Influence of Fiber Stiffness on Meniscal Cell Migration into Dense Fibrous Networks

Precision repair of zone-specific meniscal injuries using a tunable extracellular matrix-based hydrogel system

Meniscus injuries present significant therapeutic challenges due to their limited self-healing capacity and the diverse biological and mechanical properties across the tissue. Conventional repair strategies do not replicate the complex zonal characteristics within the meniscus, resulting in suboptimal outcomes. In this study, we introduce an innovative fetal/adult and stiffness-tunable meniscus decellularized extracellular matrix (DEM)-based hydrogel system designed for precision repair of heterogeneous, zonal-dependent meniscus injuries. By synthesizing fetal and adult DEM hydrogels, we identified distinct cellular responses, including that hydrogels with adult meniscus-derived DEM promote more fibrochondrogenic phenotypes. The incorporation of methacrylated hyaluronic acid (MeHA) further refined the mechanical properties and injectability of the DEM-based hydrogels. The combination of fetal and adult DEM with MeHA allowed for precise tuning of stiffness, influencing cell differentiation and closely mimicking native tissue environments. In vivo tests confirmed the biocompatibility of hydrogels and their integration with native meniscus tissues. Furthermore, advanced 3D bioprinting techniques enabled the fabrication of hybrid hydrogels with biomaterial and mechanical gradients, effectively emulating the zonal properties of meniscus tissue and enhancing cell integration. This study represents a significant advance in meniscus tissue engineering, providing a promising platform for customized regenerative therapies across a range of heterogeneous fibrous connective tissues.

Precision Repair of Zone-Specific Meniscal Injuries Using a Tunable Extracellular Matrix-Based Hydrogel System

Meniscus injuries present significant therapeutic challenges due to their limited self-healing capacity and diverse biological and mechanical properties across meniscal tissue. Conventional repair strategies neglect to replicate the complex zonal characteristics within the meniscus, resulting in suboptimal outcomes. In this study, we introduce an innovative, age- and stiffness-tunable meniscus decellularized extracellular matrix (DEM)-based hydrogel system designed for precision repair of heterogeneous, zonal-dependent meniscus injuries. By synthesizing age-dependent DEM hydrogels, we identified distinct cellular responses: fetal bovine meniscus-derived DEM promoted chondrogenic differentiation, while adult meniscus-derived DEM supported fibrochondrogenic phenotypes. The incorporation of methacrylate hyaluronic acid (MeHA) further refined the mechanical properties and injectability of the DEM-based hydrogels. The combination of age-dependent DEM with MeHA allowed for precise stiffness tuning, influencing cell differentiation and closely mimicking native tissue environments. In vivo tests confirmed the biocompatibility of hydrogels and their integration with native meniscus tissues. Furthermore, advanced 3D bioprinting techniques enabled the fabrication of hybrid hydrogels with biomaterial and mechanical gradients, effectively emulating the zonal properties of meniscus tissue and enhancing cell integration. This study represents a significant advancement in meniscus tissue engineering, providing a promising platform for customized regenerative therapies across a range of heterogeneous fibrous connective tissues.

Polyvinyl alcohol/collagen composite scaffold reinforced with biodegradable polyesters/gelatin nanofibers for adipose tissue engineering

Breast cancer has become the most diagnosed cancer type, endangering the health of women. Patients with breast resection are likely to suffer serious physical and mental trauma. Therefore, breast reconstruction becomes an important means of postoperative patient rehabilitation. Polyvinyl alcohol hydrogel has great potential in adipose tissue engineering for breast reconstruction. However, its application is limited because of the lack of bioactive factors and poor structural stability. In this study, we prepared biodegradable polylactic acid-glycolic acid copolymer/polycaprolactone/gelatin (PPG) nanofibers. We then combined them with polyvinyl alcohol/collagen to create tissue engineering scaffolds to overcome limitations. We found that PPG fibers formed amide bonds with polyvinyl alcohol/collagen scaffolds. After chemical crosslinking, the number of amide bonds increased, leading to a significant improvement in their mechanical properties and thermal stability. The results showed that compared with pure PVA scaffolds, the maximum compressive stress of the scaffold doped with 0.9 g nanofibers increased by 500 %, and the stress loss rate decreased by 40.6 % after 10 cycles of compression. The presence of natural macromolecular gelatin and the changes in the pore structure caused by nanofibers provide cells with richer and more three-dimensional adsorption sites, allowing them to grow in three dimensions on the scaffold. So, the hydrogel scaffold by reinforcing polyvinyl alcohol hydrogel with PPG fibers is a promising breast reconstruction method.

Advanced materials technologies to unravel mechanobiological phenomena

Advancements in materials-driven mechanobiology have yielded significant progress. Mechanobiology explores how cellular and tissue mechanics impact development, physiology, and disease, where extracellular matrix (ECM) dynamically interacts with cells. Biomaterial-based platforms emulate synthetic ECMs, offering precise control over cellular behaviors by adjusting mechanical properties. Recent technological advances enable in vitro models replicating active mechanical stimuli in vivo. These models manipulate cellular mechanics even at a subcellular level. In this review we discuss recent material-based mechanomodulatory studies in mechanobiology. We highlight the endeavors to mimic the dynamic properties of native ECM during pathophysiological processes like cellular homeostasis, lineage specification, development, aging, and disease progression. These insights may inform the design of accurate in vitro mechanomodulatory platforms that replicate ECM mechanics.

Modulating pentenoate-functionalized hyaluronic acid hydrogel network properties for meniscal fibrochondrocyte mechanotransduction

Knee meniscus tears are one of the most common musculoskeletal injuries. While meniscus replacements using allografts or biomaterial-based scaffolds are available, these treatments rarely result in integrated, functional tissue. Understanding mechanotransducive signaling cues that promote a meniscal cell regenerative phenotype is critical to developing therapies that promote tissue regeneration rather than fibrosis after injury. The purpose of this study was to develop a hyaluronic acid (HA) hydrogel system with tunable crosslinked network properties by modulating the degree of substitution (DoS) of reactive-ene groups to investigate mechanotransducive cues received by meniscal fibrochondrocytes (MFCs) from their microenvironment. A thiol-ene step-growth polymerization crosslinking mechanism was employed using pentenoate-functionalized hyaluronic acid (PHA) and dithiothreitol to achieve tunability of the chemical crosslinks and resulting network properties. Increased crosslink density, reduced swelling, and increased compressive modulus (60-1020 kPa) were observed with increasing DoS. Osmotic deswelling effects were apparent in PBS and DMEM+ compared to water; swelling ratios and compressive moduli were decreased in the ionic buffers. Frequency sweep studies showed storage and loss moduli of hydrogels at 1 Hz approach reported meniscus values and showed increasing viscous response with increasing DoS. The degradation rate increased with decreasing DoS. Lastly, modulating PHA hydrogel surface modulus resulted in control of MFC morphology, suggesting relatively soft hydrogels (E = 60 ± 35 kPa) promote more inner meniscus phenotype compared to rigid hydrogels (E = 610 ± 66 kPa). Overall, these results highlight the use of -ene DoS modulation in PHA hydrogels to tune crosslink density and physical properties to understand mechanotransduction mechanisms required to promote meniscus regeneration.

Suspension bath bioprinting and maturation of anisotropic meniscal constructs

Due to limited intrinsic healing capacity of the meniscus, meniscal injuries pose a significant clinical challenge. The most common method for treatment of damaged meniscal tissues, meniscectomy, leads to improper loading within the knee joint, which can increase the risk of osteoarthritis. Thus, there is a clinical need for the development of constructs for meniscal repair that better replicate meniscal tissue organization to improve load distributions and function over time. Advanced three-dimensional bioprinting technologies such as suspension bath bioprinting provide some key advantages, such as the ability to support the fabrication of complex structures using non-viscous bioinks. In this work, the suspension bath printing process is utilized to print anisotropic constructs with a unique bioink that contains embedded hydrogel fibers that align via shear stresses during printing. Constructs with and without fibers are printed and then cultured for up to 56 din vitroin a custom clamping system. Printed constructs with fibers demonstrate increased cell and collagen alignment, as well as enhanced tensile moduli when compared to constructs printed without fibers. This work advances the use of biofabrication to develop anisotropic constructs that can be utilized for the repair of meniscal tissue.

Aberrant chromatin reorganization in cells from diseased fibrous connective tissue in response to altered chemomechanical cues

Changes in the micro-environment of fibrous connective tissue can lead to alterations in the phenotypes of tissue-resident cells, yet the underlying mechanisms are poorly understood. Here, by visualizing the dynamics of histone spatial reorganization in tenocytes and mesenchymal stromal cells from fibrous tissue of human donors via super-resolution microscopy, we show that physiological and pathological chemomechanical cues can directly regulate the spatial nanoscale organization and density of chromatin in these tissue-resident cell populations. Specifically, changes in substrate stiffness, altered oxygen tension and the presence of inflammatory signals drive chromatin relocalization and compaction into the nuclear boundary, mediated by the activity of the histone methyltransferase EZH2 and an intact cytoskeleton. In healthy cells, chemomechanically triggered changes in the spatial organization and density of chromatin are reversible and can be attenuated by dynamically stiffening the substrate. In diseased human cells, however, the link between mechanical or chemical inputs and chromatin remodelling is abrogated. Our findings suggest that aberrant chromatin organization in fibrous connective tissue may be a hallmark of disease progression that could be leveraged for therapeutic intervention.

Natural biopolymer scaffold for meniscus tissue engineering

Meniscal injuries caused by trauma, degeneration, osteoarthritis, or other diseases always result in severe joint pain and motor dysfunction. Due to the unique anatomy of the human meniscus, the damaged meniscus lacks the ability to repair itself. Moreover, current clinical treatments for meniscal injuries, including meniscal suturing or resection, have significant limitations and drawbacks. With developments in tissue engineering, biopolymer scaffolds have shown promise in meniscal injury repair. They act as templates for tissue repair and regeneration, interacting with surrounding cells and providing structural support for newly formed meniscal tissue. Biomaterials offer tremendous advantages in terms of biocompatibility, bioactivity, and modifiable mechanical and degradation kinetics. In this study, the preparation and composition of meniscal biopolymer scaffolds, as well as their properties, are summarized. The current status of research and future research prospects for meniscal biopolymer scaffolds are reviewed in terms of collagen, silk, hyaluronic acid, chitosan, and extracellular matrix (ECM) materials. Overall, such a comprehensive summary provides constructive suggestions for the development of meniscal biopolymer scaffolds in tissue engineering.

Bio-inspired hydrogels with fibrous structure: A review on design and biomedical applications

Numerous tissues in the human body have fibrous structures, including the extracellular matrix, muscles, and heart, which perform critical biological functions and have exceptional mechanical strength. Due to their high-water content, softness, biocompatibility and elastic nature, hydrogels resemble biological tissues. Traditional hydrogels, on the other hand, have weak mechanical properties and lack tissue-like fibrous structures, limiting their potential applications. Thus, bio-inspired hydrogels with fibrous architectures have piqued the curiosity of biomedical researchers. Here, we review fabrication strategies for fibrous hydrogels and their recent progress in the biomedical fields of wound dressings, drug delivery, tissue engineering scaffolds and bioadhesives. Challenges and future perspectives are also discussed.

Lose the stress: Viscoelastic materials for cell engineering

Biomaterials are widely used to study and control a variety of cell behaviors, including stem cell differentiation, organogenesis, and tumor invasion. While considerable attention has historically been paid to biomaterial elastic (storage) properties, it has recently become clear that viscous (loss) properties can also powerfully influence cell behavior. Here we review advances in viscoelastic materials for cell engineering. We begin by discussing collagen, an abundant naturally occurring biomaterial that derives its viscoelastic properties from its fibrillar architecture, which enables dissipation of applied stresses. We then turn to two other naturally occurring biomaterials that are more frequently modified for engineering applications, alginate and hyaluronic acid, whose viscoelastic properties may be tuned by modulating network composition and crosslinking. We also discuss the potential of exploiting engineered fibrous materials, particularly electrospun fiber-based materials, to control viscoelastic properties. Finally, we review mechanisms through which cells process viscous and viscoelastic cues as they move along and within these materials. The ability of viscoelastic materials to relax cell-imposed stresses can dramatically alter migration on two-dimensional surfaces and confinement-imposed barriers to engraftment and infiltration in three-dimensional scaffolds. STATEMENT OF SIGNIFICANCE: Most tissues and many biomaterials exhibit some viscous character, a property that is increasingly understood to influence cell behavior in profound ways. This review discusses the origin and significance of viscoelastic properties of common biomaterials, as well as how these cues are processed by cells to influence migration. A deeper understanding of the mechanisms of viscoelastic behavior in biomaterials and how cells interpret these inputs should aid the design and selection of biomaterials for specific applications.

Accelerating Cell Migration along Radially Aligned Nanofibers through the Addition of Electrosprayed Nanoparticles in a Radial Density Gradient

Scaffolds capable of promoting cell migration from the periphery towards the center along the radial direction hold promises for tissue regeneration. Here we report a simple and general method based on masked electrospray for the fabrication of such scaffolds by depositing collagen nanoparticles on radially-aligned nanofibers in a radial density gradient. Placed between the metallic needle and the collector, an aperture with tunable opening sizes serves as the mask. By increasing the size of the opening at a fixed speed, the electrosprayed particles take a radial density gradient that decreases from the center to the periphery. When deposited on a glass slide, the radial density gradient of collagen nanoparticles promotes the migration of fibroblasts from the periphery towards the center. By replacing the glass slide with a scaffold comprised of radially-aligned nanofibers, a synergetic effect arises to further accelerate cell migration along the radial direction. The synergistic effect can be attributed to a unique combination of the topographic cue arising from the aligned nanofibers and the haptotactic cue enabled by the graded nanoparticles. This work demonstrates a method to maximize cell migration from the periphery towards the center through a combination of topographic and haptotactic cues.

Advances in Engineered Three-Dimensional (3D) Body Articulation Unit Models

Indeed, the body articulation units, commonly referred to as body joints, play significant roles in the musculoskeletal system, enabling body flexibility. Nevertheless, these articulation units suffer from several pathological conditions, such as osteoarthritis (OA), rheumatoid arthritis (RA), ankylosing spondylitis, gout, and psoriatic arthritis. There exist several treatment modalities based on the utilization of anti-inflammatory and analgesic drugs, which can reduce or control the pathophysiological symptoms. Despite the success, these treatment modalities suffer from major shortcomings of enormous cost and poor recovery, limiting their applicability and requiring promising strategies. To address these limitations, several engineering strategies have been emerged as promising solutions in fabricating the body articulation as unit models towards local articulation repair for tissue regeneration and high-throughput screening for drug development. In this article, we present challenges related to the selection of biomaterials (natural and synthetic sources), construction of 3D articulation models (scaffold-free, scaffold-based, and organ-on-a-chip), architectural designs (microfluidics, bioprinting, electrospinning, and biomineralization), and the type of culture conditions (growth factors and active peptides). Then, we emphasize the applicability of these articulation units for emerging biomedical applications of drug screening and tissue repair/regeneration. In conclusion, we put forward the challenges and difficulties for the further clinical application of the in vitro 3D articulation unit models in terms of the long-term high activity of the models.

Cellular modulation by the mechanical cues from biomaterials for tissue engineering

Mechanical cues from the extracellular matrix (ECM) microenvironment are known to be significant in modulating the fate of stem cells to guide developmental processes and maintain bodily homeostasis. Tissue engineering has provided a promising approach to the repair or regeneration of damaged tissues. Scaffolds are fundamental in cell-based regenerative therapies. Developing artificial ECM that mimics the mechanical properties of native ECM would greatly help to guide cell functions and thus promote tissue regeneration. In this review, we introduce various mechanical cues provided by the ECM including elasticity, viscoelasticity, topography, and external stimuli, and their effects on cell behaviours. Meanwhile, we discuss the underlying principles and strategies to develop natural or synthetic biomaterials with different mechanical properties for cellular modulation, and explore the mechanism by which the mechanical cues from biomaterials regulate cell function toward tissue regeneration. We also discuss the challenges in multimodal mechanical modulation of cell behaviours and the interplay between mechanical cues and other microenvironmental factors.

Advances in electrospun scaffolds for meniscus tissue engineering and regeneration

The meniscus plays a critical role in maintaining the homeostasis, biomechanics, and structural stability of the knee joint. Unfortunately, it is predisposed to damages either from sports-related trauma or age-related degeneration. The meniscus has an inherently limited capacity for tissue regeneration. Self-healing of injured adult menisci only occurs in the peripheral vascularized portion, while the spontaneous repair of the inner avascular region seems never happens. Repair, replacement, and regeneration of menisci through tissue engineering strategies are promising to address this problem. Recently, many scaffolds for meniscus tissue engineering have been proposed for both experimental and preclinical investigations. Electrospinning is a feasible and versatile technique to produce nano- to micro-scale fibers that mimic the microarchitecture of native extracellular matrix and is an effective approach to prepare nanofibrous scaffolds for constructing engineered meniscus. Electrospun scaffolds are reported to be capable of inducing colonization of meniscus cells by modulating local extracellular density and stimulating endogenous regeneration by driving reprogramming of meniscus wound microenvironment. Electrospun nanofibrous scaffolds with tunable mechanical properties, controllable anisotropy, and various porosities have shown promises for meniscus repair and regeneration and will undoubtedly inspire more efforts in exploring effective therapeutic approaches towards clinical applications. In this article, we review the current advances in the use of electrospun nanofibrous scaffolds for meniscus tissue engineering and repair and discuss prospects for future studies.

Chemically Modified Biopolymers for the Formation of Biomedical Hydrogels

Biopolymers are natural polymers sourced from plants and animals, which include a variety of polysaccharides and polypeptides. The inclusion of biopolymers into biomedical hydrogels is of great interest because of their inherent biochemical and biophysical properties, such as cellular adhesion, degradation, and viscoelasticity. The objective of this Review is to provide a detailed overview of the design and development of biopolymer hydrogels for biomedical applications, with an emphasis on biopolymer chemical modifications and cross-linking methods. First, the fundamentals of biopolymers and chemical conjugation methods to introduce cross-linking groups are described. Cross-linking methods to form biopolymer networks are then discussed in detail, including (i) covalent cross-linking (e.g., free radical chain polymerization, click cross-linking, cross-linking due to oxidation of phenolic groups), (ii) dynamic covalent cross-linking (e.g., Schiff base formation, disulfide formation, reversible Diels-Alder reactions), and (iii) physical cross-linking (e.g., guest-host interactions, hydrogen bonding, metal-ligand coordination, grafted biopolymers). Finally, recent advances in the use of chemically modified biopolymer hydrogels for the biofabrication of tissue scaffolds, therapeutic delivery, tissue adhesives and sealants, as well as the formation of interpenetrating network biopolymer hydrogels, are highlighted.

Colonization versus encapsulation in cell-laden materials design: porosity and process biocompatibility determine cellularization pathways

Seeding materials with living cells has been-and still is-one of the most promising approaches to reproduce the complexity and the functionality of living matter. The strategies to associate living cells with materials are limited to cell encapsulation and colonization, however, the requirements for these two approaches have been seldom discussed systematically. Here we propose a simple two-dimensional map based on materials' pore size and the cytocompatibility of their fabrication process to draw, for the first time, a guide to building cellularized materials. We believe this approach may serve as a straightforward guideline to design new, more relevant materials, able to seize the complexity and the function of biological materials. This article is part of the theme issue 'Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 1)'.

Click-functionalized hydrogel design for mechanobiology investigations

The advancement of click-functionalized hydrogels in recent years has coincided with rapid growth in the fields of mechanobiology, tissue engineering, and regenerative medicine. Click chemistries represent a group of reactions that possess high reactivity and specificity, are cytocompatible, and generally proceed under physiologic conditions. Most notably, the high level of tunability afforded by these reactions enables the design of user-controlled and tissue-mimicking hydrogels in which the influence of important physical and biochemical cues on normal and aberrant cellular behaviors can be independently assessed. Several critical tissue properties, including stiffness, viscoelasticity, and biomolecule presentation, are known to regulate cell mechanobiology in the context of development, wound repair, and disease. However, many questions still remain about how the individual and combined effects of these instructive properties regulate the cellular and molecular mechanisms governing physiologic and pathologic processes. In this review, we discuss several click chemistries that have been adopted to design dynamic and instructive hydrogels for mechanobiology investigations. We also chart a path forward for how click hydrogels can help reveal important insights about complex tissue microenvironments.

Meniscal Regenerative Scaffolds Based on Biopolymers and Polymers: Recent Status and Applications

Knee menisci are structurally complex components that preserve appropriate biomechanics of the knee. Meniscal tissue is susceptible to injury and cannot heal spontaneously from most pathologies, especially considering the limited regenerative capacity of the inner avascular region. Conventional clinical treatments span from conservative therapy to meniscus implantation, all with limitations. There have been advances in meniscal tissue engineering and regenerative medicine in terms of potential combinations of polymeric biomaterials, endogenous cells and stimuli, resulting in innovative strategies. Recently, polymeric scaffolds have provided researchers with a powerful instrument to rationally support the requirements for meniscal tissue regeneration, ranging from an ideal architecture to biocompatibility and bioactivity. However, multiple challenges involving the anisotropic structure, sophisticated regenerative process, and challenging healing environment of the meniscus still create barriers to clinical application. Advances in scaffold manufacturing technology, temporal regulation of molecular signaling and investigation of host immunoresponses to scaffolds in tissue engineering provide alternative strategies, and studies have shed light on this field. Accordingly, this review aims to summarize the current polymers used to fabricate meniscal scaffolds and their applications in vivo and in vitro to evaluate their potential utility in meniscal tissue engineering. Recent progress on combinations of two or more types of polymers is described, with a focus on advanced strategies associated with technologies and immune compatibility and tunability. Finally, we discuss the current challenges and future prospects for regenerating injured meniscal tissues.

Meniscal repair: The current state and recent advances in augmentation

Meniscal injuries represent one of the most common orthopedic injuries. The most frequent treatment is partial resection of the meniscus, or meniscectomy, which can affect joint mechanics and health. For this reason, the field has shifted gradually towards suture repair, with the intent of preservation of the tissue. "Save the Meniscus" is now a prolific theme in the field; however, meniscal repair can be challenging and ineffective in many scenarios. The objectives of this review are to present the current state of surgical management of meniscal injuries and to explore current approaches being developed to enhance meniscal repair. Through a systematic literature review, we identified meniscal tear classifications and prevalence, approaches being used to improve meniscal repair, and biological- and material-based systems being developed to promote meniscal healing. We found that biologic augmentation typically aims to improve cellular incorporation to the wound site, vascularization in the inner zones, matrix deposition, and inflammatory relief. Furthermore, materials can be used, both with and without contained biologics, to further support matrix deposition and tear integration, and novel tissue adhesives may provide the mechanical integrity that the meniscus requires. Altogether, evaluation of these approaches in relevant in vitro and in vivo models provides new insights into the mechanisms needed to salvage meniscal tissue, and along with regulatory considerations, may justify translation to the clinic. With the need to restore long-term function to injured menisci, biologists, engineers, and clinicians are developing novel approaches to enhance the future of robust and consistent meniscal reparative techniques.

Electrospun hydrogels for dynamic culture systems: advantages, progress, and opportunities

The extracellular matrix (ECM) is a water-swollen, tissue-specific material environment in which biophysiochemical signals are organized and influence cell behaviors. Electrospun nanofibrous substrates have been pursued as platforms for tissue engineering and cell studies that recapitulate features of the native ECM, in particular its fibrous nature. In recent years, progress in the design of electrospun hydrogel systems has demonstrated that molecular design also enables unique studies of cellular behaviors. In comparison to the use of hydrophobic polymeric materials, electrospinning hydrophilic materials that crosslink to form hydrogels offer the potential to achieve the water-swollen, nanofibrous characteristics of endogenous ECM. Although electrospun hydrogels require an additional crosslinking step to stabilize the fibers (allowing fibers to swell with water instead of dissolving) in comparison to their hydrophobic counterparts, researchers have made significant advances in leveraging hydrogel chemistries to incorporate biochemical and dynamic functionalities within the fibers. Consequently, dynamic biophysical and biochemical properties can be engineered into hydrophilic nanofibers that would be difficult to engineer in hydrophobic systems without strategic and sometimes intensive post-processing techniques. This Review describes common methodologies to control biophysical and biochemical properties of both electrospun hydrophobic and hydrogel nanofibers, with an emphasis on highlighting recent progress using hydrogel nanofibers with engineered dynamic complexities to develop culture systems for the study of biological function, dysfunction, development, and regeneration.

Bioscaffolds embedded with regulatory modules for cell growth and tissue formation: A review

The demand for artificial organs has greatly increased because of various aging-associated diseases and the wide need for organ transplants. A recent trend in tissue engineering is the precise reconstruction of tissues by the growth of cells adhering to bioscaffolds, which are three-dimensional (3D) structures that guide tissue and organ formation. Bioscaffolds used to fabricate bionic tissues should be able to not only guide cell growth but also regulate cell behaviors. Common regulation methods include biophysical and biochemical stimulations. Biophysical stimulation cues include matrix hardness, external stress and strain, surface topology, and electromagnetic field and concentration, whereas biochemical stimulation cues include growth factors, proteins, kinases, and magnetic nanoparticles. This review discusses bioink preparation, 3D bioprinting (including extrusion-based, inkjet, and ultraviolet-assisted 3D bioprinting), and regulation of cell behaviors. In particular, it provides an overview of state-of-the-art methods and devices for regulating cell growth and tissue formation and the effects of biophysical and biochemical stimulations on cell behaviors. In addition, the fabrication of bioscaffolds embedded with regulatory modules for biomimetic tissue preparation is explained. Finally, challenges in cell growth regulation and future research directions are presented.

Injectable Fibrin/Polyethylene Oxide Semi-IPN Hydrogel for a Segmental Meniscal Defect Regeneration

BACKGROUND

Meniscal deficiency from meniscectomy is a common situation in clinical practices. Regeneration of the deficient meniscal portion, however, is still not feasible.

PURPOSE

To develop an injectable hydrogel system consisting of fibrin (Fb) and polyethylene oxide (PEO) and to estimate its clinical potential for treating a segmental defect of the meniscus in a rabbit meniscal defect model.

STUDY DESIGN

Controlled laboratory study.

METHODS

The Fb/PEO hydrogel was fabricated by extruding 100 mg·mL^-1 of fibrinogen solution and 2,500 U·mL^-1 of thrombin solution containing 100 mg·mL^-1 of PEO through a dual-syringe system. The hydrogels were characterized by rheological analysis and biodegradation tests. The meniscal defects of New Zealand White male rabbits were generated by removing 60% of the medial meniscus from the anterior side. The removed portion included the central portion. The Fb/PEO hydrogel was injected into the meniscal defect of the experimental knee through the joint space between the femoral condyle and tibial plateau at the anterior knee without a skin incision. The entire medial menisci from both knees of each rabbit were collected and photographed before placement in formalin for histological processing. Hematoxylin and eosin, safranin O, and immunohistochemical staining for type II collagen was performed. The biomechanical property of the regenerated meniscus was evaluated using a universal tensile machine.

RESULTS

The Fb/PEO hydrogel was fabricated by an in situ gelation process, and the hydrogel displayed a semi-interpenetrating polymer network structure. We demonstrated that the mechanical properties of Fb-based hydrogels increased in a PEO-dependent manner. Furthermore, the addition of PEO delayed the biodegradation of the hydrogel. Our in vivo data demonstrated that, as compared with Fb hydrogel, Fb/PEO hydrogel injection into the meniscectomy model showed improved tissue regeneration. The regenerated meniscal tissue by Fb/PEO hydrogel showed enhanced tissue quality, which was supported by the histological and biomechanical properties.

CONCLUSION

The Fb/PEO hydrogel had an effective tissue-regenerative ability through injection into the in vivo rabbit meniscal defect model.

CLINICAL RELEVANCE

This injectable hydrogel system can promote meniscal repair and be readily utilized in clinical application.

3D-Printed, Modular, and Parallelized Microfluidic System with Customizable Scaffold Integration to Investigate the Roles of Basement Membrane Topography on Endothelial Cells

Because dysfunctions of endothelial cells are involved in many pathologies, in vitro endothelial cell models for pathophysiological and pharmaceutical studies have been a valuable research tool. Although numerous microfluidic-based endothelial models have been reported, they had the cells cultured on a flat surface without considering the possible three-dimensional (3D) structure of the native extracellular matrix (ECM). Endothelial cells rest on the basement membrane in vivo, which contains an aligned microfibrous topography. To better understand and model the cells, it is necessary to know if and how the fibrous topography can affect endothelial functions. With conventional fully integrated microfluidic apparatus, it is difficult to include additional topographies in a microchannel. Therefore, we developed a modular microfluidic system by 3D-printing and electrospinning, which enabled easy integration and switching of desired ECM topographies. Also, with standardized designs, the system allowed for high flow rates up to 4000 μL/min, which encompassed the full shear stress range for endothelial studies. We found that the aligned fibrous topography on the ECM altered arginine metabolism in endothelial cells and thus increased nitric oxide production. There has not been an endothelial model like this, and the new knowledge generated thereby lays a groundwork for future endothelial research and modeling.

Nanofiber Scaffolds by Electrospinning for Rotator Cuff Tissue Engineering

Rotator cuff tears continue to be at risk of retear or failure to heal after surgical repair, despite the use of various surgical techniques, which stimulate development of novel scaffolding strategies. They should be able to address the known causes of failure after the conventional rotator cuff repair: (1) failure to reproduce the normal tendon healing process, (2) resultant failure to reproduce four zones of the enthesis, and (3) failure to attain sufficient mechanical strength after repair. Nanofiber scaffolds are suited for this application because they can be engineered to mimic the ultrastructure and properties of the native rotator cuff tendon. Among various methods for tissue-engineered nanofibers, electrospinning has recently been highlighted in the rotator cuff field. Electrospinning can create fibrous and porous structures that resemble natural tendon's extracellular matrix. Other advantages include the ability to create relatively large surface-to-volume ratios, the ability to control fiber size from the micro to the nano scale, and the flexibility of material choices. In this review, we will discuss the anatomical and mechanical features of the rotator cuff tendon, their potential impacts on improper healing after repair, and the current knowledge of the use of electrospinning for rotator cuff tissue engineering.

Advanced Polymer-Based Drug Delivery Strategies for Meniscal Regeneration

The meniscus plays a critical role in maintaining knee joint homeostasis. Injuries to the meniscus, especially considering the limited self-healing capacity of the avascular region, continue to be a challenge and are often treated by (partial) meniscectomy, which has been identified to cause osteoarthritis. Currently, meniscus tissue engineering focuses on providing extracellular matrix (ECM)-mimicking scaffolds to direct the inherent meniscal regeneration process, and it has been found that various stimuli are essential. Numerous bioactive factors present benefits in regulating cell fate, tissue development, and healing, but lack an optimal delivery system. More recently, bioengineers have developed various polymer-based drug delivery systems (PDDSs), which are beneficial in terms of the favorable properties of polymers as well as novel delivery strategies. Engineered PDDSs aim to provide not only an ECM-mimicking microenvironment but also the controlled release of bioactive factors with release profiles tailored according to the biological concerns and properties of the factors. In this review, both different polymers and bioactive factors involved in meniscal regeneration are discussed, as well as potential candidate systems, with examples of recent progress. This article aims to summarize drug delivery strategies in meniscal regeneration, with a focus on novel delivery strategies rather than on specific delivery carriers. The current challenges and future prospects for the structural and functional regeneration of the meniscus are also discussed. Impact statement Meniscal injury remains a clinical Gordian knot owing to the limited healing potential of the region, restricted surgical approaches, and risk of inducing osteoarthritis. Existing tissue engineering scaffolds that provide mechanical support and a favorable microenvironment also lack biological cues. Advanced polymer-based delivery strategies consisting of polymers incorporating bioactive factors have emerged as a promising direction. This article primarily reviews the types and applications of biopolymers and bioactive factors in meniscal regeneration. Importantly, various carrier systems and drug delivery strategies are discussed with the hope of inspiring further advancements in this field.

Success Criteria and Preclinical Testing of Multifunctional Hydrogels for Tendon Regeneration

Tendon injuries are difficult to heal, in part, because intrinsic tendon healing, which is dominated by scar tissue formation, does not effectively regenerate the native structure and function of healthy tendon. Further, many current treatment strategies also fall short of producing regenerated tendon with the native properties of healthy tendon. There is increasing interest in the use of cell-instructive strategies to limit the intrinsic fibrotic response following injury and improve the regenerative capacity of tendon in vivo. We have established multifunctional, cell-instructive hydrogels for treating injured tendon that afford tunable control over the biomechanical, biochemical, and structural properties of the cell microenvironment. Specifically, we incorporated integrin-binding domains (RGDS) and assembled multifunctional collagen mimetic peptides that enable cell adhesion and elongation of stem cells within synthetic hydrogels of designed biomechanical properties and evaluated these materials using targeted success criteria developed for testing in mechanically demanding environments such as tendon healing. The in vitro and in situ success criteria were determined based on systematic reviews of the most commonly reported outcome measures of hydrogels for tendon repair and established standards for testing of biomaterials. We then showed, using validation experiments, that multifunctional and synthetic hydrogels meet these criteria. Specifically, these hydrogels have mechanical properties comparable to developing tendon; are noncytotoxic both in two-dimensional bolus exposure (hydrogel components) and three-dimensional encapsulation (full hydrogel); are formed, retained, and visualized within tendon defects over time (2-weeks); and provide mechanical support to tendon defects at the time of in situ gel crosslinking. Ultimately, the in vitro and in situ success criteria evaluated in this study were designed for preclinical research to rigorously test the potential to achieve successful tendon repair before in vivo testing and indicate the promise of multifunctional and synthetic hydrogels for continued translation. Impact statement Tendon healing results in a weak scar that forms due to poor cell-mediated repair of the injured tissue. Treatments that tailor the instructions experienced by cells during healing afford opportunities to regenerate the healthy tendon. Engineered cell-instructive cues, including the biomechanical, biochemical, and structural properties of the cell microenvironment, within multifunctional synthetic hydrogels are promising therapeutic strategies for tissue regeneration. In this article, the preclinical efficacy of multifunctional synthetic hydrogels for tendon repair is tested against rigorous in vitro and in situ success criteria. This study indicates the promise for continued preclinical translation of synthetic hydrogels for tissue regeneration.

Meniscal tissue repair with nanofibers: future perspectives

The knee menisci are critical to the long-term health of the knee joint. Because of the high incidence of injury and degeneration, replacing damaged or lost meniscal tissue is extremely clinically relevant. The multiscale architecture of the meniscus results in unique biomechanical properties. Nanofibrous scaffolds are extremely attractive to replicate the biochemical composition and ultrastructural features in engineered meniscus tissue. We review recent advances in electrospinning to generate nanofibrous scaffolds and the current state-of-the-art of electrospun materials for meniscal regeneration. We discuss the importance of cellular function for meniscal tissue engineering and the application of cells derived from multiple sources. We compare experimental models necessary for proof of concept and to support translation. Finally, we discuss future directions and potential for technological innovations.