Bioprinting pattern-dependent electrical/mechanical behavior of cardiac alginate implants: characterization and ex-vivo phase-contrast microtomography assessment
Three-dimensional (3D)-bioprinting techniques may be used to modulate electrical/mechanical properties and porosity of hydrogel constructs for fabrication of suitable cardiac implants. Notably, characterization of these properties after implantation remains a challenge, raising the need for the development of novel quantitative imaging techniques for monitoring hydrogel implant behavior in-situ. This study aims to (i) assess the influence of hydrogel bioprinting patterns on electrical/mechanical behavior of cardiac implants based on a 3D-printing technique and (ii) investigate the potential of synchrotron X-ray phase contrast computed tomography (PCI-CT) for estimating elastic modulus/impedance/porosity and microstructural features of 3D-printed cardiac implants in-situ via an ex-vivo study. Alginate laden with human coronary artery endothelial cells was bioprinted layer-by-layer, forming cardiac constructs with varying architectures. The elastic modulus, impedance, porosity and other structural features, along with cell viability and degradation of printed implants were examined in-vitro over 25 days. Two selected cardiac constructs were surgically implanted onto the myocardium of rats and ten days later, the rat hearts with implants were imaged ex-vivo using PCI-CT at varying X-ray energies and CT-scan times. The elastic modulus/impedance, porosity and structural features of the implant were inferred from the PCI-CT images using statistical models and compared to measured values. The printing patterns had significant effects on implant porosity, elastic modulus and impedance. A particular 3D-printing pattern with interstrand distance of 900 μm and strand alignment angle of 0/45/90/135° provided relatively higher stiffness and electrical conductivity with a suitable porosity, maintaining high cell viability over seven days. The X-ray photon energy of 30-33 keV utilizing CT-scan time of 1-1.2 h resulted in a low-dose PCI-CT which provided a good visibility of the low-X-ray absorbent alginate implants. Following ten days post-implantation, the PCI-CT provided reasonably accurate estimation of implant strand thickness and alignment, pore size and interconnectivity, porosity, elastic modulus and impedance, which were consistent with our measurements. Findings from this study suggest that 3D-printing patterns can be used to modulate electrical/mechanical behavior of alginate implants, and PCI-CT can be potentially used as a 3D quantitative imaging tool for assessing structural and electrical/mechanical behavior of hydrogel cardiac implants in small animal models.