Sodium alginate hydrogel, stabilized with gelatin, is a suitable, biologically 
inert matrix that can be used for encapsulating and 3D bioprinting of 
bone-related SaOS-2 cells. However, the cells, embedded in this matrix, remain 
in a non-proliferating state. Here we show that addition of an overlay onto the 
bioprinted alginate/gelatine/SaOS-2 cell scaffold, consisting of agarose and the 
calcium salt of polyphosphate [polyP·Ca(2+)-complex], resulted in a marked 
increase in cell proliferation. In the presence of 100 μm polyP·Ca(2+)-complex, 
the cells proliferate with a generation time of approximately 47-55 h. In 
addition, the hardness of the alginate/gelatin hydrogel substantially increases 
in the presence of the polymer. The reduced Young's modulus for the 
alginate/gelatin hydrogel is approximately 13-14 kPa, and this value drops to 
approximately 0.5 kPa after incubation of the cell containing scaffolds for 5 d. 
In the presence of 100 μm polyP·Ca(2+)-complex, the reduced Young's modulus 
increases to about 22 kPa. The hardness of the polyP·Ca(2+)-complex containing 
hydrogel remains essentially constant if cells are absent in the matrix, but it 
drops to 3.2 kPa after a 5 d incubation period in the presence of SaOS-2 cells, 
indicating that polyP·Ca(2+)-complex becomes metabolized, degraded, by the 
cells. The alginate/gelatine-agarose system with polyP·Ca(2+)-complex cause a 
significant increase in the mineralization of the cells. SEM analyses revealed 
that the morphology of the mineral nodules formed on the surface of the cells 
embedded in the alginate/gelatin hydrogel do not significantly differ from the 
nodules on cells growing in monolayer cultures. The newly developed technique, 
using cells encapsulated into an alginate/gelatin hydrogel and a secondary layer 
containing the morphogenetically active, growth promoting polymer 
polyP·Ca(2+)-complex opens new possibilities for the application of 3D 
bioprinting in bone tissue engineering.