Manufacturing road map for tissue engineering and regenerative medicine technologies

Hijacking plant skeletons for biomedical applications: from regenerative medicine and drug delivery to biosensing

The field of biomedical engineering continually seeks innovative technologies to address complex healthcare challenges, ranging from tissue regeneration to drug delivery and biosensing. Plant skeletons offer promising opportunities for these applications due to their unique hierarchical structures, desirable porosity, inherent biocompatibility, and adjustable mechanical properties. This review comprehensively discusses chemical principles underlying the utilization of plant-based scaffolds in biomedical engineering. Highlighting their structural integrity, tunable properties, and possibility of chemical modification, the review explores diverse preparation strategies to tailor plant skeleton properties for bone, neural, cardiovascular, skeletal muscle, and tendon tissue engineering. Such applications stem from the cellulosic three-dimensional structure of different parts of plants, which can mimic the complexity of native tissues and extracellular matrices, providing an ideal environment for cell adhesion, proliferation, and differentiation. We also discuss the application of plant skeletons as carriers for drug delivery due to their structural diversity and versatility in encapsulating and releasing therapeutic agents with controlled kinetics. Furthermore, we present the emerging role played by plant-derived materials in biosensor development for diagnostic and monitoring purposes. Challenges and future directions in the field are also discussed, offering insights into the opportunities for future translation of sustainable plant-based technologies to address critical healthcare needs.

3D Bioprinting for Engineered Tissue Constructs and Patient-Specific Models: Current Progress and Prospects in Clinical Applications

Advancements in bioprinting technology are driving the creation of complex, functional tissue constructs for use in tissue engineering and regenerative medicine. Various methods, including extrusion, jetting, and light-based bioprinting, have their unique advantages and drawbacks. Over the years, researchers and industry leaders have made significant progress in enhancing bioprinting techniques and materials, resulting in the production of increasingly sophisticated tissue constructs. Despite this progress, challenges still need to be addressed in achieving clinically relevant, human-scale tissue constructs, presenting a hurdle to widespread clinical translation. However, with ongoing interdisciplinary research and collaboration, the field is rapidly evolving and holds promise for personalized medical interventions. Continued development and refinement of bioprinting technologies have the potential to address complex medical needs, enabling the development of functional, transplantable tissues and organs, as well as advanced in vitro tissue models.

Thermo-Responsive Hyaluronan-Based Hydrogels Combined with Allogeneic Cytotherapeutics for the Treatment of Osteoarthritis

Thermo-responsive hyaluronan-based hydrogels and FE002 human primary chondroprogenitor cell sources have both been previously proposed as modern therapeutic options for the management of osteoarthritis (OA). For the translational development of a potential orthopedic combination product based on both technologies, respective technical aspects required further optimization phases (e.g., hydrogel synthesis upscaling and sterilization, FE002 cytotherapeutic material stabilization). The first aim of the present study was to perform multi-step in vitro characterization of several combination product formulas throughout the established and the optimized manufacturing workflows, with a strong focus set on critical functional parameters. The second aim of the present study was to assess the applicability and the efficacy of the considered combination product prototypes in a rodent model of knee OA. Specific characterization results (i.e., spectral analysis, rheology, tribology, injectability, degradation assays, in vitro biocompatibility) of hyaluronan-based hydrogels modified with sulfo-dibenzocyclooctyne-PEG4-amine linkers and poly(N-isopropylacrylamide) (HA-L-PNIPAM) containing lyophilized FE002 human chondroprogenitors confirmed the suitability of the considered combination product components. Specifically, significantly enhanced resistance toward oxidative and enzymatic degradation was shown in vitro for the studied injectable combination product prototypes. Furthermore, extensive multi-parametric (i.e., tomography, histology, scoring) in vivo investigation of the effects of FE002 cell-laden HA-L-PNIPAM hydrogels in a rodent model revealed no general or local iatrogenic adverse effects, whereas it did reveal some beneficial trends against the development of knee OA. Overall, the present study addressed key aspects of the preclinical development process for novel biologically-based orthopedic combination products and shall serve as a robust methodological basis for further translational investigation and clinical work.

Advanced Graft Development Approaches for ACL Reconstruction or Regeneration

The Anterior Cruciate Ligament (ACL) is one of the major knee ligaments, one which is greatly exposed to injuries. According to the British National Health Society, ACL tears represent around 40% of all knee injuries. The number of ACL injuries has increased rapidly over the past ten years, especially in people from 26-30 years of age. We present a brief background in currently used ACL treatment strategies with a description of surgical reconstruction techniques. According to the well-established method, the PubMed database was then analyzed to scaffold preparation methods and materials. The number of publications and clinical trials over the last almost 30 years were analyzed to determine trends in ACL graft development. Finally, we described selected ACL scaffold development publications of engineering, medical, and business interest. The systematic PubMed database analysis indicated a high interest in collagen for the purpose of ACL graft development, an increased interest in hybrid grafts, a numerical balance in the development of biodegradable and nonbiodegradable grafts, and a low number of clinical trials. The investigation of selected publications indicated that only a few suggest a real possibility of creating healthy tissue. At the same time, many of them focus on specific details and fundamental science. Grafts exhibit a wide range of mechanical properties, mostly because of polymer types and graft morphology. Moreover, most of the research ends at the in vitro stage, using non-certificated polymers, thus requiring a long time before the medical device can be placed on the market. In addition to scientific concerns, official regulations limit the immediate introduction of artificial grafts onto the market.

Lyophilized Progenitor Tenocyte Extracts: Sterilizable Cytotherapeutic Derivatives with Antioxidant Properties and Hyaluronan Hydrogel Functionalization Effects

Cultured primary progenitor tenocytes in lyophilized form were previously shown to possess intrinsic antioxidant properties and hyaluronan-based hydrogel viscosity-modulating effects in vitro. The aim of this study was to prepare and functionally characterize several stabilized (lyophilized) cell-free progenitor tenocyte extracts for inclusion in cytotherapy-inspired complex injectable preparations. Fractionation and sterilization methods were included in specific biotechnological manufacturing workflows of such extracts. Comparative and functional-oriented characterizations of the various extracts were performed using several orthogonal descriptive, colorimetric, rheological, mechanical, and proteomic readouts. Specifically, an optimal sugar-based (saccharose/dextran) excipient formula was retained to produce sterilizable cytotherapeutic derivatives with appropriate functions. It was shown that extracts containing soluble cell-derived fractions possessed conserved and significant antioxidant properties (TEAC) compared to the freshly harvested cellular starting materials. Progenitor tenocyte extracts submitted to sub-micron filtration (0.22 µm) and 60Co gamma irradiation terminal sterilization (5−50 kGy) were shown to retain significant antioxidant properties and hyaluronan-based hydrogel viscosity modulating effects. Hydrogel combination products displayed important efficacy-related characteristics (friction modulation, tendon bioadhesivity) with significant (p < 0.05) protective effects of the cellular extracts in oxidative environments. Overall, the present study sets forth robust control methodologies (antioxidant assays, H2O2-challenged rheological setups) for stabilized cell-free progenitor tenocyte extracts. Importantly, it was shown that highly sensitive phases of cytotherapeutic derivative manufacturing process development (purification, terminal sterilization) allowed for the conservation of critical biological extract attributes.

Retrospectives on Three Decades of Safe Clinical Experience with Allogeneic Dermal Progenitor Fibroblasts: High Versatility in Topical Cytotherapeutic Care

Allogeneic dermal progenitor fibroblasts constitute cytotherapeutic contenders for modern cutaneous regenerative medicine. Based on advancements in the relevant scientific, technical, and regulatory fields, translational developments have slowly yet steadily led to the clinical application of such biologicals and derivatives. To set the appropriate general context, the first aim of this study was to provide a current global overview of approved cell and gene therapy products, with an emphasis on cytotherapies for cutaneous application. Notable advances were shown for North America, Europe, Iran, Japan, and Korea. Then, the second and main aim of this study was to perform a retrospective analysis on the various applications of dermal progenitor fibroblasts and derivatives, as clinically used under the Swiss progenitor cell transplantation program for the past three decades. Therein, the focus was set on the extent and versatility of use of the therapies under consideration, their safety parameters, as well as formulation options for topical application. Quantitative and illustrative data were summarized and reported for over 300 patients treated with various cell-based or cell-derived preparations (e.g., progenitor biological bandages or semi-solid emulsions) in Lausanne since 1992. Overall, this study shows the strong current interest in biological-based approaches to cutaneous regenerative medicine from a global developmental perspective, as well as the consolidated local clinical experience gathered with a specific and safe allogeneic cytotherapeutic approach. Taken together, these current and historical elements may serve as tangible working bases for the further optimization of local and modern translational pathways for the provision of topical cytotherapeutic care.

Highlights on Advancing Frontiers in Tissue Engineering

The field of tissue engineering continues to advance, sometimes in exponential leaps forward, but also sometimes at a rate that does not fulfill the promise that the field imagined a few decades ago. This review is in part a catalog of success in an effort to inform the process of innovation. Tissue engineering has recruited new technologies and developed new methods for engineering tissue constructs that can be used to mitigate or model disease states for study. Key to this antecedent statement is that the scientific effort must be anchored in the needs of a disease state and be working toward a functional product in regenerative medicine. It is this focus on the wildly important ideas coupled with partnered research efforts within both academia and industry that have shown most translational potential. The field continues to thrive and among the most important recent developments are the use of three-dimensional bioprinting, organ-on-a-chip, and induced pluripotent stem cell technologies that warrant special attention. Developments in the aforementioned areas as well as future directions are highlighted in this article. Although several early efforts have not come to fruition, there are good examples of commercial profitability that merit continued investment in tissue engineering. Impact statement Tissue engineering led to the development of new methods for regenerative medicine and disease models. Among the most important recent developments in tissue engineering are the use of three-dimensional bioprinting, organ-on-a-chip, and induced pluripotent stem cell technologies. These technologies and an understanding of them will have impact on the success of tissue engineering and its translation to regenerative medicine. Continued investment in tissue engineering will yield products and therapeutics, with both commercial importance and simultaneous disease mitigation.

State-of-the-art knowledge on the regulation of advanced therapy medicinal products

Advanced therapy medicinal products (ATMPs) constitute therapeutic agents based on obtained cells, tissues or genes representing a novel treatment opportunity in medicine. In addition, ATMPs are administered into the cells or tissues of humans from the patient's own cells, donors, or genetically modified cells. Recently, the field of developing ATMPs has become a point of attention due to the clinical efficacy expected in defeating incurable diseases such as cancers and neurodegenerative disorders. Currently, there are two modes regarding the distribution of ATMPs. First, ATMPs that might be legally authorized for marketing. Second, the patients are able to access unapproved ATMPs through the hospital exemption (HE) or clinical practice program or through the compassionate use and expanded access program. The aim of this review is to discuss state-of-the-art knowledge on the regulation of ATMPs and provide regulatory recommendations.

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.

Back to the Cradle of Cytotherapy: Integrating a Century of Clinical Research and Biotechnology-Based Manufacturing for Modern Tissue-Specific Cellular Treatments in Switzerland

Empirically studied by Dr. Brown-Séquard in the late 1800s, cytotherapies were later democratized by Dr. Niehans during the twentieth century in Western Switzerland. Many local cultural landmarks around the Léman Riviera are reminiscent of the inception of such cell-based treatments. Despite the discreet extravagance of the remaining heirs of "living cell therapy" and specific enforcements by Swiss health authorities, current interest in modern and scientifically sound cell-based regenerative medicine has never been stronger. Respective progress made in bioengineering and in biotechnology have enabled the clinical implementation of modern cell-based therapeutic treatments within updated medical and regulatory frameworks. Notably, the Swiss progenitor cell transplantation program has enabled the gathering of two decades of clinical experience in Lausanne for the therapeutic management of cutaneous and musculoskeletal affections, using homologous allogeneic cell-based approaches. While striking conceptual similarities exist between the respective works of the fathers of cytotherapy and of modern highly specialized clinicians, major and important iterative updates have been implemented, centered on product quality and risk-analysis-based patient safety insurance. This perspective article highlights some historical similarities and major evolutive differences, particularly regarding product safety and quality issues, characterizing the use of cell-based therapies in Switzerland over the past century. We outline the vast therapeutic potential to be harnessed for the benefit of overall patient health and the importance of specific scientific methodological aspects.

3D bioprinting: current status and trends—a guide to the literature and industrial practice

The multidisciplinary research field of bioprinting combines additive manufacturing, biology and material sciences to create bioconstructs with three-dimensional architectures mimicking natural living tissues. The high interest in the possibility of reproducing biological tissues and organs is further boosted by the ever-increasing need for personalized medicine, thus allowing bioprinting to establish itself in the field of biomedical research, and attracting extensive research efforts from companies, universities, and research institutes alike. In this context, this paper proposes a scientometric analysis and critical review of the current literature and the industrial landscape of bioprinting to provide a clear overview of its fast-changing and complex position. The scientific literature and patenting results for 2000–2020 are reviewed and critically analyzed by retrieving 9314 scientific papers and 309 international patents in order to draw a picture of the scientific and industrial landscape in terms of top research countries, institutions, journals, authors and topics, and identifying the technology hubs worldwide. This review paper thus offers a guide to researchers interested in this field or to those who simply want to understand the emerging trends in additive manufacturing and 3D bioprinting.

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Microfluidic Platform to Transduce Cell Viability to Distinct Flow Pathways for High-Accuracy Sensing

Mechanical properties of cells such as stiffness can act as biomarkers to sort or detect cell functional properties such as viability. In this study, we report the use of a microfluidic device as a high-sensitivity sensor that transduces cell biomechanics to cell separation to accurately detect viability. Cell populations are flowed and deflected at a number of skew ridges such that deflection per ridge, cell-ridge interaction time, and cell size can all be used as sensor inputs to accurately determine the cell state. The angle of the ridges was evaluated to optimize the differences in cell translation between viable and nonviable cells while allowing continuous flow. In the first mode of operation, we flowed viable and nonviable cells through the device and conducted a sensitivity analysis by recording the cell's total deflection as a binary classifier that differentiates viable from nonviable cells. The performance of the sensor was assessed using an area under the curve (AUC) analysis to be 0.97. By including additional sensor inputs in the second mode of operation, we conducted a principal component analysis (PCA) to further improve the identification of the cell state by clustering populations with little overlap between viable and nonviable cells. We therefore found that microfluidic separation devices can be used to efficiently sort cells and accurately sense viability in a label-free manner.

GMP Tiered Cell Banking of Non-enzymatically Isolated Dermal Progenitor Fibroblasts for Allogenic Regenerative Medicine

Non-enzymatically isolated primary dermal progenitor fibroblasts derived from fetal organ donations are ideal cell types for allogenic musculoskeletal regenerative therapeutic applications. These cell types are differentiated, highly proliferative in standard in vitro culture conditions and extremely stable throughout their defined lifespans. Technical simplicity, robustness of bioprocessing and relatively small therapeutic dose requirements enable pragmatic and efficient production of clinical progenitor fibroblast lots under cGMP standards. Herein we describe optimized and standardized monolayer culture expansion protocols using dermal progenitor fibroblasts isolated under a Fetal Transplantation Program for the establishment of GMP tiered Master, Working and End of Production cryopreserved Cell Banks. Safety, stability and quality parameters are assessed through stringent testing of progeny biological materials, in view of clinical application to human patients suffering from diverse cutaneous chronic and acute affections. These methods and approaches, coupled to adequate cell source optimization, enable the obtention of a virtually limitless source of highly consistent and safe biological therapeutic material to be used for innovative regenerative medicine applications.

3-Dimensional Bioprinting of Cardiovascular Tissues: Emerging Technology

Three-dimensional (3D) bioprinting may overcome challenges in tissue engineering. Unlike conventional tissue engineering approaches, 3D bioprinting has a proven ability to support vascularization of larger scale constructs and has been used for several cardiovascular applications. An overview of 3D bioprinting techniques, in vivo translation, and challenges are described.

Evaluating Oxygen Tensions Related to Bone Marrow and Matrix for MSC Differentiation in 2D and 3D Biomimetic Lamellar Scaffolds

The physiological O2 microenvironment of mesenchymal stem cells (MSCs) and osteoblasts and the dimensionality of a substrate are known to be important in regulating cell phenotype and function. By providing the physiologically normoxic environments of bone marrow (5%) and matrix (12%), we assessed their potential to maintain stemness, induce osteogenic differentiation, and enhance the material properties in the micropatterned collagen/silk fibroin scaffolds that were produced in 2D or 3D. Expression of osterix (OSX) and vascular endothelial growth factor A (VEGFA) was significantly enhanced in the 3D scaffold in all oxygen environments. At 21% O2, OSX and VEGFA expressions in the 3D scaffold were respectively 13,200 and 270 times higher than those of the 2D scaffold. Markers for assessing stemness were significantly more pronounced on tissue culture polystyrene and 2D scaffold incubated at 5% O2. At 21% O2, we measured significant increases in ultimate tensile strength (p < 0.0001) and Young's modulus (p = 0.003) of the 3D scaffold compared to the 2D scaffold, whilst 5% O2 hindered the positive effect of cell seeding on tensile strength. In conclusion, we demonstrated that the 3D culture of MSCs in collagen/silk fibroin scaffolds provided biomimetic cues for bone progenitor cells toward differentiation and enhanced the tensile mechanical properties.

Regenerative medicine clinical readiness

Regenerative medicine, poised to transform 21st century healthcare, has aspired to enrich care options by bringing cures to patients in need. Science-driven responsible and regulated translation of innovative technology has enabled the launch of previously unimaginable care pathways adopted prudently for select serious diseases and disabilities. The collective resolve to advance the design, manufacture and validity of affordable regenerative solutions aims to democratize such health benefits for all. The objective of this Review is to outline the framework and prerequisites that underpin clinical readiness of regenerative care. Integrated research and development, specialized workforce education and accessible evidence-based practice implementation are at the core of realizing an equitable regenerative medicine vision.

Holistic Approach of Swiss Fetal Progenitor Cell Banking: Optimizing Safe and Sustainable Substrates for Regenerative Medicine and Biotechnology

Safety, quality, and regulatory-driven iterative optimization of therapeutic cell source selection has constituted the core developmental bedrock for primary fetal progenitor cell (FPC) therapy in Switzerland throughout three decades. Customized Fetal Transplantation Programs were pragmatically devised as straightforward workflows for tissue procurement, traceability maximization, safety, consistency, and robustness of cultured progeny cellular materials. Whole-cell bioprocessing standardization has provided plethoric insights into the adequate conjugation of modern biotechnological advances with current restraining legislative, ethical, and regulatory frameworks. Pioneer translational advances in cutaneous and musculoskeletal regenerative medicine continuously demonstrate the therapeutic potential of FPCs. Extensive technical and clinical hindsight was gathered by managing pediatric burns and geriatric ulcers in Switzerland. Concomitant industrial transposition of dermal FPC banking, following good manufacturing practices, demonstrated the extensive potential of their therapeutic value. Furthermore, in extenso, exponential revalorization of Swiss FPC technology may be achieved via the renewal of integrative model frameworks. Consideration of both longitudinal and transversal aspects of simultaneous fetal tissue differential processing allows for a better understanding of the quasi-infinite expansion potential within multi-tiered primary FPC banking. Multiple fetal tissues (e.g., skin, cartilage, tendon, muscle, bone, lung) may be simultaneously harvested and processed for adherent cell cultures, establishing a unique model for sustainable therapeutic cellular material supply chains. Here, we integrated fundamental, preclinical, clinical, and industrial developments embodying the scientific advances supported by Swiss FPC banking and we focused on advances made to date for FPCs that may be derived from a single organ donation. A renewed model of single organ donation bioprocessing is proposed, achieving sustained standards and potential production of billions of affordable and efficient therapeutic doses. Thereby, the aim is to validate the core therapeutic value proposition, to increase awareness and use of standardized protocols for translational regenerative medicine, potentially impacting millions of patients suffering from cutaneous and musculoskeletal diseases. Alternative applications of FPC banking include biopharmaceutical therapeutic product manufacturing, thereby indirectly and synergistically enhancing the power of modern therapeutic armamentariums. It is hypothesized that a single qualifying fetal organ donation is sufficient to sustain decades of scientific, medical, and industrial developments, as technological optimization and standardization enable high efficiency.

Improving patient outcomes with regenerative medicine: How the Regenerative Medicine Manufacturing Society plans to move the needle forward in cell manufacturing, standards, 3D bioprinting, artificial intelligence-enabled automation, education, and training

The Regenerative Medicine Manufacturing Society (RMMS) is the first and only professional society dedicated toward advancing manufacturing solutions for the field of regenerative medicine. RMMS's vision is to provide greater patient access to regenerative medicine therapies through innovative manufacturing solutions. Our mission is to identify unmet needs and gaps in regenerative medicine manufacturing and catalyze the generation of new ideas and solutions by working with private and public stakeholders. We aim to accomplish our mission through outreach and education programs and securing grants for public-private collaborations in regenerative medicine manufacturing. This perspective will cover four impact areas that the society's leadership team has identified as critical: (a) cell manufacturing and scale-up/out, respectively, for allogeneic and autologous cell therapies, (b) standards for regenerative medicine, (c) 3D bioprinting, and (d) artificial intelligence-enabled automation. In addition to covering these areas and ways in which the society intends to advance the field in a collaborative nature, we will also discuss education and training. Education and training is an area that is critical for communicating the current challenges, developing solutions to accelerate the commercialization of the latest technological advances, and growing the workforce in the rapidly expanding sector of regenerative medicine.

Bringing Safe and Standardized Cell Therapies to Industrialized Processing for Burns and Wounds

Cultured primary progenitor cell types are worthy therapeutic candidates for regenerative medicine. Clinical translation, industrial transposition, and commercial implementation of products based on such cell sources are mainly hindered by economic or technical barriers and stringent regulatory requirements. Applied research in allogenic cellular therapies in the Lausanne University Hospital focuses on cell source selection technique optimization. Use of fetal progenitor cell sources in Switzerland is regulated through Federal Transplantation Programs and associated Fetal Biobanks. Clinical applications of cultured primary progenitor dermal fibroblasts have been optimized since the 1990s as “Progenitor Biological Bandages” for pediatric burn patients and adults presenting chronic wounds. A single organ donation procured in 2009 enabled the establishment of a standardized cell source for clinical and industrial developments to date. Non-enzymatically isolated primary dermal progenitor fibroblasts (FE002-SK2 cell type) served for the establishment of a clinical-grade Parental Cell Bank, based on a patented method. Optimized bioprocessing methodology for the FE002-SK2 cell type has demonstrated that extensive and consistent progenitor cell banks can be established. In vitro mechanistic characterization and in vivo preclinical studies have confirmed potency, preliminary safety and efficacy of therapeutic progenitor cells. Most importantly, highly successful industrial transposition and up-scaling of biobanking enabled the establishment of tiered Master and Working Cell Banks using Good Manufacturing Practices. Successive and successful transfers of technology, know-how and materials to different countries around the world have been performed. Extensive developments based on the FE002-SK2 cell source have led to clinical trials for burns and wound dressing. Said trials were approved in Japan, Taiwan, USA and are continuing in Switzerland. The Swiss Fetal Transplantation Program and pioneer clinical experience in the Lausanne Burn Center over three decades constitute concrete indicators that primary progenitor dermal fibroblasts should be considered as therapeutic flagships in the domain of wound healing and for regenerative medicine in general. Indeed, one single organ donation potentially enables millions of patients to benefit from high-quality, safe and effective regenerative therapies. This work presents a technical and translational overview of the described progenitor cell technology harnessed in Switzerland as cellular therapies for treatment of burns and wounds around the globe.

The Progression of Regenerative Medicine and its Impact on Therapy Translation

Despite regenerative medicine (RM) being one of the hottest topics in biotechnology for the past 3 decades, it is generally acknowledged that the field's performance at the bedside has been somewhat disappointing. This may be linked to the novelty of these technologies and their disruptive nature, which has brought an increasing level of complexity to translation. Therefore, we look at how the historical development of the RM field has changed the translational strategy. Specifically, we explore how the pursuit of such novel regenerative therapies has changed the way experts aim to translate their ideas into clinical applications, and then identify areas that need to be corrected or reinforced in order for these therapies to eventually be incorporated into the standard-of-care. This is then linked to a discussion of the preclinical and postclinical challenges remaining today, which offer insights that can contribute to the future progression of RM.

Mesenchymal stem cells in neurodegenerative diseases: Opinion review on ethical dilemmas

The treatment of neurodegenerative diseases presents a growing need for innovation in relation to recent evidence in the field of reconstructive therapy using stem cells. Understanding the molecular mechanisms underlying neurodegenerative disorders, and the advent of methods able to induce neuronal stem cell differentiation allowed to develop innovative therapeutic approaches offering the prospect of healthy and perfectly functional cell transplants, able to replace the sick ones. Hence the importance of deepening the state of the art regarding the clinical applications of advanced cell therapy products for the regeneration of nerve tissue. Besides representing a promising area of tissue transplant surgery and a great achievement in the field of neurodegenerative disease, stem cell research presents certain critical issues that need to be carefully examined from the ethical perspective. In fact, a subject so complex and not entirely explored requires a detailed scientific and ethical evaluation aimed at avoiding improper and ineffective use, rather than incorrect indications, technical inadequacies, and incongruous expectations. In fact, the clinical usefulness of stem cells will only be certain if able to provide the patient with safe, long-term and substantially more effective strategies than any other treatment available. The present paper provides an ethical assessment of tissue regeneration through mesenchymal stem cells in neurodegenerative diseases with the aim to rule out the fundamental issues related to research and clinical translation.

Workshop on the characterization of fiber-based scaffolds: Challenges, progress, and future directions

A critical component of many tissue-engineered medical products (TEMPs) is the scaffold or biomaterial. The industry's understanding of scaffold properties and their influence on cell behavior has advanced, but our technical capability to reliably characterize scaffolds requires improvement, especially to enable large-scale manufacturing. In response to the key findings from the 2013 ASTM International Workshop of Standards and Measurements for Tissue Engineering Scaffolds, the National Institute of Standards and Technology (NIST), ASTM International, BiofabUSA, and the Standards Coordinating Body (SCB) organized a workshop in 2018 titled, "Characterization of Fiber-Based Scaffolds". The goal was to convene a group of 40 key industry stakeholders to identify major roadblocks in measurements of fiber-based scaffold properties. This report provides an overview of the findings from this collaborative workshop. The four major consensus findings were that (a) there is need for a documentary standard guide that would aid developers in the selection of test methods for characterizing fiber-based scaffolds; (b) there is a need for a strategy to assess the quality of porosity and pore size measurements, which could potentially be ameliorated by the development of a reference material; (b) there are challenges with the lexicon used to describe and assess scaffolds; and (d) the vast array of product applications makes it challenging to identify consensus test methods. As a result of these findings, a working group was formed to develop an ASTM Standard Guide for Characterizing Fiber-Based Constructs that will provide developers guidance on selecting measurements for characterizing fiber-based scaffolds.

Improving the Rate of Translation of Tissue Engineering Products

Over 100 000 research articles and 9000 patents have been published on tissue engineering (TE) in the past 20 years. Yet, very few TE products have made their way to the market during the same period. Experts have proposed a variety of strategies to address the lack of translation of TE products. However, since these proposals are guided by qualitative insights, they are limited in scope and impact. Machine learning is utilized in the current study to analyze the entire body of patents that have been published over the past twenty years and understand patenting trends, topics, areas of application, and exemplifications. This analysis yields surprising and little-known insights about the differences in research priorities and perceptions of innovativeness of tissue engineers in academia and industry, as well as aids to chart true advances in the field during the past twenty years. It is hoped that this analysis and subsequent proposal to improve translational rates of TE products will spur much needed dialogue about this important pursuit.

In Vivo Tracking of Tissue Engineered Constructs

To date, the fields of biomaterials science and tissue engineering have shown great promise in creating bioartificial tissues and organs for use in a variety of regenerative medicine applications. With the emergence of new technologies such as additive biomanufacturing and 3D bioprinting, increasingly complex tissue constructs are being fabricated to fulfill the desired patient-specific requirements. Fundamental to the further advancement of this field is the design and development of imaging modalities that can enable visualization of the bioengineered constructs following implantation, at adequate spatial and temporal resolution and high penetration depths. These in vivo tracking techniques should introduce minimum toxicity, disruption, and destruction to treated tissues, while generating clinically relevant signal-to-noise ratios. This article reviews the imaging techniques that are currently being adopted in both research and clinical studies to track tissue engineering scaffolds in vivo, with special attention to 3D bioprinted tissue constructs.

Human Pluripotent Stem Cell-Derived Engineered Tissues: Clinical Considerations

The combined power of human pluripotent stem cells and tissue engineering promises to revolutionize medicine by building tissue patches and artificial replacement organs for patients battling diverse diseases. Here, we articulate some big questions that need to be addressed before such engineered tissues become mainstream in the clinic.

Mechanical Characterization by Mesoscale Indentation: Advantages and Pitfalls for Tissue and Scaffolds

Regenerative medicine and tissue engineering are hindered by the lack of consistent measurements and standards for the mechanical characterization of tissue and scaffolds. Indentation methods for soft matter are favored because of their compatibility with small, arbitrarily shaped samples, but contact mechanics models required to interpret data are often inappropriate for soft, viscous materials. In this study, we demonstrate indentation experiments on a variety of human biopsies, animal tissue, and engineered scaffolds, and we explore the complexities of fitting analytical models to these data. Although objections exist to using Hertz contact models for soft, viscoelastic biological materials since soft matter violates their original assumptions, we demonstrate the experimental conditions that enable consistency and comparability (regardless of arguable misappropriation). Appropriate experimental conditions involving sample hydration, the indentation depth, and the ratio of the probe size to sample thickness enable repeatable metrics that are valuable when comparing synthetic scaffolds and host tissue, and bounds on these parameters are carefully described and discussed. We have also identified a reliable quasistatic parameter that can be derived from indentation data to help researchers compare results across materials and experiments. Although Hertz contact mechanics and linear viscoelastic models may constitute oversimplification for biological materials, the reporting of such simple metrics alongside more complex models is expected to support researchers in tissue engineering and regenerative medicine by providing consistency across efforts to characterize soft matter. Impact Statement To engineer replacement tissue requires a deep understanding of its biomechanical properties. Mesoscale indentation (between micron and millimeter length scales) is well-suited to characterize tissue and engineered replacements as it accommodates small, oddly shaped samples. However, it is easy to run afoul of the assumptions for common contact models when working with biological materials. In this study, we describe experimental procedures and modeling approaches that allow researchers to take advantage of indentation for biomechanical characterization while minimizing its weaknesses.

Prediction of key transcription factors during skin regeneration by combining gene expression data and regulatory network information analysis

Purpose: Burn is one of the most common injuries in clinical practice. The use of transcription factors (TFs) has been reported to reverse the epigenetic rewiring process and has great promise for skin regeneration. To better identify key TFs for skin reprogramming, we proposed a predictive system that conjoint analyzed gene expression data and regulatory network information. Methods: Firstly, the gene expression data in skin tissues were downloaded and the LIMMA package was used to identify differential-expressed genes (DEGs). Then three ways, including identification of TFs from the DEGs, enrichment analysis of TFs by a Fisher’s test, the direct and network-based influence degree analysis of TFs, were used to identify the key TFs related to skin regeneration. Finally, to obtain most comprehensive combination of TFs, the coverage extent of all the TFs were analyzed by Venn diagrams. Results: The top 30 TFs combinations with higher coverage were acquired. Especially, TFAP2A, ZEB1, and NFKB1 exerted greater regulatory influence on other DEGs in the local network and presented relatively higher degrees in the protein–protein interaction (PPI) networks. Conclusion: These TFs identification could give a deeper understanding of the molecular mechanism of cell trans-differentiation, and provide a reference for the skin regeneration and burn treatment.

Skin bioprinting: the future of burn wound reconstruction?

Burns are a significant cause of trauma, and over the years, the focus of patient care has shifted from just survival to facilitation of improved functional outcomes. Typically, burn treatment, especially in the case of extensive burn injuries, involves surgical excision of injured skin and reconstruction of the burn injury with the aid of skin substitutes. Conventional skin substitutes do not contain all skin cell types and do not facilitate recapitulation of native skin physiology. Three-dimensional (3D) bioprinting for reconstruction of burn injuries involves layer-by-layer deposition of cells along with scaffolding materials over the injured areas. Skin bioprinting can be done either in situ or in vitro. Both these approaches are similar except for the site of printing and tissue maturation. There are technological and regulatory challenges that need to be overcome for clinical translation of bioprinted skin for burn reconstruction. However, the use of bioprinting for skin reconstruction following burns is promising; bioprinting will enable accurate placement of cell types and precise and reproducible fabrication of constructs to replace the injured or damaged sites. Overall, 3D bioprinting is a very transformative technology, and its use for wound reconstruction will lead to a paradigm shift in patient outcomes. In this review, we aim to introduce bioprinting, the different stages involved, in vitro and in vivo skin bioprinting, and the various clinical and regulatory challenges in adoption of this technology.

Combinatorial cassettes to systematically evaluate tissue-engineered constructs in recipient mice

Ectopic bone formation in mice is the gold standard for evaluation of osteogenic constructs. By regular procedures, usually only 4 constructs can be accommodated per mouse, limiting screening power. Combinatorial cassettes (combi-cassettes) hold up to 19 small, uniform constructs from the time of surgery, through time in vivo, and subsequent evaluation. Two types of bone tissue engineering constructs were tested in the combi-cassettes: i) a cell-scaffold construct containing primary human bone marrow stromal cells with hydroxyapatite/tricalcium phosphate particles (hBMSCs + HA/TCP) and ii) a growth factor-scaffold construct containing bone morphogenetic protein 2 in a gelatin sponge (BMP2+GS). Measurements of bone formation by histology, bone formation by X-ray microcomputed tomography (μCT) and gene expression by quantitative polymerase chain reaction (qPCR) showed that constructs in combi-cassettes were similar to those created by regular procedures. Combi-cassettes afford placement of multiple replicates of multiple formulations into the same animal, which enables, for the first time, rigorous statistical assessment of: 1) the variability for a given formulation within an animal (intra-animal variability), 2) differences between different tissue-engineered formulations within the same animal and 3) the variability for a given formulation in different animals (inter-animal variability). Combi-cassettes enable a more high-throughput, systematic approach to in vivo studies of tissue engineering constructs.

An Industry-Driven Roadmap for Manufacturing in Regenerative Medicine

Regenerative medicine is poised to become a significant industry within the medical field. As such, the development of strategies and technologies for standardized and automated regenerative medicine clinical manufacturing has become a priority. An industry‐driven roadmap toward industrial scale clinical manufacturing was developed over a 3‐year period by a consortium of companies with significant investment in the field of regenerative medicine. Additionally, this same group identified critical roadblocks that stand in the way of advanced, large‐scale regenerative medicine clinical manufacturing. This perspective article details efforts to reach a consensus among industry stakeholders on the shortest pathway for providing access to regenerative medicine therapies for those in need, both within the United States and around the world. Stem Cells Translational Medicine2018;7:564–568

3D biofabrication for tubular tissue engineering

The therapeutic replacement of diseased tubular tissue is hindered by the availability and suitability of current donor, autologous and synthetically derived protheses. Artificially created, tissue engineered, constructs have the potential to alleviate these concerns with reduced autoimmune response, high anatomical accuracy, long-term patency and growth potential. The advent of 3D bioprinting technology has further supplemented the technological toolbox, opening up new biofabrication research opportunities and expanding the therapeutic potential of the field. In this review, we highlight the challenges facing those seeking to create artificial tubular tissue with its associated complex macro- and microscopic architecture. Current biofabrication approaches, including 3D printing techniques, are reviewed and future directions suggested.

Process System Engineering Methodologies Applied to Tissue Development and Regenerative Medicine

Tissue engineering and the manufacturing of regenerative medicine products demand strict control over the production process and product quality monitoring. In this chapter, the application of process systems engineering (PSE) approaches in the production of cell-based products has been discussed. Mechanistic, empirical, continuum and discrete models are compared and their use in describing cellular phenomena is reviewed. In addition, model-based optimization strategies employed in the field of tissue engineering and regenerative medicine are discussed. An introduction to process control theory is given and the main applications of classical and advanced methods in cellular production processes are described. Finally, new nondestructive and noninvasive monitoring techniques have been reviewed, focusing on large-scale manufacturing systems for cell-based constructs and therapeutic products. The application of the PSE methodologies presented here offers a promising alternative to overcome the main challenges in manufacturing engineered tissue and regeneration products.

Liver tissue engineering: From implantable tissue to whole organ engineering

The term “liver tissue engineering” summarizes one of the ultimate goals of modern biotechnology: the possibility of reproducing in total or in part the functions of the liver in order to treat acute or chronic liver disorders and, ultimately, create a fully functional organ to be transplanted or used as an extracorporeal device. All the technical approaches in the area of liver tissue engineering are based on allocating adult hepatocytes or stem cell‐derived hepatocyte‐like cells within a three‐dimensional structure able to ensure their survival and to maintain their functional phenotype. The hosting structure can be a construct in which hepatocytes are embedded in alginate and/or gelatin or are seeded in a pre‐arranged scaffold made with different types of biomaterials. According to a more advanced methodology termed three‐dimensional bioprinting, hepatocytes are mixed with a bio‐ink and the mixture is printed in different forms, such as tissue‐like layers or spheroids. In the last decade, efforts to engineer a cell microenvironment recapitulating the dynamic native extracellular matrix have become increasingly successful, leading to the hope of satisfying the clinical demand for tissue (or organ) repair and replacement within a reasonable timeframe. Indeed, the preclinical work performed in recent years has shown promising results, and the advancement in the biotechnology of bioreactors, ex vivo perfusion machines, and cell expansion systems associated with a better understanding of liver development and the extracellular matrix microenvironment will facilitate and expedite the translation to technical applications. (Hepatology Communications 2018;2:131–141)

Mesenchymal Stromal Cell Therapy in Bronchopulmonary Dysplasia: Systematic Review and Meta-Analysis of Preclinical Studies

Extreme prematurity is the leading cause of death among children under 5 years of age. Currently, there is no treatment for bronchopulmonary dysplasia (BPD), the most common complication of extreme prematurity. Experimental studies in animal models of BPD suggest that mesenchymal stromal cells (MSCs) are lung protective. To date, no systematic review and meta-analysis has evaluated the preclinical evidence of this promising therapy. Our protocol was registered with Collaborative Approach to Meta-Analysis and Review of Animal Data from Experimental Studies prior to searching MEDLINE (1946 to June 1, 2015), Embase (1947 to 2015 Week 22), Pubmed, Web of Science, and conference proceedings (1990 to present) for controlled comparative studies of neonatal animal models that received MSCs or cell free MSC-derived conditioned media (MSC-CM). Lung alveolarization was the primary outcome. We used random effects models for data analysis and followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses reporting guidelines. We screened 990 citations; 25 met inclusion criteria. All used hyperoxia-exposed neonatal rodents to model BPD. MSCs significantly improved alveolarization (Standardized mean difference of -1.330, 95% confidence interval [CI -1.724, -0.94, I2 69%]), irrespective of timing of treatment, source, dose, or route of administration. MSCs also significantly ameliorated pulmonary hypertension, lung inflammation, fibrosis, angiogenesis, and apoptosis. Similarly, MSC-CM significantly improved alveolarization, angiogenesis, and pulmonary artery remodeling. MSCs, tested exclusively in hyperoxic rodent models of BPD, show significant therapeutic benefit. Unclear risk of bias and incomplete reporting in the primary studies highlights nonadherence to reporting standards. Overall, safety and efficacy in other species/large animal models may provide useful information for guiding the design of clinical trials. Stem Cells Translational Medicine 2017;6:2079-2093.

Microfluidic Sorting of Cells by Viability Based on Differences in Cell Stiffness

The enrichment of viable cells is an essential step to obtain effective products for cell therapy. While procedures exist to characterize the viability of cells, most methods to exclude nonviable cells require the use of density gradient centrifugation or antibody-based cell sorting with molecular labels of cell viability. We report a label-free microfluidic technique to separate live and dead cells that exploits differences in cellular stiffness. The device uses a channel with repeated ridges that are diagonal with respect to the direction of cell flow. Stiff nonviable cells directed through the channel are compressed and translated orthogonally to the channel length, while soft live cells follow hydrodynamic flow. As a proof of concept, Jurkat cells are enriched to high purity of viable cells by a factor of 185-fold. Cell stiffness was validated as a sorting parameter as nonviable cells were substantially stiffer than live cells. To highlight the utility for hematopoietic stem cell transplantation, frozen samples of cord blood were thawed and the purity of viable nucleated cells was increased from 65% to over 94% with a recovery of 73% of the viable cells. Thus, the microfluidic stiffness sorting can simply and efficiently obtain highly pure populations of viable cells.

Preparation, characterization, and banking of clinical-grade cells for neural transplantation: Scale up, fingerprinting, and genomic stability of stem cell lines

Parkinson's disease is a complex and progressive neurodegenerative condition that is characterized by the severe loss of midbrain dopaminergic (mDA) neurons, which innervate the striatum. Cell transplantation therapies to rebuild this dopaminergic network have been attempted for over 30 years. The most promising outcomes were observed when human fetal mesencephalic tissue was used as the source of cells for transplantation. However, reliance on terminations for a Parkinson's therapy presents significant logistical and ethical hurdles. An alternative source of transplantable mDA neurons is urgently needed, and the solution may come from human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs). Protocols to differentiate hESCs/iPSCs toward mDA neurons are now robust and efficient, and upon grafting the cells rescue preclinical animal models of Parkinson's disease. The challenge now is to apply Good Manufacturing Practice (GMP) to the academic discoveries and protocols to produce clinical-grade transplantable mDA cells. Major technical and logistical considerations include (i) source of hESC or iPSC line, (ii) GMP compliance of the differentiation protocol and all reagents, (iii) characterization of the cell product in terms of identity, safety, and efficacy, (iv) characterization of genomic state and stability, and (v) banking of a transplantation-ready cell product. Approaches and solutions to these challenges are reviewed here.

Tissue-Engineered Solutions in Plastic and Reconstructive Surgery: Principles and Practice

Recent advances in microsurgery, imaging, and transplantation have led to significant refinements in autologous reconstructive options; however, the morbidity of donor sites remains. This would be eliminated by successful clinical translation of tissue-engineered solutions into surgical practice. Plastic surgeons are uniquely placed to be intrinsically involved in the research and development of laboratory engineered tissues and their subsequent use. In this article, we present an overview of the field of tissue engineering, with the practicing plastic surgeon in mind. The Medical Research Council states that regenerative medicine and tissue engineering “holds the promise of revolutionizing patient care in the twenty-first century.” The UK government highlighted regenerative medicine as one of the key eight great technologies in their industrial strategy worthy of significant investment. The long-term aim of successful biomanufacture to repair composite defects depends on interdisciplinary collaboration between cell biologists, material scientists, engineers, and associated medical specialties; however currently, there is a current lack of coordination in the field as a whole. Barriers to translation are deep rooted at the basic science level, manifested by a lack of consensus on the ideal cell source, scaffold, molecular cues, and environment and manufacturing strategy. There is also insufficient understanding of the long-term safety and durability of tissue-engineered constructs. This review aims to highlight that individualized approaches to the field are not adequate, and research collaboratives will be essential to bring together differing areas of expertise to expedite future clinical translation. The use of tissue engineering in reconstructive surgery would result in a paradigm shift but it is important to maintain realistic expectations. It is generally accepted that it takes 20–30 years from the start of basic science research to clinical utility, demonstrated by contemporary treatments such as bone marrow transplantation. Although great advances have been made in the tissue engineering field, we highlight the barriers that need to be overcome before we see the routine use of tissue-engineered solutions.

Structural determinants of hydration, mechanics and fluid flow in freeze-dried collagen scaffolds

Freeze-dried scaffolds provide regeneration templates for a wide range of tissues, due to their flexibility in physical and biological properties. Control of structure is crucial for tuning such properties, and therefore scaffold functionality. However, the common approach of modeling these scaffolds as open-cell foams does not fully account for their structural complexity. Here, the validity of the open-cell model is examined across a range of physical characteristics, rigorously linking morphology to hydration and mechanical properties. Collagen scaffolds with systematic changes in relative density were characterized using Scanning Electron Microscopy, X-ray Micro-Computed Tomography and spherical indentation analyzed in a time-dependent poroelastic framework. Morphologically, all scaffolds were mid-way between the open- and closed-cell models, approaching the closed-cell model as relative density increased. Although pore size remained constant, transport pathway diameter decreased. Larger collagen fractions also produced greater volume swelling on hydration, although the change in pore diameter was constant, and relatively small at ∼6%. Mechanically, the dry and hydrated scaffold moduli varied quadratically with relative density, as expected of open-cell materials. However, the increasing pore wall closure was found to determine the time-dependent nature of the hydrated scaffold response, with a decrease in permeability producing increasingly elastic rather than viscoelastic behavior. These results demonstrate that characterizing the deviation from the open-cell model is vital to gain a full understanding of scaffold biophysical properties, and provide a template for structural studies of other freeze-dried biomaterials.

STATEMENT OF SIGNIFICANCE

Freeze-dried collagen sponges are three-dimensional microporous scaffolds that have been used for a number of exploratory tissue engineering applications. The characterization of the structure-properties relationships of these scaffolds is necessary to understand their biophysical behavior in vivo. In this work, the relationship between morphology and physical properties in the dry and hydrated states was investigated across a range of solid concentrations in the scaffolds. The quantitative results provided can aid the design of scaffolds with a target trade-off between mechanical properties and structural features important for their biological activity.

Strategies for Achieving Measurement Assurance for Cell Therapy Products

The cell therapy industry has identified the inability to reliably characterize cells as possibly its greatest challenge and has called for standards and reference materials to provide assurance for measurements of cell properties. The challenges in characterization of cell therapy products can be largely addressed with systematic approaches for assessing sources of uncertainty and improving confidence in key measurements. This article presents the many strategies that can be used to ensure measurement confidence and discusses them in terms of how they can be applied to characterization of cell therapy products.

SIGNIFICANCE

Application of these strategies to cell measurements will help to establish qualified assays for cell characterization, which may help streamline regulatory approval and enable more efficient development of cell therapy products.