A biocompatible diazosulfonate initiator for direct encapsulation of human stem cells via two-photon polymerization

Direct cell encapsulation is a powerful tool for fabrication of biomimetic 3D cell culture models in vitro. This method allows more precise recapitulation of the natural environment and physiological functions of cells compared to classical 2D cultures. In contrast to seeding cells on prefabricated scaffolds, cell encapsulation offers benefits regarding high initial cell loading, uniformity of cell distribution and more defined cell-matrix contact. Two-photon polymerization (2PP) based 3D printing enables the precise engineering of cell-containing hydrogel constructs as tissue models. Two-photon initiators (2PIs) specifically developed for this purpose still exhibit considerable cyto-and phototoxicity, impairing the viability of encapsulated cells. This work reports the development of the first cleavable diazosulfonate 2PI DAS, largely overcoming these limitations. The material was characterized by standard spectroscopic methods, white light continuum two-photon absorption cross-section measurements, and its photosensitization of cytotoxic singlet oxygen was compared to the well-established 2PI P2CK. When DAS is used at double concentration to compensate for the lower two-photon cross section, its performance in 2PP-printing of hydrogels is similar to P2CK based on structuring threshold and structure swelling measurements. PrestoBlue metabolic assay showed vastly improved cytocompatibility of DAS in 2D. Cell survival in 3D direct encapsulation via 2PP was up to five times higher versus P2CK, further demonstrating the excellent biocompatibility of DAS and its potential as superior material for laser-based biofabrication.