Composite Three‐Dimensional Woven Scaffolds with Interpenetrating Network Hydrogels to Create Functional Synthetic Articular Cartilage

The development of synthetic biomaterials that possess mechanical properties mimicking those of native tissues remains an important challenge to the field of materials. In particular, articular cartilage is a complex nonlinear, viscoelastic, and anisotropic material that exhibits a very low coefficient of friction, allowing it to withstand millions of cycles of joint loading over decades of wear. Here, a three‐dimensionally woven fiber scaffold that is infiltrated with an interpenetrating network hydrogel can build a functional biomaterial that provides the load‐bearing and tribological properties of native cartilage. An interpenetrating dual‐network “tough‐gel” consisting of alginate and polyacrylamide was infused into a porous three‐dimensionally woven poly(?‐caprolactone) fiber scaffold, providing a versatile fiber‐reinforced composite structure as a potential acellular or cell‐based replacement for cartilage repair.     

3D Printed Cartilage‐Like Tissue Constructs with Spatially Controlled Mechanical Properties

Developing biomimetic cartilaginous tissues that support locomotion while maintaining chondrogenic behavior is a major challenge in the tissue engineering field. Specifically, while locomotive forces demand tissues with strong mechanical properties, chondrogenesis requires a soft microenvironment. To address this challenge, 3D cartilage‐like tissue is fabricated using two biomaterials with different mechanical properties: a hard biomaterial to reflect the macromechanical properties of native cartilage, and a soft biomaterial to create a chondrogenic microenvironment. To this end, a bath composed of an interpenetrating polymer network (IPN) of polyethylene glycol (PEG) and alginate hydrogel (MPa order compressive modulus) is developed as an extracellular matrix (ECM) with self‐healing properties. Within this bath supplemented with thrombin, human mesenchymal stem cell (hMSC) spheroids embedded in fibrinogen are 3D bioprinted, creating a soft microenvironment composed of fibrin (kPa order compressive modulus) that simulate cartilage's pericellular matrix and allow a fast diffusion of nutrients. The bioprinted hMSC spheroids present high viability and chondrogenic‐like behavior without adversely affecting the macromechanical properties of the tissue. Therefore, the ability to locally bioprint a soft and cell stimulating biomaterial inside of a mechanically robust hydrogel is demonstrated, thereby uncoupling the micro‐ and macromechanical properties of the 3D printed tissues such as cartilage.     

Glycocalyx‐Like Hydrogel Coatings for Small Diameter Vascular Grafts

Novel biological vascular conduits, such as decellularized tissue engineered vascular grafts (TEVGs) are hindered by high thrombogenicity. To mimic the antithrombogenic surface of native vessels with a continuous glycosaminoglycan layer that is present on endothelial cells (ECs), a hyaluronic acid (HA) modified surface is established, to effectively shield blood platelets from collagen‐triggered activation. Using the amine groups present on 4 mm diameter decellularized TEVGs, a continuous HA hydrogel coating is built via a bifunctional thiol‐reactive cross‐linker, thereby avoiding nonspecific collagen matrix cross‐linking. The HA hydrogel layer recreates a luminal wall, “hiding” exposed collagen from the bloodstream. In vitro blood tests show that adhered platelets, fibrinogen absorption, and fibrin formation on HA‐coated decellularized TEVGs are significantly lower than on uncoated decellularized TEVGs. The HA surface also inhibits macrophage adhesion in vitro. HA‐coated decellularized syngeneic rat aortae (≈1.5 mm diameter), and TEVGs in rat and canine models, respectively, are protected from aggressive thrombus formation, and preserve normal blood flow. Re‐endothelialization is also observed. HA‐coated TEVGs may be an off‐the‐shelf small‐diameter vascular graft with dual benefits: antithrombogenic protection and promotion of endothelium.     

Processed Tissue–Derived Extracellular Matrices: Tailored Platforms Empowering Diverse Therapeutic Applications

Tissue‐derived decellularized extracellular matrices (dECM) have gradually become the gold standard of scaffolds for tissue engineering, owing to their close mirroring of the intricate composition, architecture, and topology of the native extracellular matrix (ECM). Intriguingly, further manipulation of these acellular tissues through various processing techniques has been demonstrated to be an effective strategy to control their characteristics and impart them with ample valuable new traits, thereby expanding their applicability to a significantly wider spectrum of research and translational applications. Herein, state‐of‐the‐art processed dECM platforms and their potential applications are focused on. The ECM characteristics that make it so appealing for tissue engineering are presented, followed by a concise discussion on the main considerations for choosing a dECM source for such applications. The key methodologies for dECM processing, including hydrogel production, bioprinting, electrospinning, and production of porous scaffolds, microcarriers, and microcapsules, as well as their inherent advantages and challenges, are introduced. To demonstrate the use of processed dECM platforms for tissue engineering, selected in vivo and in vitro applications recently developed utilizing these platforms are highlighted. Finally, concluding remarks and a prospective outlook for future developments and improvements in the field of processed dECM‐based devices are given.     

Self‐Assembling Peptides as Cell‐Interactive Scaffolds

Cell and tissue engineering therapies for regenerative medicine as well as cell‐based assays require an understanding of the interactions between cells with the surrounding microenvironment at the nanoscale. Engineering a cell‐interactive scaffold therefore entails control over the nanostructure of the biomaterial. Peptides that are able to self‐assemble into 3D scaffolds have emerged as interesting biomaterials for directing cell behavior, with desirable properties such as the capability of tuning the nanostructure by modulating the amino acid composition. Here, an overview of the development of self‐assembling peptide hydrogels as functional cell scaffolds is presented, highlighting recent work on incorporating features such as bioactive ligands, growth factor delivery, controlled degradation, and formulation into microgels for defined cell microenvironments.     

Electrospun Extracellular Matrix: Paving the Way to Tailor‐Made Natural Scaffolds for Cardiac Tissue Regeneration

Biomimetic scaffolds generally aim at structurally and compositionally imitating native tissue, thus providing a supportive microenvironment to the transplanted or recruited cells in the tissue. Native decellularized porcine extracellular matrix (ECM) is becoming the ultimate bioactive material for the regeneration of different organs. Particularly for cardiac regeneration, ECM is studied as a patch and injectable scaffolds, which improve cardiac function, yet lack reproducibility and are difficult to control or fine‐tune for the desired properties, like most natural materials. Seeking to harness the natural advantages of ECM in a reproducible, scalable, and controllable scaffold, for the first time, a matrix that is produced from whole decellularized porcine cardiac ECM using electrospinning technology, is developed. This unique electrospun cardiac ECM mat preserves the composition of ECM, self‐assembles into the same microstructure of cardiac ECM ,and ,above all, preserves key cardiac mechanical properties. It supports cell growth and function, and demonstrates biocompatibility in vitro and in vivo. Importantly, this work reveals the great potential of electrospun ECM‐based platforms for a wide span of biomedical applications, thus offering the possibility to produce complex natural materials as tailor‐made, well‐defined structures.     

Ultrarapid Engineering of Biomimetic Materials and Tissues: Fabrication of Nano‐ and Microstructures by Plastic Compression

Currently, the concept of engineered tissues depends on the ability of cultured cells to fabricate new tissue around a scaffold. This is inherently slow and expensive and has had limited success so far. We report here a new process for the cell‐independent, controlled engineering of biomimetic scaffolds by rapid removal of fluid from hyperhydrated collagen gel (or other) constructs, using plastic compression (PC). PC fabrication produces dense, cellular, mechanically strong native collagen structures with controllable nano‐ and microscale biomimetic structures. The huge‐scale shrinkage (> 100‐fold) provides the ability to introduce controllable mechanical properties, microlayering, and embossed interface topography without cell participation, but with high cell viability. Critically, this takes minutes rather than the conventional days and weeks. The rapidity and biomimetic potential of the PC fabrication process at the mesoscale opens a new route for the production of biomaterials and patient‐customized tissues. It also represents a new concept in ‘engineering’ tissues.     

Additive Manufacturing Approaches for Hydroxyapatite‐Reinforced Composites

Additive manufacturing (AM) techniques have gained interest in the tissue engineering field, thanks to their versatility and unique possibilities of producing constructs with complex macroscopic geometries and defined patterns. Recently, composite materials—namely, heterogeneous biomaterials identified as continuous phase (matrix) and reinforcement (filler)—have been proposed as inks that can be processed by AM to obtain scaffolds with improved biomimetic and bioactive properties. Significant efforts have been dedicated to hydroxyapatite (HA)‐reinforced composites, especially targeting bone tissue engineering, thanks to the chemical similarities of HA with respect to mineral components of native mineralized tissues. Herein, applications of AM techniques to process HA‐reinforced composites and biocomposites for the production of scaffolds with biological matrices, including cellular tissues, are reviewed. The primary outcomes of recent investigations in terms of morphological, structural, and in vitro and in vivo biological properties of the materials are discussed. The approaches based on the nature of the matrices employed to embed the HA reinforcements and produce the tissue substitutes are classified, and a critical discussion is provided on the presented state of the art as well as the future perspectives, to offer a comprehensive picture of the strategies investigated as well as challenges in this emerging field of materiomics.     

Protein Corona Influences Cell–Biomaterial Interactions in Nanostructured Tissue Engineering Scaffolds

Biomaterials are extensively used to restore damaged tissues, in the forms of implants (e.g., tissue engineered scaffolds) or biomedical devices (e.g., pacemakers). Once in contact with the physiological environment, nanostructured biomaterials undergo modifications as a result of endogenous proteins binding to their surface. The formation of this macromolecular coating complex, known as “protein corona,” onto the surface of nanoparticles and its effect on cell–particle interactions are currently under intense investigation. In striking contrast, protein corona constructs within nanostructured porous tissue engineering scaffolds remain poorly characterized. As organismal systems are highly dynamic, it is conceivable that the formation of distinct protein corona on implanted scaffolds might itself modulate cell–extracellular matrix interactions. Here, it is reported that corona complexes formed onto the fibrils of engineered collagen scaffolds display specific, distinct, and reproducible compositions that are a signature of the tissue microenvironment as well as being indicative of the subject's health condition. Protein corona formed on collagen matrices modulated cellular secretome in a context‐specific manner ex vivo, demonstrating their role in regulating scaffold–cellular interactions. Together, these findings underscore the importance of custom‐designing personalized nanostructured biomaterials, according to the biological milieu and disease state. The use of protein corona as in situ biosensor of temporal and local biomarkers is proposed.     

Tissue Beads: Tissue‐Specific Extracellular Matrix Microbeads to Potentiate Reprogrammed Cell‐Based Therapy

Microbeads have been utilized as efficient cell culture carriers and injectable scaffolds for cell transplantation. However, various polymers currently used to generate microbeads have limited applicability due to loss of biological functions and tissue‐specific effects. Here, a tissue bead platform is reported that can provide a tissue‐specific microenvironment to facilitate cell culture and potentiate cell therapy. Using a flow‐focusing microfluidic device, uniform‐sized tissue microbeads are fabricated with extracellular matrix (ECM) from various decellularized tissues. The tissue microbeads are tested for tissue‐specific encapsulation of induced hepatic (iHep), induced cardiac (iCar), and induced myogenic (iMyo) cells, which are directly reprogrammed from mouse primary fibroblasts. Tissue‐specific microbeads significantly enhanced the viability, lineage‐specific maturation, and functionality of each type of reprogrammed cell, as compared to functionality when using conventional microbeads from a single ECM component (collagen). Finally, tissue microbeads are confirmed to mediate the successful in vivo engraftment of reprogrammed cells (iHep and iMyo) after transplantation, potentiating cell therapy and promoting functional tissue regeneration in tissue defective animal models. The study suggests that the use of a decellularized tissue matrix combined with a microfluidic technique can be employed to produce tissue‐specific ECM microbeads with increased versatility and efficacy for reprogrammed cell‐based therapy.     

Functionally Graded,Bone‐ and Tendon‐Like Polyurethane for Rotator Cuff Repair

Critical considerations in engineering biomaterials for rotator cuff repair include bone‐tendon‐like mechanical properties to support physiological loading and biophysicochemical attributes that stabilize the repair site over the long‐term. In this study, UV‐crosslinkable polyurethane based on quadrol (Q), hexamethylene diisocyante (H), and methacrylic anhydride (M; QHM polymers), which are free of solvent, catalyst, and photoinitiator, is developed. Mechanical characterization studies demonstrate that QHM polymers possesses phototunable bone‐ and tendon‐like tensile and compressive properties (12–74 MPa tensile strength, 0.6–2.7 GPa tensile modulus, 58–121 MPa compressive strength, and 1.5–3.0 GPa compressive modulus), including the capability to withstand 10 000 cycles of physiological tensile loading and reduce stress concentrations via stiffness gradients. Biophysicochemical studies demonstrate that QHM polymers have clinically favorable attributes vital to rotator cuff repair stability, including slow degradation profiles (5–30% mass loss after 8 weeks) with little‐to‐no cytotoxicity in vitro, exceptional suture retention ex vivo (2.79–3.56‐fold less suture migration relative to a clinically available graft), and competent tensile properties (similar ultimate load but higher normalized tensile stiffness relative to a clinically available graft) as well as good biocompatibility for augmenting rat supraspinatus tendon repair in vivo. This work demonstrates functionally graded, bone‐tendon‐like biomaterials for interfacial tissue engineering.     

Design of an “Active Defense” System as Drug Carriers for Cancer Therapy

A novel intelligent “active defense” system that can specially respond to cancerous tissues for drug release was designed and prepared. The “active defense” system consists of a biodegradable dextran microgel core cross‐linked by a Schiff's base and a surrounding layer formed by Layer‐by‐Layer (LbL) assembly. The loading and release of macromolecular model drug, dex‐FITC, as well as antineoplastic drug, DOX, was investigated. The in vitro cell inhibition and drug release behavior of the drug delivery system were studied and the results showed that the entrapped drug could be explosively released from the microcapsules and thereafter taken up by cancer cells upon the trigger of the acidic environment around tumor tissues.     

Cell‐Tethered Ligands Modulate Bone Remodeling by Osteoblasts and Osteoclasts

The goals of the present study are to establish an in vitro co‐culture model of osteoblast and osteoclast function and to quantify the resulting bone remodeling. The bone is tissue engineered using well‐defined silk protein biomaterials in 2D and 3D formats in combination with human cells. Parathyroid hormone (PTH) and glucose‐dependent insulinotropic peptide (GIP) are selected because of their roles in bone remodeling for expression in tethered format on human mesenchymal stem cells (hMSCs). The cell‐modified biomaterial surfaces are reconstructed from scanning electron microscopy images into 3D models for quantitative measurement of surface characteristics. Increased calcium deposition and surface roughness are found in 3D surface models of silk protein films remodeled by co‐cultures containing tethered PTH, and decreased surface roughness is found for the films remodeled by tethered GIP co‐cultures. Increased surface roughness is not found in monocultures of hMSCs expressing tethered PTH, suggesting that osteoclast‐osteoblast interactions in the presence of PTH signaling are responsible for the increased mineralization. These data point towards the design of in vitro bone models in which osteoblast‐osteoclast interactions are mimicked for a better understanding of bone remodeling.     

Biomimetic Elastomeric Bioactive Siloxane‐Based Hybrid Nanofibrous Scaffolds with miRNA Activation: A Joint Physico‐Chemical‐Biological Strategy for Promoting Bone Regeneration

Rapid and efficient disease‐induced or critical‐size bone regeneration remains a challenge in tissue engineering due to the lack of highly bioactive biomaterial scaffolds. Physical structures such as nanostructures, chemical components such as silicon elements, and biological factors such as genes have shown positive effects on bone regeneration. Herein, a bioactive photoluminescent elastomeric silicate‐based nanofibrous scaffold with sustained miRNA release is reported for promoting bone regeneration based on a joint physico‐chemical‐biological strategy. Bioactive nanofibrous scaffolds are fabricated by cospinning poly (ε‐caprolactone) (PCL), elastomeric poly (citrates‐siloxane) (PCS), and bioactive osteogenic miRNA nanocomplexes (denoted PPM nanofibrous scaffolds). The PPM scaffolds possess uniform nanostructures, significantly enhanced tensile stress (≈15 MPa) and modulus (≈32 MPa), improved hydrophilicity (30–60°), controlled biodegradation, and strong blue fluorescence. Bioactive miRNA complexes are efficiently loaded into the nanofibrous matrix and exhibit long‐term release for up to 70 h. The PPM scaffolds significantly promote the adhesion, proliferation, and osteoblast differentiation of bone marrow stem cells in vitro and enhanced rat cranial defect restoration (12 weeks) in vivo. This work reports an attractive joint physico‐chemical‐biological strategy for the design of novel cell/protein‐free bioactive scaffolds for synergistic tissue regeneration.     

Silk Hydrogels as Soft Substrates for Neural Tissue Engineering

There is great need for soft biomaterials that match the stiffness of human tissues for tissue engineering and regeneration. Hydrogels are frequently employed for extracellular matrix functionalization and to provide appropriate mechanical cues. It is challenging, however, to achieve structural integrity and retain bioactive molecules in hydrogels for complex tissue formation that may take months to develop. This work aims to investigate mechanical and biochemical characteristics of silk hydrogels for soft tissue engineering, specifically for the nervous system. The stiffness of 1 to 8% silk hydrogels, measured by atomic force microscopy, is 4 to 33 kPa. The structural integrity of silk gels is maintained throughout embryonic chick dorsal root ganglion (cDRG) explant culture over 4 days whereas fibrin and collagen gels decrease in mass over time. Neurite extension of cDRGs cultured on 2 and 4% silk hydrogels exhibit greater growth than softer or stiffer gels. Silk hydrogels release <5% of neurotrophin‐3 (NT‐3) over 2 weeks and 11‐day old gels show maintenance of growth factor bioactivity. Finally, fibronectin‐ and NT‐3‐functionalized silk gels elicit increased axonal bundling suggesting their use in bridging nerve injuries. These results support silk hydrogels as soft and sustainable biomaterials for neural tissue engineering.     

Recent Advances in Smart Biomaterials for the Detection and Treatment of Autoimmune Diseases

Autoimmune diseases are a group of debilitating illnesses that are often idiopathic in nature. The steady rise in the prevalence of these conditions warrants new approaches for diagnosis and treatment. Stimuli‐responsive biomaterials also known as “smart,” “intelligent,” or “recognitive” biomaterials are widely studied for their applications in drug delivery, biosensing, and tissue engineering due to their ability to produce thermal, optical, chemical, or structural changes upon interacting with the biological environment. Studies within the last decade that harness the recognitive capabilities of these biomaterials toward the development of novel detection and treatment options for autoimmune diseases are critically analyzed.     

Mechanically Activated Microcapsules for “On‐Demand” Drug Delivery in Dynamically Loaded Musculoskeletal Tissues

Delivery of biofactors in a precise and controlled fashion remains a clinical challenge. Stimuli‐responsive delivery systems can facilitate “on‐demand” release of therapeutics in response to a variety of physiologic triggering mechanisms (e.g., pH, temperature). However, few systems to date have taken advantage of mechanical inputs from the microenvironment to initiate drug release. Here, mechanically activated microcapsules (MAMCs) are designed to deliver therapeutics in response to the mechanically loaded environment of regenerating musculoskeletal tissues, with the ultimate goal of furthering tissue repair. To establish a suite of microcapsules with different thresholds for mechanoactivation, MAMC physical dimensions and composition are first manipulated, and their mechano‐response under both direct 2D compression and in 3D matrices mimicking the extracellular matrix properties and dynamic loading environment of regenerating tissue, is evaluated. To demonstrate the feasibility of this delivery system, an engineered cartilage model is used to test the efficacy of mechanically instigated release of transforming growth factor‐β3 on the chondrogenesis of mesenchymal stem cells. These data establish a novel platform by which to tune the release of therapeutics and/or regenerative factors based on the physiologic mechanical loading environment and will find widespread application in the repair and regeneration of musculoskeletal tissues.     

Creating a Living Hyaline Cartilage Graft Free from Non‐Cartilaginous Constituents: An Intermediate Role of a Biomaterial Scaffold

A novel living hyaline cartilage graft (LhCG) with controllable dimensions and free of non‐cartilaginous constituents for articular regeneration is developed. As a living graft for regenerative medicine, LhCG is purely living tissue based and truly scaffold‐free. The process of neotissue formation in LhCG is mediated by an interim biomaterial‐based novel scaffolding system. This design highlights a philosophy of using biomaterials in engineered regenerative medicine as a transient guiding facility rather than a permanent part of substitute. The fabrication is designed and practiced in a continuous and integrated process, which attributes to its simplicity in operation. Because of the intrinsic non‐cell‐adhesive property of hydrogel scaffolds, articular chondrocytes’ phenotype is always preserved throughout the whole procedure, which has been tested and approved both in vitro and in vivo. In situ grafting trials in a rabbit model showcase high success rates in both cartilage repair and graft‐host integration. Beyond cartilage repair, this LhCG model may provide a living‐tissue‐based open platform or niche for multi‐tissue regenerations.     

Charge‐Tunable Autoclaved Silk‐Tropoelastin Protein Alloys That Control Neuron Cell Responses

Tunable protein composites are important for constructing extracellular matrix mimics of human nerve tissues with control of charge, structural, and mechanical properties. Molecular interaction mechanisms between silk fibroin protein and recombinant human tropoelastin, based on charge, are utilized to generate a new group of multifunctional protein alloys with different net charges. These new biomaterials are then utilized as a biomaterial platform to control neuron cell response. With a +38 net charge in water, tropoelastin molecules provide extraordinary elasticity and selective interactions with cell surface integrins. In contrast, negatively charged silk fibroin protein (net charge ?36) provides remarkable toughness and stiffness with morphologic stability in material formats via autoclaving‐induced beta‐sheet crystal physical crosslinks. The combination of these properties in alloy format extends the versatility of both structural proteins, providing a new biocompatible, biodegradable, and charge‐tunable biomaterial platform for neural repair. The data point to these protein alloys as an alternative to commonly used charged synthetic polymers, particularly with regard to the versatility of material formats (e.g., gels, sponges, films, fibers). The results also provide a practical example of physically designed protein materials with control of net charge to direct biological outcomes, in this case for neuronal tissue engineering.     

Silk Biomaterials with Vascularization Capacity

Functional vascularization is critical for the clinical regeneration of complex tissues such as kidney, liver, or bone. The immobilization or delivery of growth factors has been explored to improve vascularization capacity of tissue‐engineered constructs; however, the use of growth factors has inherent problems such as the loss of signaling capability and the risk of complications including immunological responses and cancer. Here, a new method of preparing water‐insoluble silk protein scaffolds with vascularization capacity using an all‐aqueous process is reported. Acid is added temporally to tune the self‐assembly of silk in the lyophilization process, resulting in water‐insoluble scaffold formation directly. These biomaterials are mainly noncrystalline, offering improved cell proliferation than previously reported silk materials. These systems also have an appropriate softer mechanical property that could provide physical cues to promote cell differentiation into endothelial cells, and enhance neovascularization and tissue ingrowth in vivo without the addition of growth factors. Therefore, silk‐based degradable scaffolds represent an exciting biomaterial option, with vascularization capacity for soft tissue engineering and regenerative medicine.