Clickable PEG hydrogel microspheres as building blocks for 3D bioprintingElectronic supplementary information (ESI) available: Details of microgel formulations, experimental data of cell growth and adhesion on microgels, videos of microgel printing. See DOI: 10.1039/c8bm01286e

Three-dimensional (3D) bioprinting is important in the development of complex tissue structures for tissue engineering and regenerative medicine. However, the materials used for bioprinting, referred to as bioinks, must have a balance between a high viscosity for rapid solidification after extrusion and low shear force for cytocompatibility, which is difficult to achieve. Here, a novel bioink consisting of poly(ethylene glycol) (PEG) microgels prepared via off-stoichiometry thiol-ene click chemistry is introduced. Importantly, the microgel bioink is easily extruded, exhibits excellent stability after printing due to interparticle adhesion forces, and can be photochemically annealed with a second thiol-ene click reaction to confer long-term stability to printed constructs. The modularity of the bioink is also an advantage, as the PEG microgels have highly tunable physicochemical properties. The low force required for extrusion and cytocompatibility of the thiol-ene annealing reaction also permit cell incorporation during printing with high viability, and cells are able to spread and proliferate in the interstitial spaces between the microgels after the constructs have been annealed. Overall, these results indicate that our microgel bioink is a promising and versatile platform that could be leveraged for bioprinting and regenerative manufacturing. Clickable poly(ethylene glycol) hydrogel microspheres can be 3D printed and photochemically annealed to produce complex cell-laden structures.