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Abstract
To progress cardiac tissue engineering strategies closer to the clinic, thicker constructs are required to meet the functional need following a cardiac event. Consequently, pre-vascularization of these constructs needs to be investigated to ensure survival and optimal performance of implantable engineered heart tissue. The aim of this research is to investigate the potential of combining extrusion-based bioprinting (EBB) and melt electrowriting for the fabrication of a myocardial construct with a precisely patterned pre-vascular pathway. Gelatin methacryloyl (GelMA) was investigated as a base hydrogel for the respective myocardial and vascular bioinks with collagen, Matrigel and fibrinogen as interpenetrating polymers to support myocardial functionality. Subsequently, extrusion-based printability and viability were investigated to determine the optimal processing parameters for printing into melt electrowritten meshes. Finally, an anatomically inspired vascular pathway was implemented in a dual EBB set-up into melt electrowritten meshes, creating a patterned pre-vascularized myocardial construct. It was determined that a blend of 5% GelMA and 0.8 mg·ml−1 collagen with a low crosslinked density was optimal for myocardial cellular arrangement and alignment within the constructs. For the vascular fraction, the optimized formulation consisted of 5% GelMA, 0.8 mg·ml−1 collagen and 1 mg·ml−1 fibrinogen with a higher crosslinked density, which led to enhanced vascular cell connectivity. Printability assessment confirmed that the optimized bioinks could effectively fill the microfiber mesh while supporting cell viability (∼70%). Finally, the two bioinks were applied using a dual EBB system for the fabrication of a pre-vascular pathway with the shape of a left anterior descending artery within a myocardial construct, whereby the distinct cell populations could be visualized in their respective patterns up to D14. This research investigated the first step towards developing a thick engineered cardiac tissue construct in which a pre-vascularization pathway is fabricated within a myocardial construct.

Abstract
Abstract Three dimensional (3D) printing of heart patches usually provides the ability to precisely control cell location in 3D space. Here, one-step 3D printing of cardiac patches with built-in soft and stretchable electronics is reported. The tissue is simultaneously printed using three distinct bioinks for the cells, for the conducting parts of the electronics and for the dielectric components. It is shown that the hybrid system can withstand continuous physical deformations as those taking place in the contracting myocardium. The electronic patch is flexible, stretchable, and soft, and the electrodes within the printed patch are able to monitor the function of the engineered tissue by providing extracellular potentials. Furthermore, the system allowed controlling tissue function by providing electrical stimulation for pacing. It is envisioned that such transplantable patches may regain heart contractility and allow the physician to monitor the implant function as well as to efficiently intervene from afar when needed.

Abstract
Three-dimensional (3D) biofabrication techniques enable the production of multicellular tissue models as assay platforms for drug screening. The increased cellular and physiological complexity in these 3D tissue models should recapitulate the relevant biological environment found in the body. Here we describe the use of 3D bioprinting techniques to fabricate skin equivalent tissues of varying physiological complexity, including human epidermis, non-vascularized and vascularized full-thickness skin tissue equivalents, in a multi-well platform to enable drug screening. Human keratinocytes, fibroblasts, and pericytes, and induced pluripotent stem cell (iPSC)-derived endothelial cells were used in the biofabrication process to produce the varying complexity. The skin equivalents exhibit the correct structural markers of dermis and epidermis stratification, with physiological functions of the skin barrier. The robustness, versatility and reproducibility of the biofabrication techniques are further highlighted by the generation of atopic dermatitis (AD)-disease like tissues. These AD models demonstrate several clinical hallmarks of the disease, including: (i) spongiosis and hyperplasia; (ii) early and terminal expression of differentiation proteins; and (iii) increases in levels of pro-inflammatory cytokines. We show the pre-clinical relevance of the biofabricated AD tissue models to correct disease phenotype by testing the effects of dexamethasone, an anti-inflammatory corticosteroid, and three Janus Kinase inhibitors from clinical trials for AD. This study demonstrates the development of a versatile and reproducible bioprinting approach to create human skin equivalents with a range of cellular complexity for disease modelling. In addition, we establish several assay readouts that are quantifiable, robust, AD relevant, and can be scaled up for compound screening. The results show that the cellular complexity of the tissues develops a more physiologically relevant AD disease model. Thus, the skin models in this study offer an in vitro approach for the rapid understanding of pathological mechanisms, and testing for efficacy of action and toxic effects of drugs.

Abstract
AbstractTendons and ligaments have a poor innate healing capacity, yet account for 50% of musculoskeletal injuries in the US. Full structure and function restoration post-injury remains an unmet clinical need. This study aimed to assess the application of novel 3D printed scaffolds and induced pluripotent stem cell-derived mesenchymal stem cells (iMSCs) overexpressing the transcription factor Scleraxis (SCX, iMSCSCX+) as a new strategy for tendon defect repair. The polycaprolactone (PCL) scaffolds were fabricated by extrusion through a patterned nozzle or conventional round nozzle. Scaffolds were seeded with iMSCSCX+ and outcomes were assessed in vitro via gene expression analysis and immunofluorescence. In vivo, rat Achilles tendon defects were repaired with iMSCSCX+-seeded microgrooved scaffolds, microgrooved scaffolds only, or suture only and assessed via gait, gene expression, biomechanical testing, histology, and immunofluorescence.iMSCSCX+-seeded on microgrooved scaffolds showed upregulation of tendon markers and increased organization and linearity of cells compared to non-patterned scaffolds in vitro. In vivo gait analysis showed improvement in the Scaffold+iMSCSCX+-treated group compared to the controls. Tensile testing of the tendons demonstrated improved biomechanical properties of the Scaffold+iMSCSCX+ group compared to the controls. Histology and immunofluorescence demonstrated more regular tissue formation in the Scaffold+iMSCSCX+ group.This article is protected by copyright. All rights reserved.

Abstract
Induced pluripotent stem cells (iPSCs) are essentially produced by the genetic reprogramming of adult cells. Moreover, iPSC technology prevents the genetic manipulation of embryos. Hence, with the ensured element of safety, they rarely cause ethical concerns when utilized in tissue engineering. Several cumulative outcomes have demonstrated the functional superiority and potency of iPSCs in advanced regenerative medicine. Recently, an emerging trend in 3D bioprinting technology has been a more comprehensive approach to iPSC-based tissue engineering. The principal aim of this review is to provide an understanding of the applications of 3D bioprinting in iPSC-based tissue engineering. This review discusses the generation of iPSCs based on their distinct purpose, divided into two categories: (1) undifferentiated iPSCs applied with 3D bioprinting; (2) differentiated iPSCs applied with 3D bioprinting. Their significant potential is analyzed. Lastly, various applications for engineering tissues and organs have been introduced and discussed in detail.

Abstract
Abstract Electrically conductive materials that mimic physical and biological properties of tissues are urgently required for seamless brain–machine interfaces. Here, a multinetwork hydrogel combining electrical conductivity of 26 S m−1, stretchability of 800%, and tissue-like elastic modulus of 15 kPa with mimicry of the extracellular matrix is reported. Engineering this unique set of properties is enabled by a novel in-scaffold polymerization approach. Colloidal hydrogels of the nanoclay Laponite are employed as supports for the assembly of secondary polymer networks. Laponite dramatically increases the conductivity of in-scaffold polymerized poly(ethylene-3,4-diethoxy thiophene) in the absence of other dopants, while preserving excellent stretchability. The scaffold is coated with a layer containing adhesive peptide and polysaccharide dextran sulfate supporting the attachment, proliferation, and neuronal differentiation of human induced pluripotent stem cells directly on the surface of conductive hydrogels. Due to its compatibility with simple extrusion printing, this material promises to enable tissue-mimetic neurostimulating electrodes.

Abstract
Here, we present a concise review of current 3D bioprinting technologies applied to induced pluripotent stem cells (iPSC). iPSC have recently received a great deal of attention from the scientific and clinical communities for their unique properties, which include abundant adult cell sources, ability to indefinitely self-renew and differentiate into any tissue of the body. Bioprinting of iPSC and iPSC derived cells combined with natural or synthetic biomaterials to fabricate tissue mimicked constructs, has emerged as a technology that might revolutionize regenerative medicine and patient-specific treatment. This review covers the advantages and disadvantages of bioprinting techniques, influence of bioprinting parameters and printing condition on cell viability, and commonly used iPSC sources, and bioinks. A clear distinction is made for bioprinting techniques used for iPSC at their undifferentiated stage or when used as adult stem cells or terminally differentiated cells. This review presents state of the art data obtained from major searching engines, including Pubmed/MEDLINE, Google Scholar, and Scopus, concerning iPSC generation, undifferentiated iPSC, iPSC bioprinting, bioprinting techniques, cartilage, bone, heart, neural tissue, skin, and hepatic tissue cells derived from iPSC.

Abstract
Cartilage lesions can progress into secondary osteoarthritis and cause severe clinical problems in numerous patients. As a prospective treatment of such lesions, human-derived induced pluripotent stem cells (iPSCs) were shown to be 3D bioprinted into cartilage mimics using a nanofibrillated cellulose (NFC) composite bioink when co-printed with irradiated human chondrocytes. Two bioinks were investigated: NFC with alginate (NFC/A) or hyaluronic acid (NFC/HA). Low proliferation and phenotypic changes away from pluripotency were seen in the case of NFC/HA. However, in the case of the 3D-bioprinted NFC/A (60/40, dry weight % ratio) constructs, pluripotency was initially maintained, and after five weeks, hyaline-like cartilaginous tissue with collagen type II expression and lacking tumorigenic Oct4 expression was observed in 3D -bioprinted NFC/A (60/40, dry weight % relation) constructs. Moreover, a marked increase in cell number within the cartilaginous tissue was detected by 2-photon fluorescence microscopy, indicating the importance of high cell densities in the pursuit of achieving good survival after printing. We conclude that NFC/A bioink is suitable for bioprinting iPSCs to support cartilage production in co-cultures with irradiated chondrocytes.