution (pH 7.2, 37C). The initial Lecture 7-Hydrogels 1 10of15
BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Chemical structure of biodegradable hydrogels Mechanism I: (non-degradable water-soluble polymers with degradable cross-links) • Degradable cross-links o e.g. dextran hydrogels9 • bacterial exo-polysaccharide • branched polymer composed of α-1,6-linked D-glucopyranose residues with a low % of α-1,2 and 1,3 side chainsDextran with polylactide crosslinks: hydrolyzable crosslinks9 • dextran can be functionalized with methacrylate and then crosslinked in the presence of a small amount of vinyl monomer: d degradable gels show first swelling then dissolution as cross-links are hydrolyzed: Lecture 7 – Hydrogels 1 10 of 15
