RESOURCES

Abstract
Abstract Despite advances in biomaterials engineering, a large gap remains between the weak mechanical properties that can be achieved with natural materials and the strength of synthetic materials. Here, we present a method for reinforcing an engineered cardiac tissue fabricated from differentiated iPSCs and an ECM-based hydrogel in a manner that is fully biocompatible. The reinforcement occurs as a post-fabrication step, which allows for the use of 3D printing technology to generate thick, fully cellularized, and vascularized cardiac tissues. After tissue assembly and during the maturation process in a soft hydrogel, a small, tissue-penetrating reinforcer is deployed, leading to a significant increase in the tissue's mechanical properties. The tissue's robustness is demonstrated by injecting the tissue in a simulated minimally invasive procedure and showing that the tissue is functional and undamaged at the nano-, micro-, and macro-scales. This article is protected by copyright. All rights reserved

Abstract
The survival and function of tissues depend on appropriate vascularization. Blood vessels of the tissues supply oxygen, and nutrients and remove waste and byproducts. Incorporating blood vessels into engineered tissues is essential for overcoming diffusion limitations, improving tissue function, and thus facilitating the fabrication of thick tissues. Here, we present a modified ECM bioink, with enhanced mechanical properties and endothelial cell-specific adhesion motifs, to serve as a building material for 3D printing of a multiscale blood vessel network. The bioink is composed of natural ECM and alginate conjugated with a laminin adhesion molecule motif (YIGSR). The hybrid hydrogel was characterized for its mechanical properties, biochemical content, and ability to interact with endothelial cells. The pristine and modified hydrogels were mixed with induced pluripotent stem cells derived endothelial cells (iPSCs-ECs) and used to print large blood vessels with capillary beds in between.

Abstract
Preclinical biomedical and pharmaceutical research on disease causes, drug targets, and side effects increasingly relies on in vitro models of human tissue. 3D printing offers unique opportunities for generating models of superior physiological accuracy, as well as for automating their fabrication. Towards these goals, we here describe a simple and scalable methodology for generating physiologically relevant models of skeletal muscle. Our approach relies on dual-material micro-extrusion of two types of gelatin hydrogel into patterned soft substrates with locally alternating stiffness. We identify minimally complex patterns capable of guiding the large-scale self-assembly of aligned, extended, and contractile human and murine skeletal myotubes. Interestingly, we find high-resolution patterning is not required, as even patterns with feature sizes of several hundred micrometers is sufficient. Consequently, the procedure is rapid and compatible with any low-cost extrusion-based 3D printer. The generated myotubes easily span several millimeters, and various myotube patterns can be generated in a predictable and reproducible manner. The compliant nature and adjustable thickness of the hydrogel substrates, serves to enable extended culture of contractile myotubes. The method is further readily compatible with standard cell-culturing platforms as well as commercially available electrodes for electrically induced exercise and monitoring of the myotubes.