Three-dimensional (3-D) bioprinting is the layer-by-layer deposition of 
biological material with the aim of achieving stable 3-D constructs for 
application in tissue engineering. It is a powerful tool for the spatially 
directed placement of multiple materials and/or cells within the 3-D sample. 
Encapsulated cells are protected by the bioink during the printing process. Very 
few materials are available that fulfill requirements for bioprinting as well as 
provide adequate properties for cell encapsulation during and after the printing 
process. A hydrogel composite including alginate and gelatin precursors was 
tuned with different concentrations of hydroxyapatite (HA) and characterized in 
terms of rheology, swelling behavior and mechanical properties to assess the 
versatility of the system. Instantaneous as well as long-term structural 
integrity of the printed hydrogel was achieved with a two-step mechanism 
combining the thermosensitive properties of gelatin with chemical crosslinking 
of alginate. Novel syringe tip heaters were developed for improved temperature 
control of the bioink to avoid clogging. Human mesenchymal stem cells mixed into 
the hydrogel precursor survived the printing process and showed high cell 
viability of 85% living cells after 3 days of subsequent in vitro culture. HA 
enabled the visualization of the printed structures with micro-computed 
tomography. The inclusion of HA also favors the use of the bioink for bone 
tissue engineering applications. By adding factors other than HA, the composite 
could be used as a bioink for applications in drug delivery, microsphere 
deposition or soft tissue engineering.