Bioengineering lungs: An overview of current methods, requirements, and challenges for constructing scaffolds

How do we expand the lung donor pool?

PURPOSE OF REVIEW

Lung transplantation activity continues to be limited by the availability of timely quality donor lungs. It is apparent though that progress has been made. The steady evolution of clinical practice, combined with painstaking scientific discovery and innovation are described.

RECENT FINDINGS

There have been successful studies reporting innovations in the wider use and broader consideration of donation after circulatory death donor lungs, including an increasing number of transplants from each of the controlled, uncontrolled and medically assisted dying donor descriptive categories. Donors beyond age 70 years are providing better than expected long-term outcomes. Hepatitis C PCR positive donor lungs can be safely used if treated postoperatively with appropriate antivirals. Donor lung perfusion at a constant 10 degrees appears capable of significantly improving donor logistics and ex-vivo lung perfusion offers the potential of an ever-increasing number of novel donor management roles. Bioartificial and xenografts remain distant possibilities only at present.

SUMMARY

Donor lungs have proved to be surprisingly robust and combined with clinical, scientific and engineering innovations, the realizable lung donor pool is proving to be larger than previously thought.

Orthotopic transplantation of the bioengineered lung using a mouse-scale perfusion-based bioreactor and human primary endothelial cells

Whole lung engineering and the transplantation of its products is an ambitious goal and ultimately a viable solution for alleviating the donor-shortage crisis for lung transplants. There are several limitations currently impeding progress in the field with a major obstacle being efficient revascularization of decellularized scaffolds, which requires an extremely large number of cells when using larger pre-clinical animal models. Here, we developed a simple but effective experimental pulmonary bioengineering platform by utilizing the lung as a scaffold. Revascularization of pulmonary vasculature using human umbilical cord vein endothelial cells was feasible using a novel in-house developed perfusion-based bioreactor. The endothelial lumens formed in the peripheral alveolar area were confirmed using a transmission electron microscope. The quality of engineered lung vasculature was evaluated using box-counting analysis of histological images. The engineered mouse lungs were successfully transplanted into the orthotopic thoracic cavity. The engineered vasculature in the lung scaffold showed blood perfusion after transplantation without significant hemorrhage. The mouse-based lung bioengineering system can be utilized as an efficient ex-vivo screening platform for lung tissue engineering.

Development of lung tissue models and their applications

The lungs are important organs that play a critical role in the development of specific diseases, as well as responding to the effects of drugs, chemicals, and environmental pollutants. Due to the ethical concerns around animal testing, alternative methods have been sought which are more time-effective, do not pose ethical issues for animals, do not involve species differences, and provide easy investigation of the pathobiology of lung diseases. Several national and international organizations are working to accelerate the development and implementation of structurally and functionally complex tissue models as alternatives to animal testing, particularly for the lung. Unfortunately, to date, there is no lung tissue model that has been accepted by regulatory agencies for use in inhalation toxicology. This review discusses the challenges involved in developing a relevant lung tissue model derived from human cells such as cell lines, primary cells, and pluripotent stem cells. It also introduces examples of two-dimensional (2D) air-liquid interface and monocultured and co-cultured three-dimensional (3D) culture techniques, particularly organoid culture and 3D bioprinting. Furthermore, it reviews development of the lung-on-a-chip model to mimic the microenvironment and physiological performance. The applications of lung tissue models in various studies, especially disease modeling, viral respiratory infection, and environmental toxicology will be also introduced. The development of a relevant lung tissue model is extremely important for standardizing and validation the in vitro models for inhalation toxicity and other studies in the future.

Epidemiology and Long-Term Outcomes in Thoracic Transplantation

Over the past five decades, outcomes for lung transplantation have significantly improved in the early post-operative period, such that lung transplant is now the gold standard treatment for end-stage respiratory disease. The major limitation that impacts lung transplant survival rates is the development of chronic lung allograft dysfunction (CLAD). CLAD affects around 50% of lung transplant recipients within five years of transplantation. We must also consider other factors impacting the survival rate such as the surgical technique (single versus double lung transplant), along with donor and recipient characteristics. The future is promising, with more research looking into ex vivo lung perfusion (EVLP) and bioengineered lungs, with the hope of increasing the donor pool and decreasing the risk of graft rejection.

Global hotspots and emerging trends in 3D bioprinting research

Three-dimensional (3D) bioprinting is an advanced tissue engineering technique that has received a lot of interest in the past years. We aimed to highlight the characteristics of articles on 3D bioprinting, especially in terms of research hotspots and focus. Publications related to 3D bioprinting from 2007 to 2022 were acquired from the Web of Science Core Collection database. We have used VOSviewer, CiteSpace, and R-bibliometrix to perform various analyses on 3,327 published articles. The number of annual publications is increasing globally, a trend expected to continue. The United States and China were the most productive countries with the closest cooperation and the most research and development investment funds in this field. Harvard Medical School and Tsinghua University are the top-ranked institutions in the United States and China, respectively. Dr. Anthony Atala and Dr. Ali Khademhosseini, the most productive researchers in 3D bioprinting, may provide cooperation opportunities for interested researchers. Tissue Engineering Part A contributed the largest publication number, while Frontiers in Bioengineering and Biotechnology was the most attractive journal with the most potential. As for the keywords in 3D bioprinting, Bio-ink, Hydrogels (especially GelMA and Gelatin), Scaffold (especially decellularized extracellular matrix), extrusion-based bioprinting, tissue engineering, and in vitro models (organoids particularly) are research hotspots analyzed in the current study. Specifically, the research topics "new bio-ink investigation," "modification of extrusion-based bioprinting for cell viability and vascularization," "application of 3D bioprinting in organoids and in vitro model" and "research in personalized and regenerative medicine" were predicted to be hotspots for future research.