We recently developed a cell printer (Wilson and Boland, 2003) that enables us 
to place cells in positions that mimic their respective positions in organs. 
However, this technology was limited to the printing of two-dimensional (2D) 
tissue constructs. Here we describe the use of thermosensitive gels to generate 
sequential layers for cell printing. The ability to drop cells on previously 
printed successive layers provides a real opportunity for the realization of 
three-dimensional (3D) organ printing. Organ printing will allow us to print 
complex 3D organs with computer-controlled, exact placing of different cell 
types, by a process that can be completed in several minutes. To demonstrate the 
feasibility of this novel technology, we showed that cell aggregates can be 
placed in the sequential layers of 3D gels close enough for fusion to occur. We 
estimated the optimum minimal thickness of the gel that can be reproducibly 
generated by dropping the liquid at room temperature onto a heated substrate. 
Then we generated cell aggregates with the corresponding (to the minimal 
thickness of the gel) size to ensure a direct contact between printed cell 
aggregates during sequential printing cycles. Finally, we demonstrated that 
these closely-placed cell aggregates could fuse in two types of thermosensitive 
3D gels. Taken together, these data strongly support the feasibility of the 
proposed novel organ-printing technology.