Tissue Engineering and Regenerative Medicine 2017: A Year in Review

Polyethersulfone Polymer for Biomedical Applications and Biotechnology

Polymers stand out as promising materials extensively employed in biomedicine and biotechnology. Their versatile applications owe much to the field of tissue engineering, which seamlessly integrates materials engineering with medical science. In medicine, biomaterials serve as prototypes for organ development and as implants or scaffolds to facilitate body regeneration. With the growing demand for innovative solutions, synthetic and hybrid polymer materials, such as polyethersulfone, are gaining traction. This article offers a concise characterization of polyethersulfone followed by an exploration of its diverse applications in medical and biotechnological realms. It concludes by summarizing the significant roles of polyethersulfone in advancing both medicine and biotechnology, as outlined in the accompanying table.

Graphene-Based Material-Mediated Immunomodulation in Tissue Engineering and Regeneration: Mechanism and Significance

Tissue engineering and regenerative medicine hold promise for improving or even restoring the function of damaged organs. Graphene-based materials (GBMs) have become a key player in biomaterials applied to tissue engineering and regenerative medicine. A series of cellular and molecular events, which affect the outcome of tissue regeneration, occur after GBMs are implanted into the body. The immunomodulatory function of GBMs is considered to be a key factor influencing tissue regeneration. This review introduces the applications of GBMs in bone, neural, skin, and cardiovascular tissue engineering, emphasizing that the immunomodulatory functions of GBMs significantly improve tissue regeneration. This review focuses on summarizing and discussing the mechanisms by which GBMs mediate the sequential regulation of the innate immune cell inflammatory response. During the process of tissue healing, multiple immune responses, such as the inflammatory response, foreign body reaction, tissue fibrosis, and biodegradation of GBMs, are interrelated and influential. We discuss the regulation of these immune responses by GBMs, as well as the immune cells and related immunomodulatory mechanisms involved. Finally, we summarize the limitations in the immunomodulatory strategies of GBMs and ideas for optimizing GBM applications in tissue engineering. This review demonstrates the significance and related mechanism of the immunomodulatory function of GBM application in tissue engineering; more importantly, it contributes insights into the design of GBMs to enhance wound healing and tissue regeneration in tissue engineering.

Whole Liver Derived Acellular Extracellular Matrix for Bioengineering of Liver Constructs: An Updated Review

Biomaterial templates play a critical role in establishing and bioinstructing three-dimensional cellular growth, proliferation and spatial morphogenetic processes that culminate in the development of physiologically relevant in vitro liver models. Various natural and synthetic polymeric biomaterials are currently available to construct biomimetic cell culture environments to investigate hepatic cell-matrix interactions, drug response assessment, toxicity, and disease mechanisms. One specific class of natural biomaterials consists of the decellularized liver extracellular matrix (dECM) derived from xenogeneic or allogeneic sources, which is rich in bioconstituents essential for the ultrastructural stability, function, repair, and regeneration of tissues/organs. Considering the significance of the key design blueprints of organ-specific acellular substrates for physiologically active graft reconstruction, herein we showcased the latest updates in the field of liver decellularization-recellularization technologies. Overall, this review highlights the potential of acellular matrix as a promising biomaterial in light of recent advances in the preparation of liver-specific whole organ scaffolds. The review concludes with a discussion of the challenges and future prospects of liver-specific decellularized materials in the direction of translational research.

From materials to clinical use: advances in 3D-printed scaffolds for cartilage tissue engineering

Osteoarthritis caused by articular cartilage defects is a particularly common orthopedic disease that can involve the entire joint, causing great pain to its sufferers. A global patient population of approximately 250 million people has an increasing demand for new therapies with excellent results, and tissue engineering scaffolds have been proposed as a potential strategy for the repair and reconstruction of cartilage defects. The precise control and high flexibility of 3D printing provide a platform for subversive innovation. In this perspective, cartilage tissue engineering (CTE) scaffolds manufactured using different biomaterials are summarized from the perspective of 3D printing strategies, the bionic structure strategies and special functional designs are classified and discussed, and the advantages and limitations of these CTE scaffold preparation strategies are analyzed in detail. Finally, the application prospect and challenges of 3D printed CTE scaffolds are discussed, providing enlightening insights for their current research.

Scaffolds for Cartilage Tissue Engineering from a Blend of Polyethersulfone and Polyurethane Polymers

In recent years, one of the main goals of cartilage tissue engineering has been to find appropriate scaffolds for hyaline cartilage regeneration, which could serve as a matrix for chondrocytes or stem cell cultures. The study presents three types of scaffolds obtained from a blend of polyethersulfone (PES) and polyurethane (PUR) by a combination of wet-phase inversion and salt-leaching methods. The nonwovens made of gelatin and sodium chloride (NaCl) were used as precursors of macropores. Thus, obtained membranes were characterized by a suitable structure. The top layers were perforated, with pores over 20 µm, which allows cells to enter the membrane. The use of a nonwoven made it possible to develop a three-dimensional network of interconnected macropores that is required for cell activity and mobility. Examination of wettability (contact angle, swelling ratio) showed a hydrophilic nature of scaffolds. The mechanical test showed that the scaffolds were suitable for knee joint applications (stress above 10 MPa). Next, the scaffolds underwent a degradation study in simulated body fluid (SBF). Weight loss after four weeks and changes in structure were assessed using scanning electron microscopy (SEM) and MeMoExplorer Software, a program that estimates the size of pores. The porosity measurements after degradation confirmed an increase in pore size, as expected. Hydrolysis was confirmed by Fourier-transform infrared spectroscopy (FT-IR) analysis, where the disappearance of ester bonds at about 1730 cm−1 wavelength is noticeable after degradation. The obtained results showed that the scaffolds meet the requirements for cartilage tissue engineering membranes and should undergo further testing on an animal model.

The Progress of Stem Cell Therapy in Myocardial-Infarcted Heart Regeneration: Cell Sheet Technology

Various tissues, including the heart, cornea, bone, esophagus, bladder and liver, have been vascularized using the cell sheet technique. It overcomes the limitations of existing techniques by allowing small layers of the cell sheet to generate capillaries on their own, and it can also be used to vascularize tissue-engineered transplants. Cell sheets eliminate the need for traditional tissue engineering procedures such as isolated cell injections and scaffold-based technologies, which have limited applicability. While cell sheet engineering can eliminate many of the drawbacks, there are still a few challenges that need to be addressed. The number of cell sheets that can be layered without triggering core ischemia or hypoxia is limited. Even when scaffold-based technologies are disregarded, strategies to tackle this problem remain a substantial impediment to the efficient regeneration of thick, living three-dimensional cell sheets. In this review, we summarize the cell sheet technology in myocardial infarcted tissue regeneration.

Novel nanofibrous membrane-supporting stem cell sheets for plasmid delivery and cell activation to accelerate wound healing

The integration of biomaterials with cells for high overall performances is vitally important in tissue engineering, as scaffold-free cell sheet lacks enough mechanical performance and cell viability while cell-free scaffold possesses limited biological functions. In this study, we propose a new strategy to strengthen cell sheets and enhance cell activity for accelerating wound healing based on a novel sandwich structure of cell sheet-plasmid@membrane-cell sheet (CpMC). Specifically, the CpMC contains two adipose-derived stem cell (ADSC) sheets on outer surfaces and an electrospun gelatin/chitosan nanofibrous membrane (NFM) encapsulating vascular endothelial growth factor (VEGF) plasmids in between. The physicochemical properties of NFM including swelling, stiffness, strength, elasticity, and biodegradation can be tailored by simply adjusting the ratio between gelatin and chitosan to be 7:3 which is optimal for most effectively supporting ADSCs adhesion and proliferation. The swelling/biodegradation of NFM mediates the sustained release of encapsulated VEGF plasmids into adjacent ADSCs, and NFM assists VEGF plasmids to promote the differentiation of ADSCs into endothelial, epidermal, and fibroblast cells, in support of the neoangiogenesis and regeneration of cutaneous tissues within 2 weeks. The proposed membrane-supporting cell sheet strategy provides a new route to tissue engineering, and the developed CpMC demonstrates a high potential for clinical translation.

Evaluating potential of tissue-engineered cryogels and chondrocyte derived exosomes in articular cartilage repair

Treatment of articular cartilage injuries especially osteochondral tissue requires intervention of bioengineered scaffold. In this study, we investigated the potential of the tissue-engineered cryogel scaffold fabricated using cryogelation technology. Two types of cryogels viz. chitosan-gelatin-chondroitin sulfate (CGC) for articular cartilage and nano-hydroxyapatite-gelatin (HG) for subchondral bone were fabricated. Further, novel bilayer cryogel designed using single process fabrication of two layers (CGC as top layer and HG as the lower layer) was designed to mimic osteochondral unit. CGC cryogel was tested for their biocompatibility using the enzymatically isolated chondrcoytes from goat articular cartilage while HG cryogel was tested using pre-osteoblast cell line. Extracellular vesicles, specifically exosomes were isolated from the spent media of chondrocytes to validate their effect over cell proliferation and migration which are required for defect healing and infiltration respectively. These isolated exosomes were characterized and analyzed for confirming their size distribution profile and visualized morphologically using advanced microscopy techniques. For cartilage part, CGC cryogels were examined as delivery system for delivering exosomes at defect site, where 80% of release was observed in 72 h. Release of 18.7 µg chondroitin sulfate/mg cryogel was obtained in a period of one week from CGC cryogel (termed cryogel extract) which has chondroprotective effect. Further, effect of exosome concentration (10 and 20 µg/ml), CGC extract and combination of exosome and CGC extract (Exo-Ex) were assessed over the chondrocytes. In addition, in vitro scratch wound assay was performed to analyse the migration capacity over the micro-injury when treated with exosomes, cryogel extract and Exo-Ex. The overall results thus answer key questions of therapeutic potential of chondrocyte exosomes, cryogel extract in addition to potential of CGC and HG cryogel for osteochondral repair.

Research Advances in the Application of Adipose-Derived Stem Cells Derived Exosomes in Cutaneous Wound Healing

Cutaneous wound healing has always been an intractable medical problem for both clinicians and researchers, with an urgent need for more efficacious methods to achieve optimal outcomes morphologically and functionally. Stem cells, the body's rapid response ‘road repair crew,’ being on standby to combat tissue injuries, are an essential part of regenerative medicine. Currently, the use of adipose-derived stem cells (ADSCs), a kind of mesenchymal stem cells with multipotent differentiation and self-renewal capacity, is surging in the field of cutaneous wound healing. ADSCs may exert influences either by releasing paracrine signalling factors or differentiating into mature adipose cells to provide the ‘building blocks’ for engineered tissue. As an important paracrine substance released from ADSCs, exosomes are a kind of extracellular vesicles and carrying various bioactive molecules mediating adjacent or distant intercellular communication. Previous studies have indicated that ADSCs derived exosomes (ADSCs-Exos) promoted skin wound healing by affecting all stages of wound healing, including regulating inflammatory response, promoting proliferation and migration of fibroblasts or keratinocytes, facilitating angiogenesis, and regulating remodeling of extracellular matrix, which have provided new opportunities for understanding how ADSCs-Exos mediate intercellular communication in pathological processes of the skin and therapeutic strategies for cutaneous wound repair. In this review, we focus on elucidating the role of ADSCs-Exos at various stages of cutaneous wound healing, detailing the latest developments, and presenting some challenges necessary to be addressed in this field, with the expectation of providing a new perspective on how to best utilize this powerful cell-free therapy in the future.

Optimization of Oxygen Delivery Within Hydrogels

The field of tissue engineering has been continuously evolving since its inception over three decades ago with numerous new advancements in biomaterials and cell sources and widening applications to most tissues in the body. Despite the substantial promise and great opportunities for the advancement of current medical therapies and procedures, the field has yet to capture wide clinical translation due to some remaining challenges, including oxygen availability within constructs, both in vitro and in vivo. While this insufficiency of nutrients, specifically oxygen, is a limitation within the current frameworks of this field, the literature shows promise in new technological advances to efficiently provide adequate delivery of nutrients to cells. This review attempts to capture the most recent advances in the field of oxygen transport in hydrogel-based tissue engineering, including a comparison of current research as it pertains to the modeling, sensing, and optimization of oxygen within hydrogel constructs as well as new technological innovations to overcome traditional diffusion-based limitations. The application of these findings can further the advancement and development of better hydrogel-based tissue engineered constructs for future clinical translation and adoption.

Bioscaffolds embedded with regulatory modules for cell growth and tissue formation: A review

The demand for artificial organs has greatly increased because of various aging-associated diseases and the wide need for organ transplants. A recent trend in tissue engineering is the precise reconstruction of tissues by the growth of cells adhering to bioscaffolds, which are three-dimensional (3D) structures that guide tissue and organ formation. Bioscaffolds used to fabricate bionic tissues should be able to not only guide cell growth but also regulate cell behaviors. Common regulation methods include biophysical and biochemical stimulations. Biophysical stimulation cues include matrix hardness, external stress and strain, surface topology, and electromagnetic field and concentration, whereas biochemical stimulation cues include growth factors, proteins, kinases, and magnetic nanoparticles. This review discusses bioink preparation, 3D bioprinting (including extrusion-based, inkjet, and ultraviolet-assisted 3D bioprinting), and regulation of cell behaviors. In particular, it provides an overview of state-of-the-art methods and devices for regulating cell growth and tissue formation and the effects of biophysical and biochemical stimulations on cell behaviors. In addition, the fabrication of bioscaffolds embedded with regulatory modules for biomimetic tissue preparation is explained. Finally, challenges in cell growth regulation and future research directions are presented.

Assessing the potential use of chitosan scaffolds for the sustained localized delivery of vitamin D

Vitamin D is a commonly used bone modulator in regenerative medicine. Several modalities have been explored for the delivery of vitamin D including nanoparticles and scaffold. The present study aimed to assess the potential use of a bio-degradable chitosan scaffold for the delivery of vitamin D. The objectives included fabrication of a bio-degradable chitosan scaffold, integration of vitamin D into the scaffold, characterization of the vitamin D integrated scaffold. Characterization was carried out using, X-ray diffraction, Fourier transform infrared spectroscopy, and differential scanning calorimetry. The structure of the scaffold was assessed by scanning electron microscopy. The scaffold was placed in phosphate buffer saline and the release duration of vitamin D was observed using UV spectrophotometry. Dental pulp mesenchymal stem cells were added to the scaffold to study the scaffold associated toxicity and the functionality of the scaffold released vitamin D. The vitamin D release period from the scaffold was estimated to be for 80 hrs. MTT assay of the stem cells was comparable to that of the control group (stem cells cultured in media) inferring that the scaffold is not toxic towards the stem cells. The positive alizarin red S staining, a higher expression of alkaline phosphatase, osteocalcin, and RunX2 confirmed the functional capability (osteogenic differentiation of the stem cells) of the released vitamin D. Based on the data from the present study, it can be inferred that chitosan scaffold can be used for the sustained delivery of functional vitamin D for 3-5 days.

Delivery of transcription factors as modulators of cell differentiation

Fundamental studies performed during the last decades have shown that cell fate is much more plastic than previously considered, and technologies for its manipulation are a keystone for many new tissue regeneration therapies. Transcription factors (TFs) are DNA-binding proteins that control gene expression, and they have critical roles in the control of cell fate and other cellular behavior. TF-based therapies have much medical potential, but their use as drugs depends on the development of suitable delivery technologies that can help them reach their action site inside of the cells. TFs can be used either as proteins or encoded in polynucleotides. When used in protein form, many TFs require to be associated to a cell-penetrating peptide or another transduction domain. As polynucleotides, they can be delivered either by viral carriers or by non-viral systems such as polyplexes and lipoplexes. TF-based therapies have extensively shown their potential to solve many tissue-engineering problems, including bone, cartilage and cardiac regeneration. Yet, their use has expanded beyond regenerative medicine to other prominent disease areas such as cancer therapy and immunomodulation. This review summarizes some of the delivery options for effective TF-based therapies and their current main applications.

Chemical Degradation of PSF-PUR Blend Hollow Fiber Membranes-Assessment of Changes in Properties and Morphology after Hydrolysis

In this study, we focused on obtaining polysulfone-polyurethane (PSF-PUR) blend partly degradable hollow fiber membranes (HFMs) with different compositions while maintaining a constant PSF:PUR = 8:2 weight ratio. It was carried out through hydrolysis, and evaluation of the properties and morphology before and after the hydrolysis process while maintaining a constant cut-off. The obtained membranes were examined for changes in ultrafiltration coefficient (UFC), retention, weight loss, morphology assessment using scanning electron microscopy (SEM) and MeMoExplorer™ Software, as well as using the Fourier-transform infrared spectroscopy (FT-IR) method. The results of the study showed an increase in the UFC value after the hydrolysis process, changes in retention, mass loss, and FT-IR spectra. The evaluation in MeMoExplorer™ Software showed the changes in membranes' morphology. It was confirmed that polyurethane (PUR) was partially degraded, the percentage of ester bonds has an influence on the degradation process, and PUR can be used as a pore precursor instead of superbly known polymers.

Review of Synthetic and Hybrid Scaffolds in Cartilage Tissue Engineering

Cartilage tissue is under extensive investigation in tissue engineering and regenerative medicine studies because of its limited regenerative potential. Currently, many scaffolds are undergoing scientific and clinical research. A key for appropriate scaffolding is the assurance of a temporary cellular environment that allows the cells to function as in native tissue. These scaffolds should meet the relevant requirements, including appropriate architecture and physicochemical and biological properties. This is necessary for proper cell growth, which is associated with the adequate regeneration of cartilage. This paper presents a review of the development of scaffolds from synthetic polymers and hybrid materials employed for the engineering of cartilage tissue and regenerative medicine. Initially, general information on articular cartilage and an overview of the clinical strategies for the treatment of cartilage defects are presented. Then, the requirements for scaffolds in regenerative medicine, materials intended for membranes, and methods for obtaining them are briefly described. We also describe the hybrid materials that combine the advantages of both synthetic and natural polymers, which provide better properties for the scaffold. The last part of the article is focused on scaffolds in cartilage tissue engineering that have been confirmed by undergoing preclinical and clinical tests.

Perspectives on Synthetic Materials to Guide Tissue Regeneration for Osteochondral Defect Repair

Regenerative engineering holds the potential to treat clinically pervasive osteochondral defects (OCDs). In a synthetic materials-guided approach, the scaffold's chemical and physical properties alone instruct cellular behavior in order to effect regeneration, referred to herein as "instructive" properties. While this alleviates the costs and off-target risks associated with exogenous growth factors, the scaffold must be potently instructive to achieve tissue growth. Moreover, toward achieving functionality, such a scaffold should also recapitulate the spatial complexity of the osteochondral tissues. Thus, in addition to the regeneration of the articular cartilage and underlying cancellous bone, the complex osteochondral interface, composed of calcified cartilage and subchondral bone, should also be restored. In this Perspective, we highlight recent synthetic-based, instructive osteochondral scaffolds that have leveraged new material chemistries as well as innovative fabrication strategies. In particular, scaffolds with spatially complex chemical and morphological features have been prepared with electrospinning, solvent-casting-particulate-leaching, freeze-drying, and additive manufacturing. While few synthetic scaffolds have advanced to clinical studies to treat OCDs, these recent efforts point to the promising use of the chemical and physical properties of synthetic materials for regeneration of osteochondral tissues.

Composite Hydrogels in Three-Dimensional in vitro Models

3-dimensional (3D) in vitro models were developed in order to mimic the complexity of real organ/tissue in a dish. They offer new possibilities to model biological processes in more physiologically relevant ways which can be applied to a myriad of applications including drug development, toxicity screening and regenerative medicine. Hydrogels are the most relevant tissue-like matrices to support the development of 3D in vitro models since they are in many ways akin to the native extracellular matrix (ECM). For the purpose of further improving matrix relevance or to impart specific functionalities, composite hydrogels have attracted increasing attention. These could incorporate drugs to control cell fates, additional ECM elements to improve mechanical properties, biomolecules to improve biological activities or any combinations of the above. In this Review, recent developments in using composite hydrogels laden with cells as biomimetic tissue- or organ-like constructs, and as matrices for multi-cell type organoid cultures are highlighted. The latest composite hydrogel systems that contain nanomaterials, biological factors, and combinations of biopolymers (e.g., proteins and polysaccharide), such as Interpenetrating Networks (IPNs) and Soft Network Composites (SNCs) are also presented. While promising, challenges remain. These will be discussed in light of future perspectives toward encompassing diverse composite hydrogel platforms for an improved organ environment in vitro.

Designer Self-Assembling Peptide Hydrogels to Engineer 3D Cell Microenvironments for Cell Constructs Formation and Precise Oncology Remodeling in Ovarian Cancer

Designer self-assembling peptides form the entangled nanofiber networks in hydrogels by ionic-complementary self-assembly. This type of hydrogel has realistic biological and physiochemical properties to serve as biomimetic extracellular matrix (ECM) for biomedical applications. The advantages and benefits are distinct from natural hydrogels and other synthetic or semisynthetic hydrogels. Designer peptides provide diverse alternatives of main building blocks to form various functional nanostructures. The entangled nanofiber networks permit essential compositional complexity and heterogeneity of engineering cell microenvironments in comparison with other hydrogels, which may reconstruct the tumor microenvironments (TMEs) in 3D cell cultures and tissue-specific modeling in vitro. Either ovarian cancer progression or recurrence and relapse are involved in the multifaceted TMEs in addition to mesothelial cells, fibroblasts, endothelial cells, pericytes, immune cells, adipocytes, and the ECM. Based on the progress in common hydrogel products, this work focuses on the diverse designer self-assembling peptide hydrogels for instructive cell constructs in tissue-specific modeling and the precise oncology remodeling for ovarian cancer, which are issued by several research aspects in a 3D context. The advantages and significance of designer peptide hydrogels are discussed, and some common approaches and coming challenges are also addressed in current complex tumor diseases.

Five Decades Later, Are Mesenchymal Stem Cells Still Relevant?

Mesenchymal stem cells are culture-derived mesodermal progenitors isolatable from all vascularized tissues. In spite of multiple fundamental, pre-clinical and clinical studies, the native identity and role in tissue repair of MSCs have long remained elusive, with MSC selection in vitro from total cell suspensions essentially unchanged as a mere primary culture for half a century. Recent investigations have helped understand the tissue origin of these progenitor cells, and uncover alternative effects of MSCs on tissue healing via growth factor secretion and interaction with the immune system. In this review, we describe current trends in MSC biology and discuss how these may improve the use of these therapeutic cells in tissue engineering and regenerative medicine.

Effect of 'in air' freezing on post-thaw recovery of Callithrix jacchus mesenchymal stromal cells and properties of 3D collagen-hydroxyapatite scaffolds

Through enabling an efficient supply of cells and tissues in the health sector on demand, cryopreservation is increasingly becoming one of the mainstream technologies in rapid translation and commercialization of regenerative medicine research. Cryopreservation of tissue-engineered constructs (TECs) is an emerging trend that requires the development of practically competitive biobanking technologies. In our previous studies, we demonstrated that conventional slow-freezing using dimethyl sulfoxide (Me₂SO) does not provide sufficient protection of mesenchymal stromal cells (MSCs) frozen in 3D collagen-hydroxyapatite scaffolds. After simple modifications to a cryopreservation protocol, we report on significantly improved cryopreservation of TECs. Porous 3D scaffolds were fabricated using freeze-drying of a mineralized collagen suspension and following chemical crosslinking. Amnion-derived MSCs from common marmoset monkey Callithrix jacchus were seeded onto scaffolds in static conditions. Cell-seeded scaffolds were subjected to 24 h pre-treatment with 100 mM sucrose and slow freezing in 10% Me₂SO/20% FBS alone or supplemented with 300 mM sucrose. Scaffolds were frozen 'in air' and thawed using a two-step procedure. Diverse analytical methods were used for the interpretation of cryopreservation outcome for both cell-seeded and cell-free scaffolds. In both groups, cells exhibited their typical shape and well-preserved cell-cell and cell-matrix contacts after thawing. Moreover, viability test 24 h post-thaw demonstrated that application of sucrose in the cryoprotective solution preserves a significantly greater portion of sucrose-pretreated cells (more than 80%) in comparison to Me₂SO alone (60%). No differences in overall protein structure and porosity of frozen scaffolds were revealed whereas their compressive stress was lower than in the control group. In conclusion, this approach holds promise for the cryopreservation of 'ready-to-use' TECs.

Tissue Engineering and Regenerative Medicine 2019: The Role of Biofabrication-A Year in Review

Despite its relative youth, biofabrication is unceasingly expanding by assimilating the contributions from various disciplinary areas and their technological advances. Those developments have spawned the range of available options to produce structures with complex geometries while accurately manipulating and controlling cell behavior. As it evolves, biofabrication impacts other research fields, allowing the fabrication of tissue models of increased complexity that more closely resemble the dynamics of living tissue. The recent blooming and evolutions in biofabrication have opened new windows and perspectives that could aid the translational struggle in tissue engineering and regenerative medicine (TERM) applications. Based on similar methodologies applied in past years' reviews, we identified the most high-impact publications and reviewed the major concepts, findings, and research outcomes in the context of advancement beyond the state-of-the-art in the field. We first aim to clarify the confusion in terminology and concepts in biofabrication to therefore introduce the striking evolutions in three-dimensional and four-dimensional bioprinting of tissues. We conclude with a short discussion on the future outlooks for innovation that biofabrication could bring to TERM research.

Fully amino acid-based hydrogel as potential scaffold for cell culturing and drug delivery

Polymer hydrogels are ideal scaffolds for both tissue engineering and drug delivery. A great advantage of poly(amino acid)-based hydrogels is their high similarity to natural proteins. However, their expensive and complicated synthesis often limits their application. The use of poly(aspartic acid) (PASP) seems an appropriate solution for this problem due to the relatively cheap and simple synthesis of PASP. Using amino acids not only as building blocks in the polymer backbone but also as cross-linkers can improve the biocompatibility and the biodegradability of the hydrogel. In this paper, PASP cross-linked with cystamine (CYS) and lysine-methylester (LYS) was introduced as fully amino acid-based polymer hydrogel. Gels were synthesized employing six different ratios of CYS and LYS. The pH dependent swelling degree and the concentration of the elastically active chain were determined. After reduction of the disulfide bonds of CYS, the presence of thiol side groups was also detected. To determine the concentration of the reactive cross-linkers in the hydrogels, a new method based on the examination of the swelling behavior was established. Using metoprolol as a model drug, cell proliferation and drug release kinetics were studied at different LYS contents and in the presence of thiol groups. The optimal ratio of cross-linkers for the proliferation of periodontal ligament cells was found to be 60-80% LYS and 20-40% CYS. The reductive conditions resulted in an increased drug release due to the cleavage of disulfide bridges in the hydrogels. Consequently, these hydrogels provide new possibilities in the fields of both tissue engineering and controlled drug delivery.

Force modeling to develop a novel method for fabrication of hollow channels inside a gel structure

Fabrication of hollow channels with user-defined dimensions and patterns inside viscoelastic, gel-type materials is required for several applications, especially in biomedical engineering domain. These include objectives of obtaining vascularized tissues and enclosed or subsurface microfluidic devices. However, presently there is no suitable manufacturing technology that can create such channels and networks in a gel structure. The advent of three-dimensional bioprinting has opened new possibilities for fabricating structures with complex geometries. However, application of this technique to fabricate internal hollow channels in viscoelastic material has not been yet explored to a great extent. In this article, we present the theoretical modeling/background of a proposed manufacturing paradigm through which hollow channels can be conveniently fabricated inside a gel structure. We propose that a tip connected to a robotic arm can be moved in X-, Y-, and Z-axis as per the desired design. The tip can be moved by a magnet or mechanical force. If the tip is further trailed with porous tube and moved inside the viscoelastic material, corresponding internal channels can be fabricated. To achieve this, however, force modeling to understand the forces that will be required to move the tip inside viscoelastic material should be known and understood. Therefore, in our first attempt, we developed the computational force modeling of the tip movement inside gels with different viscoelastic properties to create the channels.

Synthesis and Characterizations of a Collagen-Rich Biomembrane with Potential for Tissue-Guided Regeneration

Objectives  In this study, a collagen-rich biomembrane obtained from porcine ­intestinal submucosa for application in guided bone regeneration was developed and characterized. Then, its biological and mechanical properties were compared with that of commercial products ( GenDerm [Baumer], Lumina-Coat [Critéria], Surgitime PTFE [Bionnovation], and Surgidry Dental F [Technodry]).

Materials and Methods  The biomembrane was extracted from porcine intestinal submucosa. Scanning electron microscopy, spectroscopic dispersive energy, glycosaminoglycan quantification, and confocal microscopy by intrinsic fluorescence were used to evaluate the collagen structural patterns of the biomembrane. Mechanical tensile and deformation tests were also performed.

Statistical Analysis   The results of the methods used for experimental membrane characterizations were compared with that obtained by the commercial membranes and statistically analyzed (significance of 5%).

Results  The collagen-rich biomembrane developed also exhibited a more organized, less porous collagen fibril network, with the presence of glycosaminoglycans. The experimental biomembrane exhibited mechanical properties, tensile strength, and deformation behavior with improved average stress/strain when compared with other commercial membranes tested. Benefits also include a structured, flexible, and ­bioresorbable characteristics scaffold.

Conclusions  The experimental collagen-rich membrane developed presents physical–chemical, molecular, and mechanical characteristics similar to or better than that of the commercial products tested, possibly allowing it to actively participating in the process of bone neoformation.

Exceptional Mechanical Properties of Phase-Separation-Free Mo3Se3--Chain-Reinforced Hydrogel Prepared by Polymer Wrapping Process

As Mo₃Se₃^- chain nanowires have dimensions comparable to those of natural hydrogel chains (molecular-level diameters of ∼0.6 nm and lengths of several micrometers) and excellent mechanical strength and flexibility, they have large potential to reinforce hydrogels and improve their mechanical properties. When a Mo₃Se₃^--chain-nanowire-gelatin composite hydrogel is prepared simply by mixing Mo₃Se₃^- nanowires with gelatin, phase separation of the Mo₃Se₃^- nanowires from the gelatin matrix occurs in the micronetwork, providing only small improvements in their mechanical properties. In contrast, when the surface of the Mo₃Se₃^- nanowire is wrapped with the gelatin polymer, the chemical compatibility of the Mo₃Se₃^- nanowire with the gelatin matrix is significantly improved, which enables the fabrication of a phase-separation-free Mo₃Se₃^--reinforced gelatin hydrogel. The composite gelatin hydrogel exhibits significantly improved mechanical properties, including a tensile strength of 27.6 kPa, fracture toughness of 26.9 kJ/m³, and elastic modulus of 54.8 kPa, which are 367%, 868%, and 378% higher than those of the pure gelatin hydrogel, respectively. Furthermore, the amount of Mo₃Se₃^- nanowires added in the composite hydrogel is as low as 0.01 wt %. The improvements in the mechanical properties are significantly larger than those for other reported composite hydrogels reinforced with one-dimensional materials.

Fabrication of Human Keratinocyte Cell Clusters for Skin Graft Applications by Templating Water-in-Water Pickering Emulsions

Most current methods for the preparation of tissue spheroids require complex materials, involve tedious physical steps and are generally not scalable. We report a novel alternative, which is both inexpensive and up-scalable, to produce large quantities of viable human keratinocyte cell clusters (clusteroids). The method is based on a two-phase aqueous system of incompatible polymers forming a stable water-in-water (w/w) emulsion, which enabled us to rapidly fabricate cell clusteroids from HaCaT cells. We used w/w Pickering emulsion from aqueous solutions of the polymers dextran (DEX) and polyethylene oxide (PEO) and a particle stabilizer based on whey protein (WP). The HaCaT cells clearly preferred to distribute into the DEX-rich phase and this property was utilized to encapsulate them in the water-in-water (DEX-in-PEO) emulsion drops then osmotically shrank to compress them into clusters. Prepared formulations of HaCaT keratinocyte clusteroids in alginate hydrogel were grown where the cells percolated to mimic 3D tissue. The HaCaT cell clusteroids grew faster in the alginate film compared to the individual cells formulated in the same matrix. This methodology could potentially be utilised in biomedical applications.

Beyond the Present Constraints That Prevent a Wide Spread of Tissue Engineering and Regenerative Medicine Approaches

Despite the success of tissue engineered medical products (TEMPs) in preclinical translational research, very few have had success in the clinical market place. This gap, referred to as the “valley of death” is due to the large number of ventures that failed to attract or retain investor funding, promotion, and clinical acceptance of their products. This loss can be attributed to a focus on a bench to bedside flow of ideas and technology, which does not account for the multitude of adoption, commercial, and regulatory constraints. The implementation of an alternative bedside to bench and back again approach permits investigators to focus on a specific unmet clinical need, defining crucial translation related questions early in the research process. Investigators often fail to accurately identify critical clinical adoption criteria due to their focus on improved patient outcomes. Other adoption criteria (such as price, time, ethical concerns, and place in the workflow) can cause a product to fail despite improved patient outcomes. By applying simplified business principles such as the build-measure-learn loop and the business model canvas to early-stage research projects, investigators can narrow in on appropriate research topics and define design constraints. Additionally, 86% of all clinical trials fail to result in Federal Drug Administration approval, resulting in significant economic burdens. On the reverse side, approval through the European Medical Agency is widely considered to be more direct but has its challenges. The Committee for Advanced Therapies within the European Medical Agency has received 22 market authorization applications for advanced therapy medicinal products, of which only 10 received authorization. A thorough understanding of the various regulatory pathways permits investigators to plan for future regulatory obstacles and potentially increase their chances of success. By utilizing a bedside to bench and back again approach, investigators can improve the odds that their research will have a meaningful clinical impact.

On the Choice of the Extracellular Vesicles for Therapeutic Purposes

Extracellular vesicles (EVs) are lipid membrane vesicles released by all human cells and are widely recognized to be involved in many cellular processes, both in physiological and pathological conditions. They are mediators of cell-cell communication, at both paracrine and systemic levels, and therefore they are active players in cell differentiation, tissue homeostasis, and organ remodeling. Due to their ability to serve as a cargo for proteins, lipids, and nucleic acids, which often reflects the cellular source, they should be considered the future of the natural nanodelivery of bio-compounds. To date, natural nanovesicles, such as exosomes, have been shown to represent a source of disease biomarkers and have high potential benefits in regenerative medicine. Indeed, they deliver both chemical and bio-molecules in a way that within exosomes drugs are more effective that in their exosome-free form. Thus, to date, we know that exosomes are shuttle disease biomarkers and probably the most effective way to deliver therapeutic molecules within target cells. However, we do not know exactly which exosomes may be used in therapy in avoiding side effects as well. In regenerative medicine, it will be ideal to use autologous exosomes, but it seems not ideal to use plasma-derived exosomes, as they may contain potentially dangerous molecules. Here, we want to present and discuss a contradictory relatively unmet issue that is the lack of a general agreement on the choice for the source of extracellular vesicles for therapeutic use.

Bioinks and bioprinting technologies to make heterogeneous and biomimetic tissue constructs

The native tissues are complex structures consisting of different cell types, extracellular matrix materials, and biomolecules. Traditional tissue engineering strategies have not been able to fully reproduce biomimetic and heterogeneous tissue constructs because of the lack of appropriate biomaterials and technologies. However, recently developed three-dimensional bioprinting techniques can be leveraged to produce biomimetic and complex tissue structures. To achieve this, multicomponent bioinks composed of multiple biomaterials (natural, synthetic, or hybrid natural-synthetic biomaterials), different types of cells, and soluble factors have been developed. In addition, advanced bioprinting technologies have enabled us to print multimaterial bioinks with spatial and microscale resolution in a rapid and continuous manner, aiming to reproduce the complex architecture of the native tissues. This review highlights important advances in heterogeneous bioinks and bioprinting technologies to fabricate biomimetic tissue constructs. Opportunities and challenges to further accelerate this research area are also described.

The inevitability of change

Change is an absolute so long as time does not stand still. We should expect it, embrace it, and try to predict its direction. Dermatology, as a specialty practice, has been changing rapidly over the past 30 years concurrent with the changes in medicine. What are these changes, how did they come about, and what may be the consequences? The goal of this review is to follow the march of time, as we move from one era to the other in step with what is happening in the world as a whole and the United States in particular. The growth of our specialty, Dermatology, is divided into 3 eras which are quite different in generational cultures. The first era spanning the 1980s and 1990s is dubbed as "old school." The second era begins with the new century, 2000 until today. This era will forever be remembered as the business era, the rise of elite cultures, and the losses and threats to academia. The third era begins now; it is that of technology which is fast progressing into the future. One can theoretically project what may occur during this technologic revolution and the directions in medicine as a whole. Dermatology can be at the forefront of this era or it could be lost as a whole if we do nothing to keep up. These eras are based on my personal experience as a dermatologist in a large academic institution in the United States and may not apply to other communities or societies elsewhere. The United States serves as a good example of a western technologically oriented society that is often emulated by others.