Solid freeform fabrication of designer scaffolds of hyaluronic acid for nerve tissue engineering

The role of chondroitin sulphate as a potential biomaterial for hepatic tissue regeneration: A comprehensive review

Chondroitin sulphate is an anionic hetero-polysaccharide, having numerous structural affinities for building the bio-active components. In addition to biodegradable/biocompatible activities, chondroitin sulphate also possesses anti-coagulant/anti-thrombogenic, anti-inflammatory, anti-oxidant as well as anti-tumor activities. Chondroitin sulphate has an inherited affinity for glycosylation enzymes and receptors, which are overexpressed over degenerated cells and organelles. Because of this affinity, chondroitin sulphate is nominated as an active cellular/subcellular targeted biological macromolecule to assist in site-specific delivery. Chondroitin sulphate is mainly considered a promising biomaterial for drug targeting and tissue engineering due to its specific physicochemical, mechanical, bio-degradation, and biological characteristics. In this review, the fundamental applications of chondroitin sulphate in hepatic tissue engineering are discussed. Chondroitin sulphate along with mesenchymal stem cells (MSCs) based scaffold and hydrogels for biopharmaceuticals' delivery in hepatic tissue engineering are critically discussed. In addition, the manuscript also describes leading features and markers involved in hepatic damage, and the potential role of chondroitin sulphate-based delivery systems in hepatic tissue engineering.

Bioprinted research models of urological malignancy

Urological malignancy (UM) is among the leading threats to health care worldwide. Recent years have seen much investment in fundamental UM research, including mechanistic investigation, early diagnosis, immunotherapy, and nanomedicine. However, the results are not fully satisfactory. Bioprinted research models (BRMs) with programmed spatial structures and functions can serve as powerful research tools and are likely to disrupt traditional UM research paradigms. Herein, a comprehensive review of BRMs of UM is presented. It begins with a brief introduction and comparison of existing UM research models, emphasizing the advantages of BRMs, such as modeling real tissues and organs. Six kinds of mainstream bioprinting techniques used to fabricate such BRMs are summarized with examples. Thereafter, research advances in the applications of UM BRMs, such as culturing tumor spheroids and organoids, modeling cancer metastasis, mimicking the tumor microenvironment, constructing organ chips for drug screening, and isolating circulating tumor cells, are comprehensively discussed. At the end of this review, current challenges and future development directions of BRMs and UM are highlighted from the perspective of interdisciplinary science.

3D Bioprinting Highly Elastic PEG-PCL-DA Hydrogel for Soft Tissue Fabrication and Biomechanical Stimulation

3-D bioprinting is a promising technology to fabricate custom geometries for tissue engineering. However, most bioprintable hydrogels are weak and fragile, difficult to handle and cannot mimetic the mechanical behaviors of the native soft elastic tissues. We have developed a visible light crosslinked, single-network, elastic and biocompatible hydrogel system based on an acrylated triblock copolymer of poly(ethylene glycol) PEG and polycaprolactone (PCL) (PEG-PCL-DA). To enable its application in bioprinting of soft tissues, we have modified the hydrogel system on its printability and biodegradability. Furthermore, we hypothesize that this elastic material can better transmit pulsatile forces to cells, leading to enhanced cellular response under mechanical stimulation. This central hypothesis was tested using vascular conduits with smooth muscle cells (SMCs) cultured under pulsatile forces in a custom-made bioreactor. The results showed that vascular conduits made of PEG-PCL-DA hydrogel faithfully recapitulate the rapid stretch and recoil under the pulsatile pressure from 1 to 3 Hz frequency, which induced a contractile SMC phenotype, consistently upregulated the core contractile transcription factors. In summary, our work demonstrates the potential of elastic hydrogel for 3D bioprinting of soft tissues by fine tuning the printability, biodegradability, while possess robust elastic property suitable for manual handling and biomechanical stimulation.

Nanotechnology Innovations in Pediatric Cardiology and Cardiovascular Medicine: A Comprehensive Review

(1) Background: Nanomedicine, incorporating various nanoparticles and nanomaterials, offers significant potential in medical practice. Its clinical adoption, however, faces challenges like safety concerns, regulatory hurdles, and biocompatibility issues. Despite these, recent advancements have led to the approval of many nanotechnology-based products, including those for pediatric use. (2) Methods: Our approach included reviewing clinical, preclinical, and animal studies, as well as literature reviews from the past two decades and ongoing trials. (3) Results: Nanotechnology has introduced innovative solutions in cardiovascular care, particularly in managing myocardial ischemia. Key developments include drug-eluting stents, nitric oxide-releasing coatings, and the use of magnetic nanoparticles in cardiomyocyte transplantation. These advancements are pivotal for early detection and treatment. In cardiovascular imaging, nanotechnology enables noninvasive assessments. In pediatric cardiology, it holds promise in assisting the development of biological conduits, synthetic valves, and bioartificial grafts for congenital heart defects, and offers new treatments for conditions like dilated cardiomyopathy and pulmonary hypertension. (4) Conclusions: Nanomedicine presents groundbreaking solutions for cardiovascular diseases in both adults and children. It has the potential to transform cardiac care, from enhancing myocardial ischemia treatment and imaging techniques to addressing congenital heart issues. Further research and guideline development are crucial for optimizing its clinical application and revolutionizing patient care.

Repair and regeneration of peripheral nerve injuries that ablate branch points

The peripheral nervous system has an extensive branching organization, and peripheral nerve injuries that ablate branch points present a complex challenge for clinical repair. Ablations of linear segments of the PNS have been extensively studied and routinely treated with autografts, acellular nerve allografts, conduits, wraps, and nerve transfers. In contrast, segmental-loss peripheral nerve injuries, in which one or more branch points are ablated so that there are three or more nerve endings, present additional complications that have not been rigorously studied or documented. This review discusses: (1) the branched anatomy of the peripheral nervous system, (2) case reports describing how peripheral nerve injuries with branched ablations have been surgically managed, (3) factors known to influence regeneration through branched nerve structures, (4) techniques and models of branched peripheral nerve injuries in animal models, and (5) conclusions regarding outcome measures and studies needed to improve understanding of regeneration through ablated branched structures of the peripheral nervous system.

Immunomodulatory nano-preparations for rheumatoid arthritis

Rheumatoid arthritis (RA) is a systemic autoimmune disease (AD) caused by the aberrant attack of the immune system on its own joint tissues. Genetic and environmental factors are the main reasons of immune system impairment and high incidence of RA. Although there are medications on the market that lessen disease activity, there is no known cure for RA, and patients are at risk in varying degrees of systemic immunosuppression. By transporting (encapsulating or surface binding) RA-related self-antigens, nucleic acids, immunomodulators, or cytokines, tolerogenic nanoparticles-also known as immunomodulatory nano-preparations-have the potential to gently regulate local immune responses and ultimately induce antigen-specific immune tolerance. We review the recent advances in immunomodulatory nano-preparations for delivering self-antigen or self-antigen plus immunomodulator, simulating apoptotic cell avatars in vivo, acting as artificial antigen-presenting cells, and based on scaffolds and gels, to provide a reference for developing new immunotherapies for RA.

Fabrication of Multi-Channel Nerve Guidance Conduits Containing Schwann Cells Based on Multi-Material 3D Bioprinting

Nerve guidance conduits (NGCs) are an essential solution for peripheral nerve repair and regeneration in tissue engineering and medicine. However, the ability of current NGCs is limited to repairing longer nerve gap (i.e., >20 mm) because it cannot meet the following two conditions simultaneously: (1) directional guidance of the axial high-density channels and (2) regenerative stimulation of the extracellular matrix secreted by Schwann cells (SCs). Therefore, we propose a multi-material 3D bioprinting process to fabricate multi-channel nerve guide conduits (MNGCs) containing SCs. In the article, cell-laden methacrylate gelatin (GelMA) was used as the bulk material of MNGCs. To improve the printing accuracy of the axial channels and the survival rate of SCs, we systematically optimized the printing temperature parameter based on hydrogel printability analysis. The multi-material bioprinting technology was used to realize the alternate printing of supporting gelatin and cell-laden GelMA. Then, the high-accuracy channels were fabricated through the UV cross-linking of GelMA and the dissolving technique of gelatin. The SCs distributed around the channels with a high survival rate, and the cell survival rate maintained above 90%. In general, the study on multi-material 3D printing was carried out from the fabricating technology and material analysis, which will provide a potential solution for the fabrication of MNGCs containing SCs.

Zein nanospheres assisting inorganic and organic drug combination to overcome stent implantation-induced thrombosis and infection

The complication of stent implantation is the biggest obstacle to the success of its clinical application. In this study, we developed a combination way of 3D printing and the coating technique for preparation of functional polyurethane stents against stent implantation-induced thrombosis and postoperative infection. SEM, XPS, static water contact angle, and XRD demonstrated that the functional polyurethane stent had a 37 μm-thickness membrane composed of zein nanospheres (250-350 nm). Meanwhile, ZnO nanoparticles were encapsulated in zein nanospheres while heparin was adsorbed on the surface, causing 97.1 ± 6.4 % release of heparin in 120 min (first-order kinetic model) and 62.7 ± 5.6 % release of Zn²⁺ in 9 days (Korsmeyer-Peppas model). The mechanical analysis revealed that the functional polyurethane stents had about 8.61 MPa and 2.5 MPa tensile strength and bending strength, respectively. The in vitro biological analysis showed that the functional polyurethane stents had good EA.hy926 cells compatibility (97.9 ± 3.8 %), anti-coagulation response (comparable plasma protein, platelet adhesion and suppressed clotting) and sustained antibacterial activities by comparison with the bare polyurethane stent. The preliminary evaluation by rabbit ex vivo carotid artery intervention experiment demonstrated that the functional polyurethane stents could maintain blood circulation under the continuous stresses of blood flow. Meanwhile, the detailed data from the simulated implant infection experiment in vivo showed the functional polyurethane stents could effectively reduce microbial infection by 3-6 times lower and improve fibrosis and macrophage infiltration.

High cell density and high-resolution 3D bioprinting for fabricating vascularized tissues

Three-dimensional (3D) bioprinting techniques have emerged as the most popular methods to fabricate 3D-engineered tissues; however, there are challenges in simultaneously satisfying the requirements of high cell density (HCD), high cell viability, and fine fabrication resolution. In particular, bioprinting resolution of digital light processing-based 3D bioprinting suffers with increasing bioink cell density due to light scattering. We developed a novel approach to mitigate this scattering-induced deterioration of bioprinting resolution. The inclusion of iodixanol in the bioink enables a 10-fold reduction in light scattering and a substantial improvement in fabrication resolution for bioinks with an HCD. Fifty-micrometer fabrication resolution was achieved for a bioink with 0.1 billion per milliliter cell density. To showcase the potential application in tissue/organ 3D bioprinting, HCD thick tissues with fine vascular networks were fabricated. The tissues were viable in a perfusion culture system, with endothelialization and angiogenesis observed after 14 days of culture.

Solid-state NMR methods for the characterization of bioconjugations and protein-material interactions

Protein solid-state NMR has evolved dramatically over the last two decades, with the development of new hardware and sample preparation methodologies. This technique is now ripe for complex applications, among which one can count bioconjugation, protein chemistry and functional biomaterials. In this review, we provide our account on this aspect of protein solid-state NMR.

Advances of 3D Printing in Vascularized Organ Construction

In the past several decades, three-dimensional (3D) printing has provided some viable tissues and organs for repairing or replacing damaged tissues and organs. However, the construction of sufficient vascular networks in a bioartificial organ has proven to be challenging. To make a fully functional bioartificial organ with a branched vascular network that can substitute its natural counterparts, various studies have been performed to surmount the limitations. Significant progress has been achieved in 3D printing of vascularized liver, heart, bone, and pancreas. It is expected that this technology can be used more widely in other bioartificial organ manufacturing. In this review, we summarize the specific applications of 3D printing vascularized organs through several rapid prototyping technologies. The limitations and future directions are also discussed.

Next Evolution in Organ-Scale Biofabrication: Bioresin Design for Rapid High-Resolution Vat Polymerization

The field of bioprinting has made significant advancements in recent years and allowed for the precise deposition of biomaterials and cells. However, within this field lies a major challenge, which is developing high resolution constructs, with complex architectures. In an effort to overcome these challenges a biofabrication technique known as vat polymerization is being increasingly investigated due to its high fabrication accuracy and control of resolution (µm scale). Despite the progress made in developing hydrogel precursors for bioprinting techniques, such as extrusion-based bioprinting, there is a major lack in developing hydrogel precursor bioresins for vat polymerization. This is due to the specific unique properties and characteristics required for vat polymerization, from lithography to the latest volumetric printing. This is of major concern as the shortage of bioresins available has a significant impact on progressing this technology and exploring its full potential, including speed, resolution, and scale. Therefore, this review discusses the key requirements that need to be addressed in successfully developing a bioresin. The influence of monomer architecture and bioresin composition on printability is described, along with key fundamental parameters that can be altered to increase printing accuracy. Finally, recent advancements in bioresins are discussed together with future directions.

3D Printed Personalized Nerve Guide Conduits for Precision Repair of Peripheral Nerve Defects

The treatment of peripheral nerve defects has always been one of the most challenging clinical practices in neurosurgery. Currently, nerve autograft is the preferred treatment modality for peripheral nerve defects, while the therapy is constantly plagued by the limited donor, loss of donor function, formation of neuroma, nerve distortion or dislocation, and nerve diameter mismatch. To address these clinical issues, the emerged nerve guide conduits (NGCs) are expected to offer effective platforms to repair peripheral nerve defects, especially those with large or complex topological structures. Up to now, numerous technologies are developed for preparing diverse NGCs, such as solvent casting, gas foaming, phase separation, freeze-drying, melt molding, electrospinning, and three-dimensional (3D) printing. 3D printing shows great potential and advantages because it can quickly and accurately manufacture the required NGCs from various natural and synthetic materials. This review introduces the application of personalized 3D printed NGCs for the precision repair of peripheral nerve defects and predicts their future directions.

From 3D printing to 3D bioprinting: the material properties of polymeric material and its derived bioink for achieving tissue specific architectures

The application of 3D printing technologies fields for biological tissues, organs, and cells in the context of medical and biotechnology applications requires a significant amount of innovation in a narrow printability range. 3D bioprinting is one such way of addressing critical design challenges in tissue engineering. In a more general sense, 3D printing has become essential in customized implant designing, faithful reproduction of microenvironmental niches, sustainable development of implants, in the capacity to address issues of effective cellular integration, and long-term stability of the cellular constructs in tissue engineering. This review covers various aspects of 3D bioprinting, describes the current state-of-the-art solutions for all aforementioned critical issues, and includes various illustrative representations of technologies supporting the development of phases of 3D bioprinting. It also demonstrates several bio-inks and their properties crucial for being used for 3D printing applications. The review focus on bringing together different examples and current trends in tissue engineering applications, including bone, cartilage, muscles, neuron, skin, esophagus, trachea, tympanic membrane, cornea, blood vessel, immune system, and tumor models utilizing 3D printing technology and to provide an outlook of the future potentials and barriers.

A review on 3D printing functional brain model

Modern neuroscience increasingly relies on 3D models to study neural circuitry, nerve regeneration, and neural disease. Several different biofabrication approaches have been explored to create 3D neural tissue model structures. Among them, 3D bioprinting has shown to have great potential to emerge as a high-throughput/high precision biofabrication strategy that can address the growing need for 3D neural models. Here, we have reviewed the design principles for neural tissue engineering. The main challenge to adapt printing technologies for biofabrication of neural tissue models is the development of neural bioink, i.e., a biomaterial with printability and gelation properties and also suitable for neural tissue culture. This review shines light on a vast range of biomaterials as well as the fundamentals of 3D neural tissue printing. Also, advances in 3D bioprinting technologies are reviewed especially for bioprinted neural models. Finally, the techniques used to evaluate the fabricated 2D and 3D neural models are discussed and compared in terms of feasibility and functionality.

Evaluation of the Higher Order Structure of Biotherapeutics Embedded in Hydrogels for Bioprinting and Drug Release

Biocompatible hydrogels for tissue regeneration/replacement and drug release with specific architectures can be obtained by three-dimensional bioprinting techniques. The preservation of the higher order structure of the proteins embedded in the hydrogels as drugs or modulators is critical for their biological activity. Solution nuclear magnetic resonance (NMR) experiments are currently used to investigate the higher order structure of biotherapeutics in comparability, similarity, and stability studies. However, the size of pores in the gel, protein-matrix interactions, and the size of the embedded proteins often prevent the use of this methodology. The recent advancements of solid-state NMR allow for the comparison of the higher order structure of the matrix-embedded and free isotopically enriched proteins, allowing for the evaluation of the functionality of the material in several steps of hydrogel development. Moreover, the structural information at atomic detail on the matrix-protein interactions paves the way for a structure-based design of these biomaterials.

3D Bioprinting of Neural Tissues

The human nervous system is a remarkably complex physiological network that is inherently challenging to study because of obstacles to acquiring primary samples. Animal models offer powerful alternatives to study nervous system development, diseases, and regenerative processes, however, they are unable to address some species-specific features of the human nervous system. In vitro models of the human nervous system have expanded in prevalence and sophistication, but still require further advances to better recapitulate microenvironmental and cellular features. The field of neural tissue engineering (TE) is rapidly adopting new technologies that enable scientists to precisely control in vitro culture conditions and to better model nervous system formation, function, and repair. 3D bioprinting is one of the major TE technologies that utilizes biocompatible hydrogels to create precisely patterned scaffolds, designed to enhance cellular responses. This review focuses on the applications of 3D bioprinting in the field of neural TE. Important design parameters are considered when bioprinting neural stem cells are discussed. The emergence of various bioprinted in vitro platforms are also reviewed for developmental and disease modeling and drug screening applications within the central and peripheral nervous systems, as well as their use as implants for in vivo regenerative therapies.

Biomaterials for Neural Tissue Engineering

The therapy of neural nerve injuries that involve the disruption of axonal pathways or axonal tracts has taken a new dimension with the development of tissue engineering techniques. When peripheral nerve injury (PNI), spinal cord injury (SCI), traumatic brain injury (TBI), or neurodegenerative disease occur, the intricate architecture undergoes alterations leading to growth inhibition and loss of guidance through large distance. To improve the limitations of purely cell-based therapies, the neural tissue engineering philosophy has emerged. Efforts are being made to produce an ideal scaffold based on synthetic and natural polymers that match the exact biological and mechanical properties of the tissue. Furthermore, through combining several components (biomaterials, cells, molecules), axonal regrowth is facilitated to obtain a functional recovery of the neural nerve diseases. The main objective of this review is to investigate the recent approaches and applications of neural tissue engineering approaches.

Complex 3D bioprinting methods

3D bioprinting technology is evolving in complexity to enable human-scale, high-resolution, and multi-cellular constructs to better mimic the native tissue microenvironment. The ultimate goal is to achieve necessary complexity in the bioprinting process to biomanufacture fully-functional tissues and organs to address organ shortage and lack of patient-specific disease models. In this Review, we presented an in-depth overview of complex 3D bioprinting approaches including evolution of complex bioprinting, from simple gel-casting approach to multi-material bioprinting to omnidirectional bioprinting approaches, and emerging bioprinting approaches, including 4D bioprinting and in situ bioprinting technologies.

Biomaterials and strategies for repairing spinal cord lesions

Spinal cord injury (SCI) causes intractable disease and leads to inevitable physical, financial, and psychological burdens on patients and their families. SCI is commonly divided into primary and secondary injury. Primary injury occurs upon direct impact to the spinal cord, which leads to cell necrosis, axon disruption, and vascular loss. This triggers pathophysiological secondary injury, which has several phases: acute, subacute, intermediate, and chronic. These phases are dependent on post-injury time and pathophysiology and have various causes, such as the infiltration of inflammatory cells and release of cytokines that can act as a barrier to neural regeneration. Another unique feature of SCI is the glial scar produced from the reactive proliferation of astrocytes, which acts as a barrier to axonal regeneration. Interdisciplinary research is investigating the use of biomaterials and tissue-engineered fabrication to overcome SCI. In this review, we discuss representative biomaterials, including natural and synthetic polymers and nanomaterials. In addition, we describe several strategies to repair spinal cord injuries, such as fabrication and the delivery of therapeutic biocomponents. These biomaterials and strategies may offer beneficial information to enhance the repair of spinal cord lesions.

Hydrogels: The Next Generation Body Materials for Microfluidic Chips?

The integration of microfluidics with biomedical research is confronted with considerable limitations due to its body materials. With high content of water, hydrogels own superior biocompatibility and degradability. Can hydrogels become another material choice for the construction of microfluidic chips, particularly biofluidics? The present review aims to systematically establish the concept of hydrogel-based microfluidic chips (HMCs) and address three main concerns: i) why choosing hydrogels? ii) how to fabricate HMCs?, and iii) in which fields to apply HMCs? It is envisioned that hydrogels may be used increasingly as substitute for traditional materials and gradually act as the body material for microfluidic chips. The modifications of conventional process are highlighted to overcome issues arising from the incompatibility between the construction methods and hydrogel materials. Specifically targeting at the "soft and wet" hydrogels, an efficient flowchart of "i) high resolution template printing; ii) damage-free demolding; iii) twice-crosslinking bonding" is proposed. Accordingly, a broader microfluidic chip concept is proposed in terms of form and function. Potential biomedical applications of HMCs are discussed. This review also highlights the challenges arising from the material replacement, as well as the future directions of the proposed concept. Finally, the authors' viewpoints and perspectives for this emerging field are discussed.

Additive Manufacturing of Nerve Guidance Conduits for Regeneration of Injured Peripheral Nerves

As a common and frequent clinical disease, peripheral nerve defect has caused a serious social burden, which is characterized by poor curative effect, long course of treatment and high cost. Nerve autografting is first-line treatment of peripheral nerve injuries (PNIs) but can result in loss of function of the donor site, neuroma formation, and prolonged operative time. Nerve guidance conduit (NGC) serves as the most promising alternative to autologous transplantation, but its production process is complicated and it is difficult to effectively combine growth factors and bioactive substances. In recent years, additive manufacturing of NGCs has effectively solved the above problems due to its simple and efficient manufacturing method, and it can be used as the carrier of bioactive substances. This review examines recent advances in additive manufacture of NGCs for PNIs as well as insight into how these approaches could be improved in future studies.

Hyaluronic acid containing scaffolds ameliorate stem cell function for tissue repair and regeneration

Recent evidence based studies have proposed hyaluronic acid (HA) as an emerging biopolymer for various tissue engineering application. Meanwhile, stem cells (SCs) have also gained immense popularity for their tissue regenerative capacity. Thus, combining HA and stem cells for tissue engineering application have shown to foster tissue repair and regeneration process. HA possesses the ability to interact with SCs via cellular surface receptors along with the capacity to elicit the process of differentiation. The influence of HA on stem cells has been widely investigated in cartilage and bone repair but their properties of reducing inflammation has also been explored in various other tissue repair processes. In this review, we have provided an insight to the effect of crosslinked and non-crosslinked HA on various stem cells. Further, HA based scaffolds combined with stem cells have shown to have a synergistic effect in the regeneration capacity. Also, various chemically modified HA and biomolecules conjugated HA as a suitable carrier or matrix for stem cells delivery and the effect of HA in fine tuning the stem cells function is discussed.

Direct 3D bioprinting of cardiac micro-tissues mimicking native myocardium

The heart possesses a complex three-dimensional (3D) laminar myofiber organization; however, because engineering physiologically relevant 3D tissues remains a technical challenge, the effects of cardiomyocyte alignment on excitation-contraction coupling, shortening and force development have not been systematically studied. Cellular shape and orientations in 3D can be controlled by engineering scaffold microstructures and encapsulating cells near these geometric cues. Here, we show that a novel method of cell encapsulation in 3D methacrylated gelatin (GelMA) scaffolds patterned via Microscale Continuous Optical Printing (μCOP) can rapidly micropattern neonatal mouse ventricular cardiomyocytes (NMVCMs) in photocrosslinkable hydrogels. Encapsulated cardiomyocytes preferentially align with the engineered microarchitecture and can display morphology and myofibril alignment phenotypic of myocardium in vivo. Utilizing the μCOP system, an asymmetric, multi-material, cantilever-based scaffold was directly printed, so that the force produced by the microtissue was transmitted onto a single deformable pillar. Aligned 3D encapsulated NMVCM scaffolds produced nearly 2 times the force compared to aligned 2D seeded samples. To further highlight the flexibility of μCOP, NMVCMs were encapsulated in several patterns to compare the effects of varying degrees of alignment on tissue displacement and synchronicity. Well aligned myofiber cultured patterns generated 4-10 times the contractile force of less anisotropically patterned constructs. Finally, normalized fluo-4 fluorescence of NMVCM-encapsulated structures showed characteristic calcium transient waveforms that increased in magnitude and rate of decline during treatment with 100 nM isoproterenol. This novel instrumented 3D cardiac microtissue serves as a physiologically relevant in vitro model system with great potential for use in cardiac disease modeling and drug screening.

Tunable methacrylated hyaluronic acid-based hydrogels as scaffolds for soft tissue engineering applications

Hyaluronic acid (HA)-based biomaterials have been explored for a number of applications in biomedical engineering, particularly as tissue regeneration scaffolds. Crosslinked forms of HA are more robust and provide tunable mechanical properties and degradation rates that are critical in regenerative medicine; however, crosslinking modalities reported in the literature vary and there are few comparisons of different scaffold properties for various crosslinking approaches. In this study, we offer direct comparison of two methacrylation techniques for HA (glycidyl methacrylate HA [GMHA] or methacrylic anhydride HA [MAHA]). The two methods for methacrylating HA provide degrees of methacrylation ranging from 2.4 to 86%, reflecting a wider range of properties than is possible using only a single methacrylation technique. We have also characterized mechanical properties for nine different tissues isolated from rat (ranging from lung at the softest to muscle at the stiffest) using indentation techniques and show that we can match the full range of mechanical properties (0.35-6.13 kPa) using either GMHA or MAHA. To illustrate utility for neural tissue engineering applications, functional hydrogels with adhesive proteins (either GMHA or MAHA base hydrogels with collagen I and laminin) were designed with effective moduli mechanically matched to rat sciatic nerve (2.47 ± 0.31 kPa). We demonstrated ability of these hydrogels to support three-dimensional axonal elongation from dorsal root ganglia cultures. Overall, we have shown that methacrylated HA provides a tunable platform with a wide range of properties for use in soft tissue engineering.

PRP and BMAC for Musculoskeletal Conditions via Biomaterial Carriers

Platelet-rich plasma (PRP) and bone marrow aspirate concentrate (BMAC) are orthobiologic therapies considered as an alternative to the current therapies for muscle, bone and cartilage. Different formulations of biomaterials have been used as carriers for PRP and BMAC in order to increase regenerative processes. The most common biomaterials utilized in conjunction with PRP and BMAC clinical trials are organic scaffolds and natural or synthetic polymers. This review will cover the combinatorial strategies of biomaterial carriers with PRP and BMAC for musculoskeletal conditions (MsCs) repair and regeneration in clinical trials. The main objective is to review the therapeutic use of PRP and BMAC as a treatment option for muscle, bone and cartilage injuries.

Nanogroove-Enhanced Hydrogel Scaffolds for 3D Neuronal Cell Culture: An Easy Access Brain-on-Chip Model

In order to better understand the brain and brain diseases, in vitro human brain models need to include not only a chemically and physically relevant microenvironment, but also structural network complexity. This complexity reflects the hierarchical architecture in brain tissue. Here, a method has been developed that adds complexity to a 3D cell culture by means of nanogrooved substrates. SH-SY5Y cells were grown on these nanogrooved substrates and covered with Matrigel, a hydrogel. To quantitatively analyze network behavior in 2D neuronal cell cultures, we previously developed an automated image-based screening method. We first investigated if this method was applicable to 3D primary rat brain cortical (CTX) cell cultures. Since the method was successfully applied to these pilot data, a proof of principle in a reductionist human brain cell model was attempted, using the SH-SY5Y cell line. The results showed that these cells also create an aligned network in the 3D microenvironment by maintaining a certain degree of guidance by the nanogrooved topography in the z-direction. These results indicate that nanogrooves enhance the structural complexity of 3D neuronal cell cultures for both CTX and human SH-SY5Y cultures, providing a basis for further development of an easy access brain-on-chip model.

3D bioprinting of complex channels within cell-laden hydrogels

3D bioprinting is an emerging manufacturing approach to fabricate (cell-laden) hydrogel constructs with embedded microchannels, which are potentially useful for fundamental studies to understand vascularization and angiogenesis, and for developing organ-on-a-chip devices for disease modeling. Although numerous printing approaches have been developed, novel approaches are still needed that enable printing of channels with user-defined and tunable size, morphology, and complexity. Here, we report a novel bioprinting approach enabling printing of a sacrificial ink within commonly used photocurable hydrogels using a sequential printing approach. To achieve this, photocurable hydrogel is printed layer-by-layer as usual, but each layer is exposed to light briefly (seconds) to create partially crosslinked, self-supporting layers. At a desired thickness, immediately after the layer is printed (prior to partial crosslinking step), sacrificial hydrogel is directly printed within this viscous uncrosslinked layer. The layer was then exposed to light to confine and support the sacrificial hydrogel. After fully crosslinking the system, the sacrificial hydrogel is washed away, forming a channel. This approach allows bioprinting of cells with the matrix material and seeding of cells into channels after the sacrificial ink is removed. This approach can potentially provide a robust platform for fabricating vascularized tissues and studying cell behaviors on diverse channel surfaces. STATEMENT OF SIGNIFICANCE: 3D bioprinting is an emerging platform for the fabrication of hydrogel-based constructs for in vitro tissue/disease modelling or tissue and organ printing. Although several approaches have been developed to print channels within these constructs, it is still challenging to incorporate microchannels (for vascularization) within 3D bioprinted constructs. This study presents a novel bioprinting approach to create user-defined and tunable channels embedded within cell-laden hydrogel constructs. We report an important advance as our approach does not require complex device modifications for bioprinters or complex synthesis and processing hurdles for the inks. Since our approach does not require special chemistries, there are potentially a greater number of commercially available options for ink materials.

Recent advances in 3D printing: vascular network for tissue and organ regeneration

Over the past years, the fabrication of adequate vascular networks has remained the main challenge in engineering tissues due to technical difficulties, while the ultimate objective of tissue engineering is to create fully functional and sustainable organs and tissues to transplant in the human body. There have been a number of studies performed to overcome this limitation, and as a result, 3D printing has become an emerging technique to serve in a variety of applications in constructing vascular networks within tissues and organs. 3D printing incorporated technical approaches allow researchers to fabricate complex and systematic architecture of vascular networks and offer various selections for fabrication materials and printing techniques. In this review, we will discuss materials and strategies for 3D printed vascular networks as well as specific applications for certain vascularized tissue and organ regeneration. We will also address the current limitations of vascular tissue engineering and make suggestions for future directions research may take.

Hydrogels for Liver Tissue Engineering

Bioengineered livers are promising in vitro models for drug testing, toxicological studies, and as disease models, and might in the future be an alternative for donor organs to treat end-stage liver diseases. Liver tissue engineering (LTE) aims to construct liver models that are physiologically relevant. To make bioengineered livers, the two most important ingredients are hepatic cells and supportive materials such as hydrogels. In the past decades, dozens of hydrogels have been developed to act as supportive materials, and some have been used for in vitro models and formed functional liver constructs. However, currently none of the used hydrogels are suitable for in vivo transplantation. Here, the histology of the human liver and its relationship with LTE is introduced. After that, significant characteristics of hydrogels are described focusing on LTE. Then, both natural and synthetic materials utilized in hydrogels for LTE are reviewed individually. Finally, a conclusion is drawn on a comparison of the different hydrogels and their characteristics and ideal hydrogels are proposed to promote LTE.

Rapid 3D bioprinting of in vitro cardiac tissue models using human embryonic stem cell-derived cardiomyocytes

There is a great need for physiologically relevant 3D human cardiac scaffolds for both short-term, the development of drug testing platforms to screen new drugs across different genetic backgrounds, and longer term, the replacement of damaged or non-functional cardiac tissue after injury or infarction. In this study, we have designed and printed a variety of scaffolds for in vitro diagnostics using light based Micro-Continuous Optical Printing (μCOP). Human embryonic stem cell-derived cardiomyocyte (hESC-CMs) were directly printed into gelatin hydrogel on glass to determine their viability and ability to align. The incorporation of Green Fluorescent Protein/Calmodulin/M13 Peptide (GCaMP3)-hESC-CMs allowed the ability to continuously monitor calcium transients over time. Normalized fluorescence of GCaMP3-hESCCMs increased by 18 ± 6% and 40 ± 5% when treated with 500 nM and 1 μM of isoproterenol, respectively. Finally, GCaMP3-hESC-CMs were printed across a customizable 3D printed cantilever-based force system. Along with force tracking by visualizing the displacement of the cantilever, calcium transients could be observed in a non-destructive manner, allowing the samples to be examined over several days. Our μCOP-printed cardiac models presented here can be used as a powerful tool for drug screening and to analyze cardiac tissue maturation.

3D Printing Applied to Tissue Engineered Vascular Grafts

The broad clinical use of synthetic vascular grafts for vascular diseases is limited by their thrombogenicity and low patency rate, especially for vessels with a diameter inferior to 6 mm. Alternatives such as tissue-engineered vascular grafts (TEVGs), have gained increasing interest. Among the different manufacturing approaches, 3D bioprinting presents numerous advantages and enables the fabrication of multi-scale, multi-material, and multicellular tissues with heterogeneous and functional intrinsic structures. Extrusion-, inkjet- and light-based 3D printing techniques have been used for the fabrication of TEVG out of hydrogels, cells, and/or solid polymers. This review discusses the state-of-the-art research on the use of 3D printing for TEVG with a focus on the biomaterials and deposition methods.

Coaxial Cell Printing of Freestanding, Perfusable, and Functional In Vitro Vascular Models for Recapitulation of Native Vascular Endothelium Pathophysiology

3D printing technology is used to produce channels within hydrogels followed by endothelial cells (ECs)-seeding to establish in vitro vascular models. However, as built-in bulk hydrogels, it is difficult to incorporate additional cells and molecules into the crosslinked matrix to study the pathophysiological responses of healthy endothelium. In this study, freestanding in vitro vascular models (VMs) are developed using the coaxial cell printing technique and a vascular tissue-specific bioink. It has various advantages in plotting tubular cell-laden vessels with designed patterns, providing pump-driven circulating perfusion, generating endothelium without ECs-seeding, and implementing further expansions to study vascular pathophysiology. Following the maturation of endothelium, the VMs exhibit representative vascular functions (i.e., selective permeability, antiplatelets/leukocytes adhesion, and vessel remodeling under shear stress). Moreover, with the expansions of the VMs, the directional angiogenesis and inflammatory responses are demonstrated by giving asymmetric distributions of proangiogenic factors and an airway inflammatory ambience, respectively. Therefore, the freestanding, perfusable, and functional VMs can be useful devices to engineer diverse in vitro platforms for a wide range of biomedical applications, from modeling blood vessel relevant diseases to building vascularized tissues/organs.

Rapid continuous 3D printing of customizable peripheral nerve guidance conduits

Engineered nerve guidance conduits (NGCs) have been demonstrated for repairing peripheral nerve injuries. However, there remains a need for an advanced biofabrication system to build NGCs with complex architectures, tunable material properties, and customizable geometrical control. Here, a rapid continuous 3D-printing platform was developed to print customizable NGCs with unprecedented resolution, speed, flexibility, and scalability. A variety of NGC designs varying in complexity and size were created including a life-size biomimetic branched human facial NGC. In vivo implantation of NGCs with microchannels into complete sciatic nerve transections of mouse models demonstrated the effective directional guidance of regenerating sciatic nerves via branching into the microchannels and extending toward the distal end of the injury site. Histological staining and immunostaining further confirmed the progressive directional nerve regeneration and branching behavior across the entire NGC length. Observational and functional tests, including the von Frey threshold test and thermal test, showed promising recovery of motor function and sensation in the ipsilateral limbs grafted with the 3D-printed NGCs.

3D Printed Polymeric Hydrogels for Nerve Regeneration

The human nervous system lacks an inherent ability to regenerate its components upon damage or diseased conditions. During the last decade, this has motivated the development of a number of strategies for nerve regeneration. However, most of those approaches have not been used in clinical applications till today. For instance, although biomaterial-based scaffolds have been extensively used for nerve reparation, the lack of more customized structures have hampered their use in vivo. This highlight focuses mainly on how 3D bioprinting technology, using polymeric hydrogels as bio-inks, can be used for the development of new nerve guidance channels or devices for peripheral nerve cell regeneration. In this concise contribution, some of the most recent and representative examples are highlighted to discuss the challenges involved in various aspects of 3D bioprinting for nerve cell regeneration, specifically when using polymeric hydrogels.

A review on fabricating tissue scaffolds using vat photopolymerization

Vat Photopolymerization (stereolithography, SLA), an Additive Manufacturing (AM) or 3D printing technology, holds particular promise for the fabrication of tissue scaffolds for use in regenerative medicine. Unlike traditional tissue scaffold fabrication techniques, SLA is capable of fabricating designed scaffolds through the selective photopolymerization of a photopolymer resin on the micron scale. SLA offers unprecedented control over scaffold porosity and permeability, as well as pore size, shape, and interconnectivity. Perhaps even more significantly, SLA can be used to fabricate vascular networks that may encourage angio and vasculogenesis. Fulfilling this potential requires the development of new photopolymers, the incorporation of biochemical factors into printed scaffolds, and an understanding of the effects scaffold geometry have on cell viability, proliferation, and differentiation. This review compares SLA to other scaffold fabrication techniques, highlights significant advances in the field, and offers a perspective on the field's challenges and future directions.

STATEMENT OF SIGNIFICANCE

Engineering de novo tissues continues to be challenging due, in part, to our inability to fabricate complex tissue scaffolds that can support cell proliferation and encourage the formation of developed tissue. The goal of this review is to first introduce the reader to traditional and Additive Manufacturing scaffold fabrication techniques. The bulk of this review will then focus on apprising the reader of current research and provide a perspective on the promising use of vat photopolymerization (stereolithography, SLA) for the fabrication of complex tissue scaffolds.

Scaffolds for peripheral nerve repair and reconstruction

Trauma-associated peripheral nerve defect is a widespread clinical problem. Autologous nerve grafting, the current gold standard technique for the treatment of peripheral nerve injury, has many internal disadvantages. Emerging studies showed that tissue engineered nerve graft is an effective substitute to autologous nerves. Tissue engineered nerve graft is generally composed of neural scaffolds and incorporating cells and molecules. A variety of biomaterials have been used to construct neural scaffolds, the main component of tissue engineered nerve graft. Synthetic polymers (e.g. silicone, polyglycolic acid, and poly(lactic-co-glycolic acid)) and natural materials (e.g. chitosan, silk fibroin, and extracellular matrix components) are commonly used along or together to build neural scaffolds. Many other materials, including the extracellular matrix, glass fabrics, ceramics, and metallic materials, have also been used to construct neural scaffolds. These biomaterials are fabricated to create specific structures and surface features. Seeding supporting cells and/or incorporating neurotrophic factors to neural scaffolds further improve restoration effects. Preliminary studies demonstrate that clinical applications of these neural scaffolds achieve satisfactory functional recovery. Therefore, tissue engineered nerve graft provides a good alternative to autologous nerve graft and represents a promising frontier in neural tissue engineering.

3D bio-printing technology for body tissues and organs regeneration

In the last decade, the use of new technologies in the reconstruction of body tissues has greatly developed. Utilising stem cell technology, nanotechnology and scaffolding design has created new opportunities in tissue regeneration. The use of accurate engineering design in the creation of scaffolds, including 3D printers, has been widely considered. Three-dimensional printers, especially high precision bio-printers, have opened up a new way in the design of 3D tissue engineering scaffolds. In this article, a review of the latest applications of this technology in this promising area has been addressed.

Three-Dimensional Printing of Bisphenol A-Free Polycarbonates

Polycarbonates are widely used in food packages, drink bottles, and various healthcare products such as dental sealants and tooth coatings. However, bisphenol A (BPA) and phosgene used in the production of commercial polycarbonates pose major concerns to public health safety. Here, we report a green pathway to prepare BPA-free polycarbonates (BFPs) by thermal ring-opening polymerization and photopolymerization. Polycarbonates prepared from two cyclic carbonates in different mole ratios demonstrated tunable mechanical stiffness, excellent thermal stability, and high optical transparency. Three-dimensional (3D) printing of the new BFPs was demonstrated using a two-photon laser direct writing system and a rapid 3D optical projection printer to produce structures possessing complex high-resolution geometries. Seeded C3H10T1/2 cells also showed over 95% viability with potential applications in biological studies. By combining biocompatible BFPs with 3D printing, novel safe and high-performance biomedical devices and healthcare products could be developed with broad long-term benefits to society.

3-D Bioprinting of Neural Tissue for Applications in Cell Therapy and Drug Screening

Neurodegenerative diseases affect millions of individuals in North America and cost the health-care industry billions of dollars for treatment. Current treatment options for degenerative diseases focus on physical rehabilitation or drug therapies, which temporarily mask the effects of cell damage, but quickly lose their efficacy. Cell therapies for the central nervous system remain an untapped market due to the complexity involved in growing neural tissues, controlling their differentiation, and protecting them from the hostile environment they meet upon implantation. Designing tissue constructs for the discovery of better drug treatments are also limited due to the resolution needed for an accurate cellular representation of the brain, in addition to being expensive and difficult to translate to biocompatible materials. 3-D printing offers a streamlined solution for engineering brain tissue for drug discovery or, in the future, for implantation. New microfluidic and bioplotting devices offer increased resolution, little impact on cell viability and have been tested with several bioink materials including fibrin, collagen, hyaluronic acid, poly(caprolactone), and poly(ethylene glycol). This review details current efforts at bioprinting neural tissue and highlights promising avenues for future work.

Polymeric scaffolds for three-dimensional culture of nerve cells: a model of peripheral nerve regeneration

Understanding peripheral nerve repair requires the evaluation of 3D structures that serve as platforms for 3D cell culture. Multiple platforms for 3D cell culture have been developed, mimicking peripheral nerve growth and function, in order to study tissue repair or diseases. To recreate an appropriate 3D environment for peripheral nerve cells, key factors are to be considered including: selection of cells, polymeric biomaterials to be used, and fabrication techniques to shape and form the 3D scaffolds for cellular culture. This review focuses on polymeric 3D platforms used for the development of 3D peripheral nerve cell cultures.

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

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

* A 3D Tissue-Printing Approach for Validation of Diffusion Tensor Imaging in Skeletal Muscle

The ability to noninvasively assess skeletal muscle microstructure, which predicts function and disease, would be of significant clinical value. One method that holds this promise is diffusion tensor magnetic resonance imaging (DT-MRI), which is sensitive to the microscopic diffusion of water within tissues and has become ubiquitous in neuroimaging as a way of assessing neuronal structure and damage. However, its application to the assessment of changes in muscle microstructure associated with injury, pathology, or age remains poorly defined, because it is difficult to precisely control muscle microstructural features in vivo. However, recent advances in additive manufacturing technologies allow precision-engineered diffusion phantoms with histology informed skeletal muscle geometry to be manufactured. Therefore, the goal of this study was to develop skeletal muscle phantoms at relevant size scales to relate microstructural features to MRI-based diffusion measurements. A digital light projection based rapid 3D printing method was used to fabricate polyethylene glycol diacrylate based diffusion phantoms with (1) idealized muscle geometry (no geometry; fiber sizes of 30, 50, or 70 μm or fiber size of 50 μm with 40% of walls randomly deleted) or (2) histology-based geometry (normal and after 30-days of denervation) containing 20% or 50% phosphate-buffered saline (PBS). Mean absolute percent error (8%) of the printed phantoms indicated high conformity to templates when "fibers" were >50 μm. A multiple spin-echo echo planar imaging diffusion sequence, capable of acquiring diffusion weighted data at several echo times, was used in an attempt to combine relaxometry and diffusion techniques with the goal of separating intracellular and extracellular diffusion signals. When fiber size increased (30-70 μm) in the 20% PBS phantom, fractional anisotropy (FA) decreased (0.32-0.26) and mean diffusivity (MD) increased (0.44 × 10^-3 mm²/s-0.70 × 10^-3 mm²/s). Similarly, when fiber size increased from 30 to 70 μm in the 50% PBS diffusion phantoms, a small change in FA was observed (0.18-0.22), but MD increased from 0.86 × 10^-3 mm²/s to 1.79 × 10^-3 mm²/s. This study demonstrates a novel application of tissue engineering to understand complex diffusion signals in skeletal muscle. Through this work, we have also demonstrated the feasibility of 3D printing for skeletal muscle with relevant matrix geometries and physiologically relevant tissue characteristics.

Direct 3D bioprinting of prevascularized tissue constructs with complex microarchitecture

Living tissues rely heavily on vascular networks to transport nutrients, oxygen and metabolic waste. However, there still remains a need for a simple and efficient approach to engineer vascularized tissues. Here, we created prevascularized tissues with complex three-dimensional (3D) microarchitectures using a rapid bioprinting method - microscale continuous optical bioprinting (μCOB). Multiple cell types mimicking the native vascular cell composition were encapsulated directly into hydrogels with precisely controlled distribution without the need of sacrificial materials or perfusion. With regionally controlled biomaterial properties the endothelial cells formed lumen-like structures spontaneously in vitro. In vivo implantation demonstrated the survival and progressive formation of the endothelial network in the prevascularized tissue. Anastomosis between the bioprinted endothelial network and host circulation was observed with functional blood vessels featuring red blood cells. With the superior bioprinting speed, flexibility and scalability, this new prevascularization approach can be broadly applicable to the engineering and translation of various functional tissues.