Materials used in the construction of an in vivo bioreactor space vary widely depending on the type of substrate, type of tissue, and mechanical demands of said tissue being grown. At its simplest, a bioreactor space will be created between tissue layers through the use of hydrogel injections to create a bioreactor space. Early models used an impermeable silicone shroud to encase a scaffold, though more recent studies have begun 3D printing custom bioreactor molds to further enhance the mechanical growth properties of the bioreactors. The choice of bioreactor chamber material generally requires that it is nontoxic and medical grade, examples include: "silicon, polycarbonate, and acrylic polymer". Recently both Teflon and titanium have been used in the growth of bone. One study utilized Polymethyl methacrylate as a chamber material and 3D printed hollow rectangular blocks. Yet another study pushed the limits of the in vivo bioreactor by proving that the omentum is suitable as a bioreactor space and chamber. Specifically, highly vascularized and functional bladder tissue was grown within the omentum space.