Tissues and organs comprise multiple functional and supporting cell types, and accurate replication at both the cellular and macro level is required for normal function of engineered tissues. The fabrication of tissues has generally been achieved through the encapsulation or seeding of cells within biodegradable materials. Most of the biomaterials used in tissue-engineering applications are either synthetic or naturally derived. The biomaterials used in tissue-engineered constructs have been largely dictated by their physical properties, such as stiffness, flexibility, and porosity, as well as biological properties such as bioactivity and degradation timelines. New developments in material sciences have facilitated the ability to tailor the material properties to meet the requirements of the tissue-engineered construct. For example, the design of biomimetic and peptide-conjugated synthetic materials has significantly improved their cellular compatibility and bioactivity, whereas the chemical functionalization of “natural” materials has produced materials with adaptable mechanical properties, degradation times, and cross-linking options.3
The design and fabrication of tissue-engineered constructs have been greatly influenced by modern manufacturing techniques. As the complexity of the native tissues increases, from flat structures such as skin, to tubular structures such as blood vessels, to hollow nontubular organs such as the bladder, and to solid organs such as the heart, the complexity of manufacturing also increases. For the nonsolid tissues, fabrication can be achieved using electrospinning, casting, or other traditional techniques, and tissue-engineered skin, cartilage, tracheas, blood vessels, urethras, bladders, and vaginas have been implanted in patients using these strategies.
Complex solid organs require newer approaches to replicate the complex 3-dimensional structures. The use of decellularized solid organs that can be reseeded with the patient’s own cells has been proposed, although preserving patency in the end capillaries remains a challenge for transferring these technologies to patients. The rapid adoption of bioprinting has resulted in the development of custom-made hardware capable of precise layer-by-layer organization of biological materials, biochemicals, and living cells into complex 3-dimensional structures. There are various design strategies used for 3-dimensional bioprinting, such as biomimicry, autonomous self-assembly, and mini-tissue building blocks.4 While 3-dimensional bioprinting is still a relatively recent approach to engineering tissues and organs, successful application for the fabrication of tracheal splints, skin, bone, cartilage, vascular grafts, and heart tissue demonstrates the wide application of this technique.