A Physiology-Inspired Multifactorial Toolbox in Soft-to-Hard Musculoskeletal Interface Tissue Engineering

Fullerol Initiates Stem Cell-Nanomaterials Interactions for Enhanced Tissue Regeneration via Clathrin-Mediated Endocytosis and Nuclear Factor Erythroid 2-Related Factor 2 Signaling

The advancement of nanomedicine requires a thorough understanding of the intrinsic bioactivity and molecular interactions of nanomaterials for safe and effective clinical applications, which remains lacking for most currently developed nanomaterials. Here, we uncover the unique intrinsic bioactivity and regulatory mechanisms of carbon-based fullerol nanomaterials through high-throughput molecular analysis and explore their therapeutic potential for tissue regeneration using tissue engineering approaches. Fullerol exhibits intrinsic pro-differentiation and antioxidant properties that enhance the osteogenesis and chondrogenesis of MSCs. Mechanistically, proteomic analysis combined with small-molecule inhibition studies reveals that fullerol is internalized by MSCs via clathrin-mediated endocytosis and activates NRF2 signaling, thereby exerting antioxidant effects that restore impaired MSC viability and differentiation under oxidative stress. Leveraging these unique bioactivities, we develop a fullerol-functionalized hydrogel with feasible physicochemical properties and triple biological functions in antioxidant, pro-osteogenic, and pro-chondrogenic effects and confirm its great regenerative capacity for both cartilage and subchondral bone by promoting structural restoration and improving functional recovery in a rat osteochondral defect model. Our findings offer new insights into the intricate interactions between stem cells and nanomaterials at the cellular and molecular levels and broaden the potential biomedical applications of fullerol for future cartilage and bone regeneration therapies.

Harnessing native blueprints for designing bioinks to bioprint functional cardiac tissue

Cardiac tissue lacks regenerative capacity, making heart transplantation the primary treatment for end-stage heart failure. Engineered cardiac tissues developed through three-dimensional bioprinting (3DBP) offer a promising alternative. However, reproducing the native structure, cellular diversity, and functionality of cardiac tissue requires advanced cardiac bioinks. Major obstacles in CTE (cardiac tissue engineering) include accurately characterizing bioink properties, replicating the cardiac microenvironment, and achieving precise spatial organization. Optimizing bioink properties to closely mimic the extracellular matrix (ECM) is essential, as deviations may result in pathological effects. This review encompasses the rheological and electromechanical properties of bioinks and the function of the cardiac microenvironment in the design of functional cardiac constructs. Furthermore, it focuses on improving the rheological characteristics, printability, and functionality of bioinks, offering valuable perspectives for developing new bioinks especially designed for CTE.

Tissue-engineered collagen matrix loaded with rat adipose-derived stem cells/human amniotic mesenchymal stem cells for rotator cuff tendon-bone repair

The rotator cuff tendon-bone interface tissue exhibits high heterogeneity in its composition and structure, with collagen being its primary component. Here, we prepared tissue-engineered decellularized live hyaline cartilage grafts (dLHCG), this dLHCG scaffold's bioactive ECM mainly consists of collagen II, proteoglycans, and fibronectin, presenting a cartilage-like lacuna microstructure. The dLHCG scaffold loaded human amniotic mesenchymal stem cells (hAMSCs) and adipose stem cells (ADSCs) were implanted into the interface. The dLHCG scaffold could maintain the pluripotency of stem cells, supporting the proliferation, osteogenic differentiation, and tenogenic differentiation of the MSCs. The collagen II, through the integrin α2β1-FAK-JNK signaling axis, promotes Runx-2 activation, playing a better regulatory role in the early osteogenic differentiation of MSCs, enhancing bone defect repair through an endochondral ossification process. The in vivo rat model demonstrated that 12 weeks post-operation, the MSC-loaded dLHCG scaffold group exhibited continuous aligned collagen fibers at the tendon-bone interface, with significantly enhanced biomechanical function compared to the control group. The dLHCG scaffold create an efficient interface, which promoting the restoration of the soft-hard gradient structure tissue at the junction between the scaffold and the host tissue, thereby providing a rational and promising strategy for the rapid healing of the rotator cuff injury.

Decoding the mechanical characteristics of the human anterior cruciate ligament entheses through graduated mineralization interfaces

The anterior cruciate ligament is anchored to the femur and tibia via specialized interfaces known as entheses. These play a critical role in ligament homeostasis and joint stability by transferring forces, varying in magnitude and direction between structurally and functionally dissimilar tissues. However, the precise structural and mechanical characteristics underlying the femoral and tibial entheses and their intricate interplay remain elusive. In this study, two thin-graduated mineralization regions in the femoral enthesis (~21 μm) and tibial enthesis (~14 μm) are identified, both exhibiting distinct biomolecular compositions and mineral assembly patterns. Notably, the femoral enthesis interface exhibits progressively maturing hydroxyapatites, whereas the mineral at the tibial enthesis interface region transitions from amorphous calcium phosphate to hydroxyapatites with increasing crystallinity. Proteomics results reveal that Matrix Gla protein uniquely enriched at the tibial enthesis interface, may stabilize amorphous calcium phosphate, while C-type lectin domain containing 11 A, enriched at the femoral enthesis interface, could facilitate the interface mineralization. Moreover, the finite element analysis indicates that the femoral enthesis model exhibited higher resistance to shearing, whereas the tibial enthesis model contributes to tensile resistance, suggesting that the discrepancy in biomolecular expression and the corresponding mineral assembly heterogeneities collectively contribute to the superior mechanical properties of both the femoral enthesis and tibial enthesis models. These findings provide novel perspectives on the structure-function relationships of anterior cruciate ligament entheses, paving the way for improved management of anterior cruciate ligament injury and regeneration.

Structure, ingredient, and function-based biomimetic scaffolds for accelerated healing of tendon-bone interface

Background

Tendon-bone interface (TBI) repair is slow and challenging owing to its hierarchical structure, gradient composition, and complex function. In this work, enlightened by the natural characteristics of TBI microstructure and the demands of TBI regeneration, a structure, composition, and function-based scaffold was fabricated. Methods: The biomimetic scaffold was designed based on the "tissue-inducing biomaterials" theory: (1) a porous scaffold was created with poly-lactic-co-glycolic-acid, nano-hydroxyapatite and loaded with BMP2-gelatinmp to simulate the bone (BP); (2) a hydrogel was produced from sodium alginate, type I collagen, and loaded with TGF-β3 to simulate the cartilage (CP); (3) the L-poly-lactic-acid fibers were oriented to simulate the tendon (TP). The morphology of tri-layered constructs, gelation kinetics, degradation rate, release kinetics and mechanical strength of the scaffold were characterized. Then, bone marrow mesenchymal stem cells (MSCs) and tenocytes (TT-D6) were cultured on the scaffold to evaluate its gradient differentiation inductivity. A rat Achilles tendon defect model was established, and BMSCs seeded on scaffolds were implanted into the lesionsite. The tendon-bone lesionsite of calcaneus at 4w and 8w post-operation were obtained for gross observation, radiological evaluation, biomechanical and histological assessment.

Results

The hierarchical microstructures not only endowed the scaffold with gradual composition and mechanical properties for matching the regional biophysical characteristics of TBI but also exhibited gradient differentiation inductivity through providing regional microenvironment for cells. Moreover, the scaffold seeded with cells could effectively accelerate healing in rat Achilles tendon defects, attributable to its enhanced differentiation performance.

Conclusion

The hierarchical scaffolds simulating the structural, compositional, and cellular heterogeneity of natural TBI tissue performed therapeutic effects on promoting regeneration of TBI and enhancing the healing quality of Achilles tendon.

The translational potential of this article

The novel scaffold showed the great efficacy on tendon to bone healing by offering a structural and compositional microenvironment. The results meant that the hierarchical scaffold with BMSCs may have a great potential for clinical application.

Harnessing Macromolecular Chemistry to Design Hydrogel Micro- and Macro-Environments

Cell encapsulation within three-dimensional hydrogels is a promising approach to mimic tissues. However, true biomimicry of the intricate microenvironment, biophysical and biochemical gradients, and the macroscale hierarchical spatial organizations of native tissues is an unmet challenge within tissue engineering. This review provides an overview of the macromolecular chemistries that have been applied toward the design of cell-friendly hydrogels, as well as their application toward controlling biophysical and biochemical bulk and gradient properties of the microenvironment. Furthermore, biofabrication technologies provide the opportunity to simultaneously replicate macroscale features of native tissues. Biofabrication strategies are reviewed in detail with a particular focus on the compatibility of these strategies with the current macromolecular toolkit described for hydrogel design and the challenges associated with their clinical translation. This review identifies that the convergence of the ever-expanding macromolecular toolkit and technological advancements within the field of biofabrication, along with an improved biological understanding, represents a promising strategy toward the successful tissue regeneration.

Enthesis repair - State of play

The fibrocartilaginous enthesis is a highly specialised tissue interface that ensures a smooth mechanical transfer between tendon or ligament and bone through a fibrocartilage area. This tissue is prone to injury and often does not heal, even after surgical intervention. Enthesis augmentation approaches are challenging due to the complexity of the tissue that is characterised by the coexistence of a range of cellular and extracellular components, architectural features and mechanical properties within only hundreds of micrometres. Herein, we discuss enthesis repair and regeneration strategies, with particular focus on elegant interfacial and functionalised scaffold-based designs.

Full-thickness osteochondral defect repair using a biodegradable bilayered scaffold of porous zinc and chondroitin sulfate hydrogel

The regeneration of osteochondral tissue necessitates the re-establishment of a gradient owing to the unique characteristics and healing potential of the chondral and osseous phases. As the self-healing capacity of hyaline cartilage is limited, timely mechanical support during neo-cartilage formation is crucial to achieving optimal repair efficacy. In this study, we devised a biodegradable bilayered scaffold, comprising chondroitin sulfate (CS) hydrogel to regenerate chondral tissue and a porous pure zinc (Zn) scaffold for regeneration of the underlying bone as mechanical support for the cartilage layer. The photocured CS hydrogel possessed a compressive strength of 82 kPa, while the porous pure Zn scaffold exhibited a yield strength of 11 MPa and a stiffness of 0.8 GPa. Such mechanical properties are similar to values reported for cancellous bone. In vitro biological experiments demonstrated that the bilayered scaffold displayed favorable cytocompatibility and promoted chondrogenic and osteogenic differentiation of bone marrow stem cells. Upon implantation, the scaffold facilitated the simultaneous regeneration of bone and cartilage tissue in a porcine model, resulting in (i) a smoother cartilage surface, (ii) more hyaline-like cartilage, and (iii) a superior integration into the adjacent host tissue. Our bilayered scaffold exhibits significant potential for clinical application in osteochondral regeneration.

Multileveled Hierarchical Hydrogel with Continuous Biophysical and Biochemical Gradients for Enhanced Repair of Full-Thickness Osteochondral Defect

The repair of hierarchical osteochondral defect requires sophisticated gradient reestablishment; however, few strategies for continuous gradient casting consider the relevance to clinical practice regarding cell adaptability, multiple gradient elements, and precise gradient mirroring native tissue. Here, a hydrogel with continuous gradients in nano-hydroxyapatite (HA) content, mechanical, and magnetism is developed using synthesized superparamagnetic HA nanorods (MagHA) that easily respond to a brief magnetic field. To precisely reconstruct osteochondral tissue, the optimized gradient mode is calculated according to magnetic resonance imaging (MRI) of healthy rabbit knees. Then, MagHA are patterned to form continuous biophysical and biochemical gradients, consequently generating incremental HA, mechanical, and electromagnetic cues under an external magnetic stimulus. To make such depth-dependent biocues work, an adaptable hydrogel is developed to facilitate cell infiltration. Furthermore, this approach is applied in rabbit full-thickness osteochondral defects equipped with a local magnetic field. Surprisingly, this multileveled gradient composite hydrogel repairs osteochondral unit in a perfect heterogeneous feature, which mimics the gradual cartilage-to-subchondral transition. Collectively, this is the first study that combines an adaptable hydrogel with magneto-driven MagHA gradients to achieve promising outcomes in osteochondral regeneration.

Design and bioprinting for tissue interfaces

Tissue interfaces include complex gradient structures formed by transitioning of biochemical and mechanical properties in micro-scale. This characteristic allows the communication and synchronistic functioning of two adjacent but distinct tissues. It is particularly challenging to restore the function of these complex structures by transplantation of scaffolds exclusively produced by conventional tissue engineering methods. Three-dimensional (3D) bioprinting technology has opened an unprecedented approach for precise and graded patterning of chemical, biological and mechanical cues in a single construct mimicking natural tissue interfaces. This paper reviews and highlights biochemical and biomechanical design for 3D bioprinting of various tissue interfaces, including cartilage-bone, muscle-tendon, tendon/ligament-bone, skin, and neuro-vascular/muscular interfaces. Future directions and translational challenges are also provided at the end of the paper.

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.

Strain Gradient Programming in 3D Fibrous Hydrogels to Direct Graded Cell Alignment

Biological tissues experience various stretch gradients which act as mechanical signaling from the extracellular environment to cells. These mechanical stimuli are sensed by cells, triggering essential signaling cascades regulating cell migration, differentiation, and tissue remodeling. In most previous studies, a simple, uniform stretch to 2D elastic substrates has been applied to analyze the response of living cells. However, induction of nonuniform strains in controlled gradients, particularly in biomimetic 3D hydrogels, has proven challenging. In this study, 3D fibrin hydrogels of manipulated geometry are stretched by a silicone carrier to impose programmable strain gradients along different chosen axes. The resulting strain gradients are analyzed and compared to finite element simulations. Experimentally, the programmed strain gradients result in similar gradient patterns in fiber alignment within the gels. Additionally, temporal changes in the orientation of fibroblast cells embedded in the stretched fibrin gels correlate to the strain and fiber alignment gradients. The experimental and simulation data demonstrate the ability to custom-design mechanical gradients in 3D biological hydrogels and to control cell alignment patterns. It provides a new technology for mechanobiology and tissue engineering studies.

Junctional epithelium and hemidesmosomes: Tape and rivets for solving the "percutaneous device dilemma" in dental and other permanent implants

The percutaneous device dilemma describes etiological factors, centered around the disrupted epithelial tissue surrounding non-remodelable devices, that contribute to rampant percutaneous device infection. Natural percutaneous organs, in particular their extracellular matrix mediating the "device"/epithelium interface, serve as exquisite examples to inspire longer lasting long-term percutaneous device design. For example, the tooth's imperviousness to infection is mediated by the epithelium directly surrounding it, the junctional epithelium (JE). The hallmark feature of JE is formation of hemidesmosomes, cell/matrix adhesive structures that attach surrounding oral gingiva to the tooth's enamel through a basement membrane. Here, the authors survey the multifaceted functions of the JE, emphasizing the role of the matrix, with a particular focus on hemidesmosomes and their five main components. The authors highlight the known (and unknown) effects dental implant - as a model percutaneous device - placement has on JE regeneration and synthesize this information for application to other percutaneous devices. The authors conclude with a summary of bioengineering strategies aimed at solving the percutaneous device dilemma and invigorating greater collaboration between clinicians, bioengineers, and matrix biologists.

Tissue Engineering Cartilage with Deep Zone Cytoarchitecture by High-Resolution Acoustic Cell Patterning

The ultimate objective of tissue engineering is to fabricate artificial living constructs with a structural organization and function that faithfully resembles their native tissue counterparts. For example, the deep zone of articular cartilage possesses a distinctive anisotropic architecture with chondrocytes organized in aligned arrays ≈1-2 cells wide, features that are oriented parallel to surrounding extracellular matrix fibers and orthogonal to the underlying subchondral bone. Although there are major advances in fabricating custom tissue architectures, it remains a significant technical challenge to precisely recreate such fine cellular features in vitro. Here, it is shown that ultrasound standing waves can be used to remotely organize living chondrocytes into high-resolution anisotropic arrays, distributed throughout the full volume of agarose hydrogels. It is demonstrated that this cytoarchitecture is maintained throughout a five-week course of in vitro tissue engineering, producing hyaline cartilage with cellular and extracellular matrix organization analogous to the deep zone of native articular cartilage. It is anticipated that this acoustic cell patterning method will provide unprecedented opportunities to interrogate in vitro the contribution of chondrocyte organization to the development of aligned extracellular matrix fibers, and ultimately, the design of new mechanically anisotropic tissue grafts for articular cartilage regeneration.

Assessing the combined effect of surface topography and substrate rigidity in human bone marrow stem cell cultures

The combined effect of surface topography and substrate rigidity in stem cell cultures is still under-investigated, especially when biodegradable polymers are used. Herein, we assessed human bone marrow stem cell response on aliphatic polyester substrates as a function of anisotropic grooved topography and rigidity (7 and 12 kPa). Planar tissue culture plastic (TCP, 3 GPa) and aliphatic polyester substrates were used as controls. Cell morphology analysis revealed that grooved substrates caused nuclei orientation/alignment in the direction of the grooves. After 21 days in osteogenic and chondrogenic media, the 3 GPa TCP and the grooved 12 kPa substrate induced significantly higher calcium deposition and alkaline phosphatase (ALP) activity and glycosaminoglycan (GAG) deposition, respectively, than the other groups. After 14 days in tenogenic media, the 3 GPa TCP upregulated four and downregulated four genes; the planar 7 kPa substrate upregulated seven genes and downregulated one gene; and the grooved 12 kPa substrate upregulated seven genes and downregulated one gene. After 21 days in adipogenic media, the softest (7 kPa) substrates induced significantly higher oil droplet deposition than the other substrates and the grooved substrate induced significantly higher droplet deposition than the planar. Our data pave the way for more rational design of bioinspired constructs.

The tendon microenvironment: Engineered in vitro models to study cellular crosstalk

Tendinopathy is a multi-faceted pathology characterized by alterations in tendon microstructure, cellularity and collagen composition. Challenged by the possibility of regenerating pathological or ruptured tendons, the healing mechanisms of this tissue have been widely researched over the past decades. However, so far, most of the cellular players and processes influencing tendon repair remain unknown, which emphasizes the need for developing relevant in vitro models enabling to study the complex multicellular crosstalk occurring in tendon microenvironments. In this review, we critically discuss the insights on the interaction between tenocytes and the other tendon resident cells that have been devised through different types of existing in vitro models. Building on the generated knowledge, we stress the need for advanced models able to mimic the hierarchical architecture, cellularity and physiological signaling of tendon niche under dynamic culture conditions, along with the recreation of the integrated gradients of its tissue interfaces. In a forward-looking vision of the field, we discuss how the convergence of multiple bioengineering technologies can be leveraged as potential platforms to develop the next generation of relevant in vitro models that can contribute for a deeper fundamental knowledge to develop more effective treatments.

Texturing Hierarchical Tissues by Gradient Assembling of Microengineered Platelet-Lysates Activated Fibers

The heterogeneity of hierarchical tissues requires designing multipart engineered constructs as suitable tissue replacements. Herein, the incorporation of platelet lysate (PL) within an electrospun fiber core is proposed aiming for the fabrication of functionally graded 3D scaffolds for heterotypic tissues regeneration, such as tendon-to-bone interfaces. First, anisotropic yarns (A-Yarns) and isotropic threads with nanohydroxyapatite (I-Threads/PL@nHAp) are fabricated to recreate the tendon- and bone-microstructures and both incorporated with PL using emulsion electrospinning for a sustained and local delivery of growth factors, cytokines, and chemokines. Biological performance using human adipose-derived stem cells demonstrates that A-Yarns/PL induce a higher expression of scleraxis, a tenogenic-marker, while in I-Threads/PL@nHAp, higher alkaline phosphatase activity and matrix mineralization suggest an osteogenic commitment without the need for biochemical supplementation compared to controls. As a proof-of-concept, functional 3D gradient scaffolds are fabricated using a weaving technique, resulting in 3D textured hierarchical constructs with gradients in composition and topography. Additionally, the precise delivery of bioactive cues together with in situ biophysical features guide the commitment into a phenotypic gradient exhibiting chondrogenic and osteochondrogenic profiles in the interface of scaffolds. Overall, a promising patch solution for the regeneration of tendon-to-bone tissue interface through the fabrication of PL-functional 3D gradient constructs is demonstrated.

Identification of an Ultrathin Osteochondral Interface Tissue with Specific Nanostructure at the Human Knee Joint

Cartilage adheres to subchondral bone via a specific osteochondral interface tissue where forces are transferred from soft cartilage to hard bone without conferring fatigue damage over a lifetime of load cycles. However, the fine structure and mechanical properties of the osteochondral interface tissue remain unclear. Here, we identified an ultrathin ∼20-30 μm graded calcified region with two-layered micronano structures of osteochondral interface tissue in the human knee joint, which exhibited characteristic biomolecular compositions and complex nanocrystals assembly. Results from finite element simulations revealed that within this region, an exponential increase of modulus (3 orders of magnitude) was conducive to force transmission. Nanoscale heterogeneity in the hydroxyapatite, coupled with enrichment of elastic-responsive protein-titin, which is usually present in muscle, endowed the osteochondral tissue with excellent mechanical properties. Collectively, these results provide novel insights into the potential design for high-performance interface materials for osteochondral interface regeneration.

Microfluidic acoustic sawtooth metasurfaces for patterning and separation using traveling surface acoustic waves

We demonstrate a sawtooth-based metasurface approach for flexibly orienting acoustic fields in a microfluidic device driven by surface acoustic waves (SAW), where sub-wavelength channel features can be used to arbitrarily steer acoustic fringes in a microchannel. Compared to other acoustofluidic methods, only a single travelling wave is used, the fluidic pressure field is decoupled from the fluid domain's shape, and steerable pressure fields are a function of a simply constructed polydimethylsiloxane (PDMS) metasurface shape. Our results are relevant to microfluidic applications including the patterning, concentration, focusing, and separation of microparticles and cells.

Biomimetic Approaches for the Design and Fabrication of Bone-to-Soft Tissue Interfaces

Bone-to-soft tissue interfaces are responsible for transferring loads between tissues with significantly dissimilar material properties. The examples of connective soft tissues are ligaments, tendons, and cartilages. Such natural tissue interfaces have unique microstructural properties and characteristics which avoid the abrupt transitions between two tissues and prevent formation of stress concentration at their connections. Here, we review some of the important characteristics of these natural interfaces. The native bone-to-soft tissue interfaces consist of several hierarchical levels which are formed in a highly specialized anisotropic fashion and are composed of different types of heterogeneously distributed cells. The characteristics of a natural interface can rely on two main design principles, namely by changing the local microarchitectural features (e.g., complex cell arrangements, and introducing interlocking mechanisms at the interfaces through various geometrical designs) and changing the local chemical compositions (e.g., a smooth and gradual transition in the level of mineralization). Implementing such design principles appears to be a promising approach that can be used in the design, reconstruction, and regeneration of engineered biomimetic tissue interfaces. Furthermore, prominent fabrication techniques such as additive manufacturing (AM) including 3D printing and electrospinning can be used to ease these implementation processes. Biomimetic interfaces have several biological applications, for example, to create synthetic scaffolds for osteochondral tissue repair.

Multifaceted Design and Emerging Applications of Tissue Adhesives

Tissue adhesives can form appreciable adhesion with tissues and have found clinical use in a variety of medical settings such as wound closure, surgical sealants, regenerative medicine, and device attachment. The advantages of tissue adhesives include ease of implementation, rapid application, mitigation of tissue damage, and compatibility with minimally invasive procedures. The field of tissue adhesives is rapidly evolving, leading to tissue adhesives with superior mechanical properties and advanced functionality. Such adhesives enable new applications ranging from mobile health to cancer treatment. To provide guidelines for the rational design of tissue adhesives, here, existing strategies for tissue adhesives are synthesized into a multifaceted design, which comprises three design elements: the tissue, the adhesive surface, and the adhesive matrix. The mechanical, chemical, and biological considerations associated with each design element are reviewed. Throughout the report, the limitations of existing tissue adhesives and immediate opportunities for improvement are discussed. The recent progress of tissue adhesives in topical and implantable applications is highlighted, and then future directions toward next-generation tissue adhesives are outlined. The development of tissue adhesives will fuse disciplines and make broad impacts in engineering and medicine.

Transforming eukaryotic cell culture with macromolecular crowding

In multicellular organisms, the intracellular and extracellular spaces are considerably packed with a diverse range of macromolecular species. Yet, standard eukaryotic cell culture is performed in dilute, and deprived of macromolecules culture media, that barely imitate the density and complex macromolecular composition of tissues. Essentially, we drown cells in a sea of media and then expect them to perform physiologically. Herein, we argue the use of macromolecular crowding (MMC) in eukaryotic cell culture for regenerative medicine and drug discovery purposes.

Fabricating spatially functionalized 3D-printed scaffolds for osteochondral tissue engineering

Three-dimensional (3D) printing of biodegradable polymers has rapidly become a popular approach to create scaffolds for tissue engineering. This technique enables fabrication of complex architectures and layer-by-layer spatial control of multiple components with high resolution. The resulting scaffolds can also present distinct chemical groups or bioactive cues on the surface to guide cell behavior. However, surface functionalization often includes one or more post-fabrication processing steps, which typically produce biomaterials with homogeneously distributed chemistries that fail to mimic the biochemical organization found in native tissues. As an alternative, our laboratory developed a novel method that combines solvent-cast 3D printing with peptide-polymer conjugates to spatially present multiple biochemical cues in a single scaffold without requiring post-fabrication modification. Here, we describe a detailed, stepwise protocol to fabricate peptide-functionalized scaffolds and characterize their physical architecture and biochemical spatial organization. We used these 3D-printed scaffolds to direct human mesenchymal stem cell differentiation and osteochondral tissue formation by controlling the spatial presentation of cartilage-promoting and bone-promoting peptides. This protocol also describes how to seed scaffolds and evaluate matrix deposition driven by peptide organization.

Biomimetic strategies for tendon/ligament-to-bone interface regeneration

Tendon/ligament-to-bone healing poses a formidable clinical challenge due to the complex structure, composition, cell population and mechanics of the interface. With rapid advances in tissue engineering, a variety of strategies including advanced biomaterials, bioactive growth factors and multiple stem cell lineages have been developed to facilitate the healing of this tissue interface. Given the important role of structure-function relationship, the review begins with a brief description of enthesis structure and composition. Next, the biomimetic biomaterials including decellularized extracellular matrix scaffolds and synthetic-/natural-origin scaffolds are critically examined. Then, the key roles of the combination, concentration and location of various growth factors in biomimetic application are emphasized. After that, the various stem cell sources and culture systems are described. At last, we discuss unmet needs and existing challenges in the ideal strategies for tendon/ligament-to-bone regeneration and highlight emerging strategies in the field.

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.

Hierarchical and heterogeneous hydrogel system as a promising strategy for diversified interfacial tissue regeneration

A state-of-the-art review on the design and preparation of hierarchical and heterogeneous hydrogel systems for interfacial tissue regeneration.

Engineering next-generation bioinks with nanoparticles: moving from reinforcement fillers to multifunctional nanoelements

The application of additive manufacturing in the biomedical field has become a hot topic in the last decade owing to its potential to provide personalized solutions for patients. Different bioinks have been designed trying to obtain a unique concoction that addresses all the needs for tissue engineering and drug delivery purposes, among others. Despite the remarkable progress made, the development of suitable bioinks which combine printability, cytocompatibility, and biofunctionality is still a challenge. In this sense, the well-established synthetic and functionalization routes to prepare nanoparticles with different functionalities make them excellent candidates to be combined with polymeric systems in order to generate suitable multi-functional bioinks. In this review, we briefly discuss the most recent advances in the design of functional nanocomposite hydrogels considering their already evaluated or potential use as bioinks. The scientific development over the last few years is reviewed, focusing the discussion on the wide range of functionalities that can be incorporated into 3D bioprinted constructs through the addition of multifunctional nanoparticles in order to increase their regenerative potential in the field of tissue engineering.

Regeneration of Damaged Tendon-Bone Junctions (Entheses)-TAK1 as a Potential Node Factor

Musculoskeletal dysfunctions are highly prevalent due to increasing life expectancy. Consequently, novel solutions to optimize treatment of patients are required. The current major research focus is to develop innovative concepts for single tissues. However, interest is also emerging to generate applications for tissue transitions where highly divergent properties need to work together, as in bone-cartilage or bone-tendon transitions. Finding medical solutions for dysfunctions of such tissue transitions presents an added challenge, both in research and in clinics. This review aims to provide an overview of the anatomical structure of healthy adult entheses and their development during embryogenesis. Subsequently, important scientific progress in restoration of damaged entheses is presented. With respect to enthesis dysfunction, the review further focuses on inflammation. Although molecular, cellular and tissue mechanisms during inflammation are well understood, tissue regeneration in context of inflammation still presents an unmet clinical need and goes along with unresolved biological questions. Furthermore, this review gives particular attention to the potential role of a signaling mediator protein, transforming growth factor beta-activated kinase-1 (TAK1), which is at the node of regenerative and inflammatory signaling and is one example for a less regarded aspect and potential important link between tissue regeneration and inflammation.

Advances in the Fabrication of Biomaterials for Gradient Tissue Engineering

Natural tissues and organs exhibit an array of spatial gradients, from the polarized neural tube during embryonic development to the osteochondral interface present at articulating joints. The strong structure-function relationships in these heterogeneous tissues have sparked intensive research into the development of methods that can replicate physiological gradients in engineered tissues. In this Review, we consider different gradients present in natural tissues and discuss their critical importance in functional tissue engineering. Using this basis, we consolidate the existing fabrication methods into four categories: additive manufacturing, component redistribution, controlled phase changes, and postmodification. We have illustrated this with recent examples, highlighted prominent trends in the field, and outlined a set of criteria and perspectives for gradient fabrication.

Electrospun Polymers in Cartilage Engineering-State of Play

Articular cartilage defects remain a clinical challenge. Articular cartilage defects progress to osteoarthritis, which negatively (e.g., remarkable pain, decreased mobility, distress) affects millions of people worldwide and is associated with excessive healthcare costs. Surgical procedures and cell-based therapies have failed to deliver a functional therapy. To this end, tissue engineering therapies provide a promise to deliver a functional cartilage substitute. Among the various scaffold fabrication technologies available, electrospinning is continuously gaining pace, as it can produce nano- to micro- fibrous scaffolds that imitate architectural features of native extracellular matrix supramolecular assemblies and can deliver variable cell populations and bioactive molecules. Herein, we comprehensively review advancements and shortfalls of various electrospun scaffolds in cartilage engineering.