Abstract: An in vitro method of preparing insulin-producing cell clusters for transplantation into a subject, comprising (a) seeding pancreatic progenitor cells onto a three-dimensional, porous scaffold at a seeding density greater than about 12.5 million cells per cm3 of scaffold and less than about 150 million cells per cm3 of scaffold, wherein the scaffold comprises a plurality of pores having an average pore diameter greater than about 225 ?m and less than about 550 pm, and (b) culturing the cells on the scaffold for more than about 3 days in culture medium to obtain insulin-producing cell clusters within the pores of the scaffold for transplantation into a subject. In exemplary aspects, the pancreatic progenitor cells are cells derived from pluripotent stem cells.