The group of authors, led by Wei Sun, PhD, of Drexel University in Philadelphia and Tsinghua University in Beijing, delved into specific bioprinting applications and shared their visions for the advances in science and technology that could address the technology's current challenges and opportunities.

Bioprinting utilizes cells, proteins, and biomaterials as building blocks for 3D-printed biological models, biological systems, and therapeutic products. The technique can be used for a number of biological applications, including printing biomaterials for tissue scaffolds and implants, printing cells or organoids for 3D biological models, and printing micro-organ chips for physiological platforms and engineered living systems.

Biomedical applications include the study of in vitro regenerative and physiological function, disease and pathogenesis development (including cancer), and drug screening with intended in vitro cell or tissue models. Work in this field has stimulated the development of novel bioinks, translational tissue engineering, personalized cancer treatments, and drug discoveries.

Sun noted several challenges that need to be addressed:

• The need for a new generation of novel bioinks with multifunctional properties to better transport, protect, and grow cells during and after printing
• Better printing processes and printers to deliver cells with high survivability and high precision
• Efficient and effective crosslinking techniques and crosslinkers to maintain bioink structural integrity and stability after printing
• Integration with microfluidic devices to provide a long-term and a simulated physiological environment in which to culture printed models

Other authors reported on specific issues relating to the following: cell expansion to 3D cell printing, bioinks for 3D bioprinting, bioprinting of stem cells, large-scale and efficient production of organoids or cell aggregates, strategy for bioprinting of tissue vascular system and tissue assembly, 3D-printed biohybrid tissues as in vitro biological models for disease study, 3D-printing for organ-on-a-chip development, biomanufacturing of multicellular engineered living systems, bioprinting in space, and bioprinting technologies.

Among the highlights:

• Cell expansion is a critical upstream process step for cell and tissue manufacturing. Improvement in bioreactor-based cell-expansion systems is required to lower barriers to the adoption of bioprinting in regenerative medicine and tissue engineering product markets. Many bioreactor systems are based on designs used for biopharmaceutical-based production. One key difference is that in biofabrication, live cells are the final product. There has been work to build cell-expansion reactors that address low wall shear requirements of working with stem cells and improve upon scaling up cell number.
• Through bioprinting, stem cells can be particularly positioned in 3D in relation to other cell types and/or biomaterials. 3D printing will be essential in realizing the potential basic science and therapeutic applications of stem cells, allowing production of more tissue‐like structures. New bioprinting technologies in combination with other platforms, such as bioreactors and organs-on-a-chip, can allow more efficient and cost-effective cell expansion and differentiation compared with standard 2D methods. Bioprinting could eventually be the preferred platform to utilize human stem cells to produce artificial solid tissues and organs.
• The combination of 3D bioprinting with microfluidics allows the development of the next generation of organ-on-a-chip platforms. To mimic the biological relationships in vitro, microfluidic systems and perfusion chambers have been integrated to simulate in vivo microenvironments. Researchers are working to link several chips to create a human-on-a-chip platform for drug testing and disease studies. The author suggests that in the future, it may be possible to develop in vivo-like organs in vitro to revolutionize modern medicine and healthcare by developing organ-on-a-chip platforms used in personalized drug screening, drug development, and improving drug safety.

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