Introduction: Hydrogels closely resemble natural soft tissue, both mechanically as well as physicochemically, making them ideal biomaterials. Hydrogels made of synthetic polymers pose an advantage over natural polymers, largely owing to the increased amount of structural control, purity and reproducibility. Poly(ethylene glycol) (PEG) is a suitable material as it is hydrophilic and does not elicit an immune response. To covalently cross-link PEG chains, the Strain-Promoted Alkyne-Azide Cycloaddition (SPAAC) reaction can occur at the ends[1]. This reaction is fast, efficient, does not produce any by-products and does not require a catalyst. Here we describe the synthesis of a strained cyclooctyne, aza-dibenzocyclooctyne (DIBAC)[2], its attachment to the ends of a PEG chain, and the reaction of the resulting polymer with an azide-functionalized PEG. Hydrogels are formed within minutes at room temperature, without the need for external stimuli or catalysts.
Materials and Methods: A linear poly(ethylene glycol) (PEG) chain was functionalized at its ends with either strained cyclooctynes or azides. The strained cyclooctyne, aza-dibenzocyclooctyne (DIBAC), was synthesized as previously reported using a modified Popik synthesis[2]. Injectable PEG hydrogels were made using a double barrel syringe. Equal volumes of both alkyne and azide polymers were added to separate barrels of the syringe and injected into a pre-made silicone mold.
Results: Improving the synthesis of the cyclooctyne, DIBAC, was the key component in being able to create hydrogels efficiently. We were able to substantially increase the overall yield from 21%, as previously reported in the literature, to 67%[3]. We were able to reduce purification to only one chromatographic step, making this procedure possible to complete in two to three laboratory days. Having a feasible route to a strained cyclooctyne made the hydrogel cross-linking reaction ideal, as it is one of the more promising reactions for hydrogel chemistry, but often ignored due to the tedious synthetic procedures previously required. One DIBAC was attached to either end of a PEG chain to produce the alkyne-functionalized polymer, and multiple azide groups were incorporated at either end of PEG to form the azide-functionalized polymer. The resulting hydrogels exhibited various properties and gelled at various rates depending on the alkyne and azide polymer combinations that were used.
Discussion and Conclusion: The Strain-Promoted Alkyne-Azide Cycloaddition (SPAAC) reaction is an effective way to make PEG hydrogels quickly and efficiently without the need for organic solvent, catalysts, UV light, high temperature or high pressure. No other reagents are necessary for gelation to occur, other than the two functionalized PEG chains in aqueous solution. The system described has the potential to be very useful for tissue engineering applications. Depending on the desired application, the hydrogel system with the most appropriate properties can be chosen.
Financial support for this work was provided by the National Science and Engineering Research Council of Canada (NSERC); S.M.H. is grateful for support through the Ontario Graduate Scholarship (OGS) program
References:
[1] Xu, J.; Filion, T. M.; Prifti, F.; Song, J. Cytocompatible Poly(Ethylene Glycol)-co-Polycarbonate Hydrogels Cross-Linked by Copper-Free, Strain-Promoted Click Chemistry, Chem. Asian J. 2011, 6, 2730-2737
[2] Chadwick, R. C.; Van Gyzen, S.; Liogier, S.; Adronov, A. Scalable Synthesis of Strained Cyclooctyne Derivatives, Synthesis. 2014, 46, 669-677
[3] Campbell-Verduyn, L. S.; Mirfeizi, L.; Schoonen, A. K.; Dierckx, R. A.; Elsinga, P. H.; Feringa, B. L. Strain-Promoted Copper-Free “Click” Chemistry for 18F Bombesin. Angew. Chem. Int. Ed. 2011, 50, 11117-11120