Enabling volumetric printing of low viscosity hyaluronic acid-based resins through fast crosslinking reactions

Most (bio)resins for volumetric printing (VP) are based on (meth)acrylated polymer precursors mixed with gelatine or Pluronic as thermoreversible thickening agents. Here we investigate if fast photocrosslinking reactions, like the thiol-ene, allow VP of low viscosity bioresins without the need of viscosity modifiers. We used a 4% w/v hyaluronic acid norbornene (HA-NB) resin with viscosity <0.1 Pa s, a frequently used matrix for encapsulation of cells. Using light doses in the range of 150-200 mJ cm^-2, stable 8 × 5 × 2 mm³crosslinked scaffolds with internal bifurcated channels down to ∼200µm dimensions were printed in <30 s. At this concentration, HA methacrylate (HA-MA) resins with comparable viscosity and functionalization degree did not form stable structures. RGD-functionalized HA-NB scaffolds were printed in the presence of fibroblasts and showed homogeneous cell distribution across the scaffold, with cell viability >90%. Encapsulated cells spread and showed fibroblast-like morphology in 7 d cultures. We demonstrate volumetric bioprinting of low molecular weight HA-based bioresins without viscosity modifiers and in the absence of post-printing steps.