Evolution of Bioinks and Additive Manufacturing Technologies for 3D Bioprinting

3D printing is an additive manufacturing (AM) technique that has quickly disrupted traditional design and manufacturing strategies. New structures can be manufactured that could not be fabricated using other methods. These new capabilities are considered by many to hallmark a historic shift representative of a new industrial revolution. Exciting utilities of this evolving technology are the fields of biomedical engineering and translational medicine, particularly in applying three-dimensional (3D) printing toward enabling on-demand fabrication of customized tissue scaffolds and medical device geometries. AM techniques are promising a future where on-demand production of patient-specific living tissues is a reality. In this review, we cover the rapid evolution and widespread concepts of a bio-"ink" and bioprinted devices and tissues from the past two decades as well as review the various additive manufacturing methods that have been used toward 3D bioprinting of cells and scaffolds with a special look at the benefits and practical considerations for each method. Despite being a young technology, the evolution and impact of AM in the fields of tissue engineering and regenerative medicine has progressed rapidly. We finish the review by looking toward the future of bioprinting and identify some of the current bottlenecks facing the blossoming industry.