Bioprinting silk fibroin using two-photon lithography enables control over the physico-chemical material properties and cellular response

Two-photon lithography (2 PL) has been proposed as an additive manufacturing method to fabricate structures at the microscale, including curing protein-based bioinks. In this study, we explored the use of 2 PL for biomedical applications to photo-cross-link silk fibroin solution into a solid hydrogel with directly tuneable physico-chemical properties. 2 PL allowed control of the cross-linking degree and the porosity of the resulting hydrogel at the micrometric scale by regulating the intensity of the IR laser exposure, tested between 800 and 2400 mW, during the 3D printing process. The regulation of the cross-linking degree in this way allowed for the production of hydrogels with significantly different degradation rates, ranging from 0.5 to 2 h during accelerated enzymatic degradation assay, while their β-sheet content, assessed by Fourier-transform infra-red spectroscopy (FTIR), did not show significant changes. The hydrogels Young’s modulus, assessed by quantitative microelastography, could also be controlled within a 9.6 to 47.2 kPa range and was found to increase with the rate of cross-linking. When human dermal fibroblasts were included in the silk fibroin solution for bioprinting testing at different laser exposures, cell viability was assessed over 95% during 21 days of in vitro culture. Moreover, cells showed a higher proliferation rate in less cross-linked hydrogels and were able to align and migrate to areas of increased stiffness printed in gradient hydrogels. Overall, this versatile additive manufacturing method represents an advancement of silk fibroin biomaterials for soft tissue engineering, enabling control of the microstructure and degradation rate of the products and the proliferation of the engrafted cells.