Vaterite, a metastable modification of calcium carbonate, embedded in a flexible microgel packaging with adjustable mechanical properties, functionality, and biocompatibility, provides a powerful scaffolding for bone tissue regeneration, as it is easily convertible to bone-like hydroxyapatite (HA). In this study, the synthesis and physical analysis of a packaging material to encapsulate vaterite particles and osteoblast cells into monodisperse, sub-millimeter-sized microgels, is described whereby a systematic approach is used to tailor the microgel properties. The size and shape of the microgels is controlled via droplet-based microfluidics. Key requirements for the polymer system, such as absence of cytotoxicity as well as biocompatibility and biodegradability, are accomplished with functionalized poly(ethylene glycol) (PEG), which reacts in a cytocompatible thiol-ene Michael addition. On a mesoscopic level, the microgel stiffness and gelation times are adjusted to obtain high cellular viabilities. The co-encapsulation of living cells provides i) an in vitro platform for the study of cellular metabolic processes which can be applied to bone formation and ii) an in vitro foundation for novel tissue-regenerative therapies. Finally, the degradability of the microgels at physiological conditions caused by hydrolysis-sensitive ester groups in the polymer network is examined.

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In article number 1901820 by Sebastian Seiffert and co‐workers, degradable vaterite/PEG‐composite microgels are templated by droplet‐based microfluidic technology. A key aspect of the material development is the fundamental understanding of the gel structure‐property relationships, and their connection to tissue engineering, by tailoring the microgel properties on micro‐, meso‐, and macroscopic levels. The co‐encapsulation of cells provides an in vitro platform for metabolic bone regeneration studies and a step towards new bone tissue regeneration materials.

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