3D bioprinting for lungs and hollow organs

Tissue-specific gelatin bioink as a rheology modifier for high printability and adjustable tissue properties

Decellularized extracellular matrix (dECM) has emerged as an exceptional biomaterial that effectively recapitulates the native tissue microenvironment for enhanced regenerative potential. Although various dECM bioinks derived from different tissues have shown promising results, challenges persist in achieving high-resolution printing of flexible tissue constructs because of the inherent limitations of dECM's weak mechanical properties and poor printability. Attempts to enhance mechanical rigidity through chemical modifications, photoinitiators, and nanomaterial reinforcement have often compromised the bioactivity of dECM and mismatched the desired mechanical properties of target tissues. In response, this study proposes a novel method involving a tissue-specific rheological modifier, gelatinized dECM. This modifier autonomously enhances bioink modulus pre-printing, ensuring immediate and precise shape formation upon extrusion. The hybrid bioink with GeldECM undergoes a triple crosslinking system-physical entanglement for pre-printing, visible light photocrosslinking during printing for increased efficiency, and thermal crosslinking post-printing during tissue culture. A meticulous gelatinization process preserves the dECM protein components, and optimal hybrid ratios modify the mechanical properties, tailoring them to specific tissues. The application of this sequential multiple crosslinking designs successfully yielded soft yet resilient tissue constructs capable of withstanding vigorous agitation with high shape fidelity. This innovative method, founded on mechanical modulation by GeldECM, holds promise for the fabrication of flexible tissues with high resilience.

Advances in 3D bioprinting for urethral tissue reconstruction

Urethral conditions affect children and adults, increasing the risk of urinary tract infections, voiding and sexual dysfunction, and renal failure. Current tissue replacements differ from healthy urethral tissues in structural and mechanical characteristics, causing high risk of postoperative complications. 3D bioprinting can overcome these limitations through the creation of complex, layered architectures using materials with location-specific biomechanical properties. This review highlights prior research and describes the potential for these emerging technologies to address ongoing challenges in urethral tissue engineering, including biomechanical and structural mismatch, lack of individualized repair solutions, and inadequate wound healing and vascularization. In the future, the integration of 3D bioprinting technology with advanced biomaterials, computational modeling, and 3D imaging could transform personalized urethral surgical procedures.

Future trends of additive manufacturing in medical applications: An overview

Additive Manufacturing (AM) has recently demonstrated significant medical progress. Due to advancements in materials and methodologies, various processes have been developed to cater to the medical sector's requirements, including bioprinting and 4D, 5D, and 6D printing. However, only a few studies have captured these emerging trends and their medical applications. Therefore, this overview presents an analysis of the advancements and achievements obtained in AM for the medical industry, focusing on the principal trends identified in the annual report of AM3DP.

Advanced 3D In Vitro Lung Fibrosis Models: Contemporary Status, Clinical Uptake, and Prospective Outlooks

Fibrosis has been characterized as a global health problem and ranks as one of the primary causes of organ dysfunction. Currently, there is no cure for pulmonary fibrosis, and limited therapeutic options are available due to an inadequate understanding of the disease pathogenesis. The absence of advanced in vitro models replicating dynamic temporal changes observed in the tissue with the progression of the disease is a significant impediment in the development of novel antifibrotic treatments, which has motivated research on tissue-mimetic three-dimensional (3D) models. In this review, we summarize emerging trends in preparing advanced lung models to recapitulate biochemical and biomechanical processes associated with lung fibrogenesis. We begin by describing the importance of in vivo studies and highlighting the often poor correlation between preclinical research and clinical outcomes and the limitations of conventional cell culture in accurately simulating the 3D tissue microenvironment. Rapid advancement in biomaterials, biofabrication, biomicrofluidics, and related bioengineering techniques are enabling the preparation of in vitro models to reproduce the epithelium structure and operate as reliable drug screening strategies for precise prediction. Improving and understanding these model systems is necessary to find the cross-talks between growing cells and the stage at which myofibroblasts differentiate. These advanced models allow us to utilize the knowledge and identify, characterize, and hand pick medicines beneficial to the human community. The challenges of the current approaches, along with the opportunities for further research with potential for translation in this field, are presented toward developing novel treatments for pulmonary fibrosis.

Application of tissue engineering techniques in tracheal repair: a bibliometric study

Transplantation of tissue-engineered trachea is an effective treatment for long-segment tracheal injury. This technology avoids problems associated with a lack of donor resources and immune rejection, generating an artificial trachea with good biocompatibility. To our knowledge, a systematic summary of basic and clinical research on tissue-engineered trachea in the last 20 years has not been conducted. Here, we analyzed the development trends of tissue-engineered trachea research by bibliometric means and outlined the future perspectives in this field. The Web of Science portal was selected as the data source. CiteSpace, VOSviewer, and the Bibliometric Online Analysis Platform were used to analyze the number of publications, journals, countries, institutions, authors, and keywords from 475 screened studies. Between 2000 and 2023, the number of published studies on tissue-engineered trachea has been increasing. Biomaterials published the largest number of papers. The United States and China have made the largest contributions to this field. University College London published the highest number of studies, and the most productive researcher was an Italian scholar, Paolo Macchiarini. However, close collaborations between various researchers and institutions from different countries were generally lacking. Despite this, keyword analysis showed that manufacturing methods for tracheal stents, hydrogel materials, and 3D bioprinting technology are current popular research topics. Our bibliometric study will help scientists in this field gain an in-depth understanding of the current research progress and development trends to guide their future work, and researchers in related fields will benefit from the introduction to transplantation methods of tissue-engineered trachea.

3D Bioprinting for Tumor Metastasis Research

Tumor metastasis is a multiple cascade process where tumor cells disseminate from the primary site to distant organs and subsequently adapt to the foreign microenvironment. Simulating the physiology of tumor metastatic events in a realistic and three-dimensional (3D) manner is a challenge for in vitro modeling. 3D bioprinting strategies, which can generate well-customized and bionic structures, enable the exploration of dynamic tumor metastasis process in a species-homologous, high-throughput and reproducible way. In this review, we summarize the recent application of 3D bioprinting in constructing in vitro tumor metastatic models and discuss its advantages and current limitations. Further perspectives on how to harness the potential of accessible 3D bioprinting strategies to better model tumor metastasis and guide anti-cancer therapies are also provided.

Regenerative medicine: current research and perspective in pediatric surgery

The field of regenerative medicine, encompassing several disciplines including stem cell biology and tissue engineering, continues to advance with the accumulating research on cell manipulation technologies, gene therapy and new materials. Recent progress in preclinical and clinical studies may transcend the boundaries of regenerative medicine from laboratory research towards clinical reality. However, for the ultimate goal to construct bioengineered transplantable organs, a number of issues still need to be addressed. In particular, engineering of elaborate tissues and organs requires a fine combination of different relevant aspects; not only the repopulation of multiple cell phenotypes in an appropriate distribution but also the adjustment of the host environmental factors such as vascularisation, innervation and immunomodulation. The aim of this review article is to provide an overview of the recent discoveries and development in stem cells and tissue engineering, which are inseparably interconnected. The current status of research on tissue stem cells and bioengineering, and the possibilities for application in specific organs relevant to paediatric surgery have been specifically focused and outlined.

Comparing species-different responses in pulmonary fibrosis research: Current understanding of in vitro lung cell models and nanomaterials

Pulmonary fibrosis (PF) is a chronic, irreversible lung disease that is typically fatal and characterized by an abnormal fibrotic response. As a result, vast areas of the lungs are gradually affected, and gas exchange is impaired, making it one of the world's leading causes of death. This can be attributed to a lack of understanding of the onset and progression of the disease, as well as a poor understanding of the mechanism of adverse responses to various factors, such as exposure to allergens, nanomaterials, environmental pollutants, etc. So far, the most frequently used preclinical evaluation paradigm for PF is still animal testing. Nonetheless, there is an urgent need to understand the factors that induce PF and find novel therapeutic targets for PF in humans. In this regard, robust and realistic in vitro fibrosis models are required to understand the mechanism of adverse responses. Over the years, several in vitro and ex vivo models have been developed with the goal of mimicking the biological barriers of the lung as closely as possible. This review summarizes recent progress towards the development of experimental models suitable for predicting fibrotic responses, with an emphasis on cell culture methods, nanomaterials, and a comparison of results from studies using cells from various species.

What is the need and why is it time for innovative models for understanding lung repair and regeneration?

Advances in tissue engineering continue at a rapid pace and have provided novel methodologies and insights into normal cell and tissue homeostasis, disease pathogenesis, and new potential therapeutic strategies. The evolution of new techniques has particularly invigorated the field and span a range from novel organ and organoid technologies to increasingly sophisticated imaging modalities. This is particularly relevant for the field of lung biology and diseases as many lung diseases, including chronic obstructive pulmonary disease (COPD) and idiopathic fibrosis (IPF), among others, remain incurable with significant morbidity and mortality. Advances in lung regenerative medicine and engineering also offer new potential avenues for critical illnesses such as the acute respiratory distress syndrome (ARDS) which also continue to have significant morbidity and mortality. In this review, an overview of lung regenerative medicine with focus on current status of both structural and functional repair will be presented. This will serve as a platform for surveying innovative models and techniques for study, highlighting the need and timeliness for these approaches.

Recent Developments in 3D Bio-Printing and Its Biomedical Applications

The fast-developing field of 3D bio-printing has been extensively used to improve the usability and performance of scaffolds filled with cells. Over the last few decades, a variety of tissues and organs including skin, blood vessels, and hearts, etc., have all been produced in large quantities via 3D bio-printing. These tissues and organs are not only able to serve as building blocks for the ultimate goal of repair and regeneration, but they can also be utilized as in vitro models for pharmacokinetics, drug screening, and other purposes. To further 3D-printing uses in tissue engineering, research on novel, suitable biomaterials with quick cross-linking capabilities is a prerequisite. A wider variety of acceptable 3D-printed materials are still needed, as well as better printing resolution (particularly at the nanoscale range), speed, and biomaterial compatibility. The aim of this study is to provide expertise in the most prevalent and new biomaterials used in 3D bio-printing as well as an introduction to the associated approaches that are frequently considered by researchers. Furthermore, an effort has been made to convey the most pertinent implementations of 3D bio-printing processes, such as tissue regeneration, etc., by providing the most significant research together with a comprehensive list of material selection guidelines, constraints, and future prospects.

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.

Multifunctional 3D platforms for rapid hemostasis and wound healing: Structural and functional prospects at biointerfaces

354Fabrication of multifunctional hemostats is indispensable against chronic blood loss and accelerated wound healing. Various hemostatic materials that aid wound repair or rapid tissue regeneration has been developed in the last 5 years. This review provides an overview of the three-dimensional (3D) hemostatic platforms designed through the latest technologies like electrospinning, 3D printing, and lithography, solely or in combination, for application in rapid wound healing. We critically discuss the pivotal role of micro/nano-3D topography and biomaterial properties in mediating rapid blood clots and healing at the hemostat-biointerface. We also highlight the advantages and limitations of the designed 3D hemostats. We anticipate that this review will guide the fabrication of smart hemostats of the future for tissue engineering applications.

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

Chronic respiratory diseases remain a significant health burden worldwide. The only option for individuals with end-stage lung failure remains Lung Transplantation. However, suitable organ donor shortages and immune rejection following transplantation remain a challenge. Since alternative options are urgently required to increase tissue availability for lung transplantation, researchers have been exploring lung bioengineering extensively, to generate functional, transplantable organs and tissue. Additionally, the development of physiologically-relevant artificial tissue models for testing novel therapies also represents an important step toward finding a definite clinical solution for different chronic respiratory diseases. This mini-review aims to highlight some of the most common methodologies used in bioengineering lung scaffolds, as well as the benefits and disadvantages associated with each method in conjunction with the current areas of research devoted to solving some of these challenges in the area of lung bioengineering.

Recent advances in biofabricated gut models to understand the gut-brain axis in neurological diseases

Increasing evidence has accumulated that gut microbiome dysbiosis could be linked to neurological diseases, including both neurodegenerative and psychiatric diseases. With the high prevalence of neurological diseases, there is an urgent need to elucidate the underlying mechanisms between the microbiome, gut, and brain. However, the standardized aniikmal models for these studies have critical disadvantages for their translation into clinical application, such as limited physiological relevance due to interspecies differences and difficulty interpreting causality from complex systemic interactions. Therefore, alternative in vitro gut–brain axis models are highly required to understand their related pathophysiology and set novel therapeutic strategies. In this review, we outline state-of-the-art biofabrication technologies for modeling in vitro human intestines. Existing 3D gut models are categorized according to their topographical and anatomical similarities to the native gut. In addition, we deliberate future research directions to develop more functional in vitro intestinal models to study the gut–brain axis in neurological diseases rather than simply recreating the morphology.

Additive manufacturing in respiratory sciences - Current applications and future prospects

Additive Manufacturing (AM) comprises a variety of techniques that enable fabrication of customised objects with specific attributes. The versatility of AM procedures and constant technological improvements allow for their application in the development of medicinal products and medical devices. This review provides an overview of AM applications related to respiratory sciences. For this purpose, both fields of research are briefly introduced and the potential benefits of integrating AM to respiratory sciences at different levels of pharmaceutical development are highlighted. Tailored manufacturing of microstructures as a particle design approach in respiratory drug delivery will be discussed. At the dosage form level, we exemplify AM as an important link in the iterative loop of data driven inhaler design, rapid prototyping and in vitro testing. This review also presents the application of bioprinting in the respiratory field for design of biorelevant in vitro cellular models, followed by an overview of AM-related processes in preventive and therapeutic care. Finally, this review discusses future prospects of AM as a component in a digital health environment.

Construction and Application of in vitro Alveolar Models Based on 3D Printing Technology

Increasing lung diseases, mutating coronaviruses, and the development of new compounds urgently require biomimetic in vitro lung models for lung pathology, toxicology, and pharmacology. The current construction strategies for lung models mainly include animal models, 2D cell culture, lung-on-a-chip, and lung organoids. However, current models face difficulties in reproducing in vivo-like alveolar size and vesicle-like structures, and are unable to contain multiple cell types. In this study, a strategy for constructing alveolar models based on degradable hydrogel microspheres is proposed. Hydrogel microspheres, 200–250 µm in diameter, were prepared using a self-developed printing technique driven by alternating viscous and inertial forces. Microcapsules were further constructed using a coacervation-based layer-by-layer technique and core liquefaction. Three types of cells were inoculated and co-cultured on hydrogel capsules based on optimized microcapsule surface treatment strategies. Finally, an in vitro three-dimensional endothelial alveolar model with a multicellular composition and vesicle-like structure with a diameter of approximately 230 µm was successfully constructed. Cells in the constructed alveolar model maintained a high survival rate. The LD₅₀ values of glutaraldehyde based on the constructed models were in good agreement with the reference values, validating the potential of the model for future toxicant and drug detection.

3D Bioprinting of Human Hollow Organs

3D bioprinting is a rapidly evolving technique that has been found to have extensive applications in disease research, tissue engineering, and regenerative medicine. 3D bioprinting might be a solution to global organ shortages and the growing aversion to testing cell patterning for novel tissue fabrication and building superior disease models. It has the unrivaled capability of layer-by-layer deposition using different types of biomaterials, stem cells, and biomolecules with a perfectly regulated spatial distribution. The tissue regeneration of hollow organs has always been a challenge for medical science because of the complexities of their cell structures. In this mini review, we will address the status of the science behind tissue engineering and 3D bioprinting of epithelialized tubular hollow organs. This review will also cover the current challenges and prospects, as well as the application of these complicated 3D-printed organs.

The promising rise of bioprinting in revolutionalizing medical science: Advances and possibilities

Bioprinting is a relatively new yet evolving technique predominantly used in regenerative medicine and tissue engineering. 3D bioprinting techniques combine the advantages of creating Extracellular Matrix (ECM)like environments for cells and computer-aided tailoring of predetermined tissue shapes and structures. The essential application of bioprinting is for the regeneration or restoration of damaged and injured tissues by producing implantable tissues and organs. The capability of bioprinting is yet to be fully scrutinized in sectors like the patient-specific spatial distribution of cells, bio-robotics, etc. In this review, currently developed experimental systems and strategies for the bioprinting of different types of tissues as well as for drug delivery and cancer research are explored for potential applications. This review also digs into the most recent opportunities and future possibilities for the efficient implementation of bioprinting to restructure medical and technological practices.

3D Bioprinting Strategies, Challenges, and Opportunities to Model the Lung Tissue Microenvironment and Its Function

Human lungs are organs with an intricate hierarchical structure and complex composition; lungs also present heterogeneous mechanical properties that impose dynamic stress on different tissue components during the process of breathing. These physiological characteristics combined create a system that is challenging to model in vitro. Many efforts have been dedicated to develop reliable models that afford a better understanding of the structure of the lung and to study cell dynamics, disease evolution, and drug pharmacodynamics and pharmacokinetics in the lung. This review presents methodologies used to develop lung tissue models, highlighting their advantages and current limitations, focusing on 3D bioprinting as a promising set of technologies that can address current challenges. 3D bioprinting can be used to create 3D structures that are key to bridging the gap between current cell culture methods and living tissues. Thus, 3D bioprinting can produce lung tissue biomimetics that can be used to develop in vitro models and could eventually produce functional tissue for transplantation. Yet, printing functional synthetic tissues that recreate lung structure and function is still beyond the current capabilities of 3D bioprinting technology. Here, the current state of 3D bioprinting is described with a focus on key strategies that can be used to exploit the potential that this technology has to offer. Despite today's limitations, results show that 3D bioprinting has unexplored potential that may be accessible by optimizing bioink composition and looking at the printing process through a holistic and creative lens.

Bioprinting Technology in Skin, Heart, Pancreas and Cartilage Tissues: Progress and Challenges in Clinical Practice

Bioprinting is an emerging additive manufacturing technique which shows an outstanding potential for shaping customized functional substitutes for tissue engineering. Its introduction into the clinical space in order to replace injured organs could ideally overcome the limitations faced with allografts. Presently, even though there have been years of prolific research in the field, there is a wide gap to bridge in order to bring bioprinting from "bench to bedside". This is due to the fact that bioprinted designs have not yet reached the complexity required for clinical use, nor have clear GMP (good manufacturing practices) rules or precise regulatory guidelines been established. This review provides an overview of some of the most recent and remarkable achievements for skin, heart, pancreas and cartilage bioprinting breakthroughs while highlighting the critical shortcomings for each tissue type which is keeping this technique from becoming widespread reality.

Immune response against the biomaterials used in 3D bioprinting of organs

Regenerative medicine has developed promising approaches for healing and replacing defective and damaged organs or tissues with functional ones. Three-dimensional (3D) bioprinting innovation has integrated a potential to design organs or tissues specific to the patient with the capability of rapid construction to fulfill the storage of organs and the need for transplantation. 3D bioprinting of organs has the main goal to develop a structural and functional organ or tissue mimic to the original one. The highly complex fabrication of tissue engineering scaffolds containing biomaterials, tissue models, and biomedical devices has made it possible to print small blood vessels to mimic organs to reduce organ or tissue rejection. 3D bioprinting has the concept of bioinks containing biomaterials that may trigger the immune responses in the body. Nevertheless, foreign body response (FBR) is mediated by various cell types such as B-cells, dendritic cells, macrophages, natural killer cells, neutrophils, and T-cells, and molecular signals such as antibodies (Abs), cytokines, and reactive radical species. Typically, the biomaterial is shielded by the fibrous encapsulation that is regulated by molecular signals. This review explored the progress in 3D bioprinting of vital organs and basic immune response against the biomaterials used in this approach. Thus, evaluating immune response against biomaterials used in 3D printed organs is necessary to mitigate tissue rejection after the transplantation.

Support Diminution Design for Layered Manufacturing of Manifold Surface Based on Variable Orientation Tracking

This article proposes a support diminution design method for layered manufacturing of manifold surface based on variable orientation tracking (VOT). We aim at reducing the external support or upholders to a minimum with maximum possibility theoretically to save material and diminish material stripping effect (MSE), thereby improving the bilateral surface precision either exterior or interior. The cosmic gravity effect criterion is first used to extract surface need support from manifold surface with various materials by considering the balance force involving material characteristics and inclination angle. In the light of this criterion theory, varying the substrate normal orientation (SNO), namely workbench, for each layer in printing coordinate system, may break the balance between gravity and its equilibrium force. Therefore, the optimal SNO can be rigorously calculated using mathematical harmonic analysis among the continuous domain. To serve for the multidegree of freedom (DOF) on account of SNO, a reconfigurable VOT robot with six-axis DOF is developed for 3D printing (3DP). The matched servo controller is successfully implemented to accurate tracking of both orientation and Cartesian coordinates, using forward kinematic chains as well as reverse kinematic tracking. What is more, the end-effector (extruder) is holding perpendicular to the substrate workbench. The physical experiment that takes human external ear auricle, for example, using a layer-based process is implemented via VOT. The MSE due to supporting material can be clearly observed and diminished using an optical microscope. The stripped material from external support via diminution design can be evaluated quantitatively by electronic weighting balance. All of which indicate the findings that external support in 3DP can be virtually reckoned and diminished using VOT rather than the so-called build orientation traversal method. The VOT method upon which we touched can be widely applied to various layered manufacturing of accurate structure, for instance, cantilever, sandwich, and scaffolds in the occasion needing precise curtailment of outer support multimaterial.

A Paradigm Shift in Tissue Engineering: From a Top–Down to a Bottom–Up Strategy

Tissue engineering (TE) was initially designed to tackle clinical organ shortage problems. Although some engineered tissues have been successfully used for non-clinical applications, very few (e.g., reconstructed human skin) have been used for clinical purposes. As the current TE approach has not achieved much success regarding more broad and general clinical applications, organ shortage still remains a challenging issue. This very limited clinical application of TE can be attributed to the constraints in manufacturing fully functional tissues via the traditional top–down approach, where very limited cell types are seeded and cultured in scaffolds with equivalent sizes and morphologies as the target tissues. The newly proposed developmental engineering (DE) strategy towards the manufacture of fully functional tissues utilises a bottom–up approach to mimic developmental biology processes by implementing gradual tissue assembly alongside the growth of multiple cell types in modular scaffolds. This approach may overcome the constraints of the traditional top–down strategy as it can imitate in vivo-like tissue development processes. However, several essential issues must be considered, and more mechanistic insights of the fundamental, underpinning biological processes, such as cell–cell and cell–material interactions, are necessary. The aim of this review is to firstly introduce and compare the number of cell types, the size and morphology of the scaffolds, and the generic tissue reconstruction procedures utilised in the top–down and the bottom–up strategies; then, it will analyse their advantages, disadvantages, and challenges; and finally, it will briefly discuss the possible technologies that may overcome some of the inherent limitations of the bottom–up strategy.

Modern World Applications for Nano-Bio Materials: Tissue Engineering and COVID-19

Over the past years, biomaterials-based nano cues with multi-functional characteristics have been engineered with high interest. The ease in fine tunability with maintained compliance makes an array of nano-bio materials supreme candidates for the biomedical sector of the modern world. Moreover, the multi-functional dimensions of nano-bio elements also help to maintain or even improve the patients' life quality most securely by lowering or diminishing the adverse effects of in practice therapeutic modalities. Therefore, engineering highly efficient, reliable, compatible, and recyclable biomaterials-based novel corrective cues with multipurpose applications is essential and a core demand to tackle many human health-related challenges, e.g., the current COVID-19 pandemic. Moreover, robust engineering design and properly exploited nano-bio materials deliver wide-ranging openings for experimentation in the field of interdisciplinary and multidisciplinary scientific research. In this context, herein, it is reviewed the applications and potential on tissue engineering and therapeutics of COVID-19 of several biomaterials. Following a brief introduction is a discussion of the drug delivery routes and mechanisms of biomaterials-based nano cues with suitable examples. The second half of the review focuses on the mainstream applications changing the dynamics of 21st century materials. In the end, current challenges and recommendations are given for a healthy and foreseeable future.

All-Inkjet-Printed 3D Alveolar Barrier Model with Physiologically Relevant Microarchitecture

With the outbreak of new respiratory viruses and high mortality rates of pulmonary diseases, physiologically relevant models of human respiratory system are urgently needed to study disease pathogenesis, drug efficacy, and pharmaceutics. In this paper, a 3D alveolar barrier model fabricated by printing four human alveolar cell lines, namely, type I and II alveolar cells (NCI-H1703 and NCI-H441), lung fibroblasts (MRC5), and lung microvascular endothelial cells (HULEC-5a) is presented. Automated high-resolution deposition of alveolar cells by drop-on-demand inkjet printing enables to fabricate a three-layered alveolar barrier model with an unprecedented thickness of ≈10 µm. The results show that the 3D structured model better recapitulate the structure, morphologies, and functions of the lung tissue, compared not only to a conventional 2D cell culture model, as expected, but also a 3D non-structured model of a homogeneous mixture of the alveolar cells and collagen. Finally, it is demonstrated that this thin multilayered model reproduce practical tissue-level responses to influenza infection. Drop-on-demand inkjet-printing is an enabling technology for customization, scalable manufacturing, and standardization of their size and growth, and it is believed that this 3D alveolar barrier model can be used as an alternative to traditional test models for pathological and pharmaceutical applications.

Advanced single-cell technologies to guide the development of bioengineered lungs

Lung transplantation remains the only viable option for individuals suffering from end-stage lung failure. However, a number of current limitations exist including a continuing shortage of suitable donor lungs and immune rejection following transplantation. To address these concerns, engineering a decellularized biocompatible lung scaffold from cadavers reseeded with autologous lung cells to promote tissue regeneration is being explored. Proof-of-concept transplantation of these bioengineered lungs into animal models has been accomplished. However, these lungs were incompletely recellularized with resulting epithelial and endothelial leakage and insufficient basement membrane integrity. Failure to repopulate lung scaffolds with all of the distinct cell populations necessary for proper function remains a significant hurdle for the progression of current engineering approaches and precludes clinical translation. Advancements in 3D bioprinting, lung organoid models, and microfluidic device and bioreactor development have enhanced our knowledge of pulmonary lung development, as well as important cell-cell and cell-matrix interactions, all of which will help in the path to a bioengineered transplantable lung. However, a significant gap in knowledge of the spatiotemporal interactions between cell populations as well as relative quantities and localization within each compartment of the lung necessary for its proper growth and function remains. This review will provide an update on cells currently used for reseeding decellularized scaffolds with outcomes of recent lung engineering attempts. Focus will then be on how data obtained from advanced single-cell analyses, coupled with multiomics approaches and high-resolution 3D imaging, can guide current lung bioengineering efforts for the development of fully functional, transplantable lungs.

Reconstruction of Vascular and Urologic Tubular Grafts by Tissue Engineering

Tissue engineering is one of the most promising scientific breakthroughs of the late 20th century. Its objective is to produce in vitro tissues or organs to repair and replace damaged ones using various techniques, biomaterials, and cells. Tissue engineering emerged to substitute the use of native autologous tissues, whose quantities are sometimes insufficient to correct the most severe pathologies. Indeed, the patient’s health status, regulations, or fibrotic scars at the site of the initial biopsy limit their availability, especially to treat recurrence. This new technology relies on the use of biomaterials to create scaffolds on which the patient’s cells can be seeded. This review focuses on the reconstruction, by tissue engineering, of two types of tissue with tubular structures: vascular and urological grafts. The emphasis is on self-assembly methods which allow the production of tissue/organ substitute without the use of exogenous material, with the patient’s cells producing their own scaffold. These continuously improved techniques, which allow rapid graft integration without immune rejection in the treatment of severely burned patients, give hope that similar results will be observed in the vascular and urological fields.

Building three-dimensional lung models for studying pharmacokinetics of inhaled drugs

Drug development is a critical step in the development pipeline of pharmaceutical industry, commonly performed in traditional cell culture and animal models. Though, those models hold critical gapsin the prediction and the translation of human pharmacokinetic (PK) and pharmacodynamics (PD) parameters. The advances in tissue engineering have allowed the combination of cell biology with microengineering techniques, offering alternatives to conventional preclinical models. Organ-on-a-chips and three-dimensional (3D) bioprinting models present the potentialityof simulating the physiological and pathological microenvironment of living organs and tissues, constituting this way,more realistic models for the assessment of absorption, distribution, metabolism and excretion (ADME) of drugs. Therefore, this review will focus on lung-on-a-chip and 3D bioprinting techniques for developing lung models that can be usedfor predicting PK/PD parameters.

Engineering of tissue constructs using coaxial bioprinting

Bioprinting is a rapidly developing technology for the precise design and manufacture of tissues in various biological systems or organs. Coaxial extrusion bioprinting, an emergent branch, has demonstrated a strong potential to enhance bioprinting's engineering versatility. Coaxial bioprinting assists in the fabrication of complex tissue constructs, by enabling concentric deposition of biomaterials. The fabricated tissue constructs started with simple, tubular vasculature but have been substantially developed to integrate complex cell composition and self-assembly, ECM patterning, controlled release, and multi-material gradient profiles. This review article begins with a brief overview of coaxial printing history, followed by an introduction of crucial engineering components. Afterward, we review the recent progress and untapped potential in each specific organ or biological system, and demonstrate how coaxial bioprinting facilitates the creation of tissue constructs. Ultimately, we conclude that this growing technology will contribute significantly to capabilities in the fields of in vitro modeling, pharmaceutical development, and clinical regenerative medicine.

Bioengineered in Vitro Tissue Models to Study SARS-CoV-2 Pathogenesis and Therapeutic Validation

Given the various viral outbreaks in the 21st century, specifically the present pandemic situation arising from SARS-CoV-2 or the coronavirus, of unknown magnitude, there is an unmet clinical need to develop effective therapeutic and diagnostic strategies to combat this infectious disease worldwide. To develop precise anticoronavirus drugs and prophylactics, tissue engineering and biomaterial research strategies can serve as a suitable alternative to the conventional treatment options. Therefore, in this Review, we have highlighted various tissue engineering-based diagnostic systems for SARS-CoV-2 and suggested how these strategies involving organ-on-a-chip, organoids, 3D bioprinting, and advanced bioreactor models can be employed to develop in vitro human tissue models, for more efficient diagnosis, drug/vaccine development, and focusing on the need for patient-specific therapy. We believe that combining the basics of virology with tissue engineering techniques can help the researchers to understand the molecular mechanism underlying viral infection, which is critical for effective drug design. In addition, it can also serve to be a suitable platform for drug testing and delivery of small molecules that can lead to therapeutic tools in this dreaded pandemic situation. Additionally, we have also discussed the essential biomaterial properties which polarize the immune system, including dendritic cells and macrophages, toward their inflammatory phenotype, which can thus serve as a reference for exhibiting the role of biomaterial in influencing the adaptive immune response involving B and T lymphocytes to foster a regenerative tissue microenvironment. The situation arising from SARS-CoV-2 poses a challenge to scientists from almost all disciplines, and we feel that tissue engineers can thus provide new translational opportunities in this dreadful pandemic situation.

Building Scaffolds for Tubular Tissue Engineering

Hollow organs and tissue systems drive various functions in the body. Many of these hollow or tubular systems, such as vasculature, the intestines, and the trachea, are common targets for tissue engineering, given their relevance to numerous diseases and body functions. As the field of tissue engineering has developed, numerous benchtop models have been produced as platforms for basic science and drug testing. Production of tubular scaffolds for different tissue engineering applications possesses many commonalities, such as the necessity for producing an intact tubular opening and for formation of semi-permeable epithelia or endothelia. As such, the field has converged on a series of manufacturing techniques for producing these structures. In this review, we discuss some of the most common tissue engineered applications within the context of tubular tissues and the methods by which these structures can be produced. We provide an overview of the general structure and anatomy for these tissue systems along with a series of general design criteria for tubular tissue engineering. We categorize methods for manufacturing tubular scaffolds as follows: casting, electrospinning, rolling, 3D printing, and decellularization. We discuss state-of-the-art models within the context of vascular, intestinal, and tracheal tissue engineering. Finally, we conclude with a discussion of the future for these fields.

3D Printing of Bioinspired Biomaterials for Tissue Regeneration

Biological systems, which possess remarkable functions and excellent properties, are gradually becoming a source of inspiration for the fabrication of advanced tissue regeneration biomaterials due to their hierarchical structures and novel compositions. It would be meaningful to learn and transfer the characteristics of creatures to biomaterials design. However, traditional strategies cannot satisfy the design requirements of the complicated bioinspired materials for tissue regeneration. 3D printing, as a rapidly developing new technology that can accurately achieve multimaterial and multiscale fabrication, is capable of optimizing the fabrication of bioinspired materials with complex composition and structure. This review summarizes the recent developments in 3D-printed bioinspired biomaterials for multiple tissue regeneration, and especially highlights the progresses on i) traditional bioinspired designs for biomaterials fabrication, ii) biological composition inspired designs for the 3D-printed biomaterials, and iii) biological structure inspired designs for the 3D-printed biomaterials. Finally, the challenges and prospects for the development of 3D-printed bioinspired biomaterials are discussed.

Standardization of Microcomputed Tomography for Tracheal Tissue Engineering Analysis

Tracheal tissue engineering has become an active area of interest among clinical and scientific communities; however, methods to evaluate success of in vivo tissue-engineered solutions remain primarily qualitative. These evaluation methods have generally relied on the use of photographs to qualitatively demonstrate tracheal patency, endoscopy to image healing over time, and histology to determine the quality of the regenerated extracellular matrix. Although those generally qualitative methods are valuable, they alone may be insufficient. Therefore, to quantitatively assess tracheal regeneration, we recommend the inclusion of microcomputed tomography (μCT) to quantify tracheal patency as a standard outcome analysis. To establish a standard of practice for quantitative μCT assessment for tracheal tissue engineering, we recommend selecting a constant length to quantify airway volume. Dividing airway volumes by a constant length provides an average cross-sectional area for comparing groups. We caution against selecting a length that is unjustifiably large, which may result in artificially inflating the average cross-sectional area and thereby diminishing the ability to detect actual differences between a test group and a healthy control. Therefore, we recommend selecting a length for μCT assessment that corresponds to the length of the defect region. We further recommend quantifying the minimum cross-sectional area, which does not depend on the length, but has functional implications for breathing. We present empirical data to elucidate the rationale for these recommendations. These empirical data may at first glance appear as expected and unsurprising. However, these standard methods for performing μCT and presentation of results do not yet exist in the literature, and are necessary to improve reporting within the field. Quantitative analyses will better enable comparisons between future publications within the tracheal tissue engineering community and empower a more rigorous assessment of results. Impact statement The current study argues for the standardization of microcomputed tomography (μCT) as a quantitative method for evaluating tracheal tissue-engineered solutions in vivo or ex vivo. The field of tracheal tissue engineering has generally relied on the use of qualitative methods for determining tracheal patency. A standardized quantitative evaluation method currently does not exist. The standardization of μCT for evaluation of in vivo studies would enable a more robust characterization and allow comparisons between groups within the field. The impact of standardized methods within the tracheal tissue engineering field as presented in the current study would greatly improve the quality of published work.

3D Printing and NIR Fluorescence Imaging Techniques for the Fabrication of Implants

Three-dimensional (3D) printing technology holds great potential to fabricate complex constructs in the field of regenerative medicine. Researchers in the surgical fields have used 3D printing techniques and their associated biomaterials for education, training, consultation, organ transplantation, plastic surgery, surgical planning, dentures, and more. In addition, the universal utilization of 3D printing techniques enables researchers to exploit different types of hardware and software in, for example, the surgical fields. To realize the 3D-printed structures to implant them in the body and tissue regeneration, it is important to understand 3D printing technology and its enabling technologies. This paper concisely reviews 3D printing techniques in terms of hardware, software, and materials with a focus on surgery. In addition, it reviews bioprinting technology and a non-invasive monitoring method using near-infrared (NIR) fluorescence, with special attention to the 3D-bioprinted tissue constructs. NIR fluorescence imaging applied to 3D printing technology can play a significant role in monitoring the therapeutic efficacy of 3D structures for clinical implants. Consequently, these techniques can provide individually customized products and improve the treatment outcome of surgeries.

Bioengineering lungs - current status and future prospects

INTRODUCTION

Once pulmonary disease progresses to end-stage pulmonary disease, treatment options are very limited. An important advance in the field is the development of a bioartificial lung derived from a generic matrix scaffold populated with patients' own cells. Significant progress has already been made in the engineering of bioartificial lungs.

AREAS COVERED

This review explains how previous and current research contributes to the goal of creating a successful bioartificial lung, and the barriers faced in doing so. We will also highlight some of the design considerations being explored to optimize bioartificial lungs and considerations for clinical translation.

EXPERT OPINION

While current bioartificial lungs are able to provide short-term gas exchange in large-animal studies, much work is still required to combine the disciplines of cell biology, materials science, and tissue engineering to create such clinically useful and functioning artificial lungs.

Trends in 3D bioprinting for esophageal tissue repair and reconstruction

In esophageal pathologies, such as esophageal atresia, cancers, caustic burns, or post-operative stenosis, esophageal replacement is performed by using parts of the gastrointestinal tract to restore nutritional autonomy. However, this surgical procedure most often does not lead to complete functional recovery and is instead associated with many complications resulting in a decrease in the quality of life and survival rate. Esophageal tissue engineering (ETE) aims at repairing the defective esophagus and is considered as a promising therapeutic alternative. Noteworthy progress has recently been made in the ETE research area but strong challenges remain to replicate the structural and functional integrity of the esophagus with the approaches currently being developed. Within this context, 3D bioprinting is emerging as a new technology to facilitate the patterning of both cellular and acellular bioinks into well-organized 3D functional structures. Here, we present a comprehensive overview of the recent advances in tissue engineering for esophageal reconstruction with a specific focus on 3D bioprinting approaches in ETE. Current biofabrication techniques and bioink features are highlighted, and these are discussed in view of the complexity of the native esophagus that the designed substitute needs to replace. Finally, perspectives on recent strategies for fabricating other tubular organ substitutes via 3D bioprinting are discussed briefly for their potential in ETE applications.

What Have In Vitro Co-Culture Models Taught Us about the Contribution of Epithelial-Mesenchymal Interactions to Airway Inflammation and Remodeling in Asthma?

As the lung develops, epithelial-mesenchymal crosstalk is essential for the developmental processes that drive cell proliferation, differentiation, and extracellular matrix (ECM) production within the lung epithelial-mesenchymal trophic unit (EMTU). In asthma, a number of the lung EMTU developmental signals have been associated with airway inflammation and remodeling, which has led to the hypothesis that aberrant activation of the asthmatic EMTU may lead to disease pathogenesis. Monoculture studies have aided in the understanding of the altered phenotype of airway epithelial and mesenchymal cells and their contribution to the pathogenesis of asthma. However, 3-dimensional (3D) co-culture models are needed to enable the study of epithelial-mesenchymal crosstalk in the setting of the in vivo environment. In this review, we summarize studies using 3D co-culture models to assess how defective epithelial-mesenchymal communication contributes to chronic airway inflammation and remodeling within the asthmatic EMTU.

Bioprinting stem cells: building physiological tissues one cell at a time

Bioprinting aims to direct the spatial arrangement in three dimensions of cells, biomaterials, and growth factors. The biofabrication of clinically relevant constructs for the repair or modeling of either diseased or damaged tissues is rapidly advancing, resulting in the ability to three-dimensional (3D) print biomimetic platforms which imitate a large number of tissues in the human body. Primary tissue-specific cells are typically isolated from patients and used for the fabrication of 3D models for drug screening or tissue repair purposes. However, the lack of resilience of these platforms, due to the difficulties in harnessing, processing, and implanting patient-specific cells can limit regeneration ability. The printing of stem cells obviates these hurdles, producing functional in vitro models or implantable constructs. Advancements in biomaterial science are helping the development of inks suitable for the encapsulation and the printing of stem cells, promoting their functional growth and differentiation. This review specifically aims to investigate the most recent studies exploring innovative and functional approaches for the printing of 3D constructs to model disease or repair damaged tissues. Key concepts in tissue physiology are highlighted, reporting stem cell applications in biofabrication. Bioprinting technologies and biomaterial inks are listed and analyzed, including recent advancements in biomaterial design for bioprinting applications, commenting on the influence of biomaterial inks on the encapsulated stem cells. Ultimately, most recent successful efforts and clinical potentials for the manufacturing of functional physiological tissue substitutes are reported here, with a major focus on specific tissues, such as vasculature, heart, lung and airways, liver, bone and muscle.

Advances in the Fabrication of Biomaterials for Gradient Tissue Engineering

Natural tissues and organs exhibit an array of spatial gradients, from the polarized neural tube during embryonic development to the osteochondral interface present at articulating joints. The strong structure-function relationships in these heterogeneous tissues have sparked intensive research into the development of methods that can replicate physiological gradients in engineered tissues. In this Review, we consider different gradients present in natural tissues and discuss their critical importance in functional tissue engineering. Using this basis, we consolidate the existing fabrication methods into four categories: additive manufacturing, component redistribution, controlled phase changes, and postmodification. We have illustrated this with recent examples, highlighted prominent trends in the field, and outlined a set of criteria and perspectives for gradient fabrication.

Epithelial-mesenchymal crosstalk in COPD: An update from in vitro model studies

Chronic Obstructive Pulmonary disease (COPD) involves airway inflammation and remodeling leading to small airways disease and emphysema, which results in irreversible airflow obstruction. During lung development, reciprocal interactions between the endoderm and mesoderm (epithelial-mesenchymal trophic unit (EMTU)) are essential for morphogenetic cues that direct cell proliferation, differentiation, and extracellular (ECM) production. In COPD, a significant number of the inflammation and remodeling mediators resemble those released during lung development, which has led to the hypothesis that aberrant activation of the EMTU may occur in the disease. Studies assessing lung epithelial and fibroblast function in COPD, have been primarily focused on monoculture studies. To capture the in vivo environment of the human lung and aid in the understanding of mechanisms and mediators involved in abnormal epithelial-fibroblast communication in COPD, complex co-culture models are required. In this review, we describe the studies that have used co-culture models to assess epithelial-fibroblast interactions and their role in the pathogenesis of COPD.

3D Bioprinting Strategies for the Regeneration of Functional Tubular Tissues and Organs

It is difficult to fabricate tubular-shaped tissues and organs (e.g., trachea, blood vessel, and esophagus tissue) with traditional biofabrication techniques (e.g., electrospinning, cell-sheet engineering, and mold-casting) because these have complicated multiple processes. In addition, the tubular-shaped tissues and organs have their own design with target-specific mechanical and biological properties. Therefore, the customized geometrical and physiological environment is required as one of the most critical factors for functional tissue regeneration. 3D bioprinting technology has been receiving attention for the fabrication of patient-tailored and complex-shaped free-form architecture with high reproducibility and versatility. Printable biocomposite inks that can facilitate to build tissue constructs with polymeric frameworks and biochemical microenvironmental cues are also being actively developed for the reconstruction of functional tissue. In this review, we delineated the state-of-the-art of 3D bioprinting techniques specifically for tubular tissue and organ regeneration. In addition, this review described biocomposite inks, such as natural and synthetic polymers. Several described engineering approaches using 3D bioprinting techniques and biocomposite inks may offer beneficial characteristics for the physiological mimicry of human tubular tissues and organs.

Bioengineered 3D Models to Recapitulate Tissue Fibrosis

Fibrosis, characterized by progressive tissue stiffening resulting in organ failure, is a growing health problem affecting millions of people worldwide. Currently, therapeutic options for tissue fibrosis are severely limited and organ transplantation is the only effective treatment for the end-stage fibrotic diseases with inherent limitations. Recent advancements in engineered 3D in vitro human disease mimic models, recapitulating the tissue pathophysiology, have provided unique state-of-the-art platforms for: (i) understanding the biological mechanisms involved in the disease pathogenesis; and (ii) high-throughput and reproducible drug screening. This review focuses on the recent multidisciplinary developments made towards advanced 3D biomimetic fibrotic tissue (liver, kidney, and lung) models that combine highly precision manufacturing techniques with high cellular functionality and biophysical (mechanical) properties.

Force modeling to develop a novel method for fabrication of hollow channels inside a gel structure

Fabrication of hollow channels with user-defined dimensions and patterns inside viscoelastic, gel-type materials is required for several applications, especially in biomedical engineering domain. These include objectives of obtaining vascularized tissues and enclosed or subsurface microfluidic devices. However, presently there is no suitable manufacturing technology that can create such channels and networks in a gel structure. The advent of three-dimensional bioprinting has opened new possibilities for fabricating structures with complex geometries. However, application of this technique to fabricate internal hollow channels in viscoelastic material has not been yet explored to a great extent. In this article, we present the theoretical modeling/background of a proposed manufacturing paradigm through which hollow channels can be conveniently fabricated inside a gel structure. We propose that a tip connected to a robotic arm can be moved in X-, Y-, and Z-axis as per the desired design. The tip can be moved by a magnet or mechanical force. If the tip is further trailed with porous tube and moved inside the viscoelastic material, corresponding internal channels can be fabricated. To achieve this, however, force modeling to understand the forces that will be required to move the tip inside viscoelastic material should be known and understood. Therefore, in our first attempt, we developed the computational force modeling of the tip movement inside gels with different viscoelastic properties to create the channels.