Co-axial Printing of Convoluted Proximal Tubule for Kidney Disease Modeling

Kidney Fibrosis In Vitro and In Vivo Models: Path Toward Physiologically Relevant Humanized Models

Chronic kidney disease (CKD) affects over 10% of the global population and is a leading cause of mortality. Kidney fibrosis, a key endpoint of CKD, disrupts nephron tubule anatomy and filtration function, and disease pathomechanisms are not fully understood. Kidney fibrosis is currently investigated with in vivo models, that gradually support the identification of possible mechanisms of fibrosis, but with limited translational research, as they do not fully recapitulate human kidney physiology, metabolism, and molecular pathways. In vitro 2D cell culture models are currently used, as a starting point in disease modeling and pharmacology, however, they lack the 3D kidney architecture complexity and functions. The failure of several therapies and drugs in clinical trials highlights the urgent need for advanced 3D in vitro models. This review discusses the urinary system's anatomy, associated diseases, and diagnostic methods, including biomarker analysis and tissue biopsy. It evaluates 2D and in vivo models, highlighting their limitations. The review explores the state-of-the-art 3D-humanized in vitro models, such as 3D cell aggregates, on-chip models, biofabrication techniques, and hybrid models, which aim to mimic kidney morphogenesis and functions. These advanced models hold promise for translating new therapies and drugs for kidney fibrosis into clinics.

Recent trends in the development ofin vitro3D kidney models

The kidneys are vital for maintaining bodily homeostasis and are susceptible to various diseases that disrupt their function. Traditionally, research on kidney diseases has relied on animal models and simplistic two-dimensional cell cultures, which do not fully replicate human tissue pathology. To address this, recent advances focus on developing advanced 3D biomimeticin vitromodels using human-derived cells. These models mimic healthy and diseased kidney tissues with specificity, replicating key elements like glomerular and tubular structures through tissue engineering. By closely mimicking human physiology, they provide a promising platform for studying renal disorders, drug-induced nephrotoxicity, and evaluating new therapies. However, the challenges include optimizing scalability, reproducibility, and long-term stability to enhance reliability in research and clinical applications. This review highlights the transformative potential of 3D biomimeticin vitrokidney models in advancing biomedical research and clinical applications. By focusing on human-specific cell cultures and tissue engineering techniques, these models aim to overcome the limitations of conventional animal models and simplistic 2D cell cultures. The review discusses in detail the various types of biomimetic kidney models currently under development, their specific applications, and the innovative approaches used to construct them. It also addresses the challenges and limitations associated with these models for their widespread adoption and reliability in research settings.

Interparticle Crosslinked Ion-responsive Microgels for 3D and 4D (Bio)printing Applications

Microgels offer unique advantages over bulk hydrogels due to their improved diffusion limits for oxygen and nutrients. Particularly, stimuli-responsive microgels with inherently bioactive and self-supporting properties emerge as highly promising biomaterials. This study unveils the development of interparticle-crosslinked, self-supporting, ion-responsive microgels tailored for 3D and 4D (bio)printing applications. A novel strategy was proposed to develop microgels that enabled interparticle crosslinking, eliminating the need for filler hydrogels and preserving essential microscale void spaces to support cell migration and vascularization. Additionally, these microgels possessed unique, ion-responsive shrinking behavior primarily by the Hofmeister effect, reversible upon the removal of the stimulus. Two types of microgels, spherical (µS) and random-shaped (µR), were fabricated, with µR exhibiting superior mechanical properties and higher packing density. Fabricated microgel-based constructs supported angiogenesis with tunable vessel size based on interstitial void spaces while demonstrating excellent shear-thinning and self-healing properties and high print fidelity. Various bioprinting techniques were employed and validated using these microgels, including extrusion-based, embedded, intraembedded, and aspiration-assisted bioprinting, facilitating the biofabrication of scalable constructs. Multi-material 4D printing was achieved by combining ion-responsive microgels with non-responsive microgels, enabling programmable shape transformations upon exposure to ionic solutions. Utilizing 4D printing, complex, dynamic structures were generated such as coiling filaments, grippers, and folding sheets, providing a foundation for the development of advanced tissue models and devices for regenerative medicine and soft robotics, respectively.

Thermoforming for Small Feature Replication in Melt Electrowritten Membranes to Model Kidney Proximal Tubule

A novel approach merging melt electrowriting (MEW) with matched die thermoforming to achieve scaffolds with micron-sized curvatures (200 - 800 µm versus 1000 µm of mandrel printing) for in vitro modeling of the kidney proximal tubule (PT) is proposed. Recent advances in this field emphasize the relevance of accurately replicating the intricate tissue microenvironment, particularly the curvature of the nephrons' tubular segments. While MEW offers promising capabilities for fabricating highly and porous precise 3D structures mimicking the PT, challenges persist in approximating the diameter of tubular scaffolds to match the actual PT. The thermoformed MEW membranes retain the initial MEW printing design parameters (rhombus geometry, porosity > 45%) while accurately following the imprinted curvature (ratios between 0.67-0.95). PT epithelial cells cultured on these membranes demonstrate the ability to fill in the large pores of the membrane by secreting their own collagen IV-rich extracellular matrix and form an organized, functional, and tight monolayer expressing characteristic PT markers. Besides approximating PT architecture, this setup maximizes the usable surface area for cell culture and molecular readouts. By closely mimicking the structural intricacies of native tissue architecture, this approach enhances the biomimetic fidelity of engineered scaffolds, offering potential applications beyond kidney tissue engineering.

Collagen-based materials in male genitourinary diseases and tissue regeneration

Male genitourinary dysfunction causes serious physical or mental distress, such as infertility and psychological harm, which leads to impaired quality of life. Current conventional treatments involving drug therapy, surgical repair, and tissue grafting have a limited effect on recovering the function and fertility of the genitourinary organs. To address these limitations, various biomaterials have been explored, with collagen-based materials increasingly gaining attention for reconstructing the male genitourinary system due to their superior biocompatibility, biodegradability, low antigenicity, biomimetic 3D matrix characteristics, hemostatic efficacy, and tissue regeneration capabilities. This review covers the recent biomedical applications of collagen-based materials including treatment of erectile dysfunction, premature ejaculation, penile girth enlargement, prostate cancer, Peyronie's disease, chronic kidney disease, etc. Although there are relatively few clinical trials, the promising results of the existing studies on animal models reveal a bright future for collagen-based materials in the treatment of male genitourinary diseases.

Graphic Abstract

Engineered Microfibers for Tissue Engineering

Hydrogel microfibers are hydrogel materials engineered into fiber structures. Techniques such as wet spinning, microfluidic spinning, and 3D bioprinting are often used to prepare microfibers due to their ability to precisely control the size, morphology, and structure of the microfibers. Microfibers with different structural morphologies have different functions; they provide a flow-through culture environment for cells to improve viability, and can also be used to induce the differentiation of cells such as skeletal muscle and cardiac muscle cells to eventually form functional organs in vitro through special morphologies. This Review introduces recent advances in microfluidics, 3D bioprinting, and wet spinning in the preparation of microfibers, focusing on the materials and fabrication methods. The applications of microfibers in tissue engineering are highlighted by summarizing their contributions in engineering biomimetic blood vessels, vascularized tissues, bone, heart, pancreas, kidney, liver, and fat. Furthermore, applications of engineered fibers in tissue repair and drug screening are also discussed.

Electrospinning and Cell Fibers in Biomedical Applications

Human body tissues such as muscle, blood vessels, tendon/ligaments, and nerves have fiber-like fascicle morphologies, where ordered organization of cells and extracellular matrix (ECM) within the bundles in specific 3D manners orchestrates cells and ECM to provide tissue functions. Through engineering cell fibers (which are fibers containing living cells) as living building blocks with the help of emerging "bottom-up" biomanufacturing technologies, it is now possible to reconstitute/recreate the fiber-like fascicle morphologies and their spatiotemporally specific cell-cell/cell-ECM interactions in vitro, thereby enabling the modeling, therapy, or repair of these fibrous tissues. In this article, a concise review is provided of the "bottom-up" biomanufacturing technologies and materials usable for fabricating cell fibers, with an emphasis on electrospinning that can effectively and efficiently produce thin cell fibers and with properly designed processes, 3D cell-laden structures that mimic those of native fibrous tissues. The importance and applications of cell fibers as models, therapeutic platforms, or analogs/replacements for tissues for areas such as drug testing, cell therapy, and tissue engineering are highlighted. Challenges, in terms of biomimicry of high-order hierarchical structures and complex dynamic cellular microenvironments of native tissues, as well as opportunities for cell fibers in a myriad of biomedical applications, are discussed.

Sequentially suspended 3D bioprinting of multiple-layered vascular models with tunable geometries forin vitromodeling of arterial disorders initiation

As the main precursor of arterial disorders, endothelial dysfunction preferentially occurs in regions of arteries prone to generating turbulent flow, particularly in branched regions of vasculatures. Although various diseased models have been engineered to investigate arterial pathology, producing a multiple-layered vascular model with branched geometries that can recapitulate the critical physiological environments of human arteries, such as intercellular communications and local turbulent flows, remains challenging. This study develops a sequentially suspended three-dimensional bioprinting (SSB) strategy and a visible-light-curable decellularized extracellular matrix bioink (abbreviated as 'VCD bioink') to construct a biomimetic human arterial model with tunable geometries. The engineered multiple-layered arterial models with compartmentalized vascular cells can exhibit physiological functionality and pathological performance under defined physiological flows specified by computational fluid dynamics simulation. Using different configurations of the vascular models, we investigated the independent and synergetic effects of cellular crosstalk and abnormal hemodynamics on the initiation of endothelial dysfunction, a hallmark event of arterial disorder. The results suggest that the arterial model constructed using the SSB strategy and VCD bioinks has promise in establishing diagnostic/analytic platforms for understanding the pathophysiology of human arterial disorders and relevant abnormalities, such as atherosclerosis, aneurysms, and ischemic diseases.

Combining rotary wet-spinning biofabrication and electro-mechanical stimulation for thein vitroproduction of functional myo-substitutes

In this work, we present an innovative, high-throughput rotary wet-spinning biofabrication method for manufacturing cellularized constructs composed of highly-aligned hydrogel fibers. The platform is supported by an innovative microfluidic printing head (MPH) bearing a crosslinking bath microtank with a co-axial nozzle placed at the bottom of it for the immediate gelation of extruded core/shell fibers. After a thorough characterization and optimization of the new MPH and the fiber deposition parameters, we demonstrate the suitability of the proposed system for thein vitroengineering of functional myo-substitutes. The samples produced through the described approach were first characterizedin vitroand then used as a substrate to ascertain the effects of electro-mechanical stimulation on myogenic maturation. Of note, we found a characteristic gene expression modulation of fast (MyH1), intermediate (MyH2), and slow (MyH7) twitching myosin heavy chain isoforms, depending on the applied stimulation protocol. This feature should be further investigated in the future to biofabricate engineered myo-substitutes with specific functionalities.

Development of lumen-based perfusable 3D liver in vitro model using single-step bioprinting with composite bioinks

Hepatic sinusoids are uniquely organized structures that help maintain a spectrum of hepatic functions. Although several in vitro liver models have been developed to replicate liver sinusoids, most of these platforms require complex, multi-step fabrication methods making it difficult to achieve truly three-dimensional (3D) channel geometries. In this study, a single-step bioprinting technique was demonstrated to simultaneously print a chip platform and develop a perfusable vascularized liver sinusoid in vitro model. The integrated system uses a co-axial-based bioprinting approach to develop a liver sinusoid-like model that consists of a sacrificial core compartment containing a perfusable pre-vascular structure and an alginate-collagen-based shell compartment containing hepatocytes. The lumen-based perfusable 3D liver sinusoid-on-a-chip (LSOC-P) demonstrated significantly better hepatocyte viability, proliferation, and liver-specific gene and protein expression compared to a 3D hepatocyte-based core/shell model with static media and the standard hepatocyte-based 2D sandwich culture system. A drug toxicity evaluation of hepatotoxins highlighted the comparatively higher sensitivity of the LSOC system with a close estimation of the therapeutic range of safe drug concentrations for humans. In conclusion, the current findings indicate that the combinatorial single-step co-axial bioprinting technique is a promising fabrication approach for the development of a perfusable LSOC platform for drug screening applications.

Placing biofabrication into the context of human disease modeling

The field of biofabrication has seen tremendous advances in the past decade. More recently, the emerging role of biofabrication in allowing faithful generation of models of human tissues in their healthy and diseased states has been demonstrated and has rapidly expanded. These biomimetic models are potentially widely applicable in a range of research and translational areas including but not limited to fundamental biology studies as well as screening of chemical compounds, such as therapeutic agents. The United States Food and Drug Administration Modernization Act 2.0, which now no longer requires animal tests before approving human drug trials, will likely further boost the field in the years to come. This Special Issue, with a collection of 11 excellent research articles, thus focuses on showcasing the latest developments of biofabrication towards human disease modeling, spanning from 3D (bo)printing to organ-on-a-chip as well as their integration.

Indirect 3D Bioprinting of a Robust Trilobular Hepatic Construct with Decellularized Liver Matrix Hydrogel

The liver exhibits complex geometrical morphologies of hepatic cells arranged in a hexagonal lobule with an extracellular matrix (ECM) organized in a specific pattern on a multi-scale level. Previous studies have utilized 3D bioprinting and microfluidic perfusion systems with various biomaterials to develop lobule-like constructs. However, they all lack anatomical relevance with weak control over the size and shape of the fabricated structures. Moreover, most biomaterials lack liver-specific ECM components partially or entirely, which might limit their biomimetic mechanical properties and biological functions. Here, we report 3D bioprinting of a sacrificial PVA framework to impart its trilobular hepatic structure to the decellularized liver extracellular matrix (dLM) hydrogel with polyethylene glycol-based crosslinker and tyrosinase to fabricate a robust multi-scale 3D liver construct. The 3D trilobular construct exhibits higher crosslinking, viscosity (182.7 ± 1.6 Pa·s), and storage modulus (2554 ± 82.1 Pa) than non-crosslinked dLM. The co-culture of HepG2 liver cells and NIH 3T3 fibroblast cells exhibited the influence of fibroblasts on liver-specific activity over time (7 days) to show higher viability (90-91.5%), albumin secretion, and increasing activity of four liver-specific genes as compared to the HepG2 monoculture. This technique offers high lumen patency for the perfusion of media to fabricate a densely populated scaled-up liver model, which can also be extended to other tissue types with different biomaterials and multiple cells to support the creation of a large functional complex tissue.