Engineering Cardiac Muscle Tissue

The origin of the term tissue engineering is not clear, but a 1985 proposal of Y.C. Fung to the National Science Foundation of the United States for a Center for the Engineering of Living Tissues and 2 conferences organized by the National Science Foundation in 1987 and 1988 are generally accepted as the origin of this lively and rapidly growing research field (The Emergence of Tissue Engineering as a Research Field; https:// www.nsf.gov/pubs/2004/nsf0450/emergence.htm). In its 1988 definition, tissue engineering is the application of principles and methods of engineering and life sciences toward fundamental understanding of structure–function relationship in normal and pathological mammalian tissues and the development of biological substitutes to restore, maintain, or improve functions. Besides bone and cartilage, cardiovascular tissue engineering is the most proliferative discipline in the field (>9200 PubMed entries) and can be traced back to 1986, when the first report on tissue-engineered vasculature was published. The first engineered heart tissue (EHT) was generated 1994 by seeding chicken embryonic heart cells in collagen I between 2 Velcrocoated glass tubes to form coherently beating and force-generating biconcaval lattices. Today, 20 years later, the field is in a maturating state with EHT patches moving toward first-in-man applications, and EHT assays being on the cusp of industrial application. In the present article, we will concentrate on 3 major questions: (1) How closely do EHTs currently resemble native myocardium and how can this be improved? (2) Pros and cons of using tissue engineered heart muscle assays in drug development and disease modeling. (3) Tissue engineered heart muscle patches for cardiac regeneration. For more information also on 3-dimensional (3D) printing and organ-on-the-chip approaches, the reader is referred to recent reviews. Technologies to Engineer Heart Muscles In Vitro In a normal organism, parenchymal cells including cardiomyocytes are never alone. Instead, they are organized in a complex 3-dimensional tissue that, in the case of the heart muscle, is comprised mechanically and electrically connected cardiomyocytes intimately coupled to capillary endothelial cells, fibroblasts, vascular smooth muscle cells, and macrophages. A mammalian heart is composed of ≈20% to 30% cardiomyocytes and 70% to 80% nonmyocytes. The principal promise of the field is that engineered 3D heart muscles recapitulate normal tissue organization in vitro and allow the study of cell–cell interactions and heart muscle function under normal and pathological conditions. Two principally different strategies are pursued to generate tissues—(1) to capitalize on and promote the natural ability of cells to assemble and form organized 3D structures or to use the natural organ structure as blueprint and (2) to engineer the 3D organization by technical or chemical means. This review will focus on the first strategy because it is the field the authors have actively contributed. A schematic overview of different strategies is given in Figure 1. The latter is attractive particularly for engineers and offers the advantage that the desired geometric form of the final construct can be technically designed. Although seeding of cells in preformed matrices had been an issue in early work, more recent work showed good tissue formation with novel materials such as porous collagen sponges or poly(glycerol sebacate) scaffolds. Organ-on-the-chip (the heart on a chip), 3D-printing approaches, and intelligent multifunctional synthetic polymers are currently developed to advance the field in terms of miniaturization, high-throughput, simulation of organ-like cell–cell Review