Porous Three-Dimensional Silk Fibroin Scaffolds for Tracheal Epithelial Regeneration in Vitro and in Vivo

The Development of Lung Tissue Engineering: From Biomaterials to Multicellular Systems

The challenge of the treatment of end-stage lung disease poses an urgent clinical demand for lung tissue engineering. Over the past few years, various lung tissue-engineered constructs are developed for lung tissue regeneration and respiratory pathology study. In this review, an overview of recent achievements in the field of lung tissue engineering is proposed. The introduction of lung structure and lung injury are stated briefly at first. After that, the lung tissue-engineered constructs are categorized into three types: acellular, monocellular, and multicellular systems. The different bioengineered constructs included in each system that can be applied to the reconstruction of the trachea, airway epithelium, alveoli, and even whole lung are described in detail, followed by the highlight of relevant representative research. Finally, the challenges and future directions of biomaterials, manufacturing technologies, and cells involved in lung tissue engineering are discussed. Overall, this review can provide referable ideas for the realization of functional lung regeneration and permanent lung substitution.

Silk-Based 3D Porous Scaffolds for Tissue Engineering

Silk fibroin, extracted from the silk of the Bombyx mori silkworm, stands out as a biomaterial due to its nontoxic nature, excellent biocompatibility, and adjustable biodegradability. Porous scaffolds, a type of biomaterial, are crucial for creating an optimal microenvironment that supports cell adhesion and proliferation, thereby playing an essential role in tissue remodeling and repair. Therefore, this review focuses on 3D porous silk fibroin-based scaffolds, first summarizing their preparation methods and then detailing their regenerative effects on bone, cartilage, tendon, vascular, neural, skin, hepatic, and tracheal epithelial tissue engineering in recent years.

Role of Biomaterials in the Development of Epithelial Support in 3D In Vitro Airway Epithelium Development: A Systematic Review

Respiratory diseases have a major impact on global health. The airway epithelium, which acts as a frontline defence, is one of the most common targets for inhaled allergens, irritants, or micro-organisms to enter the respiratory system. In the tissue engineering field, biomaterials play a crucial role. Due to the continuing high impact of respiratory diseases on society and the emergence of new respiratory viruses, in vitro airway epithelial models with high microphysiological similarities that are also easily adjustable to replicate disease models are urgently needed to better understand those diseases. Thus, the development of biomaterial scaffolds for the airway epithelium is important due to their function as a cell-support device in which cells are seeded in vitro and then are encouraged to lay down a matrix to form the foundations of a tissue for transplantation. Studies conducted in in vitro models are necessary because they accelerate the development of new treatments. Moreover, in comparatively controlled conditions, in vitro models allow for the stimulation of complex interactions between cells, scaffolds, and growth factors. Based on recent studies, the biomaterial scaffolds that have been tested in in vitro models appear to be viable options for repairing the airway epithelium and avoiding any complications. This review discusses the role of biomaterial scaffolds in in vitro airway epithelium models. The effects of scaffold, physicochemical, and mechanical properties in recent studies were also discussed.

Silk fibroin-hydroxyapatite scaffolds promote the proliferation of adipose-derived mesenchymal stem cells by activating the ERK signal

Adipose-derived mesenchymal stem cell (Ad-MSC) with capacities of releasing trophic factors and chondrogenic differentiation was a promising candidate for tracheal reconstruction. Silk fibroin (SF)- hydroxyapatite (HA) scaffolds were fabricated by the freeze-drying method. And Ad-MSCs were co-cultured on the scaffolds for 14 days in vitro. The role of the SF-HA scaffold in regulating the adhesion, growth, and proliferation of Ad-MSCs, and its potential mechanisms were investigated. The identity of Ad-MSCs was confirmed by cell morphology, surface markers, and differentiation characteristics. Cell proliferation, viability, and morphology were observed via CCK-8, live/dead assay, and scanning electron microscopy (SEM). Gene mRNA and protein levels were examined using quantitative real-time polymerase chain reaction and western blotting, respectively. SF-HA scaffolds showed excellent properties of promoting Ad-MSCs adhesion, growth, and proliferation for at least 14 days. In the CCK-8 assay, the relative OD value of Ad-MSCs cultured on SF-HA scaffolds increased (p < 0.001). Furthermore, live/dead staining showed that the fluorescent coverage increased with time (p < 0.05). SEM also showed that 3 days after inoculation, the coverage of Ad-MSCs on the SF-HA scaffolds was 78.15%, increased to 92.91% on day 7, and reached a peak of 94.38% on day 14. Extracellular signal-regulated kinase (ERK) mRNA and phosphorylated ERK (pERK) protein expression increased at day 3 (p < 0.05), followed by a significant decline at day 7 (p < 0.05). And ERK mRNA expression was positively correlated with Ad-MSCs proliferation (p < 0.05). In summary, the SF-HA scaffold co-cultured with Ad-MSCs is a promising biomaterial for tracheal repair by activating the ERK signal pathway.

The interplay of cells, polymers, and vascularization in three-dimensional lung models and their applications in COVID-19 research and therapy

Millions of people have been affected ever since the emergence of the corona virus disease of 2019 (COVID-19) outbreak, leading to an urgent need for antiviral drug and vaccine development. Current experimentation on traditional two-dimensional culture (2D) fails to accurately mimic the in vivo microenvironment for the disease, while in vivo animal model testing does not faithfully replicate human COVID-19 infection. Human-based three-dimensional (3D) cell culture models such as spheroids, organoids, and organ-on-a-chip present a promising solution to these challenges. In this report, we review the recent 3D in vitro lung models used in COVID-19 infection and drug screening studies and highlight the most common types of natural and synthetic polymers used to generate 3D lung models.

Fabrication of vitamin K3-carnosine peptide-loaded spun silk fibroin fibers/collagen bi-layered architecture for bronchopleural fistula tissue repair and regeneration applications

Bronchial and pleural injuries with persistent air leak pose a threat in the repair and regeneration of pulmonary diseases. The need to arrive at a highly efficient therapy for closure of bronchopleural fistula (BPF) so as to effectively suppress inflammation, infection and repair the damaged pleural space caused by cancer as well as contractile restoration of bronchopleural scars remain a significant clinical challenge. Herein, we have designed and developed potent bioactive vitamin K3 carnosine peptide (VKC)-loaded spun SF fibroin fibers/collagen bi-layered 3D scaffold for bronchopleural fistula tissue engineering applications. The VKC drug showed excellent cell viability in human bronchial epithelial cells (HBECs), in addition to its pronounced higher cytotoxicity against the A549 lung cancer cell line with an IC₅₀ of 5 μg/mL. Furthermore, VKC displayed a strong affinity with the catalytic site of EGFR (PDB ID: 1M17) and VEGFR2 (PDB ID: 4AGD, 4ASD) receptors in molecular docking studies. Following which the spun SF-VKC (primary layer) and collagen film (top layer) constructed bi-layered CSVKC were structurally elucidated and its morphological, physicochemical and biological characterizations were well examined. The bi-layered scaffold showed superior biocompatibility and cell migration ability in HBECs than other scaffolds. Interestingly, the CSVKC revealed rapid HBECs motility towards scratched regions for fast healing in vitro bronchial tissue engineering. In vivo biocompatibility and angiogenesis studies of the prepared scaffolds were evaluated and the results obtained demonstrated excellent new tissue formation and neovascularization in the bi-layered architecture rather than others. Therefore, our results suggest that the potent antibacterial and anticancer therapeutic agent (VKC)-impregnated silk fibroin fibers/collagen bi-layered 3D biomaterial could be useful in treating cancerous BPF and pulmonary diseases in future.

Tissue Engineering for Tracheal Replacement: Strategies and Challenges

The critical feature in trachea replacement is to provide a hollow cylindrical framework that is laterally stable and longitudinally flexible, facilitating cartilage and epithelial tissue formation. Despite advanced techniques and sources of materials used, most inherent challenges are related to the complexity of its anatomy. Limited blood supply leads to insufficient regenerative capacity for cartilage and epithelium. Natural and synthetic scaffolds, different types of cells, and growth factors are part of tissue engineering approaches with varying outcomes. Pre-vascularization remains one of the crucial factors to expedite the regenerative process in tracheal reconstruction. This review discusses the challenges and strategies used in tracheal tissue engineering, focusing on scaffold implantation in clinical and preclinical studies conducted in recent decades.

The Contribution of Silk Fibroin in Biomedical Engineering

Silk fibroin (SF) is a natural protein (biopolymer) extracted from the cocoons of Bombyx mori L. (silkworm). It has many properties of interest in the field of biotechnology, the most important being biodegradability, biocompatibility and robust mechanical strength with high tensile strength. SF is usually dissolved in water-based solvents and can be easily reconstructed into a variety of material formats, including films, mats, hydrogels, and sponges, by various fabrication techniques (spin coating, electrospinning, freeze-drying, and physical or chemical crosslinking). Furthermore, SF is a feasible material used in many biomedical applications, including tissue engineering (3D scaffolds, wounds dressing), cancer therapy (mimicking the tumor microenvironment), controlled drug delivery (SF-based complexes), and bone, eye and skin regeneration. In this review, we describe the structure, composition, general properties, and structure-properties relationship of SF. In addition, the main methods used for ecological extraction and processing of SF that make it a green material are discussed. Lastly, technological advances in the use of SF-based materials are addressed, especially in healthcare applications such as tissue engineering and cancer therapeutics.

Injectable Desferrioxamine-Laden Silk Nanofiber Hydrogels for Accelerating Diabetic Wound Healing

Dysangiogenesis and chronic inflammation are two critical reasons for diabetic foot ulcers. Desferrioxamine (DFO) was used clinically in the treatment of diabetic foot ulcers by repeated injections because of its capacity to induce vascularization. Biocompatible carriers that release DFO slowly and facilitate healing simultaneously are preferable options to accelerate the healing of diabetic wounds. Here, DFO-laden silk nanofiber hydrogels that provided a sustained release of DFO for more than 40 days were used to treat diabetic wounds. The DFO-laden hydrogels stimulated the healing of diabetic wounds. In vitro cell studies revealed that the DFO-laden hydrogels modulated the migration and gene expression of endothelial cells, and they also tuned the inflammation behavior of macrophages. These results were confirmed in an in vivo diabetic wound model. The DFO-laden hydrogels alleviated dysangiogenesis and chronic inflammation in the diabetic wounds, resulting in a more rapid wound healing and increased collagen deposition. Both in vitro and in vivo studies suggested potential clinical applications of these DFO-laden hydrogels in the treatment of diabetic ulcers.

Influence of the Mechanical Environment on the Regeneration of Osteochondral Defects

Articular cartilage is a highly specialised connective tissue of diarthrodial joints which provides a smooth, lubricated surface for joint articulation and plays a crucial role in the transmission of loads. In vivo cartilage is subjected to mechanical stimuli that are essential for cartilage development and the maintenance of a chondrocytic phenotype. Cartilage damage caused by traumatic injuries, ageing, or degradative diseases leads to impaired loading resistance and progressive degeneration of both the articular cartilage and the underlying subchondral bone. Since the tissue has limited self-repairing capacity due its avascular nature, restoration of its mechanical properties is still a major challenge. Tissue engineering techniques have the potential to heal osteochondral defects using a combination of stem cells, growth factors, and biomaterials that could produce a biomechanically functional tissue, representative of native hyaline cartilage. However, current clinical approaches fail to repair full-thickness defects that include the underlying subchondral bone. Moreover, when tested in vivo, current tissue-engineered grafts show limited capacity to regenerate the damaged tissue due to poor integration with host cartilage and the failure to retain structural integrity after insertion, resulting in reduced mechanical function. The aim of this review is to examine the optimal characteristics of osteochondral scaffolds. Additionally, an overview on the latest biomaterials potentially able to replicate the natural mechanical environment of articular cartilage and their role in maintaining mechanical cues to drive chondrogenesis will be detailed, as well as the overall mechanical performance of grafts engineered using different technologies.

Biodegradable Polymers as the Pivotal Player in the Design of Tissue Engineering Scaffolds

Biodegradable polymers play a pivotal role in in situ tissue engineering. Utilizing various technologies, researchers have been able to fabricate 3D tissue engineering scaffolds using biodegradable polymers. They serve as temporary templates, providing physical and biochemical signals to the cells and determining the successful outcome of tissue remodeling. Furthermore, a biodegradable scaffold also presents the fourth dimension for tissue engineering, namely time. The properties of the biodegradable polymer change over time, presenting continuously changing features during the degradation process. These changes become more complicated when different materials are combined together to fabricate a composite or heterogeneous scaffold. This review undertakes a systematic analysis of the basic characteristics of biodegradable polymers and describe recent advances in making composite biodegradable scaffolds for in situ tissue engineering and regenerative medicine. The interaction between implanted biodegradable biomaterials and the in vivo environment are also discussed, including the properties and functional changes of the degradable scaffold, the local effect of degradation on the contiguous tissue and their evaluation using both in vitro and in vivo models.

Entrapped in cage (EiC) scaffolds of 3D-printed polycaprolactone and porous silk fibroin for meniscus tissue engineering

The meniscus has critical functions in the knee joint kinematics and homeostasis. Injuries of the meniscus are frequent, and the lack of a functional meniscus between the femur and tibial plateau can cause articular cartilage degeneration leading to osteoarthritis development and progression. Regeneration of meniscus tissue has outstanding challenges to be addressed. In the current study, novel Entrapped in cage (EiC) scaffolds of 3D-printed polycaprolactone (PCL) and porous silk fibroin were proposed for meniscus tissue engineering. As confirmed by micro-structural analysis the entrapment of silk fibroin was successful, and all scaffolds had excellent interconnectivity (≥99%). The EiC scaffolds had more favorable micro-structure compared with the PCL cage scaffolds by improving the pore size while keeping the interconnectivity almost the same. When compared with the PCL cage, the entrapment of porous silk fibroin into the PCL cage decreased the high compressive modulus in a favorable matter in the wet state thanks to the silk fibroin's high swelling properties. The in vitro studies with human stem cells or meniscocytes seeded constructs, demonstrated that the EiC scaffolds had superior cell adhesion, metabolic activity, and proliferation compared to the PCL cage scaffolds. Upon subcutaneous implantation of scaffolds in nude mice, all groups were free of adverse incidents, and mildly invaded by inflammatory cells with neovascularization, while the EiC scaffolds showed better tissue infiltration. The results of this work indicated that the EiC scaffolds of PCL and silk fibroin are favorable for meniscus tissue engineering, and the findings are encouraging for further studies using a larger animal model.