Development of open-source in situ photocrosslinking printhead attachments for extrusion bioprinting of hydrogel matrices

This study presents an open-source, modular in situ photocrosslinking system (“ISX-Linker”) designed as clip-on printhead attachments for the commercially available CELLINK BioX extrusion bioprinter, enabling targeted 405 nm LED exposure during extrusion to improve print fidelity of low-viscosity photocrosslinkable hydrogels. Two attachment variants were developed to fit both standard pneumatic and temperature-controlled BioX printheads, with adjustable dual-LED alignment and an external control unit; optical performance was validated via ANSYS Speos simulations correlated to measured irradiance at the nozzle outlet. Using exemplar bioinks (10% GelMA + 0.25% LAP; and PEGDA/alginate + LAP), photorheology and printability testing showed that increasing in situ irradiance (30–90 mW/cm2) improved crosshatch shape fidelity and printability, particularly for GelMA. The platform supported cell-laden printing with human MSCs, maintaining high day-1 viability (generally >80%) across practical irradiance conditions while enabling stiffness tunability through selection of in situ intensity and optional post-crosslinking. Overall, the work provides a broadly accessible, customizable in situ photocrosslinking approach for extrusion bioprinting of hydrogel matrices that improves construct stability and fidelity without requiring rheology modifiers.