Vascularization strategies for tissue engineering

Microfluidic and Static Organotypic Culture Systems to Support Ex Vivo Spermatogenesis From Prepubertal Porcine Testicular Tissue: A Comparative Study

Background:In vitro maturation of immature testicular tissue (ITT) cryopreserved for fertility preservation is a promising fertility restoration strategy. Organotypic tissue culture proved successful in mice, leading to live births. In larger mammals, including humans, efficiently reproducing spermatogenesis ex vivo remains challenging. With advances in biomaterials technology, culture systems are becoming more complex to better mimic in vivo conditions. Along with improving culture media components, optimizing physical culture conditions (e.g., tissue perfusion, oxygen diffusion) also needs to be considered. Recent studies in mice showed that by using silicone-based hybrid culture systems, the efficiency of spermatogenesis can be improved. Such systems have not been reported for ITT of large mammals.

Methods: Four different organotypic tissue culture systems were compared: static i.e., polytetrafluoroethylene membrane inserts (OT), agarose gel (AG) and agarose gel with polydimethylsiloxane chamber (AGPC), and dynamic i.e., microfluidic (MF). OT served as control. Porcine ITT fragments were cultured over a 30-day period using a single culture medium. Analyses were performed at days (d) 0, 5, 10, 20 and 30. Seminiferous tubule (ST) integrity, diameters, and tissue core integrity were evaluated on histology. Immunohistochemistry was used to identify germ cells (PGP9.5, VASA, SYCP3, CREM), somatic cells (SOX9, INSL3) and proliferating cells (Ki67), and to assess oxidative stress (MDA) and apoptosis (C-Caspase3). Testosterone was measured in supernatants using ELISA.

Results: ITT fragments survived and grew in all systems. ST diameters, and Sertoli cell (SOX9) numbers increased, meiotic (SYCP3) and post-meiotic (CREM) germ cells were generated, and testosterone was secreted. When compared to control (OT), significantly larger STs (d10 through d30), better tissue core integrity (d5 through d20), higher numbers of undifferentiated spermatogonia (d30), meiotic and post-meiotic germ cells (SYCP3: d20 and 30, CREM: d20) were observed in the AGPC system. Apoptosis, lipid peroxidation (MDA), ST integrity, proliferating germ cell (Ki67/VASA) numbers, Leydig cell (INSL3) numbers and testosterone levels were not significantly different between systems.

Conclusions: Using a modified culture system (AGPC), germ cell survival and the efficiency of porcine germ cell differentiation were moderately improved ex vivo. We assume that further optimization can be obtained with concomitant modifications in culture media components.

Current methods for fabricating 3D cardiac engineered constructs

3D cardiac engineered constructs have yielded not only the next generation of cardiac regenerative medicine but also have allowed for more accurate modeling of both healthy and diseased cardiac tissues. This is critical as current cardiac treatments are rudimentary and often default to eventual heart transplants. This review serves to highlight the various cell types found in cardiac tissues and how they correspond with current advanced fabrication methods for creating cardiac engineered constructs capable of shedding light on various pathologies and providing the therapeutic potential for damaged myocardium. In addition, insight is given toward the future direction of the field with an emphasis on the creation of specialized and personalized constructs that model the region-specific microtopography and function of native cardiac tissues.

High-Throughput Production of Microsponges from Platelet Lysate for Tissue Engineering Applications

Cell-based therapies require a large number of cells, as well as appropriate methods to deliver the cells to damaged tissue. Microcarriers provide an optimal platform for large-scale cell culture while also improving cell retention during cell delivery. However, this technology still presents significant challenges due to low-throughput fabrication methods and an inability of the microcarriers to recreate the properties of human tissue. This work proposes, for the first time, the use of methacryloyl platelet lysates (PLMA), a photocrosslinkable material derived from human platelet lysates, to produce porous microcarriers. Initially, high quantities of PLMA/alginate core-shell microcapsules are produced using coaxial electrospray. Subsequently, the microcapsules are collected, irradiated with ultraviolet light, washed, and freeze dried yielding PLMA microsponges. These microsponges are able to support the adhesion and proliferation of human adipose-derived stem cells, while also displaying potential in the assembly of autologous microtissues. Cell-laden microsponges were shown to self-organize into aggregates, suggesting possible applications in bottom-up tissue engineering applications. Impact Statement Microcarriers have increasingly been used as delivery platforms in cell therapy. Herein, the encapsulation of human-derived proteins in alginate microcapsules is proposed as a method to produce microcarriers from photopolymerizable materials. The capsules function as a template structure, which is then processed into spherical microparticles, which can be used in cell culture, cell delivery, and bottom-up assembly. As a proof of concept, this method was combined with lyophilization to process methacryloyl platelet lysates into injectable microsponges for cell delivery.

The Growing Medical Need for Tracheal Replacement: Reconstructive Strategies Should Overcome Their Limits

Breathing, being predominantly an automatic action, is often taken for granted. However, respiratory diseases affect millions of people globally, emerging as one of the major causes of disability and death overall. Among the respiratory dysfunctions, tracheal alterations have always represented a primary challenge for clinicians, biologists, and engineers. Indeed, in the case of wide structural alterations involving more than 50% of the tracheal length in adults or 30% in children, the available medical treatments are ineffective or inapplicable. So far, a plethora of reconstructive approaches have been proposed and clinically applied to face this growing, unmet medical need. Unfortunately, none of them has become a well-established and routinely applied clinical procedure to date. This review summarizes the main clinical reconstructive attempts and classifies them as non-tissue engineering and tissue engineering strategies. The analysis of the achievements and the main difficulties that still hinder this field, together with the evaluation of the forefront preclinical experiences in tracheal repair/replacement, is functional to promote a safer and more effective clinical translation in the near future.

Human Adipose-Derived Stem Cell-Conditioned Medium Promotes Vascularization of Nanostructured Scaffold Transplanted into Nude Mice

Several studies have been conducted on the interaction between three-dimensional scaffolds and mesenchymal stem cells for the regeneration of damaged tissues. Considering that stem cells do not survive for sufficient time to directly sustain tissue regeneration, it is essential to develop cell-free systems to be applied in regenerative medicine. In this work, by in vivo experiments, we established that a collagen-nanostructured scaffold, loaded with a culture medium conditioned with mesenchymal stem cells derived from adipose tissue (hASC-CM), exerts a synergic positive effect on angiogenesis, fundamental in tissue regeneration. To this aim, we engrafted athymic BALB-C nude mice with four different combinations: scaffold alone; scaffold with hASCs; scaffold with hASC crude protein extract; scaffold with hASC-CM. After their removal, we verified the presence of blood vessels by optical microscopy and confirmed the vascularization evaluating, by real-time PCR, several vascular growth factors: CD31, CD34, CD105, ANGPT1, ANGPT2, and CDH5. Our results showed that blood vessels were absent in the scaffold grafted alone, while all the other systems appeared vascularized, a finding supported by the over-expression of CD31 and CDH5 mRNA. In conclusion, our data sustain the capability of hASC-CM to be used as a therapeutic cell-free approach for damaged tissue regeneration.

Surface channel patterned and endothelialized poly(glycerol sebacate) based elastomers

Prevascularization of tissue equivalents is critical for fulfilling the need for sufficient vascular organization for nutrient and gas transport. Hence, endothelial cell culture on biomaterials is of great importance for researchers. Numerous alternate strategies have been suggested in this sense, with cell-based methods being the most commonly employed. In this study, poly (glycerol sebacate) (PGS) elastomers with varying crosslinking ratios were synthesized and their surfaces were patterned with channels by using laser ablation technique. In order to determine an ideal material for cell culture studies, the elastomers were subsequently mechanically, chemically, and biologically characterized. Following that, human umbilical vein endothelial cells (HUVECs) were seeded into the channels established on the PGS membranes and cultured under various culture conditions to establish the optimal culture parameters. Lastly, the endothelial cell responses to the synthesized PGS elastomers were evaluated. Remarkable cell proliferation and impressive cellular organizations were noticed on the constructs created as part of the investigation. On the concrete output of this research, arrangements in various geometries can be created by laser ablation method and the effects of various molecules, drugs or agents on endothelial cells can be evaluated. The platforms produced can be employed as an intermediate biomaterial layer containing endothelial cells for vascularization of tissue-engineered structures, particularly in layer-by-layer tissue engineering approaches.

Biomimetic models of the glomerulus

The use of biomimetic models of the glomerulus has the potential to improve our understanding of the pathogenesis of kidney diseases and to enable progress in therapeutics. Current in vitro models comprise organ-on-a-chip, scaffold-based and organoid approaches. Glomerulus-on-a-chip designs mimic components of glomerular microfluidic flow but lack the inherent complexity of the glomerular filtration barrier. Scaffold-based 3D culture systems and organoids provide greater microenvironmental complexity but do not replicate fluid flows and dynamic responses to fluidic stimuli. As the available models do not accurately model the structure or filtration function of the glomerulus, their applications are limited. An optimal approach to glomerular modelling is yet to be developed, but the field will probably benefit from advances in biofabrication techniques. In particular, 3D bioprinting technologies could enable the fabrication of constructs that recapitulate the complex structure of the glomerulus and the glomerular filtration barrier. The next generation of in vitro glomerular models must be suitable for high(er)-content or/and high(er)-throughput screening to enable continuous and systematic monitoring. Moreover, coupling of glomerular or kidney models with those of other organs is a promising approach to enable modelling of partial or full-body responses to drugs and prediction of therapeutic outcomes.

Evaluation of ethylene oxide, gamma radiation, dry heat and autoclave sterilization processes on extracellular matrix of biomaterial dental scaffolds

Scaffolds used to receive stem cells are a promising perspective of tissue regeneration research, and one of the most effective solutions to rebuild organs. In the near future will be possible to reconstruct a natural tooth using stems cells, but to avoid an immune-defensive response, sterilize the scaffold is not only desired, but also essential to be successful. A study confirmed stem cells extracted from rat's natural teeth, and implanted into the alveolar bone, could differentiate themselves in dental cells, but the scaffold's chemistry, geometry, density, morphology, adherence, biocompatibility and mechanical properties remained an issue. This study intended to produce a completely sterilized dental scaffold with preserved extracellular matrix. Fifty-one samples were collected, kept in formaldehyde, submitted to partial demineralization and decellularization processes and sterilized using four different methods: dry heating; autoclave; ethylene-oxide and gamma-radiation. They were characterized through optical images, micro-hardness, XRD, EDS, XRF, SEM, histology and sterility test. The results evidenced the four sterilization methods were fully effective with preservation of ECM molecular arrangements, variation on chemical composition (proportion of Ca/P) was compatible with Ca/P proportional variation between enamel and dentine regions. Gamma irradiation and ethylene oxide presents excellent results, but their viability are compromised by the costs and technology's accessibility (requires very expensive equipment and/or consumables). Excepted gamma irradiation, all the sterilization methods more than sterilizing also reduced the remaining pulp. Autoclave presents easy equipment accessibility, lower cost consumables, higher reduction of remaining pulp and complete sterilization, reason why was considered the most promising technique.

Investigation of Lysozyme Diffusion in Agarose Hydrogels Employing a Microfluidics-Based UV Imaging Approach

Hydrogels are polymer-based materials with a high water content. Due to their biocompatible and cell-friendly nature, they play a major role in a variety of biotechnological applications. For many of these applications, diffusibility is an essential property influencing the choice of material. We present an approach to estimate diffusion coefficients in hydrogels based on absorbance measurements of a UV area imaging system. A microfluidic chip with a y-junction was employed to generate a fluid-hydrogel interface and the diffusion of lysozyme from the fluid into the hydrogel phase was monitored. Employing automated image and data processing, analyte concentration profiles were generated from the absorbance measurements and fits with an analytical solution of Fick's second law of diffusion were applied to estimate diffusion coefficients. As a case study, the diffusion of lysozyme in hydrogels made from different concentrations (0.5-1.5% (w/w)) of an unmodified and a low-melt agarose was investigated. The estimated diffusion coefficients for lysozyme were between 0.80 ± 0.04×10-10 m2 s-1 for 1.5% (w/w) low-melt agarose and 1.14 ± 0.02×10-10 m2 s-1 for 0.5% (w/w) unmodified agarose. The method proved sensitive enough to resolve significant differences between the diffusion coefficients in different concentrations and types of agarose. The microfluidic approach offers low consumption of analyte and hydrogel and requires only relatively simple instrumentation.

Study of sacrificial ink-assisted embedded printing for 3D perfusable channel creation for biomedical applications

For an engineered thick tissue construct to be alive and sustainable, it should be perfusable with respect to nutrients and oxygen. Embedded printing and then removing sacrificial inks in a cross-linkable yield-stress hydrogel matrix bath can serve as a valuable tool for fabricating perfusable tissue constructs. The objective of this study is to investigate the printability of sacrificial inks and the creation of perfusable channels in a cross-linkable yield-stress hydrogel matrix during embedded printing. Pluronic F-127, methylcellulose, and polyvinyl alcohol are selected as three representative sacrificial inks for their different physical and rheological properties. Their printability and removability performances have been evaluated during embedded printing in a gelatin microgel-based gelatin composite matrix bath, which is a cross-linkable yield-stress bath. The ink printability during embedded printing is different from that during printing in air due to the constraining effect of the matrix bath. Sacrificial inks with a shear-thinning property are capable of printing channels with a broad range of filaments by simply tuning the extrusion pressure. Bi-directional diffusion may happen between the sacrificial ink and matrix bath, which affects the sacrificial ink removal process and final channel diameter. As such, sacrificial inks with a low diffusion coefficient for gelatin precursor are desirable to minimize the diffusion from the gelatin precursor solution to minimize the post-printing channel diameter variation. For feasibility demonstration, a multi-channel perfusable alveolar mimic has been successfully designed, printed, and evaluated. The study results in the knowledge of the channel diameter controllability and sacrificial ink removability during embedded printing.

Precision-engineered niche for directed differentiation of MSCs to lineage-restricted mineralized tissues

The major difference between tissue healing and regeneration is the extent of instructional cues available to precisely direct the biological response. A classic example is reparative or osteodentin that is seen in response to physicochemical injury to the pulp-dentin complex. Dentin regeneration can direct the differentiation of dental stem cells using concerted actions of both soluble (biomolecules, agonists, and antagonists) and insoluble (matrix topology) cues. The major purpose of this study was to examine the synergistic combination of two discrete biomaterial approaches by utilizing nanofiber scaffolds in discrete configurations (aligned or random) with incorporated polymeric microspheres capable of controlled release of growth factors. Further, to ensure appropriate disinfection for clinical use, Radio-Frequency Glow Discharge (RFGD) treatments were utilized, followed by seeding with a mesenchymal stem cell (MSC) line. SEM analysis revealed electrospinning generated controlled architectural features that significantly improved MSC adhesion and proliferation on the aligned nanofiber scaffolds compared to randomly oriented scaffolds. These responses were further enhanced by RFGD pre-treatments. These enhanced cell adhesion and proliferative responses could be attributed to matrix-induced Wnt signaling that was abrogated by pre-treatments with anti-Wnt3a neutralizing antibodies. Next, we incorporated controlled-release microspheres within these electrospun scaffolds with either TGF-β1 or BMP4. We observed that these scaffolds could selectively induce dentinogenic or osteogenic markers (DSPP, Runx2, and BSP) and mineralization. This work demonstrates the utility of a novel, modular combinatorial scaffold system capable of lineage-restricted differentiation into bone or dentin. Future validation of this scaffold system in vivo as a pulp capping agent represents an innovative dentin regenerative approach capable of preserving tooth pulp vitality.

Decellularized Alstroemeria flower stem modified with chitosan for tissue engineering purposes: A cellulose/chitosan scaffold

Utilizing plant-based scaffolds has pulled in the consideration of tissue engineers. Plant tissues own different structures with particular porosity and structure. In this study, the stem of the Alstroemeria flower was designated for decellularization to fabricate a new scaffold. The stems were decellularized and called AFSP and then modified by chitosan and named AFSPC. Osteoblast precursor cell line was employed to assess the biological potential of the final scaffolds. The results uncovered that AFSP owns linear microchannels with a smooth surface. AFSPC delineated uniform chitosan coating on the walls with appropriate roughness. AFSPC showed higher potential in swelling, degradation, diffusion, and having a porous structure than AFSP. Modification with chitosan improved mechanical behavior. Biological assays depicted no cytotoxicity for AFSP and AFSPC. AFSPC showed good cell attachment, proliferation, and migration. In conclusion, modified tissue plants can be a good candidate for tissue engineering of both soft and hard tissues.

Left Ventricular Wall Reconstruction with Autologous Vascularized Gastric Graft in a Porcine Pilot Model

INTRODUCTION

Surgical replacement of dysfunctional cardiac muscle with regenerative tissue is an important option to combat heart failure. But current available myocardial prostheses like a Dacron or a pericardium patch neither have a regenerative capacity nor do they actively contribute to the heart`s pump function. This study aimed to show the feasibility of utilizing a vascularized stomach patch for transmural left ventricular wall reconstruction.

METHODS

A left ventricular transmural myocardial defect was reconstructed by performing transdiaphragmatic autologous transplantation of a vascularized stomach segment in six Lewe minipigs. Three further animals received a conventional Dacron patch as a control treatment. The first three animals were followed up for 3 months until planned euthanasia, whereas the observation period for the remaining three animals was scheduled 6 months following surgery. Functional assessment of the grafts was carried out via cardiac magnetic resonance tomography (MRI) and angiography. Physiological remodeling was evaluated histologically and immunohistochemically after heart explanation.

RESULTS

Five out of six test animals and all control animals survived the complex surgery and completed the follow-up without clinical complications. One animal died intraoperatively due to excessive bleeding. No animal experienced a rupture of the stomach graft. Functional integration of the heterotopically transplanted stomach into the surrounding myocardium was observed. Angiography showed the development of connections between the gastric graft vasculature and the coronary system of the host cardiac tissue.

CONCLUSIONS

The clinical results and the observed physiological integration of gastric grafts into the cardiac structure demonstrate the feasibility of vascularized stomach tissue as a myocardial prosthesis. The physiological remodeling indicates a regenerative potential of the graft. Above all, the connection of the gastric vessels with the coronary system constitutes a rationale for the use of vascularized and therefore viable stomach tissue for versatile tissue engineering applications.

3D Bio-printing For Skin Tissue Regeneration: Hopes and Hurdles

Abstract:

For many years, discovering the appropriate methods for the treatment of skin irritation has been challenging for specialists and researchers. Bio-printing can be extensively applied to address the demand for proper skin substitutes to improve skin damage. Nowadays, to make more effective bio-mimicking of natural skin, many research teams have developed cell-seeded bio-inks for bioprinting of skin substitutes. These loaded cells can be single or co-cultured in these structures. The present review gives a comprehensive overview of the methods, substantial parameters of skin bioprinting, examples of in vitro and in vivo studies, and current advances and challenges for skin tissue engineering.

Reconstruction of the trachea and carina: Surgical reconstruction, autologous tissue transplantation, allograft transplantation, and bioengineering

There have been significant advancements in medical techniques in the present epoch, with the emergence of some novel operative substitutes. However, the treatment of tracheal defects still faces tremendous challenges and there is, as yet, no consensus on tracheal and carinal reconstruction. In addition, surgical outcomes vary in different individuals, which results in an ambiguous future for tracheal surgery. Although transplantation was once an effective and promising method, it is limited by a shortage of donors and immune rejection. The development of bioengineering has provided an alternative for the treatment of tracheal defects, but this discipline is full of ethical controversy and hindered by limited cognition in this area. Meanwhile, progression of this technique is blocked by a deficiency in ideal materials. The trachea together with the carina is still the last unpaired organ in thoracic surgery and propososal of a favorable scheme to remove this dilemma is urgently required. In this review, four main tracheal reconstruction methods, especially surgical techniques, are evaluated, and a thorough interpretation conducted.

Tissue Engineered Neurovascularization Strategies for Craniofacial Tissue Regeneration

Craniofacial tissue injuries, diseases, and defects, including those within bone, dental, and periodontal tissues and salivary glands, impact an estimated 1 billion patients globally. Craniofacial tissue dysfunction significantly reduces quality of life, and successful repair of damaged tissues remains a significant challenge. Blood vessels and nerves are colocalized within craniofacial tissues and act synergistically during tissue regeneration. Therefore, the success of craniofacial regenerative approaches is predicated on successful recruitment, regeneration, or integration of both vascularization and innervation. Tissue engineering strategies have been widely used to encourage vascularization and, more recently, to improve innervation through host tissue recruitment or prevascularization/innervation of engineered tissues. However, current scaffold designs and cell or growth factor delivery approaches often fail to synergistically coordinate both vascularization and innervation to orchestrate successful tissue regeneration. Additionally, tissue engineering approaches are typically investigated separately for vascularization and innervation. Since both tissues act in concert to improve craniofacial tissue regeneration outcomes, a revised approach for development of engineered materials is required. This review aims to provide an overview of neurovascularization in craniofacial tissues and strategies to target either process thus far. Finally, key design principles are described for engineering approaches that will support both vascularization and innervation for successful craniofacial tissue regeneration.

Chest Feminization in Male-to-Female Transgender Patients: A Review of Options

Management of a transgender (TG) woman's gender dysphoria is individualized to address the sources of her distress. This typically involves some combination of psychological therapy, hormone modulation, and surgical intervention. Breast enhancement is the most commonly pursued physical modification in this population. Because hormone manipulation provides disappointing results for most TG women, surgical treatment is frequently required to achieve the goal of a feminine chest. Creating a female breast from natal male chest anatomy poses significant challenges; the sexual dimorphism requires a different approach than that used in cisgender breast augmentation. The options and techniques used continue to evolve as experience in this field grows.

Hydrogel based tissue engineering and its future applications in personalized disease modeling and regenerative therapy

BACKGROUND

Evolution in the in vitro cell culture from conventional 2D to 3D technique has been a significant accomplishment. The 3D culture models have provided a close and better insight into the physiological study of the human body. The increasing demand for organs like liver, kidney, and pancreas for transplantation, rapid anti-cancer drug screening, and the limitations associated with the use of animal models have attracted the interest of researchers to explore 3D organ culture.

MAIN BODY

Natural, synthetic, and hybrid material-based hydrogels are being used as scaffolds in 3D culture and provide 'close-to-in vivo' structures. Organoids: the stem cell-derived small size 3D culture systems are now favored due to their ability to mimic the in-vivo conditions of organ or tissue and this characteristic has made it eligible for a variety of clinical applications, drug discovery and regenerative medicine are a few of the many areas of application. The use of animal models for clinical applications has been a long-time ethical and biological challenge to get accurate outcomes. 3D bioprinting has resolved the issue of vascularization in organoid culture to a great extent by its layer-by-layer construction approach. The 3D bioprinted organoids have a popular application in personalized disease modeling and rapid drug development and therapeutics.

SHORT CONCLUSIONS

This review paper, focuses on discussing the novel organoid culture approach, its advantages and limitations, and potential applications in a variety of life science areas namely cancer research, cell therapy, tissue engineering, and personalized medicine and drug discovery.

GRAPHICAL ABSTRACT

Mechanistic characterization of endothelial sprouting mediated by pro-angiogenic signaling

Objective

We aim to quantitatively characterize the crosstalk between VEGF‐ and FGF‐mediated angiogenic signaling and endothelial sprouting, to gain mechanistic insights and identify novel therapeutic strategies.

Methods

We constructed an experimentally validated hybrid agent‐based mathematical model that characterizes endothelial sprouting driven by FGF‐ and VEGF‐mediated signaling. We predicted the total sprout length, number of sprouts, and average length by the mono‐ and co‐stimulation of FGF and VEGF.

Results

The experimentally fitted and validated model predicts that FGF induces stronger angiogenic responses in the long‐term compared with VEGF stimulation. Also, FGF plays a dominant role in the combination effects in endothelial sprouting. Moreover, the model suggests that ERK and Akt pathways and cellular responses contribute differently to the sprouting process. Last, the model predicts that the strategies to modulate endothelial sprouting are context‐dependent, and our model can identify potential effective pro‐ and anti‐angiogenic targets under different conditions and study their efficacy.

Conclusions

The model provides detailed mechanistic insight into VEGF and FGF interactions in sprouting angiogenesis. More broadly, this model can be utilized to identify targets that influence angiogenic signaling leading to endothelial sprouting and to study the effects of pro‐ and anti‐angiogenic therapies.

In Vivo Efficacy of Neutrophil-Mediated Bone Regeneration Using a Rabbit Calvarial Defect Model

Reconstruction of bone due to surgical removal or disease-related bony defects is a clinical challenge. It is known that the immune system exerts positive immunomodulatory effects on tissue repair and regeneration. In this study, we evaluated the in vivo efficacy of autologous neutrophils on bone regeneration using a rabbit calvarial defect model. Methods: Twelve rabbits, each with two surgically created calvarial bone defects (10 mm diameter), were randomly divided into two groups; (i) single application of neutrophils (SA-NP) vs. SA-NP control, and (ii) repetitive application of neutrophils (RA-NP) vs. RA-NP control. The animals were euthanized at 4 and 8 weeks post-operatively and the treatment outcomes were evaluated by micro-computed tomography, histology, and histomorphometric analyses. Results: The micro-CT analysis showed a significantly higher bone volume fraction (bone volume/total volume) in the neutrophil-treated groups, i.e., median interquartile range (IQR) SA-NP (18) and RA-NP (24), compared with the untreated controls, i.e., SA-NP (7) and RA-NP (14) at 4 weeks (p < 0.05). Similarly, new bone area fraction (bone area/total area) was significantly higher in neutrophil-treated groups at 4 weeks (p < 0.05). Both SA-NP and RA-NP had a considerably higher bone volume and bone area at 8 weeks, although the difference was not statistically significant. In addition, immunohistochemical analysis at 8 weeks revealed a higher expression of osteocalcin in both SA-NP and RA-NP groups. Conclusions: The present study provides first hand evidence that autologous neutrophils may have a positive effect on promoting new bone formation. Future studies should be performed with a larger sample size in non-human primate models. If proven feasible, this new promising strategy could bring clinical benefits for bone defects to the field of oral and maxillofacial surgery.

Repair of peripheral nerve injuries using a prevascularized cell-based tissue-engineered nerve conduit

One of the major challenges in the development of a larger and longer nerve conduit for peripheral nerve repair is the limitation in oxygen and nutrient diffusion within the tissue after transplantation preventing Schwann cell and axonal migration. This restriction is due to the slow neovascularization process of the graft starting from both nerve endings. To overcome this limitation, we propose the design of a living tissue-engineered nerve conduit made of an internal tube with a three-dimensional structure supporting axonal migration, which is inserted inside a hollow external tube that plays the role of an epineurium and is strong enough to be stitched to the severed nerve stumps. The internal tube is made of a rolled living fibroblast sheet and can be seeded with endothelial cells to promote the formation of a network containing capillary-like structures which allow rapid inosculation with the host nerve microvasculature after grafting. Human nerve conduits were grafted in immunodeficient rats to bridge a 15 mm sciatic nerve gap. Human capillaries within the pre-vascularized nerve conduit successfully connected to the host circulation 2 weeks after grafting. Twenty-two weeks after surgery, rats transplanted with the nerve conduits had a similar motor function recovery compared to the autograft group. By promoting rapid vascularization of the internal nerve tube from both ends of the nerve stumps, this endothelialized nerve conduit model displays a favorable environment to enhance axonal migration in both larger caliber and longer nerve grafts.

Towards clinical translation of 'second-generation' regenerative stroke therapies: hydrogels as game changers?

Stroke is an unmet clinical need with a paucity of treatments, at least in part because chronic stroke pathologies are prohibitive to 'first-generation' stem cell-based therapies. Hydrogels can remodel the hostile stroke microenvironment to aid endogenous and exogenous regenerative repair processes. However, no clinical trials have yet been successfully commissioned for these 'second-generation' hydrogel-based therapies for chronic ischaemic stroke regeneration. This review recommends a path forward to improve hydrogel technology for future clinical translation for stroke. Specifically, we suggest that a better understanding of human host stroke tissue-hydrogel interactions in addition to the effects of scaling up hydrogel volume to human-sized cavities would help guide translation of these second-generation regenerative stroke therapies.

Effect of biochemical and biomechanical factors on vascularization of kidney organoid-on-a-chip

Kidney organoids derived from the human pluripotent stem cells (hPSCs) recapitulating human kidney are the attractive tool for kidney regeneration, disease modeling, and drug screening. However, the kidney organoids cultured by static conditions have the limited vascular networks and immature nephron-like structures unlike human kidney. Here, we developed a kidney organoid-on-a-chip system providing fluidic flow mimicking shear stress with optimized extracellular matrix (ECM) conditions. We demonstrated that the kidney organoids cultured in our microfluidic system showed more matured podocytes and vascular structures as compared to the static culture condition. Additionally, the kidney organoids cultured in microfluidic systems showed higher sensitivity to nephrotoxic drugs as compared with those cultured in static conditions. We also demonstrated that the physiological flow played an important role in maintaining a number of physiological functions of kidney organoids. Therefore, our kidney organoid-on-a-chip system could provide an organoid culture platform for in vitro vascularization in formation of functional three-dimensional (3D) tissues.

Craniofacial Bone Tissue Engineering: Current Approaches and Potential Therapy

Craniofacial bone defects can result from various disorders, including congenital malformations, tumor resection, infection, severe trauma, and accidents. Successfully regenerating cranial defects is an integral step to restore craniofacial function. However, challenges managing and controlling new bone tissue formation remain. Current advances in tissue engineering and regenerative medicine use innovative techniques to address these challenges. The use of biomaterials, stromal cells, and growth factors have demonstrated promising outcomes in vitro and in vivo. Natural and synthetic bone grafts combined with Mesenchymal Stromal Cells (MSCs) and growth factors have shown encouraging results in regenerating critical-size cranial defects. One of prevalent growth factors is Bone Morphogenetic Protein-2 (BMP-2). BMP-2 is defined as a gold standard growth factor that enhances new bone formation in vitro and in vivo. Recently, emerging evidence suggested that Megakaryocytes (MKs), induced by Thrombopoietin (TPO), show an increase in osteoblast proliferation in vitro and bone mass in vivo. Furthermore, a co-culture study shows mature MKs enhance MSC survival rate while maintaining their phenotype. Therefore, MKs can provide an insight as a potential therapy offering a safe and effective approach to regenerating critical-size cranial defects.

Gelatin hydrogel nonwoven fabrics of a cell culture scaffold to formulate 3-dimensional cell constructs

The objective of this study is to evaluate the possibility of gelatin hydrogel nonwoven fabrics (GHNF) of a cell culture scaffold to formulate 3-dimensional (3D) cell construct. The thickness of cell construct is about 1 mm and the cells inside are live and bio-active, irrespective of their internal distribution. The GHNF were prepared by the solution blow method of gelatin, following by dehydrothermal crosslinking. The GHNF showed a mechanical strength strong enough not to allow the shape to deform even in a wet state. The wet GHNF also showed resistance against repeated compression. After human mesenchymal stromal cells (hMSC) were seeded and cultured, the inner distribution in GHNF, the apoptosis, hypoxia inducible factor (HIF)-1α, Ki67, collagen or sulfated glycosaminoglycan (sGAG) secretion of cells were evaluated. The hMSC proliferated inside the GHNF with time while a homogeneous distribution in the number of cells proliferated from the surface to the 1000 μm depth of GHNF was observed. The number of apoptosis and HIF-1α positive cells was significantly low compared with that of polypropylene nonwoven fabrics with the similar fiber diameters and intra-structure. The GHNF were degraded during cell culture, and completely replaced by collagen and sGAG secreted. It is concluded that the GHNF is a promising cell culture scaffold for 3D cell constructs.

Long non-coding RNA MALAT1 enhances angiogenesis during bone regeneration by regulating the miR-494/SP1 axis

Bone regeneration is a coordinated process involving connections between blood vessels and osteocytes. Angiogenesis and osteogenesis are tightly connected throughout the progression of bone regeneration. This study aimed to explore the underlying mechanism of metastasis-associated lung adenocarcinoma transcript 1 (MALAT1)-regulated angiogenesis during bone regeneration. Gene and protein expression was detected by quantitative real-time PCR and western blot assay. Vascular endothelial growth factor (VEGFA) secretion was assessed by enzyme-linked immunosorbent assay. To evaluate the effect of osteogenic differentiation, alkaline phosphatase (ALP) and alizarin red staining assays were performed. Proliferation was detected by 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Migration and angiogenesis were measured using Transwell and tube formation assays. A dual luciferase reporter assay was performed to confirm the binding relationship among MALAT1, miR-494, and specificity protein 1 (SP1). Expression levels of MALAT1, SP1, and VEGFA were elevated and miR-494 was suppressed in MC3T3-E1 cells after culture in osteogenic medium. MALAT1 knockdown suppressed the osteogenic differentiation of MC3T3-E1, since ALP activity, mineralized nodules, and expression of the osteodifferentiated markers runt-related transcription factor 2 and osterix were restrained. In addition, MALAT1 silencing inhibited angiogenesis during bone regeneration, as the proliferation, migration, and capillary tube formation of human umbilical vein endothelial cells were blocked. Furthermore, miR-494 was directly targeted by MALAT1 and regulated the SP1/Toll-like receptor 2 (TLR2)/bone morphogenetic protein 2 (BMP2) axis by targeting SP1. Furthermore, miR-494 overexpression inhibited angiogenesis and osteogenic differentiation. Moreover, SP1 overexpression or miR-494 inhibition rescued the regulatory effect of sh-MALAT1 on angiogenesis and osteogenic differentiation. Taken together, these findings indicate that MALAT1 promotes angiogenesis and osteogenic differentiation by targeting miR-494 and activating the SP1/TLR2/BMP2 pathway, suggesting a novel target for bone regeneration therapy by promoting angiogenesis.

Self-Oxygenation of Tissues Orchestrates Full-Thickness Vascularization of Living Implants

Bioengineering of tissues and organs has the potential to generate functional replacement organs. However, achieving the full-thickness vascularization that is required for long-term survival of living implants has remained a grand challenge, especially for clinically sized implants. During the pre-vascular phase, implanted engineered tissues are forced to metabolically rely on the diffusion of nutrients from adjacent host-tissue, which for larger living implants results in anoxia, cell death, and ultimately implant failure. Here it is reported that this challenge can be addressed by engineering self-oxygenating tissues, which is achieved via the incorporation of hydrophobic oxygen-generating micromaterials into engineered tissues. Self-oxygenation of tissues transforms anoxic stresses into hypoxic stimulation in a homogenous and tissue size-independent manner. The in situ elevation of oxygen tension enables the sustained production of high quantities of angiogenic factors by implanted cells, which are offered a metabolically protected pro-angiogenic microenvironment. Numerical simulations predict that self-oxygenation of living tissues will effectively orchestrate rapid full-thickness vascularization of implanted tissues, which is empirically confirmed via in vivo experimentation. Self-oxygenation of tissues thus represents a novel, effective, and widely applicable strategy to enable the vascularization living implants, which is expected to advance organ transplantation and regenerative medicine applications.

Using type III recombinant human collagen to construct a series of highly porous scaffolds for tissue regeneration

As an alternative biopolymer material without the risks of the use of animal-derived collagens in soft tissue engineering applications, recombinant human collagen polypeptide (RHC) was used to construct three-dimensional porous scaffolds. RHC and RHC-chitosan (RHC-CHI) porous scaffolds were fabricated using a freeze-drying method to create highly porous internal structures and then cross-linked with 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide (EDC). All scaffolds had interconnected porous structures with high porosity (90%), and pore size that ranged from 111 µm to 159 µm. The swelling ability and in vitro degradation of the prepared scaffolds were investigated. The mechanical properties could be tailored to meet the requirements of end-use application by adjusting the concentrations of the polymer or cross-linking agent, and the resulting mechanical strengths were comparable to those of biological soft tissues. The cytocompatibility of the fabricated porous scaffolds was investigated by seeding 3T3 fibroblasts into the porous structures, and then cell proliferation, distribution, morphology, and synthesis of extra cellular matrix-associated proteins were assessed. The results indicated that RHC-based porous scaffolds were non-cytotoxic and promoted the attachment and proliferation of the seeded cells. Finally, the in vivo study proved these porous scaffolds were able to accelerate the cell infiltration and collagen deposition that promoted the wound closure. Overall, the results indicate that RHC-based porous scaffolds show promise for use in soft tissue engineering due to their excellent in vitro cytocompatibility and adjustable mechanical properties.

Near-field electrospinning polycaprolactone microfibers to mimic arteriole-capillary-venule structure

The ability to create three-dimensional (3D) cell-incorporated constructs for tissue engineering has progressed tremendously. One of the major challenges that limit the clinical applications of tissue engineering is the inability to form sufficient vascularization of capillary vessels in the 3D constructs. The lack of a functional capillary network for supplying nutrients and oxygen leads to poor cell viability. This paper presents the near-field electrospinning (ES) technique to fabricate a branched microfiber structure that mimics the morphology of capillaries. Polycaprolactone solution was electrospun onto a sloped collector that resulted in morphological and geometric variation of the fibers. With proper control over the solution viscosity and the electrospinning voltage, a single fiber was scattered into a branched fiber network and then converged back to a single fiber on the collector. The obtained fibers have a diameter of less than 100 microns at the two ends with coiled and branched fibers of less than 10 microns that mimics the arteriole-capillary-venule structure. The formation of such a structure in the near-field ES strongly depends on the solution viscosity. Low viscosity solutions form beads and discontinuous lines thus cannot be used to achieve the desired structure. The branching of PCL fiber occurs due to an electrohydrodynamic instability. The transition from the straight large fiber to smaller coiled/branched fibers is not instantaneous and stretches over a horizontal region of 1.5 cm. The current work shows the feasibility of electrospinning the stem-branch-stem fibrous structure by adopting a valley-shaped collector with potentials for tissue engineering applications.

Oxygen Delivery Approaches to Augment Cell Survival After Myocardial Infarction: Progress and Challenges

Myocardial infarction (MI), triggered by blockage of a coronary artery, remains the most common cause of death worldwide. After MI, the capability of providing sufficient blood and oxygen significantly decreases in the heart. This event leads to depletion of oxygen from cardiac tissue and consequently leads to massive cardiac cell death due to hypoxemia. Over the past few decades, many studies have been carried out to discover acceptable approaches to treat MI. However, very few have addressed the crucial role of efficient oxygen delivery to the injured heart. Thus, various strategies were developed to increase the delivery of oxygen to cardiac tissue and improve its function. Here, we have given an overall discussion of the oxygen delivery mechanisms and how the current technologies are employed to treat patients suffering from MI, including a comprehensive view on three major technical approaches such as oxygen therapy, hemoglobin-based oxygen carriers (HBOCs), and oxygen-releasing biomaterials (ORBs). Although oxygen therapy and HBOCs have shown promising results in several animal and clinical studies, they still have a few drawbacks which limit their effectiveness. More recent studies have investigated the efficacy of ORBs which may play a key role in the future of oxygenation of cardiac tissue. In addition, a summary of conducted studies under each approach and the remaining challenges of these methods are discussed.

Converging functionality: Strategies for 3D hybrid-construct biofabrication and the role of composite biomaterials for skeletal regeneration

The evolution of additive manufacturing (AM) technologies, biomaterial development and our increasing understanding of cell biology has created enormous potential for the development of personalized regenerative therapies. In the context of skeletal tissue engineering, physical and biological demands play key roles towards successful construct implantation and the achievement of bone, cartilage and blood vessel tissue formation. Nevertheless, meeting such physical and biological demands to mimic the complexity of human tissues and their functionality is still a significant ongoing challenge. Recent studies have demonstrated that combination of AM technologies and advanced biomaterials has great potential towards skeletal tissue engineering. This review aims to analyze how the most prominent technologies and discoveries in the field converge towards the development of advanced constructs for skeletal regeneration. Particular attention is placed on hybrid biofabrication strategies, combining bioinks for cell delivery with biomaterial inks providing physical support. Hybrid biofabrication has been the focus of recent emerging strategies, however there has been limited review and analysis of these techniques and the challenges involved. Furthermore, we have identified that there are multiple hybrid fabrication strategies, here we present a category system where each strategy is reviewed highlighting their distinct advantages, challenges and potential applications. In addition, bioinks and biomaterial inks are the main components of the hybrid biofabrication strategies, where it is recognized that such platforms still lack optimal physical and biological functionality. Thus, this review also explores the development of composite materials specifically targeting the enhancement of physical and biological functionality towards improved skeletal tissue engineering. STATEMENT OF SIGNIFICANCE: Biofabrication strategies capable of recreating the complexity of native tissues could open new clinical possibilities towards patient-specific regenerative therapies and disease models. Several reviews target the existing additive manufacturing (AM) technologies that may be utilised for biomedical purposes. However, this work presents a unique perspective, describing how such AM technologies have been recently translated towards hybrid fabrication strategies, targeting the fabrication of constructs with converging physical and biological properties. Furthermore, we address composite bioinks and biomaterial inks that have been engineered to overcome traditional limitations, and might be applied to the hybrid fabrication strategies outlined. This work offers ample perspectives and insights into the current and future challenges for the fabrication of skeletal tissues aiming towards clinical and biomedical applications.

New Bone Formation in the Whole Decellularized Cortical Bone Scaffold Using the Model of Revitalizing a Haversian System

ABSTRACT

Decellularized allogeneic bone chips act as scaffolds for bone tissue regeneration. Owing to their lack of osteogenic potentials compared to autologous bone graft, decellularized bone scaffolds (DBSs) have applied only to small partial bone defects in clinical settings. Furthermore, only decellularized cancellous bone chips have been limitedly used for the purpose of bone regeneration. The cortical bone has less porosity and less osteogenic materials such as bone morphogenetic proteins in comparison with cancellous bone. In this study, we tried to accelerate new bone formation within the decellularized cortical bone scaffold using a vascular pedicle as an in vivo bioreactor.Forty DBSs were divided into 4 groups with different conditionings (DBS+ demineralized bone matrix [DBM], DBS+DBM+mesenchymal stem cells, DBS+DBM+vascular pedicle, and DBS+DBM+vascular pedicle+mesenchymal stem cells) and implanted into the back of 5 rabbits. Half of the DBSs were examined at 8 weeks and the other half at 16 weeks to determine vascularization level and osteogenesis within each group. New bone formation and bone-forming cells related to osteogenesis were observed via histological staining. Inclusion of the vascular pedicle resulted in larger areas of bone regeneration. With time, osteon structures became more prominent in groups containing the vascular pedicle.In summary, vascularized DBSs combined with a vascular pedicle have shown promising results for bone regeneration, thereby representing potential therapeutic alternatives for autologous bone grafts or bone tissue free transfer in large or segmental bone defects. In addition, demineralized whole cortical bone matrix along with vascular pedicle and various bone inductive materials, such as DBM and recombinant human bone morphogenetic protein-2, may be an additional new option of an ideal osteoinductive system.

Methods for fabricating oxygen releasing biomaterials

Sustained external supply of oxygen (O₂) to engineered tissue constructs is important for their survival in the body while angiogenesis is taking place. In the recent years, the trend towards the fabrication of various O₂-generating materials that can provide prolonged and controlled O₂ source to the large volume tissue constructs resulted in preventing necrosis associated with the lack of O₂ supply. In this review, we explain different methods employed in the fabrication of O₂-generating materials such as emulsion, microfluidics, solvent casting, freeze drying, electrospraying, gelation, microfluidic and three-dimensional (3D) bioprinting methods. After discussing pros and cons of each method, we review physical, chemical, and biological characterisation techniques used to analyse the resulting product. Finally, the challenges and future directions in the field are discussed.

Heparin-modified alginate microspheres enhance neovessel formation in hiPSC-derived endothelial cells and heterocellular in vitro models by controlled release of vascular endothelial growth factor

A formidable challenge in regenerative medicine is the development of stable microvascular networks to restore adequate blood flow or to sustain graft viability and long-term function in implanted or ischemic tissues. In this work, we develop a biomimetic approach to increase the binding affinity of the extracellular matrix for the class of heparin-binding growth factors to localize and control the release of proangiogenic cues while maintaining their bioactivity. Sulfate and heparin moieties are covalently coupled to alginate, and alginate microspheres are produced and used as local delivery depots for vascular endothelial growth factor (VEGF). Release of VEGF from sulfate-alginate and heparin-alginate bulk hydrogels and microspheres was sustained over 14 days. In vitro evaluation with human induced pluripotent stem cell (hiPSC)-derived endothelial cells and aortic ring assay in a chemically defined hydrogel demonstrates development of primitive three-dimensional vessel-like networks in the presence of VEGF released from the chemically modified alginate microspheres. Furthermore, our results suggest that the sulfate groups available on the chemically modified alginate microspheres promote some new vessel formation even in VEGF-free samples. Based on this evidence, we conclude that sulfate- and heparin-alginate hydrogels are adaptive and bioactive delivery systems for revascularization therapy and translational vascular tissue engineering.

Cell-based therapies for vascular regeneration: Past, present and future

Tissue vascularization remains one of the outstanding challenges in regenerative medicine. Beyond its role in circulating oxygen and nutrients, the vasculature is critical for organ development, function and homeostasis. Importantly, effective vascular regeneration is key in generating large 3D tissues for regenerative medicine applications to enable the survival of cells post-transplantation, organ growth, and integration into the host system. Therefore, the absence of clinically applicable means of (re)generating vessels is one of the main obstacles in cell replacement therapy. In this review, we highlight cell-based vascularization strategies which demonstrate clinical potential, discuss their strengths and limitations and highlight the main obstacles hindering cell-based therapeutic vascularization.

Engineering a Vascularized Hypoxic Tumor Model for Therapeutic Assessment

Solid tumors in advanced cancer often feature a structurally and functionally abnormal vasculature through tumor angiogenesis, which contributes to cancer progression, metastasis, and therapeutic resistances. Hypoxia is considered a major driver of angiogenesis in tumor microenvironments. However, there remains a lack of in vitro models that recapitulate both the vasculature and hypoxia in the same model with physiological resemblance to the tumor microenvironment, while allowing for high-content spatiotemporal analyses for mechanistic studies and therapeutic evaluations. We have previously constructed a hypoxia microdevice that utilizes the metabolism of cancer cells to generate an oxygen gradient in the cancer cell layer as seen in solid tumor sections. Here, we have engineered a new composite microdevice-microfluidics platform that recapitulates a vascularized hypoxic tumor. Endothelial cells were seeded in a collagen channel formed by viscous fingering, to generate a rounded vascular lumen surrounding a hypoxic tumor section composed of cancer cells embedded in a 3-D hydrogel extracellular matrix. We demonstrated that the new device can be used with microscopy-based high-content analyses to track the vascular phenotypes, morphology, and sprouting into the hypoxic tumor section over a 7-day culture, as well as the response to different cancer/stromal cells. We further evaluated the integrity/leakiness of the vascular lumen in molecular delivery, and the potential of the platform to study the movement/trafficking of therapeutic immune cells. Therefore, our new platform can be used as a model for understanding tumor angiogenesis and therapeutic delivery/efficacy in vascularized hypoxic tumors.

Hypoxia-responsive expression of vascular endothelial growth factor for induction of angiogenesis in artificial three-dimensional tissues

Constructing three-dimensional (3D) tissues is an important process to improve cellular functions in tissue engineering. When transplanting artificially constructed tissues, a poor vascular network restricts oxygen and nutrient supplies to the tissue cells, which leads to cell death and reduced rates of tissue engraftment. Therefore, it is necessary to develop a system that builds a vascular network within 3D tissues. Here, we developed a hypoxia-responsive gene expression system for production of an angiogenic factor, vascular endothelial growth factor (VEGF), to improve hypoxia and nutrition deficiencies inside artificial 3D tissues. We demonstrated that cells into which the hypoxia-responsive VEGF gene expression system had been introduced autonomously controlled VEGF expression in a hypoxic stress-dependent manner. Next, we confirmed that VEGF expression within a 3D cell sheet was induced in response to a hypoxic environment in vitro. The genetically modified cell sheet was subcutaneously transplanted into mice to evaluate the feasibility of the hypoxia-responsive VEGF gene expression system in vivo. The results suggest that the hypoxia-responsive VEGF gene expression system is promising to prepare artificial 3D tissues in regenerative medicine.

Combining tunable proteolytic sequences and a VEGF-mimetic peptide for the spatiotemporal control of angiogenesis within Elastin-Like Recombinamer scaffolds

One of the main challenges in regenerative medicine is the spatiotemporal control of angiogenesis, which is key for the successful repair of many tissues, and determines the proper integration of the implant through the generation of a functional vascular network. To this end, we have designed a three-dimensional (3D) model consisting of a coaxial binary elastin-like recombinamer (ELR) tubular construct. It displays fast and slow proteolytic hydrogels on its inner and outer part, respectively, both sensitive to the urokinase plasminogen activator protease. The ELRs used to build the scaffold included crosslinkable domains to stabilize the structure and a conjugated VEGF-derived peptide (QK) to induce angiogenesis. The mechanical and morphological evaluation of the ELR hydrogels proved their suitability for soft tissue regeneration. In addition, in vitro studies evidenced the effect of the QK peptide on endothelial cell spreading and anastomosis. Moreover, immunohistochemical analyses after subcutaneous implantation of the ELR hydrogels in mice showed the induction of a low macrophage response that resolved over time. The implantation of the 3D model constructs evidenced the ability of the fast proteolytic sequence and the QK peptide to guide cell infiltration and capillary formation in the pre-designed arrangement of the constructs. These results set the basis for the application of this type of scaffolds in regenerative medicine, where spatiotemporally controlled vascularization will help in the promotion of an optimal tissue repair. STATEMENT OF SIGNIFICANCE: Herein, we show the spatiotemporal control of angiogenesis in vivo by the combination of proteolytic sequences, with fast and slow degradation kinetics, and VEGF-mimetic peptide (QK) in a coaxial binary elastin-like recombinamer (ELR) tubular scaffold. These two bioactivities have been previously described for angiogenesis purposes, but have never been combined. This work demonstrates that the bioactivities act synergistically in promoting cell infiltration and subsequent vascularization, thus leading to a controlled evolution in space and time of the vascular microstructure within the hydrogel-like tubular scaffold. This effect has not been showed before and holds great potential for future vascular applications, which might be of great interest for a substantial part of Acta Biomaterialia readership.

In Situ Oxygen Generation Enhances the SCAP Survival in Hydrogel Constructs

Prolonged and severe hypoxia is the main cause of death of transplanted cells prior to the establishment of functional circulation. In situ generation of oxygen by oxygen-producing scaffolds-a unique solution that could produce and deliver oxygen to the adjacent cells independently of blood perfusion-has attracted considerable attention to enhance the survivability of the transplanted cells. However, the application of oxygen-generating scaffolds for facilitating cell survival in pulp-like tissue regeneration is yet to be explored. In this study, gelatin methacryloyl (GelMA)-a biocompatible scaffolding material that closely mimics the native extracellular matrix and is conducive to cell proliferation and differentiation-was used to fabricate oxygen-generating scaffolds by loading various concentrations of CaO₂. The CaO₂ distribution, topography, swelling, and pore size of CaO₂-GelMA hydrogels were characterized in detail. The release of O₂ by the scaffold and the viability, spreading, and proliferation of stem cells from apical papilla (SCAPs) encapsulated in the GelMA hydrogels with various concentrations of CaO₂ under hypoxia were evaluated. In addition, cellular constructs were engineered into root canals, and cell viability within the apical, middle, and coronal portions was assessed. Our findings showed that 0.5% CaO₂-GelMA was sufficient to supply in situ oxygen for maintaining the embedded SCAP viability for 1 wk. Furthermore, the 0.5% CaO₂-GelMA hydrogels improved the survivability of SCAPs within the coronal portion of the engineered cellular constructs within the root canals. This work demonstrated that 0.5% CaO₂-GelMA hydrogels offer a potential promising scaffold that enhances survival of the embedded SCAPs in endodontic regeneration.

Tissue-Specific Decellularized Extracellular Matrix Bioinks for Musculoskeletal Tissue Regeneration and Modeling Using 3D Bioprinting Technology

The musculoskeletal system is a vital body system that protects internal organs, supports locomotion, and maintains homeostatic function. Unfortunately, musculoskeletal disorders are the leading cause of disability worldwide. Although implant surgeries using autografts, allografts, and xenografts have been conducted, several adverse effects, including donor site morbidity and immunoreaction, exist. To overcome these limitations, various biomedical engineering approaches have been proposed based on an understanding of the complexity of human musculoskeletal tissue. In this review, the leading edge of musculoskeletal tissue engineering using 3D bioprinting technology and musculoskeletal tissue-derived decellularized extracellular matrix bioink is described. In particular, studies on in vivo regeneration and in vitro modeling of musculoskeletal tissue have been focused on. Lastly, the current breakthroughs, limitations, and future perspectives are described.

A novel engineered vascular construct of stem cell-laden 3D-printed PGSA scaffold enhances tissue revascularization

Development of transplantable engineered tissue has been hampered by lacking vascular network within the engineered tissue. Three-dimensional (3D) printing has emerged as a new technology with great potential in fabrication and customization of geometric microstructure. In this study, utilizing digital light processing system, we manufactured a recently designed novel 3D architecture scaffold with poly(glycerol sebacate) acrylate (PGSA). Vascular construct was subsequently generated by seeding stem cells within this scaffold. PGSA provided inductive substrate in terms of supporting three-germ layer differentiation of embryonic stem cells (ESCs) and also promoting ESCs-derived vascular progenitor cells (VPCs) differentiation into endothelial cells (ECs). Furthermore, the differentiation efficiency of VPCs into ECs on PGSA was much higher than that on collagen IV or fibronectin. The results from seeding VPCs in the rotating hexagonal PGSA scaffold suggest that this architectural framework is highly efficient for cell engraftment in 3D structures. After long-term suspension culture of the VPCs in scaffold under directed EC differentiation condition, VPC-differentiated ECs were populated in the scaffold and expressed EC markers. Transplantation of the vascular construct in mice resulted in formation of new vascular network and integration of the microvasculature within the scaffold into the existing vasculature of host tissue. Importantly, in a mouse model of wound healing, ECs from the transplanted vascular construct directly contributed to revascularization and enhanced blood perfusion at the injured site. Collectively, this transplantable vascular construct provides an innovative alternative therapeutic strategy for vascular tissue engineering.

Investigating the Viability of Epithelial Cells on Polymer Based Thin-Films

The development of novel polymer-based materials opens up possibilities for several novel applications, such as advanced wound dressings, bioinks for 3D biofabrication, drug delivery systems, etc. The aim of this study was to evaluate the viability of vascular and intestinal epithelial cells on different polymers as a selection procedure for more advanced cell-polymer applications. In addition, possible correlations between increased cell viability and material properties were investigated. Twelve polymers were selected, and thin films were prepared by dissolution and spin coating on silicon wafers. The prepared thin films were structurally characterized by Fourier transform infrared spectroscopy, atomic force microscopy, and goniometry. Their biocompatibility was determined using two epithelial cell lines (human umbilical vein endothelial cells and human intestinal epithelial cells), assessing the metabolic activity, cell density, and morphology. The tested cell lines showed different preferences regarding the culture substrate. No clear correlation was found between viability and individual substrate characteristics, suggesting that complex synergistic effects may play an important role in substrate design. These results show that a systematic approach is required to compare the biocompatibility of simple cell culture substrates as well as more complex applications (e.g., bioinks).

Engineering Vascularized Composite Allografts Using Natural Scaffolds: A Systematic Review

INTRODUCTION

Vascularized Composite Allotransplantation refers to the transplantation of multiple tissues as a functional unit from a deceased donor to a recipient with a severe injury. These grafts serve as potential replacements for traumatic tissue losses. The main problems are the consequences of the long immunosuppressive drugs medications and the lake of compatible donor. To avoid these limitations, decellularization/recellularization constitute an attractive approach. The aim of decellularization/recellularization technology is to develop immunogenic free biological substitutes that will restore, maintain, or improve tissue and organ's function.

METHODS

A PubMed search was performed for articles on decellularization and recellularization of composite tissue allografts between March and February 2021, with no restrictions in publication year. The selected reports were evaluated in terms of decellularization protocols, assessment of decellularized grafts, and evaluation of their biocompatibility and repopulation with cells both in vitro and in vivo.

RESULTS

The search resulted in a total of 88 articles. Each article was reviewed, 77 were excluded and the remaining 11 articles reported decellularization of 12 different vascular composite allografts in humans (four), large animals (three), and small animals (rodents) (five). The decellularization protocol for vascularized composite allotransplantation varies slightly between studies, but majority of the reports employ 1% sodium dodecyl sulfate as the main reagent for decellularization. The immunological response of the decellularized scaffolds remains poorly evaluated. Few authors have been able to attempt the recellularization and transplantation of these scaffolds. Successful transplantation seems to require prior recellularization.

CONCLUSION

Decellularization/recellularization is a promising, growing, emerging developing research field in vascular composite allotransplantation.

3D bioprinting in cardiac tissue engineering

Heart disease is the main cause of death worldwide. Because death of the myocardium is irreversible, it remains a significant clinical challenge to rescue myocardial deficiency. Cardiac tissue engineering (CTE) is a promising strategy for repairing heart defects and offers platforms for studying cardiac tissue. Numerous achievements have been made in CTE in the past decades based on various advanced engineering approaches. 3D bioprinting has attracted much attention due to its ability to integrate multiple cells within printed scaffolds with complex 3D structures, and many advancements in bioprinted CTE have been reported recently. Herein, we review the recent progress in 3D bioprinting for CTE. After a brief overview of CTE with conventional methods, the current 3D printing strategies are discussed. Bioink formulations based on various biomaterials are introduced, and strategies utilizing composite bioinks are further discussed. Moreover, several applications including heart patches, tissue-engineered cardiac muscle, and other bionic structures created via 3D bioprinting are summarized. Finally, we discuss several crucial challenges and present our perspective on 3D bioprinting techniques in the field of CTE.

Recent Progress on Cellulose-Based Ionic Compounds for Biomaterials

Glycans play important roles in all major kingdoms of organisms, such as archea, bacteria, fungi, plants, and animals. Cellulose, the most abundant polysaccharide on the Earth, plays a predominant role for mechanical stability in plants, and finds a plethora of applications by humans. Beyond traditional use, biomedical application of cellulose becomes feasible with advances of soluble cellulose derivatives with diverse functional moieties along the backbone and modified nanocellulose with versatile functional groups on the surface due to the native features of cellulose as both cellulose chains and supramolecular ordered domains as extractable nanocellulose. With the focus on ionic cellulose-based compounds involving both these groups primarily for biomedical applications, a brief introduction about glycoscience and especially native biologically active glycosaminoglycans with specific biomedical application areas on humans is given, which inspires further development of bioactive compounds from glycans. Then, both polymeric cellulose derivatives and nanocellulose-based compounds synthesized as versatile biomaterials for a large variety of biomedical applications, such as for wound dressings, controlled release, encapsulation of cells and enzymes, and tissue engineering, are separately described, regarding the diverse routes of synthesis and the established and suggested applications for these highly interesting materials.

Induced Pluripotent Stem Cell-Derived Endothelial Networks Accelerate Vascularization But Not Bone Regeneration

Vascularization is critical for engineering mineralized tissues. It has been previously shown that biomaterials containing preformed endothelial networks anastomose to host vasculature following implantation. However, the networks alone may not increase regeneration. In addition, a clinically applicable source of cells for vascularization is needed. In this study, vascular networks were generated from endothelial cells (ECs) derived from human induced pluripotent stem cells (iPSCs). Network formation by iPSC-ECs within fibrin gels was investigated in a mesenchymal stem cells (MSCs) coculture spheroid model. Statistical design of experiments technique was evaluated for its predicting capability during the optimization of experimental parameters. The prevascularized units were combined with hydroxyapatite nanoparticles to develop a vascularized composite hydrogel that was implanted in a rodent critical-sized cranial defect model. Immunohistological staining for human-specific CD31 at week 1 indicated the presence and maintenance of the implanted vessels. At 8 weeks, the prevascularized systems resulted in higher vessel density over MSC-only scaffolds. The implanted vessels appeared to establish flow with host vasculature. While there was a slight increase in bone volume in the prevascularized bone construct compared to MSC-only bone constructs, there was not a profound increase in bone regeneration. These results show that scaffolds with network structures can be generated from ECs derived from iPSC and that the networks survive and inosculate with the host postimplantation in a bone model. Impact statement Vascularization is critical for engineering bone. Prevascularized scaffolds have been shown to improve postimplantation vascularization. Herein, vascularized networks were generated from induced pluripotent cells derived from endothelial cells. These vascularized units were combined with a fibrin/hydroxyapatite scaffold to develop a prevascularized construct for bone regeneration. Implantation of these scaffolds in a small animal cranial defect model resulted in network inosculation and increased vascularization, but exhibited only a limited effect on bone formation. This study provides insight into the challenges of generating vascularized bone.

Magnetic stimulation of the angiogenic potential of mesenchymal stromal cells in vascular tissue engineering

The growing prevalence of vascular diseases worldwide has emphasized the need for novel tissue-engineered options concerning the development of vascularized 3D constructs. This study reports, for the first time, the use of external magnetic fields to stimulate mesenchymal stromal cells (MSCs) to increase the production of vascular endothelial growth factor-A (VEGF-A). Polyvinylalcohol and gelatin-based scaffolds, containing iron oxide nanoparticles, were designed for optimal cell magnetic stimulation. While the application of static magnetic fields over 24 h did not impact on MSCs proliferation, viability and phenotypic identity, it significantly increased the production of VEGF-A and guided MSCs morphology and alignment. The ability to enhance MSCs angiogenic potential was demonstrated by the increase in the number of new vessels formed in the presence of MSCs conditioned media through in vitro and in vivo models. Ultimately, this study uncovers the potential to manipulate cellular processes through short-term magnetic stimulation.

Scaffolds for the manufacture of cultured meat

The cultured meat market has been growing at an accelerated space since the first creation of cultured meat burger back in 2013. Substantial efforts have been made to reduce costs by eliminating serum in growth media and improving process efficiency by employing bioreactors. In parallel, efforts are also being made on scaffolding innovations to offer better cells proliferation, differentiation and tissue development. So far, scaffolds used in cultured meat research are predominantly collagen and gelatin, which are animal-derived. To align with cell-based meat vision i.e. environment conservation and animal welfare, plant-derived biomaterials for scaffolding are being intensively explored. This paper reviews and discusses the advantages and disadvantages of scaffold materials and potential scaffolding related to scale-up solution for the production of cultured meat.