Microfluidic 3D Printing Responsive Scaffolds with Biomimetic Enrichment Channels for Bone Regeneration

Tissue-engineered scaffolds have been extensively explored for treating bone defects; however, slow and insufficient vascularization throughout the scaffolds remains a key challenge for further application. Herein, a versatile microfluidic 3D printing strategy to fabricate black phosphorus (BP) incorporated fibrous scaffolds with photothermal responsive channels for improving vascularization and bone regeneration is proposed. The thermal channeled scaffolds display reversible shrinkage and swelling behavior controlled by near-infrared irradiation, which facilitates the penetration of suspended cells into the scaffold channels and promotes the prevascularization. Furthermore, the embedded BP nanosheets exhibit intrinsic properties for in situ biomineralization and improve in vitro cell proliferation and osteogenic differentiation. Following transplantation in vivo, these channels also promote host vessel infiltration deep into the scaffolds and effectively accelerate the healing process of bone defects. Thus, it is believed that these near-infrared responsive channeled scaffolds are promising candidates for tissue/vascular ingrowth in diverse tissue engineering applications.     

Concentration Gradients in Material Sciences: Methods to Design and Biomedical Applications

Concentration gradients are at the basis of many natural phenomena and hold great importance in human and plant biology. This basic phenomenon has inspired many material scientists to confer novel and original properties to materials with numerous biomedical and pharmaceutical applications such as drug discovery, controlled release of drugs, tissue engineering, orthopedic implants and time–temperature indicators, amongst others. While the generation of gradients in solution is well described, their incorporation in solid materials remains challenging. This study reviews concepts and methods reported to generate gradients in respective solutions and (at the surface or in) solid materials like polymer scaffolds and matrices. This article also discusses how innovative materials possessing such gradients may be applied in biomedicine and pharmacy.     

A Single Component Conducting Polymer Hydrogel as a Scaffold for Tissue Engineering

Conducting polymers (CPs) have exciting potential as scaffolds for tissue engineering, typically applied in regenerative medicine applications. In particular, the electrical properties of CPs has been shown to enhance nerve and muscle cell growth and regeneration. Hydrogels are particularly suitable candidates as scaffolds for tissue engineering because of their hydrated nature, their biocompatibility, and their tissue‐like mechanical properties. This study reports the development of the first single component CP hydrogel that is shown to combine both electro‐properties and hydrogel characteristics. Poly(3‐thiopheneacetic acid) hydrogels were fabricated by covalently crosslinking the polymer with 1,1′‐carbonyldiimidazole (CDI). Their swelling behavior was assessed and shown to display remarkable swelling capabilities (swelling ratios up to 850%). The mechanical properties of the networks were characterized as a function of the crosslinking density and were found to be comparable to those of muscle tissue. Hydrogels were found to be electroactive and conductive at physiological pH. Fibroblast and myoblast cells cultured on the hydrogel substrates were shown to adhere and proliferate. This is the first time that the potential of a single component CP hydrogel has been demonstrated for cell growth, opening the way for the development of new tissue engineering scaffolds.     

Micropatterned Polypyrrole: A Combination of Electrical and Topographical Characteristics for the Stimulation of Cells

Electrically conducting polymers such as polypyrrole (PPy) are important biomaterials in neural engineering applications, including neural probes, nerve conduits, and scaffolds for tissue and nerve regeneration. Surface modification of these polymers can introduce other valuable characteristics for neural interfacing in addition to electrical conductivity, such as topographical features and chemical bioactivity. Here, the patterning of PPy to create topographical cues for cells is reported. In particular, 1 and 2 μm wide PPy microchannels are fabricated using electron‐beam (e‐beam) lithography and electropolymerization. A systematic analysis of parameters controlling PPy micropatterning is performed, and finds that microchannel depth, roughness, and morphology are highly dependent on the e‐beam writing current, polymerization current, PPy/dopant concentrations, and the polymerization time. Embryonic hippocampal neurons cultured on patterned PPy polarize (i.e., defined an axon) faster on this modified material, with a twofold increase in the number of cells with axons compared to cells cultured on unmodified PPy. These topographical features also have an effect on axon orientation but do not have a significant effect on overall axon length. This is the first investigation that studies controlled PPy patterning with small dimensions (i.e., less than 5 μm) for biological applications, which demonstrates the relevance of expanding microelectronic materials and techniques to the biomedical field.     

Self‐Supporting Graphene Hydrogel Film as an Experimental Platform to Evaluate the Potential of Graphene for Bone Regeneration

Graphene, a two dimensional carbonaceous material possessing a range of extraordinary properties, is considered promising for biomedical applications. Here, a simple form of graphene‐based bulk material–self‐supporting graphene hydrogel (SGH) film is used as a suitable platform to study the intrinsic properties of graphene both in vitro and in vivo. The free‐standing film show good cell adhesion, spreading, and proliferation. Films are implanted into subcutaneous sites of rats, and produce minimal fibrous capsule formation, and mild host tissue response in vivo. New blood vessel formation is also seen. The films swell and cracked in vivo, indicating the beginning of degradation. Of particular interest is that the film alone is found to be able to stimulate osteogenic differentiation of stem cells, without additional inducer, both in vitro and in vivo. Thus, this SGH film appears to be highly biocompatible and osteoinductive, demonstrating graphene's potential for bone regenerative medicine.     

Biomimetic Ultralight,Highly Porous,Shape‐Adjustable,and Biocompatible 3D Graphene Minerals via Incorporation of Self‐Assembled Peptide Nanosheets

Hybrid nanomaterials with tailored functions, consisting of self‐assembled peptides, are intensively applied in nanotechnology, tissue engineering, and biomedical applications due to their unique structures and properties. Herein, a peptide‐mediated biomimetic strategy is adopted to create the multifunctional 3D graphene foam (GF)‐based hybrid minerals. First, 2D peptide nanosheets (PNSs), obtained by self‐assembling a motif‐specific peptide molecule (LLVFGAKMLPHHGA), are expected to exhibit biofunctionality, such as the biomimetic mineralization of hydroxyapatite (HA) minerals. Subsequently, the noncovalent conjugation of PNSs onto GF support is utilized to form 3D GF‐PNSs hybrid scaffolds, which are suitable for the growth of HA minerals. The fabricated biomimetic 3D GF‐PNSs‐HA minerals exhibit adjustable shape, superlow weight (0.017 g cm^?3), high porosity (5.17 m² g^?1), and excellent biocompatibility, proving potential applications in both bone tissue engineering and biomedical engineering. To the best of the authors' knowledge, it is the first time to combine 2D PNSs and GF to fabricate 3D organic–inorganic hybrid scaffold. Further development of these hybrid GF‐PNSs scaffolds can potentially lead to materials used as matrices for drug delivery or bone tissue engineering as proven via successful 3D scaffold formation exhibiting interconnected pore‐size structures suitable for vascularization and medium transport.     

Piezoelectric Nanotopography Induced Neuron‐Like Differentiation of Stem Cells

The biophysical characteristics of the extracellular matrix, such as nanotopography and bioelectricity, have a profound influence on cell proliferation, adhesion, differentiation, etc. Recognition of the function of a certain biophysical cue and fabrication of biomaterial scaffolds with specific properties would have important implications and significant applications in tissue engineering. Herein, nanotopographic and piezoelectric biomaterials are fabricated and the combination effect of and individual contribution to proliferation, adhesion, and neuron‐like differentiation of rat bone marrow‐derived mesenchymal stem cells (rbMSCs) are clarified via nanotopography and piezoelectricity. Piezoelectric polyvinylidene fluoride with nanostripe array structures is fabricated, which can generate a surface piezoelectric potential up to millivolt by cell movement and traction. The results reveal a more favorable effect on neuron‐like differentiation of rbMSCs from the combination of piezoelectricity and nanotopography rather than nanotopography alone, whereas nanotopography can increase cellular adhesion. This research provides a new insight into designing biomaterials for the potential application in neural tissue engineering.     

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.     

Conductive Scaffolds for Cardiac and Neuronal Tissue Engineering: Governing Factors and Mechanisms

Tissue engineering is a promising therapeutic approach in medicine, targeting the replacement of a diseased tissue with a healthy one grown within an artificial scaffold. Due to the high prevalence of cardiac and brain‐related ailments that involve some necrosis of tissue, cardiac and neuronal tissue engineering are intensely studied fields in regenerative medicine. A growing trend in the use of conductive scaffolds for the growth of these tissues has been witnessed recently. While the results are irrefutable, the mechanism of how an electrically conducting scaffold interacts with an electroactive tissue remains has remained elusive. An up‐to‐date summary of all work done in the field is reported, with a special focus on the specific contribution of the conductive scaffold on the performance of the formed tissue. The cell–scaffold electronic interface is then explored from an electrical engineering perspective. The electronic configuration of the system and the mechanisms and governing factors controlling the ability of a conductive scaffold to support cardiac and neuronal tissues are discussed. Using several simulations, the required conductivity of the scaffold in order for it to be suitable for tissue engineering—which also depends on the nature of the charge carriers—is also discussed.     

Advanced Collagen‐Based Biomaterials for Regenerative Biomedicine

In recent decades, collagen is one of the most versatile biomaterials used in biomedical applications, mostly due to its biomimetic and structural composition in the extracellular matrix (ECM). Several attempts are proposed for designing innovative collagen‐based biomaterials and applying them in tissue regeneration. The regeneration of different tissues is prompted by different types and diverse physical forms of collagen‐based biomaterials prepared by various methods. Based on such concepts, the source, structure, and classification of collagen are briefly introduced in this review. Here, the commonly used physical forms and modification methods of collagen‐based biomaterials are reviewed, including hydrogels, scaffolds, and microspheres, followed by their applications in the regeneration of tissues and organs. Important proof‐of‐concept examples are described to demonstrate the outcomes on material characteristics, cellular reactions, and tissue regeneration. A concise assessment of the limitations that still exist and the developing trends in the future of collagen‐based biomaterials are put forward.     

Development of Citrate‐Based Dual‐Imaging Enabled Biodegradable Electroactive Polymers

Increasing occurrences of degenerative diseases, defective tissues, and severe cancers heighten the importance of advanced biomedical treatments, which in turn enhance the need for improved biomaterials with versatile theranostic functionalities yet using minimal design complexity. Leveraging the advantages of citrate chemistry, a multifunctional citrate‐based biomaterial platform is developed with both imaging and therapeutic capabilities utilizing a facile and efficient one‐pot synthesis. The resulting aniline tetramer doped biodegradable photoluminescent polymers (BPLPATs) not only possess programmable degradation profiles (<1 to > 6 months) and mechanical strengths (≈20 MPa to >400 MPa), but also present a combination of intrinsic fluorescence, photoacoustic (PA), and electrical conductivity properties. BPLPAT nanoparticles are able to label cells for fluorescence imaging and perform deep tissue detection with PA imaging. Coupled with significant photothermal performance, BPLPAT nanoparticles demonstrate great potential for thermal treatment and in vivo real‐time detection of cancers. The results on BPLPAT scaffolds demonstrate 3D high‐spatial‐resolution deep tissue PA imaging (23 mm), as well as promote growth and differentiation of PC‐12 nerve cells. It is envisioned that the biodegradable dual‐imaging‐enabled electroactive citrate‐based biomaterial platform will expand the currently available theranostic material systems and open new avenues for diversified biomedical and biological applications via the demonstrated multifunctionality.     

Recent Advances in Electrospun Nanofibrous Scaffolds for Cardiac Tissue Engineering

Cardiovascular diseases remain the leading cause of human mortality worldwide. Some severe symptoms, including myocardial infarction and heart failure, are difficult to heal spontaneously or under systematic treatment due to the limited regenerative capacity of the native myocardium. Cardiac tissue engineering has emerged as a practical strategy to culture functional cardiac tissues and relieve the disorder in myocardium when implanted. In cardiac tissue engineering, the design of a scaffold is closely relevant to the function of the regenerated cardiac tissues. Nanofibrous materials fabricated by electrospinning have been developed as desirable scaffolds for tissue engineering applications because of the biomimicking structure of protein fibers in native extra cellular matrix. The versatilities of electrospinning on the polymer component, the fiber structure, and the functionalization with bioactive molecules have made the fabrication of nanofibrous scaffolds with suitable mechanical strength and biological properties for cardiac tissue engineering feasible. Here, an overview of recent advances in various electrospun scaffolds for engineering cardiac tissues, including the design of advanced electrospun scaffolds and the performance of the scaffolds in functional cardiac tissue regeneration, is provided with the aim to offer guidance in the innovation of novel electrospun scaffolds and methods for improving their potential for cardiac tissue engineering applications.     

Black‐Phosphorus‐Incorporated Hydrogel as a Conductive and Biodegradable Platform for Enhancement of the Neural Differentiation of Mesenchymal Stem Cells

Conductive hydrogel scaffolds have important applications for electroactive tissue repairs. However, the development of conductive hydrogel scaffolds tends to incorporate nonbiodegradable conductive nanomaterials that will remain in the human body as foreign matters. Herein, a biodegradable conductive hybrid hydrogel is demonstrated based on the integration of black phosphorus (BP) nanosheets into the hydrogel matrix. To address the challenge of applying BP nanosheets in tissue engineering due to its intrinsic instability, a polydopamine (PDA) modification method is developed to improve the stability. Moreover, PDA modification also enhances interfacial bonding between pristine BP nanosheets and the hydrogel matrix. The incorporation of polydopamine‐modified black phosphorous (BP@PDA) nanosheets into the gelatin methacryloyl (GelMA) hydrogels significantly enhances the electrical conductivity of the hydrogels and improves the cell migration of mesenchymal stem cells (MSCs) within the 3D scaffolds. On the basis of the gene expression and protein level assessments, the BP@PDA‐incorporated GelMA scaffold can significantly promote the differentiation of MSCs into neural‐like cells under the synergistic electrical stimulation. This strategy of integrating biodegradable conductive BP nanomaterials within a biocompatible hydrogel provides a new insight into the design of biomaterials for broad applications in tissue engineering of electroactive tissues, such as neural, cardiac, and skeletal muscle tissues.     

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.     

Polymer Fiber Scaffolds for Bone and Cartilage Tissue Engineering

Successful regeneration of weight‐bearing bone defects and critical‐sized cartilage defects remains a major challenge in clinical orthopedics. In the past decades, biodegradable polymer materials with biomimetic chemical and physical properties have been rapidly developed as ideal candidates for bone and cartilage tissue engineering scaffolds. Due to their unique advantages over other materials of high specific‐surface areas, suitable mechanical strength, and tailorable characteristics, scaffolds made of polymer fibers have been increasingly used for the repair of bone and cartilage defects. This Review summarizes the preparation and compositions of polymer fibers, as well as their characteristics. More importantly, the applications of polymer fiber scaffolds with well‐designed structures or unique properties in bone, cartilage, and osteochondral tissue engineering have been comprehensively highlighted. On the whole, such a comprehensive summary affords constructive suggestions for the development of polymer fiber scaffolds in bone and cartilage tissue engineering.     

Sequence‐Optimized Peptide Nanofibers as Growth Stimulators for Regeneration of Peripheral Neurons

There is an urgent need for biomaterials that support tissue healing, particularly neuronal regeneration. In a medium throughput screen novel self‐assembling peptide (SAP) sequences that form fibrils and stimulated nerve fiber growth of peripheral nervous system (PNS)‐derived neurons are identified. Based on the peptide sequences and fibril morphologies and by applying rational data‐mining, important structural parameters stimulating neuronal activity are elucidated. Three SAPs (SAP^1e, SAP^2e, and SAP^5c) enhance adhesion and growth of PNS neurons. These SAPs form 2D and 3D matrices that serve as bioactive scaffolds stimulating cell adhesion and growth. The newly discovered SAPs also support the growth of CNS neurons and glia cells. Subsequently, the potential of SAPs to enhance PNS regeneration in vivo is analyzed. For this, the facial nerve driving whisker movement in mice is injured. Notably, SAPs persist for up to 3 weeks in the injury site indicating highly adhesive properties and stability. After SAP administration, more motor neurons incorporating markers for successive regeneration are observed. Recovery of whisker movement is elevated in SAP‐injected mice. In summary, short peptides that form fibrils are identified and the adhesion, growth, and regeneration of neurons have been efficiently enhanced without the necessity to attach hormones or growth factors.     

Ascidian‐Inspired Fast‐Forming Hydrogel System for Versatile Biomedical Applications: Pyrogallol Chemistry for Dual Modes of Crosslinking Mechanism

Exploitation of unique biochemical and biophysical properties of marine organisms has led to the development of functional biomaterials for various biomedical applications. Recently, ascidians have received great attention, owing to their extraordinary properties such as strong underwater adhesion and rapid self‐regeneration. Specific polypeptides containing 3,4,5‐trihydroxyphenylalanine (TOPA) in the blood cells of ascidians are associated with such intrinsic properties generated through complex oxidative processes. In this study, a bioinspired hydrogel platform is developed, demonstrating versatile applicability for tissue engineering and drug delivery, by conjugating pyrogallol (PG) moiety resembling ascidian TOPA to hyaluronic acid (HA). The HA–PG conjugate can be rapidly crosslinked by dual modes of oxidative mechanisms using an oxidant or pH control, resulting in hydrogels with different mechanical and physical characteristics. The versatile utility of HA–PG hydrogels formed via different crosslinking mechanisms is tested for different biomedical platforms, including microparticles for sustained drug delivery and tissue adhesive for noninvasive cell transplantation. With extraordinarily fast and different routes of PG oxidation, ascidian‐inspired HA–PG hydrogel system may provide a promising biomaterial platform for a wide range of biomedical applications.     

Interactions between Primary Neurons and Graphene Films with Different Structure and Electrical Conductivity

Graphene-based materials represent a useful tool for the realization of novel neural interfaces. Several studies have demonstrated the biocompatibility of graphene-based supports, but the biological interactions between graphene and neurons still pose open questions. In this work, the influence of graphene films with different characteristics on the growth and maturation of primary cortical neurons is investigated. Graphene films are grown by chemical vapor deposition progressively lowering the temperature range from 1070 to 650 °C to change the lattice structure and corresponding electrical conductivity. Two graphene-based films with different electrical properties are selected and used as substrate for growing primary cortical neurons: i) highly crystalline and conductive (grown at 1070 °C) and ii) highly disordered and 140-times less conductive (grown at 790 °C). Electron and fluorescence microscopy imaging reveal an excellent neuronal viability and the development of a mature, structured, and excitable network onto both substrates, regardless of their microstructure and electrical conductivity. The results underline that high electrical conductivity by itself is not fundamental for graphene-based neuronal interfaces, while other physico–chemical characteristics, including the atomic structure, should be also considered in the design of functional, bio-friendly templates. This finding widens the spectrum of carbon-based materials suitable for neuroscience applications.     

Direct 3D Printing of Ultralight Graphene Oxide Aerogel Microlattices

Graphene aerogel microlattices (GAMs) hold great prospects for many multifunctional applications due to their low density, high porosity, designed lattice structures, good elasticity, and tunable electrical conductivity. Previous 3D printing approaches to fabricate GAMs require either high content of additives or complex processes, limiting their wide applications. Here, a facile ion‐induced gelation method is demonstrated to directly print GAMs from graphene oxide (GO) based ink. With trace addition of Ca²⁺ ions as gelators, aqueous GO sol converts to printable gel ink. Self‐standing 3D structures with programmable microlattices are directly printed just in air at room temperature. The rich hierarchical pores and high electrical conductivity of GAMs bring admirable capacitive performance for supercapacitors. The gravimetric capacitance (C_s) of GAMs is 213 F g^?1 at 0.5 A g^?1 and 183 F g^?1 at 100 A g^?1, and retains over 90% after 50 000 cycles. The facile, direct 3D printing of neat graphene oxide can promote wide applications of GAMs from energy storage to tissue engineering scaffolds.     

Magnetic Hydrogels and Their Potential Biomedical Applications

Hydrogels find widespread applications in biomedical engineering due to their hydrated environment and tunable properties (e.g., mechanical, chemical, biocompatible) similar to the native extracellular matrix (ECM). However, challenges still exist regarding utilizing hydrogels in applications such as engineering 3D tissue constructs and active targeting in drug delivery, due to the lack of controllability, actuation, and quick‐response properties. Recently, magnetic hydrogels have emerged as a novel biocomposite for their active response properties and extended applications. In this review, the state‐of‐the‐art methods for magnetic hydrogel preparation are presented and their advantages and drawbacks in applications are discussed. The applications of magnetic hydrogels in biomedical engineering are also reviewed, including tissue engineering, drug delivery and release, enzyme immobilization, cancer therapy, and soft actuators. Concluding remarks and perspectives for the future development of magnetic hydrogels are addressed.