RESOURCES

Abstract
Abstract Soft and stretchable electronic materials have a number of unique applications, not least within sensors for monitoring human health. Through development of appropriate inks, micro-extrusion 3D printing offers an appealing route for integrating soft electronic materials within wearable garments. Toward this objective, here a series of conductive inks based on soft thermoplastic styrene–ethylene–butylene–styrene elastomers combined with silver micro-flakes, carbon black nanoparticles, or poly(3,4-ethylenedioxythiophene) (PEDOT) conducting polymer additives, is developed. Their electrical and mechanical properties are systematically compared and found to be highly dependent on additive amount and type. Thus, while silver composites offer the highest conductivity, their stretchability is far inferior to carbon black composites, which can maintain conductivity beyond 400% strain. The PEDOT composites are the least conductive and stretchable but display unique properties due to their propensity for ionic conductivity. To integrate these inks, as well as insulating counterparts, into functional designs, a multi-material micro-extrusion 3D printing routine for direct deposition onto stretchable, elastic fabrics is established. As demonstration, prototypes are produced for sensing common health markers including strain, physiological temperatures, and electrocardiograms. Collectively, this work demonstrates multi-material 3D printing of soft styrene–ethylene–butylene–styrene elastomer composites as a versatile method for fabricating soft bio-sensors.

Abstract
The successful translation of organ-on-a-chip devices requires the development of an automated workflow for device fabrication, which is challenged by the need for precise deposition of multiple classes of materials in micro-meter scaled configurations. Many current heart-on-a-chip devices are produced manually, requiring the expertise and dexterity of skilled operators. Here, we devised an automated and scalable fabrication method to engineer a Biowire II multiwell platform to generate human iPSC-derived cardiac tissues. This high-throughput heart-on-a-chip platform incorporated fluorescent nanocomposite microwires as force sensors, produced from quantum dots and thermoplastic elastomer, and 3D printed on top of a polystyrene tissue culture base patterned by hot embossing. An array of built-in carbon electrodes was embedded in a single step into the base, flanking the microwells on both sides. The facile and rapid 3D printing approach efficiently and seamlessly scaled up the Biowire II system from an 8-well chip to a 24-well and a 96-well format, resulting in an increase of platform fabrication efficiency by 17,5000–69,000% per well. The device's compatibility with long-term electrical stimulation in each well facilitated the targeted generation of mature human iPSC-derived cardiac tissues, evident through a positive force-frequency relationship, post-rest potentiation, and well-aligned sarcomeric apparatus. This system's ease of use and its capacity to gauge drug responses in matured cardiac tissue make it a powerful and reliable platform for rapid preclinical drug screening and development.

Abstract
Abstract Epicardial cells (EPIs) form the outer layer of the heart and play an important role in development and disease. Current heart-on-a-chip platforms still do not fully mimic the native cardiac environment due to the absence of relevant cell types, such as EPIs. Here, using the Biowire II platform, engineered cardiac tissues with an epicardial outer layer and inner myocardial structure are constructed, and an image analysis approach is developed to track the EPI cell migration in a beating myocardial environment. Functional properties of EPI cardiac tissues improve over two weeks in culture. In conditions mimicking ischemia reperfusion injury (IRI), the EPI cardiac tissues experience less cell death and a lower impact on functional properties. EPI cell coverage is significantly reduced and more diffuse under normoxic conditions compared to the post-IRI conditions. Upon IRI, migration of EPI cells into the cardiac tissue interior is observed, with contributions to alpha smooth muscle actin positive cell population. Altogether, a novel heart-on-a-chip model is designed to incorporate EPIs through a formation process that mimics cardiac development, and this work demonstrates that EPI cardiac tissues respond to injury differently than epicardium-free controls, highlighting the importance of including EPIs in heart-on-a-chip constructs that aim to accurately mimic the cardiac environment.

Abstract
Yolk sac macrophages are the first to seed the developing heart; however, owing to a lack of accessible tissue, there is no understanding of their roles in human heart development and function. In this study, we bridge this gap by differentiating human embryonic stem (hES) cells into primitive LYVE1+ macrophages (hESC-macrophages) that stably engraft within contractile cardiac microtissues composed of hESC-cardiomyocytes and fibroblasts. Engraftment induces a human fetal cardiac macrophage gene program enriched in efferocytic pathways. Functionally, hESC-macrophages trigger cardiomyocyte sarcomeric protein maturation, enhance contractile force and improve relaxation kinetics. Mechanistically, hESC-macrophages engage in phosphatidylserine-dependent ingestion of apoptotic cardiomyocyte cargo, which reduces microtissue stress, leading hESC-cardiomyocytes to more closely resemble early human fetal ventricular cardiomyocytes, both transcriptionally and metabolically. Inhibiting hESC-macrophage efferocytosis impairs sarcomeric protein maturation and reduces cardiac microtissue function. Together, macrophage-engineered human cardiac microtissues represent a considerably improved model for human heart development and reveal a major beneficial role for human primitive macrophages in enhancing early cardiac tissue function.