Figures
- FIGURE 1
Lung bioengineering approaches. In most approaches, a lung scaffold is seeded with autologous or allogeneic cells for bioengineering a lung. The cells can be expanded to appropriate numbers in bioreactors for cell expansion. Different lung scaffolds have been explored, including decellularised scaffolds and synthetic scaffolds. An emerging idea is the use of hybrid scaffolds that combine biological materials such as extracellular matrix (ECM) components with synthetic scaffolds in order to create a hybrid lung scaffold. Bioreactors for organ culture can then be used to mature and evaluate the repopulated lung scaffold before lung transplantation.
- FIGURE 2
Recapitulating the complexity of the lung architecture from proximal (trachea) to distal (bronchi and alveoli): haematoxylin/eosin (HE; left column) staining histology, transmission electron microscopy (TEM; centre column) and scanning electron microscopy (SEM; right column) images from proximal and distal native lung tissue. The lung architecture varies dramatically within the lung: moving from proximal to distal, the resolution required to mimic the native structures is higher (centimetres (cm)→micrometres (μm)→nanometres (nm)). To date, biological scaffolds are the only scaffolds that have a resolution that mimics that of the native lung across all length scales. The bronchi TEM micrograph of a thin section of the mucous membrane of a small human bronchus shows a ciliated cell (CC) with cilia (C) and microvilli, a goblet cell (GC) with an apical mucous plug (MU), basal cell (BC), fibres and fibroblasts (FB) in connective tissue and macrophages (MP). The bronchi SEM micrograph of the epithelial surface shows ciliary tufts (C) and a mucous plug (MU) of a goblet cell in the process of extrusion. The distal lung TEM and SEM micrographs of the structure of the alveolar septum in human lungs show a septal fibroblast (FB), capillary endothelium (EN), alveolar epithelium (EP) and fibre strands (F). HE histology scale bars: 100 µm; bronchi TEM scale bar: 5 µm; bronchi SEM scale bar: 10 µm; distal lung TEM scale bar: 2 µm; distal lung SEM scale bar: 10 µm. Electron microscopy images reproduced and modified from [129, 130] with permission.
Tables
- TABLE 1
Challenges within the field of ex vivo lung tissue engineering
Area Focus for future research/future perspectives Scaffold source Can a suitable acellular xenograft source be identified? Can allogeneic human scaffolds be used? Does the age (neonatal or aged) of the scaffold impact the biomaterial? Evaluate immunogenicity of scaffold with and without cells. Cell sources Are all of the more than 40 cell types found within the lung required to make a functional lung? Can allogeneic cells be used or do we need to use autologous cells? Where will we source cells for patients with chronic or genetic lung diseases? What types of cell sources can be used (e.g. endogenous progenitor cells, induced pluripotent stem cells)? Manufacturing Which Good Manufacturing Practice manufacturing method will be suitable for scaffold generation, storage and maturation? Which standardised approaches for the characterisation and validation of the scaffold will be required? How can we obtain enough cells to recellularise and how will they be re-introduced into the scaffold? Will bioengineered lungs need to be tailored for patients with specific lung diseases (e.g. the main lung transplant recipients (chronic obstructive pulmonary disease, idiopathic pulmonary fibrosis, pulmonary arterial hypertension, cystic fibrosis and α1-antitrypsin deficiency)). Maturation Will different bioreactors be needed for the different cell types in the lung? What time span and/or maturation level will be required? What degree of vascularisation of the scaffold will be required? Surgical and clinical approach How will we assess the functionality prior to transplantation? Will ex vivo lung perfusion parameters be enough to predict success? What surgical techniques could be used for pieces of bioengineered lung tissue? Will special post-operative care be required? Will the patients need to be immunosuppressed? - TABLE 2
Compilation of studies of breakthrough advances within the field of ex vivo lung tissue engineering
Year Material Method Scaffold Significant advance End-points Reference Synthetic 2006 Polyglycolic acid and pluronic F-127 hydrogel Microfabrication techniques Alveoli-like structures Growth of lung progenitor cells on a synthetic scaffold and transplanted In vitro,
in vivo[79] 2006 Poly-dl-lactic acid Microfilm templates and 3D foam Alveoli-like structures Alveolar epithelial cells can be grown on porous synthetic materials In vitro [120] 2012 Decorin-containing matrices Electrospinning Trachea Electrospinning decorin matrices for a tissue-engineered trachea In vitro [73] 2013 Hydroxyethyl methacrylate–alginate–gelatine cryogel Cryogelation Alveoli-like structures Macroporous matrix with ability to recruit cells when implanted in vivo In vitro,
in vivo[84] 2014 Gelatine/microbubble scaffold Microfluidics Alveoli-like structures Differentiation of lung stem/progenitor cells into alveolar pneumocytes and induction of angiogenesis within a manufactured scaffold In vitro,
in vivo[82] 2015 Polyethylene glycol-based hydrogel Microsphere templates Alveoli-like structures Cytocompatible manufacturing method for co-culture of alveoli-like structures In vitro [86] 2016 POSS-PCU (polyhedral oligomeric silsesquioxane poly(carbonate-urea) urethane) Dispersion of porogens Trachea Use of engineered pores to improve integration capacity of a synthetic scaffold In vitro,
in vivo[72] 2017 Alginate beads Alginate beads Alveoli-like structures Self-assembled alveoli-like structures with human cells In vitro [87] 2018 Matrix metalloproteinase-degradable polyethylene glycol-based hydrogel Microsphere templates Alveoli-like structures Controlled degradation with specific matrix metalloproteinase-cleavable sites in alveolar-like structures In vitro [85] Acellular 1981 Alveolar basement membrane (various origins) Decellularisation Alveolar basement membrane First decellularisation attempt to obtain alveolar basement membrane In vitro [121] 1986 Human alveolar and amniotic matrix Decellularisation Acellular alveolar versus amniotic basement membranes First repopulation experiment on acellular lung tissue; differentiation on various basement membranes In vitro [122] 2010 Rat lung Decellularisation Rat acellular lung Orthotopic transplantation In vitro,
in vivo[68] 2010 Rat lung Decellularisation Rat acellular lung Orthotopic transplantation and first report of decellularisation of human lung In vitro,
in vivo[48] 2011 Rat lung and liver Decellularisation Rat acellular lung and liver Cellular differentiation on the scaffolds In vitro [123] 2012 Mouse lung Decellularisation Mouse acellular lung and slices Comparison of different detergent-based protocols for mouse lung de- and recellularisation In vitro [52] 2012 Human lung Decellularisation Human acellular lung and slices De- and recellularisation of human normal and fibrotic lungs In vitro [49] 2013 Human and porcine lung Decellularisation Human and porcine lung and slices De- and recellularisation of human and porcine lungs In vitro [33] 2013 Mouse lung Decellularisation Mouse acellular lung and slices Effects of age and emphysematous and fibrotic injury on murine recellularisation In vitro [124] 2013 Mouse lung Decellularisation Mouse acellular lung and slices Effects of storage and sterilisation on de- and recellularised whole lung In vitro [42] 2014 Human and porcine lung Decellularisation 3D lung segments Small segments to retain 3D lung structure in acellular scaffolds from large animals and human origin for physiological recellularisation In vitro [24] 2014 Rat and human lung Decellularisation Rat and human acellular lung Transplant of iPSC-derived re-epithelialised and re-endothelialised scaffold In vitro,
in vivo[51] 2015 Rat and human lung Decellularisation Rat and human acellular lung Regeneration of functional pulmonary vasculature In vitro,
in vivo[65] 2016 Porcine lung Decellularisation Porcine lung extracellular matrix hydrogel First extracellular matrix hydrogel derived from acellular lung In vitro,
in vivo[125] 2016 Porcine lung: wild-type and α1,3-galactosyltransferase knockout Decellularisation Porcine lung Comparison of de- and recellularisation of wild-type and α1,3-galactosyltransferase knockout pig lungs; identification of residual immunogens in wild-type lungs In vitro [25] 2017 Porcine lung Decellularisation Porcine lung Orthotopic transplantation of porcine scaffold recellularised with human cells In vitro,
in vivo[35] Hybrid 2006 Polylactic-co-glycolic acid, poly-l-lactic acid and Matrigel porous foam and nanofibrous matrix Microfabrication techniques Alveoli-like structures First hybrid material attempt for lung tissue engineering In vitro [80] 2008 Matrigel plug combined with fibroblast growth factor 2-loaded polyvinyl sponge Microfabrication techniques Vascularised pulmonary tissue constructs Distal pulmonary epithelial differentiation can be maintained in vivo; donor-derived endothelial cells contribute to the formation of vessels In vitro,
in vivo[126] 2011 Lung extract-coated poly-ε-caprolactone nanofibres Electrospinning Electrospun nanofibres coated with lung extracts from fibrotic or nonfibrotic mice Ex vivo system to recapitulate the 3D fibrotic lung microenvironment In vitro [22] 2011 Collagen–Matrigel/alginate microcapsules Microsphere encapsulation Alveoli-like structures Fibroblasts, epithelial cells and alveolar type II form alveolus-like structures in collagen–Matrigel/alginate-poly-l-lysine-alginate microcapsule engineered scaffolds In vitro [127] 2017 Poly-ε-caprolactone and decellularised aorta Electrospinning, decellularisation Electrospun poly-ε-caprolactone stents in acellular rabbit aorta Hybrid trachea scaffold for tracheal replacement In vitro,
in vivo[128] Cells 2014 Human pluripotent stem cells NA NA Functional human pluripotent stem cell-derived distal lung epithelial cells seeded onto human scaffold In vitro,
in vivo[59] 2015 Human endothelial and perivascular cells NA NA Regeneration of functional pulmonary vasculature In vitro,
in vivo[65] 2016 Human respiratory epithelial cells NA NA Scalable cell culture system In vitro,
in vivo[107] 2016 KRT5+TP63+ basal epithelial stem cells NA NA Recellularisation In vitro [64] 2018 Chondrocytes, endothelial cells and mesenchymal stem cells 3D bioprinting None Scaffold-free manufacturing of a rat trachea mimic In vitro,
in vivo[101] 3D: three-dimensional; iPSC: induced pluripotent stem cell; NA: not applicable.
Jump To
- Article
- Abstract
- Abstract
- Introduction
- Manufacturing lung scaffolds
- Acellular lung scaffolds
- Artificial lung scaffolds
- Cell types and scaling-up cell culture methods
- Bioreactor strategies for lung bioengineering
- Regulatory and ethical implications for translating lung bioengineering approaches
- Discussion and outlook
- Acknowledgements
- Footnotes
- References
- Figures & Data
- Info & Metrics