High density type I collagen gels for tissue engineering of whole menisci

Photo-Polymerizable Autologous Growth-Factor Loaded Silk-Based Biomaterial-Inks toward 3D Printing-Based Regeneration of Meniscus Tears

Meniscus tears in the avascular region undergoing partial or full meniscectomy lead to knee osteoarthritis and concurrent lifestyle hindrances in the young and aged alike. Here they reported ingenious photo-polymerizable autologous growth factor loaded 3D printed scaffolds to potentially treat meniscal defects . A shear-thinning photo-crosslinkable silk fibroin methacrylate-gelatin methacrylate-polyethylene glycol dimethacrylate biomaterial-ink is formulated and loaded with freeze-dried growth factor rich plasma (GFRP) . The biomaterial-ink exhibits optimal rheological properties and shape fidelity for 3D printing. Initial evaluation revealed that the 3D printed scaffolds mimic mechanical characteristics of meniscus, possess favourable porosity and swelling characteristics, and demonstrate sustained GFRP release. GFRP laden 3D scaffolds are screened with human neo-natal stem cells in vitro and biomaterial-ink comprising of 25 mg mL-1 of GFRP (GFRP25) is found to be amicable for meniscus tissue engineering. GFRP25 ink demonstrated rigorous rheological compliance, and printed constructs demonstrated long term degradability (>6 weeks), GFRP release (>5 weeks), and mechanical durability (3 weeks). GFRP25 scaffolds aided in proliferation of seeded human neo-natal stem cellsand their meniscus-specific fibrochondrogenic differentiation . GFRP25 constructs show amenable inflammatory response in vitro and in vivo. GFRP25 biomaterial-ink and printed GFRP25 scaffolds could be potential patient-specific treatment modalities for meniscal defects.

Intermittent Cyclic Stretch of Engineered Ligaments Drives Hierarchical Collagen Fiber Maturation in a Dose- and Organizational-Dependent Manner

Hierarchical collagen fibers are the primary source of strength in tendons and ligaments, however these fibers do not regenerate after injury or with repair, resulting in limited treatment options. We previously developed a culture system that guides ACL fibroblasts to produce native-sized fibers and fascicles by 6 weeks. These constructs are promising ligament replacements, but further maturation is needed. Mechanical cues are critical for development in vivo and in engineered tissues; however, the effect on larger fiber and fascicle formation is largely unknown. Our objective was to investigate whether intermittent cyclic stretch, mimicking rapid muscle activity, drives further maturation in our system to create stronger engineered replacements and to explore whether cyclic loading has differential effects on cells at different degrees of collagen organization to better inform engineered tissue maturation protocols. Constructs were loaded with an established intermittent cyclic loading regime at 5 or 10% strain for up to 6 weeks and compared to static controls. Cyclic loading drove cells to increase hierarchical collagen organization, collagen crimp, and tissue mechanics, ultimately producing constructs that matched or exceeded immature ACL properties. Further, the effect of loading on cells varied depending on degree of organization. Specifically, 10% load drove early improvements in mechanics and composition, while 5% load was more beneficial later in culture, suggesting a cellular threshold response and a shift in mechanotransduction. This study provides new insight into how cyclic loading affects cell-driven hierarchical fiber formation and maturation, which will help to develop better rehabilitation protocols and engineer stronger replacements.

Current advances in engineering meniscal tissues: insights into 3D printing, injectable hydrogels and physical stimulation based strategies

The knee meniscus is the cushioning fibro-cartilage tissue present in between the femoral condyles and tibial plateau of the knee joint. It is largely avascular in nature and suffers from a wide range of tears and injuries caused by accidents, trauma, active lifestyle of the populace and old age of individuals. Healing of the meniscus is especially difficult due to its avascularity and hence requires invasive arthroscopic approaches such as surgical resection, suturing or implantation. Though various tissue engineering approaches are proposed for the treatment of meniscus tears, three-dimensional (3D) printing/bioprinting, injectable hydrogels and physical stimulation involving modalities are gaining forefront in the past decade. A plethora of new printing approaches such as direct light photopolymerization and volumetric printing, injectable biomaterials loaded with growth factors and physical stimulation such as low-intensity ultrasound approaches are being added to the treatment portfolio along with the contemporary tear mitigation measures. This review discusses on the necessary design considerations, approaches for 3D modeling and design practices for meniscal tear treatments within the scope of tissue engineering and regeneration. Also, the suitable materials, cell sources, growth factors, fixation and lubrication strategies, mechanical stimulation approaches, 3D printing strategies and injectable hydrogels for meniscal tear management have been elaborated. We have also summarized potential technologies and the potential framework that could be the herald of the future of meniscus tissue engineering and repair approaches.

Enthesis maturation in engineered ligaments is differentially driven by loads that mimic slow growth elongation and rapid cyclic muscle movement

Entheses are complex attachments that translate load between elastic-ligaments and stiff-bone via organizational and compositional gradients. Neither natural healing, repair, nor engineered replacements restore these gradients, contributing to high re-tear rates. Previously, we developed a culture system which guides ligament fibroblasts in high-density collagen gels to develop early postnatal-like entheses, however further maturation is needed. Mechanical cues, including slow growth elongation and cyclic muscle activity, are critical to enthesis development in vivo but these cues have not been widely explored in engineered entheses and their individual contribution to maturation is largely unknown. Our objective here was to investigate how slow stretch, mimicking ACL growth rates, and intermittent cyclic loading, mimicking muscle activity, individually drive enthesis maturation in our system so to shed light on the cues governing enthesis development, while further developing our tissue engineered replacements. Interestingly, we found these loads differentially drive organizational maturation, with slow stretch driving improvements in the interface/enthesis region, and cyclic load improving the ligament region. However, despite differentially affecting organization, both loads produced improvements to interface mechanics and zonal composition. This study provides insight into how mechanical cues differentially affect enthesis development, while producing some of the most organized engineered enthesis to date. STATEMENT OF SIGNIFICANCE: Entheses attach ligaments to bone and are critical to load transfer; however, entheses do not regenerate with repair or replacement, contributing to high re-tear rates. Mechanical cues are critical to enthesis development in vivo but their individual contribution to maturation is largely unknown and they have not been widely explored in engineered replacements. Here, using a novel culture system, we provide new insight into how slow stretch, mimicking ACL growth rates, and intermittent cyclic loading, mimicking muscle activity, differentially affect enthesis maturation in engineered ligament-to-bone tissues, ultimately producing some of the most organized entheses to date. This system is a promising platform to explore cues regulating enthesis formation so to produce functional engineered replacements and better drive regeneration following repair.

Proton Transport Across Collagen Fibrils and Scaffolds: The Role of Hydroxyproline

Collagen is one of the most studied proteins due to its fundamental role in creating fibrillar structures and supporting tissues in our bodies. Accordingly, collagen is also one of the most used proteins for making tissue-engineered scaffolds for various types of tissues. To date, the high abundance of hydroxyproline (Hyp) within collagen is commonly ascribed to the structure and stability of collagen. Here, we hypothesize a new role for the presence of Hyp within collagen, which is to support proton transport (PT) across collagen fibrils. For this purpose, we explore here three different collagen-based hydrogels: the first is prepared by the self-assembly of natural collagen fibrils, and the second and third are based on covalently linking between collagen via either a self-coupling method or with an additional cross-linker. Following the formation of the hydrogel, we introduce here a two-step reaction, involving (1) attaching methanesulfonyl to the -OH group of Hyp, followed by (2) removing the methanesulfonyl, thus reverting Hyp to proline (Pro). We explore the PT efficiency at each step of the reaction using electrical measurements and show that adding the methanesulfonyl group vastly enhances PT, while reverting Hyp to Pro significantly reduces PT efficiency (compared with the initial point) with different efficiencies for the various collagen-based hydrogels. The role of Hyp in supporting the PT can assist in our understanding of the physiological roles of collagen. Furthermore, the capacity to modulate conductivity across collagen is very important to the use of collagen in regenerative medicine.

Harnessing Biofabrication Strategies to Re-Surface Osteochondral Defects: Repair, Enhance, and Regenerate

Osteochondral tissue (OC) is a complex and multiphasic system comprising cartilage and subchondral bone. The discrete OC architecture is layered with specific zones characterized by different compositions, morphology, collagen orientation, and chondrocyte phenotypes. To date, the treatment of osteochondral defects (OCD) remains a major clinical challenge due to the low self-regenerative capacity of damaged skeletal tissue, as well as the critical lack of functional tissue substitutes. Current clinical approaches fail to fully regenerate damaged OC recapitulating the zonal structure while granting long-term stability. Thus, the development of new biomimetic treatment strategies for the functional repair of OCDs is urgently needed. Here, we review recent developments in the preclinical investigation of novel functional approaches for the resurfacing of skeletal defects. The most recent studies on preclinical augmentation of OCDs and highlights on novel studies for the in vivo replacement of diseased cartilage are presented.

Temporal application of lysyl oxidase during hierarchical collagen fiber formation differentially effects tissue mechanics

The primary source of strength in menisci, tendons, and ligaments are hierarchical collagen fibers; however, these fibers are not regenerated after injury nor in engineered replacements, resulting in limited repair options. Collagen strength is reliant on fiber alignment, density, diameter, and crosslinking. Recently, we developed a culture system which guides cells in high-density collagen gels to develop native-like hierarchically organized collagen fibers, which match native fiber alignment and diameters by 6 weeks. However, tensile moduli plateau at 1MPa, suggesting crosslinking may be lacking. Collagen crosslinking is regulated by lysyl oxidase (LOX) which forms immature crosslinks that condense into mature trivalent crosslinks. Trivalent crosslinks are thought to be the primarily source of strength in fibers, but it's not well understood how they form. The objective of this study was to evaluate the effect of exogenous LOX in our culture system at different stages of hierarchical fiber formation to produce stronger replacements and to better understand factors affecting crosslink maturation. We found treatment with LOX isoform LOXL2 did not restrict hierarchical fiber formation, with constructs still forming aligned collagen fibrils by 2 weeks, larger fibers by 4 weeks, and early fascicles by 6 weeks. However, LOXL2 treatment did significantly increase mature pyridinium (PYD) crosslink accumulation and tissue mechanics, with timing of LOXL2 supplementation during fiber formation having a significant effect. Overall, we found one week of LOXL2 supplementation at 4 weeks produced constructs with native-like fiber organization, increased PYD accumulation, and increased mechanics, ultimately matching the tensile modulus of immature bovine menisci. STATEMENT OF SIGNIFICANCE: Collagen fibers are the primary source of strength and function in connective tissues throughout the body, however it remains a challenge to develop these fibers in engineered replacements, greatly reducing treatment options. Here we demonstrate lysyl oxidase like 2 (LOXL2) can be used to significantly improve the mechanics of tissue engineered constructs, but timing of application is important and will most likely depend on degree of collagen organization or maturation. Currently there is limited understanding of how collagen crosslinking is regulated, and this system is a promising platform to further investigate cellular regulation of LOX crosslinking. Understanding the mechanism that regulates LOX production and activity is needed to ultimately regenerate functional repair or replacements for connective tissues throughout the body.

Controlling collagen gelation pH to enhance biochemical, structural, and biomechanical properties of tissue-engineered menisci

Collagen-based hydrogels have been widely used in biomedical applications due to their biocompatibility. Enhancing mechanical properties of collagen gels remains challenging while maintaining biocompatibility. Here, we demonstrate that gelation pH has profound effects on cellular activity, collagen fibril structure, and mechanical properties of the fibrochondrocyte-seeded collagen gels in both short- and long-terms. Acidic and basic gelation pH, below pH 7.0 and above 8.5, resulted in dramatic cell death. Gelation pH ranging from 7.0 to 8.5 showed a relatively high cell viability. Furthermore, physiologic gelation (pH 7.5) showed the greatest collagen deposition while glycosaminoglycan deposition appeared independent of gelation pH. Scanning electron microscopy showed that neutral and physiologic gelation pH, 7.0 and 7.5, exhibited well-aligned collagen fibril structure on day 0 and enhanced collagen fibril structure with laterally joined fibrils on day 30. However, basic pH, 8.0 and 8.5, displayed a densely packed collagen fibril structure on day 0, which was also persistent on day 30. Initial equilibrium modulus increased with increasing gelation pH. Notably, after 30 days of culture, gelation pH of 7.5 and 8.0 showed the highest equilibrium modulus, reaching 150 -160 kPa. While controlling gelation pH is simply achieved compared with other strategies to improve mechanical properties, its influences on biochemical and biomechanical properties of the collagen gel are long-lasting. As such, gelation pH is a useful means to modulate both biochemical and biomechanical properties of the collagen-based hydrogels and can be utilized for diverse types of tissue engineering due to its simple application.

A comparative study on various cell sources for constructing tissue-engineered meniscus

Injury to the meniscus is a common occurrence in the knee joint and its management remains a significant challenge in the clinic. Appropriate cell source is essential to cell-based tissue regeneration and cell therapy. Herein, three commonly used cell sources, namely, bone marrow mesenchymal stem cell (BMSC), adipose-derived stem cell (ADSC), and articular chondrocyte, were comparatively evaluated to determine their potential for engineered meniscus tissue in the absence of growth factor stimulus. Cells were seeded on electrospun nanofiber yarn scaffolds that share similar aligned fibrous configurations with native meniscus tissue for constructing meniscus tissue in vitro. Our results show that cells proliferated robustly along nanofiber yarns to form organized cell-scaffold constructs, which recapitulate the typical circumferential fiber bundles of native meniscus. Chondrocytes exhibited different proliferative characteristics and formed engineered tissues with distinct biochemical and biomechanical properties compared to BMSC and ADSC. Chondrocytes maintained good chondrogenesis gene expression profiles and produced significantly increased chondrogenic matrix and form mature cartilage-like tissue as revealed by typical cartilage lacunae. In contrast, stem cells underwent predominately fibroblastic differentiation and generated greater collagen, which contributes to improved tensile strengths of cell-scaffold constructs in comparison to the chondrocyte. ADSC showed greater proliferative activity and increased collagen production than BMSC. These findings indicate that chondrocytes are superior to stem cells for constructing chondrogenic tissues while the latter is feasible to form fibroblastic tissue. Combination of chondrocytes and stem cells might be a possible solution to construct fibrocartilage tissue and meniscus repair and regeneration.

Removal of GAGs Regulates Mechanical Properties, Collagen Fiber Formation, and Alignment in Tissue Engineered Meniscus

The complex fibrillar architecture of native meniscus is essential for proper function and difficult to recapitulate in vitro. In the native meniscus, proteoglycan content is low during the development of collagen fibers and progressively increases with aging. In vitro, fibrochondrocytes produce glycosaminoglycans (GAGs) early in culture, in contrast to native tissue, where they are deposited after collagen fibers have formed. This difference in the timing of GAG production hinders the formation of a mature fiber network in such in vitro models. In this study, we removed GAGs from collagen gel-based tissue engineered constructs using chondroitinase ABC (cABC) and evaluated the effect on the formation and alignment of collagen fibers and the subsequent effect on tensile and compressive mechanical properties. Removal of GAGs during maturation of in vitro constructs improved collagen fiber alignment in tissue engineered meniscus constructs. Additionally, removal of GAGs during maturation improved fiber alignment without compromising compressive strength, and this removal improved not only fiber alignment and formation but also tensile properties. The increased fiber organization in cABC-treated groups also appeared to influence the size, shape, and location of defects in these constructs, suggesting that treatment may prevent the propagation of large defects under loading. This data gives another method of modulating the ECM for improved collagen fiber formation and mechanical properties in tissue engineered constructs.

An inducible model for unraveling the effects of advanced glycation end-product accumulation in aging connective tissues

PURPOSE

In connective tissues there is a clear link between increasing age and degeneration. Advanced glycation end-products (AGEs) are believed to play a central role. AGEs are sugar-induced non-enzymatic crosslinks which accumulate in collagen with age and diabetes, altering tissue mechanics and cellular function. Despite ample correlative evidence linking collagen glycation to tissue degeneration, little is known how AGEs impact cell-matrix interactions, limiting therapeutic options. One reason for this limited understanding is that AGEs are typically induced using high concentrations of ribose which decrease cell viability, making it impossible to investigate cell-matrix interactions. The objective of this study was to develop a system to trigger AGE accumulation while maintaining cell viability.

MATERIALS AND METHODS

Using cell-seeded high density collagen gels, we investigated the effect of two systems for AGE induction, ribose at low concentrations (30, 100, and 200 mM) over 15 days of culture and riboflavin (0.25 and 0.75 mM) induced with blue light for 40 seconds (riboflavin-465 nm).

RESULTS

We found ribose and riboflavin-465 nm treatment produces fluorescent AGE quantities which match and/or exceed human fluorescent AGE levels for various tissues, ages, and diseases, without affecting cell viability or metabolism. Interestingly, a 40 second treatment of riboflavin-465 nm produced similar levels of fluorescent AGEs as 3 days of 100 mM ribose treatment.

CONCLUSIONS

Riboflavin-465 nm is a promising method to trigger AGEs on demand in vivo or in vitro without impacting cell viability and offers potential for unraveling the mechanism of AGEs in age and diabetes related tissue damage.

Driving native-like zonal enthesis formation in engineered ligaments using mechanical boundary conditions and β-tricalcium phosphate

Fibrocartilaginous entheses are structurally complex tissues that translate load from elastic ligaments to stiff bone via complex zonal gradients in the organization, mineralization, and cell phenotype. Currently, these complex gradients necessary for long-term mechanical function are not recreated in soft tissue-to-bone healing or engineered replacements, contributing to high failure rates. Previously, we developed a culture system that guides ligament fibroblasts to develop aligned native-sized collagen fibers using high-density collagen gels and mechanical boundary conditions. These constructs are promising ligament replacements, however functional ligament-to-bone attachments, or entheses, are required for long-term function in vivo. The objective of this study was to investigate the effect of compressive mechanical boundary conditions and the addition of beta-tricalcium phosphate (βTCP), a known osteoconductive agent, on the development of zonal ligament-to-bone entheses. We found that compressive boundary clamps, that restrict cellular contraction and produce a zonal tensile-compressive environment, guide ligament fibroblasts to produce 3 unique zones of collagen organization and zonal accumulation of glycosaminoglycans (GAGs), type II, and type X collagen. Ultimately, by 6 weeks of culture these constructs had similar organization and composition as immature bovine entheses. Further, βTCP applied under the clamp enhanced maturation of these entheses, leading to significantly increased tensile moduli, and zonal GAG accumulation, ALP activity, and calcium-phosphate accumulation, suggesting the initiation of endochondral ossification. This culture system produced some of the most organized entheses to date, closely mirroring early postnatal enthesis development, and provides an in vitro platform to better understand the cues that drive enthesis maturation in vivo. STATEMENT OF SIGNIFICANCE: Ligaments are attached to bone via entheses. Entheses are complex tissues with gradients in organization, composition, and cell phenotype. Entheses are necessary for proper transfer of load from ligament-to-bone, but currently are not restored with healing or replacements. Here, we provide new insight into how tensile-compressive boundary conditions and βTCP drive zonal gradients in collagen organization, mineralization, and matrix composition, producing tissues similar to immature ligament-to-bone attachments. Collectively, this culture system uses a bottom-up approach with mechanical and biochemical cues to produce engineered replacements which closely mirror postnatal enthesis development. This culture system is a promising platform to better understanding the cues that regulate enthesis formation so to better drive enthesis regeneration following graft repair and in engineered replacements.

Regional-specific meniscal extracellular matrix hydrogels and their effects on cell-matrix interactions of fibrochondrocytes

Decellularized meniscal extracellular matrix (ECM) material holds great potential for meniscus repair and regeneration. Particularly, injectable ECM hydrogel is highly desirable for the minimally invasive treatment of irregularly shaped defects. Although regional-specific variations of the meniscus are well documented, no ECM hydrogel has been reported to simulate zonally specific microenvironments of the native meniscus. To fill the gap, different (outer, middle, and inner) zones of porcine menisci were separately decellularized. Then the regionally decellularized meniscal ECMs were solubilized by pepsin digestion, neutralized, and then form injectable hydrogels. The hydrogels were characterized in gelation behaviors and mechanical properties and seeded with bovine fibrochondrocytes to evaluate the regionally biochemical effects on the cell-matrix interactions. Our results showed that the decellularized inner meniscal ECM (IM) contained the greatest glycosaminoglycan (GAG) content and the least collagen content compared with the decellularized outer meniscal ECM (OM) and middle meniscal ECM (MM). The IM hydrogel showed lower compressive strength than the OM hydrogel. When encapsulated with fibrochondrocytes, the IM hydrogel accumulated more GAG, contracted to a greater extent and reached higher compressive strength than that of the OM hydrogel at 28 days. Our findings demonstrate that the regionally specific meniscal ECMs present biochemical variation and show various effects on the cell behaviors, thus providing information on how meniscal ECM hydrogels may be utilized to reconstruct the microenvironments of the native meniscus.

3D bioprinting of a trachea-mimetic cellular construct of a clinically relevant size

Despite notable advances in extrusion-based 3D bioprinting, it remains a challenge to create a clinically-sized cellular construct using extrusion-based 3D printing due to long printing times adversely affecting cell viability and functionality. Here, we present an advanced extrusion-based 3D bioprinting strategy composed of a two-step printing process to facilitate creation of a trachea-mimetic cellular construct of clinically relevant size. A porous bellows framework is first printed using typical extrusion-based 3D printing. Selective printing of cellular components, such as cartilage rings and epithelium lining, is then performed on the outer grooves and inner surface of the bellows framework by a rotational printing process. With this strategy, 3D bioprinting of a trachea-mimetic cellular construct of clinically relevant size is achieved in significantly less total printing time compared to a typical extrusion-based 3D bioprinting strategy which requires printing of an additional sacrificial material. Tracheal cartilage formation was successfully demonstrated in a nude mouse model through a subcutaneous implantation study of trachea-mimetic cellular constructs wrapped with a sinusoidal-patterned tubular mesh preventing rapid resorption of cartilage rings in vivo. This two-step 3D bioprinting for a trachea-mimetic cellular construct of clinically relevant size can provide a fundamental step towards clinical translation of 3D bioprinting based tracheal reconstruction.

Bioprinting of Cartilage with Bioink Based on High-Concentration Collagen and Chondrocytes

The study was aimed at the applicability of a bioink based on 4% collagen and chondrocytes for de novo cartilage formation. Extrusion-based bioprinting was used for the biofabrication. The printing parameters were tuned to obtain stable material flow. In vivo data proved the ability of the tested bioink to form a cartilage within five to six weeks after the subcutaneous scaffold implantation. Certain areas of cartilage formation were detected as early as in one week. The resulting cartilage tissue had a distinctive structure with groups of isogenic cells as well as a high content of glycosaminoglycans and type II collagen.

A review of strategies for development of tissue engineered meniscal implants

The meniscus is a key stabilizing tissue of the knee that facilitates proper tracking and movement of the knee joint and absorbs stresses related to physical activity. This review article describes the biology, structure, and functions of the human knee meniscus, common tears and repair approaches, and current research and development approaches using modern methods to fabricate a scaffold or tissue engineered meniscal replacement. Meniscal tears are quite common, often resulting from sports or physical training, though injury can result without specific contact during normal physical activity such as bending or squatting. Meniscal injuries often require surgical intervention to repair, restore basic functionality and relieve pain, and severe damage may warrant reconstruction using allograft transplants or commercial implant devices. Ongoing research is attempting to develop alternative scaffold and tissue engineered devices using modern fabrication techniques including three-dimensional (3D) printing which can fabricate a patient-specific meniscus replacement. An ideal meniscal substitute should have mechanical properties that are close to that of natural human meniscus, and also be easily adapted for surgical procedures and fixation. A better understanding of the organization and structure of the meniscus as well as its potential points of failure will lead to improved design approaches to generate a suitable and functional replacement.

Combining TGF-β1 and Mechanical Anchoring to Enhance Collagen Fiber Formation and Alignment in Tissue-Engineered Menisci

Recapitulating the collagen fiber structure of native menisci is one of the major challenges in the development of tissue-engineered menisci. Native collagen fibers are developed by the complex interplay of biochemical and biomechanical signals. In this study, we optimized glucose and transforming growth factor-β1 (TGF-β1) concentrations in combination with mechanical anchoring to balance contributions of proteoglycan synthesis and contractile behavior in collagen fiber assembly. Glucose had a profound effect on the final dimensions of collagen-based constructs. TGF-β1 influenced construct contraction rate and glycosaminoglycan (GAG) production with two half-maximal effective concentration (EC₅₀) ranges, which are 0.23 to 0.28 and 0.53 to 1.71 ng/mL, respectively. At concentrations less than the EC₅₀, for the GAG production and contraction rate, TGF-β1 treatment resulted in less organized collagen fibers. At concentrations greater than the EC₅₀, TGF-β1 led to dense, disorganized collagen fibers. Between the two EC₅₀ values, collagen fiber diameter and length increased. The effects of TGF-β1 on fiber development were enhanced by mechanical anchoring, leading to peaks in fiber diameter, length, and alignment index. Fiber diameter and length increased from 7.9 ± 1.4 and 148.7 ± 16.4 to 17.5 ± 2.1 and 262.0 ± 13.0 μm, respectively. The alignment index reached 1.31, comparable to that of native tissue, 1.40. These enhancements in fiber architecture resulted in significant increases in tensile modulus and ultimate tensile stress (UTS) by 1.6- and 1.4-fold. Correlation analysis showed that tensile modulus and UTS strongly correlated with collagen fiber length, diameter, and alignment, while compressive modulus correlated with GAG content. These outcomes highlight the need for optimization of both biochemical and biomechanical cues in the culture environment for enhancing fiber development within tissue-engineered constructs.

Driving Hierarchical Collagen Fiber Formation for Functional Tendon, Ligament, and Meniscus Replacement

Hierarchical collagen fibers are the primary source of strength in musculoskeletal tendons, ligaments, and menisci. It has remained a challenge to develop these large fibers in engineered replacements or in vivo after injury. The objective of this study was to investigate the ability of restrained cell-seeded high density collagen gels to drive hierarchical fiber formation for multiple musculoskeletal tissues. We found boundary conditions applied to high density collagen gels were capable of driving tenocytes, ligament fibroblasts, and meniscal fibrochondrocytes to develop native-sized hierarchical collagen fibers 20-40 μm in diameter. The fibers organize similar to bovine juvenile collagen with native fibril banding patterns and hierarchical fiber bundles 50-350 μm in diameter by 6 weeks. Mirroring fiber organization, tensile properties of restrained samples improved significantly with time, reaching ~1 MPa. Additionally, tendon, ligament, and meniscal cells produced significantly different sized fibers, different degrees of crimp, and different GAG concentrations, which corresponded with respective juvenile tissue. To our knowledge, these are some of the largest, most organized fibers produced to date in vitro. Further, cells produced tissue specific hierarchical fibers, suggesting this system is a promising tool to better understand cellular regulation of fiber formation to better stimulate it in vivo after injury.

Bone Marrow Mesenchymal Stem Cell-Derived Tissues are Mechanically Superior to Meniscus Cells

Bone marrow-derived mesenchymal stem cells (BMSCs) have the potential to form the mechanically responsive matrices of joint tissues, including the menisci of the knee joint. The purpose of this study is to assess BMSC's potential to engineer meniscus-like tissue relative to meniscus fibrochondrocytes (MFCs). MFCs were isolated from castoffs of partial meniscectomy from nonosteoarthritic knees. BMSCs were developed from bone marrow aspirates of the iliac crest. All cells were of human origin. Cells were cultured in type I collagen scaffolds under normoxia (21% O₂) for 2 weeks followed by hypoxia (3% O₂) for 3 weeks. The structural and functional assessment of the generated meniscus constructs were based on glycosaminoglycan (GAG) content, histological appearance, gene expression, and mechanical properties. The tissues formed by both cell types were histologically positive for Safranin O stain and appeared more intense in the BMSC constructs. This observation was confirmed by a 2.7-fold higher GAG content. However, there was no significant difference in collagen I (COL1A2) expression in BMSC- and MFC-based constructs (p = 0.17). The expression of collagen II (COL2A1) and aggrecan (ACAN) were significantly higher in BMSCs than MFC (p ≤ 0.05). Also, the gene expression of the hypertrophic marker collagen X (COL10A1) was 199-fold higher in BMSCs than MFC (p < 0.001). Moreover, relaxation moduli were significantly higher in BMSC-based constructs at 10-20% strain step than MFC-based constructs. BMSC-based constructs expressed higher COL2A1, ACAN, COL10A1, contained higher GAG content, and exhibited higher relaxation moduli at 10-20% strain than MFC-based construct. Impact statement Cell-based tissue engineering (TE) has the potential to produce functional tissue replacements for irreparably damaged knee meniscus. But the source of cells for the fabrication of the tissue replacements is currently unknown and of research interest in orthopedic TE. In this study, we fabricated tissue-engineered constructs using type I collagen scaffolds and two candidate cell sources in meniscus TE. We compared the mechanical properties of the tissues formed from human meniscus fibrochondrocytes and bone marrow-derived mesenchymal stem cells (BMSCs). Our data show that the tissues engineered from the BMSC are mechanically superior in relaxation modulus.

A Parameter Study for 3D-Printing Organized Nanofibrous Collagen Scaffolds Using Direct-Write Electrospinning

Collagen-based scaffolds are gaining more prominence in the field of tissue engineering. However, readily available collagen scaffolds either lack the rigid structure (hydrogels) and/or the organization (biopapers) seen in many organ tissues, such as the cornea and meniscus. Direct-write electrospinning is a promising potential additive manufacturing technique for constructing highly ordered fibrous scaffolds for tissue engineering and foundational studies in cellular behavior, but requires specific process parameters (voltage, relative humidity, solvent) in order to produce organized structures depending on the polymer chosen. To date, no work has been done to optimize direct-write electrospinning parameters for use with pure collagen. In this work, a custom electrospinning 3D printer was constructed to derive optimal direct write electrospinning parameters (voltage, relative humidity and acetic acid concentrations) for pure collagen. A LabVIEW program was built to automate control of the print stage. Relative humidity and electrospinning current were monitored in real-time to determine the impact on fiber morphology. Fiber orientation was analyzed via a newly defined parameter (spin quality ratio (SQR)). Finally, tensile tests were performed on electrospun fibrous mats as a proof of concept.

Regulation of proteoglycan production by varying glucose concentrations controls fiber formation in tissue engineered menisci

Fibrillar collagens are highly prevalent in the extracellular matrix of all connective tissues and therefore commonly used as a biomaterial in tissue engineering applications. In the native environment, collagen fibers are arranged in a complex hierarchical structure that is often difficult to recreate in a tissue engineered construct. Small leucine rich proteoglycans as well as hyaluronan binding proteoglycans, aggrecan and versican, have been implicated in regulating fiber formation. In this study, we modified proteoglycan production in vitro by altering culture medium glucose concentrations (4500, 1000, 500, 250, and 125 mg/L), and evaluated its effect on the formation of collagen fibers inside tissue engineered meniscal constructs. Reduction of extracellular glucose resulted in a dose dependent decrease in total sulfated glycosaminoglycan (GAG) production, but minimal decreases of decorin and biglycan. However, fibromodulin doubled in production between 125 and 4500 mg/L glucose concentration. A peak in fiber formation was observed at 500 mg/L glucose concentration and corresponded with reductions in total GAG production. Fiber formation reduction at 125 and 250 mg/L glucose concentrations are likely due to changes in metabolic activity associated with a limited supply of glucose. These results point to proteoglycan production as a means to manipulate fiber architecture in tissue engineered constructs. STATEMENT OF SIGNIFICANCE: Fibrillar collagens are highly prevalent in the extracellular matrix of all connective tissues; however achieving appropriate assembly and organization of collagen fibers in engineered connective tissues is a persistent challenge. Proteoglycans have been implicated in regulating collagen fiber organization both in vivo and in vitro, however little is known about methods to control proteoglycan production and the subsequent fiber organization in tissue engineered menisci. Here, we show that media glucose content can be optimized to control proteoglycan production and collagen fiber assembly, with optimal collagen fiber assembly occurring at sub-physiologic levels of glucose.

High density cell seeding affects the rheology and printability of collagen bioinks

An advantage of bioprinting is the ability to incorporate cells into the hydrogel bioink allowing for precise control over cell placement within a construct. Previous work found that the printability of collagen bioinks is highly dependent on their rheological properties. The effect of cell density on collagen rheological properties and, therefore, printability has not been assessed. Therefore, the objective of this study was to determine the effects of incorporating cells on the rheology and printability of collagen bioinks. Primary chondrocytes, at densities relevant to cartilage tissue engineering (up to 100 × 10⁶ cells ml^-1), were incorporated into 8 mg ml^-1 collagen bioinks. Bioink rheological properties before, during, and after gelation as well as printability were assessed. Cell-laden printed constructs were also cultured for up to 14 d to assess longer-term cell behavior. The addition of cells resulted in an increase in the storage modulus and viscosity of the collagen before gelation. However, the storage modulus after gelation and the rate of gelation decreased with increasing cell density. Theoretical models were compared to the rheological data to suggest frameworks that could be used to predict the rheological properties of cell-laden bioinks. Printability testing showed that improved printability could be achieved with higher cell densities. Fourteen-day culture studies showed that the printing process had no adverse effects on the viability or function of printed cells. Overall, this study shows that collagen bioinks are conducive to bioprinting with a wide range of cell densities while maintaining high printability and chondrocyte viability and function.

Cellular and Chemical Gradients to Engineer the Meniscus-to-Bone Insertion

Tissue-engineered menisci hold promise as an alternative to allograft procedures but require a means of robust fixation to the native bone. The insertion of the meniscus into bone is critical for meniscal function and inclusion of a soft tissue-to-bone interface in a tissue engineered implant can aid in the fixation process. The native insertion is characterized by gradients in composition, tissue architecture, and cellular phenotype, which are all difficult to replicate. In this study, a soft tissue-to-bone interface is tissue engineered with a cellular gradient of fibrochondrocytes and mesenchymal stem cells and subjected to a biochemical gradient through a custom media diffusion bioreactor. These constructs, consisting of interpenetrating collagen and boney regions, display improved mechanical performance and collagen organization compared to controls without a cellular or chemical gradient. Media gradient exposure produces morphological features in the constructs that appear similar to the native tissue. Collectively, these data show that cellular and biochemical gradients improve integration between collagen and bone in a tissue engineered soft tissue-to-bone construct.

Biomechanical characterization of a novel collagen-hyaluronan infused 3D-printed polymeric device for partial meniscus replacement

The menisci transmit load by increasing the contact area and decreasing peak contact stresses on the articular surfaces. Meniscal lesions are among the most common orthopedic injuries, and resulting meniscectomies are associated with adverse polycaprolactone contact mechanics changes and, ultimately, an increased likelihood of osteoarthritis. Meniscus scaffolds were fabricated by 3D-printing a network of circumferential and radial filaments of resorbable polymer (poly(desaminotyrosyl-tyrosine dodecyl ester dodecanoate)) and infused with collagen-hyaluronan. The scaffold demonstrated an instantaneous compressive modulus (1.66 ± 0.44 MPa) comparable to native meniscus (1.52 ± 0.59 MPa). The scaffold aggregate modulus (1.33 ± 0.51 MPa) was within 2% of the native value (1.31 ± 0.36 MPa). In tension, the scaffold displayed a comparable stiffness to native tissue (127.6-97.1 N/mm) and an ultimate load of 33% of the native value. Suture pull-out load of scaffolds (83.1 ± 10.0 N) was within 10% of native values (91.5 ± 15.4 N). Contact stress analysis demonstrated the scaffold reduced peak contact stress by 60-67% and increased contact area by 38%, relative to partial meniscectomy. This is the first meniscal scaffold to match both the axial compressive properties and the circumferential tensile stiffness of the native meniscus. The improvement of joint contact mechanics, relative to partial meniscectomy alone, motivates further investigation using a large animal model. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B:2457-2465, 2019.

In situ formation of hydrophobic clusters to enhance mechanical performance of biodegradable poly(l-glutamic acid)/poly(ε-caprolactone) hydrogel towards meniscus tissue engineering

The preparation of scaffolds represents a huge challenge for meniscus tissue engineering, as there are particular biomechanical and biodegradability requirements. Here, an amphiphilic polymer was prepared by grafting hydrophobic poly(ε-caprolactone) (PCL) with short chains onto hydrophilic biodegradable poly(l-glutamic acid) (PLGA). PLGA-g-PCL was then crosslinked with triethylene glycol (TEG) to form a chemically crosslinked network (CCN), followed by the in situ formation of stable hydrophobic PCL clusters inside the preformed CCN to increase the strength and elasticity of the hydrogel. The spindle-like PCL clusters in the hydrogel were observed to possess a mean size of 5-10 μm on the long axis and 3-4 μm on the minor axis, with no crystallization according to WAXS. Owing to the hydrophobic association of PCL, the swollen CCN was secondarily crosslinked and stiffened. Dynamic mechanical analysis (DMA) demonstrated excellent rebound resilience, and the maximum compressive strength of all the PLGA-g-PCL hydrogels was greater than 0.60 MPa with a maximum strain of about 70%, which represented significant increases in comparison with a PLGA hydrogel. Owing to the short chains of PCL, degradation of the PLGA-g-PCL hydrogel was accelerated within 3 months in vivo. A hydrogel carrying adipose-derived stem cells (ASCs) effectively regenerated meniscus-like tissue in vivo and preserved the corresponding articular cartilage from degeneration over a 16 week period.

Tissue engineering the human auricle by auricular chondrocyte-mesenchymal stem cell co-implantation

Children suffering from microtia have few options for auricular reconstruction. Tissue engineering approaches attempt to replicate the complex anatomy and structure of the ear with autologous cartilage but have been limited by access to clinically accessible cell sources. Here we present a full-scale, patient-based human ear generated by implantation of human auricular chondrocytes and human mesenchymal stem cells in a 1:1 ratio. Additional disc construct surrogates were generated with 1:0, 1:1, and 0:1 combinations of auricular chondrocytes and mesenchymal stem cells. After 3 months in vivo, monocellular auricular chondrocyte discs and 1:1 disc and ear constructs displayed bundled collagen fibers in a perichondrial layer, rich proteoglycan deposition, and elastin fiber network formation similar to native human auricular cartilage, with the protein composition and mechanical stiffness of native tissue. Full ear constructs with a 1:1 cell combination maintained gross ear structure and developed a cartilaginous appearance following implantation. These studies demonstrate the successful engineering of a patient-specific human auricle using exclusively human cell sources without extensive in vitro tissue culture prior to implantation, a critical step towards the clinical application of tissue engineering for auricular reconstruction.

In vivo annular repair using high-density collagen gel seeded with annulus fibrosus cells

OBJECTIVE

The aim is assessing the in vivo efficacy of annulus fibrosus (AF) cells seeded into collagen by enhancing the reparative process around annular defects and preventing further degeneration in a rat-tail model.

SUMMARY OF BACKGROUND DATA

Treating disc herniation with discectomy may relieve the related symptoms but does not address the underlying pathology. The persistent annular defect may lead to re-herniation and further degeneration. We recently demonstrated that riboflavin crosslinked high-density collagen gels (HDC) can facilitate annular repair in vivo.

METHODS

42 rats, tail disc punctured with an 18-gauge needle, were divided into 3 groups: untreated (n = 6), injected with crosslinked HDC (n = 18), and injected with AF cell-laden crosslinked HDC (n = 18). Ovine AF cells were mixed with HDC gels prior to injection. X-rays and MRIs were conducted over 5 weeks, determining disc height index (DHI), nucleus pulposus (NP) size, and hydration. Histological assessments evaluated the viability of implanted cells and degree of annular repair.

RESULTS

Although average DHIs of both HDC gel groups were higher than those of the puncture control group at 5 weeks, the retention of disc height, NP size and hydration at 1 and 5 weeks was significant for the cellular group compared to the punctured, and at 5 weeks to the acellular group. Histological assessment indicated that AF cell-laden HDC gels have accelerated reparative sealing compared to acellular HDC gels.

CONCLUSIONS

AF cell-laden HDC gels have the ability of better repairing annular defects than acellular gels after needle puncture.

STATEMENT OF SIGNIFICANCE

This project addresses the compelling demand of a sufficient treatment strategy for degenerative disc disease (DDD) perpetuated by annulus fibrosus (AF) injury, a major cause of morbidity and burden to health care systems. Our study is designed to answer the question of whether injectable, photo-crosslinked, high density collagen gels can seal defects in the annulus fibrosus of rats and prevent disc degeneration. Furthermore, we investigated whether the healing of AF defects will be enhanced by the delivery of AF cells (fibrochondrocytes) to these defects. The use of cell-laden collagen gels in spine surgery holds promise for a wide array of applications, from current discectomy procedures to future nucleus pulposus reparative therapies, and our group is excited about this potential.

Resorbable plating system stabilizes tissue-engineered intervertebral discs implanted ex vivo in canine cervical spines

Total disc replacement using tissue-engineered intervertebral discs (TE-IVDs) may offer a biological alternative to treat radiculopathy caused by disc degeneration. A composite TE-IVD was previously developed and evaluated in rat tail and beagle cervical spine models in vivo. Although cell viability and tissue integration into host tissue were promising, significant implant displacement occurred at multiple spinal levels. The goal of the present study was to assess the effects of a resorbable plating system on the stiffness of motion segments and stability of tissue-engineered implants subjected to axial compression. Canine motion segments from levels C2/C3 to C5/C6 were assessed as intact (CTRL), after discectomy (Dx), with an implanted TE-IVD only (PLATE-), and with a TE-IVD combined with an attached resorbable plate (PLATE+). Segments under PLATE+ conditions fully restored separation between endplates and showed significantly higher compressive stiffness than segments under PLATE- conditions. Plated segments partially restored more than 25% of the CTRL motion segment stiffness. Plate attachment also prevented implant extrusion from the disc space at 50% compressive strain, and this effect was more significant in segments from levels C3/C4 when compared to segments from level C5/C6. These results suggest that stabilization of motion segments via resorbable plating assists TE-IVD retention in the disc space while allowing the opportunity for implants to fully integrate into the host tissue and achieve optimal restoration of spine biomechanics.

A low friction, biphasic and boundary lubricating hydrogel for cartilage replacement

Partial joint repair is a surgical procedure where an artificial material is used to replace localised chondral damage. These artificial bearing surfaces must articulate against cartilage, but current materials do not replicate both the biphasic and boundary lubrication mechanisms of cartilage. A research challenge therefore exists to provide a material that mimics both boundary and biphasic lubrication mechanisms of cartilage. In this work a polymeric network of a biomimetic boundary lubricant, poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), was incorporated into an ultra-tough double network (DN) biphasic (water phase + polymer phase) gel, to form a PMPC triple network (PMPC TN) hydrogel with boundary and biphasic lubrication capability. The presence of this third network of MPC was confirmed using ATR-FTIR. The PMPC TN hydrogel had a yield stress of 26 MPa, which is an order of magnitude higher than the peak stresses found in the native human knee. A preliminary pin on plate tribology study was performed where both the DN and PMPC TN hydrogels experienced a reduction in friction with increasing sliding speed which is consistent with biphasic lubrication. In the physiological sliding speed range, the PMPC TN hydrogel halved the friction compared to the DN hydrogel indicating the boundary lubricating PMPC network was working. A biocompatible, tough, strong and chondral lubrication imitating PMPC TN hydrogel was synthesised in this work. By complementing the biphasic and boundary lubrication mechanisms of cartilage, PMPC TN hydrogel could reduce the reported incidence of chondral damage opposite partial joint repair implants, and therefore increase the clinical efficacy of partial joint repair.

STATEMENT OF SIGNIFICANCE

This paper presents the synthesis, characterisation and preliminary tribological testing of a new biomaterial that aims to recreate the primary chondral lubrication mechanisms: boundary and biphasic lubrication. This work has demonstrated that the introduction of an established zwitterionic, biomimetic boundary lubricant can improve the frictional properties of an ultra-tough hydrogel. This new biomaterial, when used as a partial joint replacement bearing material, may help avoid damage to the opposing chondral surface-which has been reported as an issue for other non-biomimetic partial joint replacement materials. Alongside the synthesis of a novel biomaterial focused on complementing the lubrication mechanisms of cartilage, your readership will gain insights into effective mechanical and tribological testing methods and materials characterisation methods for their own biomaterials.

Large Animal Models of Meniscus Repair and Regeneration: A Systematic Review of the State of the Field

Injury to the meniscus is common, but few viable strategies exist for its repair or regeneration. To address this, animal models have been developed to translate new treatment strategies toward the clinic. However, there is not yet a regulatory document guiding such studies. The purpose of this study was to carry out a systematic review of the literature on meniscus treatment methods and outcomes to define the state of the field. Public databases were queried by using search terms related to animal models and meniscus injury and/or repair over the years 1980-2015. Identified peer-reviewed manuscripts were screened by using Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. One of nine reviewers read each manuscript and scored them based on whether the publication described a series of predefined study descriptors and outcome measures. Additional data were extracted to identify common assays used. A total of 128 full-length peer-reviewed manuscripts were identified. The number of publications increased over the time frame analyzed, with 48% focused on augmented repair. Rabbit was, by far, the most prevalent species utilized (46%), with dog (21%) and sheep (20%) being the next most common. Analysis of study descriptors revealed that most studies appropriately documented details of the animal used, the surgical approach, and defect and implant characteristics (e.g., 63% of studies identified clearly the defect size). In terms of outcome parameters, most studies carried out macroscopic (85%), histologic (90%), and healing/integration (83%) analyses of the meniscus. However, many studies did not provide further analysis beyond these fundamental measures, and less than 40% reported on the adjacent cartilage and synovium, as well as joint function. There is intense interest in the field of meniscus repair. However, given the current lack of guidance documentation in this area, preclinical animal models are not performed in a standardized fashion. The development of a "Best Practices" document would increase reproducibility and external validity of experiments, while accelerating advancements in translational research. Advancement is of paramount importance given the high prevalence of meniscal injuries and the paucity of effective repair or regenerative strategies.

A model system for developing a tissue engineered meniscal enthesis

The meniscus acts as a stabilizer, lubricator, and load distributer in the knee joint. The mechanical stability of the meniscus depends on its connection to the underlying bone by a fibrocartilage to bone transition zone called the meniscal enthesis. Tissue engineered menisci hold great promise as a treatment alternative however lack a means of integrated fixation to the underlying bone needed in order for a tissue engineered meniscal replacement to be successful. Tissue engineering the meniscal enthesis is a difficult task given the complex gradients of cell type, mineral, and extracellular matrix molecules. Therefore, there is a need for a simplified and high throughput enthesis model to test experimental parameters. The goal of this study was to develop a simplified enthesis model to test collagen integration with decellularized bone. We found that injection molding collagen into tubing loaded with decellularized bone plugs resulted in a scaffold with three regions: bone, bone-collagen, and collagen. Furthermore, collagen formation was directed in the axial direction by using mechanical fixation at the bony ends. The results of this study showed that this technique can be used to mimic the native enthesis morphology and serves as ideal test platform to generate a model tissue engineered enthesis.

STATEMENT OF SIGNIFICANCE

The meniscal enthesis is a complex structure that is essential to mechanical stability of the meniscus and the knee joint. Several studies document the development of anatomically shaped tissue engineered meniscus constructs, but none have focused on how to integrate such tissues with underlying bone. This study establishes a simplified construct to model the meniscal enthesis composed of a collagen gel seeded with meniscal fibrochondrocytes integrated with decellularized cancellous bone. Mechanical fixation at the bony ends induced tissue integration of fibers into the bony tissue, which is critical for mechanical performance and has yet to be shown in enthesis literature. Our test platform is amenable to targeted experiments investigating mineralization gradients, collagen fiber alignment, cell population phenotype, and media conditioning with experimental impact on enthesis studies for meniscus, tendon, and ligament.

Anatomical region-dependent enhancement of 3-dimensional chondrogenic differentiation of human mesenchymal stem cells by soluble meniscus extracellular matrix

Extracellular matrix (ECM) derived from decellularized tissues has been found to promote tissue neogenesis, most likely mediated by specific biochemical and physical signaling motifs that promote tissue-specific differentiation of progenitor cells. Decellularized ECM has been suggested to be efficacious for the repair of tissue injuries. However, decellularized meniscus contains a dense collagenous structure, which impedes cell seeding and infiltration and is not readily applicable for meniscus repair. In addition, the meniscus consists of two distinct anatomical regions that differ in vascularity and cellular phenotype. The purpose of this study was to explore the region-specific bioactivity of solubilized ECM derived from the inner and outer meniscal regions as determined in 2D and 3D cultures of adult mesenchymal stem cells (MSCs). When added as a medium supplement to 2D cultures of MSCs, urea-extracted fractions of the inner (imECM) and outer meniscal ECM (omECM) enhanced cell proliferation while imECM most strongly upregulated fibrochondrogenic differentiation on the basis of gene expression profiles. When added to 3D cultures of MSCs seeded in photocrosslinked methacrylated gelatin (GelMA) hydrogels, both ECM fractions upregulated chondrogenic differentiation as determined by gene expression and protein analyses, as well as elevated sulfated glycosaminoglycan sGAG content, compared to ECM-free controls. The chondrogenic effect at day 21 was most pronounced with imECM supplementation, but equivalent between ECM groups by day 42. Despite increased cartilage matrix, imECM and omECM constructs possessed compressive moduli similar to controls. In conclusion, soluble meniscal ECM may be considered for use as a tissue-specific reagent to enhance chondrogenesis for MSC-based 3D cartilage tissue engineering.

STATEMENT OF SIGNIFICANCE

The inner region of the knee meniscus is frequently injured and possesses a poor intrinsic healing capacity. Solubilized extracellular matrix (ECM) derived from decellularized meniscus tissue may promote homologous differentiation of progenitor cells, thereby enhancing fibrocartilage formation within a meniscal lesion. However, the meniscus possesses regional variation in ultrastructure, biochemical composition, and cell phenotype, which may affect the bioactivity of soluble ECM derived from different regions of decellularized menisci. In this study, we demonstrate that urea-extracted fractions of ECM derived from the inner and outer regions of menisci enhance chondrogenesis in mesenchymal stem cells seeded in 3-dimensional photocrosslinkable hydrogels and that this effect is more strongly mediated by inner meniscal ECM. These findings suggest region-specific bioactivity of decellularized meniscal ECM.

Fiber development and matrix production in tissue-engineered menisci using bovine mesenchymal stem cells and fibrochondrocytes

Mesenchymal stem cells (MSCs) have been investigated with promising results for meniscus healing and tissue engineering. While MSCs are known to contribute to extracellular matrix (ECM) production, less is known about how MSCs produce and align large organized fibers for application to tissue engineering the meniscus. The goal of this study was to investigate the capability of MSCs to produce and organize ECM molecules compared to meniscal fibrochondrocytes (FCCs). Bovine FCCs and MSCs were encapsulated in an anatomically accurate collagen meniscus using monoculture and co-culture of each cell type. Each meniscus was mechanically anchored at the horns to mimic the physiological fixation by the meniscal entheses. Mechanical fixation generates a static mechanical boundary condition previously shown to induce formation of oriented fiber by FCCs. Samples were cultured for 4 weeks and then evaluated for biochemical composition and fiber development. MSCs increased the glycosaminoglycan (GAG) and collagen production in both co-culture and monoculture groups compared to FCC monoculture. Collagen organization was greatest in the FCC monoculture group. While MSCs had increased matrix production, they lacked the fiber organization capabilities of FCCs. This study suggests that GAG production and fiber formation are linked. Co-culture can be used as a means of balancing the synthetic properties of MSCs and the matrix remodeling capabilities of FCCs for tissue engineering applications.

Engineering 3D-Bioplotted scaffolds to induce aligned extracellular matrix deposition for musculoskeletal soft tissue replacement

PURPOSE

Tissue engineering and regenerative medicine approaches have the potential to overcome the challenges associated with current treatment strategies for meniscus injuries. 3D-Bioplotted scaffolds are promising, but have not demonstrated the ability to guide the formation of aligned collagenous matrix in vivo, which is critical for generating functional meniscus tissue. In this study, we evaluate the ability of 3D-Bioplotted scaffold designs with varying interstrand spacing to induce the deposition of aligned matrix in vivo.

MATERIALS AND METHODS

3D-Bioplotted polycaprolactone scaffolds with 100, 200, or 400 μm interstrand spacing were implanted subcutaneously in a rat model for 4, 8, or 12 weeks. Scaffolds were harvested, paraffin-embedded, sectioned, and stained to visualize cell nuclei and collagen. Quantitative image analysis was used to evaluate cell density, matrix fill, and collagen fiber alignment within the scaffolds.

RESULTS

By 4 weeks, cells had infiltrated the innermost scaffold regions. Similarly, collagenous matrix filled interstrand regions nearly completely by 4 weeks. By 12 weeks, aligned collagen was present in all scaffolds. Generally, alignment along the scaffold strands increased over time for all three interstrand spacing groups. Distribution of collagen fiber alignment angles narrowed as interstrand spacing decreased.

CONCLUSIONS

3D-Bioplotted scaffolds allow for complete cell infiltration and collagenous matrix production throughout the scaffold. The ability to use interstrand spacing as a means of controlling the formation of aligned collagen in vivo was demonstrated, which helps establish a design space for scaffold-based meniscus tissue engineering.

Anatomic Mesenchymal Stem Cell-Based Engineered Cartilage Constructs for Biologic Total Joint Replacement

Cartilage has a poor healing response, and few viable options exist for repair of extensive damage. Hyaluronic acid (HA) hydrogels seeded with mesenchymal stem cells (MSCs) polymerized through UV crosslinking can generate functional tissue, but this crosslinking is not compatible with indirect rapid prototyping utilizing opaque anatomic molds. Methacrylate-modified polymers can also be chemically crosslinked in a cytocompatible manner using ammonium persulfate (APS) and N,N,N',N'-tetramethylethylenediamine (TEMED). The objectives of this study were to (1) compare APS/TEMED crosslinking with UV crosslinking in terms of functional maturation of MSC-seeded HA hydrogels; (2) generate an anatomic mold of a complex joint surface through rapid prototyping; and (3) grow anatomic MSC-seeded HA hydrogel constructs using this alternative crosslinking method. Juvenile bovine MSCs were suspended in methacrylated HA (MeHA) and crosslinked either through UV polymerization or chemically with APS/TEMED to generate cylindrical constructs. Minipig porcine femoral heads were imaged using microCT, and anatomic negative molds were generated by three-dimensional printing using fused deposition modeling. Molded HA constructs were produced using the APS/TEMED method. All constructs were cultured for up to 12 weeks in a chemically defined medium supplemented with TGF-β3 and characterized by mechanical testing, biochemical assays, and histologic analysis. Both UV- and APS/TEMED-polymerized constructs showed increasing mechanical properties and robust proteoglycan and collagen deposition over time. At 12 weeks, APS/TEMED-polymerized constructs had higher equilibrium and dynamic moduli than UV-polymerized constructs, with no differences in proteoglycan or collagen content. Molded HA constructs retained their hemispherical shape in culture and demonstrated increasing mechanical properties and proteoglycan and collagen deposition, especially at the edges compared to the center of these larger constructs. Immunohistochemistry showed abundant collagen type II staining and little collagen type I staining. APS/TEMED crosslinking can be used to produce MSC-seeded HA-based neocartilage and can be used in combination with rapid prototyping techniques to generate anatomic MSC-seeded HA constructs for use in filling large and anatomically complex chondral defects or for biologic joint replacement.

Redifferentiation of High-Throughput Generated Fibrochondrocyte Micro-Aggregates: Impact of Low Oxygen Tension

In meniscus tissue engineering strategies, enhancing the matrix quality of the neomeniscal tissue is important. When the differentiated phenotype of fibrochondrocytes is lost, the quality of the matrix becomes compromised. The objective of this study was to produce uniform fibrochondrocyte micro-aggregates with desirable phenotype and tissue homogeneity in large quantities using a simple and reproducible method. Furthermore, we investigated if hypoxia could enhance the matrix quality. Porcine fibrochondrocytes were expanded at 21% oxygen until passage 3 (P3) and a gene expression profile was determined. P3 fibrochondrocytes were cultivated in chondrogenic medium at 5 and 21% oxygen in high-throughput agarose chips containing 2,865 microwells 200 µm in diameter. Evaluation included live/dead staining, histological examination, immunohistochemistry, dimethylmethylene blue assay and real-time reverse transcriptase quantitative polymerase chain reaction of the micro-aggregates. Gene expression analysis showed a drastic decline in collagen II and high expression of collagen I during monolayer culture. After 4 days, uniform and stable micro-aggregates could be produced. The redifferentiation and matrix quality of the hypoxic cultured micro-aggregates were enhanced relative to the normoxic cultures. Sulfated glycosaminoglycan synthesis was significantly higher, and collagen II expression and the collagen II/collagen I ratio were significantly upregulated in the hypoxic cultures. High-throughput production of uniform microtissues holds promise for the generation of larger-scale tissue engineering constructs or optimization of redifferentiation mechanisms for clinical applications.

Physiologically Distributed Loading Patterns Drive the Formation of Zonally Organized Collagen Structures in Tissue-Engineered Meniscus

The meniscus is a dense fibrocartilage tissue that withstands the complex loads of the knee via a unique organization of collagen fibers. Attempts to condition engineered menisci with compression or tensile loading alone have failed to reproduce complex structure on the microscale or anatomic scale. Here we show that axial loading of anatomically shaped tissue-engineered meniscus constructs produced spatial distributions of local strain similar to those seen in the meniscus when the knee is loaded at full extension. Such loading drove formation of tissue with large organized collagen fibers, levels of mechanical anisotropy, and compressive moduli that match native tissue. Loading accelerated the development of native-sized and aligned circumferential and radial collagen fibers. These loading patterns contained both tensile and compressive components that enhanced the major biochemical and functional properties of the meniscus, with loading significantly improved glycosaminoglycan (GAG) accumulation 200-250%, collagen accumulation 40-55%, equilibrium modulus 1000-1800%, and tensile moduli 500-1200% (radial and circumferential). Furthermore, this study demonstrates local changes in mechanical environment drive heterogeneous tissue development and organization within individual constructs, highlighting the importance of recapitulating native loading environments. Loaded menisci developed cartilage-like tissue with rounded cells, a dense collagen matrix, and increased GAG accumulation in the more compressively loaded horns, and fibrous collagen-rich tissue in the more tensile loaded outer 2/3, similar to native menisci. Loaded constructs reached a level of organization not seen in any previous engineered menisci and demonstrate great promise as meniscal replacements.

Long-Term Morphological and Microarchitectural Stability of Tissue-Engineered, Patient-Specific Auricles In Vivo

Current techniques for autologous auricular reconstruction produce substandard ear morphologies with high levels of donor-site morbidity, whereas alloplastic implants demonstrate poor biocompatibility. Tissue engineering, in combination with noninvasive digital photogrammetry and computer-assisted design/computer-aided manufacturing technology, offers an alternative method of auricular reconstruction. Using this method, patient-specific ears composed of collagen scaffolds and auricular chondrocytes have generated auricular cartilage with great fidelity following 3 months of subcutaneous implantation, however, this short time frame may not portend long-term tissue stability. We hypothesized that constructs developed using this technique would undergo continued auricular cartilage maturation without degradation during long-term (6 month) implantation. Full-sized, juvenile human ear constructs were injection molded from high-density collagen hydrogels encapsulating juvenile bovine auricular chondrocytes and implanted subcutaneously on the backs of nude rats for 6 months. Upon explantation, constructs retained overall patient morphology and displayed no evidence of tissue necrosis. Limited contraction occurred in vivo, however, no significant change in size was observed beyond 1 month. Constructs at 6 months showed distinct auricular cartilage microstructure, featuring a self-assembled perichondrial layer, a proteoglycan-rich bulk, and rounded cellular lacunae. Verhoeff's staining also revealed a developing elastin network comparable to native tissue. Biochemical measurements for DNA, glycosaminoglycan, and hydroxyproline content and mechanical properties of aggregate modulus and hydraulic permeability showed engineered tissue to be similar to native cartilage at 6 months. Patient-specific auricular constructs demonstrated long-term stability and increased cartilage tissue development during extended implantation, and offer a potential tissue-engineered solution for the future of auricular reconstructions.

Characterization of mesenchymal stem cells and fibrochondrocytes in three-dimensional co-culture: analysis of cell shape, matrix production, and mechanical performance

Background

Bone marrow mesenchymal stem cells (MSCs) have shown positive therapeutic effects for meniscus regeneration and repair. Preliminary in vitro work has indicated positive results for MSC applications for meniscus tissue engineering; however, more information is needed on how to direct MSC behavior. The objective of this study was to examine the effect of MSC co-culture with primary meniscal fibrochondrocytes (FCCs) in a three-dimensional collagen scaffold in fibrochondrogenic media. Co-culture of MSCs and FCCs was hypothesized to facilitate the transition of MSCs to a FCC cell phenotype as measured by matrix secretion and morphology.

Methods

MSCs and FCCs were isolated from bovine bone marrow and meniscus, respectively. Cells were seeded in a 20 mg/mL high-density type I collagen gel at MSC:FCC ratios of 0:100, 25:75, 50:50, 75:25, and 100:0. Constructs were cultured for up to 2 weeks and then analyzed for cell morphology, glycosaminoglycan content, collagen content, and production of collagen type I, II, and X.

Results

Cells were homogeneously mixed throughout the scaffold and cells had limited direct cell–cell contact. After 2 weeks in culture, MSCs transitioned from a spindle-like morphology toward a rounded phenotype, while FCCs remained rounded throughout culture. Although MSC shape changed with culture, the overall size was significantly larger than FCCs throughout culture. While 75:25 and 100:0 (MSC mono-culture) culture groups produced significantly more glycosaminoglycan (GAG)/DNA than FCCs in mono-culture, GAG retention was highest in 50:50 co-cultures. Similarly, the aggregate modulus was highest in 100:0 and 50:50 co-cultures. All samples contained both collagen types I and II after 2 weeks, and collagen type X expression was evident only in MSC mono-culture gels.

Conclusions

MSCs shift to a FCC morphology in both mono- and co-culture. Co-culture reduced hypertrophy by MSCs, indicated by collagen type X. This study shows that MSC phenotype can be influenced by indirect homogeneous cell culture in a three-dimensional gel, demonstrating the applicability of MSCs in meniscus tissue engineering applications.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-016-0301-8) contains supplementary material, which is available to authorized users.

Acellular and cellular high-density, collagen-fibril constructs with suprafibrillar organization

Collagen is used extensively for tissue engineering due to its prevalence in connective tissues and its role in defining tissue biophysical and biological signalling properties. However, traditional collagen-based materials fashioned from atelocollagen and telocollagen have lacked collagen densities, multi-scale organization, mechanical integrity, and proteolytic resistance found within tissues in vivo. Here, highly interconnected low-density matrices of D-banded fibrils were created from collagen oligomers, which exhibit fibrillar as well as suprafibrillar assembly. Confined compression then was applied to controllably reduce the interstitial fluid while maintaining fibril integrity. More specifically, low-density (3.5 mg mL(-1)) oligomer matrices were densified to create collagen-fibril constructs with average concentrations of 12.25 mg mL(-1) and 24.5 mg mL(-1). Control and densified constructs exhibited nearly linear increases in ultimate stress, Young's modulus, and compressive modulus over the ranges of 65 to 213 kPa, 400 to 1.26 MPa, and 20 to 150 kPa, respectively. Densification also increased construct resistance to collagenase degradability. Finally, this process was amenable to creating high-density cellularized tissues; all constructs maintained high cell viability (at least 97%) immediately following compression as well as after 1 day and 7 days of culture. This method, which integrates the suprafibrillar assembly capacity of oligomers and controlled fluid reduction by confined compression, supports the rational and scalable design of a broad range of collagen-fibril materials and cell-encapsulated tissue constructs for tissue engineering applications.

Advancing biomaterials of human origin for tissue engineering

Biomaterials have played an increasingly prominent role in the success of biomedical devices and in the development of tissue engineering, which seeks to unlock the regenerative potential innate to human tissues/organs in a state of deterioration and to restore or reestablish normal bodily function. Advances in our understanding of regenerative biomaterials and their roles in new tissue formation can potentially open a new frontier in the fast-growing field of regenerative medicine. Taking inspiration from the role and multi-component construction of native extracellular matrices (ECMs) for cell accommodation, the synthetic biomaterials produced today routinely incorporate biologically active components to define an artificial in vivo milieu with complex and dynamic interactions that foster and regulate stem cells, similar to the events occurring in a natural cellular microenvironment. The range and degree of biomaterial sophistication have also dramatically increased as more knowledge has accumulated through materials science, matrix biology and tissue engineering. However, achieving clinical translation and commercial success requires regenerative biomaterials to be not only efficacious and safe but also cost-effective and convenient for use and production. Utilizing biomaterials of human origin as building blocks for therapeutic purposes has provided a facilitated approach that closely mimics the critical aspects of natural tissue with regard to its physical and chemical properties for the orchestration of wound healing and tissue regeneration. In addition to directly using tissue transfers and transplants for repair, new applications of human-derived biomaterials are now focusing on the use of naturally occurring biomacromolecules, decellularized ECM scaffolds and autologous preparations rich in growth factors/non-expanded stem cells to either target acceleration/magnification of the body's own repair capacity or use nature's paradigms to create new tissues for restoration. In particular, there is increasing interest in separating ECMs into simplified functional domains and/or biopolymeric assemblies so that these components/constituents can be discretely exploited and manipulated for the production of bioscaffolds and new biomimetic biomaterials. Here, following an overview of tissue auto-/allo-transplantation, we discuss the recent trends and advances as well as the challenges and future directions in the evolution and application of human-derived biomaterials for reconstructive surgery and tissue engineering. In particular, we focus on an exploration of the structural, mechanical, biochemical and biological information present in native human tissue for bioengineering applications and to provide inspiration for the design of future biomaterials.

Riboflavin crosslinked high-density collagen gel for the repair of annular defects in intervertebral discs: An in vivo study

Open annular defects compromise the ability of the annulus fibrosus to contain nuclear tissue in the disc space, and therefore lead to disc herniation with subsequent degenerative changes to the entire intervertebral disc. This study reports the use of riboflavin crosslinked high-density collagen gel for the repair of annular defects in a needle-punctured rat-tail model. High-density collagen has increased stiffness and greater hydraulic permeability than conventional low-density gels; riboflavin crosslinking further increases these properties. This study found that treating annular defects with crosslinked high-density collagen inhibited the progression of disc degeneration over 18 weeks compared to untreated control discs. Histological sections of FITC-labeled collagen gel revealed an early tight attachment to host annular tissue. The gel was subsequently infiltrated by host fibroblasts which remodeled it into a fibrous cap that bridged the outer disrupted annular fibers and partially repaired the defect. This repair tissue enhanced retention of nucleus pulposus tissue, maintained physiological disc hydration, and preserved hydraulic permeability, according to MRI, histological, and mechanical assessments. Degenerative changes were partially reversed in treated discs, as indicated by an increase in nucleus pulposus size and hydration between weeks 5 and 18. The collagen gel appeared to work as an instant sealant and by enhancing the intrinsic healing capabilities of the host tissue.

Passive strain-induced matrix synthesis and organization in shape-specific, cartilaginous neotissues

Tissue-engineered musculoskeletal soft tissues typically lack the appropriate mechanical robustness of their native counterparts, hindering their clinical applicability. With structure and function being intimately linked, efforts to capture the anatomical shape and matrix organization of native tissues are imperative to engineer functionally robust and anisotropic tissues capable of withstanding the biomechanically complex in vivo joint environment. The present study sought to tailor the use of passive axial compressive loading to drive matrix synthesis and reorganization within self-assembled, shape-specific fibrocartilaginous constructs, with the goal of developing functionally anisotropic neotissues. Specifically, shape-specific fibrocartilaginous neotissues were subjected to 0, 0.01, 0.05, or 0.1 N axial loads early during tissue culture. Results found the 0.1-N load to significantly increase both collagen and glycosaminoglycan synthesis by 27% and 67%, respectively, and to concurrently reorganize the matrix by promoting greater matrix alignment, compaction, and collagen crosslinking compared with all other loading levels. These structural enhancements translated into improved functional properties, with the 0.1-N load significantly increasing both the relaxation modulus and Young's modulus by 96% and 255%, respectively, over controls. Finite element analysis further revealed the 0.1-N uniaxial load to induce multiaxial tensile and compressive strain gradients within the shape-specific neotissues, with maxima of 10.1%, 18.3%, and -21.8% in the XX-, YY-, and ZZ-directions, respectively. This indicates that strains created in different directions in response to a single axis load drove the observed anisotropic functional properties. Together, results of this study suggest that strain thresholds exist within each axis to promote matrix synthesis, alignment, and compaction within the shape-specific neotissues. Tailoring of passive axial loading, thus, presents as a simple, yet effective way to drive in vitro matrix development in shape-specific neotissues toward more closely achieving native structural and functional properties.

Engineering meniscus structure and function via multi-layered mesenchymal stem cell-seeded nanofibrous scaffolds

Despite advances in tissue engineering for the knee meniscus, it remains a challenge to match the complex macroscopic and microscopic structural features of native tissue, including the circumferentially and radially aligned collagen bundles essential for mechanical function. To mimic this structural hierarchy, this study developed multi-lamellar mesenchymal stem cell (MSC)-seeded nanofibrous constructs. Bovine MSCs were seeded onto nanofibrous scaffolds comprised of poly(ε-caprolactone) with fibers aligned in a single direction (0° or 90° to the scaffold long axis) or circumferentially aligned (C). Multi-layer groups (0°/0°/0°, 90°/90°/90°, 0°/90°/0°, 90°/0°/90°, and C/C/C) were created and cultured for a total of 6 weeks under conditions favoring fibrocartilaginous tissue formation. Tensile testing showed that 0° and C single layer constructs had stiffness values several fold higher than 90° constructs. For multi-layer groups, the stiffness of 0°/0°/0° constructs was higher than all other groups, while 90°/90°/90° constructs had the lowest values. Data for collagen content showed a general positive interactive effect for multi-layers relative to single layer constructs, while a positive interaction for stiffness was found only for the C/C/C group. Collagen content and cell infiltration occurred independent of scaffold alignment, and newly formed collagenous matrix followed the scaffold fiber direction. Structural hierarchies within multi-lamellar constructs dictated biomechanical properties, and only the C/C/C constructs with non-orthogonal alignment within layers featured positive mechanical reinforcement as a consequence of the layered construction. These multi-layer constructs may serve as functional substitutes for the meniscus as well as test beds to understand the complex mechanical principles that enable meniscus function.

Dose-dependent response of tissue-engineered intervertebral discs to dynamic unconfined compressive loading

Because of the limitations of current surgical methods in the treatment of degenerative disc disease, tissue-engineered intervertebral discs (TE-IVDs) have become an important target. This study investigated the biochemical and mechanical responses of composite TE-IVDs to dynamic unconfined compression. TE-IVDs were manufactured by floating an injection molded alginate nucleus pulposus (NP) in a type I collagen annulus fibrosus (AF) that was allowed to contract for 2 weeks before loading. The discs were mechanically stimulated at a range of strain amplitude (1-10%) for 2 weeks with a duty cycle of 1 h on-1 h off-1 h on before being evaluated for their biochemical and mechanical properties. Mechanical loading increased all properties in a dose-dependent manner. Glycosaminoglycans (GAGs) increased between 2.8 and 2.2 fold in the AF and NP regions, respectively, whereas the hydroxyproline content increased between 1.2 and 1.8 fold. The discs also experienced a 2-fold increase in the equilibrium modulus and a 4.3-fold increase in the instantaneous modulus. Full effects for all properties were seen by 5% strain amplitude. These data suggest that dynamic loading increases the functionality of our TE-IVDs with region-dependent responses using a method that may be scaled up to larger disc models to expedite maturation for implantation.

Critical seeding density improves the properties and translatability of self-assembling anatomically shaped knee menisci

A recent development in the field of tissue engineering is the rise of all-biologic, scaffold-free engineered tissues. Since these biomaterials rely primarily upon cells, investigation of initial seeding densities constitutes a particularly relevant aim for tissue engineers. In this study, a scaffold-free method was used to create fibrocartilage in the shape of the rabbit knee meniscus. The objectives of this study were to: (i) determine the minimum seeding density, normalized by an area of 44 mm(2), necessary for the self-assembling process of fibrocartilage to occur; (ii) examine relevant biomechanical properties of engineered fibrocartilage, such as tensile and compressive stiffness and strength, and their relationship to seeding density; and (iii) identify a reduced, or optimal, number of cells needed to produce this biomaterial. It was found that a decreased initial seeding density, normalized by the area of the construct, produced superior mechanical and biochemical properties. Collagen per wet weight, glycosaminoglycans per wet weight, tensile properties and compressive properties were all significantly greater in the 5 million cells per construct group as compared to the historical 20 million cells per construct group. Scanning electron microscopy demonstrated that a lower seeding density results in a denser tissue. Additionally, the translational potential of the self-assembling process for tissue engineering was improved though this investigation, as fewer cells may be used in the future. The results of this study underscore the potential for critical seeding densities to be investigated when researching scaffold-free engineered tissues.