Cell-based therapeutics from an economic perspective: primed for a commercial success or a research sinkhole?

Research Hotspots and Trends of Bone Xenograft in Clinical Procedures: A Bibliometric and Visual Analysis of the Past Decade

BACKGROUND

Bone defect therapy is a common clinical challenge for orthopedic and clinical physicians worldwide, and the therapeutic effect affects the physiological function and healthy life quality of millions of patients. Compared with traditional autogenous bone transplants, bone xenografts are attracting attention due to their advantages of unlimited availability and avoidance of secondary damage. However, there is currently a lack of bibliometric analysis on bone xenograft. This study aimed to use bibliometric methods to analyze the literature on bone xenograft from 2013 to 2023, to explore the current status, hotspots, and future trends of research in this field, and to promote its development and progress.

METHODS

Using the Web of Science Core Collection database, we retrieved and collected publication data related to xenogeneic bone grafting materials worldwide from January 2013 to March 2023. Origin (2021), CiteSpace (6.2.R2 standard), and an online bibliometric platform were used for bibliometric analysis and data visualization.

RESULTS

A total of 3395 documents were retrieved, and 686 eligible papers were selected. The country and institutions with the highest number of publications and centrality were the United States (125 papers, centrality = 0.44) and the University of Zurich (29 papers, centrality = 0.28), respectively. The most cited author was Araujo MG (163 times), and the author with the most significant centrality was Froum SJ (centrality = 0.09). The main keyword clusters were "tissue engineering", "sinus floor elevation", "dental implants", "tooth extraction", and "bone substitutes". The most significant bursting keywords in the last three years were "platelet rich fibrin".

CONCLUSIONS

Research on bone xenograft is steadily growing and will continue to rise. Currently, research hotspots and directions are mainly focused on dental implants related to bone-augmentation techniques and bone tissue engineering. In the future, research hotspots and directions may focus on decellularization technology and investigations involving platelet-rich fibrin.

Development of a vascular substitute produced by weaving yarn made from human amniotic membrane

Because synthetic vascular prostheses perform poorly in small-diameter revascularization, biological vascular substitutes are being developed as an alternative. Although their in vivo results are promising, their production involves long, complex, and expensive tissue engineering methods. To overcome these limitations, we propose an innovative approach that combines the human amniotic membrane (HAM), which is a widely available and cost-effective biological raw material, with a rapid and robust textile-inspired assembly strategy. Fetal membranes were collected after cesarean deliveries at term. Once isolated by dissection, HAM sheets were cut into ribbons that could be further processed by twisting into threads. Characterization of the HAM yarns (both ribbons and threads) showed that their physical and mechanical properties could be easily tuned. Since our clinical strategy will be to provide an off-the-shelf allogeneic implant, we studied the effects of decellularization and/or gamma sterilization on the histological, mechanical, and biological properties of HAM ribbons. Gamma irradiation of hydrated HAMs, with or without decellularization, did not interfere with the ability of the matrix to support endothelium formation in vitro. Finally, our HAM-based, woven tissue-engineered vascular grafts (TEVGs) exhibited clinically relevant mechanical properties. Thus, this study demonstrates that human, completely biological, allogeneic, small-diameter TEVGs can be produced from HAM, thereby avoiding costly cell culture and bioreactors.

A blueprint for translational regenerative medicine

The past few decades have produced a large number of proof-of-concept studies in regenerative medicine. However, the route to clinical adoption is fraught with technical and translational obstacles that frequently consign promising academic solutions to the so-called "valley of death." Here, we present a proposed blueprint for translational regenerative medicine. We offer principles to help guide the selection of cells and materials, present key in vivo imaging modalities, and argue that the host immune response should be considered throughout design and development. Last, we suggest a pathway to navigate the often complex regulatory and manufacturing landscape of translational regenerative medicine.

A review of biomimetic scaffolds for bone regeneration: Toward a cell-free strategy

In clinical terms, bone grafting currently involves the application of autogenous, allogeneic, or xenogeneic bone grafts, as well as natural or artificially synthesized materials, such as polymers, bioceramics, and other composites. Many of these are associated with limitations. The ideal scaffold for bone tissue engineering should provide mechanical support while promoting osteogenesis, osteoconduction, and even osteoinduction. There are various structural complications and engineering difficulties to be considered. Here, we describe the biomimetic possibilities of the modification of natural or synthetic materials through physical and chemical design to facilitate bone tissue repair. This review summarizes recent progresses in the strategies for constructing biomimetic scaffolds, including ion-functionalized scaffolds, decellularized extracellular matrix scaffolds, and micro- and nano-scale biomimetic scaffold structures, as well as reactive scaffolds induced by physical factors, and other acellular scaffolds. The fabrication techniques for these scaffolds, along with current strategies in clinical bone repair, are described. The developments in each category are discussed in terms of the connection between the scaffold materials and tissue repair, as well as the interactions with endogenous cells. As the advances in bone tissue engineering move toward application in the clinical setting, the demonstration of the therapeutic efficacy of these novel scaffold designs is critical.

Effects of a Bioactive SPPEPS Peptide on Chondrogenic Differentiation of Mesenchymal Stem Cells

A synthetic 'chondroinductive' biomaterial that could induce chondrogenesis without the need for growth factors, extracellular matrix, or pre-seeded cells could revolutionize orthopedic regenerative medicine. The objective of the current study was thus to introduce a synthetic SPPEPS peptide and evaluate its ability to induce chondrogenic differentiation. In the current study, dissolving a synthetic chondroinductive peptide candidate (100 ng/mL SPPEPS) in the culture medium of rat bone marrow-derived mesenchymal stem cells (rBMSCs) elevated collagen type II gene expression compared to the negative control (no growth factor or peptide in the cell culture medium) after 3 days. In addition, proteomic analyses indicated similarities in pathways and protein profiles between the positive control (10 ng/mL TGF-β₃) and peptide group (100 ng/mL SPPEPS), affirming the potential of the peptide for chondroinductivity. Incorporating the SPPEPS peptide in combination with the RGD peptide in pentenoate-functionalized hyaluronic acid (PHA) hydrogels elevated the collagen type II gene expression of the rBMSCs cultured on top of the hydrogels compared to using either peptide alone. The evidence suggests that SPPEPS may be a chondroinductive peptide, which may be enhanced in combination with an adhesion peptide.

Current and Future Perspectives on Skin Tissue Engineering: Key Features of Biomedical Research, Translational Assessment, and Clinical Application

The skin is responsible for several important physiological functions and has enormous clinical significance in wound healing. Tissue engineered substitutes may be used in patients suffering from skin injuries to support regeneration of the epidermis, dermis, or both. Skin substitutes are also gaining traction in the cosmetics and pharmaceutical industries as alternatives to animal models for product testing. Recent biomedical advances, ranging from cellular-level therapies such as mesenchymal stem cell or growth factor delivery, to large-scale biofabrication techniques including 3D printing, have enabled the implementation of unique strategies and novel biomaterials to recapitulate the biological, architectural, and functional complexity of native skin. This progress report highlights some of the latest approaches to skin regeneration and biofabrication using tissue engineering techniques. Current challenges in fabricating multilayered skin are addressed, and perspectives on efforts and strategies to meet those limitations are provided. Commercially available skin substitute technologies are also examined, and strategies to recapitulate native physiology, the role of regulatory agencies in supporting translation, as well as current clinical needs, are reviewed. By considering each of these perspectives while moving from bench to bedside, tissue engineering may be leveraged to create improved skin substitutes for both in vitro testing and clinical applications.

Disruptive technological advances in vascular access for dialysis: an overview

End-stage kidney disease (ESKD), one of the most prevalent diseases in the world and with increasing incidence, is associated with significant morbidity and mortality. Current available modes of renal replacement therapy (RRT) include dialysis and renal transplantation. Though renal transplantation is the preferred and ideal mode of RRT, this modality may not be available to all patients with ESKD. Moreover, renal transplant recipients are constantly at risk of complications associated with immunosuppression and immunosuppressant use, and posttransplant lymphoproliferative disorder. Dialysis may be the only available modality in certain patients. However, dialysis has its limitations, which include issues associated with lack of vascular access, risks of infections and vascular thrombosis, decreased quality of life, and absence of biosynthetic functions of the kidney. In particular, the creation and maintenance of hemodialysis vascular access in children poses a unique set of challenges to the pediatric nephrologist owing to the smaller vessel diameters and vascular hyperreactivity compared with adult patients. Vascular access issues continue to be one of the major limiting factors prohibiting the delivery of adequate dialysis in ESKD patients and is the Achilles' heel of hemodialysis. This review aims to provide a critical overview of disruptive technological advances and innovations for vascular access. Novel strategies in preventing neointimal hyperplasia, novel bioengineered products, grafts and devices for vascular access will be discussed. The potential impact of these solutions on improving the morbidity encountered by dialysis patients will also be examined.

Building a Total Bioartificial Heart: Harnessing Nature to Overcome the Current Hurdles

Engineering a bioartificial heart has become a possibility in part because of the regenerative medicine approaches to repairing or replacing damaged organs that have evolved over the past two decades. With the advent of inducible pluripotent stem cell technology, it is now possible to generate personalized cells that make the concept of autologous tissue engineering imaginable. Scaffolds that provide form, function, and biological cues to cells likewise potentially enable the engineering of biocompatible vascularized solid organs. Decellularized organs or tissue matrices retain organ complexity and structure at the macro and micro scales, contain biologically active molecules that support cell phenotype and function, and are vascularized allowing full thickness tissue generation. There is also dynamic reciprocity between the extracellular matrix and cells, which does not occur with synthetic scaffolds and allows both to evolve as functional need changes, making it a unique scaffold. Yet, building a whole heart from decellularized scaffolds and cells requires delivering hundreds of billions of multiple types of cardiac cells appropriately and providing a milieu where they can survive and mature. We propose a novel type of in vivo organ engineering utilizing pre-clinical models where decellularized hearts are heterotopically transplanted with the intent to harness the capability of the body to at least in part repopulate the scaffold. By adding load and electrical input, possibly via temporary mechanical assistance, we posit that vascular and parenchymal cell maturation can occur. In this study, we implanted porcine decellularized hearts acutely and chronically in living recipients in a heterotopic position. We demonstrated that the surgical procedure is critical to prevent coagulation and to increase graft patency. We also demonstrated that short-term implantation promotes endothelial cell adhesion to the vessel lumens and that long-term implantation also promotes tissue formation with evidence of cardiomyocytes and endothelial cells present within the graft. Utilizing endogenous repair capabilities of the recipient in response to a naked ECM, we allowed the transplanted scaffold to direct host cells-both organizationally and functionally. Thus, the scaffold provided necessary cues for cell organization and remodeling within the transplanted organ. Future work would involve culturing partially recellularized engineered organs in bioreactors where mechanical and electrical stimulation can be controlled to promote organ development and then transplanting these after a minimal level of maturation has been achieved.

Generating vascular conduits: from tissue engineering to three-dimensional bioprinting

Vascular disease - including coronary artery disease, carotid artery disease, and peripheral vascular disease - is a leading cause of morbidity and mortality worldwide. The standard of care for restoring patency or bypassing occluded vessels involves using autologous grafts, typically the saphenous veins or internal mammary arteries. Yet, many patients who need life- or limb-saving procedures have poor outcomes, and a third of patients who need vascular intervention have multivessel disease and therefore lack appropriate vasculature to harvest autologous grafts from. Given the steady increase in the prevalence of vascular disease, there is great need for grafts with the biological and mechanical properties of native vessels that can be used as vascular conduits. In this review, we present an overview of methods that have been employed to generate suitable vascular conduits, focusing on the advances in tissue engineering methods and current three-dimensional (3D) bioprinting methods. Tissue-engineered vascular grafts have been fabricated using a variety of approaches such as using preexisting scaffolds and acellular organic compounds. We also give an extensive overview of the novel use of 3D bioprinting as means of generating new vascular conduits. Different strategies have been employed in bioprinting, and the use of cell-based inks to create de novo structures offers a promising solution to bridge the gap of paucity of optimal donor grafts. Lastly, we provide a glimpse of our work to create scaffold-free, bioreactor-free, 3D bioprinted vessels from a combination of rat vascular smooth muscle cells and fibroblasts that remain patent and retain the tensile and mechanical strength of native vessels.

Tissue-Engineered Vascular Graft of Small Diameter Based on Electrospun Polylactide Microfibers

Tubular vascular grafts 1.1 mm in diameter based on poly(L-lactide) microfibers were obtained by electrospinning. X-ray diffraction and scanning electron microscopy data demonstrated that the samples treated at T = 70°C for 1 h in the fixed state on a cylindrical mandrel possessed dense fibrous structure; their degree of crystallinity was approximately 44%. Strength and deformation stability of these samples were higher than those of the native blood vessels; thus, it was possible to use them in tissue engineering as bioresorbable vascular grafts. The experiments on including implantation into rat abdominal aorta demonstrated that the obtained vascular grafts did not cause pathological reactions in the rats; in four weeks, inner side of the grafts became completely covered with endothelial cells, and fibroblasts grew throughout the wall. After exposure for 12 weeks, resorption of PLLA fibers started, and this process was completed in 64 weeks. Resorbed synthetic fibers were replaced by collagen and fibroblasts. At that time, the blood vessel was formed; its neointima and neoadventitia were close to those of the native vessel in structure and composition.

Recent advances in cell-laden 3D bioprinting: materials, technologies and applications

Fabrication of 3D scaffolds with patient-specific designs, high structural and component complexity, and rapid on-demand production at a low-cost by printing technique has attracted ever-increasing interests in tissue engineering. Cell-laden 3D bioprinting offers good prospects for future organ transplantation. Compared with nonbiological 3D printing, cell-laden 3D bioprinting involves more complex factors, including the choice of printing materials, the strategy of gelling, cell viability and technical challenges. Although cell-populated 3D bioprinting has so many complex factors, it has proven to be a useful and exciting tool with wide potential applications in regenerative medicine to generate a variety of transplantable tissues. In this review, we first overview the bioprinting materials, gelling strategies and some major applications of cell-laden 3D bioprinting, with main focus on the recent advances and current challenges of the field. Finally, we propose some future directions of the cell-populated 3D bioprinting in tissue engineering and regenerative medicine.

[Formula: see text]

In this review, we first overview the bioprinting materials, gelling strategies and some major applications of cell-populated 3D bioprinting, with main focus on the recent advances and current challenges of the field. Finally, we propose some future directions of the cell-laden 3D bioprinting in tissue engineering and regenerative medicine.

Bone marrow mesenchymal stem cell-derived acellular matrix-coated chitosan/silk scaffolds for neural tissue regeneration

A novel tissue engineered nerve graft (TENG) was used for the first time to bridge a 60 mm long nerve gap in a dog sciatic nerve and achieved satisfactory results.

Induced pluripotent stem cell-derived vascular smooth muscle cells: methods and application

Vascular smooth muscle cells (VSMCs) play a major role in the pathophysiology of cardiovascular diseases. The advent of induced pluripotent stem cell (iPSC) technology and the capability of differentiating into virtually every cell type in the human body make this field a ray of hope for vascular regenerative therapy and understanding of the disease mechanism. In the present review, we first discuss the recent iPSC technology and vascular smooth muscle development from an embryo and then examine different methodologies to derive VSMCs from iPSCs, and their applications in regenerative therapy and disease modelling.

Tissue engineered scaffolds for an effective healing and regeneration: reviewing orthotopic studies

It is commonly stated that tissue engineering is the most promising approach to treat or replace failing tissues/organs. For this aim, a specific strategy should be planned including proper selection of biomaterials, fabrication techniques, cell lines, and signaling cues. A great effort has been pursued to develop suitable scaffolds for the restoration of a variety of tissues and a huge number of protocols ranging from in vitro to in vivo studies, the latter further differentiating into several procedures depending on the type of implantation (i.e., subcutaneous or orthotopic) and the model adopted (i.e., animal or human), have been developed. All together, the published reports demonstrate that the proposed tissue engineering approaches spread toward multiple directions. The critical review of this scenario might suggest, at the same time, that a limited number of studies gave a real improvement to the field, especially referring to in vivo investigations. In this regard, the present paper aims to review the results of in vivo tissue engineering experimentations, focusing on the role of the scaffold and its specificity with respect to the tissue to be regenerated, in order to verify whether an extracellular matrix-like device, as usually stated, could promote an expected positive outcome.

Implantation of completely biological engineered grafts following decellularization into the sheep femoral artery

The performance of completely biological, decellularized engineered allografts in a sheep model was evaluated to establish clinical potential of these unique arterial allografts. The 4-mm-diameter, 2-3-cm-long grafts were fabricated from fibrin gel remodeled into an aligned tissue tube in vitro by ovine dermal fibroblasts. Decellularization and subsequent storage had little effect on graft properties, with burst pressure exceeding 4000 mmHg and the same compliance as the ovine femoral artery. Grafts were implanted interpositionally in the femoral artery of six sheep (n=9), with contralateral sham controls (n=3). At 8 weeks (n=5) and 24 weeks (n=4), all grafts were patent and showed no evidence of dilatation or mineralization. Mid-graft lumen diameter was unchanged. Extensive recellularization occurred, with most cells expressing αSMA. Endothelialization was complete by 24 weeks with elastin deposition evident. These completely biological grafts possessed circumferential alignment/mechanical anisotropy characteristic of native arteries and were cultured only 5 weeks prior to decellularization and storage as "off-the-shelf" grafts.

Treating the whole not the hole: necessary coupling of technologies for diabetic foot ulcer treatment

Type 2 diabetes is the epidemic of our generation, and diabetic foot ulcers (DFUs) are a major complication. Although DFU formation itself can indicate disease progression, the failure to effectively treat ulcers contributes further to a decay in patient quality of life and increased mortality. Herein we discuss the development of next-generation DFU therapies including: (i) topical growth factors, (ii) scaffolds, and (iii) cellular therapies. Individually these therapies have yielded measurable but modest improvements in DFU repair. Because DFUs arise as a result of multiple biochemical deficiencies, a singular treatment modality is unlikely to be effective. Next-generation DFU technologies must be combined to address effectively the complex underlying pathology and enable reliable DFU repair.

Acellular biomaterials: an evolving alternative to cell-based therapies

Acellular biomaterials can stimulate the local environment to repair tissues without the regulatory and scientific challenges of cell-based therapies. A greater understanding of the mechanisms of such endogenous tissue repair is furthering the design and application of these biomaterials. We discuss recent progress in acellular materials for tissue repair, using cartilage and cardiac tissues as examples of applications with substantial intrinsic hurdles, but where human translation is now occurring.

Improved recellularization of ex vivo vascular scaffolds using directed transport gradients to modulate ECM remodeling

The regeneration of functional, clinically viable, tissues from acellular ex vivo tissues has been problematic largely due to poor nutrient transport conditions that limit cell migration and integration. Compounding these issues are subcellular pore sizes that necessarily requires extracellular matrix (ECM) remodeling in order for cells to migrate and regenerate the tissue. The aim of the present work was to create a directed growth environment that allows cells to fully populate an ex vivo-derived vascular scaffold and maintain viability over extended periods. Three different culture conditions using single (one nutrient source) or dual perfusion bioreactor systems (two nutrients sources) were designed to assess the effect of pressure and nutrient gradients under either low (50/30 mmHg) or high (120/80) relative pressure conditions. Human myofibroblasts were seeded to the ablumenal periphery of an ex vivo-derived vascular scaffold using a collagen/hydrogel cell delivery system. After 30 days culture, total cell density was consistent between groups; however, significant variation was noted in cell distribution and construct mechanics as a result of differing perfusion conditions. The most aggressive transport gradient was developed by the single perfusion low-pressure circuits and resulted in a higher proportion of cells migrating across the scaffold toward the vessel lumen (nutrient source). These investigations illustrate the influence of directed nutrient gradients where precisely controlled perfusion conditions significantly affects cell migration, distribution and function, resulting in pronounced effects on construct mechanics during early remodeling events.

The expanding world of tissue engineering: the building blocks and new applications of tissue engineered constructs

The field of tissue engineering has been growing in the recent years as more products have made it to the market and as new uses for the engineered tissues have emerged, motivating many researchers to engage in this multidisciplinary field of research. Engineered tissues are now not only considered as end products for regenerative medicine, but also have emerged as enabling technologies for other fields of research ranging from drug discovery to biorobotics. This widespread use necessitates a variety of methodologies for production of tissue engineered constructs. In this review, these methods together with their non-clinical applications will be described. First, we will focus on novel materials used in tissue engineering scaffolds; such as recombinant proteins and synthetic, self assembling polypeptides. The recent advances in the modular tissue engineering area will be discussed. Then scaffold-free production methods, based on either cell sheets or cell aggregates will be described. Cell sources used in tissue engineering and new methods that provide improved control over cell behavior such as pathway engineering and biomimetic microenvironments for directing cell differentiation will be discussed. Finally, we will summarize the emerging uses of engineered constructs such as model tissues for drug discovery, cancer research and biorobotics applications.

Effect of bone marrow-derived extracellular matrix on cardiac function after ischemic injury

Ischemic heart disease is a leading cause of death, with few options to retain ventricular function following myocardial infarction. Hematopoietic-derived progenitor cells contribute to angiogenesis and tissue repair following ischemia reperfusion injury. Motivated by the role of bone marrow extracellular matrix (BM-ECM) in supporting the proliferation and regulation of these cell populations, we investigated BM-ECM injection in myocardial repair. In BM-ECM isolated from porcine sternum, we identified several factors important for myocardial healing, including vascular endothelial growth factor, basic fibroblast growth factor-2, and platelet-derived growth factor-BB. We further determined that BM-ECM serves as an adhesive substrate for endothelial cell proliferation. Bone marrow ECM was injected in a rat model of myocardial infarction, with and without a methylcellulose carrier gel. After one day, reduced infarct area was noted in rats receiving BM-ECM injection. After seven days we observed improved fractional shortening, decreased apoptosis, and significantly lower macrophage counts in the infarct border. Improvements in fractional shortening, sustained through 21 days, as well as decreased fibrotic area, enhanced angiogenesis, and greater c-kit-positive cell presence were associated with BM-ECM injection. Notably, the concentrations of BM-ECM growth factors were 10(3)-10(8) fold lower than typically required to achieve a beneficial effect, as reported in pre-clinical studies that have administered single growth factors alone.

Regenerative medicine in rheumatic disease-progress in tissue engineering

Joint destruction occurs in both osteoarthritis and rheumatoid arthritis. Even in the era of biologic agents, this destruction can be delayed but not averted. As cartilage has limited ability to self-regenerate, joint arthroplasty is required. Here, we outline current tissue engineering procedures (including autologous chondrocyte implantation and in situ mesenchymal stem cell recruitment) that are routinely applied for the regenerative treatment of injured or early osteoarthritic cartilage. Potential future regenerative therapies, including administration of multipotent or pluripotent stem cells, are also discussed. In the future, cell-free, material-based (for cartilage lesions) or cell-free, factor-based (for osteoarthritic cartilage) therapies to facilitate the recruitment of repair cells and improve cartilage metabolism are likely to become more important. Moreover, delivery of anti-inflammatory factors or immunomodulatory cells could be a regenerative treatment option for rheumatoid arthritis. Tissue engineering faces a crucial phase to translate products into clinical routine and the regulatory framework for cell-based products in particular is an important issue.

New biological solutions for hemodialysis access

Since Scribner described the first prosthetic chronic dialysis shunt in 1961, the surgical techniques and strategies to maintain vascular access have improved dramatically. Today, hundreds of thousands of patients worldwide are treated with some combination of native vein fistula, synthetic vascular graft, or synthetic semipermanent catheter. Despite significantly lower efficacy compared with autologous fistulae, the basic materials used for synthetic shunts and catheters have evolved surprisingly slowly. The disparity between efficacy rates and concomitant maintenance costs has driven a strong campaign to decrease the use of synthetic grafts and catheters in favor of native fistulae. Whether arguing the benefits of Fistula First or "Catheter Last," the fact that clinicians are in need of an alternative to expanded polytetrafluoroethylene (ePTFE) is irrefutable. The poor performance of synthetic materials has a significant economic impact as well. End-stage renal disease (ESRD) accounts for approximately 6% of Medicare's overall budget, despite a prevalence of about 0.17%. Of that, 15%-25% is spent on access maintenance, making hemodialysis access a critical priority for Medicare. This clinical and economic situation has spawned an aggressive effort to improve clinical care strategies to reduce overall cost and complications. While the bulk of this effort has historically focused on developing new synthetic biomaterials, more recently, investigators have developed a variety of cell-based strategies to create tissue-engineered vascular grafts. In this article, we review the evolution of the field of cardiovascular tissue engineering. We also present an update on the Lifeline™ vascular graft, an autologous, biological, and tissue-engineered vascular graft, which was the first tissue-engineered graft to be used clinically in dialysis patients.

From banking to international governance: fostering innovation in stem cell research

Stem cell banks are increasingly recognized as an essential resource of biological materials for both basic and translational stem cell research. By providing transnational access to quality controlled and ethically sourced stem cell lines, stem cell banks seek to foster international collaboration and innovation. However, given that national stem cell banks operate under different policy, regulatory and commercial frameworks, the transnational sharing of stem cell materials and data can be complicating. This paper will provide an overview of the most pressing challenges regarding the governance of stem cell banks, and the difficulties in designing regulatory and commercial frameworks that foster stem cell research. Moreover, the paper will shed light on the numerous international initiatives that have arisen to help harmonize and standardize stem cell banking and research processes to overcome such challenges.

Whole-organ tissue engineering: decellularization and recellularization of three-dimensional matrix scaffolds

The definitive treatment for end-stage organ failure is orthotopic transplantation. However, the demand for transplantation far exceeds the number of available donor organs. A promising tissue-engineering/regenerative-medicine approach for functional organ replacement has emerged in recent years. Decellularization of donor organs such as heart, liver, and lung can provide an acellular, naturally occurring three-dimensional biologic scaffold material that can then be seeded with selected cell populations. Preliminary studies in animal models have provided encouraging results for the proof of concept. However, significant challenges for three-dimensional organ engineering approach remain. This manuscript describes the fundamental concepts of whole-organ engineering, including characterization of the extracellular matrix as a scaffold, methods for decellularization of vascular organs, potential cells to reseed such a scaffold, techniques for the recellularization process and important aspects regarding bioreactor design to support this approach. Critical challenges and future directions are also discussed.

Tissue Engineering of Blood Vessels: Functional Requirements, Progress, and Future Challenges

Vascular disease results in the decreased utility and decreased availability of autologus vascular tissue for small diameter (< 6 mm) vessel replacements. While synthetic polymer alternatives to date have failed to meet the performance of autogenous conduits, tissue-engineered replacement vessels represent an ideal solution to this clinical problem. Ongoing progress requires combined approaches from biomaterials science, cell biology, and translational medicine to develop feasible solutions with the requisite mechanical support, a non-fouling surface for blood flow, and tissue regeneration. Over the past two decades interest in blood vessel tissue engineering has soared on a global scale, resulting in the first clinical implants of multiple technologies, steady progress with several other systems, and critical lessons-learned. This review will highlight the current inadequacies of autologus and synthetic grafts, the engineering requirements for implantation of tissue-engineered grafts, and the current status of tissue-engineered blood vessel research.

Enhancing Cell therapies from the Outside In: Cell Surface Engineering Using Synthetic Nanomaterials

Therapeutic treatments based on the injection of living cells are in clinical use and preclinical development for diseases ranging from cancer to cardiovascular disease to diabetes. To enhance the function of therapeutic cells, a variety of chemical and materials science strategies are being developed that engineer the surface of therapeutic cells with new molecules, artificial receptors, and multifunctional nanomaterials, synthetically endowing donor cells with new properties and functions. These approaches offer a powerful complement to traditional genetic engineering strategies for enhancing the function of living cells.

Readily available tissue-engineered vascular grafts

Autologous or synthetic vascular grafts are used routinely for providing access in hemodialysis or for arterial bypass in patients with cardiovascular disease. However, some patients either lack suitable autologous tissue or cannot receive synthetic grafts. Such patients could benefit from a vascular graft produced by tissue engineering. Here, we engineer vascular grafts using human allogeneic or canine smooth muscle cells grown on a tubular polyglycolic acid scaffold. Cellular material was removed with detergents to render the grafts nonimmunogenic. Mechanical properties of the human vascular grafts were similar to native human blood vessels, and the grafts could withstand long-term storage at 4 °C. Human engineered grafts were tested in a baboon model of arteriovenous access for hemodialysis. Canine grafts were tested in a dog model of peripheral and coronary artery bypass. Grafts demonstrated excellent patency and resisted dilatation, calcification, and intimal hyperplasia. Such tissue-engineered vascular grafts may provide a readily available option for patients without suitable autologous tissue or for those who are not candidates for synthetic grafts.

Towards organ printing: engineering an intra-organ branched vascular tree

IMPORTANCE OF THE FIELD

Effective vascularization of thick three-dimensional engineered tissue constructs is a problem in tissue engineering. As in native organs, a tissue-engineered intra-organ vascular tree must be comprised of a network of hierarchically branched vascular segments. Despite this requirement, current tissue-engineering efforts are still focused predominantly on engineering either large-diameter macrovessels or microvascular networks.

AREAS COVERED IN THIS REVIEW

We present the emerging concept of organ printing or robotic additive biofabrication of an intra-organ branched vascular tree, based on the ability of vascular tissue spheroids to undergo self-assembly.

WHAT THE READER WILL GAIN

The feasibility and challenges of this robotic biofabrication approach to intra-organ vascularization for tissue engineering based on organ-printing technology using self-assembling vascular tissue spheroids including clinically relevantly vascular cell sources are analyzed.

TAKE HOME MESSAGE

It is not possible to engineer 3D thick tissue or organ constructs without effective vascularization. An effective intra-organ vascular system cannot be built by the simple connection of large-diameter vessels and microvessels. Successful engineering of functional human organs suitable for surgical implantation will require concomitant engineering of a 'built in' intra-organ branched vascular system. Organ printing enables biofabrication of human organ constructs with a 'built in' intra-organ branched vascular tree.

Assessing the value of autologous and allogeneic cells for regenerative medicine

The advantages and disadvantages of autologous and allogeneic human cells for regenerative medicine are summarized. The comparison of relative advantages includes: ease and cost of treating large numbers of patients, the speed of availability of therapy and the differing complexity of the development pathways. The comparison of relative disadvantages deals with issues such as variability of source material, the risks of cell abnormality and of viral and prion contamination, and the sensitive issues surrounding use of embryo-derived cells. From the comparisons, several potentially decisive issues are drawn out, such as possible immune response and teratoma formation, the impact of patents and the virtues of hospital versus industry-centered development.