The development of collagen-GAG scaffold-membrane composites for tendon tissue engineering

Sweet but Challenging: Tackling the Complexity of GAGs with Engineered Tailor-Made Biomaterials

Glycosaminoglycans (GAGs) play a crucial role in tissue homeostasis by regulating the activity and diffusion of bioactive molecules. Incorporating GAGs into biomaterials has emerged as a widely adopted strategy in medical applications, owing to their biocompatibility and ability to control the release of bioactive molecules. Nevertheless, immobilized GAGs on biomaterials can elicit distinct cellular responses compared to their soluble forms, underscoring the need to understand the interactions between GAG and bioactive molecules within engineered functional biomaterials. By controlling critical parameters such as GAG type, density, and sulfation, it becomes possible to precisely delineate GAG functions within a biomaterial context and to better mimic specific tissue properties, enabling tailored design of GAG-based biomaterials for specific medical applications. However, this requires access to pure and well-characterized GAG compounds, which remains challenging. This review focuses on different strategies for producing well-defined GAGs and explores high-throughput approaches employed to investigate GAG-growth factor interactions and to quantify cellular responses on GAG-based biomaterials. These automated methods hold considerable promise for improving the understanding of the diverse functions of GAGs. In perspective, the scientific community is encouraged to adopt a rational approach in designing GAG-based biomaterials, taking into account the in vivo properties of the targeted tissue for medical applications.

Directing ligament-mimetic bi-directional cell organization in scaffolds through zone-specific microarchitecture for ligament tissue engineering

Ligament tissues exhibit zone-specific anisotropic cell organization. The cells in ligament-proper are longitudinally oriented, whereas, the cells in epiligament are circumferentially oriented. Therefore, scaffolds developed to regenerate ligament tissues should possess adequate architectural features to govern ligament-mimetic bi-directional cell organization. The scaffold architectural features along with ligament-mimetic cell organization may ultimately yield neo-tissues with ligament-like extracellular matrix (ECM) structure and biomechanical properties. Towards this goal, we fabricated a silk/gelatin-based core-shell scaffold (csSG) with zone-specific anisotropic architectural features, wherein, the core of the scaffold possessed longitudinally aligned pores while the shell of the scaffold possessed parallel microgrooves that are aligned circumferentially around the surface of the scaffold. The ligament-mimetic architectural features significantly improved the mechanical properties of the scaffold. Moreover, architectural features of the csSG scaffold governed zone-specific anisotropic organization of cells. The cells in the core were longitudinally oriented as observed in the ligament-proper and the cells on the shell were circumferentially oriented as observed in epiligament. This bi-directional cell orientation partially mimicked the complex cellular network in native ligament tissue. Additionally, both the core and the shell individually supported fibrogenic differentiation of stem cells which further improved their potential for ligament tissue engineering. Further, the aligned pores of the core could govern unidirectional organization of ECM deposited by cells which is crucial for regenerating anisotropic tissues like ligaments. Finally, when implanted subcutaneously in mice, the scaffolds retained their anisotropic architecture for at least 2 weeks, were biocompatible, supported cell infiltration and governed anisotropic organization of cells and ECM. Taken together, the fabricated biomimetic csSG scaffold, through its zone-specific architectural features, could govern ligament-mimetic cellular and ECM organization which is ultimately expected to achieve regeneration of ligament tissues with native-like hierarchical structure and biomechanical properties. Consequently, this study introduces bi-directional structural parameters as design criteria for developing scaffolds for ligament tissue engineering.

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.

Aligned Ice Templated Biomaterial Strategies for the Musculoskeletal System

Aligned pore structures present many advantages when conceiving biomaterial strategies for treatment of musculoskeletal disorders. Aligned ice templating (AIT) is one of the many different techniques capable of producing anisotropic porous scaffolds; its high versatility allows for the formation of structures with tunable pore sizes, as well as the use of many different materials. AIT has been found to yield improved compressive properties for bone tissue engineering (BTE), as well as higher tensile strength and optimized cellular alignment and proliferation in tendon and muscle repair applications. This review evaluates the work that has been done in the last decade toward the production of aligned pore structures by AIT with an outlook on the musculoskeletal system. This work describes the fundamentals of the AIT technique and focuses on the research carried out to optimize the biomechanical properties of scaffolds by modifying the pore structure, categorizing by material type and application. Related topics including growth factor incorporation into AIT scaffolds, drug delivery applications, and studies about immune system response will be discussed.

Biomaterials and tissue engineering approaches using glycosaminoglycans for tissue repair: Lessons learned from the native extracellular matrix

Glycosaminoglycans (GAGs) are an important component of the extracellular matrix as they influence cell behavior and have been sought for tissue regeneration, biomaterials, and drug delivery applications. GAGs are known to interact with growth factors and other bioactive molecules and impact tissue mechanics. This review provides an overview of native GAGs, their structure, and properties, specifically their interaction with proteins, their effect on cell behavior, and their mechanical role in the ECM. GAGs' function in the extracellular environment is still being understood however, promising studies have led to the development of medical devices and therapies. Native GAGs, including hyaluronic acid, chondroitin sulfate, and heparin, have been widely explored in tissue engineering and biomaterial approaches for tissue repair or replacement. This review focuses on orthopaedic and wound healing applications. The use of GAGs in these applications have had significant advances leading to clinical use. Promising studies using GAG mimetics and future directions are also discussed. STATEMENT OF SIGNIFICANCE: Glycosaminoglycans (GAGs) are an important component of the native extracellular matrix and have shown promise in medical devices and therapies. This review emphasizes the structure and properties of native GAGs, their role in the ECM providing biochemical and mechanical cues that influence cell behavior, and their use in tissue regeneration and biomaterial approaches for orthopaedic and wound healing applications.

The Use of Collagen-Based Materials in Bone Tissue Engineering

Synthetic bone substitute materials (BSMs) are becoming the general trend, replacing autologous grafting for bone tissue engineering (BTE) in orthopedic research and clinical practice. As the main component of bone matrix, collagen type I has played a critical role in the construction of ideal synthetic BSMs for decades. Significant strides have been made in the field of collagen research, including the exploration of various collagen types, structures, and sources, the optimization of preparation techniques, modification technologies, and the manufacture of various collagen-based materials. However, the poor mechanical properties, fast degradation, and lack of osteoconductive activity of collagen-based materials caused inefficient bone replacement and limited their translation into clinical reality. In the area of BTE, so far, attempts have focused on the preparation of collagen-based biomimetic BSMs, along with other inorganic materials and bioactive substances. By reviewing the approved products on the market, this manuscript updates the latest applications of collagen-based materials in bone regeneration and highlights the potential for further development in the field of BTE over the next ten years.

Triphasic 3D In Vitro Model of Bone-Tendon-Muscle Interfaces to Study Their Regeneration

The transition areas between different tissues, known as tissue interfaces, have limited ability to regenerate after damage, which can lead to incomplete healing. Previous studies focussed on single interfaces, most commonly bone-tendon and bone-cartilage interfaces. Herein, we develop a 3D in vitro model to study the regeneration of the bone-tendon-muscle interface. The 3D model was prepared from collagen and agarose, with different concentrations of hydroxyapatite to graduate the tissues from bones to muscles, resulting in a stiffness gradient. This graduated structure was fabricated using indirect 3D printing to provide biologically relevant surface topographies. MG-63, human dermal fibroblasts, and Sket.4U cells were found suitable cell models for bones, tendons, and muscles, respectively. The biphasic and triphasic hydrogels composing the 3D model were shown to be suitable for cell growth. Cells were co-cultured on the 3D model for over 21 days before assessing cell proliferation, metabolic activity, viability, cytotoxicity, tissue-specific markers, and matrix deposition to determine interface formations. The studies were conducted in a newly developed growth chamber that allowed cell communication while the cell culture media was compartmentalised. The 3D model promoted cell viability, tissue-specific marker expression, and new matrix deposition over 21 days, thereby showing promise for the development of new interfaces.

Natural, synthetic and commercially-available biopolymers used to regenerate tendons and ligaments

Tendon and ligament (TL) injuries affect millions of people annually. Biopolymers play a significant role in TL tissue repair, whether the treatment relies on tissue engineering strategies or using artificial tendon grafts. The biopolymer governs the mechanical properties, biocompatibility, degradation, and fabrication method of the TL scaffold. Many natural, synthetic and hybrid biopolymers have been studied in TL regeneration, often combined with therapeutic agents and minerals to engineer novel scaffold systems. However, most of the advanced biopolymers have not advanced to clinical use yet. Here, we aim to review recent biopolymers and discuss their features for TL tissue engineering. After introducing the properties of the native tissue, we discuss different types of natural, synthetic and hybrid biopolymers used in TL tissue engineering. Then, we review biopolymers used in commercial absorbable and non-absorbable TL grafts. Finally, we explain the challenges and future directions for the development of novel biopolymers in TL regenerative treatment.

Development of tropoelastin-functionalized anisotropic PCL scaffolds for musculoskeletal tissue engineering

The highly organized extracellular matrix (ECM) of musculoskeletal tissues, encompassing tendons, ligaments and muscles, is structurally anisotropic, hierarchical and multi-compartmental. These features collectively contribute to their unique function. Previous studies have investigated the effect of tissue-engineered scaffold anisotropy on cell morphology and organization for musculoskeletal tissue repair and regeneration, but the hierarchical arrangement of ECM and compartmentalization are not typically replicated. Here, we present a method for multi-compartmental scaffold design that allows for physical mimicry of the spatial architecture of musculoskeletal tissue in regenerative medicine. This design is based on an ECM-inspired macromolecule scaffold. Polycaprolactone (PCL) scaffolds were fabricated with aligned fibers by electrospinning and mechanical stretching, and then surface-functionalized with the cell-supporting ECM protein molecule, tropoelastin (TE). TE was attached using two alternative methods that allowed for either physisorption or covalent attachment, where the latter was achieved by plasma ion immersion implantation (PIII). Aligned fibers stimulated cell elongation and improved cell alignment, in contrast to randomly oriented fibers. TE coatings bound by physisorption or covalently following 200 s PIII treatment promoted fibroblast proliferation. This represents the first cytocompatibility assessment of novel PIII-treated TE-coated PCL scaffolds. To demonstrate their versatility, these 2D anisotropic PCL scaffolds were assembled into 3D hierarchical constructs with an internally compartmentalized structure to mimic the structure of musculoskeletal tissue.

Tendon mechanical properties are enhanced via recombinant lysyl oxidase treatment

Tendon mechanical properties are significantly compromised in adult tendon injuries, tendon-related birth defects, and connective tissue disorders. Unfortunately, there currently is no effective treatment to restore native tendon mechanical properties after postnatal tendon injury or abnormal fetal development. Approaches to promote crosslinking of extracellular matrix components in tendon have been proposed to enhance insufficient mechanical properties of fibrotic tendon after healing. However, these crosslinking agents, which are not naturally present in the body, are associated with toxicity and significant reductions in metabolic activity at concentrations that enhance tendon mechanical properties. In contrast, we propose that an effective method to restore tendon mechanical properties would be to promote lysyl oxidase (LOX)-mediated collagen crosslinking in tendon during adult tissue healing or fetal tissue development. LOX is naturally occurring in the body, and we previously demonstrated LOX-mediated collagen crosslinking to be a critical regulator of tendon mechanical properties during new tissue formation. In this study, we examined the effects of recombinant LOX treatment on tendon at different stages of development. We found that recombinant LOX treatment significantly enhanced tensile and nanoscale tendon mechanical properties without affecting cell viability or collagen content, density, and maturity. Interestingly, both tendon elastic modulus and LOX-mediated collagen crosslink density plateaued at higher recombinant LOX concentrations, which may have been due to limited availability of adjacent lysine residues that are near enough to be crosslinked together. The plateau in crosslink density at higher concentrations of recombinant LOX treatments may have implications for preventing over-stiffening of tendon, though this requires further investigation. These findings demonstrate the exciting potential for a LOX-based therapeutic to enhance tendon mechanical properties via a naturally occurring crosslinking mechanism, which could have tremendous implications for an estimated 32 million acute and chronic tendon and ligament injuries each year in the U.S.

The Role of the Non-Collagenous Extracellular Matrix in Tendon and Ligament Mechanical Behavior: A Review

Tendon is a connective tissue that transmits loads from muscle to bone, while ligament is a similar tissue that stabilizes joint articulation by connecting bone to bone. The 70-90% of tendon and ligament's extracellular matrix (ECM) is composed of a hierarchical collagen structure that provides resistance to deformation primarily in the fiber direction, and the remaining fraction consists of a variety of non-collagenous proteins, proteoglycans, and glycosaminoglycans (GAGs) whose mechanical roles are not well characterized. ECM constituents such as elastin, the proteoglycans decorin, biglycan, lumican, fibromodulin, lubricin, and aggrecan and their associated GAGs, and cartilage oligomeric matrix protein (COMP) have been suggested to contribute to tendon and ligament's characteristic quasi-static and viscoelastic mechanical behavior in tension, shear, and compression. The purpose of this review is to summarize existing literature regarding the contribution of the non-collagenous ECM to tendon and ligament mechanics, and to highlight key gaps in knowledge that future studies may address. Using insights from theoretical mechanics and biology, we discuss the role of the non-collagenous ECM in quasi-static and viscoelastic tensile, compressive, and shear behavior in the fiber direction and orthogonal to the fiber direction. We also address the efficacy of tools that are commonly used to assess these relationships, including enzymatic degradation, mouse knockout models, and computational models. Further work in this field will foster a better understanding of tendon and ligament damage and healing as well as inform strategies for tissue repair and regeneration.

Silkworm Gut Fibres from Silk Glands of Samia cynthia ricini-Potential Use as a Scaffold in Tissue Engineering

High-performance fibroin fibres are ideal candidates for the manufacture of scaffolds with applications in tissue engineering due to the excellent mechanical properties and optimal biocompatibility of this protein. In this work, the manufacture of high-strength fibres made from the silk glands of Samia cynthia ricini is explored. The glands were subjected to soaking in aqueous dissolutions of acetic acid and stretched to manufacture the fibres. The materials produced were widely characterized, in terms of morphology, mechanical properties, crystallinity and content of secondary structures, comparing them with those produced by the standard procedure published for Bombyx mori. In addition, mechanical properties and biocompatibility of a braided scaffold produced from these fibres was evaluated. The results obtained show that the fibres from B. mori present a higher degree of crystallinity than those from S. c. ricini, which is reflected in higher values of elastic modulus and lower values of strain at break. Moreover, a decrease in the elongation values of the fibres from S. c. ricini was observed as the concentration of acetic acid was increased during the manufacture. On the other hand, the study of the braided scaffolds showed higher values of tensile strength and strain at break in the case of S. c. ricini materials and similar values of elastic modulus, compared to those of B. mori, displaying both scaffolds optimal biocompatibility using a fibroblast cell line.

Natural Hydrogel-Based Bio-Inks for 3D Bioprinting in Tissue Engineering: A Review

Three-dimensional (3D) printing is well acknowledged to constitute an important technology in tissue engineering, largely due to the increasing global demand for organ replacement and tissue regeneration. In 3D bioprinting, which is a step ahead of 3D biomaterial printing, the ink employed is impregnated with cells, without compromising ink printability. This allows for immediate scaffold cellularization and generation of complex structures. The use of cell-laden inks or bio-inks provides the opportunity for enhanced cell differentiation for organ fabrication and regeneration. Recognizing the importance of such bio-inks, the current study comprehensively explores the state of the art of the utilization of bio-inks based on natural polymers (biopolymers), such as cellulose, agarose, alginate, decellularized matrix, in 3D bioprinting. Discussions regarding progress in bioprinting, techniques and approaches employed in the bioprinting of natural polymers, and limitations and prospects concerning future trends in human-scale tissue and organ fabrication are also presented.

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.

Mechanically and biologically promoted cell-laden constructs generated using tissue-specific bioinks for tendon/ligament tissue engineering applications

Tendon and ligament tissues provide stability and mobility crucial for musculoskeletal function, but are particularly prone to injury. Owing to poor innate healing capacity, the regeneration of mature and functional tendon/ligament (T/L) poses a formidable clinical challenge. Advanced bioengineering strategies to develop biomimetic tissue implants are highly desired for the treatment of T/L injuries. Here, we presented a cell-based tissue engineering strategy to generate cell-laden tissue constructs comprising stem cells and tissue-specific bioinks using 3D cell-printing technology. We implemented an in vitro preconditioning approach to guide semi-organized T/L-like formation before the in vivo application of cell-printed implants. During in vitro maturation, tissue-specific decellularized extracellular matrix-based cellular constructs facilitated long-term in vitro culture with high cell viability and promoted tenogenesis with enhanced cellular/structural anisotropy. Moreover, we demonstrated improved cell survival/retention upon in vivo implantation of pre-matured constructs in nude mice with de novo tendon formation and improved mechanical strength. Although in vivo mechanical properties of the cell-printed implants were lower than those of human T/L tissues, the results of this study may have significant implications for future cell-based therapies in tendon and ligament regeneration and translational medicine.

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

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

Development of wound healing scaffolds with precisely-triggered sequential release of therapeutic nanoparticles

Natural bioactive cue profiles are generally transient with cues switching on/off to coordinate successful outcomes. Dysregulation of these sequences typically leads to disease. Successful wound healing, for example, should progress sequentially through hemostasis, inflammation, granulation tissue formation, and maturation. Chronic wounds, such as diabetic foot ulcers, suffer from uncoordinated signaling, and arrest and cycle between the inflammation and granulation stages. Traditionally, therapeutic delivery in tissue engineering has focused on sustaining delivery of key signaling factors; however, temporal and sequential delivery have increasingly come into focus. To fully take advantage of these signaling systems, a scaffold or matrix material that can house the delivery system is desirable. In this work, we functionalized a collagen-based scaffold - which has proven regenerative potential in wounds - with on-demand delivery of nanoparticles. Building on our previous work with ultrasound-responsive alginate that shows near-zero baseline release and a rapid release in response to an ultrasound trigger, we developed two novel scaffolds. In the first version, homogeneously-distributed microparticles of alginate were incorporated within the collagen-glycosaminoglycan (GAG) scaffold; ultrasound-triggered release of platelet derived growth factor (PDGF) loaded gold nanoparticles was demonstrated; and their maintained bioactivity confirmed. In the second version, pockets of alginate that can be individually loaded and triggered with ultrasound, were incorporated. The ability to sequentially release multiple therapeutics within these scaffolds using ultrasound was successfully confirmed. These platforms offer a precise and versatile way to deliver therapeutic nanoparticles within a proven regenerative template, and can be used to deliver and probe timed therapeutic delivery in wound healing and other tissue engineering applications.

Aligned and electrically conductive 3D collagen scaffolds for skeletal muscle tissue engineering

Skeletal muscle is characterized by its three-dimensional (3D) anisotropic architecture composed of highly aligned and electrically-excitable muscle fibers that enable normal movement. Biomaterial-based tissue engineering approaches to repair skeletal muscle are limited due to difficulties combining 3D structural alignment (to guide cell/matrix organization) and electrical conductivity (to enable electrically-excitable myotube assembly and maturation). In this work we successfully produced aligned and electrically conductive 3D collagen scaffolds using a freeze-drying approach. Conductive polypyrrole (PPy) nanoparticles were synthesized and directly mixed into a suspension of type I collagen and chondroitin sulfate followed by directional lyophilization. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and confocal microscopy showed that directional solidification resulted in scaffolds with longitudinally aligned pores with homogeneously-distributed PPy content. Chronopotentiometry verified that PPy incorporation resulted in a five-fold increase in conductivity compared to non-PPy-containing collagen scaffolds without detrimentally affecting myoblast metabolic activity. Furthermore, the aligned scaffold microstructure provided contact guidance cues that directed myoblast growth and organization. Incorporation of PPy also promoted enhanced myotube formation and maturation as measured by myosin heavy chain (MHC) expression and number of nuclei per myotube. Together these data suggest that aligned and electrically conductive 3D collagen scaffolds could be useful for skeletal muscle tissue engineering.

A Tissue-Engineered Tracheobronchial In Vitro Co-Culture Model for Determining Epithelial Toxicological and Inflammatory Responses

Translation of novel inhalable therapies for respiratory diseases is hampered due to the lack of in vitro cell models that reflect the complexity of native tissue, resulting in many novel drugs and formulations failing to progress beyond preclinical assessments. The development of physiologically-representative tracheobronchial tissue analogues has the potential to improve the translation of new treatments by more accurately reflecting in vivo respiratory pharmacological and toxicological responses. Herein, advanced tissue-engineered collagen hyaluronic acid bilayered scaffolds (CHyA-B) previously developed within our group were used to evaluate bacterial and drug-induced toxicity and inflammation for the first time. Calu-3 bronchial epithelial cells and Wi38 lung fibroblasts were grown on either CHyA-B scaffolds (3D) or Transwell® inserts (2D) under air liquid interface (ALI) conditions. Toxicological and inflammatory responses from epithelial monocultures and co-cultures grown in 2D or 3D were compared, using lipopolysaccharide (LPS) and bleomycin challenges to induce bacterial and drug responses in vitro. The 3D in vitro model exhibited significant epithelial barrier formation that was maintained upon introduction of co-culture conditions. Barrier integrity showed differential recovery in CHyA-B and Transwell® epithelial cultures. Basolateral secretion of pro-inflammatory cytokines to bacterial challenge was found to be higher from cells grown in 3D compared to 2D. In addition, higher cytotoxicity and increased basolateral levels of cytokines were detected when epithelial cultures grown in 3D were challenged with bleomycin. CHyA-B scaffolds support the growth and differentiation of bronchial epithelial cells in a 3D co-culture model with different transepithelial resistance in comparison to the same co-cultures grown on Transwell® inserts. Epithelial cultures in an extracellular matrix like environment show distinct responses in cytokine release and metabolic activity compared to 2D polarised models, which better mimic in vivo response to toxic and inflammatory stimuli offering an innovative in vitro platform for respiratory drug development.

Cell-Free Book-Shaped Decellularized Tendon Matrix Graft Capable of Controlled Release of BMP-12 to Improve Tendon Healing in a Rat Model

BACKGROUND

Achilles tendon (AT) defects often occur in traumatic and chronic injuries. Currently, no graft can satisfactorily regenerate parallel tendinous tissue at the defect site to completely restore AT function.

PURPOSE

To develop a cell-free functional graft by tethering bone morphogenetic protein 12 (BMP-12) on a book-shaped decellularized tendon matrix (BDTM) and to determine whether this graft is more beneficial for AT defect healing than an autograft.

STUDY DESIGN

Controlled laboratory study.

METHODS

Canine patellar tendon was sectioned into a book shape and decellularized to fabricate a BDTM. The collagen-binding domain (CBD) was fused into the N-terminus of BMP-12 to synthesize a recombinant BMP-12 (CBD-BMP-12), which was tethered to the BDTM to prepare a cell-free functional graft (CBD-BMP-12/BDTM). After its tensile resistance, tenogenic inducibility, and BMP-12 release dynamics were evaluated, the efficacy of the graft for tendon regeneration was determined in a rat model. A total of 140 mature male Sprague-Dawley rats underwent AT tenotomy. The defect was reconstructed with reversed AT (autograft group), native BMP-12 tethered to an intact decellularized tendon matrix (IDTM; NAT-BMP-12/IDTM group), native BMP-12 tethered to a BDTM (NAT-BMP-12/BDTM group), CBD-BMP-12 tethered on an IDTM (CBD-BMP-12/IDTM group), and CBD-BMP-12 tethered on a BDTM (CBD-BMP-12/BDTM group). The rats were sacrificed 4 or 8 weeks after surgery to harvest AT specimens. Six specimens from each group at each time point were used for histological evaluation; the remaining 8 specimens were used for biomechanical testing.

RESULTS

In vitro CBD-BMP-12/BDTM was noncytotoxic, showed high biomimetics with native tendons, was suitable for cell adhesion and growth, and had superior tenogenic inducibility. In vivo the defective AT in the CBD-BMP-12/BDTM group regenerated more naturally than in the other groups, as indicated by more spindle-shaped fibroblasts embedded in a matrix of parallel fibers. The biomechanical properties of the regenerated AT in the CBD-BMP-12/BDTM group also increased more significantly than in the other groups.

CONCLUSION

CBD-BMP-12/BDTM is more beneficial than autograft for healing AT defects in a rat model.

CLINICAL RELEVANCE

The findings of this study demonstrate that CBD-BMP-12/BDTM can serve as a practical graft for reconstructing AT defects.

Biofabrication and Signaling Strategies for Tendon/Ligament Interfacial Tissue Engineering

Tendons and ligaments (TL) have poor healing capability, and for serious injuries like tears or ruptures, surgical intervention employing autografts or allografts is usually required. Current tissue replacements are nonideal and can lead to future problems such as high retear rates, poor tissue integration, or heterotopic ossification. Alternatively, tissue engineering strategies are being pursued using biodegradable scaffolds. As tendons connect muscle and bone and ligaments attach bones, the interface of TL with other tissues represent complex structures, and this intricacy must be considered in tissue engineered approaches. In this paper, we review recent biofabrication and signaling strategies for biodegradable polymeric scaffolds for TL interfacial tissue engineering. First, we discuss biodegradable polymeric scaffolds based on the fabrication techniques as well as the target tissue application. Next, we consider the effect of signaling factors, including cell culture, growth factors, and biophysical stimulation. Then, we discuss human clinical studies on TL tissue healing using commercial synthetic scaffolds that have occurred over the past decade. Finally, we highlight the challenges and future directions for biodegradable scaffolds in the field of TL and interface tissue engineering.

Collagen and chondroitin sulfate functionalized bioinspired fibers for tendon tissue engineering application

Functional tendon tissue engineering depends on harnessing the biochemical and biophysical cues of the native tendon extracellular matrix. In this study, we fabricated highly-aligned poly(L-lactic acid) (PLLA) fibers with surfaces decorated by two of the crucial tendon ECM components, type 1 collagen (COL1) and chondroitin sulfate (CS), through a coaxial stable jet electrospinning approach. Effects of the biomimetic COL1-CS (shell)/PLLA (core) fibers on the tenogenic differentiation of human mesenchymal stem cells (hMSCs) in vitro were investigated. Higher rates of cell spreading and proliferation are observed on the aligned COL1-CS/PLLA fibers compared to that on the plain PLLA fibers. Expression of the tendon-associated genes scleraxis (SCX) and COL1 as well as protein tenomodulin (TNMD) are significantly increased. Introduction of mechanical stimulation gives rise to synergistic effect on tenogenic differentiation of hMSCs. Higher expression of TGF-β2, TGFβR-II, and Smad3 by the cells on the COL1-CS/PLLA fiber substrates are observed, which indicates that COL1-CS/PLLA ultrafine fibers dictate the hMSC tenogenic differentiation through activating the TGF-β signaling pathway. Animal study in rat Achilles tendon repair model corroborated the promoting role of COL1-CS/PLLA in regenerating a tendon-like tissue. Thus, our highly aligned biomimicking fibers may serve as an efficient scaffolding system for functional tendon regeneration.

Development of collagen-poly(caprolactone)-based core-shell scaffolds supplemented with proteoglycans and glycosaminoglycans for ligament repair

Core-shell scaffolds offer a promising regenerative solution to debilitating injuries to anterior cruciate ligament (ACL) thanks to a unique biphasic structure. Nevertheless, current core-shell designs are impaired by an imbalance between permeability, biochemical and mechanical cues. This study aimed to address this issue by creating a porous core-shell construct which favors cell infiltration and matrix production, while providing mechanical stability at the site of injury. The developed core-shell scaffold combines an outer shell of electrospun poly(caprolactone) fibers with a freeze-dried core of type I collagen doped with proteoglycans (biglycan, decorin) or glycosaminoglycans (chondroitin sulphate, dermatan sulphate). The aligned fibrous shell achieved an elastic modulus akin of the human ACL, while the porous collagen core is permeable to human mesenchymal stem cell (hMSC). Doping of the core with the aforementioned biomolecules led to structural and mechanical changes in the pore network. Assessment of cellular metabolic activity and scaffold contraction shows that hMSCs actively remodel the matrix at different degrees, depending on the core's doping formulation. Additionally, immunohistochemical staining and mRNA transcript levels show that the collagen-chondroitin sulphate formulation has the highest matrix production activity, while the collagen-decorin formulation featured a matrix production profile more characteristic of the undamaged tissue. Together, this demonstrates that scaffold doping with target biomolecules leads to distinct levels of cell-mediated matrix remodeling. Overall, this work resulted in the development of a versatile and robust platform with a combination of mechanical and biochemical features that have a significant potential in promoting the repair process of ACL tissue.

Fluidic embedding of additional macroporosity in alginate-gelatin composite structure for biomimetic application

Biomimetic characteristics of hydrogel scaffold are tuned in this study utilizing the synergy of alginate, gelatin, and microfluidically embedded voids. Superposition of alginate and gelatin polymer networks results in additional rigidity, which can be tuned by introduction of voids, and thereby allowing faster release of pore pressure through movement of aqueous phase through the pore network. More importantly, voids enabled the cells to penetrate from the surface of seeding into the depth of the scaffold and proliferate there, as demonstrated for MDA MB 231 breast cancer cells. The uniform voids, generated by the microfluidic device, self-align creating uniform macroporosity within the gel structure, get readily filled by the media due to hydrophilicity, and extend the characteristics of composite uniformly across the entire scaffold.

Scaffolds Functionalized with Matrix from Induced Pluripotent Stem Cell Fibroblasts for Diabetic Wound Healing

Diabetic foot ulcers (DFUs) are chronic wounds, with 20% of cases resulting in amputation, despite intervention. A recently approved tissue engineering product-a cell-free collagen-glycosaminoglycan (GAG) scaffold-demonstrates 50% success, motivating its functionalization with extracellular matrix (ECM). Induced pluripotent stem cell (iPSC) technology reprograms somatic cells into an embryonic-like state. Recent findings describe how iPSCs-derived fibroblasts ("post-iPSF") are proangiogenic, produce more ECM than their somatic precursors ("pre-iPSF"), and their ECM has characteristics of foetal ECM (a wound regeneration advantage, as fetuses heal scar-free). ECM production is 45% higher from post-iPSF and has favorable components (e.g., Collagen I and III, and fibronectin). Herein, a freeze-dried scaffold using ECM grown by post-iPSF cells (Post-iPSF Coll) is developed and tested vs precursors ECM-activated scaffolds (Pre-iPSF Coll). When seeded with healthy or DFU fibroblasts, both ECM-derived scaffolds have more diverse ECM and more robust immune responses to cues. Post-iPSF-Coll had higher GAG, higher cell content, higher Vascular Endothelial Growth Factor (VEGF) in DFUs, and higher Interleukin-1-receptor antagonist (IL-1ra) vs. pre-iPSF Coll. This work constitutes the first step in exploiting ECM from iPSF for tissue engineering scaffolds.

Hierarchical biofabrication of biomimetic collagen-elastin vascular grafts with controllable properties via lyophilisation

This article describes the development of a hierarchical biofabrication technique suitable to create large but complex structures, such as vascular mimicking grafts, using facile lyophilisation technology amenable to multiple other biomaterial classes. The combination of three fabrication techniques together, namely solvent evaporation, lyophilisation, and crosslinking together allows highly tailorable structures from the microstructure up to the macrostructure, and with the ability to independently crosslink each layer it allows great flexibility to match desired native mechanical properties independently of the micro/macrostructure. We have demonstrated the flexibility of this biofabrication technique by independently optimising each of the layers to create a multi-layered arterial structure with tailored architectural and biophysical/biochemical properties using a collagen-elastin composite. Taken together, the facile biofabrication methodology developed has led to the development of a biomimetic bilayered scaffold suitable for use as a tissue engineered vascular graft (for haemodialysis access or peripheral/coronary bypass), or as an in vitro test platform to examine disease progression, pharmacological toxicity, or cardiovascular medical device testing. STATEMENT OF SIGNIFICANCE: The ability to grow large complex tissues such as blood vessels for transplantation is often hampered by the limitations of the selected biofabrication technique. Here, we sought to overcome some of the fabrication limitations for naturally occurring cardiovascular polymers (collagen/elastin) via a hierarchical approach to fabrication where each layer is built upon the previous. This approach enabled the flexibility to modify and tailor each layer's properties independently via control over polymer concentration, microstructure, and crosslinking. This simple approach facilitated us to fabricate multi-layered vascular grafts which were remodelled into high-density vascular tissue after 21-days. The fabrication approach could be translated to a myriad of other tissues while the engineered vascular graft could also be used as a test platform for drugs/medical devices or as a tissue engineering scaffold for vascular grafting for different indications.

Additive Biomanufacturing with Collagen Inks

Collagen is a natural polymer found abundantly in the extracellular matrix (ECM). It is easily extracted from a variety of sources and exhibits excellent biological properties such as biocompatibility and weak antigenicity. Additionally, different processes allow control of physical and chemical properties such as mechanical stiffness, viscosity and biodegradability. Moreover, various additive biomanufacturing technology has enabled layer-by-layer construction of complex structures to support biological function. Additive biomanufacturing has expanded the use of collagen biomaterial in various regenerative medicine and disease modelling application (e.g., skin, bone and cornea). Currently, regulatory hurdles in translating collagen biomaterials still remain. Additive biomanufacturing may help to overcome such hurdles commercializing collagen biomaterials and fulfill its potential for biomedicine.

Effect of ultrasound power on extraction kinetic model, and physicochemical and structural characteristics of collagen from chicken lung

Abstract

The effects of ultrasound power on extraction kinetic model, and physicochemical and structural characteristics of collagen from chicken lung were studied. Ultrasound power caused a significant increase in extraction rate and equilibrium concentration, with the maximum extraction yield (31.25%) at 150 W. The experimental data were consistent with the predicted ones in this empirical equation, in which the percentage error differences was 0.026–4.159%. Besides, ultrasound treatment did not affect their triple-helical structure. The thermal stability of pepsin-soluble collagen by ultrasound pre-treatment (UPSC) was higher, due to the higher imino acid content (20.76%). UPSC also exhibited better solubility and fibril forming capacity. Overall, the kinetic model of UPSC from chicken lung could serve the purpose of obtaining collagen, which displayed a potential alternative source to mammal collagens for application in food, biomaterials and biomedical fields.

Graphical abstract

The inclusion of zinc into mineralized collagen scaffolds for craniofacial bone repair applications

Implant osteoinduction and subsequent osteogenic activity are critical events that need improvement for regenerative healing of large craniofacial bone defects. Here we describe the augmentation of the mineral content of a class of mineralized collagen scaffolds under development for craniomaxillofacial bone regeneration via the inclusion of zinc ions to promote osteogenesis in vitro. Zinc is an essential trace element in skeletal tissue and bone, with soluble zinc being shown to promote osteogenic differentiation of porcine adipose derived stem cells. We report the development of a new class of zinc functionalized scaffolds fabricated by adding zinc sulfate to a mineralized collagen-glycosaminoglycan precursor suspension that was then freeze dried to form a porous biomaterial. We report analysis of zinc functionalized scaffolds via imaging (scanning electron microscopy), mechanical testing (compression), and compositional (X-ray diffraction, inductively coupled plasma mass spectrometry) analyses. Notably, zinc-functionalized scaffolds display morphological changes to the mineral phase and altered elastic modulus without substantially altering the composition of the brushite phase or removing the micro-scale pore morphology of the scaffold. These scaffolds also display zinc release kinetics on the order of days to weeks and promote successful growth and pro-osteogenic capacity of porcine adipose derived stem cells cultured within these zinc scaffolds. Taken together, we believe that zinc functionalized scaffolds provide a unique platform to explore strategies to improve in vivo osteogenesis in craniomaxillofacial bone injuries models. STATEMENT OF SIGNIFICANCE: Craniomaxillofacial bone defects that arise from traumatic, congenital, and post-oncologic origins cannot heal on their own and often require surgical intervention. We have developed a class of mineralized collagen scaffolds that promotes osteogenesis and bone regeneration. Here we describe the inclusion of zinc sulfate into the mineralized collagen scaffold to improve osteogenesis. Zinc functionalized scaffolds demonstrate altered crystallite microstructure but consistent Brushite chemistry, improved mechanics, and promote zinc transporter expression while supporting stem cell viability, osteogenic differentiation, and mineral biosynthesis.

Topotactic Fibrillogenesis of Freeze-Cast Microridged Collagen Scaffolds for 3D Cell Culture

Type I collagen is the main component of the extracellular matrix (ECM). In vitro, under a narrow window of physicochemical conditions, type I collagen self-assembles to form complex supramolecular architectures reminiscent of those found in native ECM. Presently, a major challenge in collagen-based biomaterials is to couple the delicate collagen fibrillogenesis events with a controlled shaping process in non-denaturating conditions. In this work, an ice-templating approach promoting the structuration of collagen into macroporous monoliths is used. Instead of common solvent removal procedures, a new topotactic conversion approach yielding self-assembled ordered fibrous materials is implemented. These collagen-only, non-cross-linked scaffolds exhibit uncommon mechanical properties in the wet state, with a Young's modulus of 33 ± 12 kPa, an ultimate tensile stress of 33 ± 6 kPa, and a strain at failure of 105 ± 28%. With the help of the ice-patterned microridge features, normal human dermal fibroblasts and C2C12 murine myoblasts successfully migrate and form highly aligned populations within the resulting three-dimensional (3D) collagen scaffolds. These results open a new pathway to the development of new tissue engineering scaffolds ordered across various organization levels from the molecule to the macropore and are of particular interest for biomedical applications where large-scale 3D cell alignment is needed such as for muscular or nerve reconstruction.

Biologic and mechanical aspects of tendon fibrosis after injury and repair

Tendon injuries of the hand that require surgical repair often heal with excess scarring and adhesions to adjacent tissues. This can compromise the natural gliding mechanics of the flexor tendons in particular, which operate within a fibro-osseous tunnel system similar to a set of pulleys. Even combining the finest suture repair techniques with optimal hand therapy protocols cannot ensure predictable restoration of hand function in these cases. To date, the majority of research regarding tendon injuries has revolved around the mechanical aspects of the surgical repair (i.e. suture techniques) and postoperative rehabilitation. The central principles of treatment gleaned from this literature include using a combination of core and epitendinous sutures during repair and initiating motion early on in hand therapy to improve tensile strength and limit adhesion formation. However, it is likely that the best clinical solution will utilize optimal biological modulation of the healing response in addition to these core strategies and, recently, the research in this area has expanded considerably. While there are no proven additive biological agents that can be used in clinical practice currently, in this review, we analyze the recent literature surrounding cytokine modulation, gene and cell-based therapies, and tissue engineering, which may ultimately lead to improved clinical outcomes following tendon injury in the future.

Incorporation of the Amniotic Membrane as an Immunomodulatory Design Element in Collagen Scaffolds for Tendon Repair

Tendon injuries often require surgical intervention and even then result in poor outcomes due to scar formation and repeated failure. Biomaterial implants offer the potential to address multiple underlying concerns preventing improved tendon repair. Here, we describe modifications to the composition of an anisotropic collagen-glycosaminoglycan (CG) scaffold biomaterial, incorporating amniotic membrane (AM)-derived matrix to alter the inflammatory response and establish conditions for improved regenerative repair. We explored two methods of AM matrix incorporation to address multiple concerns associated with tendon repair. Amniotic membrane-derived matrix was incorporated directly into the scaffold microstructure during fabrication to form a C/AM composite. Alternatively, decellularized amniotic matrix was wrapped around the traditional collagen-chondroitin sulfate (C/CS) scaffold to form a core-shell composite (C/CS plus AM wrap) in a manner similar to current collagen membrane wraps used in rotator cuff and Achilles tendon surgeries to improve the mechanical strength of the repair. Human mesenchymal stem cells (MSCs) cultured within these materials were evaluated for metabolic health and immunomodulatory gene expression in response to inflammatory media challenge of interleukin 1 β and tumor necrosis factor α. The scaffolds were able to maintain MSC metabolic activity in all media conditions over the course of a 7 day culture. Expression of genes encoding for pro-inflammatory cytokines were down-regulated in AM containing scaffolds, suggesting the potential to employ AM-modified CG scaffolds for tendon-repair applications.

Functional regeneration of tendons using scaffolds with physical anisotropy engineered via microarchitectural manipulation

Structural and hierarchical anisotropy underlies the structure-function relationship of most living tissues. Attempts to exploit the interplay between cells and their immediate environment have rarely featured macroscale, three-dimensional constructs required for clinical applications. Furthermore, compromises to biomechanical robustness during fabrication often limit the scaffold's relevance in translational medicine. We report a polymeric three-dimensional scaffold with tendon-like mechanical properties and controlled anisotropic microstructures. The scaffold was composed of two distinct portions, which enabled high porosity while retaining tendon-like mechanical properties. When tenocytes were cultured in vitro on the scaffold, phenotypic markers of tenogenesis such as type-I collagen, decorin, and tenascin were significantly expressed over nonanisotropic controls. Moreover, highly aligned intracellular cytoskeletal network and high nuclear alignment efficiencies were observed, suggesting that microstructural anisotropy might play the epigenetic role of mechanotransduction. When implanted in an in vivo micropig model, a neotissue that formed over the scaffold resembled native tendon tissue in composition and structure.

Advances in Tendon and Ligament Tissue Engineering: Materials Perspective

Introduction. Tendons are specialised, heterogeneous connective tissues, which represent a significant healthcare challenge after injury. Primary surgical repair is the gold standard modality of care; however, it is highly dependent on the extent of injuries. Tissue engineering represents an alternative solution for good tissue integration and regeneration. In this review, we look at the advanced biomaterial composites employed to improve cellular growth while providing appropriate mechanical properties for tendon and ligament repair. Methodology. Comprehensive literature searches focused on advanced composite biomaterials for tendon and ligament tissue engineering. Studies were categorised depending on the application. Results. In the literature, a range of natural and/or synthetic materials have been combined to produce composite scaffolds tendon and ligament tissue engineering. In vitro and in vivo assessment demonstrate promising cellular integration with sufficient mechanical strength. The biological properties were improved with the addition of growth factors within the composite materials. Most in vivo studies were completed in small-scale animal models. Conclusions. Advanced composite materials represent a promising solution to the challenges associated with tendon and ligament tissue engineering. Nevertheless, these approaches still demonstrate limitations, including the necessity of larger-scale animal models to ease future clinical translation and comprehensive assessment of tissue response after implantation.

Tunable Mechanical, Antibacterial, and Cytocompatible Hydrogels Based on a Functionalized Dual Network of Metal Coordination Bonds and Covalent Crosslinking

Tissue engineering has become a rapidly developing field of research because of the increased demand from regenerative medicine, and hydrogels are a promising tissue engineering scaffold because of their three-dimensional structures. In this study, we constructed novel hydrogels of gelatin methacrylate (GelMA) hydrogels modified with histidine and Zn²⁺ (GelMA-His-Zn(II)), which possessed fascinating antibacterial properties and tunable mechanical properties because of the formation of a functionalized dual network of covalent crosslinking and metal coordination bonds. The introduction of metal coordination bonds not only improves the strength of the GelMA hydrogels with covalent crosslinking but also makes their mechanical properties tunable via adjustments to the concentration of Zn²⁺. The synergistic effect of Zn²⁺ and the imidazole groups gives the GelMA-His-Zn(II) hydrogels fascinating antibacterial properties (up to 100% inhibition). Counting the colony forming units and compression tests confirmed the fascinating antibacterial abilities and tunable mechanical properties, respectively, of the GelMA-His-Zn(II) hydrogels. In addition, Cell Counting Kit-8 assays, cytoskeletal staining assays, and live/dead assays confirmed the excellent cytocompatibility of the GelMA-His-Zn(II) hydrogels. Therefore, the GelMA-His-Zn(II) hydrogels are promising for applications in tissue engineering.

Strontium and bisphosphonate coated iron foam scaffolds for osteoporotic fracture defect healing

The purpose of this work was to investigate new bone formation in macroporous iron foams coated with strontium (FeSr) or bisphosphonate (FeBiP) compared to plain iron foam (Fe) and empty defect in a critical size metaphyseal bone defect model in ovariectomized rats. 60 female rats were subjected to bilateral ovariectomy and multi-deficient diet for 3 months. A 4 mm wedge shaped metaphyseal osteotomy was created, fixed with a mini-plate and subsequently filled with Fe, FeSr, FeBiP or left empty. After 6 weeks, μCt analysis revealed a statistically significant increased bone formation at the implant interface in FeSr compared to FeBiP (p = 0.035) and Fe (p = 0.002), respectively. Increased mineralized tissue was also seen within the pores in FeSr (p = 0.023) compared to Fe. Histomorphometry revealed significantly increased bone formation at the implant interface in FeSr (p < 0.001) and FeBiP (p = 0.006) compared to plain Fe with increased osteoblast and decreased osteoclast activity in combination with increased BMP2 and decreased RANKL/OPG in immunohistochemistry. ToF-SIMS analysis showed overlapping Ca signals with Fe for both FeSr and FeBiP thereby indicating tissue in-growth into the scaffolds. In conclusion, iron foam with strontium or bisphosphonate coating are of further interest in metaphyseal fracture defects in osteopenic bone.

Bioinspired Collagen Scaffolds in Cranial Bone Regeneration: From Bedside to Bench

Calvarial defects are common reconstructive dilemmas secondary to a variety of etiologies including traumatic brain injury, cerebrovascular disease, oncologic resection, and congenital anomalies. Reconstruction of the calvarium is generally undertaken for the purposes of cerebral protection, contour restoration for psychosocial well-being, and normalization of neurological dysfunction frequently found in patients with massive cranial defects. Current methods for reconstruction using autologous grafts, allogeneic grafts, or alloplastic materials have significant drawbacks that are unique to each material. The combination of wide medical relevance and the need for a better clinical solution render defects of the cranial skeleton an ideal target for development of regenerative strategies focused on calvarial bone. With the improved understanding of the instructive properties of tissue-specific extracellular matrices and the advent of precise nanoscale modulation in materials science, strategies in regenerative medicine have shifted in paradigm. Previously considered to be simple carriers of stem cells and growth factors, increasing evidence exists for differential materials directing lineage specific differentiation of progenitor cells and tissue regeneration. In this work, we review the clinical challenges for calvarial reconstruction, the anatomy and physiology of bone, and extracellular matrix-inspired, collagen-based materials that have been tested for in vivo cranial defect healing.

Indentation across interfaces between stiff and compliant tissues

Bone-tendon, bone-ligament and bone-cartilage junctions are multi-tissue interfaces that connect materials that differ by two orders of magnitude in mechanical properties, via gradual variations in mineral content and matrix composition. These sites mediate load transfer between highly dissimilar materials and are consequently a primary site of injury during orthopedic failure. Given the large incidence rate and the lack of suitable surgical solutions for their regeneration or repair, characterization of their natural structure and subsequent replication through tissue engineering is important. Here, we evaluate the ability and accuracy of instrumented indentation to characterize the mechanical properties of both biological tissues and engineered scaffolds with interfaces between materials that contain significant changes in mechanical properties. In this study, finite element simulations and reference samples are developed that characterize how accurately indentation measures the modulus of a material as it varies with distance across a continuous interface between dissimilar tissues with multiple orders of magnitude difference in properties. Finite element simulations accurately predicted discrepancies between the modulus function across an interface observed by indentation and the true modulus function of the material and hence allow us to understand the limits of instrumented indentation as a technique for quantifying gradual changes in material properties. It was found that in order to accurately investigate mechanical property variations in tissues with significant modulus heterogeneity the indenter size should be less than 10 percent of the expected length scale of the modulus variations.

STATEMENT OF SIGNIFICANCE

The interfaces between stiff and compliant orthopedic tissues such as bone-tendon, bone-ligament and bone-cartilage are frequent sites of failure during both acute and chronic orthopedic injury and as such their replication via tissue engineering is of importance. The characterization and understanding of these tissue interfaces on a mechanical basis is a key component of elucidating the structure-function relationships that allow them to function naturally and hence a core component of efforts to replicate them. This work uses finite element models and exeperiments to outline the ability of instrumented indentation to characterize the elastic modulus variations across tissue interfaces and provides guidelines for investigators seeking to use this method to understand any interface between dissimilar tissues.

Polymer structure-property requirements for stereolithographic 3D printing of soft tissue engineering scaffolds

This review highlights the synthesis, properties, and advanced applications of synthetic and natural polymers 3D printed using stereolithography for soft tissue engineering applications. Soft tissue scaffolds are of great interest due to the number of musculoskeletal, cardiovascular, and connective tissue injuries and replacements humans face each year. Accurately replacing or repairing these tissues is challenging due to the variation in size, shape, and strength of different types of soft tissue. With advancing processing techniques such as stereolithography, control of scaffold resolution down to the μm scale is achievable along with the ability to customize each fabricated scaffold to match the targeted replacement tissue. Matching the advanced manufacturing technique to polymer properties as well as maintaining the proper chemical, biological, and mechanical properties for tissue replacement is extremely challenging. This review discusses the design of polymers with tailored structure, architecture, and functionality for stereolithography, while maintaining chemical, biological, and mechanical properties to mimic a broad range of soft tissue types.

A poly-L-lactic acid/ collagen/glycosaminoglycan matrix for tissue engineering applications

Adhesion of tissue cells to biomaterials is a prerequisite of paramount importance for the effectiveness of a tissue engineering construct (cell and scaffolds). Functionalization of polymeric scaffolds with organic polymers, such as collagen or proteoglycans, is a promising approach in order to improve the cytocompatibility. As a matter of fact, organic polymers, isolated directly from the extracellular matrix, contain a multitude of surface ligand (fibronectin, laminin, vitronectin) and arginine–glycine–aspartic acid-containing peptides that promote cell adhesion. In tissue engineering, the combination of organic and synthetic polymers gives rise to scaffolds characterized simultaneously by the mechanical strength of synthetic materials and the biocompatibility of natural materials. In this work, porous poly-L-lactide acid scaffolds were functionalized with a synthetic collagen–glycosaminoglycans matrix in order to improve cell adhesion. For this purpose, a protocol for collagen–glycosaminoglycans conjugation into the pores of the scaffolds was set up. Moreover, an innovative protocol for the quantification of the conjugated glycosaminoglycans inside the scaffolds was created and adopted. The results have confirmed the effectiveness of the developed protocol: a collagen–glycosaminoglycans conjugation, with an efficiency of about 21% was obtained inside the scaffold. Moreover, SEM analysis highlighted the presence of the homogeneous synthetic matrix into the bulk of porous scaffolds. Finally, cell culture assays carried out by utilizing mouse embryonic fibroblasts showed that cell proliferation on poly-L-lactide acid-collagen–glycosaminoglycans scaffold is higher than on poly-L-lactide acid collagen scaffold (utilized as control). Therefore, it can be stated that the presence of glycosaminoglycans not only increases the mechanical strength of the matrix, thanks to their cross-linking effect, but also it seems to lead to a more significant cell growth. Overall, it is reasonable to state that the concerned protocol may be proposed as a reliable route to increase the rate of proliferation and in some cases to stimulate the cell differentiation in tissue engineering devices.

Anisotropic Shape-Memory Alginate Scaffolds Functionalized with Either Type I or Type II Collagen for Cartilage Tissue Engineering

Regenerating articular cartilage and fibrocartilaginous tissue such as the meniscus is still a challenge in orthopedic medicine. While a range of different scaffolds have been developed for joint repair, none have facilitated the development of a tissue that mimics the complexity of soft tissues such as articular cartilage. Furthermore, many of these scaffolds are not designed to function in mechanically challenging joint environments. The overall goal of this study was to develop a porous, biomimetic, shape-memory alginate scaffold for directing cartilage regeneration. To this end, a scaffold was designed with architectural cues to guide cellular and neo-tissue alignment, which was additionally functionalized with a range of extracellular matrix cues to direct stem cell differentiation toward the chondrogenic lineage. Shape-memory properties were introduced by covalent cross-linking alginate using carbodiimide chemistry, while the architecture of the scaffold was modified using a directional freezing technique. Introducing such an aligned pore structure was found to improve the mechanical properties of the scaffold, and promoted higher levels of sulfated glycosaminoglycans (sGAG) and collagen deposition compared to an isotropic (nonaligned) pore geometry when seeded with adult human stem cells. Functionalization with collagen improved stem cell recruitment into the scaffold and facilitated more homogenous cartilage tissue deposition throughout the construct. Incorporating type II collagen into the scaffolds led to greater cell proliferation, higher sGAG and collagen accumulation, and the development of a stiffer tissue compared to scaffolds functionalized with type I collagen. The results of this study demonstrate how both scaffold architecture and composition can be tailored in a shape-memory alginate scaffold to direct stem cell differentiation and support the development of complex cartilaginous tissues.

Modifying the strength and strain concentration profile within collagen scaffolds using customizable arrays of poly-lactic acid fibers

The tendon-to-bone junction is a highly specialized tissue which dissipates stress concentrations between mechanically dissimilar tendon and bone. Upon injury, the local heterogeneities across this insertion are not regenerated, leading to poor functional outcomes such as formation of scar tissue at the insertion and re-failure rates exceeding 90%. Although current tissue engineering methods are moving towards the development of spatially-graded biomaterials to begin to address these injuries, significant opportunities remain to engineer the often complex local mechanical behavior of such biomaterials to enhance their bioactivity. Here, we describe the use of three-dimensional printing techniques to create customizable arrays of poly-lactic acid (PLA) fibers that can be incorporated into a collagen scaffold under development for tendon bone junction repair. Notably, we use additive manufacturing concepts to generate arrays of spatially-graded fibers from biodegradable PLA that are incorporated into collagen scaffolds to create a collagen-PLA composite. We demonstrate the ability to tune the mechanical performance of the fiber-scaffold composite at the bulk scale. We also demonstrate the incorporation of spatially-heterogeneous fiber designs to establish non-uniform local mechanical performance of the composite biomaterial under tensile load, a critical element in the design of multi-compartment biomaterials for tendon-to-bone regeneration applications. Together, this work highlights the capacity to use multi-scale composite biomaterials to control local and bulk mechanical properties, and provides key insights into design elements under consideration for mechanically competent, multi-tissue regeneration platforms.