Influence of Hydrolyzed Polyacrylamide Hydrogel Stiffness on Podocyte Morphology, Phenotype, and Mechanical Properties

Stiffness-Tunable Substrate Fabrication by DMD-Based Optical Projection Lithography for Cancer Cell Invasion Studies

OBJECTIVE

Cancer cell invasion is a critical cause of fatality in cancer patients. Physiologically relevant tumor models play a key role in revealing the mechanisms underlying the invasive behavior of cancer cells. However, most existing models only consider interactions between cells and extracellular matrix (ECM) components while neglecting the role of matrix stiffness in tumor invasion. Here, we propose an effective approach that can construct stiffness-tunable substrates using digital mirror device (DMD)-based optical projection lithography to explore the invasion behavior of cancer cells. The printability, mechanical properties, and cell viability of three-dimensional (3D) models can be tuned by the concentration of prepolymer and the exposure time. The invasion trajectories of gastric cancer cells in tumor models of different stiffness were automatically detected and tracked in real-time using a deep learning algorithm. The results show that tumor models of different mechanical stiffness can yield distinct regulatory effects. Moreover, owing to the biophysical characteristics of the 3D in vitro model, different cellular substructures of cancer cells were induced. The proposed tunable substrate construction method can be used to build various microstructures to achieve simulation of cancer invasion and antitumor screening, which has great potential in promoting personalized therapy.

Harnessing the potential of hydrogels for advanced therapeutic applications: current achievements and future directions

The applications of hydrogels have expanded significantly due to their versatile, highly tunable properties and breakthroughs in biomaterial technologies. In this review, we cover the major achievements and the potential of hydrogels in therapeutic applications, focusing primarily on two areas: emerging cell-based therapies and promising non-cell therapeutic modalities. Within the context of cell therapy, we discuss the capacity of hydrogels to overcome the existing translational challenges faced by mainstream cell therapy paradigms, provide a detailed discussion on the advantages and principal design considerations of hydrogels for boosting the efficacy of cell therapy, as well as list specific examples of their applications in different disease scenarios. We then explore the potential of hydrogels in drug delivery, physical intervention therapies, and other non-cell therapeutic areas (e.g., bioadhesives, artificial tissues, and biosensors), emphasizing their utility beyond mere delivery vehicles. Additionally, we complement our discussion on the latest progress and challenges in the clinical application of hydrogels and outline future research directions, particularly in terms of integration with advanced biomanufacturing technologies. This review aims to present a comprehensive view and critical insights into the design and selection of hydrogels for both cell therapy and non-cell therapies, tailored to meet the therapeutic requirements of diverse diseases and situations.

Metal Ion-Induced Acid Hydrolysis Strategy for the One-Step Synthesis of Tough and Highly Transparent Hydrolyzed Polyacrylamide Hydrogels

Polyacrylamide (PAM) hydrogel is hard to enhance through coordination bonds because amide groups rarely coordinate with metal ions strongly in an aqueous solution. It is known that the aqueous solution of ZrOCl2.8H2O can be strongly acidic depending on its concentration. Consequently, through a facile one-step metal ion-induced acid hydrolysis strategy (MIAHS), tough and highly transparent hydrolyzed PAM physical hydrogels are prepared by using ZrOCl2.8H2O in this work. The formation of the partially hydrolyzed PAM physical hydrogels elucidates that the side reaction of imidization during common acid hydrolysis of PAM can be perfectly overcome because the structure of the Zr(IV) ion and its interaction with amide groups promote selective acidic hydrolysis from amide to carboxyl groups. Compared to most coordination cross-linked hydrogels, which need at least two-step fabrication, the hydrolyzed PAM hydrogel via MIAHS can be obtained by one-step synthesis due to the weak interaction between amide groups and Zr(IV). The obtained PAM hydrogel cross-linked by hydrogen bonds and coordination bond between Zr(IV) and carboxyl is a multibond network (MBN) and can achieve hierarchical energy dissipation, which exhibits excellent mechanical properties (tensile strength of 3.15 MPa, elongation at break of 890%, and toughness of 17.0 MJ m-3), high transparence (transmittance of 95%), and outstanding conductivity (5.6 S m-1) at water content of 80 wt %. The high gauge factor (from 2.24 to 12.8 as the strain increases from 0 to 400%) endows the hydrolyzed PAM hydrogels with promising application as strain sensors. Furthermore, in addition to ZrOCl2.8H2O, the fact that various hydrolyzable compounds of Ti(IV), Zr(IV) Hf(IV), and Sn(IV) can also fabricate tough hydrolyzed PAM hydrogels verifies the universality of MIAHS. Therefore, the simple, efficient, and universal MIAHS will shed new light on preparing functional PAM-based hydrogels.

Role of biophysics and mechanobiology in podocyte physiology

Podocytes form the backbone of the glomerular filtration barrier and are exposed to various mechanical forces throughout the lifetime of an individual. The highly dynamic biomechanical environment of the glomerular capillaries greatly influences the cell biology of podocytes and their pathophysiology. Throughout the past two decades, a holistic picture of podocyte cell biology has emerged, highlighting mechanobiological signalling pathways, cytoskeletal dynamics and cellular adhesion as key determinants of biomechanical resilience in podocytes. This biomechanical resilience is essential for the physiological function of podocytes, including the formation and maintenance of the glomerular filtration barrier. Podocytes integrate diverse biomechanical stimuli from their environment and adapt their biophysical properties accordingly. However, perturbations in biomechanical cues or the underlying podocyte mechanobiology can lead to glomerular dysfunction with severe clinical consequences, including proteinuria and glomerulosclerosis. As our mechanistic understanding of podocyte mechanobiology and its role in the pathogenesis of glomerular disease increases, new targets for podocyte-specific therapeutics will emerge. Treating glomerular diseases by targeting podocyte mechanobiology might improve therapeutic precision and efficacy, with potential to reduce the burden of chronic kidney disease on individuals and health-care systems alike.

Mechanosensing by Piezo1 and its implications in the kidney

Piezo1 is an essential mechanosensitive transduction ion channel in mammals. Its unique structure makes it capable of converting mechanical cues into electrical and biological signals, modulating biological and (patho)physiological processes in a wide variety of cells. There is increasing evidence demonstrating that the piezo1 channel plays a vital role in renal physiology and disease conditions. This review summarizes the current evidence on the structure and properties of Piezo1, gating modulation, and pharmacological characteristics, with special focus on the distribution and (patho)physiological significance of Piezo1 in the kidney, which may provide insights into potential treatment targets for renal diseases involving this ion channel.

Effect of GelMA Hydrogel Properties on Long-Term Encapsulation and Myogenic Differentiation of C2C12 Spheroids

Skeletal muscle regeneration and engineering hold great promise for the treatment of various muscle-related pathologies and injuries. This research explores the use of gelatin methacrylate (GelMA) hydrogels as a critical component for encapsulating cellular spheroids in the context of muscle tissue engineering and regenerative applications. The preparation of GelMA hydrogels at various concentrations, ranging from 5% to 15%, was characterized and correlated with their mechanical stiffness. The storage modulus was quantified and correlated with GelMA concentration: 6.01 ± 1.02 Pa (5% GelMA), 75.78 ± 6.67 Pa (10% GelMA), and 134.69 ± 7.93 Pa (15% GelMA). In particular, the mechanical properties and swelling capacity of GelMA hydrogels were identified as key determinants affecting cell sprouting and migration from C2C12 spheroids. The controlled balance between these factors was found to significantly enhance the differentiation and functionality of the encapsulated spheroids. Our results highlight the critical role of GelMA hydrogels in orchestrating cellular dynamics and processes within a 3D microenvironment. The study demonstrates that these hydrogels provide a promising scaffold for the long-term encapsulation of spheroids while maintaining high biocompatibility. This research provides valuable insights into the design and use of GelMA hydrogels for improved muscle tissue engineering and regenerative applications, paving the way for innovative approaches to muscle tissue repair and regeneration.

Biomaterials / bioinks and extrusion bioprinting

Bioinks are formulations of biomaterials and living cells, sometimes with growth factors or other biomolecules, while extrusion bioprinting is an emerging technique to apply or deposit these bioinks or biomaterial solutions to create three-dimensional (3D) constructs with architectures and mechanical/biological properties that mimic those of native human tissue or organs. Printed constructs have found wide applications in tissue engineering for repairing or treating tissue/organ injuries, as well as in vitro tissue modelling for testing or validating newly developed therapeutics and vaccines prior to their use in humans. Successful printing of constructs and their subsequent applications rely on the properties of the formulated bioinks, including the rheological, mechanical, and biological properties, as well as the printing process. This article critically reviews the latest developments in bioinks and biomaterial solutions for extrusion bioprinting, focusing on bioink synthesis and characterization, as well as the influence of bioink properties on the printing process. Key issues and challenges are also discussed along with recommendations for future research.

Development of Organs-on-Chips and Their Impact on Precision Medicine and Advanced System Simulation

Drugs may undergo costly preclinical studies but still fail to demonstrate their efficacy in clinical trials, which makes it challenging to discover new drugs. Both in vitro and in vivo models are essential for disease research and therapeutic development. However, these models cannot simulate the physiological and pathological environment in the human body, resulting in limited drug detection and inaccurate disease modelling, failing to provide valid guidance for clinical application. Organs-on-chips (OCs) are devices that serve as a micro-physiological system or a tissue-on-a-chip; they provide accurate insights into certain functions and the pathophysiology of organs to precisely predict the safety and efficiency of drugs in the body. OCs are faster, more economical, and more precise. Thus, they are projected to become a crucial addition to, and a long-term replacement for, traditional preclinical cell cultures, animal studies, and even human clinical trials. This paper first outlines the nature of OCs and their significance, and then details their manufacturing-related materials and methodology. It also discusses applications of OCs in drug screening and disease modelling and treatment, and presents the future perspective of OCs.

Dynamic Stimulations with Bioengineered Extracellular Matrix-Mimicking Hydrogels for Mechano Cell Reprogramming and Therapy

Cells interact with their surrounding environment through a combination of static and dynamic mechanical signals that vary over stimulus types, intensity, space, and time. Compared to static mechanical signals such as stiffness, porosity, and topography, the current understanding on the effects of dynamic mechanical stimulations on cells remains limited, attributing to a lack of access to devices, the complexity of experimental set-up, and data interpretation. Yet, in the pursuit of emerging translational applications (e.g., cell manufacturing for clinical treatment), it is crucial to understand how cells respond to a variety of dynamic forces that are omnipresent in vivo so that they can be exploited to enhance manufacturing and therapeutic outcomes. With a rising appreciation of the extracellular matrix (ECM) as a key regulator of biofunctions, researchers have bioengineered a suite of ECM-mimicking hydrogels, which can be fine-tuned with spatiotemporal mechanical cues to model complex static and dynamic mechanical profiles. This review first discusses how mechanical stimuli may impact different cellular components and the various mechanobiology pathways involved. Then, how hydrogels can be designed to incorporate static and dynamic mechanical parameters to influence cell behaviors are described. The Scopus database is also used to analyze the relative strength in evidence, ranging from strong to weak, based on number of published literatures, associated citations, and treatment significance. Additionally, the impacts of static and dynamic mechanical stimulations on clinically relevant cell types including mesenchymal stem cells, fibroblasts, and immune cells, are evaluated. The aim is to draw attention to the paucity of studies on the effects of dynamic mechanical stimuli on cells, as well as to highlight the potential of using a cocktail of various types and intensities of mechanical stimulations to influence cell fates (similar to the concept of biochemical cocktail to direct cell fate). It is envisioned that this progress report will inspire more exciting translational development of mechanoresponsive hydrogels for biomedical applications.

Preparation of Crosslinked Poly(acrylic acid-co-acrylamide)-Grafted Deproteinized Natural Rubber/Silica Composites as Coating Materials for Controlled Release of Fertilizer

The crosslinked poly(acrylic acid-co-acrylamide)-grafted deproteinized natural rubber/silica ((PAA-co-PAM)-DPNR/silica) composites were prepared and applied as coating materials for fertilizer in this work. The crosslinked (PAA-co-PAM)-DPNR was prepared via emulsion graft copolymerization in the presence of MBA as a crosslinking agent. The modified DPNR was mixed with various contents of silica (10 to 30 phr) to form the composites. The existence of crosslinked (PAA-co-PAM) after modification provided a water adsorption ability to DPNR. The swelling degree values of composites were found in the range of 2217.3 ± 182.0 to 8132.3 ± 483.8%. The addition of silica in the composites resulted in an improvement in mechanical properties. The crosslinked (PAA-co-PAM)-DPNR with 20 phr of silica increased its compressive strength and compressive modulus by 1.61 and 1.55 times compared to the unloaded silica sample, respectively. There was no breakage of samples after 80% compression strain. Potassium nitrate, a model fertilizer, was loaded into chitosan beads with a loading percentage of 40.55 ± 1.03% and then coated with the modified natural rubber/silica composites. The crosslinked (PAA-co-PAM)-DPNR/silica composites as the outer layers had the ability of holding water in their structure and retarded the release of fertilizer. These composites could be promising materials for controlled release and water retention that would have potential for agricultural application.

Injectable Double-Network Hydrogel-Based Three-Dimensional Cell Culture Systems for Regenerating Dental Pulp

The regeneration of dental pulp tissue is very important, but difficult, in dentistry. The biocompatibility, water content, and viscoelastic properties of pulp-like tissue must be optimized to achieve the efficient transfer of metabolites and nutrients, a suitable degradation rate, distribution of encapsulated cells, injectability, and gelation in situ under physiological conditions. As promising materials for pulp regeneration, hydrogel scaffolds have been produced to simulate the extracellular matrix and transmit signaling molecules. It is imperative to develop hydrogels to effectively regenerate pulp tissue for clinical application. Here, two injectable double-network (DN) hydrogel-based three-dimensional (3D) cell culture systems were developed for regenerating dental pulp. The microstructure, mechanical property, rheology property, and degradation behavior of the injectable DN glycol chitosan-based hydrogels in a simulated root canal model were characterized and compared to a single-network (SN) glycol chitosan-based hydrogel. Human dental pulp stem cells (hDPSCs) were then encapsulated into the GC-based hydrogels for the regeneration of pulp tissue, and the biological performance was investigated both in vitro and in vivo. The results showed that the DN hydrogels had ideal injectability under physiological conditions due to the dynamic nature of the crosslinks. Besides, the DN hydrogels exhibited better mechanical properties and longer degradation duration than the corresponding SN hydrogel. As a 3D cell culture system, the characteristics of the DN hydrogel facilitated odontogenic differentiation and mineralization of hDPSCs in vitro. Further in vivo analysis confirmed that the chemical composition, matrix stiffness, and degradation rate of the DN hydrogel matched those of pulp-like fibrous connective tissue, which might be related to Smad3 activation. These findings demonstrate that DN glycol chitosan-based hydrogels are suitable for the regeneration of pulp tissue.

Engineering Hydrogels for Modulation of Dendritic Cell Function

Dendritic cells (DCs), the most potent antigen-presenting cells, are necessary for the effective activation of naïve T cells. DCs encounter numerous microenvironments with different biophysical properties, such as stiffness and viscoelasticity. Considering the emerging importance of mechanical cues for DC function, it is essential to understand the impacts of these cues on DC function in a physiological or pathological context. Engineered hydrogels have gained interest for the exploration of the impacts of biophysical matrix cues on DC functions, owing to their extracellular-matrix-mimetic properties, such as high water content, a sponge-like pore structure, and tunable mechanical properties. In this review, the introduction of gelation mechanisms of hydrogels is first summarized. Then, recent advances in the substantial effects of developing hydrogels on DC function are highlighted, and the potential molecular mechanisms are subsequently discussed. Finally, persisting questions and future perspectives are presented.

Research Progress of Polydopamine Hydrogel in the Prevention and Treatment of Oral Diseases

Oral diseases represent one of the most prevalent diseases globally and are associated with serious health and economic burdens, greatly altering the quality of life of affected individuals. Various biomaterials play important roles in the treatment of oral diseases. To some extent, the development of biomaterials has promoted progress in clinically available oral medicines. Hydrogels have unique tunable advantages that make them useful in the next generation of regenerative strategies and have been widely applied in both oral soft and hard tissues repair. However, most hydrogels lack self-adhesive properties, which may result in low repair efficacy. Polydopamine (PDA), the primary adhesive component, has attracted increasing attention in recent years. PDA-modified hydrogels exhibit reliable and suitable adherence to tissues and easily integrate into tissues to promote repair efficiency. This paper reviews the latest research progress on PDA hydrogels and elaborates on the mechanism of the reaction between PDA functional groups and hydrogels, and summarizes the biological properties and the applications of PDA hydrogels in the prevention and treatment of the field of oral diseases. It is also proposed that in future research we should simulate the complex microenvironment of the oral cavity as much as possible, coordinate and plan various biological events rationally, and realize the translation from scientific research to clinical practice.

Fabrication Strategies Towards Hydrogels for Biomedical Application: Chemical and Mechanical Insights

This review aims at giving selected chemical and mechanical insights on design criteria that should be taken into account in hydrogel production for biomedical applications. Particular emphasis will be given to the chemical aspects involved in hydrogel design: macromer chemical composition, cross-linking strategies and chemistry towards "conventional" and smart/stimuli responsive hydrogels. Mechanical properties of hydrogels in view of regenerative medicine applications will also be considered.

Kidney-on-a-Chip: Mechanical Stimulation and Sensor Integration

Bioengineered in vitro models of the kidney offer unprecedented opportunities to better mimic the in vivo microenvironment. Kidney-on-a-chip technology reproduces 2D or 3D features which can replicate features of the tissue architecture, composition, and dynamic mechanical forces experienced by cells in vivo. Kidney cells are exposed to mechanical stimuli such as substrate stiffness, shear stress, compression, and stretch, which regulate multiple cellular functions. Incorporating mechanical stimuli in kidney-on-a-chip is critically important for recapitulating the physiological or pathological microenvironment. This review will explore approaches to applying mechanical stimuli to different cell types using kidney-on-a-chip models and how these systems are used to study kidney physiology, model disease, and screen for drug toxicity. We further discuss sensor integration into kidney-on-a-chip for monitoring cellular responses to mechanical or other pathological stimuli. We discuss the advantages, limitations, and challenges associated with incorporating mechanical stimuli in kidney-on-a-chip models for a variety of applications. Overall, this review aims to highlight the importance of mechanical stimuli and sensor integration in the design and implementation of kidney-on-a-chip devices.

An ex vivo culture model of kidney podocyte injury reveals mechanosensitive, synaptopodin-templating, sarcomere-like structures

Chronic kidney diseases are widespread and incurable. The biophysical mechanisms underlying them are unclear, in part because material systems for reconstituting the microenvironment of relevant kidney cells are limited. A critical question is how kidney podocytes (glomerular epithelial cells) regenerate foot processes of the filtration apparatus following injury. Recently identified sarcomere-like structures (SLSs) with periodically spaced myosin IIA and synaptopodin appear in injured podocytes in vivo. We hypothesized that SLSs template synaptopodin in the initial stages of recovery in response to microenvironmental stimuli and tested this hypothesis by developing an ex vivo culture system that allows control of the podocyte microenvironment. Results supported our hypothesis. SLSs in podocytes that migrated from isolated kidney glomeruli presented periodic synaptopodin-positive clusters that nucleated peripheral, foot process-like extensions. SLSs were mechanoresponsive to actomyosin inhibitors and substrate stiffness. Results suggest SLSs as mechanobiological mediators of podocyte recovery and as potential targets for therapeutic intervention.

Engineering Hydrogels for Modulation of Material-Cell Interactions

Hydrogels are a recurrent platform for Tissue Engineering (TE) strategies. Their versatility and the variety of available methods for tuning their properties highly contribute to hydrogels' success. As a result, the design of advanced hydrogels has been thoroughly studied, in the quest for better solutions not only for drugs- and cell-based therapies but also for more fundamental studies. The wide variety of sources, crosslinking strategies, and functionalization methods, and mostly the resemblance of hydrogels to the natural extracellular matrix, make this 3D hydrated structures an excellent tool for TE approaches. The state-of-the-art information regarding hydrogel design, processing methods, and the influence of different hydrogel formulations on the final cell-biomaterial interactions are overviewed herein. This article is protected by copyright. All rights reserved.

Mimicking the Natural Basement Membrane for Advanced Tissue Engineering

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

Biomimetic models of the glomerulus

The use of biomimetic models of the glomerulus has the potential to improve our understanding of the pathogenesis of kidney diseases and to enable progress in therapeutics. Current in vitro models comprise organ-on-a-chip, scaffold-based and organoid approaches. Glomerulus-on-a-chip designs mimic components of glomerular microfluidic flow but lack the inherent complexity of the glomerular filtration barrier. Scaffold-based 3D culture systems and organoids provide greater microenvironmental complexity but do not replicate fluid flows and dynamic responses to fluidic stimuli. As the available models do not accurately model the structure or filtration function of the glomerulus, their applications are limited. An optimal approach to glomerular modelling is yet to be developed, but the field will probably benefit from advances in biofabrication techniques. In particular, 3D bioprinting technologies could enable the fabrication of constructs that recapitulate the complex structure of the glomerulus and the glomerular filtration barrier. The next generation of in vitro glomerular models must be suitable for high(er)-content or/and high(er)-throughput screening to enable continuous and systematic monitoring. Moreover, coupling of glomerular or kidney models with those of other organs is a promising approach to enable modelling of partial or full-body responses to drugs and prediction of therapeutic outcomes.

Mechanosensitive Piezo1 channels mediate renal fibrosis

Kidney fibrosis is the final common pathway of progressive kidney diseases, the underlying mechanisms of which is not fully understood. The purpose of the current study is to investigate a role of Piezo1, a mechanosensitive nonselective cation channel, in kidney fibrosis. In human fibrotic kidneys, Piezo1 protein expression was markedly upregulated. The abundance of Piezo1 protein in kidneys of mice with UUO or with folic-acid treatment was significantly increased. Inhibition of Piezo1 with GsMTx4 markedly ameliorated UUO or folic acid-induced kidney fibrosis. Mechanical stretch, compression or stiffness induced Piezo1 activation and pro-fibrotic responses in human HK2 cells and primary cultured mouse proximal tubular cells (mPTCs), which were greatly prevented by inhibition or silence of Piezo1. TGFβ-1 induced increased Piezo1 expression and pro-fibrotic phenotypic alterations in HK2 cells and mPTCs, which was again markedly prevented by inhibition of Piezo1. Activation of Piezo1 by Yoda1, a Piezo1 agonist, caused calcium influx and profibrotic responses in HK2 cells and induced calpain2 activation, followed by talin1 cleavage and upregulation of integrinβ1. Also, Yoda1 promoted the link between ECM and integrinβ1. In conclusion, Piezo1 is involved in the progression of kidney fibrosis and pro-fibrotic alterations in renal proximal tubular cells, likely through activating calcium-calpain2-integrinβ1 pathway.

A Review on the Design of Hydrogels With Different Stiffness and Their Effects on Tissue Repair

Tissue repair after trauma and infection has always been a difficult problem in regenerative medicine. Hydrogels have become one of the most important scaffolds for tissue engineering due to their biocompatibility, biodegradability and water solubility. Especially, the stiffness of hydrogels is a key factor, which influence the morphology of mesenchymal stem cells (MSCs) and their differentiation. The researches on this point are meaningful to the field of tissue engineering. Herein, this review focus on the design of hydrogels with different stiffness and their effects on the behavior of MSCs. In addition, the effect of hydrogel stiffness on the phenotype of macrophages is introduced, and then the relationship between the phenotype changes of macrophages on inflammatory response and tissue repair is discussed. Finally, the future application of hydrogels with a certain stiffness in regenerative medicine and tissue engineering has been prospected.

Bioengineering Strategies To Develop Podocyte Culture Systems

Unraveling the complex behavior of healthy and disease podocytes by analyzing the changes in their unique arrangement of foot processes, slit diaphragm and the 3D morphology is a long-standing goal in kidney-glomerular research. The complexities surrounding the podocytes' accessibility in animal models and growing evidence of differences between humans and animal systems have compelled researchers to look for alternate approaches to study podocyte behaviors. With the advent of bioengineered models, an increasingly powerful and diverse set of tools is available to develop novel podocyte culture systems. This review discusses the pertinence of various culture models of podocytes to study podocyte mechanisms in both normal physiology and disease conditions. While no one in vitro system comprehensively recapitulates podocytes' in vivo architecture, we emphasize how the existing systems can be exploited to answer targeted questions on podocyte structure and function. We highlight the distinct advantages and limitations of using these models to study podocyte behaviors and screen therapeutics. Finally, we discuss various considerations and potential engineering strategies for developing next-generation complex 3D culture models for studying podocyte behaviors in vitro.

Hydrolyzed Hydrogels with Super Stretchability, High Strength, and Fast Self-Recovery for Flexible Sensors

Polyacrylamide is widely employed in constructing functional hydrogels. However, the volume expansion of this hydrogel in water weakens its mechanical properties and restricts its application. Herein, we report a strategy to convert the swollen and weak polyacrylamide/carboxymethyl chitosan hydrogel into a strong and tough one by hydrolysis in acid solution with an elevated temperature. The obtained hydrolyzed hydrogels possess a high strength, toughness, and tearing fracture energy of 5.9 MPa, 22 MJ/m³ and 7517 J/m², which are 254, 535 and 186 times higher than those of the original swollen one, respectively. In addition, the gels demonstrate low residual strain and rapid self-recovery abilities. Moreover, the gels have good shape memory behavior controlled by temperature. Furthermore, the gels can be worked as strain sensors with a broad strain window, high sensitivity, excellent linear response, and great durability in monitoring human motions after immersing treatment in a normal saline solution. This work provides a new method for preparing the stretchable and tough polyacrylamide-based hydrogels used in the areas of soft actuators and flexible electronics.

Understanding and Regulating Cell-Matrix Interactions Using Hydrogels of Designable Mechanical Properties

Similar to natural tissues, hydrogels contain abundant water, so they are considered as promising biomaterials for studying the influence of the mechanical properties of extracellular matrices (ECM) on various cell functions. In recent years, the growing research on cellular mechanical response has revealed that many cell functions, including cell spreading, migration, tumorigenesis and differentiation, are related to the mechanical properties of ECM. Therefore, how cells sense and respond to the extracellular mechanical environment has gained considerable attention. In these studies, hydrogels are widely used as the in vitro model system. Hydrogels of tunable stiffness, viscoelasticity, degradability, plasticity, and dynamical properties have been engineered to reveal how cells respond to specific mechanical features. In this review, we summarize recent process in this research direction and specifically focus on the influence of the mechanical properties of the ECM on cell functions, how cells sense and respond to the extracellular mechanical environment, and approaches to adjusting the stiffness of hydrogels.

Biomimetic mineralized microenvironment stiffness regulated BMSCs osteogenic differentiation through cytoskeleton mediated mechanical signaling transduction

Construction of biomimetic microenvironment is vital to understand the relationship between matrix mechanical cues and cell fate, as well as to explore potential tissue engineering scaffolds for clinical application. In this study, through the enzymatic mineralizable collagen hydrogel system, we established the biomimetic bone matrix which was capable of realizing mechanical regulation independent of mineralization by incorporation of phosphorylated molecules (vinylphosphonic acid, VAP). Then, based on the biomimetic mineralized matrix with same composition but significantly different mechanical stiffness, we further investigated the effect of matrix stiffness on osteogenic differentiation of bone marrow stromal cells (BMSCs). The results clearly demonstrated that biomimetic mineralized microenvironment with higher mechanical strength promoted osteogenic differentiation of BMSCs. Further mechanism analysis demonstrated that the mineralized hydrogel with higher stiffness promoted cytoskeletal assembly, which enhanced the expression and nuclear colocalization of YAP and RUNX2, thereby promoted the osteogenic differentiation of stem cells. This study supplies a promising material platform not only for bone tissue engineering but also for exploring the mechanism of biomimetic bone matrix mechanics on osteogenesis.

Microfluidic Device for the Analysis of Angiogenic Sprouting under Bidirectional Biochemical Gradients

In this paper, we developed a spheroid culture device that can trap a spheroid in the trapping site sandwiched by two extracellular matrix gels located at the upper and lower side of the spheroid. This device can form different biochemical gradients by applying target biochemicals separately in upper and lower channels, allowing us to study the angiogenic sprouting under various biochemical gradients in different directions. In the experiments, we confirmed the trapping of the spheroids and demonstrate the investigation on the direction and extent of angiogenic sprouts under unidirectional or bidirectional biochemical gradients. We believe our device can contribute to understanding the pathophysiological phenomena driven by chemical gradients, such as tissue development and tumor angiogenesis.

Enhancement of Podocyte Attachment on Polyacrylamide Hydrogels with Gelatin-Based Polymers

Biological activities of cells such as survival and differentiation processes are mainly maintained by a specific extracellular matrix (ECM). Hydrogels have recently been employed successfully in tissue engineering applications. In particular, scaffolds made of gelatin methacrylate-based hydrogels (GelMA) showed great potential due to their biocompatibility, biofunctionality, and low mechanical strength. The development of a hydrogel having tunable and appropriate mechanical properties as well as chemical and biological cues was the aim of this work. A synthetic and biological hybrid hydrogel was developed to mimic the biological and mechanical properties of native ECM. A combination of gelatin methacrylate and acrylamide (GelMA-AAm)-based hydrogels was studied, and it showed tunable mechanical properties upon changing the polymer concentrations. Different GelMA-AAm samples were prepared and studied by varying the concentrations of GelMA and AAm (AAm_2.5% + GelMA_3%, AAm_5% + GelMA_3%, and AAm_5% + GelMA_5%). The swelling behavior, biodegradability, physicochemical and mechanical properties of GelMA-AAm were also characterized. The results showed a variation of swelling capability and a tunable elasticity ranging from 4.03 to 24.98 kPa depending on polymer concentrations. Moreover, the podocyte cell morphology, cytoskeleton reorganization and differentiation were evaluated as a function of GelMA-AAm mechanical properties. We concluded that the AAm_2.5% + GelMA_3% hydrogel sample having an elasticity of 4.03 kPa can mimic the native kidney glomerular basement membrane (GBM) elasticity and allow podocyte cell attachment without the functionalization of the gel surface with adhesion proteins compared to synthetic hydrogels (PAAm). This work will further enhance the knowledge of the behavior of podocyte cells to understand their biological properties in both healthy and diseased states.

The mechanobiology of kidney podocytes in health and disease

Chronic kidney disease (CKD) substantially reduces quality of life and leads to premature death for thousands of people each year. Dialysis and kidney organ transplants remain prevalent therapeutic avenues but carry significant medical, economic and social burden. Podocytes are responsible for blood filtration selectivity in the kidney, where they extend a network of foot processes (FPs) from their cell bodies which surround endothelial cells and interdigitate with those on neighbouring podocytes to form narrow slit diaphragms (SDs). During aging, some podocytes are lost naturally but accelerated podocyte loss is a hallmark of CKD. Insights into the origin of degenerative podocyte loss will help answer important questions about kidney function and lead to substantial health benefits. Here, approaches that uncover insights into podocyte mechanobiology are reviewed, both those that interrogate the biophysical properties of podocytes and how the external physical environment affects podocyte behaviour, and also those that interrogate the biophysical effects that podocytes exert on their surroundings.

Fully amino acid-based hydrogel as potential scaffold for cell culturing and drug delivery

Polymer hydrogels are ideal scaffolds for both tissue engineering and drug delivery. A great advantage of poly(amino acid)-based hydrogels is their high similarity to natural proteins. However, their expensive and complicated synthesis often limits their application. The use of poly(aspartic acid) (PASP) seems an appropriate solution for this problem due to the relatively cheap and simple synthesis of PASP. Using amino acids not only as building blocks in the polymer backbone but also as cross-linkers can improve the biocompatibility and the biodegradability of the hydrogel. In this paper, PASP cross-linked with cystamine (CYS) and lysine-methylester (LYS) was introduced as fully amino acid-based polymer hydrogel. Gels were synthesized employing six different ratios of CYS and LYS. The pH dependent swelling degree and the concentration of the elastically active chain were determined. After reduction of the disulfide bonds of CYS, the presence of thiol side groups was also detected. To determine the concentration of the reactive cross-linkers in the hydrogels, a new method based on the examination of the swelling behavior was established. Using metoprolol as a model drug, cell proliferation and drug release kinetics were studied at different LYS contents and in the presence of thiol groups. The optimal ratio of cross-linkers for the proliferation of periodontal ligament cells was found to be 60-80% LYS and 20-40% CYS. The reductive conditions resulted in an increased drug release due to the cleavage of disulfide bridges in the hydrogels. Consequently, these hydrogels provide new possibilities in the fields of both tissue engineering and controlled drug delivery.

Substrate Stiffness Modulates Renal Progenitor Cell Properties via a ROCK-Mediated Mechanotransduction Mechanism

Stem cell (SC)-based tissue engineering and regenerative medicine (RM) approaches may provide alternative therapeutic strategies for the rising number of patients suffering from chronic kidney disease. Embryonic SCs and inducible pluripotent SCs are the most frequently used cell types, but autologous patient-derived renal SCs, such as human CD133+CD24+ renal progenitor cells (RPCs), represent a preferable option. RPCs are of interest also for the RM approaches based on the pharmacological encouragement of in situ regeneration by endogenous SCs. An understanding of the biochemical and biophysical factors that influence RPC behavior is essential for improving their applicability. We investigated how the mechanical properties of the substrate modulate RPC behavior in vitro. We employed collagen I-coated hydrogels with variable stiffness to modulate the mechanical environment of RPCs and found that their morphology, proliferation, migration, and differentiation toward the podocyte lineage were highly dependent on mechanical stiffness. Indeed, a stiff matrix induced cell spreading and focal adhesion assembly trough a Rho kinase (ROCK)-mediated mechanism. Similarly, the proliferative and migratory capacity of RPCs increased as stiffness increased and ROCK inhibition, by either Y27632 or antisense LNA-GapmeRs, abolished these effects. The acquisition of podocyte markers was also modulated, in a narrow range, by the elastic modulus and involved ROCK activity. Our findings may aid in 1) the optimization of RPC culture conditions to favor cell expansion or to induce efficient differentiation with important implication for RPC bioprocessing, and in 2) understanding how alterations of the physical properties of the renal tissue associated with diseases could influenced the regenerative response of RPCs.