Harnessing biomechanics to develop cartilage regeneration strategies

The mechanics of tissue-engineered temporomandibular joint discs: Current status and prospects for enhancement

The temporomandibular joint (TMJ) disc is an essential protective but vulnerable fibrocartilage. Their high mechanical strength is vital in absorbing loads, reducing friction, and protecting the condylar surface. Many diseases can lead to the destruction or degeneration of the mechanical function of the TMJ disc. Unfortunately, conservative treatment is ineffective in restoring the defective mechanical properties of the discs. Tissue engineering has been investigated as a promising alternative treatment approach to approximate the properties of native tissue. However, it is difficult for tissue-engineered discs to obtain sufficient mechanical properties. Several approaches have been proposed to improve the mechanical properties of tissue-engineered constructs. In this review, we summarized the mechanical properties of native TMJ discs and discussed the current mechanical testing methods. We then summarized the current advances in improving the mechanical properties of TMJ disc tissue-engineered constructs. Moreover, existing challenges and outbreak directions are discussed. This review assists future research in better understanding the mechanical properties of both native and tissue-engineered TMJ discs. It provides new insights into future mechanical property enhancement for TMJ disc tissue engineering.

Self-Assemble Silk Fibroin Microcapsules for Cartilage Regeneration through Gene Delivery and Immune Regulation

Effective treatments for cartilage defects are currently lacking. Gene delivery using proper delivery systems has shown great potential in cartilage regeneration. However, the inflammatory microenvironment generated by the defected cartilage severely affects the system's delivery efficiency. Therefore, this study reports a silk fibroin microcapsule (SFM) structure based on layer-by-layer self-assembly, in which interleukin-4 (IL-4) is modified on silk by click chemistry and loaded with lysyl oxidase plasmid DNA (LOX pDNA). The silk microcapsules display good biocompatibility and the release rate of genes can be adjusted by controlling the number of self-assembled layers. Moreover, the functionalized SFMs mixed with methacrylated gelatin (GelMA) exhibit good injectability. The IL-4 on the outer layer of the SFM can regulate macrophages to polarize toward the M2 type, thereby promoting cartilage matrix repair and inhibiting inflammation. The LOX pDNA loaded inside can be effectively delivered into cells to promote extracellular matrix generation, significantly promoting cartilage regeneration. The results of this study provide a promising biomaterial for cartilage repair, and this novel silk-based microcapsule delivery system can also provide strategies for the treatment of other diseases.

Low-oxygen tension augments chondrocyte sensitivity to biomimetic thermomechanical cues in cartilage-engineered constructs

Chondrocytes respond to various biophysical cues, including oxygen tension, transient thermal signals, and mechanical stimuli. However, understanding how these factors interact to establish a unique regulatory microenvironment for chondrocyte function remains unclear. Herein, we explore these interactions using a joint-simulating bioreactor that independently controls the culture's oxygen concentration, evolution of temperature, and mechanical loading. Our analysis revealed significant coupling between these signals, resulting in a remarkable ∼14-fold increase in collagen type II (COL2a) and aggrecan (ACAN) mRNA expression. Furthermore, dynamic thermomechanical stimulation enhanced glycosaminoglycan and COL2a protein synthesis, with the magnitude of the biosynthetic changes being oxygen dependent. Additionally, our mechanistic study highlighted the crucial role of SRY-box transcription factor 9 (SOX9) as a major regulator of chondrogenic response, specifically expressed in response to combined biophysical signals. These findings illuminate the integration of various mechanobiological cues by chondrocytes and provide valuable insights for improving the extracellular matrix content in cartilage-engineered constructs.

Within or Without You? A Perspective Comparing In Situ and Ex Situ Tissue Engineering Strategies for Articular Cartilage Repair

Human articular cartilage has a poor ability to self-repair, meaning small injuries often lead to osteoarthritis, a painful and debilitating condition which is a major contributor to the global burden of disease. Existing clinical strategies generally do not regenerate hyaline type cartilage, motivating research toward tissue engineering solutions. Prospective cartilage tissue engineering therapies can be placed into two broad categories: i) Ex situ strategies, where cartilage tissue constructs are engineered in the lab prior to implantation and ii) in situ strategies, where cells and/or a bioscaffold are delivered to the defect site to stimulate chondral repair directly. While commonalities exist between these two approaches, the core point of distinction-whether chondrogenesis primarily occurs "within" or "without" (outside) the body-can dictate many aspects of the treatment. This difference influences decisions around cell selection, the biomaterials formulation and the surgical implantation procedure, the processes of tissue integration and maturation, as well as, the prospects for regulatory clearance and clinical translation. Here, ex situ and in situ cartilage engineering strategies are compared: Highlighting their respective challenges, opportunities, and prospects on their translational pathways toward long term human cartilage repair.

The sternum reconstruction: Present and future perspectives

Sternectomy is a procedure mainly used for removing tumor masses infiltrating the sternum or treating infections. Moreover, the removal of the sternum involves the additional challenge of performing a functional reconstruction. Fortunately, various approaches have been proposed for improving the operation and outcome of reconstruction, including allograft transplantation, using novel materials, and developing innovative surgical approaches, which promise to enhance the quality of life for the patient. This review will highlight the surgical approaches to sternum reconstruction and the new perspectives in the current literature.

Engineered artificial articular cartilage made of decellularized extracellular matrix by mechanical and IGF-1 stimulation

Cartilage engineering has the potential to overcome clinical deficiency in joint disorders. Decellularized extracellular matrix (dECM) has great biocompatibility and bioactivity and can be considered an appropriate natural scaffold for tissue engineering applications. Both insulin-like growth factor-1 (IGF-1) and mechanical compression stimulate the production of cartilage ECM, modulate mechanical properties, and gene expression. The current investigation aimed to fabricate a high-quality moldable artificial cartilage by exposing the chondrocytes in biomimicry conditions using cartilage dECM, IGF-1, and mechanical stimulations. In this study, an ad hoc bioreactor was designed to apply dynamic mechanical stimuli (10 % strain, 1 Hz) on chondrocyte-laden cartilage dECM-constructs with/without IGF-1 supplementation for 2 weeks, 3 h/day. Our data revealed that mechanical stimulation had no adverse effect on cell viability and proliferation. However, it elevated the expression of chondrogenic markers such as collagen type II (COL2A1), aggrecan (ACAN), and proteoglycan-4 (PRG-4), and reduced the expression of matrix metalloproteinase-3 (MMP-3). Mechanical stimulation also promoted higher newly formed glycosaminoglycan (GAG) and produced more aligned fibers that can be responsible for higher Young's modulus of the engineered construct. Even though IGF-1 demonstrated some extent of improvement in developing neocartilage, it was not as effective as mechanical stimulation. Neither IGF-1 nor compression elevated the collagen type I expression. Compression and IGF-1 showed a synergistic impact on boosting the level of COL2A1 but not the other factors. In conclusion, mechanical stimulation on moldable cartilage dECM can be considered a good technique to fabricate artificial cartilage with higher functionality.

Optimizing artificial meniscus by mechanical stimulation of the chondrocyte-laden acellular meniscus using ad hoc bioreactor

Background

Tissue engineering focuses on reconstructing the damaged meniscus by mimicking the native meniscus. The application of mechanical loading on chondrocyte-laden decellularized whole meniscus is providing the natural microenvironment. The goal of this study was to evaluate the effects of dynamic compression and shear load on chondrocyte-laden decellularized meniscus.

Material and methods

The fresh samples of rabbit menisci were decellularized, and the DNA removal was confirmed by histological assessments and DNA quantification. The biocompatibility, degradation and hydration rate of decellularized menisci were evaluated. The decellularized meniscus was injected at a density of 1 × 10⁵ chondrocyte per scaffold and was subjected to 3 cycles of dynamic compression and shear stimuli (1 h of 5% strain, ± 25°shear at 1 Hz followed by 1 h rest) every other day for 2 weeks using an ad hoc bioreactor. Cytotoxicity, GAG content, ultrastructure, gene expression and mechanical properties were examined in dynamic and static condition and compared to decellularized and intact menisci.

Results

Mechanical stimulation supported cell viability and increased glycosaminoglycan (GAG) accumulation. The expression of collagen-I (COL-I, 10.7-folds), COL-II (6.4-folds), aggrecan (AGG, 3.2-folds), and matrix metalloproteinase (MMP3, 2.3-folds) was upregulated compared to the static conditions. Furthermore, more aligned fibers and enhanced tensile strength were observed in the meniscus treated in dynamic condition with no sign of mineralization.

Conclusion

Compress and shear stimulation mimics the loads on the joint during walking and be able to improve cell function and ultrastructure of engineered tissue to recreate a functional artificial meniscus.

Magnetic Nanoparticles and Magnetic Field Exposure Enhances Chondrogenesis of Human Adipose Derived Mesenchymal Stem Cells But Not of Wharton Jelly Mesenchymal Stem Cells

Purpose: Iron oxide based magnetic nanoparticles (MNP) are versatile tools in biology and medicine. Adipose derived mesenchymal stem cells (ADSC) and Wharton Jelly mesenchymal stem cells (WJMSC) are currently tested in different strategies for regenerative regenerative medicine (RM) purposes. Their superiority compared to other mesenchymal stem cell consists in larger availability, and superior proliferative and differentiation potential. Magnetic field (MF) exposure of MNP-loaded ADSC has been proposed as a method to deliver mechanical stimulation for increasing conversion to musculoskeletal lineages. In this study, we investigated comparatively chondrogenic conversion of ADSC-MNP and WJMSC with or without MF exposure in order to identify the most appropriate cell source and differentiation protocol for future cartilage engineering strategies. Methods: Human primary ADSC and WJMSC from various donors were loaded with proprietary uncoated MNP. The in vitro effect on proliferation and cellular senescence (beta galactosidase assay) in long term culture was assessed. In vitro chondrogenic differentiation in pellet culture system, with or without MF exposure, was assessed using pellet histology (Safranin O staining) as well as quantitative evaluation of glycosaminoglycan (GAG) deposition per cell. Results: ADSC-MNP complexes displayed superior proliferative capability and decreased senescence after long term (28 days) culture in vitro compared to non-loaded ADSC and to WJMSC-MNP. Significant increase in chondrogenesis conversion in terms of GAG/cell ratio could be observed in ADSC-MNP. MF exposure increased glycosaminoglycan deposition in MNP-loaded ADSC, but not in WJMSC. Conclusion: ADSC-MNP display decreased cellular senescence and superior chondrogenic capability in vitro compared to non-loaded cells as well as to WJMSC-MNP. MF exposure further increases ADSC-MNP chondrogenesis in ADSC, but not in WJMSC. Loading ADSC with MNP can derive a successful procedure for obtaining improved chondrogenesis in ADSC. Further in vivo studies are needed to confirm the utility of ADSC-MNP complexes for cartilage engineering.

High resolution three-dimensional strain measurements in human articular cartilage

An unresolved challenge in osteoarthritis research is characterising the localised intra-tissue mechanical response of articular cartilage. The aim of this study was to explore whether laboratory micro-computed tomography (micro-CT) and digital volume correlation (DVC) permit non-destructive quantification of three-dimensional (3D) strain fields in human articular cartilage. Human articular cartilage specimens were harvested from the knee, mounted into a loading device and imaged in the unloaded and loaded states using a micro-CT scanner. Strain was measured throughout the cartilage volume using the micro-CT image data and DVC analysis. The volumetric DVC-measured strain was within 5% of the known applied strain. Variation in strain distribution between the superficial, middle and deep zones was observed, consistent with the different architecture of the material in these locations. These results indicate DVC method may be suitable for calculating strain in human articular cartilage.

Advances in biomechanical and biochemical engineering methods to stimulate meniscus tissue

Meniscal injuries can cause cartilage degeneration, which usually leads to the development of osteoarthritis (OA) and results in progressive destruction of the knee joint. Therefore, it is important to identify methods to stop or slow the development of OA after the onset of meniscal defects. The current surgical techniques for meniscal injuries are insufficient to prevent the progression of knee OA, which has accelerated the development of alternative tissue engineering strategies. Much progress has been made in the use of biomechanical and biochemical stimuli in the past decades to engineer neotissue akin to native meniscus. In this review, we focus on the current progress in biomechanical and biochemical stimuli-based strategies applied to meniscal tissue engineering, and explore how these factors influence meniscal regeneration. By understanding the functional mechanism that can stimulate regeneration in the meniscus, we hope that this review will provide a theoretical basis and strategies for meniscus tissue engineering.

The Challenge of Cartilage Integration: Understanding a Major Barrier to Chondral Repair

Articular cartilage defects caused by injury frequently lead to osteoarthritis, a painful and costly disease. Despite widely used surgical methods to treat articular cartilage defects and a plethora of research into regenerative strategies as treatments, long-term clinical outcomes are not satisfactory. Failure to integrate repair tissue with native cartilage is a recurring issue in surgical and tissue-engineered strategies, seeing eventual degradation of the regenerated or surrounding tissue. This review delves into the current understanding of why continuous and robust integration with native cartilage is so difficult to achieve. Both the intrinsic limitations of chondrocytes to remodel injured cartilage, and the significant challenges posed by a compromised biomechanical environment are described. Recent scaffold and cell-based techniques to repair cartilage are also discussed, and limitations of existing methods to evaluate integrative repair. In particular, the importance of evaluating the mechanical integrity of the interface between native and repair tissue is highlighted as a meaningful assessment of any strategy to repair this load-bearing tissue. Impact statement The failure to integrate grafts or biomaterials with native cartilage is a major barrier to cartilage repair. An in-depth understanding of the reasons cartilage integration remains a challenge is required to inform cartilage repair strategies. In particular, this review highlights that integration of cartilage repair strategies is frequently assessed in terms of the continuity of tissue, but not the mechanical integrity. Given the load-bearing nature of cartilage, evaluating integration in terms of interfacial strength is essential to assessing the potential success of cartilage repair methods.

3D cell-printing of biocompatible and functional meniscus constructs using meniscus-derived bioink

Meniscus injuries are prevalent in orthopedic diagnosis. The reconstruction of the structural inhomogeneity and anisotropy of the meniscus is a major challenge in clinical practice. Meniscal tissue engineering has emerged as a potential alternative for the treatment of various meniscal diseases and injuries. In this study, we developed three-dimensional (3D) cell-printed meniscus constructs using a mixture of polyurethane and polycaprolactone polymers and cell-laden decellularized meniscal extracellular matrix (me-dECM) bioink with high controllability and durable architectural integrity. The me-dECM bioink provided 3D cell-printed meniscus constructs with a conducive biochemical environment that supported growth and promoted the proliferation and differentiation of encapsulated stem cells toward fibrochondrogenic commitment. In addition, we investigated the in vivo performance of the 3D cell-printed meniscus constructs, which exhibited biocompatibility, excellent mechanical properties, and improved biological functionality. These attributes were similar to those of the native meniscus. Collectively, the 3D cell-printing technology and me-dECM bioink facilitate the recapitulation of meniscus tissue specificity in the aspect of the shape and microenvironment for meniscus regeneration. Further, the developed constructs can potentially be applied in clinical practice.

Endogenous cell recruitment strategy for articular cartilage regeneration

In the absence of timely and proper treatments, injuries to articular cartilage (AC) can lead to cartilage degeneration and ultimately result in osteoarthritis. Regenerative medicine and tissue engineering techniques are emerging as promising approaches for AC regeneration and repair. Although the use of cell-seeded scaffolds prior to implantation can regenerate and repair cartilage lesions to some extent, these approaches are still restricted by limited cell sources, excessive costs, risks of disease transmission and complex manufacturing practices. Recently developed acellular scaffold approaches that rely on the recruitment of endogenous cells to the injured sites avoid these drawbacks and offer great promise for in situ AC regeneration. Multiple endogenous stem/progenitor cells (ESPCs) are found in joint-resident niches and have the capability to migrate to sites of injury to participate in AC regeneration. However, the natural recruitment of ESPCs is insufficient, and the local microenvironment is hostile after injury. Hence, an endogenous cell recruitment strategy based on the combination of chemoattractants and acellular scaffolds to effectively and specifically recruit ESPCs and improve local microenvironment may provide new insights into in situ AC regeneration. This review provides a brief overview of: (1) the status of endogenous cell recruitment strategy; (2) the subpopulations, potential migration routes (PMRs) of joint-resident ESPCs and their immunomodulatory and reparative effects; (3) chemoattractants and their potential adverse effects; (4) scaffold-based drug delivery systems (SDDSs) that are utilized for in situ AC regeneration; and (5) the challenges and future perspectives of endogenous cell recruitment strategy for AC regeneration. STATEMENT OF SIGNIFICANCE: Although the endogenous cell recruitment strategy for articular cartilage (AC) regeneration has been investigated for several decades, much work remains to be performed in this field. Future studies should have the following aims: (1) reporting the up-to-date progress in the endogenous cell recruitment strategies; (2) determining the subpopulations of ESPCs, the cellular and molecular mechanisms underlying the migration of these cells and their anti-inflammatory, immunomodulatory and reparative effects; (3) elucidating the chemoattractants that enhance ESPC recruitment and their potential adverse effects; and (4) developing advanced SDDSs for chemoattractant dispatch. Herein, we present a systematic overview of the aforementioned issues to provide a better understanding of endogenous cell recruitment strategies for AC regeneration and repair.

Biological perspectives and current biofabrication strategies in osteochondral tissue engineering

Articular cartilage and the underlying subchondral bone are crucial in human movement and when damaged through disease or trauma impacts severely on quality of life. Cartilage has a limited regenerative capacity due to its avascular composition and current therapeutic interventions have limited efficacy. With a rapidly ageing population globally, the numbers of patients requiring therapy for osteochondral disorders is rising, leading to increasing pressures on healthcare systems. Research into novel therapies using tissue engineering has become a priority. However, rational design of biomimetic and clinically effective tissue constructs requires basic understanding of osteochondral biological composition, structure, and mechanical properties. Furthermore, consideration of material design, scaffold architecture, and biofabrication strategies, is needed to assist in the development of tissue engineering therapies enabling successful translation into the clinical arena. This review provides a starting point for any researcher investigating tissue engineering for osteochondral applications. An overview of biological properties of osteochondral tissue, current clinical practices, the role of tissue engineering and biofabrication, and key challenges associated with new treatments is provided. Developing precisely engineered tissue constructs with mechanical and phenotypic stability is the goal. Future work should focus on multi-stimulatory environments, long-term studies to determine phenotypic alterations and tissue formation, and the development of novel bioreactor systems that can more accurately resemble the in vivo environment.

Role of the lysyl oxidase family in organ development (Review)

Lysyl oxidase proteins (LOXs) are amine oxidases, which are mainly located in smooth muscle cells and fibroblasts and serve an important role in the formation of the extracellular matrix (ECM) in a copper-dependent manner. Owing to the ability of LOX proteins to modulate crosslinking between collagens and to promote the deposition of other fibers, they serve crucially in organogenesis and the subsequent organ development, as well as disease initiation and progression. In addition, ECM formation significantly influences organ morphological formation in both cancer- and non-tumor-related diseases, in addition to cellular epigenetic transformation and migration, under the influence of LOXs. A number of different signaling pathways regulate the LOXs expression and their enzymatic activation. The tissue remodeling and transformation process shares some resemblance between oncogenesis and embryogenesis. Additionally the roles that LOXs serve appeared to be stressed during oncogenesis and tumor metastasis. It has also been indicated LOXs have a noteworthy role in non-tumor diseases. Nonetheless, the role of LOXs in systemic or local organ development and disease control remains unknown. In the present study, the essential roles that LOXs play in embryogenesis were unveiled partially, whereas the role of LOXs in organ or systematic development requires further investigations. The present review aimed to discuss the roles of members of the LOX family in the context of the remodeling of organogenesis and organ development. In addition, the consequences of the malfunction of these proteins related to the development of abnormalities and resulting diseases is discussed.

Roles of TRPV4 and piezo channels in stretch-evoked Ca2+ response in chondrocytes

Chondrocyte mechanotransduction is not well understood, but recently, it has been proposed that mechanically activated ion channels such as transient receptor potential vanilloid 4 (TRPV4), Piezo1, and Piezo2 are of functional importance in chondrocyte mechanotransduction. The aim of this study was to distinguish the potential contributions of TRPV4, Piezo1, and Piezo2 in transducing different intensities of repetitive mechanical stimulus in chondrocytes. To study this, TRPV4-, Piezo1-, or Piezo2-specific siRNAs were transfected into cultured primary chondrocytes to knock down (KD) TRPV4, Piezo1, or Piezo2 expression, designated TRPV4-KD, Piezo1-KD, or Piezo2-KD cells. Then we used Flexcell® Tension System to apply cyclic tensile strains (CTS) of 3% to 18% at 0.5 Hz for 8 h to the knockdown and control siRNA-treated cells. Finally, using a Ca2+ imaging system, stretch-evoked intracellular Ca2+ ([Ca2+]i ) influx in chondrocytes was examined to investigate the roles of TRPV4, Piezo1, and Piezo2 in Ca2+ signaling in response to different intensities of repetitive mechanical stretch stimulation. The characteristics of [Ca2+]i in chondrocytes evoked by stretch stimulation were stretch intensity dependent when comparing unstretched cells. In addition, stretch-evoked [Ca2+]i changes were significantly suppressed in TRPV4-KD, Piezo1-KD, or Piezo2-KD cells compared with control siRNA-treated cells, indicating that any channel essential for Ca2+ signaling induced by stretch stimulation in chondrocytes. Of note, they played different roles in calcium oscillation induced by different intensities of stretch stimulation. More specifically, TRPV4-mediated Ca2+ signaling played a central role in the response of chondrocytes to physiologic levels of strain (3% and 8% of strain), while Piezo2-mediated Ca2+ signaling played a central role in the response of chondrocytes to injurious levels of strain (18% of strain). These results provide a basis for further examination of mechanotransduction in cartilage and raise a possibility of therapeutically targeting Piezo2-mediated mechanotransduction for the treatment of cartilage disease induced by repetitive mechanical forces.

The Scaffold-Articular Cartilage Interface: A Combined In Vitro and In Silico Analysis Under Controlled Loading Conditions

The optimal method to integrate scaffolds with articular cartilage has not yet been identified, in part because of our lack of understanding about the mechanobiological conditions at the interface. Our objective was to quantify the effect of mechanical loading on integration between a scaffold and articular cartilage. We hypothesized that increased number of loading cycles would have a detrimental effect on interface integrity. The following models were developed: (i) an in vitro scaffold-cartilage explant system in which compressive sinusoidal loading cycles were applied for 14 days at 1 Hz, 5 days per week, for either 900, 1800, 3600, or 7200 cycles per day and (ii) an in silico inhomogeneous, biphasic finite element model (bFEM) of the scaffold-cartilage construct that was used to characterize interface micromotion, stress, and fluid flow under the prescribed loading conditions. In accordance with our hypothesis, mechanical loading significantly decreased scaffold-cartilage interface strength compared to unloaded controls regardless of the number of loading cycles. The decrease in interfacial strength can be attributed to abrupt changes in vertical displacement, fluid pressure, and compressive stresses along the interface, which reach steady-state after only 150 cycles of loading. The interfacial mechanical conditions are further complicated by the mismatch between the homogeneous properties of the scaffold and the depth-dependent properties of the articular cartilage. Finally, we suggest that mechanical conditions at the interface can be more readily modulated by increasing pre-incubation time before the load is applied, as opposed to varying the number of loading cycles.

A novel construct with biomechanical flexibility for articular cartilage regeneration

BACKGROUND

Although tissue-engineered cartilage has been broadly studied, complete integration of regenerated cartilage with residual cartilage is still difficult for the inferior mechanical and biochemical feature of neocartilage. Chondrogenesis of mesenchymal stem cells can be induced by biophysical and biochemical factors.

METHODS

In this study, autologous platelet-rich fibrin (PRF) membrane was used as a growth factor-rich scaffold that may facilitate differentiation of the transplanted bone marrow mesenchymal stem cells (BMSCs). At the same time, hydrostatic pressure was adopted for pre-adjustment of the seed cells before transplantation that may promote the mechanical flexibility of neocartilage.

RESULTS

An in vitro study showed that the feasible hydrostatic pressure stimulation substantially promoted the chondrogenic potential of in vitro-cultured BMSC/PRF construct. In vivo results revealed that at every time point, the newborn tissues were the most favorable in the pressure-pretreated BMSC/PRF transplant group. Besides, the transplantation of feasible hydrostatic pressure-pretreated construct by BMSC sheet fragments and PRF granules could obviously improve the integration between the regenerated cartilage and host cartilage milieu, and thereby achieve boundaryless repair between the neocartilage and residual host cartilage tissue in rabbit temporomandibular joints. It could be concluded that feasible hydrostatic pressure may effectively promote the proliferation and chondrogenic differentiation of BMSCs in a BMSC/PRF construct.

CONCLUSION

This newly formed construct with biomechanical flexibility showed a superior capacity for cartilage regeneration by promoting the mechanical properties and integration of neocartilage.

Full biomechanical mapping of the ovine knee joint to determine creep-recovery, stiffness and thickness variation

BACKGROUND

Clinical cartilage repair strategies can be tested using the sheep model as suggest by the European Medicines Agency. To characterize variation within the joint a full biomechanical mapping is necessary. The aim of this study is to establish a loading model, to map regional differences within the knee and determine reference areas for area specific replacement techniques.

METHODS

A porous indenter was selected to evaluate 22 defined test locations (femoral condyles, tibia plateau, patella, femoral groove) on ovine knees (n = 7). A high-dynamic force-controlled micro creep and creep-recovery indentation test system applied five loading (0.11 MPa) and unloading (5.6 kPa) cycles for 60 s each and recorded creep-recovery. Needle indentation was used to measure cartilage thickness and calculate total strain.

FINDINGS

Steady state behaviour was observed from the third cycle and further evaluated. Little variation of stiffness in N/mm was found within the patella (4.3SD0.5) and femoral groove (8.1SD0.7) compared to larger variations in the femur (7.9SD2.0) and tibia (7.5SD3.2). Creep indentation showed values of 14.5%(SD2.7%) for the patella and 17.4%(SD3%) for the femoral grove opposed to 13.4%(SD4.3%) for the femoral condyles and 21.8%(SD6.6%) for the tibia plateau. Similar trends were observed analysing creep-recovery. Values were normalized to cartilage thickness which ranged between 0.36 mm and 1.14 mm.

INTERPRETATION

Our setup allows a reliable evaluation of zonal differences. Homogenous biomechanical behaviour is found within the patella and femoral groove whereas significant biomechanical variation within the femoral condyles and tibia plateau indicates the need for site-specific cartilage repair products.

Optimization strategies for ACI: A step-chronicle review

Bearing compression from adjacent joints, the articular cartilage is cumulatively pressured in daily life, thus making it prone to injuries; however, once damaged, the self-healing capacity of articular cartilage is limited owing to its low metabolic property. Autologous chondrocyte implantation, a three-step repairing technique for articular lesions, has received satisfactory short-term clinical outcomes, whereas its long-term effect remains controversial. Currently, improved stem-cell therapies and novel biomaterials have shed new lights on autologous chondrocyte implantation. We would, therefore, synthesize these optimization strategies in order of their presences in the three-step protocol, seeking to find and amplify synergic effects between these strategies.

THE TRANSLATIONAL POTENTIAL OF THIS ARTICLE

Autologous chondrocytes implantation serves as an alternative for the treatment of articular cartilage lesions to avoid potentially detrimental effects of applying microfracture. The optimized ACI should improve the cost-effectiveness of repairing articular cartilage while circumventing latent complications like osteophyte. This article synthesized optimization strategies for ACI and provided appropriate applying approaches to maximize their synergic effects. It will be a pioneering trial for combinedly using stem cells and nanotechnology to regenerate cartilage.

Novel Biomedical Applications of Crosslinked Collagen

Collagen is one of the most useful biopolymers because of its low immunogenicity and biocompatibility. The biomedical potential of natural collagen is limited by its poor mechanical strength, thermal stability, and enzyme resistance, but exogenous chemical, physical, or biological crosslinks have been used to modify the molecular structure of collagen to minimize degradation and enhance mechanical stability. Although crosslinked collagen-based materials have been widely used in biomedicine, there is no standard crosslinking protocol that can achieve a perfect balance between stability and functional remodeling of collagen. Understanding the role of crosslinking agents in the modification of collagen performance and their potential biomedical applications are crucial for developing novel collagen-based biopolymers for therapeutic gain.

Cell-Free Strategies for Repair and Regeneration of Meniscus Injuries through the Recruitment of Endogenous Stem/Progenitor Cells

The meniscus plays a vital role in protecting the articular cartilage of the knee joint. The inner two-thirds of the meniscus are avascular, and injuries to this region often fail to heal without intervention. The use of tissue engineering and regenerative medicine techniques may offer novel and effective approaches to repairing meniscal injuries. Meniscal tissue engineering and regenerative medicine typically use one of two techniques, cell-based or cell-free. While numerous cell-based strategies have been applied to repair and regenerate meniscal defects, these techniques possess certain limitations including cellular contamination and an increased risk of disease transmission. Cell-free strategies attempt to repair and regenerate the injured tissues by recruiting endogenous stem/progenitor cells. Cell-free strategies avoid several of the disadvantages of cell-based techniques and, therefore, may have a wider clinical application. This review first compares cell-based to cell-free techniques. Next, it summarizes potential sources for endogenous stem/progenitor cells. Finally, it discusses important recruitment factors for meniscal repair and regeneration. In conclusion, cell-free techniques, which focus on the recruitment of endogenous stem and progenitor cells, are growing in efficacy and may play a critical role in the future of meniscal repair and regeneration.

A Guide for Using Mechanical Stimulation to Enhance Tissue-Engineered Articular Cartilage Properties

The use of tissue-engineered articular cartilage (TEAC) constructs has the potential to become a powerful treatment option for cartilage lesions resulting from trauma or early stages of pathology. Although fundamental tissue-engineering strategies based on the use of scaffolds, cells, and signals have been developed, techniques that lead to biomimetic AC constructs that can be translated to in vivo use are yet to be fully confirmed. Mechanical stimulation during tissue culture can be an effective strategy to enhance the mechanical, structural, and cellular properties of tissue-engineered constructs toward mimicking those of native AC. This review focuses on the use of mechanical stimulation to attain and enhance the properties of AC constructs needed to translate these implants to the clinic. In vivo, mechanical loading at maximal and supramaximal physiological levels has been shown to be detrimental to AC through the development of degenerative changes. In contrast, multiple studies have revealed that during culture, mechanical stimulation within narrow ranges of magnitude and duration can produce anisotropic, mechanically robust AC constructs with high cellular viability. Significant progress has been made in evaluating a variety of mechanical stimulation techniques on TEAC, either alone or in combination with other stimuli. These advancements include determining and optimizing efficacious loading parameters (e.g., duration and frequency) to yield improvements in construct design criteria, such as collagen II content, compressive stiffness, cell viability, and fiber organization. With the advancement of mechanical stimulation as a potent strategy in AC tissue engineering, a compendium detailing the results achievable by various stimulus regimens would be of great use for researchers in academia and industry. The objective is to list the qualitative and quantitative effects that can be attained when direct compression, hydrostatic pressure, shear, and tensile loading are used to tissue-engineer AC. Our goal is to provide a practical guide to their use and optimization of loading parameters. For each loading condition, we will also present and discuss benefits and limitations of bioreactor configurations that have been used. The intent is for this review to serve as a reference for including mechanical stimulation strategies as part of AC construct culture regimens.

Optimization of Extracellular Matrix Synthesis and Accumulation by Human Articular Chondrocytes in 3-Dimensional Construct with Repetitive Hydrostatic Pressure

Objective The effects of hydrostatic pressure (HP) on the matrix synthesis by human articular chondrocytes have been reported elsewhere. In order to optimize the production of extracellular matrix, we aimed to clarify the effects of repetitive HP on metabolic function by human articular chondrocytes. Design The human articular chondrocytes were expanded and embedded within a collagen gel/sponge scaffold. We incubated these constructs with and without HP followed by atmospheric pressure (AP) and repeated the second HP followed by AP over 14 days. Genomic, biochemical, and histological evaluation were performed to compare the effects of each regimen on the constructs. Results The gene expressions of collagen type II and aggrecan core protein were significantly upregulated with repetitive HP regimens compared with a single HP or AP by 14 days ( P < 0.01 or 0.05). Matrix metalloptoteinase-13 (MMP-13) in AP was upregulated significantly compared to other HP regimens at day 14 ( P < 0.01). No significant difference was observed in tissue inhibitor of metalloproteinases-II. Immunohistology demonstrated that application of HP (both repetitive and single) promoted the accumulation of specific extracellular matrix and reduced a MMP-13. A single regimen of HP followed by AP significantly increased the amount of sulfated glycosaminoglycan than that of the AP, whereas repetitive HP remained similar level of that of the AP. Conclusions Repetitive HP had a greater effect on anabolic activity by chondrocytes than a single HP regimen, which will be advantageous for producing a matrix-rich cell construct.

Ammonium-Chloride-Potassium Lysing Buffer Treatment of Fully Differentiated Cells Increases Cell Purity and Resulting Neotissue Functional Properties

Juvenile and fetal, primary, fully differentiated cells are widely considered to be ideal cell types for tissue engineering applications. However, their use in tissue engineering may be hindered through contamination by undesirable cell types. These include blood-associated cells as well as unwanted resident cell types found both in healthy and pathologic donor tissues. Ammonium-chloride-potassium (ACK) lysing buffer is used to lyse red blood cells (RBCs) during the isolation of stem cell populations, but has not been explored for the purification of fully differentiated cells. This study sought to investigate the effect of ACK buffer treatment of freshly isolated, fully differentiated cells to increase cell purity and enhance the formation of biofunctional engineered neotissues; this was tested in the well-established cartilage tissue engineering model of the self-assembling process using fetal ovine articular chondrocytes (foACs) and juvenile bovine articular chondrocytes (jbACs). ACK buffer treatment of foACs and jbACs decreased the number of contaminating RBCs by over 60% and additionally reduced the number of apoptotic chondrocytes in the cell isolates. Reducing the number of contaminating RBCs removed cellular detractors to the self-assembling process and eliminated an apoptotic stimulus, thus improving neocartilage homogeneity, chondrocyte distribution, and extracellular matrix deposition within the neotissues. For example, in foAC neocartilage, ACK buffer treatment ultimately led to a 170% increase in compressive aggregate modulus, a 130% increase in shear modulus, an 80% increase in tensile modulus, and a 130% increase in ultimate tensile strength of the neocartilage. This work represents the first time that ACK buffer has been used to purify fully differentiated cells and subsequently increase the functional properties of neotissue.

The Self-Assembling Process and Applications in Tissue Engineering

Tissue engineering strives to create neotissues capable of restoring function. Scaffold-free technologies have emerged that can recapitulate native tissue function without the use of an exogenous scaffold. This review will survey, in particular, the self-assembling and self-organization processes as scaffold-free techniques. Characteristics and benefits of each process are described, and key examples of tissues created using these scaffold-free processes are examined to provide guidance for future tissue-engineering developments. We aim to explore the potential of self-assembly and self-organization scaffold-free approaches, detailing the recent progress in the in vitro tissue engineering of biomimetic tissues with these methods toward generating functional tissue replacements.

Tissue engineering potential of human dermis-isolated adult stem cells from multiple anatomical locations

Abundance and accessibility render skin-derived stem cells an attractive cell source for tissue engineering applications. Toward assessing their utility, the variability of constructs engineered from human dermis-isolated adult stem (hDIAS) cells was examined with respect to different anatomical locations (foreskin, breast, and abdominal skin), both in vitro and in a subcutaneous, athymic mouse model. All anatomical locations yielded hDIAS cells with multi-lineage differentiation potentials, though adipogenesis was not seen for foreskin-derived hDIAS cells. Using engineered cartilage as a model, tissue engineered constructs from hDIAS cells were compared. Construct morphology differed by location. The mechanical properties of human foreskin- and abdominal skin-derived constructs were similar at implantation, remaining comparable after 4 additional weeks of culture in vivo. Breast skin-derived constructs were not mechanically testable. For all groups, no signs of abnormality were observed in the host. Addition of aggregate redifferentiation culture prior to construct formation improved chondrogenic differentiation of foreskin-derived hDIAS cells, as evident by increases in glycosaminoglycan and collagen contents. More robust Alcian blue staining and homogeneous cell populations were also observed compared to controls. Human DIAS cells elicited no adverse host responses, reacted positively to chondrogenic regimens, and possessed multi-lineage differentiation potential with the caveat that efficacy may differ by anatomical origin of the skin. Taken together, these results suggest that hDIAS cells hold promise as a potential cell source for a number of tissue engineering applications.

Tension stimulation drives tissue formation in scaffold-free systems

Scaffold-free systems have emerged as viable approaches for engineering load-bearing tissues. However, the tensile properties of engineered tissues have remained far below the values for native tissue. Here, by using self-assembled articular cartilage as a model to examine the effects of intermittent and continuous tension stimulation on tissue formation, we show that the application of tension alone, or in combination with matrix remodelling and synthesis agents, leads to neocartilage with tensile properties approaching those of native tissue. Implantation of tension-stimulated tissues results in neotissues that are morphologically reminiscent of native cartilage. We also show that tension stimulation can be translated to a human cell source to generate anisotropic human neocartilage with enhanced tensile properties. Tension stimulation, which results in nearly sixfold improvements in tensile properties over unstimulated controls, may allow the engineering of mechanically robust biological replacements of native tissue.

A review of in-vitro fibrocartilage tissue engineered therapies with a focus on the temporomandibular joint

The inability of fibrocartilage, specifically the temporomandibular joint (TMJ) disc, to regenerate and remodel following injury presents a unique problem for clinicians. Tissue engineering then offers a potential regenerative therapy. In vitro testing provides a valuable screening tool for potential tissue engineered solutions. The conclusions drawn for TMJ in vitro research were compared against state of the art fibrocartilage studies in the knee meniscus, and annulus fibrosus of the intervertebral disc (IVD). For TMJ disc regeneration, in vitro tissue engineered approaches, focused on cellular therapies with fibrochondrocytes, have displayed an inability to produce enough collagen, as well as an inability to recapitulate native mechanical properties. Biomaterial approaches have recapitulated the native properties of the TMJ disc, but their in vivo efficacy has yet to be determined. By comparison, the knee meniscus field is the most progressive in the use of stem cells as a cell source. The knee meniscus field has moved away from measuring mechanical properties, and are instead more focused on biochemistry and gene expression. IVD studies mainly use electrospun scaffolds, and have produced the best success in mechanical properties. The TMJ field, in comparison to knee meniscus and IVD, needs to employ stem cell therapies, new biomaterials and manufacturing techniques, and cutting edge molecular assays, in future in vitro approaches to screen for viable technologies to move to in vivo studies.

Cell Therapy and Tissue Engineering Approaches for Cartilage Repair and/or Regeneration

Articular cartilage injuries caused by traumatic, mechanical and/or by progressive degeneration result in pain, swelling, subsequent loss of joint function and finally osteoarthritis. Due to the peculiar structure of the tissue (no blood supply), chondrocytes, the unique cellular phenotype in cartilage, receive their nutrition through diffusion from the synovial fluid and this limits their intrinsic capacity for healing. The first cellular avenue explored for cartilage repair involved the in situ transplantation of isolated chondrocytes. Latterly, an improved alternative for the above reparative strategy involved the infusion of mesenchymal stem cells (MSC), which in addition to a self-renewal capacity exhibit a differentiation potential to chondrocytes, as well as a capability to produce a vast array of growth factors, cytokines and extracellular matrix compounds involved in cartilage development. In addition to the above and foremost reparative options up till now in use, other therapeutic options have been developed, comprising the design of biomaterial substrates (scaffolds) capable of sustaining MSC attachment, proliferation and differentiation. The implantation of these engineered platforms, closely to the site of cartilage damage, may well facilitate the initiation of an ‘in situ’ cartilage reparation process. In this mini-review, we examined the timely and conceptual development of several cell-based methods, designed to repair/regenerate a damaged cartilage. In addition to the above described cartilage reparative options, other therapeutic alternatives still in progress are portrayed.