Finally, to demonstrate organ-scale

Finally, to demonstrate organ-scale FRESH v2.0 printing capabilities and the potential to engineer larger scaffolds, we printed a neonatal- scale human heart from collagen (Fig. 4, Q and R, and fig. S15, A to C). To highlight the micro- scale internal structure, we printed half the heart (Fig. 4S). Structures such as trabeculae were printed from collagen with the same architec- ture as defined in the G-code file (Fig. 4, T and U). The square-lattice infill pattern within the ventricular walls was similarly well defined (Fig. 4, V and W). We used mCT imaging to confirm reproduction of all the anatomical structures contained within the 3D model of the heart, in- cluding the atrial and ventricular chambers, trabeculae, and pulmonary and aortic valves (fig. S15, D to I, and movie S10).We have used the human heart for proof of concept; however, FRESH v2.0 printing of col- lagen is a platform that can build advanced tis- sue scaffolds for a wide range of organ systems. There are still many challenges to overcome, such as generating the billions of cells required to 3D-bioprint large tissues, achieving manufac- turing scale, and creating a regulatory process for clinical translation (23). Although the 3D bioprinting of a fully functional organ is yet to be achieved, we now have the ability to build constructs that start to recapitulate the struc- tural, mechanical, and biological properties of native tissues.