Basement Membrane Mimics of Biofunctionalized Nanofibers for a Bipolar-Cultured Human Primary Alveolar-Capillary Barrier Model

Biomimetic basement membranes: advances in materials, preparation techniques, and applications in in vitro biological models

In vitro biological model technology has become a cornerstone of modern biological research, driving advancements in drug screening, physiological and pathological studies, and tissue implantation applications. The natural basement membrane (BM), a homogeneous structure, provides critical physical and biological support for tissues and organs. To replicate its function, researchers have developed biomimetic BMs using advanced fabrication technologies, which are increasingly applied to in vitro models. This review explores the materials, preparation techniques, and applications of biomimetic BMs across various biological models, highlighting their advantages and limitations. Additionally, it discusses recent progress in the field and identifies current challenges in achieving BM simulations that closely mimic native structures. Future directions and recommendations are provided to guide the development of high-performance biomimetic BM materials and their manufacturing processes.

A micro-scale humanized ventilator-on-a-chip to examine the injurious effects of mechanical ventilation

Patients with compromised respiratory function frequently require mechanical ventilation to survive. Unfortunately, non-uniform ventilation of injured lungs generates complex mechanical forces that lead to ventilator induced lung injury (VILI). Although investigators have developed lung-on-a-chip systems to simulate normal respiration, modeling the complex mechanics of VILI as well as the subsequent recovery phase is a challenge. Here we present a novel humanized in vitro ventilator-on-a-chip (VOC) model of the lung microenvironment that simulates the different types of injurious forces generated in the lung during mechanical ventilation. We used transepithelial/endothelial electrical impedance measurements to investigate how individual and simultaneous application of mechanical forces alters real-time changes in barrier integrity during and after injury. We find that compressive stress (i.e. barotrauma) does not significantly alter barrier integrity while over-distention (20% cyclic radial strain, volutrauma) results in decreased barrier integrity that quickly recovers upon removal of mechanical stress. Conversely, surface tension forces generated during airway reopening (atelectrauma), result in a rapid loss of barrier integrity with a delayed recovery relative to volutrauma. Simultaneous application of cyclic stretching (volutrauma) and airway reopening (atelectrauma), indicates that the surface tension forces associated with reopening fluid-occluded lung regions are the primary driver of barrier disruption. Thus, our novel VOC system can monitor the effects of different types of injurious forces on barrier disruption and recovery in real-time and can be used to interogate the biomechanical mechanisms of VILI.

The Modulation of Respiratory Epithelial Cell Differentiation by the Thickness of an Electrospun Poly-ε-Carprolactone Mesh Mimicking the Basement Membrane

The topology of the basement membrane (BM) affects cell physiology and pathology, and BM thickening is associated with various chronic lung diseases. In addition, the topology of commercially available poly (ethylene terephthalate) (PET) membranes, which are used in preclinical in vitro models, differs from that of the human BM, which has a fibrous and elastic structure. In this study, we verified the effect of BM thickness on the differentiation of normal human bronchial epithelial (NHBE) cells. To evaluate whether the thickness of poly-ε-carprolactone (PCL) mesh affects the differentiation of NHBE cells, cells were grown on thin- (6-layer) and thick-layer (80-layer) meshes consisting of electrospun PCL nanofibers using an air-liquid interface (ALI) cell culture system. It was found that the NHBE cells formed a normal pseudostratified epithelium composed of ciliated, goblet, and basal cells on the thin-layer PCL mesh; however, goblet cell hyperplasia was observed on the thick-layer PCL mesh. Differentiated NHBE cells cultured on the thick-layer PCL mesh also demonstrated increased epithelial-mesenchymal transition (EMT) compared to those cultured on the thin-layer PCL mesh. In addition, expression of Sox9, nuclear factor (NF)-κB, and oxidative stress-related markers, which are also associated with goblet cell hyperplasia, was increased in the differentiated NHBE cells cultured on the thick-layer PCL mesh. Thus, the use of thick electrospun PCL mesh led to NHBE cells differentiating into hyperplastic goblet cells via EMT and the oxidative stress-related signaling pathway. Therefore, the topology of the BM, for example, thickness, may affect the differentiation direction of human bronchial epithelial cells.

A miniaturized multicellular platform to mimic the 3D structure of the alveolar-capillary barrier

Several diseases affect the alveoli, and the efficacy of medical treatments and pharmaceutical therapies is hampered by the lack of pre-clinical models able to recreate in vitro the diseases. Microfluidic devices, mimicking the key structural and compositional features of the alveoli, offer several advantages to medium and high-throughput analysis of new candidate therapies. Here, we developed an alveolus-on-a-chip recapitulating the microanatomy of the physiological tissue by including the epithelium, the fibrous interstitial layer and the capillary endothelium. A PDMS device was obtained assembling a top layer and a bottom layer obtained by replica molding. A polycaprolactone/gelatin (PCL-Gel) electrospun membrane was included within the two layers supporting the seeding of 3 cell phenotypes. Epithelial cells were grown on a fibroblast-laden collagen hydrogel located on the top side of the PCL-Gel mats while endothelial cells were seeded on the basolateral side of the membrane. The innovative design of the microfluidic device allows to replicate both cell-cell and cell-extracellular matrix interactions according to the in vivo cell arrangement along with the establishment of physiologically relevant air-liquid interface conditions. Indeed, high cell viability was confirmed for up to 10 days and the formation of a tight endothelial and epithelial barrier was assessed by immunofluorescence assays.

Fibroblasts and polymer composition are essential for bioengineering of airway epithelium on nonwoven scaffolds

To make tissue engineering a truly effective tool, it is necessary to understand how the patterns of specific tissue development are modulated by and depend on the artificial environment. Even the most advanced approaches still do not fully meet the requirements of practical engineering of tracheobronchial epithelium. This study aimed to test the ability of the synthetic and natural nonwoven scaffolds to support the formation of morphological sound airway epithelium including the basement membrane (BM). We also sought to identify the potential role of fibroblasts in this process. Our results showed that nonwoven scaffolds are generally suitable for producing well-differentiated tracheobronchial epithelium (with cilia and goblet cells), while the structure and functionality of the equivalents appeared to be highly dependent on the composition of the scaffolds. Unlike natural scaffolds, synthetic ones supported the formation of the epithelium only when epithelial cells were cocultured with fibroblasts. Fibroblasts also appeared to be obligatory for basal lamina formation, regardless of the type of the nonwoven material used. However, even in the presence of fibroblasts, the synthetic scaffolds were unable to support the formation of the epithelium and of the BM (in particular, basal lamina) as effectively as the natural scaffolds did.

A Micro-scale Humanized Ventilator-on-a-Chip to Examine the Injurious Effects of Mechanical Ventilation

Patients with compromised respiratory function frequently require mechanical ventilation to survive. Unfortunately, non-uniform ventilation of injured lungs generates complex mechanical forces that lead to ventilator induced lung injury (VILI). Although investigators have developed lung-on-a-chip systems to simulate normal respiration, modeling the complex mechanics of VILI as well as the subsequent recovery phase is a challenge. Here we present a novel humanized in vitro ventilator-on-a-chip (VOC) model of the lung microenvironment that simulates the different types of injurious forces generated in the lung during mechanical ventilation. We used transepithelial/endothelial electrical resistance (TEER) measurements to investigate how individual and simultaneous application of the different mechanical forces alters real-time changes in barrier integrity during and after injury. We find that compressive stress (i.e. barotrauma) does not significantly alter barrier integrity while over-distention (20% cyclic radial strain, volutrauma) results in decreased barrier integrity that quickly recovers upon removal of mechanical stress. Conversely, surface tension forces generated during airway reopening (atelectrauma), result in a rapid loss of barrier integrity with a delayed recovery relative to volutrauma. Simultaneous application of cyclic stretching (volutrauma) and airway reopening (atelectrauma), indicate that the surface tension forces associated with reopening fluid-occluded lung regions is the primary driver of barrier disruption. Thus, our novel VOC system can monitor the effects of different types of injurious forces on barrier disruption and recovery in real-time and can be used to identify the biomechanical mechanisms of VILI.

A hybrid fluorescent nanofiber membrane integrated with microfluidic chips towards lung-on-a-chip applications

Here, we report a fluorescent electrospun nanofiber membrane for integration into microfluidic devices towards lung-on-a-chip applications complemented with the results of computational fluid dynamics modelling. A proposed hybrid poly(ε-caprolactone) (PCL)-collagen membrane was developed, characterized, tested, and integrated into a prototype microfluidic chip for biocompatibility studies. The resulting membrane has a thickness of approximately 10 μm, can be adjusted for appropriate porosity, and offers excellent biocompatibility for mimicry of a basement membrane to be used in lung-on-a-chip device applications. Several membrane variations were synthesized and evaluated using SEM, FTIR, AFM, and high-resolution confocal fluorescence microscopy. A sample microfluidic chip made of cyclic olefin copolymer and polydimethylsiloxane was built and integrated with the developed PCL-collagen membrane for on-chip cell culture visualisation and biocompatibility studies. The sample chip design was modelled to determine the optimal fluidic conditions for using the membrane in the chip under fluidic conditions for future studies. The integration of the proposed membrane into microfluidic devices represents a novel strategy for improving lung-on-a-chip applications which can enhance laboratory recapitulation of the lung microenvironment.

Peptide-Functionalized Electrospun Meshes for the Physiological Cultivation of Pulmonary Alveolar Capillary Barrier Models in a 3D-Printed Micro-Bioreactor

In vitro environments that realize biomimetic scaffolds, cellular composition, physiological shear, and strain are integral to developing tissue models of organ-specific functions. In this study, an in vitro pulmonary alveolar capillary barrier model is developed that closely mimics physiological functions by combining a synthetic biofunctionalized nanofibrous membrane system with a novel three-dimensional (3D)-printed bioreactor. The fiber meshes are fabricated from a mixture of polycaprolactone (PCL), 6-armed star-shaped isocyanate-terminated poly(ethylene glycol) (sPEG-NCO), and Arg-Gly-Asp (RGD) peptides by a one-step electrospinning process that offers full control over the fiber surface chemistry. The tunable meshes are mounted within the bioreactor where they support the co-cultivation of pulmonary epithelial (NCI-H441) and endothelial (HPMEC) cell monolayers at air-liquid interface under controlled stimulation by fluid shear stress and cyclic distention. This stimulation, which closely mimics blood circulation and breathing motion, is observed to impact alveolar endothelial cytoskeleton arrangement and improve epithelial tight junction formation as well as surfactant protein B production compared to static models. The results highlight the potential of PCL-sPEG-NCO:RGD nanofibrous scaffolds in combination with a 3D-printed bioreactor system as a platform to reconstruct and enhance in vitro models to bear a close resemblance to in vivo tissues.

Topographical influence of electrospun basement membrane mimics on formation of cellular monolayer

Functional unit of many organs like lung, kidney, intestine, and eye have their endothelial and epithelial monolayers physically separated by a specialized extracellular matrix called the basement membrane. The intricate and complex topography of this matrix influences cell function, behavior and overall homeostasis. In vitro barrier function replication of such organs requires mimicking of these native features on an artificial scaffold system. Apart from chemical and mechanical features, the choice of nano-scale topography of the artificial scaffold is integral, however its influence on monolayer barrier formation is unclear. Though studies have reported improved single cell adhesion and proliferation in presence of pores or pitted topology, corresponding influence on confluent monolayer formation is not well reported. In this work, basement membrane mimic with secondary topographical cues is developed and its influence on single cells and their monolayers is investigated. We show that single cells cultured on fibers with secondary cues form stronger focal adhesions and undergo increased proliferation. Counterintuitively, absence of secondary cues promoted stronger cell-cell interaction in endothelial monolayers and promoted formation of integral tight barriers in alveolar epithelial monolayers. Overall, this work highlights the importance of choice of scaffold topology to develop basement barrier function in in vitro models.

Biomimetic In Vitro Lung Models: Current Challenges and Future Perspective

As post-COVID complications, chronic respiratory diseases are one of the foremost causes of mortality. The quest for a cure for this recent global challenge underlines that the lack of predictive in vitro lung models is one of the main bottlenecks in pulmonary preclinical drug development. Despite rigorous efforts to develop biomimetic in vitro lung models, the current cutting-edge models represent a compromise in numerous technological and biological aspects. Most advanced in vitro models are still in the "proof-of-concept" phase with a low clinical translation of the findings. On the other hand, advances in cellular and molecular studies are mainly based on relatively simple and unrealistic in vitro models. Herein, the current challenges and potential strategies toward not only bioinspired but truly biomimetic lung models are discussed.

Alternative lung cell model systems for toxicology testing strategies: Current knowledge and future outlook

Due to the current relevance of pulmonary toxicology (with focus upon air pollution and the inhalation of hazardous materials), it is important to further develop and implement physiologically relevant models of the entire respiratory tract. Lung model development has the aim to create human relevant systems that may replace animal use whilst balancing cost, laborious nature and regulatory ambition. There is an imperative need to move away from rodent models and implement models that mimic the holistic characteristics important in lung function. The purpose of this review is therefore, to describe and identify the various alternative models that are being applied towards assessing the pulmonary toxicology of inhaled substances, as well as the current and potential developments of various advanced models and how they may be applied towards toxicology testing strategies. These models aim to mimic various regions of the lung, as well as implementing different exposure methods with the addition of various physiologically relevent conditions (such as fluid-flow and dynamic movement). There is further progress in the type of models used with focus on the development of lung-on-a-chip technologies and bioprinting, as well as and the optimization of such models to fill current knowledge gaps within toxicology.

Ultrathin and handleable nanofibrous net as a novel biomimetic basement membrane material for endothelial barrier formation

Many strategies have been adopted to develop porous membranes to reconstitute basement membrane in vitro, which play a key role in the development of in vitro biomimetic models. However, the development of an artificial basement membrane combines cytocompatibility and nano-thickness is still challenging. Herein, a monolayer nanofibrous net patch was fabricated by combining microfabrication and electrospinning as a biomimetic basement membrane material, which was demonstrated for endothelial barrier formation. The nanofibrous net patches with different fiber densities were obtained by controlling electrospinning time. The net was with high porosity and ultrathin thickness approximate to the diameter of nanofibers, which is comparable to that of the native basement membrane. The morphology, proliferation and cell-cell/cell-substrate interactions of endothelial cells on the nanofibrous nets were studied and compared with track-etched polycarbonate membrane and traditional multilayer nanofibers membrane. In addition, the results of TEER measurement and permeability test demonstrated that the endothelial barrier formed on the nanofibrous net patch displayed stronger barrier integrity and function. Therefore, the proposed nanofibrous net patch shows great potential as a novel biomimetic basement membrane, which is promising to be applied for in vitro tissue mimetic applications.

Real-Time Measurement of Cell Mechanics as a Clinically Relevant Readout of an In Vitro Lung Fibrosis Model Established on a Bioinspired Basement Membrane

Lung fibrosis, one of the major post-COVID complications, is a progressive and ultimately fatal disease without a cure. Here, an organ- and disease-specific in vitro mini-lung fibrosis model equipped with noninvasive real-time monitoring of cell mechanics is introduced as a functional readout. To establish an intricate multiculture model under physiologic conditions, a biomimetic ultrathin basement (biphasic elastic thin for air-liquid culture conditions, BETA) membrane (<1 µm) is developed with unique properties, including biocompatibility, permeability, and high elasticity (<10 kPa) for cell culturing under air-liquid interface and cyclic mechanical stretch conditions. The human-based triple coculture fibrosis model, which includes epithelial and endothelial cell lines combined with primary fibroblasts from idiopathic pulmonary fibrosis patients established on the BETA membrane, is integrated into a millifluidic bioreactor system (cyclic in vitro cell-stretch, CIVIC) with dose-controlled aerosolized drug delivery, mimicking inhalation therapy. The real-time measurement of cell/tissue stiffness (and compliance) is shown as a clinical biomarker of the progression/attenuation of fibrosis upon drug treatment, which is confirmed for inhaled Nintedanib-an antifibrosis drug. The mini-lung fibrosis model allows the combined longitudinal testing of pharmacodynamics and pharmacokinetics of drugs, which is expected to enhance the predictive capacity of preclinical models and hence facilitate the development of approved therapies for lung fibrosis.

Novel electrospun chitosan/PEO membranes for more predictive nanoparticle transport studies at biological barriers

The design of safe and effective nanoparticles (NPs) for commercial and medical applications requires a profound understanding of NP translocation and effects at biological barriers. To gain mechanistic insights, physiologically relevant and accurate human in vitro biobarrier models are indispensable. However, current transfer models largely rely on artificial porous polymer membranes for the cultivation of cells, which do not provide a close mimic of the natural basal membrane and intrinsically provide limited permeability for NPs. In this study, electrospinning is exploited to develop thin chitosan/polyethylene oxide (PEO) membranes with a high porosity and nanofibrous morphology for more predictive NP transfer studies. The nanofiber membranes allow the cultivation of a tight and functional placental monolayer (BeWo trophoblasts). Translocation studies with differently sized molecules and NPs (Na-fluorescein; 40 kDa FITC-Dextran; 25 nm PMMA; 70, 180 and 520 nm polystyrene NPs) across empty and cell containing membranes reveal a considerably enhanced permeability compared to commercial microporous membranes. Importantly, the transfer data of NPs is highly similar to data from ex vivo perfusion studies of intact human placental tissue. Therefore, the newly developed membranes may decisively contribute to establish physiologically relevant in vitro biobarrier transfer models with superior permeability for a wide range of molecules and particles.

Bioactive Cell-Derived ECM Scaffold Forms a Unique Cellular Microenvironment for Lung Tissue Engineering

Chronic lung diseases are one of the leading causes of death worldwide. Lung transplantation is currently the only causal therapeutic for lung diseases, which is restricted to end-stage disease and limited by low access to donor lungs. Lung tissue engineering (LTE) is a promising approach to regenerating a replacement for at least a part of the damaged lung tissue. Currently, lung regeneration is limited to a simplified local level (e.g., alveolar−capillary barrier) due to the sophisticated and complex structure and physiology of the lung. Here, we introduce an extracellular matrix (ECM)-integrated scaffold using a cellularization−decellularization−recellularization technique. This ECM-integrated scaffold was developed on our artificial co-polymeric BETA (biphasic elastic thin for air−liquid interface cell culture conditions) scaffold, which were initially populated with human lung fibroblasts (IMR90 cell line), as the main generator of ECM proteins. Due to the interconnected porous structure of the thin (<5 µm) BETA scaffold, the cells can grow on and infiltrate into the scaffold and deposit their own ECM. After a mild decellularization procedure, the ECM proteins remained on the scaffold, which now closely mimicked the cellular microenvironment of pulmonary cells more realistically than the plain artificial scaffolds. We assessed several decellularization methods and found that 20 mM NH4OH and 0.1% Triton X100 with subsequent DNase treatment completely removed the fibroblasts (from the first cellularization) and maintains collagen I and IV as the key ECM proteins on the scaffold. We also showed the repopulation of the primary fibroblast from human (without chronic lung disease (non-CLD) donors) and human bronchial epithelial (16HBE14o−) cells on the ECM-integrated BETA scaffold. With this technique, we developed a biomimetic scaffold that can mimic both the physico-mechanical properties and the native microenvironment of the lung ECM. The results indicate the potential of the presented bioactive scaffold for LTE application.

Mimicking the Natural Basement Membrane for Advanced Tissue Engineering

Advancements in the field of tissue engineering have led to the elucidation of physical and chemical characteristics of physiological basement membranes (BM) as specialized forms of the extracellular matrix. Efforts to recapitulate the intricate structure and biological composition of the BM have encountered various advancements due to its impact on cell fate, function, and regulation. More attention has been paid to synthesizing biocompatible and biofunctional fibrillar scaffolds that closely mimic the natural BM. Specific modifications in biomimetic BM have paved the way for the development of in vitro models like alveolar-capillary barrier, airway models, skin, blood-brain barrier, kidney barrier, and metastatic models, which can be used for personalized drug screening, understanding physiological and pathological pathways, and tissue implants. In this Review, we focus on the structure, composition, and functions of in vivo BM and the ongoing efforts to mimic it synthetically. Light has been shed on the advantages and limitations of various forms of biomimetic BM scaffolds including porous polymeric membranes, hydrogels, and electrospun membranes This Review further elaborates and justifies the significance of BM mimics in tissue engineering, in particular in the development of in vitro organ model systems.

Engineering cryoelectrospun elastin-alginate scaffolds to serve as stromal extracellular matrices

Scaffold-based regenerative strategies that emulate physical, biochemical, and mechanical properties of the native extracellular matrix (ECM) of the region of interest can influence cell growth and function. Existing ECM-mimicking scaffolds, including nanofiber (NF) mats, sponges, hydrogels, and NF-hydrogel composites are unable to simultaneously mimic typical composition, topography, pore size, porosity, and viscoelastic properties of healthy soft-tissue ECM. In this work, we used cryoelectrospinning to fabricate 3D porous scaffolds with minimal fibrous backbone, pore size and mechanical properties similar to soft-tissue connective tissue ECM. We used salivary glands as our soft tissue model and found the decellularized adult salivary gland (DSG) matrix to have a fibrous backbone, 10-30μm pores, 120 Pa indentation modulus, and ∼200 s relaxation half time. We used elastin and alginate as natural, compliant biomaterials and water as the solvent for cryoelectrospinning scaffolds to mimic the structure and viscoelasticity of the connective tissue ECM of the DSG. Process parameters were optimized to produce scaffolds with desirable topography and compliance similar to DSG, with a high yield of >100 scaffolds/run. Using water as solvent, rather than organic solvents, was critical to generate biocompatible scaffolds with desirable topography; further, it permitted a green chemistry fabrication process. Here, we demonstrate that cryoelectrospun scaffolds (CESs) support penetration of NIH 3T3 fibroblasts 250-450µm into the scaffold, cell survival, and maintenance of a stromal cell phenotype. Thus, we demonstrate that elastin-alginate CESs mimic many structural and functional properties of ECM and have potential for future use in regenerative medicine applications.

A High-Throughput Distal Lung Air-Blood Barrier Model Enabled By Density-Driven Underside Epithelium Seeding

High-throughput tissue barrier models can yield critical insights on how barrier function responds to therapeutics, pathogens, and toxins. However, such models often emphasize multiplexing capability at the expense of physiologic relevance. Particularly, the distal lung's air-blood barrier is typically modeled with epithelial cell monoculture, neglecting the substantial contribution of endothelial cell feedback in the coordination of barrier function. An obstacle to establishing high-throughput coculture models relevant to the epithelium/endothelium interface is the requirement for underside cell seeding, which is difficult to miniaturize and automate. Therefore, this paper describes a scalable, low-cost seeding method that eliminates inversion by optimizing medium density to float cells so they attach under the membrane. This method generates a 96-well model of the distal lung epithelium-endothelium barrier with serum-free, glucocorticoid-free air-liquid differentiation. The polarized epithelial-endothelial coculture exhibits mature barrier function, appropriate intercellular junction staining, and epithelial-to-endothelial transmission of inflammatory stimuli such as polyinosine:polycytidylic acid (poly(I:C)). Further, exposure to influenza A virus PR8 and human beta-coronavirus OC43 initiates a dose-dependent inflammatory response that propagates from the epithelium to endothelium. While this model focuses on the air-blood barrier, the underside seeding method is generalizable to various coculture tissue models for scalable, physiologic screening.

Reconstruction of Ultra-thin Alveolar-capillary Basement Membrane Mimics

Alveolar-capillary basement membrane (BM) is ultra-thin (<2 µm) extracellular matrix that maintains integral epithelial-endothelial cell layers. In vitro reconstructions of alveolar-capillary barrier supported on synthetic scaffolds closely resembling the fibrous and ultra-thin natural BM are essential in mimicking the lung pathophysiology. Although BM topology and dimensions are well known to significantly influence cellular behavior, conventionally used BM mimics fail to recreate this natural niche. To overcome this, electrospun ultra-thin 2 µm poly(caprolactone) (PCL) nanofibrous mesh is used to establish an alveolar-capillary barrier model of lung endothelial/epithelial cells. Transepithelial electrical resistance (TEER) and permeability studies reveal integral tight junctions and improved mass transport through the highly porous PCL meshes compared to conventional dense membranes with etched pores. The chemotaxis of neutrophils is shown across the barrier in presence of inflammatory response that is naturally impeded in confined regions. Conventional requirement of 3 µm or larger pore size can lead to barrier disruption due to epithelial/endothelial cell invasion. Despite high porosity, the interconnected BM mimic prevents barrier disruption and allows neutrophil transmigration, thereby demonstrating the physiological relevance of the thin nanofibrous meshes. It is envisioned that these bipolar cultured barriers would contribute to an organ-level in vitro model for pathological disease, environmental pollutants, and nanotoxicology.

Analysis of Thickness and Roughness Effects of Artificial Basement Membranes on Endothelial Cell Functions

Various cells and tissues are highly organized in vivo by basement membranes (BMs) and thus promising artificial BMs (A-BMs) constructed by electrospinning and layer-by-layer (LbL) assembly have recently attracted much attention in the tissue engineering field. However, control of cell adhesion, morphology, and migration of the attached cells on the A-BMs has not been reported yet. In this study, we investigated both thickness and roughness-dependent effects of A-BMs on the functions of endothelial cells (ECs), which resulted from different assembly concentrations. The results indicated that the roughness of A-BMs increased gradually with the increase of nanofilm thickness. EC adhesion, spreading and proliferation were inhibited on thicker A-BM surfaces with larger roughness, while interendothelial junctions and the barrier effect of confluent EC monolayers on thicker A-BM surfaces were compensated by increasing seeding cell number and expanding culture time. Our study highlights the influence of LbL assembly conditions on endothelial functions, which offers a new criterion for the design of A-BMs in well-organized 3D tissues.

A Bioinspired in vitro Lung Model to Study Particokinetics of Nano-/Microparticles Under Cyclic Stretch and Air-Liquid Interface Conditions

Evolution has endowed the lung with exceptional design providing a large surface area for gas exchange area (ca. 100 m²) in a relatively small tissue volume (ca. 6 L). This is possible due to a complex tissue architecture that has resulted in one of the most challenging organs to be recreated in the lab. The need for realistic and robust in vitro lung models becomes even more evident as causal therapies, especially for chronic respiratory diseases, are lacking. Here, we describe the Cyclic InVItroCell-stretch (CIVIC) “breathing” lung bioreactor for pulmonary epithelial cells at the air-liquid interface (ALI) experiencing cyclic stretch while monitoring stretch-related parameters (amplitude, frequency, and membrane elastic modulus) under real-time conditions. The previously described biomimetic copolymeric BETA membrane (5 μm thick, bioactive, porous, and elastic) was attempted to be improved for even more biomimetic permeability, elasticity (elastic modulus and stretchability), and bioactivity by changing its chemical composition. This biphasic membrane supports both the initial formation of a tight monolayer of pulmonary epithelial cells (A549 and 16HBE14o⁻) under submerged conditions and the subsequent cell-stretch experiments at the ALI without preconditioning of the membrane. The newly manufactured versions of the BETA membrane did not improve the characteristics of the previously determined optimum BETA membrane (9.35% PCL and 6.34% gelatin [w/v solvent]). Hence, the optimum BETA membrane was used to investigate quantitatively the role of physiologic cyclic mechanical stretch (10% linear stretch; 0.33 Hz: light exercise conditions) on size-dependent cellular uptake and transepithelial transport of nanoparticles (100 nm) and microparticles (1,000 nm) for alveolar epithelial cells (A549) under ALI conditions. Our results show that physiologic stretch enhances cellular uptake of 100 nm nanoparticles across the epithelial cell barrier, but the barrier becomes permeable for both nano- and micron-sized particles (100 and 1,000 nm). This suggests that currently used static in vitro assays may underestimate cellular uptake and transbarrier transport of nanoparticles in the lung.

In Situ Cross-Linking of Artificial Basement Membranes in 3D Tissues and Their Size-Dependent Molecular Permeability

In the human body, highly organized tissues rely on the compartmentalization effect of basement membranes (BMs) that separate different types of cells. We recently reported an artificial basement membrane (A-BM) composed of type-IV collagen and laminin (Col-IV/LM), which are the main components of natural BMs, for cell compartmentalization in three-dimensional (3D) tissues. However, such compartmentalized structures can be maintained only for 3 days, probably due to the degradation issues. In this study, a robust A-BM was fabricated by in situ cross-linking the Col-IV/LM layer-by-layer (LbL) nanofilms in 3D tissues by transglutaminase. The effects of molecular size and configuration on the permeability of obtained A-BMs were comprehensively studied using polystyrene nanoparticles (PS NPs) and dextran with various hydrodynamic diameters, as well as albumin. The findings agreed well with the known size-selective behavior of the glomerular basement membrane. Cross-linked Col-IV/LM nanofilms demonstrate improved stability and a more powerful barrier effect to maintain cell compartmentalization for organized 3D tissues. This in vitro A-BM exhibit great potentials for the design of more complex compartmentalized 3D tissues, for understanding the unique cell-cell cross talk through BMs, and for providing a more reliable 3D tissue model for new drug screening and other in vitro physiological studies.

Fabrication of Artificial Nanobasement Membranes for Cell Compartmentalization in 3D Tissues

In recent decades, tissue engineering techniques have attracted much attention in the construction of 3D tissues or organs. However, even though precise control of cell locations in 3D has been achieved, the organized cell locations are easily destroyed because of the cell migration during the cell culture period. In human body, basement membranes (BMs) maintain the precise cell locations in 3D (compartmentalization). Constructing artificial BMs that mimic the structure and biofunctions of natural BMs remains a major challenge. Here, a nanometer-sized artificial BM through layer-by-layer assembly of collagen type IV (Col-IV) and laminin (LM), chosen because they are the main components of natural BMs, is reported. This multilayered Col-IV/LM nanofilm imitates natural BM structure closely, showing controllable and similar components, thickness, and fibrous network. The Col-IV/LM nanofilms have high cell adhesion properties and maintain the spreading morphology effectively. Furthermore, the barrier effect of preventing cell migration but permitting effective cell-cell crosstalk between fibroblasts and endothelial cells demonstrates the ability of Col-IV/LM nanofilms for cell compartmentalization in 3D tissues, providing more reliable tissue models for evaluating drug efficacy, nanotoxicology, and implantation.

Multifunctional Biomaterials: Combining Material Modification Strategies for Engineering of Cell-Contacting Surfaces

In the human body, cells in a tissue are exposed to signals derived from their specific extracellular matrix (ECM), such as architectural structure, mechanical properties, and chemical composition (proteins, growth factors). Research on biomaterials in tissue engineering and regenerative medicine aims to recreate such stimuli using engineered materials to induce a specific response of cells at the interface. Although traditional biomaterials design has been mostly limited to varying individual signals, increasing interest has arisen on combining several features in recent years to improve the mimicry of extracellular matrix properties. Tremendous progress in combinatorial surface modification exploiting, for example, topographical features or variations in mechanics combined with biochemical cues has enabled the identification of their key regulatory characteristics on various cell fate decisions. Gradients especially facilitated such research by enabling the investigation of combined continuous changes of different signals. Despite unravelling important synergies for cellular responses, challenges arise in terms of fabrication and characterization of multifunctional engineered materials. This review summarizes recent work on combinatorial surface modifications that aim to control biological responses. Modification and characterization methods for enhanced control over multifunctional material properties are highlighted and discussed. Thereby, this review deepens the understanding and knowledge of biomimetic combinatorial material modification, their challenges but especially their potential.

Bioengineering of Pulmonary Epithelium With Preservation of the Vascular Niche

The shortage of transplantable donor organs directly affects patients with end-stage lung disease, for which transplantation remains the only definitive treatment. With the current acceptance rate of donor lungs of only 20%, rescuing even one half of the rejected donor lungs would increase the number of transplantable lungs threefold, to 60%. We review recent advances in lung bioengineering that have potential to repair the epithelial and vascular compartments of the lung. Our focus is on the long-term support and recovery of the lung ex vivo, and the replacement of defective epithelium with healthy therapeutic cells. To this end, we first review the roles of the lung epithelium and vasculature, with focus on the alveolar-capillary membrane, and then discuss the available and emerging technologies for ex vivo bioengineering of the lung by decellularization and recellularization. While there have been many meritorious advances in these technologies for recovering marginal quality lungs to the levels needed to meet the standards for transplantation – many challenges remain, motivating further studies of the extended ex vivo support and interventions in the lung. We propose that the repair of injured epithelium with preservation of quiescent vasculature will be critical for the immediate blood supply to the lung and the lung survival and function following transplantation.

Fibrosis in tissue engineering and regenerative medicine: treat or trigger?

Fibrosis is a life-threatening pathological condition resulting from a dysfunctional tissue repair process. There is no efficient treatment and organ transplantation is in many cases the only therapeutic option. Here we review tissue engineering and regenerative medicine (TERM) approaches to address fibrosis in the cardiovascular system, the kidney, the lung and the liver. These strategies have great potential to achieve repair or replacement of diseased organs by cell- and material-based therapies. However, paradoxically, they might also trigger fibrosis. Cases of TERM interventions with adverse outcome are also included in this review. Furthermore, we emphasize the fact that, although organ engineering is still in its infancy, the advances in the field are leading to biomedically relevant in vitro models with tremendous potential for disease recapitulation and development of therapies. These human tissue models might have increased predictive power for human drug responses thereby reducing the need for animal testing.

Options for modeling the respiratory system: inserts, scaffolds and microfluidic chips

The human respiratory system is continuously exposed to varying levels of hazardous substances ranging from environmental toxins to purposely administered drugs. If the noxious effects exceed the inherent regenerative capacity of the respiratory system, injured tissue undergoes complex remodeling that can significantly affect lung function and lead to various diseases. Advanced near-to-native in vitro lung models are required to understand the mechanisms involved in pulmonary damage and repair and to reliably test the toxicity of compounds to lung tissue. This review is an overview of the development of in vitro respiratory system models used for study of lung diseases. It includes discussion of using these models for environmental toxin assessment and pulmonary toxicity screening.

Electrospun Nanometer to Micrometer Scale Biomimetic Synthetic Membrane Scaffolds in Drug Delivery and Tissue Engineering: A Review

The scaffold technology research utilizes biomimicry to produce efficient scaffolds that mimic the natural cell growth environment including the basement membrane for tissue engineering. Because the natural basement membrane is composed of fibrillar protein networks of nanoscale diameter, the scaffold produced should efficiently mimic the nanoscale topography at a low production cost. Electrospinning is a technique that can achieve that. This review discusses the physical and chemical characteristics of the basement membrane and its significance on cell growth and overall focuses on nanoscale biomimetic synthetic membrane scaffolds primarily generated using electrospinning and their application in drug delivery and tissue engineering.