Tissue engineers have tried a number of schemes for building replacement organs out of patients' own cells, but biophysi-cist Gabor Forgacs at the University of Missouri-Columbia has a new twist: let the cells assemble themselves. Forgacs and his team start with a printing device built by Sciperio, an R&D firm in Stillwater, OK, loaded with what he calls "bioink." The bioink consists of spherical aggregates of many thousands of cells. The printer deposits the aggregates onto successive layers of biodegradable gel; by manipulating the composition of the gel, Forgacs can coax the aggregates to grow together to form complex structures while the gel degrades. Forgacs has already built three-dimensional blood-vessel-like modules and thinks a usable blood vessel will be ready in the near future. Another application Forgacs envisions is in cancer treatment. Doctors could take tumor cells from a patient, build an artificial replica of the tumor outside of his or her body, and see which drugs worked best on the replica before barraging the patient with different treatments.