This year has seen major advances in 3D bioprinting of tissue structures, from the fabrication of complex vascular networks  to rebuilding components of the human heart . Integral to the success of 3D bioprinting is the ability to incorporate cells into an ink that supports their survival and growth both during and after the printing process. An oft-overlooked aspect is the extent to which cells can alter the architecture of bioprinted tissues over time. A recent report from Angelini and colleagues in Nature Communications (DOI: 10.1038/s41467-019-10919-1) demonstrates that cell-generated forces can predictably bend, buckle, or break bioprinted tissue beams.
This project began in 2016 when investigators in the UF Soft Matter Group set out to determine if cells could do something more than just survive in the 3D printed sacrificial materials developed by their team . Cells can exert local forces on the extracellular matrix that translate into macroscopic changes in tissue structure, as seen with the contraction of heart tissue by cardiomyocytes; however, systematically exploring these phenomena is not feasible with traditional manufacturing methods for soft solids. Angelini, Morley, and Ellison saw an opportunity to utilize their 3D printing platform to better understand if macroscopic changes in printed structural elements, such as cell-collagen beams, could be dictated by micro-scale material properties. A major obstacle came in determining the stiffness of the fibrous collagen network that the cells feel. Morley approximates the collagen network to a pile of cooked spaghetti -- as cells pull on the “noodles”, some will be pulled in tension which will provide resistance to deformation, whereas others will compress and buckle, providing negligible resistance to deformation.
To investigate these complex phenomena, Angelini, Morley, and Ellison assembled a team of UF chemists (Kabb and Summerlin), mechanical engineers (Bhattacharjee, O’Bryan, Schulze, Niemi, Zhang, Smith, and Sawyer), and cell biologists (Sebastian, Moore, Tran, Mitchell, and Flores) to develop a platform consisting of collagen-cell beams embedded within a jammed microgel support. Each domain has independently controllable mechanical properties, which enabled them to observe that cells react to their integrated mechanical environment. By varying the properties of either the beams, the microgel environment, or both, they can encode cell-mediated beam buckling, beam failure, or beam contraction, as well as conditions in which the beams remain unchanged (Figure 1). Notably, these structural changes were observed for different cell types, suggesting that they are a general property of collagen, not the cells. Further, the beams respond to cell-generated forces in accordance with classical macro-scale models of structural elements, such as steel, enabling sophisticated, user-defined control of the resulting shape change.
Figure 1: Microbeam mechanical behaviors controlled by beam and microenvironment material properties.
The authors anticipate that the ability to systematically study the highly irregular mechanical properties of bioprinted cell-collagen materials will be central to establishing a user’s handbook for future tissue fabrication efforts. Further, this work presents an exciting opportunity to push 3D printed tissues into the 4D realm by enabling the design of constructs that can dynamically reconfigure to mimic macroscopic tissue morphogenesis events that are central to developmental processes, such as the formation of the gut lumen or the neural tube.
 Grigoryan, B., et al. “Multivascular networks and functional intravascular topologies within biocompatible hydrogels.” Science 2019 DOI:10.1126/science.aav9750
 Lee, A., et al. “3D bioprinting of collagen to rebuild components of the human heart.” Science 2019 DOI:10.1126/science.aav9051
 Bhattacharjee, T., et al. “Liquid-like solids support cells in 3D.” ACS Biomaterials Science & Engineering 2016 DOI:10.1021/acsbiomaterials.6b00218