From Engineered Tissues and Microfludics to Human Eyes-On-A-Chip.

The field of tissue engineering was created in the late 1980s with the aim of restoring the function of damaged or diseased organs. Originally, tissue engineering was defined in 1987 as ‘‘the application of the principles and methods of engineering and life sciences toward the fundamental understanding of structure–function relationships in normal and pathologic mammalian tissue and the development of biological substitutes to restore, maintain, or improve function. ’’ The promise of full functional regeneration inspired a new generation of scientist and engineers to work on overcoming the key problems leading to the application of engineered tissues in the clinic. For example, it is now possible to buy living skin grafts for restoration of nonhealing wounds and large surface area burns from one of the commercial pioneers in the field, Organogenesis. Tissue engineered blood vessels will hopefully follow soon, as offthe shelf products, available in the decellularized form, as phase 3 clinical trials in atrioventricular shunt patients are nearing completion. Originally, tissue engineers focused on creating centimeter-scale tissues from primary cells, derived from the patient’s body. The original paradigm involved expansion of individual patient cells outside the body, their seeding into 3–dimensional (3D) biomaterial scaffolds followed by cultivation in bioreactors needed for the tissues to reach a certain level of functional competence. Cells were present in each of the applications, since they are true tissue engineers, secreting both molecular factors that are needed for their own function and survival and remodeling the environment through tractional forces and depositing of new extracellular matrix (ECM). Early tissue engineers were focusing on the application of a single cell type, in many cases to gain a better control of the system and due to the limitations related to cell availability. However, it was soon noted that cell coand tricultures perform better in 3D as well as upon in vivo implantation, in comparison with monocultures. In addition to the cells being tissue engineers by secreting and remodeling their own ECM, the fields of polymer chemistry and biomaterials have significantly contributed to our understanding of cell–ECM interactions as well as to the design of materials that recapitulate critical ECM functions. For example, it was demonstrated that cells from a specific tissue exhibit appropriate phenotypic features when cultivated on a matrix with stiffness corresponding to that of the native tissue. Importance of adhesive ligands for cell attachment as well as cleavable sites for matrix remodeling in 3D was also elegantly documented in a number of studies. One of the main limitations in early tissue engineering efforts was keeping the large tissues alive, as necrotic core rapidly forms in high cell density tissues once the oxygen diffusion length of *100mm is exceeded. This was one of the key impediments toward clinical translation. However, many tissue engineers have turned this limitation into an advantage. They decided to perfect cultivation of small tissues, whose size is still within oxygen diffusional limitations, motivated by the possibility to precisely control all microenvironmental cues. In parallel to tissue engineering efforts, the field of microfluidics has developed rapidly with Whitesides’ invention of soft lithography, and the ability to rapidly prototype devices using polydimethylsiloxane (PDMS), a soft crosslinkable silicone-based material. Our recent analysis clearly shows a very persistent increase in the number of articles focusing on microfluidics, from only few articles per year in early 2,000s to >16,000 articles per year in 2016 onward. The magic of organ-on-a-chip engineering happened when cells were introduced into microfluidic channel. Although these kinds of experiments were carried out as early as late 1990s, the official term, organ-on-a-chip engineering, gained a widespread recognition with the publication of a landmark lung-on-a-chip article in Science by Don Ingber’s group. In this study, an epithelial/endothelial barrier was recreated on a membrane placed in the middle of a microfluidic channel. Owing to the elastomeric nature of PDMS, it was possible to stretch this membrane through application of vacuum through 2 side channels, mimicking the breathing motion, which ultimately resulted in the establishment of better barrier function. Our group has developed strips of cardiac muscle, of advanced maturation stage, similar to the native trabecula, whose force of contraction can continuously be