Degradable Poly(ethylene glycol) Hydrogels for 2D and 3D Cell Culture

Andrea M. Kasko

Department of Bioengineering University of California, Los Angeles 410 Westwood Plaza, 5121 Eng V Los Angeles, CA 90095 United States

akasko@ucla.edu

Introduction

Progress in biotechnology fields such as tissue engineering and drug delivery is accompanied by an increasing demand for diverse functional biomaterials. For decades, research in polymeric biomaterials has focused on testing the biocompatibility of polymers developed for other applications and/or their processing (e.g., electrospinning, solvent casting/porogen leaching, 3D printing). More recently, researchers have shifted towards synthesizing materials specifically for biomedical uses, including synthetic proteins, glycomimetics and polymers compatible with aqueous media, along with chemical modification of naturally occurring polymers (e.g., to allow gelation or increased in vivo stability). In the past decade, polymer chemists have created a niche for designed biomaterials to use as cell scaffolds and to deliver therapeutic agents.

One class of biomaterials that has been the subject of intense research interest is hydrogels.1 Hydrogels are extensively investigated as two- and three-dimensional scaffolds for cells because they closely mimic the natural environment of cells, both chemically and physically.2 Hydrogels can be formed from synthetic (e.g., poly(ethylene glycol), poly (hydroxyethyl methacrylate)) and naturally occurring polymers (e.g., collagen, hyaluronan, heparin),1b and are useful 3D models of tissue culture due to their high water content and ability to form in the presence of cells, proteins and DNA. Depending on the reactivity of the constituent materials, gelation can be induced using pH3, temperature4, coulombic interactions, covalent bonding, non-covalent interactions5, or polymerization.

PEG

Poly(ethylene glycol) is a hydrophilic polymer that, when cross-linked into networks, can have a high water content. PEG is a suitable material for biological applications because it does not generally elicit an immune response.6 Since the 1970s, PEG has been used to modify therapeutic proteins and peptides to increase their solubility, lower their toxicity and to prolong their circulation half-life.7 In the late 1970s, researchers began to experiment with PEG hydrogels for cell culture. PEG hydrogels are chemically well-defined, and multiple chemistries can be used both for their formation and chemical modification.

PEG Macromers

PEG is easily synthesized by the living anionic ring-opening polymerization of ethylene oxide; well-defined (low polydispersity) PEGs with a range of molecular weights and a variety of end groups (e.g., alcohol, methyl ether, amine, N-hydroxysuccinimidyl (NHS) ester) are widely available.

In order to form a hydrogel, PEG must be cross-linked. Initially, PEG was cross-linked non-specifically using ionizing radiation.8 PEG hydrogels are now typically synthesized via covalent cross-linking of PEG macromers with reactive chain ends.

PEG macromers with reactive chain ends such as acrylate, methacrylate, allyl ether, maleimide, vinyl sulfone, NHS ester and vinyl ether groups (Chart 1) are easily synthesized from readily available starting materials. The alcohol chain ends of PEG can be esterified using acid chlorides (e.g., acryloyl chloride, methacryloyl chloride) in the presence of base. PEG chain ends can be etherified under basic conditions by reaction with alkyl halides such as 2-chloroethyl vinyl ether or allyl bromide. PEG divinyl sulfone is prepared by coupling PEG to a large excess of divinyl sulfone or by a multistep process to prepare chloroethyl sulfone chain ends that undergo basic elimination to form vinyl sulfone groups9.

Poly(ethylene glycol) is a hydrophilic polymer that, when cross-linked into networks, can have a high water content. PEG is a suitable material for biological applications because it does not generally elicit an immune response.6 Since the 1970s, PEG has been used to modify therapeutic proteins and peptides to increase their solubility, lower their toxicity and to prolong their circulation half-life.7 In the late 1970s, researchers began to experiment with PEG hydrogels for cell culture. PEG hydrogels are chemically well-defined, and multiple chemistries can be used both for their formation and chemical modification.

PEG Macromers

PEG is easily synthesized by the living anionic ring-opening polymerization of ethylene oxide; well-defined (low polydispersity) PEGs with a range of molecular weights and a variety of end groups (e.g., alcohol, methyl ether, amine, N-hydroxysuccinimidyl (NHS) ester) are widely available.

In order to form a hydrogel, PEG must be cross-linked. Initially, PEG was cross-linked non-specifically using ionizing radiation.8 PEG hydrogels are now typically synthesized via covalent cross-linking of PEG macromers with reactive chain ends.

PEG macromers with reactive chain ends such as acrylate, methacrylate, allyl ether, maleimide, vinyl sulfone, NHS ester and vinyl ether groups (Chart 1) are easily synthesized from readily available starting materials. The alcohol chain ends of PEG can be esterified using acid chlorides (e.g., acryloyl chloride, methacryloyl chloride) in the presence of base. PEG chain ends can be etherified under basic conditions by reaction with alkyl halides such as 2-chloroethyl vinyl ether or allyl bromide. PEG divinyl sulfone is prepared by coupling PEG to a large excess of divinyl sulfone or by a multistep process to prepare chloroethyl sulfone chain ends that undergo basic elimination to form vinyl sulfone groups9.

End groups of different PEG macromers

Chart 1. End groups of different PEG macromers.

Macromers can be homobifunctional or heterobifunctional. Homobifunctional macromers are typically used to form networks, while heterobifunctional macromers may be used to tether a therapeutic molecule into a hydrogel network.

Mechanisms of Hydrogel Formation

The cross-linking mechanism to form hydrogels depends on the identity of the chain ends of PEG macromers. In most cases, cross-linking occurs when the reactive vinyl chain ends are polymerized, usually with a free radical initiator. For example, polymerization of macromers can be initiated using redox-generated radicals (e.g., ammonium persulfate and TEMED), or radicals generated with light (e.g., Irgacure® 651, λ=365 nm Scheme 1). Acrylate and methacrylate chain ends undergo chain polymerization. In step growth network formation, a multifunctional (f>2) cross-linker reacts with the PEG chain ends in a stoichiometric manner; alternatively, multifunctional PEGs (f>2) can be crosslinked with difunctional crosslinkers (Scheme 1). Acrylate, methacrylate, vinyl sulfone, maleimide, vinyl ether and allyl ether are all capable of step growth network formation, through conversion to thiols depending on reaction conditions. Typical cross-linkers may include thiol or amine moieties. Mixed-mode polymerizations are the result of both mechanisms occurring in the same reaction vessel; acrylate and methacrylate groups can undergo mixed mode network formation. Both mechanisms of hydrogel formation can be used to encapsulate live cells, and both mechanisms allow for the reactive incorporation of peptides, proteins and other therapeutics.

Chain growth and step growth reactions

Scheme 1. Chain growth and step growth reactions.

The mesh structure that results from different mechanisms is depicted in Figure 1. In chain growth networks, a kinetic chain is formed at the crosslink site, while in step growth networks, the crosslink sites bear the same functionality as the multifunctional cross-linker, neglecting defects. In both chain and step growth, network defects such as loops, permanent entanglements and dangling chain ends may exist. The chemical identity of the macromer and the mechanism of hydrogel formation are both important as each influences the cross-link density of the hydrogel network. Material properties that are important to 2D and 3D culture are easily controlled through the chemistry of hydrogel formation. As cross-linking density increases, mesh size decreases, swelling ratio decreases, and storage modulus increases. Varying the molecular weight of the PEG macromer results in coarse control over hydrogel properties (large differences in cross-linking density). Varying the reaction mechanism used to produce the hydrogels results in fine control over hydrogel properties (can be used to tune cross-linking density of a system).

Formation mechanism affects hydrogel network structure

Figure 1. Formation mechanism affects hydrogel network structure and network defects.

Degradeable Hydrogels

In order to use 3D hydrogel scaffolds to study cell differentiation and tissue evolution, it is critical to be able to control the physical and chemical properties of the gel in a spatially and temporally controlled manner.10 Polymeric material properties are typically changed through polymerization/cross-linking (bond forming events) or through controlled degradation and/or release (bond breaking events). Bond forming events typically often use small molecule reagents (initiators, catalysts, monomers, ligands to be conjugated to the material) while bond breaking typically does not rely on exogenous reagents. Small molecules often have more adverse effects in vitro and in vivo than polymeric reagents so many research groups use degradation as a tool for in situ manipulation of polymeric biomaterials.

Hydrolytic Degradation

The mechanism of degradation most commonly utilized in hydrogels is hydrolysis, in which a molecule of water adds to the polymer backbone, causing chain scission. Anhydrides, esters and amides are all susceptible to hydrolysis. Anyhydrides typically hydrolyze too quickly, and the uncatalyzed hydrolysis of amides is too slow, so most hydrogels that degrade hydrolytically utilize ester linkages. In order to obtain hydrolytically degradable hydrogels with physiologically relevant time scales of degradation, researchers typically functionalize PEG with degradable ester linkages using lactide or glycolide segments.

Alcohol chain ends on PEG can initiate ring-opening reactions of 3,6-dimethyl-1,4-dioxane-2,5-dione and 1,4-Dioxane-2,5-dione to generate PEG-lactide and PEG-glycolide, respectively (Scheme 2).11 The ring-opening reaction is typically catalyzed by tin(II)-2-ethylhexanoate,12 although the reaction is also easily accomplished using dimethylaminopyridine as a catalyst,13 which may be easier to remove than the residual tin. The alcohol chain ends of PEG-lactide or PEG-glycolide are easily functionalized with reactive double bonds such as acrylate and methacrylate.

Synthesis of PEG-lactide and PEG-glycolide

Scheme 2. Synthesis of PEG-lactide and PEG-glycolide.

Enzymatic Degradation

Although ester linkages are enzymatically degradable, most researchers utilize sequence-specific enzymatic degradation of peptides incorporated into hydrogels rather than non-specific enzymatic degradation of esters and amides. Hubbell′s group pioneered this approach16 by incorporating matrix metalloproteinase (MMP) sensitive linkages into hydrogels via Michael addition of cysteine-functionalized peptides across acrylates, maleimides and vinyl sulfones (Scheme 3).17

MMP-degradable linkages have also been used to tether therapeutic agents into hydrogels. For example, growth factors such as vascular endothelial growth factor (VEG-F) can be released via enzymatic degradation of an MMP-sensitive tether to induce angiogenesis.18

In both hydrolysis and enzymolysis, the rate of degradation is predetermined by the chemistry of the macromer. In hydrolysis, the degradation rate of the material is pre-engineered through the identity (e.g., hydrophobicity or hydrophilicity) and number of the hydrolysable groups, and cannot be changed once the material is fabricated. In enzymolysis, the degradation typically occurs in an area local to the cells producing the enzyme. While hydrolysis and enzymolysis are both effective methods for sustained hydrogel degradation and sustained release of therapeutic agents, the rate of release cannot be adjusted or arrested after the hydrogel is fabricated, and release is not spatially controlled.

Enzymatically degradable hydrogels

Scheme 3. Enzymatically degradable hydrogels via Michael addition of cysteinecontaining peptides to vinyl sulfone groups.

Photodegradable Hydrogels

In contrast to hydrolytically and enzymatically degradable linkages, photodegradable linkages allow precise spatial and temporal control over degradation and release. While many researchers have reported photopolymerizable hydrogels, and photofunctionalizable hydrogels, very few reports exist of biocompatible photodegradable hydrogels. Kloxin and Kasko reported photodegradable hydrogel networks formed from 2-methoxy-5-nitro-4-(1-hydroxyethyl) phenoxybutanoate-containing PEG macromers (Scheme 4)19; the photodegradation behavior of the ortho-nitrobenzyl (o-NB) linker group is well-characterized. Hydrogels formed from the photodegradable macromer show bulk degradation upon exposure to light that is dependent on exposure time, wavelength, and light intensity. When the light is shuttered, degradation is arrested; the sample continues photolyzing once light exposure resumes. hMSCs (human mesenchymal stem cells) encapsulated in a hydrogel containing the photo-releasable cell-adhesive ligand RGDS (Arg-Gly-Asp-Ser) differentiate down the chondrogenic pathway when the RGD is released at day ten (corresponding to the downregulation of fibronectin during chondrogenesis). Surface erosion and through-gel lithography of this degradable hydrogel can be used to form features over a range of lengths scales, from 10-7 m to 10-2 m or larger.20 Partial degradation in a local area results in decreased cross-link density and increased swelling, providing a means to etch softer features onto a hydrogel that protrude out from the gel.

Photodegradable o-NB moieties

Scheme 4. Photodegradable o-NB moieties incorporated into hydrogel backbone and for therapeutic agent release.

In addition to single photon photolysis, the o-NB containing hydrogels are also susceptible to two-photon photolysis, allowing for 3D etching.19-20 In single photon reactions, any area exposed to the light will react. In contrast, multi-photon lithography should occur only where multiple photons are simultaneously absorbed, which occurs at the focal volume of the light source (inset). Typical wavelengths in single photon lithography of biomaterials range from long wave UV (≥365 nm) into the visible region, while two-photon lithography uses IR light (typically ~740–800 nm). IR light is more biocompatible and less destructive to live tissues and offers greater penetration depth. The probability of twophoton absorption occurring is also tightly limited to the focal point of the focused light, rather than along the entire path of the light, providing 3D control over excitation. Both single- and multi-photon reactions have the potential to pattern materials with features smaller than 500 nm, much smaller than the size of a mammalian cell.21 This represents an unprecedented level of spatial control over hydrogel scaffold structure and chemistry.

Single and two photon photolysis

Figure 2. Single photon photolysis (left) occurs in the entire area of the hydrogel exposed to UV-visible light, and two photon photolysis (right) results only in the area where simultaneous absorption of two photons of IR light occurs.

The o-NB linker can also be used to tether therapeutic agents into hydrogels for delivery to live cells. Griffin et al. demonstrated the controlled release of fluorescein tethered into a hydrogel through an o-NB-PEG macromer.22 The release of this model therapeutic as a function of light exposure at multiple wavelengths (365–436 nm), intensities (5–20 mW/cm2) and durations (0–20 minutes) was quantified. While the fastest release occurs at 365 nm (which corresponds to a higher molar absorptivity of the o-NB linker at that wavelength), significant release is also seen at 405 nm; the release is easily modeled from physical constants of the molecules (such as molar absorptivity). Light attenuation allows the facile formation of chemical and mechanical gradients in these systems.

Conclusion

Poly(ethylene glycol) is a readily available, easily modifiable polymer. It has found widespread use in hydrogel fabrication, including as 2D and 3D scaffolds for tissue culture. Degradable linkages are easily introduced into PEG hydrogels. Hydrolytically degradable gels allow for sustained material degradation and/or therapeutic agent release. Degradation and release is cell-dictated in enzymatically degradable gels. Photodegradation allows for real-time user tailored external manipulation of the chemical and physical properties of hydrogels.

Materials

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689882 O-(2-Aminoethyl)-O′-[2-(biotinylamino)ethyl]octaethylene glycol ≥95% (oligomer purity)
671487 O-(2-Aminoethyl)-O′-(2-carboxyethyl)polyethylene glycol hydrochloride Mp 3,000
671592 O-(2-Aminoethyl)-O′-(2-carboxyethyl)polyethylene glycol 5,000 hydrochloride Mp 5,000
672130 O-(2-Aminoethyl)polyethylene glycol Mp 5,000
671924 O-(2-Aminoethyl)polyethylene glycol Mp 10,000
07969 O-(2-Aminoethyl)polyethylene glycol 3,000 Mp 3,000
672165 O-(2-Aminoethyl)-O′-(2-(succinylamino)ethyl)polyethylene glycol hydrochloride Mp 10,000
689440 O-(2-Azidoethyl)heptaethylene glycol ≥95% (oligomer purity)
669946 O-[2-(Biotinyl-amino)ethyl]-O′-(2-carboxyethyl)polyethylene glycol Mp 3,000
689998 O-[2-(Biotinylamino)ethyl]-O′-(2-carboxyethyl)undecaethylene glycol ≥95% (oligomer purity)
670049 O-[2-(Biotinyl-amino)ethyl]-O′-[3-(N-succinimidyloxy)-3-oxopropyl]polyethylene glycol Mp 3,000
689653 O-(2-Carboxyethyl)-O′-[2-(Fmoc-amino)-ethyl]heptacosaethylene glycol ≥90% (oligomer purity)
670812 O-(2-Carboxyethyl)polyethylene glycol 3,000
671037 O-(2-Carboxyethyl)polyethylene glycol 10,000
712515 O-(3-Carboxypropyl)-O′-[2-(3-mercaptopropionylamino)ethyl]-polyethylene glycol Mw 3000
712523 O-(3-Carboxypropyl)-O′-[2-(3-mercaptopropionylamino)ethyl]-polyethylene glycol Mw 5000
259268 Hexaethylene glycol 97%
670162 O-[N-(6-Maleimidohexanoyl)aminoethyl]-O′-(2-carboxyethyl)polyethylene glycol Mp 3,000
670278 O-[N-(6-Maleimidohexanoyl)aminoethyl]-O′-[3-(N-succinimidyloxy)-3-oxopropyl]polyethylene glycol 3,000 Mp 3,000
689777 O-[N-(3-Maleimidopropionyl)aminoethyl]-O′-[3-(N-succinimidyloxy)-3-oxopropyl]heptacosaethylene glycol ≥90% (oligomer purity)
719080 PEG-ditosylate average Mn 1,300
705047 PEG-ditosylate average Mn 10,000
701750 PEG-di-p-tosylate average Mn 3,500
335754 Pentaethylene glycol 98%
81300 Poly(ethylene glycol) average Mn 20,000
202444 Poly(ethylene glycol) average Mn 3,350, powder
81210 Poly(ethylene glycol) average Mw 1,500
295906 Poly(ethylene glycol) average Mn 2,050, chips
309028 Poly(ethylene glycol) average Mn 10,000, flakes
14509 Poly(ethylene glycol) bis(amine) Mw 20,000
14501 Poly(ethylene glycol) bis(amine) Mw 2,000
14502 Poly(ethylene glycol) bis(amine) Mw 3,000
407038 Poly(ethylene glycol) bis(carboxymethyl) ether average Mn 600
406996 Poly(ethylene glycol) bis(carboxymethyl) ether average Mn 250
699810 Poly(ethylene glycol) diacetylene average Mn 2,000
475629 Poly(ethylene glycol) diacrylate average Mn 250
701963 Poly(ethylene glycol) diacrylate average Mn 6,000, contains ≤1500 ppm MEHQ as inhibitor
729094 Poly(ethylene glycol) diacrylate average Mn 10,000, contains MEHQ as inhibitor
729086 Poly(ethylene glycol) diacrylate average Mn 1,000, contains MEHQ as inhibitor
475696 Poly(ethylene glycol) diglycidyl ether average Mn 500
409510 Poly(ethylene glycol) dimethacrylate average Mn 550, contains 80-120 ppm MEHQ as inhibitor, 270-330 ppm BHT as inhibitor
687537 Poly(ethylene glycol) dimethacrylate average Mn 6,000, contains 1000 ppm 4-methoxyphenol as inhibitor
725692 Poly(ethylene glycol) dimethacrylate average Mn 20,000, contains MEHQ as inhibitor
725684 Poly(ethylene glycol) dimethacrylate average Mn 10,000, contains MEHQ as inhibitor
717142 Poly(ethylene glycol) dithiol average Mn 1,000
704539 Poly(ethylene glycol) dithiol average Mn 3,400
705004 Poly(ethylene glycol) dithiol average Mn 8,000
410195 Poly(ethylene glycol) divinyl ether average Mn 250
752398 Poly(ethylene glycol), 4 hydroxyl dendron, generation 1 average Mn 20,300
579319 Poly(ethylene glycol), α-maleimidopropionamide-ω-formyl terminated average Mn 3,000
409537 Poly(ethylene glycol) methacrylate average Mn 360, contains 500-800 ppm MEHQ as inhibitor
409529 Poly(ethylene glycol) methacrylate average Mn 500, contains 900 ppm monomethyl ether hydroquinone as inhibitor
81323 Poly(ethylene glycol) methyl ether average Mn 5,000
202487 Poly(ethylene glycol) methyl ether average Mn 550
202495 Poly(ethylene glycol) methyl ether average Mn 750
732621 Poly(ethylene glycol) methyl ether average Mn 10,000
732613 Poly(ethylene glycol) methyl ether average Mn 20,000
699802 Poly(ethylene glycol)methyl ether acetylene average Mn 2,000
730289 Poly(ethylene glycol) methyl ether acrylate average Mn 5,000, contains MEHQ as inhibitor
730270 Poly(ethylene glycol) methyl ether acrylate average Mn 2,000, contains MEHQ as inhibitor
740705 Poly(ethylene glycol) methyl ether 2-(dodecylthiocarbonothioylthio)-2-methylpropionate average Mn 1,100
731765 Poly(ethylene glycol) methyl ether maleimide average Mn 2,000
447951 Poly(ethylene glycol) methyl ether methacrylate average Mn 950, contains 300 ppm BHT as inhibitor, 100 ppm MEHQ as inhibitor
730327 Poly(ethylene glycol) methyl ether methacrylate average Mn 4,000, contains ≤300 ppm monomethyl ether hydroquinone as inhibitor
730319 Poly(ethylene glycol) methyl ether methacrylate average Mn 1500, contains ≤300 ppm monomethyl ether hydroquinone as stabilizer
729108 Poly(ethylene glycol) methyl ether thiol average Mn 800
729140 Poly(ethylene glycol) methyl ether thiol average Mn 2,000
729159 Poly(ethylene glycol) methyl ether thiol average Mn 6,000
729116 Poly(ethylene glycol) methyl ether tosylate average Mn 900
729124 Poly(ethylene glycol) methyl ether tosylate average Mn 2,000
729132 Poly(ethylene glycol) methyl ether tosylate average Mn 5,000
309524 Poly(ethylene glycol) tetrahydrofurfuryl ether average Mn 200
181986 Poly(ethylene oxide) average Mv 100,000, powder
182028 Poly(ethylene oxide) average Mv 600,000, powder
181994 Poly(ethylene oxide) average Mv 200,000, powder
110175 Tetraethylene glycol 99%
712507 O-[2-(3-Tritylthiopropionylamino)ethyl]polyethylene glycol Mp 3,000

References

  1. (a) Drury, J. L.; Mooney, D. J. Biomaterials 2003, 24, 4337-4351.
    (b) Lee, K. Y.; Mooney, D. J. Chem. Rev. 2001, 101, 1869-1879.
    (c) Hoffman, A. S. Bioartificial Organs III: Tissue Sourcing, Immunoisolation, and Clinical Trials;Hunkeler, D., Ed.; New York Academy of Sciences: New York, 2001; pp 62-73.
    (d) Hoffman, A. S. Adv. Drug Delivery Rev. 2002, 54, 3-12.
  2. (a) Tibbitt, M. W.; Anseth, K. S. Biotechnol. Bioeng. 2009, 103, 655-663.
    (b) Lin, C. C.; Anseth, K. S. Pharm. Res. 2009, 26, 631-643.
  3. (a) Richter, A.; Paschew, G.; Klatt, S.; Lienig, J.; Arndt, K. F.; Adler, H. J. P. Sensors 2008, 8, 561-581.
    (b) Gupta, P.; Vermani, K.; Garg, S. Drug Discovery Today 2002, 7, 569-579.
  4. Ruel-Gariepy, E.; Leroux, J. C. Eur. J. Pharm. Biopharm. 2004, 58, 409-426.
  5. Yang, Z.; Xu, B. J. Mater. Chem. 2007, 17, 2385-2393.
  6. Zalipsky, S.; Harris, J. M. Poly(Ethylene Glycol) 1997, 680, 1-13.
  7. Davis, F. F. Adv. Drug Deliver Rev. 2002, 54, 457-458.
  8. Merrill, E. W.; Salzman, E. W.; Wan, S.; Mahmud, N.; Kushner, L.; Lindon, J. N.; Curme, J. T. Am. Soc. Art. Int. Org. 1982, 28, 482-487.
  9. Morpurgo, M.; Veronese, F. M.; Kachensky, D.; Harris, J. M. Bioconjugate Chem. 1996, 7, 363-368.
  10. Lutolf, M. P.; Hubbell, J. A. Nat. Biotechnol. 2005, 23, 47-55.
  11. Sawhney, A. S.; Pathak, C. P.; Hubbell, J. A. Macromolecules 1993, 26, 581-587.
  12. Du, Y. J.; Lemstra, P. J.; Nijenhuis, A. J.; Vanaert, H. A. M.; Bastiaansen, C. Macromolecules 1995, 28, 2124-2132.
  13. Nederberg, F.; Connor, E. F.; Moller, M.; Glauser, T.; Hedrick, J. L. Angew. Chem. Int. Edit. 2001, 40, 2712-2715.
  14. Kim, H.; Kim, H. W.; Suh, H. Biomaterials 2003, 24, 4671-4679.
  15. Benoit, D. S. W.; Nuttelman, C. R.; Collins, S. D.; Anseth, K. S. Biomaterials 2006, 27, 6102-6110.
  16. West, J. L.; Hubbell, J. A. Macromolecules 1999, 32, 241-244.
  17. (a) Lutolf, M. P.; Hubbell, J. A. Biomacromolecules 2003, 4, 713-722.
    (b) Lutolf, M. P.; Lauer-Fields, J. L.; Schmoekel, H. G.; Metters, A. T.; Weber, F. E.; Fields, G. B.; Hubbell, J. A. P. Natl. Acad. Sci. USA 2003, 100, 5413-5418.
  18. Zisch, A. H.; Lutolf, M. P.; Ehrbar, M.; Raeber, G. P.; Rizzi, S. C.; Davies, N.; Schmokel, H.; Bezuidenhout, D.; Djonov, V.; Zilla, P.; Hubbell, J. A. Faseb. J. 2003, 17, 2260.
  19. (a) Kloxin, A. M.; Kasko, A. M.; Salinas, C. N.; Anseth, K. S. Science 2009, 324, 59-63.
    (b) Kloxin, A. M.; Tibbitt, M. W.; Kasko, A. M.; Fairbairn, J. A.; Anseth, K. S. Adv. Mater. 2010, 22, 61-66.
  20. Wong, D. Y.; Griffin, D. R.; Reed, J.; Kasko, A. M. Macromolecules 2010, 43, 2824-2831.
  21. Kasko, A. M.; Wong, D. Y. Future Med. Chem. 2010, 2, 1669-1680.
  22. Griffin, D. R.; Patterson, J. T.; Kasko, A. M. Biotechnol. Bioeng. 2010, 107, 1012-1019.

Poly(ethylene glycol) is a hydrophilic polymer that, when cross-linked into networks, can have a high water content. PEG is a suitable material for biological applications because it does not generally elicit an immune response.6 Since the 1970s, PEG has been used to modify therapeutic proteins and peptides to increase their solubility, lower their toxicity and to prolong their circulation half-life.7 In the late 1970s, researchers began to experiment with PEG hydrogels for cell culture. PEG hydrogels are chemically well-defined, and multiple chemistries can be used both for their formation and chemical modification.

PEG Macromers

PEG is easily synthesized by the living anionic ring-opening polymerization of ethylene oxide; well-defined (low polydispersity) PEGs with a range of molecular weights and a variety of end groups (e.g., alcohol, methyl ether, amine, N-hydroxysuccinimidyl (NHS) ester) are widely available.

In order to form a hydrogel, PEG must be cross-linked. Initially, PEG was cross-linked non-specifically using ionizing radiation.8 PEG hydrogels are now typically synthesized via covalent cross-linking of PEG macromers with reactive chain ends.

PEG macromers with reactive chain ends such as acrylate, methacrylate, allyl ether, maleimide, vinyl sulfone, NHS ester and vinyl ether groups (Chart 1) are easily synthesized from readily available starting materials. The alcohol chain ends of PEG can be esterified using acid chlorides (e.g., acryloyl chloride, methacryloyl chloride) in the presence of base. PEG chain ends can be etherified under basic conditions by reaction with alkyl halides such as 2-chloroethyl vinyl ether or allyl bromide. PEG divinyl sulfone is prepared by coupling PEG to a large excess of divinyl sulfone or by a multistep process to prepare chloroethyl sulfone chain ends that undergo basic elimination to form vinyl sulfone groups9.

End groups of different PEG macromers

Chart 1. End groups of different PEG macromers.

Macromers can be homobifunctional or heterobifunctional. Homobifunctional macromers are typically used to form networks, while heterobifunctional macromers may be used to tether a therapeutic molecule into a hydrogel network.

Mechanisms of Hydrogel Formation

The cross-linking mechanism to form hydrogels depends on the identity of the chain ends of PEG macromers. In most cases, cross-linking occurs when the reactive vinyl chain ends are polymerized, usually with a free radical initiator. For example, polymerization of macromers can be initiated using redox-generated radicals (e.g., ammonium persulfate and TEMED), or radicals generated with light (e.g., Irgacure® 651, λ=365 nm Scheme 1). Acrylate and methacrylate chain ends undergo chain polymerization. In step growth network formation, a multifunctional (f>2) cross-linker reacts with the PEG chain ends in a stoichiometric manner; alternatively, multifunctional PEGs (f>2) can be crosslinked with difunctional crosslinkers (Scheme 1). Acrylate, methacrylate, vinyl sulfone, maleimide, vinyl ether and allyl ether are all capable of step growth network formation, through conversion to thiols depending on reaction conditions. Typical cross-linkers may include thiol or amine moieties. Mixed-mode polymerizations are the result of both mechanisms occurring in the same reaction vessel; acrylate and methacrylate groups can undergo mixed mode network formation. Both mechanisms of hydrogel formation can be used to encapsulate live cells, and both mechanisms allow for the reactive incorporation of peptides, proteins and other therapeutics.

Chain growth and step growth reactions

Scheme 1. Chain growth and step growth reactions.

The mesh structure that results from different mechanisms is depicted in Figure 1. In chain growth networks, a kinetic chain is formed at the crosslink site, while in step growth networks, the crosslink sites bear the same functionality as the multifunctional cross-linker, neglecting defects. In both chain and step growth, network defects such as loops, permanent entanglements and dangling chain ends may exist. The chemical identity of the macromer and the mechanism of hydrogel formation are both important as each influences the cross-link density of the hydrogel network. Material properties that are important to 2D and 3D culture are easily controlled through the chemistry of hydrogel formation. As cross-linking density increases, mesh size decreases, swelling ratio decreases, and storage modulus increases. Varying the molecular weight of the PEG macromer results in coarse control over hydrogel properties (large differences in cross-linking density). Varying the reaction mechanism used to produce the hydrogels results in fine control over hydrogel properties (can be used to tune cross-linking density of a system).

Formation mechanism affects hydrogel network structure

Figure 1. Formation mechanism affects hydrogel network structure and network defects.

Degradeable Hydrogels

In order to use 3D hydrogel scaffolds to study cell differentiation and tissue evolution, it is critical to be able to control the physical and chemical properties of the gel in a spatially and temporally controlled manner.10 Polymeric material properties are typically changed through polymerization/cross-linking (bond forming events) or through controlled degradation and/or release (bond breaking events). Bond forming events typically often use small molecule reagents (initiators, catalysts, monomers, ligands to be conjugated to the material) while bond breaking typically does not rely on exogenous reagents. Small molecules often have more adverse effects in vitro and in vivo than polymeric reagents so many research groups use degradation as a tool for in situ manipulation of polymeric biomaterials.

Hydrolytic Degradation

The mechanism of degradation most commonly utilized in hydrogels is hydrolysis, in which a molecule of water adds to the polymer backbone, causing chain scission. Anhydrides, esters and amides are all susceptible to hydrolysis. Anyhydrides typically hydrolyze too quickly, and the uncatalyzed hydrolysis of amides is too slow, so most hydrogels that degrade hydrolytically utilize ester linkages. In order to obtain hydrolytically degradable hydrogels with physiologically relevant time scales of degradation, researchers typically functionalize PEG with degradable ester linkages using lactide or glycolide segments.

Alcohol chain ends on PEG can initiate ring-opening reactions of 3,6-dimethyl-1,4-dioxane-2,5-dione and 1,4-Dioxane-2,5-dione to generate PEG-lactide and PEG-glycolide, respectively (Scheme 2).11 The ring-opening reaction is typically catalyzed by tin(II)-2-ethylhexanoate,12 although the reaction is also easily accomplished using dimethylaminopyridine as a catalyst,13 which may be easier to remove than the residual tin. The alcohol chain ends of PEG-lactide or PEG-glycolide are easily functionalized with reactive double bonds such as acrylate and methacrylate.

Synthesis of PEG-lactide and PEG-glycolide

Scheme 2. Synthesis of PEG-lactide and PEG-glycolide.

Enzymatic Degradation

Although ester linkages are enzymatically degradable, most researchers utilize sequence-specific enzymatic degradation of peptides incorporated into hydrogels rather than non-specific enzymatic degradation of esters and amides. Hubbell′s group pioneered this approach16 by incorporating matrix metalloproteinase (MMP) sensitive linkages into hydrogels via Michael addition of cysteine-functionalized peptides across acrylates, maleimides and vinyl sulfones (Scheme 3).17

MMP-degradable linkages have also been used to tether therapeutic agents into hydrogels. For example, growth factors such as vascular endothelial growth factor (VEG-F) can be released via enzymatic degradation of an MMP-sensitive tether to induce angiogenesis.18

In both hydrolysis and enzymolysis, the rate of degradation is predetermined by the chemistry of the macromer. In hydrolysis, the degradation rate of the material is pre-engineered through the identity (e.g., hydrophobicity or hydrophilicity) and number of the hydrolysable groups, and cannot be changed once the material is fabricated. In enzymolysis, the degradation typically occurs in an area local to the cells producing the enzyme. While hydrolysis and enzymolysis are both effective methods for sustained hydrogel degradation and sustained release of therapeutic agents, the rate of release cannot be adjusted or arrested after the hydrogel is fabricated, and release is not spatially controlled.

Enzymatically degradable hydrogels

Scheme 3. Enzymatically degradable hydrogels via Michael addition of cysteinecontaining peptides to vinyl sulfone groups.

Photodegradable Hydrogels

In contrast to hydrolytically and enzymatically degradable linkages, photodegradable linkages allow precise spatial and temporal control over degradation and release. While many researchers have reported photopolymerizable hydrogels, and photofunctionalizable hydrogels, very few reports exist of biocompatible photodegradable hydrogels. Kloxin and Kasko reported photodegradable hydrogel networks formed from 2-methoxy-5-nitro-4-(1-hydroxyethyl) phenoxybutanoate-containing PEG macromers (Scheme 4)19; the photodegradation behavior of the ortho-nitrobenzyl (o-NB) linker group is well-characterized. Hydrogels formed from the photodegradable macromer show bulk degradation upon exposure to light that is dependent on exposure time, wavelength, and light intensity. When the light is shuttered, degradation is arrested; the sample continues photolyzing once light exposure resumes. hMSCs (human mesenchymal stem cells) encapsulated in a hydrogel containing the photo-releasable cell-adhesive ligand RGDS (Arg-Gly-Asp-Ser) differentiate down the chondrogenic pathway when the RGD is released at day ten (corresponding to the downregulation of fibronectin during chondrogenesis). Surface erosion and through-gel lithography of this degradable hydrogel can be used to form features over a range of lengths scales, from 10-7 m to 10-2 m or larger.20 Partial degradation in a local area results in decreased cross-link density and increased swelling, providing a means to etch softer features onto a hydrogel that protrude out from the gel.

Photodegradable o-NB moieties

Scheme 4. Photodegradable o-NB moieties incorporated into hydrogel backbone and for therapeutic agent release.

In addition to single photon photolysis, the o-NB containing hydrogels are also susceptible to two-photon photolysis, allowing for 3D etching.19-20 In single photon reactions, any area exposed to the light will react. In contrast, multi-photon lithography should occur only where multiple photons are simultaneously absorbed, which occurs at the focal volume of the light source (inset). Typical wavelengths in single photon lithography of biomaterials range from long wave UV (≥365 nm) into the visible region, while two-photon lithography uses IR light (typically ~740–800 nm). IR light is more biocompatible and less destructive to live tissues and offers greater penetration depth. The probability of twophoton absorption occurring is also tightly limited to the focal point of the focused light, rather than along the entire path of the light, providing 3D control over excitation. Both single- and multi-photon reactions have the potential to pattern materials with features smaller than 500 nm, much smaller than the size of a mammalian cell.21 This represents an unprecedented level of spatial control over hydrogel scaffold structure and chemistry.

Single and two photon photolysis

Figure 2. Single photon photolysis (left) occurs in the entire area of the hydrogel exposed to UV-visible light, and two photon photolysis (right) results only in the area where simultaneous absorption of two photons of IR light occurs.

The o-NB linker can also be used to tether therapeutic agents into hydrogels for delivery to live cells. Griffin et al. demonstrated the controlled release of fluorescein tethered into a hydrogel through an o-NB-PEG macromer.22 The release of this model therapeutic as a function of light exposure at multiple wavelengths (365–436 nm), intensities (5–20 mW/cm2) and durations (0–20 minutes) was quantified. While the fastest release occurs at 365 nm (which corresponds to a higher molar absorptivity of the o-NB linker at that wavelength), significant release is also seen at 405 nm; the release is easily modeled from physical constants of the molecules (such as molar absorptivity). Light attenuation allows the facile formation of chemical and mechanical gradients in these systems.

Conclusion

Poly(ethylene glycol) is a readily available, easily modifiable polymer. It has found widespread use in hydrogel fabrication, including as 2D and 3D scaffolds for tissue culture. Degradable linkages are easily introduced into PEG hydrogels. Hydrolytically degradable gels allow for sustained material degradation and/or therapeutic agent release. Degradation and release is cell-dictated in enzymatically degradable gels. Photodegradation allows for real-time user tailored external manipulation of the chemical and physical properties of hydrogels.

Materials

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689882 O-(2-Aminoethyl)-O′-[2-(biotinylamino)ethyl]octaethylene glycol ≥95% (oligomer purity)
671487 O-(2-Aminoethyl)-O′-(2-carboxyethyl)polyethylene glycol hydrochloride Mp 3,000
671592 O-(2-Aminoethyl)-O′-(2-carboxyethyl)polyethylene glycol 5,000 hydrochloride Mp 5,000
672130 O-(2-Aminoethyl)polyethylene glycol Mp 5,000
671924 O-(2-Aminoethyl)polyethylene glycol Mp 10,000
07969 O-(2-Aminoethyl)polyethylene glycol 3,000 Mp 3,000
672165 O-(2-Aminoethyl)-O′-(2-(succinylamino)ethyl)polyethylene glycol hydrochloride Mp 10,000
689440 O-(2-Azidoethyl)heptaethylene glycol ≥95% (oligomer purity)
669946 O-[2-(Biotinyl-amino)ethyl]-O′-(2-carboxyethyl)polyethylene glycol Mp 3,000
689998 O-[2-(Biotinylamino)ethyl]-O′-(2-carboxyethyl)undecaethylene glycol ≥95% (oligomer purity)
670049 O-[2-(Biotinyl-amino)ethyl]-O′-[3-(N-succinimidyloxy)-3-oxopropyl]polyethylene glycol Mp 3,000
689653 O-(2-Carboxyethyl)-O′-[2-(Fmoc-amino)-ethyl]heptacosaethylene glycol ≥90% (oligomer purity)
670812 O-(2-Carboxyethyl)polyethylene glycol 3,000
671037 O-(2-Carboxyethyl)polyethylene glycol 10,000
712515 O-(3-Carboxypropyl)-O′-[2-(3-mercaptopropionylamino)ethyl]-polyethylene glycol Mw 3000
712523 O-(3-Carboxypropyl)-O′-[2-(3-mercaptopropionylamino)ethyl]-polyethylene glycol Mw 5000
259268 Hexaethylene glycol 97%
670162 O-[N-(6-Maleimidohexanoyl)aminoethyl]-O′-(2-carboxyethyl)polyethylene glycol Mp 3,000
670278 O-[N-(6-Maleimidohexanoyl)aminoethyl]-O′-[3-(N-succinimidyloxy)-3-oxopropyl]polyethylene glycol 3,000 Mp 3,000
689777 O-[N-(3-Maleimidopropionyl)aminoethyl]-O′-[3-(N-succinimidyloxy)-3-oxopropyl]heptacosaethylene glycol ≥90% (oligomer purity)
719080 PEG-ditosylate average Mn 1,300
705047 PEG-ditosylate average Mn 10,000
701750 PEG-di-p-tosylate average Mn 3,500
335754 Pentaethylene glycol 98%
81300 Poly(ethylene glycol) average Mn 20,000
202444 Poly(ethylene glycol) average Mn 3,350, powder
81210 Poly(ethylene glycol) average Mw 1,500
295906 Poly(ethylene glycol) average Mn 2,050, chips
309028 Poly(ethylene glycol) average Mn 10,000, flakes
14509 Poly(ethylene glycol) bis(amine) Mw 20,000
14501 Poly(ethylene glycol) bis(amine) Mw 2,000
14502 Poly(ethylene glycol) bis(amine) Mw 3,000
407038 Poly(ethylene glycol) bis(carboxymethyl) ether average Mn 600
406996 Poly(ethylene glycol) bis(carboxymethyl) ether average Mn 250
699810 Poly(ethylene glycol) diacetylene average Mn 2,000
475629 Poly(ethylene glycol) diacrylate average Mn 250
701963 Poly(ethylene glycol) diacrylate average Mn 6,000, contains ≤1500 ppm MEHQ as inhibitor
729094 Poly(ethylene glycol) diacrylate average Mn 10,000, contains MEHQ as inhibitor
729086 Poly(ethylene glycol) diacrylate average Mn 1,000, contains MEHQ as inhibitor
475696 Poly(ethylene glycol) diglycidyl ether average Mn 500
409510 Poly(ethylene glycol) dimethacrylate average Mn 550, contains 80-120 ppm MEHQ as inhibitor, 270-330 ppm BHT as inhibitor
687537 Poly(ethylene glycol) dimethacrylate average Mn 6,000, contains 1000 ppm 4-methoxyphenol as inhibitor
725692 Poly(ethylene glycol) dimethacrylate average Mn 20,000, contains MEHQ as inhibitor
725684 Poly(ethylene glycol) dimethacrylate average Mn 10,000, contains MEHQ as inhibitor
717142 Poly(ethylene glycol) dithiol average Mn 1,000
704539 Poly(ethylene glycol) dithiol average Mn 3,400
705004 Poly(ethylene glycol) dithiol average Mn 8,000
410195 Poly(ethylene glycol) divinyl ether average Mn 250
752398 Poly(ethylene glycol), 4 hydroxyl dendron, generation 1 average Mn 20,300
579319 Poly(ethylene glycol), α-maleimidopropionamide-ω-formyl terminated average Mn 3,000
409537 Poly(ethylene glycol) methacrylate average Mn 360, contains 500-800 ppm MEHQ as inhibitor
409529 Poly(ethylene glycol) methacrylate average Mn 500, contains 900 ppm monomethyl ether hydroquinone as inhibitor
81323 Poly(ethylene glycol) methyl ether average Mn 5,000
202487 Poly(ethylene glycol) methyl ether average Mn 550
202495 Poly(ethylene glycol) methyl ether average Mn 750
732621 Poly(ethylene glycol) methyl ether average Mn 10,000
732613 Poly(ethylene glycol) methyl ether average Mn 20,000
699802 Poly(ethylene glycol)methyl ether acetylene average Mn 2,000
730289 Poly(ethylene glycol) methyl ether acrylate average Mn 5,000, contains MEHQ as inhibitor
730270 Poly(ethylene glycol) methyl ether acrylate average Mn 2,000, contains MEHQ as inhibitor
740705 Poly(ethylene glycol) methyl ether 2-(dodecylthiocarbonothioylthio)-2-methylpropionate average Mn 1,100
731765 Poly(ethylene glycol) methyl ether maleimide average Mn 2,000
447951 Poly(ethylene glycol) methyl ether methacrylate average Mn 950, contains 300 ppm BHT as inhibitor, 100 ppm MEHQ as inhibitor
730327 Poly(ethylene glycol) methyl ether methacrylate average Mn 4,000, contains ≤300 ppm monomethyl ether hydroquinone as inhibitor
730319 Poly(ethylene glycol) methyl ether methacrylate average Mn 1500, contains ≤300 ppm monomethyl ether hydroquinone as stabilizer
729108 Poly(ethylene glycol) methyl ether thiol average Mn 800
729140 Poly(ethylene glycol) methyl ether thiol average Mn 2,000
729159 Poly(ethylene glycol) methyl ether thiol average Mn 6,000
729116 Poly(ethylene glycol) methyl ether tosylate average Mn 900
729124 Poly(ethylene glycol) methyl ether tosylate average Mn 2,000
729132 Poly(ethylene glycol) methyl ether tosylate average Mn 5,000
309524 Poly(ethylene glycol) tetrahydrofurfuryl ether average Mn 200
181986 Poly(ethylene oxide) average Mv 100,000, powder
182028 Poly(ethylene oxide) average Mv 600,000, powder
181994 Poly(ethylene oxide) average Mv 200,000, powder
110175 Tetraethylene glycol 99%
712507 O-[2-(3-Tritylthiopropionylamino)ethyl]polyethylene glycol Mp 3,000

References

  1. (a) Drury, J. L.; Mooney, D. J. Biomaterials 2003, 24, 4337-4351.
    (b) Lee, K. Y.; Mooney, D. J. Chem. Rev. 2001, 101, 1869-1879.
    (c) Hoffman, A. S. Bioartificial Organs III: Tissue Sourcing, Immunoisolation, and Clinical Trials;Hunkeler, D., Ed.; New York Academy of Sciences: New York, 2001; pp 62-73.
    (d) Hoffman, A. S. Adv. Drug Delivery Rev. 2002, 54, 3-12.
  2. (a) Tibbitt, M. W.; Anseth, K. S. Biotechnol. Bioeng. 2009, 103, 655-663.
    (b) Lin, C. C.; Anseth, K. S. Pharm. Res. 2009, 26, 631-643.
  3. (a) Richter, A.; Paschew, G.; Klatt, S.; Lienig, J.; Arndt, K. F.; Adler, H. J. P. Sensors 2008, 8, 561-581.
    (b) Gupta, P.; Vermani, K.; Garg, S. Drug Discovery Today 2002, 7, 569-579.
  4. Ruel-Gariepy, E.; Leroux, J. C. Eur. J. Pharm. Biopharm. 2004, 58, 409-426.
  5. Yang, Z.; Xu, B. J. Mater. Chem. 2007, 17, 2385-2393.
  6. Zalipsky, S.; Harris, J. M. Poly(Ethylene Glycol) 1997, 680, 1-13.
  7. Davis, F. F. Adv. Drug Deliver Rev. 2002, 54, 457-458.
  8. Merrill, E. W.; Salzman, E. W.; Wan, S.; Mahmud, N.; Kushner, L.; Lindon, J. N.; Curme, J. T. Am. Soc. Art. Int. Org. 1982, 28, 482-487.
  9. Morpurgo, M.; Veronese, F. M.; Kachensky, D.; Harris, J. M. Bioconjugate Chem. 1996, 7, 363-368.
  10. Lutolf, M. P.; Hubbell, J. A. Nat. Biotechnol. 2005, 23, 47-55.
  11. Sawhney, A. S.; Pathak, C. P.; Hubbell, J. A. Macromolecules 1993, 26, 581-587.
  12. Du, Y. J.; Lemstra, P. J.; Nijenhuis, A. J.; Vanaert, H. A. M.; Bastiaansen, C. Macromolecules 1995, 28, 2124-2132.
  13. Nederberg, F.; Connor, E. F.; Moller, M.; Glauser, T.; Hedrick, J. L. Angew. Chem. Int. Edit. 2001, 40, 2712-2715.
  14. Kim, H.; Kim, H. W.; Suh, H. Biomaterials 2003, 24, 4671-4679.
  15. Benoit, D. S. W.; Nuttelman, C. R.; Collins, S. D.; Anseth, K. S. Biomaterials 2006, 27, 6102-6110.
  16. West, J. L.; Hubbell, J. A. Macromolecules 1999, 32, 241-244.
  17. (a) Lutolf, M. P.; Hubbell, J. A. Biomacromolecules 2003, 4, 713-722.
    (b) Lutolf, M. P.; Lauer-Fields, J. L.; Schmoekel, H. G.; Metters, A. T.; Weber, F. E.; Fields, G. B.; Hubbell, J. A. P. Natl. Acad. Sci. USA 2003, 100, 5413-5418.
  18. Zisch, A. H.; Lutolf, M. P.; Ehrbar, M.; Raeber, G. P.; Rizzi, S. C.; Davies, N.; Schmokel, H.; Bezuidenhout, D.; Djonov, V.; Zilla, P.; Hubbell, J. A. Faseb. J. 2003, 17, 2260.
  19. (a) Kloxin, A. M.; Kasko, A. M.; Salinas, C. N.; Anseth, K. S. Science 2009, 324, 59-63.
    (b) Kloxin, A. M.; Tibbitt, M. W.; Kasko, A. M.; Fairbairn, J. A.; Anseth, K. S. Adv. Mater. 2010, 22, 61-66.
  20. Wong, D. Y.; Griffin, D. R.; Reed, J.; Kasko, A. M. Macromolecules 2010, 43, 2824-2831.
  21. Kasko, A. M.; Wong, D. Y. Future Med. Chem. 2010, 2, 1669-1680.
  22. Griffin, D. R.; Patterson, J. T.; Kasko, A. M. Biotechnol. Bioeng. 2010, 107, 1012-1019.

End groups of different PEG macromers.

Chart 1. End groups of different PEG macromers.

Macromers can be homobifunctional or heterobifunctional. Homobifunctional macromers are typically used to form networks, while heterobifunctional macromers may be used to tether a therapeutic molecule into a hydrogel network.

Mechanisms of Hydrogel Formation

The cross-linking mechanism to form hydrogels depends on the identity of the chain ends of PEG macromers. In most cases, cross-linking occurs when the reactive vinyl chain ends are polymerized, usually with a free radical initiator. For example, polymerization of macromers can be initiated using redox-generated radicals (e.g., ammonium persulfate and TEMED), or radicals generated with light (e.g., Irgacure® 651, λ=365 nM Scheme 1). Acrylate and methacrylate chain ends undergo chain polymerization. In step growth network formation, a multifunctional (f>2) cross-linker reacts with the PEG chain ends in a stoichiometric manner; alternatively, multifunctional PEGs (f>2) can be crosslinked with difunctional crosslinkers (Scheme 1). Acrylate, methacrylate, vinyl sulfone, maleimide, vinyl ether and allyl ether are all capable of step growth network formation, through conversion to thiols depending on reaction conditions. Typical cross-linkers may include thiol or amine moieties. Mixed-mode polymerizations are the result of both mechanisms occurring in the same reaction vessel; acrylate and methacrylate groups can undergo mixed mode network formation. Both mechanisms of hydrogel formation can be used to encapsulate live cells, and both mechanisms allow for the reactive incorporation of peptides, proteins and other therapeutics.

Chain growth and step growth reactions

Scheme 1. Chain growth and step growth reactions.

The mesh structure that results from different mechanisms is depicted in Figure 1. In chain growth networks, a kinetic chain is formed at the crosslink site, while in step growth networks, the crosslink sites bear the same functionality as the multifunctional cross-linker, neglecting defects. In both chain and step growth, network defects such as loops, permanent entanglements and dangling chain ends may exist. The chemical identity of the macromer and the mechanism of hydrogel formation are both important as each influences the cross-link density of the hydrogel network. Material properties that are important to 2D and 3D culture are easily controlled through the chemistry of hydrogel formation. As cross-linking density increases, mesh size decreases, swelling ratio decreases, and storage modulus increases. Varying the molecular weight of the PEG macromer results in coarse control over hydrogel properties (large differences in cross-linking density). Varying the reaction mechanism used to produce the hydrogels results in fine control over hydrogel properties (can be used to tune cross-linking density of a system).

Formation mechanism affects hydrogel network structure and network defects

Figure 1. Formation mechanism affects hydrogel network structure and network defects.

Degradeable Hydrogels

In order to use 3D hydrogel scaffolds to study cell differentiation and tissue evolution, it is critical to be able to control the physical and chemical properties of the gel in a spatially and temporally controlled manner.10 Polymeric material properties are typically changed through polymerization/cross-linking (bond forming events) or through controlled degradation and/or release (bond breaking events). Bond forming events typically often use small molecule reagents (initiators, catalysts, monomers, ligands to be conjugated to the material) while bond breaking typically does not rely on exogenous reagents. Small molecules often have more adverse effects in vitro and in vivo than polymeric reagents so many research groups use degradation as a tool for in situ manipulation of polymeric biomaterials.

Hydrolytic Degradation

The mechanism of degradation most commonly utilized in hydrogels is hydrolysis, in which a molecule of water adds to the polymer backbone, causing chain scission. Anhydrides, esters and amides are all susceptible to hydrolysis. Anyhydrides typically hydrolyze too quickly, and the uncatalyzed hydrolysis of amides is too slow, so most hydrogels that degrade hydrolytically utilize ester linkages. In order to obtain hydrolytically degradable hydrogels with physiologically relevant time scales of degradation, researchers typically functionalize PEG with degradable ester linkages using lactide or glycolide segments.

Alcohol chain ends on PEG can initiate ring-opening reactions of 3,6-dimethyl-1,4-dioxane-2,5-dione and 1,4-Dioxane-2,5-dione to generate PEG-lactide and PEG-glycolide, respectively (Scheme 2).11 The ring-opening reaction is typically catalyzed by tin(II)-2-ethylhexanoate,12 although the reaction is also easily accomplished using dimethylaminopyridine as a catalyst,13 which may be easier to remove than the residual tin. The alcohol chain ends of PEG-lactide or PEG-glycolide are easily functionalized with reactive double bonds such as acrylate and methacrylate.

Synthesis of PEG-lactide and PEG-glycolide

Scheme 2. Synthesis of PEG-lactide and PEG-glycolide.

Enzymatic Degradation

Although ester linkages are enzymatically degradable, most researchers utilize sequence-specific enzymatic degradation of peptides incorporated into hydrogels rather than non-specific enzymatic degradation of esters and amides. Hubbell′s group pioneered this approach16 by incorporating matrix metalloproteinase (MMP) sensitive linkages into hydrogels via Michael addition of cysteine-functionalized peptides across acrylates, maleimides and vinyl sulfones (Scheme 3).17

MMP-degradable linkages have also been used to tether therapeutic agents into hydrogels. For example, growth factors such as vascular endothelial growth factor (VEG-F) can be released via enzymatic degradation of an MMP-sensitive tether to induce angiogenesis.18

In both hydrolysis and enzymolysis, the rate of degradation is predetermined by the chemistry of the macromer. In hydrolysis, the degradation rate of the material is pre-engineered through the identity (e.g., hydrophobicity or hydrophilicity) and number of the hydrolysable groups, and cannot be changed once the material is fabricated. In enzymolysis, the degradation typically occurs in an area local to the cells producing the enzyme. While hydrolysis and enzymolysis are both effective methods for sustained hydrogel degradation and sustained release of therapeutic agents, the rate of release cannot be adjusted or arrested after the hydrogel is fabricated, and release is not spatially controlled.

Enzymatically degradable hydrogels via Michael addition of cysteinecontaining peptides to vinyl sulfone groups

Scheme 3. Enzymatically degradable hydrogels via Michael addition of cysteinecontaining peptides to vinyl sulfone groups.

Photodegradable Hydrogels

In contrast to hydrolytically and enzymatically degradable linkages, photodegradable linkages allow precise spatial and temporal control over degradation and release. While many researchers have reported photopolymerizable hydrogels, and photofunctionalizable hydrogels, very few reports exist of biocompatible photodegradable hydrogels. Kloxin and Kasko reported photodegradable hydrogel networks formed from 2-methoxy-5-nitro-4-(1-hydroxyethyl) phenoxybutanoate-containing PEG macromers (Scheme 4)19; the photodegradation behavior of the ortho-nitrobenzyl (o-NB) linker group is well-characterized. Hydrogels formed from the photodegradable macromer show bulk degradation upon exposure to light that is dependent on exposure time, wavelength, and light intensity. When the light is shuttered, degradation is arrested; the sample continues photolyzing once light exposure resumes. hMSCs (human mesenchymal stem cells) encapsulated in a hydrogel containing the photo-releasable cell-adhesive ligand RGDS (Arg-Gly-Asp-Ser) differentiate down the chondrogenic pathway when the RGD is released at day ten (corresponding to the downregulation of fibronectin during chondrogenesis). Surface erosion and through-gel lithography of this degradable hydrogel can be used to form features over a range of lengths scales, from 10-7 m to 10-2 m or larger.20 Partial degradation in a local area results in decreased cross-link density and increased swelling, providing a means to etch softer features onto a hydrogel that protrude out from the gel.

Photodegradable o-NB moieties incorporated into hydrogel backbone and for therapeutic agent release

Scheme 4. Photodegradable o-NB moieties incorporated into hydrogel backbone and for therapeutic agent release.

In addition to single photon photolysis, the o-NB containing hydrogels are also susceptible to two-photon photolysis, allowing for 3D etching.19-20 In single photon reactions, any area exposed to the light will react. In contrast, multi-photon lithography should occur only where multiple photons are simultaneously absorbed, which occurs at the focal volume of the light source (inset). Typical wavelengths in single photon lithography of biomaterials range from long wave UV (≥365 nM) into the visible region, while two-photon lithography uses IR light (typically ~740–800 nM). IR light is more biocompatible and less destructive to live tissues and offers greater penetration depth. The probability of twophoton absorption occurring is also tightly limited to the focal point of the focused light, rather than along the entire path of the light, providing 3D control over excitation. Both single- and multi-photon reactions have the potential to pattern materials with features smaller than 500 nM, much smaller than the size of a mammalian cell.21 This represents an unprecedented level of spatial control over hydrogel scaffold structure and chemistry.

Single photon photolysis

Figure 2.Single photon photolysis (left) occurs in the entire area of the hydrogel exposed to UV-visible light, and two photon photolysis (right) results only in the area where simultaneous absorption of two photons of IR light occurs.

The o-NB linker can also be used to tether therapeutic agents into hydrogels for delivery to live cells. Griffin et al. demonstrated the controlled release of fluorescein tethered into a hydrogel through an o-NB-PEG macromer.22 The release of this model therapeutic as a function of light exposure at multiple wavelengths (365–436 nM), intensities (5–20 mW/cm2) and durations (0–20 minutes) was quantified. While the fastest release occurs at 365 nM (which corresponds to a higher molar absorptivity of the o-NB linker at that wavelength), significant release is also seen at 405 nM; the release is easily modeled from physical constants of the molecules (such as molar absorptivity). Light attenuation allows the facile formation of chemical and mechanical gradients in these systems.

Conclusion

Poly(ethylene glycol) is a readily available, easily modifiable polymer. It has found widespread use in hydrogel fabrication, including as 2D and 3D scaffolds for tissue culture. Degradable linkages are easily introduced into PEG hydrogels. Hydrolytically degradable gels allow for sustained material degradation and/or therapeutic agent release. Degradation and release is cell-dictated in enzymatically degradable gels. Photodegradation allows for real-time user tailored external manipulation of the chemical and physical properties of hydrogels.

Materials
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References

1.
Drury JL, Mooney DJ. 2003. Hydrogels for tissue engineering: scaffold design variables and applications. Biomaterials. 24(24):4337-4351. http://dx.doi.org/10.1016/s0142-9612(03)00340-5
2.
Lee KY, Mooney DJ. 2001. Hydrogels for Tissue Engineering. Chem. Rev.. 101(7):1869-1880. http://dx.doi.org/10.1021/cr000108x
3.
HOFFMAN AS. Hydrogels for Biomedical Applications. 944(1):62-73. http://dx.doi.org/10.1111/j.1749-6632.2001.tb03823.x
4.
Hoffman AS. 2002. Hydrogels for biomedical applications. Advanced Drug Delivery Reviews. 54(1):3-12. http://dx.doi.org/10.1016/s0169-409x(01)00239-3
5.
Tibbitt MW, Anseth KS. 2009. Hydrogels as extracellular matrix mimics for 3D cell culture. Biotechnol. Bioeng.. 103(4):655-663. http://dx.doi.org/10.1002/bit.22361
6.
Lin C, Anseth KS. 2009. PEG Hydrogels for the Controlled Release of Biomolecules in Regenerative Medicine. Pharm Res. 26(3):631-643. http://dx.doi.org/10.1007/s11095-008-9801-2
7.
Richter A, Paschew G, Klatt S, Lienig J, Arndt K, Adler H. Review on Hydrogel-based pH Sensors and Microsensors. Sensors. 8(1):561-581. http://dx.doi.org/10.3390/s8010561
8.
Ruel-Gariépy E, Leroux J. 2004. In situ-forming hydrogels?review of temperature-sensitive systems. European Journal of Pharmaceutics and Biopharmaceutics. 58(2):409-426. http://dx.doi.org/10.1016/j.ejpb.2004.03.019
9.
Yang Z, Xu B. 2007. Supramolecular hydrogels based on biofunctional nanofibers of self-assembled small molecules. J. Mater. Chem.. 17(23):2385. http://dx.doi.org/10.1039/b702493b
10.
Zalipsky S, Harris JM. 1997. Introduction to Chemistry and Biological Applications of Poly(ethylene glycol).1-13. http://dx.doi.org/10.1021/bk-1997-0680.ch001
11.
Davis FF. 2002. The origin of pegnology. Advanced Drug Delivery Reviews. 54(4):457-458. http://dx.doi.org/10.1016/s0169-409x(02)00021-2
12.
Merrill E, Salzman E, Wan S, Mahmud N, Kushner L, Lindon J, Curme J. 1982. Platelet-compatible hydrophilic segmented polyurethanes from polyethylene glycols and cyclohexane diisocyanate. . Transactions-American Society for Artificial Internal Organs . 28482-487.
13.
Morpurgo M, Veronese FM, Kachensky D, Harris JM. 1996. Preparation and Characterization of Poly(ethylene glycol) Vinyl Sulfone. Bioconjugate Chem.. 7(3):363-368. http://dx.doi.org/10.1021/bc9600224
14.
Lutolf MP, Hubbell JA. 2005. Synthetic biomaterials as instructive extracellular microenvironments for morphogenesis in tissue engineering. Nat Biotechnol. 23(1):47-55. http://dx.doi.org/10.1038/nbt1055
15.
Sawhney AS, Pathak CP, Hubbell JA. 1993. Bioerodible hydrogels based on photopolymerized poly(ethylene glycol)-co-poly(.alpha.-hydroxy acid) diacrylate macromers. Macromolecules. 26(4):581-587. http://dx.doi.org/10.1021/ma00056a005
16.
Du YJ, Lemstra PJ, Nijenhuis AJ, Van Aert HAM, Bastiaansen C. 1995. ABA Type Copolymers of Lactide with Poly(ethylene glycol). Kinetic, Mechanistic, and Model Studies. Macromolecules. 28(7):2124-2132. http://dx.doi.org/10.1021/ma00111a004
17.
Nederberg F, Connor E, Moller M, Glauser T, Hedrick J. 2001. Angew. Chem. Int. Edit.. 402712-2715.
18.
Kim H, Kim HW, Suh H. 2003. Sustained release of ascorbate-2-phosphate and dexamethasone from porous PLGA scaffolds for bone tissue engineering using mesenchymal stem cells. Biomaterials. 24(25):4671-4679. http://dx.doi.org/10.1016/s0142-9612(03)00358-2
19.
Benoit DS, Nuttelman CR, Collins SD, Anseth KS. 2006. Synthesis and characterization of a fluvastatin-releasing hydrogel delivery system to modulate hMSC differentiation and function for bone regeneration. Biomaterials. 27(36):6102-6110. http://dx.doi.org/10.1016/j.biomaterials.2006.06.031
20.
West JL, Hubbell JA. 1999. Polymeric Biomaterials with Degradation Sites for Proteases Involved in Cell Migration. Macromolecules. 32(1):241-244. http://dx.doi.org/10.1021/ma981296k
21.
Lutolf MP, Hubbell JA. 2003. Synthesis and Physicochemical Characterization of End-Linked Poly(ethylene glycol)-co-peptide Hydrogels Formed by Michael-Type Addition. Biomacromolecules. 4(3):713-722. http://dx.doi.org/10.1021/bm025744e
22.
Lutolf MP, Lauer-Fields JL, Schmoekel HG, Metters AT, Weber FE, Fields GB, Hubbell JA. 2003. Synthetic matrix metalloproteinase-sensitive hydrogels for the conduction of tissue regeneration: Engineering cell-invasion characteristics. Proceedings of the National Academy of Sciences. 100(9):5413-5418. http://dx.doi.org/10.1073/pnas.0737381100
23.
Zisch AH, Lutolf MP, Ehrbar M, Raeber GP, Rizzi SC, Davies N, Schmökel H, Bezuidenhout D, Djonov V, Zilla P, et al. 2003. Cell?demanded release of VEGF from synthetic, biointeractive cell?ingrowth matrices for vascularized tissue growth. FASEB j.. 17(15):2260-2262. http://dx.doi.org/10.1096/fj.02-1041fje
24.
Kloxin AM, Kasko AM, Salinas CN, Anseth KS. 2009. Photodegradable Hydrogels for Dynamic Tuning of Physical and Chemical Properties. Science. 324(5923):59-63. http://dx.doi.org/10.1126/science.1169494
25.
Kloxin AM, Tibbitt MW, Kasko AM, Fairbairn JA, Anseth KS. 2010. Tunable Hydrogels for External Manipulation of Cellular Microenvironments through Controlled Photodegradation. Adv. Mater.. 22(1):61-66. http://dx.doi.org/10.1002/adma.200900917
26.
Wong DY, Griffin DR, Reed J, Kasko AM. 2010. Photodegradable Hydrogels to Generate Positive and Negative Features over Multiple Length Scales. Macromolecules. 43(6):2824-2831. http://dx.doi.org/10.1021/ma9023679
27.
Kasko AM, Wong DY. 2010. Two-photon lithography in the future of cell-based therapeutics and regenerative medicine: a review of techniques for hydrogel patterning and controlled release. Future Medicinal Chemistry. 2(11):1669-1680. http://dx.doi.org/10.4155/fmc.10.253
28.
Griffin DR, Patterson JT, Kasko AM. 2010. Photodegradation as a mechanism for controlled drug delivery. Biotechnol. Bioeng.. 107(6):1012-1019. http://dx.doi.org/10.1002/bit.22882

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