Bioprinting stem cells: building physiological tissues one cell at a time.

Bioprinting aims to direct the spatial arrangement in three dimensions of cells, biomaterials and growth factors. The biofabrication of clinically relevant constructs for the repair or modelling of either diseased or damaged tissues is rapidly advancing, resulting in the ability to 3D print biomimetic platforms which imitate a large number of tissues in the human body. Primary tissue-specific cells are typically isolated from patients and used for the fabrication of 3D models for drug screening or tissue repair purposes. However, the lack of resilience of these platforms, due to the difficulties in harnessing, processing and implanting patient-specific cells can limit regeneration ability. The printing of stem cells obviates these hurdles, producing functional in vitro models or implantable constructs. Advancements in biomaterial science are helping the development of inks suitable for the encapsulation and the printing of stem cells, promoting their functional growth and differentiation. This review investigates recent studies exploring innovative and functional approaches for the printing of 3D constructs to model disease or repair damaged tissues. Key concepts in tissue physiology are highlighted, reporting stem cell applications in tissue engineering and biofabrication. Bioprinting technologies and biomaterial inks are listed and analysed, including recent advancements in biomaterial design for bioprinting applications. Recent successful efforts and clinical potentials for the manufacturing of functional physiological tissue substitutes are here reported, with a major focus on specific tissues such as vasculature, heart, lung and airways, liver, bone and muscle.