Designing of biologically active scaffolds with optimal characteristics is one of the key factors for successful tissue engineering. Recently, hydrogels have received a considerable interest as leading candidates for engineered tissue scaffolds due to their unique compositional and structural similarities to the natural extracellular matrix, in addition to their desirable framework for cellular proliferation and survival. More recently, the ability to control the shape, porosity, surface morphology, and size of hydrogel scaffolds has created new opportunities to overcome various challenges in tissue engineering such as vascularization, tissue architecture and simultaneous seeding of multiple cells. This review provides an overview of the different types of hydrogels, the approaches that can be used to fabricate hydrogel matrices with specific features and the recent applications of hydrogels in tissue engineering. Special attention was given to the various design considerations for an efficient hydrogel scaffold in tissue engineering. Also, the challenges associated with the use of hydrogel scaffolds were described.

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Hydrogel scaffolds for tissue engineering: Progress and challenges

The ultimate goal of most biomedical research is to gain greater insight into mechanisms of human disease or to develop new and improved therapies or diagnostics. Although great advances have been made in terms of developing disease models in animals, such as transgenic mice, many of these models fail to faithfully recapitulate the human condition. In addition, it is difficult to identify critical cellular and molecular contributors to disease or to vary them independently in whole-animal models. This challenge has attracted the interest of engineers, who have begun to collaborate with biologists to leverage recent advances in tissue engineering and microfabrication to develop novel in vitro models of disease. As these models are synthetic systems, specific molecular factors and individual cell types, including parenchymal cells, vascular cells, and immune cells, can be varied independently while simultaneously measuring system-level responses in real time. In this article, we provide some examples of these efforts, including engineered models of diseases of the heart, lung, intestine, liver, kidney, cartilage, skin and vascular, endocrine, musculoskeletal, and nervous systems, as well as models of infectious diseases and cancer. We also describe how engineered in vitro models can be combined with human inducible pluripotent stem cells to enable new insights into a broad variety of disease mechanisms, as well as provide a test bed for screening new therapies.

Engineered in vitro disease models.

The common and persistent failures to translate promising preclinical drug candidates into clinical success highlight the limited effectiveness of disease models currently used in drug discovery. An apparent reluctance to explore and adopt alternative cell- and tissue-based model systems, coupled with a detachment from clinical practice during assay validation, contributes to ineffective translational research. To help address these issues and stimulate debate, here we propose a set of principles to facilitate the definition and development of disease-relevant assays, and we discuss new opportunities for exploiting the latest advances in cell-based assay technologies in drug discovery, including induced pluripotent stem cells, three-dimensional (3D) co-culture and organ-on-a-chip systems, complemented by advances in single-cell imaging and gene editing technologies. Funding to support precompetitive, multidisciplinary collaborations to develop novel preclinical models and cell-based screening technologies could have a key role in improving their clinical relevance, and ultimately increase clinical success rates.

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Screening out irrelevant cell-based models of disease

The engineering of 3-dimensional (3D) heart muscles has undergone exciting progress for the past decade. Profound advances in human stem cell biology and technology, tissue engineering and material sciences, as well as prevascularization and in vitro assay technologies make the first clinical application of engineered cardiac tissues a realistic option and predict that cardiac tissue engineering techniques will find widespread use in the preclinical research and drug development in the near future. Tasks that need to be solved for this purpose include standardization of human myocyte production protocols, establishment of simple methods for the in vitro vascularization of 3D constructs and better maturation of myocytes, and, finally, thorough definition of the predictive value of these methods for preclinical safety pharmacology. The present article gives an overview of the present state of the art, bottlenecks, and perspectives of cardiac tissue engineering for cardiac repair and in vitro testing.

Cardiac tissue engineering: state of the art.

Forces generated by cells are critical regulators of cell adhesion, signaling, and function, and they are also essential drivers in the morphogenetic events of development. Over the past 20 years, several methods have been developed to measure these forces. However, despite recent substantial interest in understanding the contribution of these forces in biology, implementation and adoption of the developed methods by the broader biological community remain challenging because of the inherently multidisciplinary expertise required to conduct and interpret the measurements. In this review, we introduce the established methods and highlight the technical challenges associated with implementing each technique in a biological laboratory.

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Measuring cell-generated forces: a guide to the available tools

Rationale: Tissue engineering may provide advanced in vitro models for drug testing and, in combination with recent induced pluripotent stem cell technology, disease modeling, but available techniques are unsuitable for higher throughput. Objective: Here, we present a new miniaturized and automated method based on engineered heart tissue (EHT). Methods and Results: Neonatal rat heart cells are mixed with fibrinogen/Matrigel plus thrombin and pipetted into rectangular casting molds in which two flexible silicone posts are positioned from above. Contractile activity is monitored video-optically by a camera and evaluated by a custom-made software program. Fibrin-based mini-EHTs (FBMEs) (150 μL, 600 000 cells) were transferred from molds to a standard 24-well plate two hours after casting. Over time FBMEs condensed from a 12×3×3 mm gel to a muscle strip of 8 mm length and, depending on conditions, 0.2 to 1.3 mm diameter. After 8 to 10 days, FBMEs started to rhythmically deflect the posts. Post properties and the extent of post deflection allowed calculation of rate, force (0.1 to 0.3 mN), and kinetics which was validated in organ baths experiments. FBMEs exhibited a well-developed, longitudinally aligned actinin-positive cardiac muscle network and lectin-positive vascular structures interspersed homogeneously throughout the construct. Analysis of a large series of FBME (n=192) revealed high yield and reproducibility and stability for weeks. Chromanol, quinidine, and erythromycin exerted concentration-dependent increases in relaxation time, doxorubicin decreases in contractile force. Conclusions: We developed a simple technique to construct large series of EHT and automatically evaluate contractile activity. The method shall be useful for drug screening and disease modeling.

Marianne Flato

Papers

Development of a Drug Screening Platform Based on Engineered Heart Tissue