The production of patient-specific bone substitutes with an exact fit through 3D printing is emerging as an alternative to autologous bone grafting. To the success of tissue regeneration, the material characteristics such as porosity, stiffness, and surface topography have a strong influence on the cell–material interaction and require significant attention. Printing a soft hydrocolloid-based hydrogel reinforced with irregularly-shaped microporous biphasic calcium phosphate (BCP) particles (150–500 µm) is an alternative strategy for the acquisition of a complex network with good mechanical properties that could fulfill the needs of cell proliferation and regeneration. Three well-known hydrocolloids (sodium alginate, xanthan gum, and gelatin) have been combined with BCP particles to generate stable, homogenous, and printable solid dispersions. Through rheological assessment, it was determined that the crosslinking time, printing process parameters (infill density percentage and infill pattern), as well as BCP particle size and concentration all influence the stiffness of the printed matrices. Additionally, the swelling behavior on fresh and dehydrated 3D-printed structures was investigated, where it was observed that the BCP particle characteristics influenced the constructs’ water absorption, particle diffusion out of the matrix and degradability.

A biphasic calcium phosphate with submicron needle-shaped surface topography combined with a novel polyethylene glycol/polylactic acid triblock copolymer binder (BCP-EP) was investigated in this study. This study aims to evaluate the composition, degradation mechanism and bioactivity of BCP-EP in vitro, and its in vivo performance as an autograft bone graft (ABG) extender in a rabbit Posterolateral Fusion (PLF) model. The characterization of BCP-EP and its in vitro degradation products showed that the binder hydrolyses rapidly into lactic acid, lactide oligomers and unaltered PEG (polyethylene glycol) without altering the BCP granules and their characteristic submicron needle-shaped surface topography. The bioactivity of BCP-EP after immersion in SBF revealed a progressive surface mineralization. In vivo, BCP-EP was assessed in a rabbit PLF model by radiography, manual palpation, histology and histomorphometry up to 12 weeks post-implantation. Twenty skeletally mature New Zealand (NZ) White Rabbits underwent single-level intertransverse process PLF surgery at L4/5 using (1) autologous bone graft (ABG) alone or (2) by mixing in a 1:1 ratio with BCP-EP (BCP-EP/ABG). After 3 days of implantation, histology showed the BCP granules were in direct contact with tissues and cells. After 12 weeks, material resorption and mature bone formation were observed, which resulted in solid fusion between the two transverse processes, following all assessment methods. BCP-EP/ABG showed comparable fusion rates with ABG at 12 weeks, and no graft migration or adverse reaction were noted at the implantation site nor in distant organs.