Cellular spheroids are attractive for tissue fabrication due to having precise control over cell and extracellular matrix (ECM) composition, the ability for upscaled production and repeatability, their three-dimensional nature and the fact that spheroids will produce their own ECM over time. A critical process in the fabrication of complex tissue structures with cellular spheroids is related to their fusion and maturation. Tissue fusion is a self-assembly process in which two or more distinct cell populations, or tissues, make contact and coalesce to form a single cohesive structure. Maturation of tissue engineered constructs involves developing the mechanical properties and ECM compositions that mimic native vasculature. However, the fusion and maturation of spheroids and tissues composed of spheroids over time is not clearly understood. Therefore, developing methods to understand and accelerate the fusion and maturation of tissues composed of spheroids will improve upon current techniques for tissue fabrication with spheroids. Here, the fusion and maturation of vascular tissues mediated by magnetic forces was modeled using Janus Magnetic Cellular Spheroids (JMCSs). JMCSs contain two distinct domains: cells and extracellular iron oxide magnetic nanoparticles (MNPs). This separation of cells and MNPs has no adverse effects on long-term viability or cellular phenotype, allowing for magnetic manipulation of spheroids for building larger tissues.
Here, spheroid composition was manipulated, by varying ECM and cell contents, in order to study the resulting effects on JMCS fusion mediated by magnetic forces. Next, the influence of iron oxide MNPs on ECM production in JMCSs was studied over time. Further, magnetic sheets composed of JMCSs were fabricated and their maturation mediated by cyclic longitudinal stretching using magnetic forces. The objective of this work was to determine the mechanisms associated with the fusion and maturation of JMCSs and tissues composed of JMCSs. The hypotheses driving this work were that spheroid composition dictates their fusion and that magnetic forces can be utilized to dynamically condition tissues composed of JMCSs for maturation.
Results demonstrated the critical importance of magnetic forces for promoting the fusion of JMCSs, when compared to JMCSs not exposed to magnetic forces. Further, results demonstrate the critical role of cell-cell and cell-ECM interactions for mediating cellular spheroid fusion over time. Results showed that the addition of iron oxide magnetic nanoparticles in JMCSs caused a significant increase in collagen production, when compared to no iron oxide controls. Quantitative results demonstrate that cyclic longitudinal stretching of tissue sheets mediated by magnetic forces increases the Young’s modulus, enhances ECM production and induces collagen fiber alignment over 7 days, when compared to statically conditioned controls. These findings are expected to provide a strong theoretical and methodological foundation for the development of new tissue engineering technologies
