Conductive Microfibers Improve Stem Cell-Derived Cardiac Spheroid Maturation

Conventional two-dimensional (2D) cardiomyocyte differentiation protocols create cells with limited maturity, which impairs their predictive capacity and has driven interest in three-dimensional (3D) engineered cardiac tissue models of varying maturity and scalability. Cardiac spheroids are attractive high-throughput models that have demonstrated improved functional and transcriptional maturity over conventional 2D differentiations. However, these 3D models still tend to have limited contractile and electrical maturity compared to highly engineered cardiac tissues; hence, we incorporated a library of conductive polymer microfibers in cardiac spheroids to determine if fiber properties could accelerate maturation. Conductive microfibers improved contractility parameters of cardiac spheroids over time versus nonconductive fibers, specifically, when they were short, for example, 5 μm, and when there was moderate fiber mass per spheroid, for example, 20 μg. Spheroids with optimal conductive microfiber length and concentration developed a thicker ring-like perimeter and a less compacted cavity, improving their contractile work compared to control cardiac spheroids. Functional improvements correlated with increased expression of contractility and calcium handling-related cardiac proteins, as well as improved calcium handling abilities and drug response. Taken together, these data suggest that conductive microfibers can improve cardiac spheroid performance to improve cardiac disease modeling.