Biomaterial inks and bioinks for fabricating 3D biomimetic lung tissue: A delicate balancing act between biocompatibility and mechanical printability

Three dimensional (3D) bioprinting has rapidly emerged as a major fabrication tool in the bioengineering of tissue constructs. 3D bioprinting enables the precise deposition of nano, micro and macroscale tissue components to produce tissue models that more closely mimic the unique and complex architecture of their corresponding in vivo tissues. Despite the vast potential of 3D bioprinting to revolutionize tissue engineering, few biomimetic models of lung tissue have been reported in the literature to date, which is largely owed to the lung's highly complex branching tubular network that permits gaseous transport and exchange with its inextricably integrated blood supply system. This review focuses on the repertoire of natural and synthetic polymers that have been employed thus far to fabricate hydrogel-based tissue constructs of various regions of the lung airway network. The applicability of novel biocompatible materials, such as micro- and nanogels and self-assembling low molecular weight gelators, to the fabrication of lung tissue constructs are also discussed. The essential criteria that these biomaterial inks must fulfill from both biological and engineering perspectives, including biomimicry, biocompatibility, biodegradability and rheology, are emphasized within the context of current 3D bioprinting technology.