Repairing Volumetric Muscle Loss with Commercially Available Hydrogels in an Ovine Model

Volumetric muscle loss (VML) is the loss of skeletal muscle that exceeds the muscle's self-repair mechanism and leads to permanent functional deficits. In a previous study, we demonstrated the ability of our scaffold-free, multiphasic, tissue-engineered skeletal muscle units (SMUs) to restore muscle mass and force production. However, it was observed that the full recovery of muscle structure was inhibited due to increased fibrosis in the repair site. As such, novel biomaterials such as hydrogels (HGs) may have significant potential for decreasing the acute inflammation and subsequent fibrosis, as well as enhancing skeletal muscle regeneration following VML injury and repair. The goal of the current study was to assess the biocompatibility of commercially available poly(ethylene glycol), methacrylated gelatin, and hyaluronic acid (HA) HGs in combination with our SMUs to treat VML in a clinically relevant large animal model. An acute 30% VML injury created in the sheep peroneus tertius (PT) muscle was repaired with or without HGs and assessed for acute inflammation (incision swelling) and white blood cell counts in blood for 7 days. At the 7-day time point, HA was selected as the HG to use for the combined HG/SMU repair, as it exhibited a reduced inflammation response compared to the other HGs. Six weeks after implantation, all groups were assessed for gross and histological structural recovery. The results showed that the groups repaired with an SMU (SMU-Only and SMU+HA) restored muscle mass to greater degree than the groups with only HG and that the SMU groups had PT muscle masses that were statistically indistinguishable from its uninjured contralateral PT muscle. Furthermore, the HA HG, SMU-Only, and SMU+HA groups displayed notable efficacy in diminishing pro-inflammatory markers and showed an increased number of regenerating muscle fibers in the repair site. Taken together, the data demonstrates the efficacy of HA HG in decreasing acute inflammation and fibrotic response. The combination of HA and our SMUs also holds promise to decrease acute inflammation and fibrosis and increase muscle regeneration, advancing this combination therapy toward clinically relevant interventions for VML injuries in humans.

Bioprinting Using Organ Building Blocks: Spheroids, Organoids, and Assembloids

Three-dimensional (3D) bioprinting, a promising advancement in tissue engineering technology, involves the robotic, layer-by-layer additive biofabrication of functional 3D tissue and organ constructs. This process utilizes biomaterials, typically hydrogels and living cells, following digital models. Traditional tissue engineering uses a classic triad of living cells, scaffolds, and physicochemical signals in bioreactors. A scaffold is a temporary, often biodegradable, support structure. Tissue engineering primarily falls into two categories: (i) scaffold based and (ii) scaffold free. The latter, scaffold-free 3D bioprinting, is gaining increasing popularity. Organ building blocks (OBB), capable of self-assembly and self-organization, such as tissue spheroids, organoids, and assembloids, have begun to be utilized in scaffold-free bioprinting. This article discusses the expanding range of OBB, presents the rapidly evolving collection of bioprinting and bioassembly methods using these OBB, and finally, outlines the advantages, challenges, and future perspectives of using OBB in organ printing.

Three-Dimensional Bioprinting in Vascular Tissue Engineering and Tissue Vascularization of Cardiovascular Diseases

In the 21st century, significant progress has been made in repairing damaged materials through material engineering. However, the creation of large-scale artificial materials still faces a major challenge in achieving proper vascularization. To address this issue, researchers have turned to biomaterials and three-dimensional (3D) bioprinting techniques, which allow for the combination of multiple biomaterials with improved mechanical and biological properties that mimic natural materials. Hydrogels, known for their ability to support living cells and biological components, have played a crucial role in this research. Among the recent developments, 3D bioprinting has emerged as a promising tool for constructing hybrid scaffolds. However, there are several challenges in the field of bioprinting, including the need for nanoscale biomimicry, the formulation of hydrogel blends, and the ongoing complexity of vascularizing biomaterials, which requires further research. On a positive note, 3D bioprinting offers a solution to the vascularization problem due to its precise spatial control, scalability, and reproducibility compared with traditional fabrication methods. This paper aims at examining the recent advancements in 3D bioprinting technology for creating blood vessels, vasculature, and vascularized materials. It provides a comprehensive overview of the progress made and discusses the limitations and challenges faced in current 3D bioprinting of vascularized tissues. In addition, the paper highlights the future research directions focusing on the development of 3D bioprinting techniques and bioinks for creating functional materials.

Structural and Molecular Changes of Human Chondrocytes Exposed to the Rotating Wall Vessel Bioreactor

Over the last 30 years, the prevalence of osteoarthritis (OA), a disease characterized by a loss of articular cartilage, has more than doubled worldwide. Patients suffer from pain and progressive loss of joint function. Cartilage is an avascular tissue mostly consisting of extracellular matrix with embedded chondrocytes. As such, it does not regenerate naturally, which makes an early onset of OA prevention and treatment a necessity to sustain the patients' quality of life. In recent years, tissue engineering strategies for the regeneration of cartilage lesions have gained more and more momentum. In this study, we aimed to investigate the scaffold-free 3D cartilage tissue formation under simulated microgravity in the NASA-developed rotating wall vessel (RWV) bioreactor. For this purpose, we cultured both primary human chondrocytes as well as cells from the immortalized line C28/I2 for up to 14 days on the RWV and analyzed tissue morphology, development of apoptosis, and expression of cartilage-specific proteins and genes by histological staining, TUNEL-assays, immunohistochemical detection of collagen species, and quantitative real-time PCR, respectively. We observed spheroid formation in both cell types starting on day 3. After 14 days, constructs from C28/I2 cells had diameters of up to 5 mm, while primary chondrocyte spheroids were slightly smaller with 3 mm. Further inspection of the 14-day-old C28/I2 spheroids revealed a characteristic cartilage morphology with collagen-type 1, -type 2, and -type 10 positivity. Interestingly, these tissues were less susceptible to RWV-induced differential gene expression than those formed from primary chondrocytes, which showed significant changes in the regulation of IL6, ACTB, TUBB, VIM, COL1A1, COL10A1, MMP1, MMP3, MMP13, ITGB1, LAMA1, RUNX3, SOX9, and CASP3 gene expression. These diverging findings might reflect the differences between primary and immortalized cells. Taken together, this study shows that simulated microgravity using the RWV bioreactor is suitable to engineer dense 3D cartilage-like tissue without addition of scaffolds or any other artificial materials. Both primary articular cells and the stable chondrocyte cell line C28/I2 formed 3D neocartilage when exposed for 14 days to an RWV.

Engineering three-dimensional bone macro-tissues by guided fusion of cell spheroids

Introduction

Bioassembly techniques for the application of scaffold-free tissue engineering approaches have evolved in recent years toward producing larger tissue equivalents that structurally and functionally mimic native tissues. This study aims to upscale a 3-dimensional bone in-vitro model through bioassembly of differentiated rat osteoblast (dROb) spheroids with the potential to develop and mature into a bone macrotissue.

Methods

dROb spheroids in control and mineralization media at different seeding densities (1 × 104, 5 × 104, and 1 × 105 cells) were assessed for cell proliferation and viability by trypan blue staining, for necrotic core by hematoxylin and eosin staining, and for extracellular calcium by Alizarin red and Von Kossa staining. Then, a novel approach was developed to bioassemble dROb spheroids in pillar array supports using a customized bioassembly system. Pillar array supports were custom-designed and printed using Formlabs Clear Resin® by Formlabs Form2 printer. These supports were used as temporary frameworks for spheroid bioassembly until fusion occurred. Supports were then removed to allow scaffold-free growth and maturation of fused spheroids. Morphological and molecular analyses were performed to understand their structural and functional aspects.

Results

Spheroids of all seeding densities proliferated till day 14, and mineralization began with the cessation of proliferation. Necrotic core size increased over time with increased spheroid size. After the bioassembly of spheroids, the morphological assessment revealed the fusion of spheroids over time into a single macrotissue of more than 2.5 mm in size with mineral formation. Molecular assessment at different time points revealed osteogenic maturation based on the presence of osteocalcin, downregulation of Runx2 (p < 0.001), and upregulated alkaline phosphatase (p < 0.01).

Discussion

With the novel bioassembly approach used here, 3D bone macrotissues were successfully fabricated which mimicked physiological osteogenesis both morphologically and molecularly. This biofabrication approach has potential applications in bone tissue engineering, contributing to research related to osteoporosis and other recurrent bone ailments.

Cellular Interactions in Cell Sheets Enhance Mesenchymal Stromal Cell Immunomodulatory Properties

Immune-related applications of mesenchymal stromal cells (MSCs) in cell therapy seek to exploit immunomodulatory paracrine signaling pathways to reduce inflammation. A key MSC therapeutic challenge is reducing patient outcome variabilities attributed to insufficient engraftment/retention of injected heterogenous MSCs. To address this, we propose directly transplantable human single-cell-derived clonal bone marrow MSC (hcBMSC) sheets. Cell sheet technology is a scaffold-free tissue engineering strategy enabling scalable production of highly engraftable cell constructs retaining endogenous cell-cell and cell-matrix interactions, important to cell function. cBMSCs, as unique MSC subset populations, facilitate rational selection of therapeutically relevant MSC clones from donors. Here, we combine human cBMSCs with cell sheet technology, demonstrating cell sheet fabrication as a method to significantly upregulate expression of immunomodulatory molecules interleukin (IL)-10, indoleamine 2,3-dioxygenase (IDO-1), and prostaglandin E synthase 2 (PTGES2) across GMP-grade hcBMSC lines and whole human bone marrow-derived MSCs compared to respective conventional cell suspensions. When treated with carbenoxolone, a gap junction inhibitor, cell sheets downregulate IL-10 and IDO-1 expression, implicating functional roles for intercellular sheet interactions. Beyond producing directly transferable multicellular hcBMSC constructs, cell sheet technology amplifies hcBMSC expression of immunomodulatory factors important to therapeutic action. In addition, this work demonstrates the importance of cell-cell interactions as a tissue engineering design criterion to enhance consistent MSC functions.

Development of a Self-Assembled Dermal Substitute from Human Fibroblasts Using Long-term Three-Dimensional Culture

Skin substitutes have emerged as an alternative to autografts for the treatment of skin defects. Among them, scaffold-based dermal substitutes have been extensively studied; however, they have certain limitations, such as delayed vascularization, limited elasticity, and the inability to achieve permanent engraftment. Self-assembled, cell-based dermal substitutes are a promising alternative that may overcome these shortcomings but have not yet been developed. In this study, we successfully developed a cell-based dermal substitute (cultured dermis) through the long-term culture of human dermal fibroblasts using the net-mold method, which enables three-dimensional cell culture without the use of a scaffold. Spheroids prepared from human dermal fibroblasts were poured into a net-shaped mold and cultured for 2, 4, or 6 months. The dry weight, tensile strength, collagen and glycosaminoglycan levels, and cell proliferation capacity were assessed and compared among the 2-, 4-, and 6-month culture periods. We found that collagen and glycosaminoglycan levels decreased over time, while the dry weight remained unchanged. Tensile strength increased at 4 months, suggesting that remodeling had progressed. In addition, the cell proliferation capacity was maintained, even after a 6-month culture period. Unexpectedly, the internal part of the cultured dermis became fragile, resulting in the division of the cultured dermis into two collagen-rich tissues, each of which had a thickness of 400 μm and sufficient strength to be sutured during in vivo analysis. The divided 4-month cultured dermis was transplanted to skin defects of immunocompromised mice and its wound healing effects were compared to those of a clinically available collagen-based artificial dermis. The cultured dermis promoted epithelialization and angiogenesis more effectively than the collagen-based artificial dermis. Although further improvements are needed, such as the shortening of the culture period and increasing the size of the cultured dermis, we believe that the cultured dermis presented in this study has the potential to be an innovative material for permanent skin coverage.

Fabrication of shape-designable cartilage from human induced pluripotent stem cell-derived chondroprogenitors using a cell self-aggregation technique

With the advancement of tissue engineering technologies, implantable materials have been developed for use in facial plastic surgery, including auriculoplasty and rhinoplasty. Tissue-engineered cartilage comprising only cells and cell-produced extracellular matrix is considered valuable as there is no need to consider problems associated with scaffold absorption or immune responses commonly related to conventional artificial materials. However, it is exceedingly difficult to produce large-sized complex shapes of cartilage without the use of scaffolds. In this study, we describe the production of shape-designable cartilage using a novel cell self-aggregation technique (CAT) and chondroprogenitor cells derived from human induced pluripotent stem cells as the source. The method described does not require special equipment such as bio-3D printers, and the produced tissue can be induced into well-matured cartilage with abundant cartilage matrixin vitro. Using CAT, we were able to generate cartilage in the form of rings or tubes with adjustable inner diameter and curvature, over a range of several centimeters, without the use of scaffolds. Thein vitrofabrication of shape-designable cartilage using CAT is a promising development in facial plastic surgery.

Scaffold-Free Multicellular 3D Tissue Constructs Utilizing Bio-orthogonal Click Strategy

Multicellular 3D tissue constructs (MTCs) are important in biomedical research due to their capacity to accurately mimic the structure and variation found in real tissues. This study presents a novel bio-orthogonal engineering strategy (BIEN), a transformative scaffold-free approach, to create advanced MTCs. BIEN harnesses the cellular biosynthetic machinery to incorporate bio-orthogonal azide reporters into cell surface glycoconjugates, followed by a click reaction with multiarm PEG, resulting in rapid assembly of MTCs. The implementation of this cutting-edge strategy culminates in the formation of uniform, heterogeneous spheroids, characterized by a high degree of intercellular junction and pluripotency. Remarkably, MTCs simulate tumor features, ensure cell heterogeneity, and significantly improve the subcutaneous xenograft model after transplantation, thereby bolstering both in vitro and in vivo research models. In conclusion, the utilization of the bio-orthogonal engineering strategy as a scaffold-free method to generate superior MTCs holds promising potential for driving advancements in cancer research.

Current biofabrication methods for vascular tissue engineering and an introduction to biological textiles

Cardiovascular diseases are the leading cause of mortality in the world and encompass several important pathologies, including atherosclerosis. In the cases of severe vessel occlusion, surgical intervention using bypass grafts may be required. Synthetic vascular grafts provide poor patency for small-diameter applications (< 6 mm) but are widely used for hemodialysis access and, with success, larger vessel repairs. In very small vessels, such as coronary arteries, synthetics outcomes are unacceptable, leading to the exclusive use of autologous (native) vessels despite their limited availability and, sometimes, quality. Consequently, there is a clear clinical need for a small-diameter vascular graft that can provide outcomes similar to native vessels. Many tissue-engineering approaches have been developed to offer native-like tissues with the appropriate mechanical and biological properties in order to overcome the limitations of synthetic and autologous grafts. This review overviews current scaffold-based and scaffold-free approaches developed to biofabricate tissue-engineered vascular grafts (TEVGs) with an introduction to the biological textile approaches. Indeed, these assembly methods show a reduced production time compared to processes that require long bioreactor-based maturation steps. Another advantage of the textile-inspired approaches is that they can provide better directional and regional control of the TEVG mechanical properties.

Scaffold-Free Tracheal Engineering via a Modular Strategy Based on Cartilage and Epithelium Sheets

Tracheal defects lead to devastating problems, and practical clinical substitutes that have complex functional structures and can avoid adverse influences from exogenous bioscaffolds are lacking. Herein, a modular strategy for scaffold-free tracheal engineering is developed. A cartilage sheet (Cart-S) prepared by high-density culture is laminated and reshaped to construct a cartilage tube as the main load-bearing structure in which the chondrocytes exhibit a stable phenotype and secreted considerable cartilage-specific matrix, presenting a native-like grid arrangement. To further build a tracheal epithelial barrier, a temperature-sensitive technique is used to construct the monolayer epithelium sheet (Epi-S), in which the airway epithelial cells present integrated tight junctions, good transepithelial electrical resistance, and favorable ciliary differentiation capability. Epi-S can be integrally transferred to inner wall of cartilage tube, forming a scaffold-free complex tracheal substitute (SC-trachea). Interestingly, when Epi-S is attached to the cartilage surface, epithelium-specific gene expression is significantly enhanced. SC-trachea establishes abundant blood supply via heterotopic vascularization and then is pedicle transplanted for tracheal reconstruction, achieving 83.3% survival outcomes in rabbit models. Notably, the scaffold-free engineered trachea simultaneously satisfies sufficient mechanical properties and barrier function due to its matrix-rich cartilage structure and well-differentiated ciliated epithelium, demonstrating great clinical potential for long-segmental tracheal reconstruction.

Vascularization Strategies in 3D Cell Culture Models: From Scaffold-Free Models to 3D Bioprinting

The discrepancies between the findings in preclinical studies, and in vivo testing and clinical trials have resulted in the gradual decline in drug approval rates over the past decades. Conventional in vitro drug screening platforms employ two-dimensional (2D) cell culture models, which demonstrate inaccurate drug responses by failing to capture the three-dimensional (3D) tissue microenvironment in vivo. Recent advancements in the field of tissue engineering have made possible the creation of 3D cell culture systems that can accurately recapitulate the cell-cell and cell-extracellular matrix interactions, as well as replicate the intricate microarchitectures observed in native tissues. However, the lack of a perfusion system in 3D cell cultures hinders the establishment of the models as potential drug screening platforms. Over the years, multiple techniques have successfully demonstrated vascularization in 3D cell cultures, simulating in vivo-like drug interactions, proposing the use of 3D systems as drug screening platforms to eliminate the deviations between preclinical and in vivo testing. In this review, the basic principles of 3D cell culture systems are briefly introduced, and current research demonstrating the development of vascularization in 3D cell cultures is discussed, with a particular focus on the potential of these models as the future of drug screening platforms.

Clumps of mesenchymal stem cells/extracellular matrix complexes directly reconstruct the functional periodontal tissue in a rat periodontal defect model

Periodontitis is an inflammatory disease characterized by tooth-supporting periodontal tissue destruction, including the cementum, periodontal ligament, and alveolar bone. To regenerate the damaged periodontal tissue, mesenchymal stem cells (MSCs) have attracted much scientific and medical attention. Recently, we generated clumps of MSCs/extracellular matrix (ECM) complexes (C-MSCs), which consist of cells and self-produced ECM. C-MSCs can be transplanted into lesion areas without artificial scaffold to induce tissue regeneration. To develop reliable scaffold-free periodontal tissue regenerative cell therapy by C-MSCs, this study investigated the periodontal tissue regenerative capacity of C-MSCs and the behavior of the transplanted cells. Rat bone marrow-derived MSCs were isolated from rat femur. Confluent cells were scratched using a micropipette tip and then torn off. The sheet was rolled to make a three-dimensional round clump of cells, C-MSCs. Then, ten C-MSCs were grafted into a rat periodontal fenestration defect model. To trace the grafted cells in the defect, PKH26-labeled cells were also employed. Micro-CT and histological analyses demonstrated that transplantation of C-MSCs induced successful periodontal tissue regeneration in the rat periodontal defect model. Interestingly, the majority of the cells in the reconstructed tissue, including cementum, periodontal ligaments, and alveolar bone, were PKH26 positive donor cells, suggesting the direct tissue formation by MSCs. This study demonstrates a promising scaffold-free MSCs transplantation strategy for periodontal disease using C-MSCs and offers the significance of multipotency of MSCs to induce successful periodontal tissue regeneration.

Promoting and Orienting Axon Extension Using Scaffold-Free Dental Pulp Stem Cell Sheets

Current treatments of facial nerve injury result in poor functional outcomes due to slow and inefficient axon regeneration and aberrant reinnervation. To address these clinical challenges, bioactive scaffold-free cell sheets were engineered using neurotrophic dental pulp stem/progenitor cells (DPCs) and their aligned extracellular matrix (ECM). DPCs endogenously supply high levels of neurotrophic factors (NTFs), growth factors capable of stimulating axonal regeneration, and an aligned ECM provides guidance cues to direct axon extension. Human DPCs were grown on a substrate comprising parallel microgrooves, inducing the cells to align and deposit a linearly aligned, collagenous ECM. The resulting cell sheets were robust and could be easily removed from the underlying substrate. DPC sheets produced NTFs at levels previously shown capable of promoting axon regeneration, and, moreover, inducing DPC alignment increased the expression of select NTFs relative to unaligned controls. Furthermore, the aligned DPC sheets were able to stimulate functional neuritogenic effects in neuron-like cells in vitro. Neuronally differentiated neuroblastoma SH-SY5Y cells produced neurites that were significantly more oriented and less branched when cultured on aligned cell sheets relative to unaligned sheets. These data demonstrate that the linearly aligned DPC sheets can biomechanically support axon regeneration and improve axonal guidance which, when applied to a facial nerve injury, will result in more accurate reinnervation. The aligned DPC sheets generated here could be used in combination with commercially available nerve conduits to enhance their bioactivity or be formed into stand-alone scaffold-free nerve conduits capable of facilitating improved facial nerve recovery.

Scaffold-free approaches for the fabrication of engineered articular cartilage tissue

Tissue engineered cartilaginous constructs have meet great advances in the past decades as a treatment for osteoarthritis, a degenerative disease affecting people all over the world as the population ages. Scaffold-free tissue engineered constructs are designed and developed in recent years with only cells and cell-derived matrix involved. Scaffold-free tissue constructs do not require cell adherence on exogenous materials and are superior to scaffold-based constructs in (1) relying on only cells to produce matrix, (2) not interfering cell-cell signaling, cell migration or small molecules diffusion after implantation and (3) introducing no exogenous impurities. In this review, three main scaffold-free methodologies for cartilage tissue engineering, the cell sheet technology, the phase transfer cell culture-living hyaline cartilage graft (PTCC-LhCG) system and the cell aggregate-based (bottom-up) methods, were reviewed, covering mold fabrication, decellularization and 3D bioprinting. The recent advances, medical applications, superiority and drawbacks were elaborated in detail.

The Developing Field of Scaffold-Free Tissue Engineering for Articular Cartilage Repair

Articular cartilage is critical for proper joint mobility as it provides a smooth and lubricated surface between articulating bones and allows for transmission of load to underlying bones. Extended wear or injury of this tissue can result in osteoarthritis, a degenerative disease affecting millions across the globe. Because of its low regenerative capacity, articular cartilage cannot heal on its own and effective treatments for injured joint restoration remain a challenge. Strategies in tissue engineering have been demonstrated as potential therapeutic approaches to regenerate and repair damaged articular cartilage. Although many of these strategies rely on the use of an exogenous three-dimensional scaffolds to regenerate cartilage, scaffold-free tissue engineering provides numerous advantages over scaffold-based methods. This review highlights the latest advancements in scaffold-free tissue engineering for cartilage and the potential for clinical translation.

The ECM: To Scaffold, or Not to Scaffold, That Is the Question

The extracellular matrix (ECM) has pleiotropic effects, ranging from cell adhesion to cell survival. In tissue engineering, the use of ECM and ECM-like scaffolds has separated the field into two distinct areas-scaffold-based and scaffold-free. Scaffold-free techniques are used in creating reproducible cell aggregates which have massive potential for high-throughput, reproducible drug screening and disease modeling. Though, the lack of ECM prevents certain cells from surviving and proliferating. Thus, tissue engineers use scaffolds to mimic the native ECM and produce organotypic models which show more reliability in disease modeling. However, scaffold-based techniques come at a trade-off of reproducibility and throughput. To bridge the tissue engineering dichotomy, we posit that finding novel ways to incorporate the ECM in scaffold-free cultures can synergize these two disparate techniques.

Bioprinting Scaffolds for Vascular Tissues and Tissue Vascularization

In recent years, tissue engineering has achieved significant advancements towards the repair of damaged tissues. Until this day, the vascularization of engineered tissues remains a challenge to the development of large-scale artificial tissue. Recent breakthroughs in biomaterials and three-dimensional (3D) printing have made it possible to manipulate two or more biomaterials with complementary mechanical and/or biological properties to create hybrid scaffolds that imitate natural tissues. Hydrogels have become essential biomaterials due to their tissue-like physical properties and their ability to include living cells and/or biological molecules. Furthermore, 3D printing, such as dispensing-based bioprinting, has progressed to the point where it can now be utilized to construct hybrid scaffolds with intricate structures. Current bioprinting approaches are still challenged by the need for the necessary biomimetic nano-resolution in combination with bioactive spatiotemporal signals. Moreover, the intricacies of multi-material bioprinting and hydrogel synthesis also pose a challenge to the construction of hybrid scaffolds. This manuscript presents a brief review of scaffold bioprinting to create vascularized tissues, covering the key features of vascular systems, scaffold-based bioprinting methods, and the materials and cell sources used. We will also present examples and discuss current limitations and potential future directions of the technology.

Clumps of Mesenchymal Stem Cells/Extracellular Matrix Complexes Generated with Xeno-Free Chondro-Inductive Medium Induce Bone Regeneration via Endochondral Ossification

Three-dimensional clumps of mesenchymal stem cells (MSCs)/extracellular matrix (ECM) complexes (C-MSCs) can be transplanted into tissue defect site with no artificial scaffold. Importantly, most bone formation in the developing process or fracture healing proceeds via endochondral ossification. Accordingly, this present study investigated whether C-MSCs generated with chondro-inductive medium (CIM) can induce successful bone regeneration and assessed its healing process. Human bone marrow-derived MSCs were cultured with xeno-free/serum-free (XF) growth medium. To obtain C-MSCs, confluent cells that had formed on the cellular sheet were scratched using a micropipette tip and then torn off. The sheet was rolled to make a round clump of cells. The cell clumps, i.e., C-MSCs, were maintained in XF-CIM. C-MSCs generated with XF-CIM showed enlarged round cells, cartilage matrix, and hypertrophic chondrocytes genes elevation in vitro. Transplantation of C-MSCs generated with XF-CIM induced successful bone regeneration in the SCID mouse calvaria defect model. Immunofluorescence staining for human-specific vimentin demonstrated that donor human and host mouse cells cooperatively contributed the bone formation. Besides, the replacement of the cartilage matrix into bone was observed in the early period. These findings suggested that cartilaginous C-MSCs generated with XF-CIM can induce bone regeneration via endochondral ossification.

Bioengineering of a scaffold-less three-dimensional tissue using net mould

Tissue engineering has attracted attention worldwide because of its application in regenerative medicine, drug screening, and cultured meat. Numerous biofabrication techniques for producing tissues have been developed, including various scaffold and printing methods. Here, we have proposed a novel tissue engineering method using a net metal mould without the use of a scaffold. Briefly, normal human dermal fibroblasts seeded on a dimple plate were subjected to static culture technique for several days to form spheroids. Spheroids of diameter ⩾200μm were poured into a net-shaped mould of gap ⩽100μm and subjected to shake-cultivation for several weeks, facilitating their fusion to form a three-dimensional (3D) tissue. Through this study, we successfully constructed a scaffold-free 3D tissue having strength that can be easily manipulated, which was difficult to construct using conventional tissue engineering methods. We also investigated the viability of the 3D tissue and found that the condition of the tissues was completely different depending on the culture media used. Collectively, this method allows scaffold-free culture of 3D tissues of unprecedented thickness, and may contribute largely to next-generation tissue engineering products.

Dental Pulp Cell Sheets Enhance Facial Nerve Regeneration via Local Neurotrophic Factor Delivery

An effective strategy for sustained neurotrophic factor (NTF) delivery to sites of peripheral nerve injury (PNI) would accelerate healing and enhance functional recovery, addressing the major clinical challenges associated with the current standard of care. In this study, scaffold-free cell sheets were generated using human dental pulp stem/progenitor cells, that endogenously express high levels of NTFs, for use as bioactive NTF delivery systems. Additionally, the effect of fibroblast growth factor 2 (FGF2) on NTF expression by dental pulp cell (DPC) sheets was evaluated. In vitro analysis confirmed that DPC sheets express high levels of NTF messenger RNA (mRNA) and proteins, and the addition of FGF2 to DPC sheet culture increased total NTF production by significantly increasing the cellularity of sheets. Furthermore, the DPC sheet secretome stimulated neurite formation and extension in cultured neuronal cells, and these functional effects were further enhanced when DPC sheets were cultured with FGF2. These neuritogenic results were reversed by NTF inhibition substantiating that DPC sheets have a positive effect on neuronal cell activity through the production of NTFs. Further evaluation of DPC sheets in a rat facial nerve crush injury model in vivo established that in comparison with untreated controls, nerves treated with DPC sheets had greater axon regeneration through the injury site and superior functional recovery as quantitatively assessed by compound muscle action potential measurements. This study demonstrates the use of DPC sheets as vehicles for NTF delivery that could augment the current methods for treating PNIs to accelerate regeneration and enhance the functional outcome. Impact statement The major challenges associated with current treatments of peripheral nerve injuries (PNIs) are prolonged repair times and insufficient functional recovery. Dental pulp stem/progenitor cells (DPCs) are known to endogenously express high levels of neurotrophic factors (NTFs), growth factors that enhance axon regeneration. In this study, we demonstrate that scaffold-free DPC sheets can act as effective carrier systems to facilitate the delivery and retention of NTF-producing DPCs to sites of PNIs and improve functional nerve regeneration. DPC sheets have high translational feasibility and could augment the current standard of care to enhance the quality of life for patients dealing with PNIs.

Bio-3D printing iPSC-derived human chondrocytes for articular cartilage regeneration

Osteoarthritis is a leading cause of pain and joint immobility, the incidence of which is increasing worldwide. Currently, total joint replacement is the only treatment for end-stage disease. Scaffold-based tissue engineering is a promising alternative approach for joint repair but is subject to limitations such as poor cytocompatibility and degradation-associated toxicity. To overcome these limitations, a completely scaffold-free Kenzan method for bio-3D printing was used to fabricate cartilage constructs feasible for repairing large chondral defects. Human induced pluripotent stem cell (iPSC)-derived neural crest cells with high potential to undergo chondrogenesis through mesenchymal stem cell differentiation were used to fabricate the cartilage. Unified, self-sufficient, and functional cartilaginous constructs up to 6 cm²in size were assembled by optimizing fabrication time during chondrogenic induction. Maturation for 3 weeks facilitated the self-organisation of the cells, which improved the construct's mechanical strength (compressive and tensile properties) and induced changes in glycosaminoglycan and type II collagen expression, resulting in improved tissue function. The compressive modulus of the construct reached the native cartilage range of 0.88 MPa in the 5th week of maturation. This paper reports the fabrication of anatomically sized and shaped cartilage constructs, achieved by combining novel iPSCs and bio-3D printers using a Kenzan needle array technology, which may facilitate chondral resurfacing of articular cartilage defects.

An Improved Scalable Hydrogel Dish for Spheroid Culture

Research in fields studying cellular response to surface tension and mechanical forces necessitate cell culture tools with tunability of substrate stiffness. We created a scalable hydrogel dish design to facilitate scaffold-free formation of multiple spheroids in a single dish. Our novel design features inner and outer walls, allowing efficient media changes and downstream experiments. The design is easily scalable, accommodating varying numbers of microwells per plate. We report that non-adherent hydrogel stiffness affects spheroid morphology and compaction. We found that spheroid morphology and viability in our hydrogel dishes were comparable to commercially available Aggrewell™800 plates, with improved tunability of surface stiffness and imaging area. Device function was demonstrated with a migration assay using two investigational inhibitors against EMT. We successfully maintained primary-derived spheroids from murine and porcine lungs in the hydrogel dish. These features increase the ability to produce highly consistent cell aggregates for biological research.

The utility of biomedical scaffolds laden with spheroids in various tissue engineering applications

A spheroid is a complex, spherical cellular aggregate supporting cell-cell and cell-matrix interactions in an environment that mimics the real-world situation. In terms of tissue engineering, spheroids are important building blocks that replace two-dimensional cell cultures. Spheroids replicate tissue physiological activities. The use of spheroids with/without scaffolds yields structures that engage in desired activities and replicate the complicated geometry of three-dimensional tissues. In this mini-review, we describe conventional and novel methods by which scaffold-free and scaffolded spheroids may be fabricated and discuss their applications in tissue regeneration and future perspectives.

Three-Dimensional Bioprinting of Articular Cartilage: A Systematic Review

OBJECTIVE

Treatment of chondral injury is clinically challenging. Available chondral repair/regeneration techniques have significant shortcomings. A viable and durable tissue engineering strategy for articular cartilage repair remains an unmet need. Our objective was to systematically evaluate the published data on bioprinted articular cartilage with regards to scaffold-based, scaffold-free and in situ cartilage bioprinting.

DESIGN

We performed a systematic review of studies using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. PubMed and ScienceDirect databases were searched and all articles evaluating the use of 3-dimensional (3D) bioprinting in articular cartilage were included. Inclusion criteria included studies written in or translated to English, published in a peer-reviewed journal, and specifically discussing bioinks and/or bioprinting of living cells related to articular cartilage applications. Review papers, articles in a foreign language, and studies not involving bioprinting of living cells related to articular cartilage applications were excluded.

RESULTS

Twenty-seven studies for articular cartilage bioprinting were identified that met inclusion and exclusion criteria. The technologies, materials, cell types used in these studies, and the biological and physical properties of the created constructs have been demonstrated.

CONCLUSION

These 27 studies have demonstrated 3D bioprinting of articular cartilage to be a tissue engineering strategy that has tremendous potential translational value. The unique abilities of the varied techniques allow replication of mechanical properties and advances toward zonal differentiation. This review demonstrates that bioprinting has great capacity for clinical cartilage reconstruction and future in vivo implantation.

Formation of Osteochondral Organoids from Murine Induced Pluripotent Stem Cells

Osteoarthritis is a debilitating joint disease that is characterized by pathologic changes in both cartilage and bone, potentially involving cross talk between these tissues that is complicated by extraneous factors that are difficult to study in vivo. To create a model system of these cartilage-bone interactions, we developed an osteochondral organoid from murine induced pluripotent stem cells (iPSCs). Using this approach, we grew organoids from a single cell type through time-dependent sequential exposure of growth factors, namely transforming growth factor β-3 and bone morphogenic protein 2, to mirror bone development through endochondral ossification. The result is a cartilaginous region and a calcified bony region comprising an organoid with the potential for joint disease drug screening and investigation of genetic risk in a patient or disease-specific manner. Furthermore, we also investigated the possibility of the differentiated cells within the organoid to revert to a pluripotent state. It was found that while the cells themselves maintain the capacity for reinduction of pluripotency, encapsulation in the newly formed 3D matrix prevents this process from occurring, which could have implications for future clinical use of iPSCs.

Tissue Engineering of Cartilage Using a Random Positioning Machine

Articular cartilage is a skeletal tissue of avascular nature and limited self-repair capacity. Cartilage-degenerative diseases, such as osteoarthritis (OA), are difficult to treat and often necessitate joint replacement surgery. Cartilage is a tough but flexible material and relatively easy to damage. It is, therefore, of high interest to develop methods allowing chondrocytes to recolonize, to rebuild the cartilage and to restore joint functionality. Here we studied the in vitro production of cartilage-like tissue using human articular chondrocytes exposed to the Random Positioning Machine (RPM), a device to simulate certain aspects of microgravity on Earth. To screen early adoption reactions of chondrocytes exposed to the RPM, we performed quantitative real-time PCR analyses after 24 h on chondrocytes cultured in DMEM/F-12. A significant up-regulation in the gene expression of IL6, RUNX2, RUNX3, SPP1, SOX6, SOX9, and MMP13 was detected, while the levels of IL8, ACAN, PRG4, ITGB1, TGFB1, COL1A1, COL2A1, COL10A1, SOD3, SOX5, MMP1, and MMP2 mRNAs remained unchanged. The STRING (Search Tool for the Retrieval of Interacting Genes/Proteins) analysis demonstrated among others the importance of these differentially regulated genes for cartilage formation. Chondrocytes grown in DMEM/F-12 medium produced three-dimensional (3D) spheroids after five days without the addition of scaffolds. On day 28, the produced tissue constructs reached up to 2 mm in diameter. Using specific chondrocyte growth medium, similar results were achieved within 14 days. Spheroids from both types of culture media showed the typical cartilage morphology with aggrecan positivity. Intermediate filaments form clusters under RPM conditions as detected by vimentin staining after 7 d and 14 d. Larger meshes appear in the network in 28-day samples. Furthermore, they were able to form a confluent chondrocyte monolayer after being transferred back into cell culture flasks in 1 g conditions showing their suitability for transplantation into joints. Our results demonstrate that the cultivation medium has a direct influence on the velocity of tissue formation and tissue composition. The spheroids show properties that make them interesting candidates for cellular cartilage regeneration approaches in trauma and OA therapy.

Three-Dimensional Biofabrication Models of Endometriosis and the Endometriotic Microenvironment

Endometriosis occurs when endometrial-like tissue grows outside the uterine cavity, leading to pelvic pain, infertility, and increased risk of ovarian cancer. The present study describes the optimization and characterization of cellular spheroids as building blocks for Kenzan scaffold-free method biofabrication and proof-of-concept models of endometriosis and the endometriotic microenvironment. The spheroid building blocks must be of a specific diameter (~500 μm), compact, round, and smooth to withstand Kenzan biofabrication. Under optimized spheroid conditions for biofabrication, the endometriotic epithelial-like cell line, 12Z, expressed high levels of estrogen-related genes and secreted high amounts of endometriotic inflammatory factors that were independent of TNFα stimulation. Heterotypic spheroids, composed of 12Z and T-HESC, an immortalized endometrial stromal cell line, self-assembled into a biologically relevant pattern, consisting of epithelial cells on the outside of the spheroids and stromal cells in the core. 12Z spheroids were biofabricated into large three-dimensional constructs alone, with HEYA8 spheroids, or as heterotypic spheroids with T-HESC. These three-dimensional biofabricated constructs containing multiple monotypic or heterotypic spheroids represent the first scaffold-free biofabricated in vitro models of endometriosis and the endometriotic microenvironment. These efficient and innovative models will allow us to study the complex interactions of multiple cell types within a biologically relevant microenvironment.

Development of a Method for Scaffold-Free Elastic Cartilage Creation

Microtia is a congenital aplasia of the auricular cartilage. Conventionally, autologous costal cartilage grafts are collected and shaped for transplantation. However, in this method, excessive invasion occurs due to limitations in the costal cartilage collection. Due to deformation over time after transplantation of the shaped graft, problems with long-term morphological maintenance exist. Additionally, the lack of elasticity with costal cartilage grafts is worth mentioning, as costal cartilage is a type of hyaline cartilage. Medical plastic materials have been transplanted as alternatives to costal cartilage, but transplant rejection and deformation over time are inevitable. It is imperative to create tissues for transplantation using cells of biological origin. Hence, cartilage tissues were developed using a biodegradable scaffold material. However, such materials suffer from transplant rejection and biodegradation, causing the transplanted cartilage tissue to deform due to a lack of elasticity. To address this problem, we established a method for creating elastic cartilage tissue for transplantation with autologous cells without using scaffold materials. Chondrocyte progenitor cells were collected from perichondrial tissue of the ear cartilage. By using a multilayer culture and a three-dimensional rotating suspension culture vessel system, we succeeded in creating scaffold-free elastic cartilage from cartilage progenitor cells.

The Effects of Engineered Skeletal Muscle on Volumetric Muscle Loss in The Tibialis Anterior Of Rat After Three Months In Vivo

Volumetric muscle loss (VML) is traumatic, degenerative, or surgical loss of skeletal muscle that exceeds the regenerative capacity of the remaining muscle, thus resulting in impaired muscle function. In humans, the loss of 30% or more mass of any one muscle will result in permanent structural and functional loss. Current VML repair treatments are limited by donor site morbidity and graft tissue availability, necessitating alternative muscle graft sources. To address this need, our lab has fabricated tissue-engineered skeletal muscle units (SMUs) for implantation into a 30 % VML model in the tibialis anterior (TA) muscle of rat. Previous results showed that after 28 days in vivo, muscle with a 30% VML repaired with our SMUs produced significantly more force than muscle with acute VML. But repair with our SMU did not fully restore muscle force production to that of native muscle. Thus, we hypothesized that more time for in vivo tissue regeneration would allow for greater force recovery. Therefore, the purpose of this study was to examine the long-term (3-month) effects of our SMUs on a 30% VML repair. We also assessed the effects of reinnervation by redirecting a branch of the peroneal nerve to the repair site. Thirty-nine, 2-month old female F344 rats were separated into a nonsurgical control group (n=5) and four surgical experimental groups (VML Only, n=5; VML+Nerve Redirect, n=6; VML+SMU, n=5; VML+SMU+ Nerve Redirect, n=8). Experimental rats were allowed a 3-month recovery period post-surgery before undergoing in situ force testing of the surgical (left) TA. The left TA of the control animals also underwent in situ force testing. Finally, the surgical (left) and contralateral (right) TAs of the experimental animals, as well as the left TA of the control animals, were explanted for histological analysis. Results for specific force showed that while all groups recovered specific forces similar to that of native muscle, the two SMU groups had significantly higher specific forces, on average, compared to the uninjured control group. Histological staining showed small muscle fibers in the repair site in animals that received an SMU. The average cross-sectional area of the native fibers just outside the area of repair (or the equivalent area in control animals) was not significantly different between groups, indicating that hypertrophy of remaining fibers did not contribute to the recovery of force following the VML. Our results suggest that following a 30% VML of the TA muscle, all surgical groups were able to recover TA mass, maximum tetanic and specific force production. Thus, creating a 30% VML in the TA in a rat model is not enough a sufficient VML to produce the sustained VML seen in humans following similar 30% loss of muscle volume.

3D Bioprinted Osteogenic Tissue Models for In Vitro Drug Screening

Metabolic bone disease affects hundreds of millions of people worldwide, and as a result, in vitro models of bone tissue have become essential tools to help analyze bone pathogenesis, develop drug screening, and test potential therapeutic strategies. Drugs that either promote or impair bone formation are in high demand for the treatment of metabolic bone diseases. These drugs work by targeting numerous signaling pathways responsible for regulating osteogenesis such as Hedgehog, Wnt/β-catenin, and PI3K-AKT. In this study, differentiated bone marrow-derived mesenchymal stem cell (BM-MSC) scaffold-free 3D bioprinted constructs and 2D monolayer cultures were utilized to screen four drugs predicted to either promote (Icariin and Purmorphamine) or impair osteogenesis (PD98059 and U0126). Osteogenic differentiation capacity was analyzed over a four week culture period by evaluating mineralization, alkaline phosphatase (ALP) activity, and osteogenesis related gene expression. Responses to drug treatment were observed in both 3D differentiated constructs and 2D monolayer cultures. After four weeks in culture, 3D differentiated constructs and 2D monolayer cultures treated with Icariin or Purmorphamine showed increased mineralization, ALP activity, and the gene expression of bone formation markers (BGLAP, SSP1, and COL1A1), signaling molecules (MAPK1, WNT1, and AKT1), and transcription factors (RUNX2 and GLI1) that regulate osteogenic differentiation relative to untreated. 3D differentiated constructs and 2D monolayer cultures treated with PD98059 or U0126 showed decreased mineralization, ALP activity, and the expression of the aforementioned genes BGLAP, SPP1, COL1A1, MAPK1, AKT1, RUNX2, and GLI1 relative to untreated. Differences in ALP activity and osteogenesis related gene expression relative to untreated cells cultured in a 2D monolayer were greater in 3D constructs compared to 2D monolayer cultures. These findings suggest that our bioprinted bone model system offers a more sensitive, biologically relevant drug screening platform than traditional 2D monolayer in vitro testing platforms.

Osteochondral Regeneration Using Adipose Tissue-Derived Mesenchymal Stem Cells

Osteoarthritis (OA) is a major joint disease that promotes locomotor deficiency during the middle- to old-age, with the associated disability potentially decreasing quality of life. Recently, surgical strategies to reconstruct both articular cartilage and subchondral bone for OA have been diligently investigated for restoring joint structure and function. Adipose tissue-derived mesenchymal stem cells (AT-MSCs), which maintain pluripotency and self-proliferation ability, have recently received attention as a useful tool to regenerate osteocartilage for OA. In this review, several studies were described related to AT-MSC spheroids, with scaffold and scaffold-free three-dimensional (3D) constructs produced using “mold” or “Kenzan” methods for osteochondral regeneration. First, several examples of articular cartilage regeneration using AT-MSCs were introduced. Second, studies of osteochondral regeneration (not only cartilage but also subchondral bone) using AT-MSCs were described. Third, examples were presented wherein spheroids were produced using AT-MSCs for cartilage regeneration. Fourth, osteochondral regeneration following autologous implantation of AT-MSC scaffold-free 3D constructs, fabricated using the “mold” or “Kenzan” method, was considered. Finally, prospects of osteochondral regeneration by scaffold-free 3D constructs using AT-MSC spheroids were discussed.

Repairing Volumetric Muscle Loss in the Ovine Peroneus Tertius Following a 3-Month Recovery

Much effort has been made to fabricate engineered tissues on a scale that is clinically relevant to humans; however, scale-up remains one of the most significant technological challenges of tissue engineering to date. To address this limitation, our laboratory has developed tissue-engineered skeletal muscle units (SMUs) and engineered neural conduits (ENCs), and modularly scaled them to clinically relevant sizes for the treatment of volumetric muscle loss (VML). The goal of this study was to evaluate the SMUs and ENCs in vitro, and to test the efficacy of our SMUs and ENCs in restoring muscle function in a clinically relevant large animal (sheep) model. The animals received a 30% VML injury to the peroneus tertius muscle and were allowed to recover for 3 months. The animals were divided into three experimental groups: VML injury without a repair (VML only), repair with an SMU (VML+SMU), or repair with an SMU and ENC (VML+SMU+ENC). We evaluated the SMUs before implantation and found that our single scaled-up SMUs were characterized by the presence of contracting myotubes, linearly aligned extracellular matrix proteins, and Pax7⁺ satellite cells. Three months after implantation, we found that the repair groups (VML+SMU and VML+SMU+ENC) had restored muscle mass and tetanic force production to a level that was statistically indistinguishable from the uninjured contralateral muscle after 3 months in vivo. Furthermore, we demonstrated the ability of our ENCs to effectively bridge the gap between native nerve and the repair site by eliciting a muscle contraction through direct electrical stimulation of the re-routed nerve. Impact statement The fabrication of tissues of clinically relevant sizes is one of the largest obstacles preventing engineered tissues from achieving widespread use in the clinic. This study aimed to combat this limitation by developing a fabrication method to scale-up tissue-engineered skeletal muscle for the treatment of volumetric muscle loss in a large animal (sheep) model and evaluating the efficacy of the tissue-engineered constructs after a 3-month recovery.

Scaffold- and serum-free hypertrophic cartilage tissue engineering as an alternative approach for bone repair

Human adipose stem/stromal cell (ASC) spheroids were used as a serum-free in vitro model to recapitulate the molecular events and extracellular matrix organization that orchestrate a hypertrophic cartilage phenotype. Induced-ASC spheroids (ø = 450 µm) showed high cell viability throughout the period of culture. The expression of collagen type X alpha 1 chain (COLXA1) and matrix metallopeptidase 13 (MMP-13) was upregulated at week 2 in induced-ASC spheroids compared with week 5 (P < .001) evaluated by quantitative real-time PCR. In accordance, secreted levels of IL-6 (P < .0001), IL-8 (P < .0001), IL-10 (P < .0001), bFGF (P < .001), VEGF (P < .0001), and RANTES (P < .0001) were the highest at week 2. Strong in situ staining for collagen type X and low staining for TSP-1 was associated with the increase of hypertrophic genes expression at week 2 in induced-ASC spheroids. Collagen type I, osteocalcin, biglycan, and tenascin C were detected at week 5 by in situ staining, in accordance with the highest expression of alkaline phosphatase (ALPL) gene and the presence of calcium deposits as evaluated by Alizarin Red O staining. Induced-ASC spheroids showed a higher force required to compression at week 2 (P < .0001). The human ASC spheroids under serum-free inducer medium and normoxic culture conditions were induced to a hypertrophic cartilage phenotype, opening a new perspective to recapitulate endochondral ossification in vivo.

3D Co-culture of hiPSC-Derived Cardiomyocytes With Cardiac Fibroblasts Improves Tissue-Like Features of Cardiac Spheroids

Purpose: Both cardiomyocytes and cardiac fibroblasts (CF) play essential roles in cardiac development, function, and remodeling. Properties of 3D co-cultures are incompletely understood. Hence, 3D co-culture of cardiomyocytes and CF was characterized, and selected features compared with single-type and 2D culture conditions. Methods: Human cardiomyocytes derived from induced-pluripotent stem cells (hiPSC-CMs) were obtained from Cellular Dynamics or Ncardia, and primary human cardiac fibroblasts from ScienCell. Cardiac spheroids were investigated using cryosections and whole-mount confocal microscopy, video motion analysis, scanning-, and transmission-electron microscopy (SEM, TEM), action potential recording, and quantitative PCR (qPCR). Results: Spheroids formed in hanging drops or in non-adhesive wells showed spontaneous contractions for at least 1 month with frequent media changes. SEM of mechanically opened spheroids revealed a dense inner structure and no signs of blebbing. TEM of co-culture spheroids at 1 month showed myofibrils, intercalated disc-like structures and mitochondria. Ultrastructural features were comparable to fetal human myocardium. We then assessed immunostained 2D cultures, cryosections of spheroids, and whole-mount preparations by confocal microscopy. CF in co-culture spheroids assumed a small size and shape similar to the situation in ventricular tissue. Spheroids made only of CF and cultured for 3 weeks showed no stress fibers and strongly reduced amounts of alpha smooth muscle actin compared to early spheroids and 2D cultures as shown by confocal microscopy, western blotting, and qPCR. The addition of CF to cardiac spheroids did not lead to arrhythmogenic effects as measured by sharp-electrode electrophysiology. Video motion analysis showed a faster spontaneous contraction rate in co-culture spheroids compared to pure hiPSC-CMs, but similar contraction amplitudes and kinetics. Spontaneous contraction rates were not dependent on spheroid size. Applying increasing pacing frequencies resulted in decreasing contraction amplitudes without positive staircase effect. Gene expression analysis of selected cytoskeleton and myofibrillar proteins showed more tissue-like expression patterns in co-culture spheroids than with cardiomyocytes alone or in 2D culture. Conclusion: We demonstrate that the use of 3D co-culture of hiPSC-CMs and CF is superior over 2D culture conditions for co-culture models and more closely mimicking the native state of the myocardium with relevance to drug development as well as for personalized medicine.

Computational fluid dynamic analysis of bioprinted self-supporting perfused tissue models

Natural tissues are incorporated with vasculature, which is further integrated with a cardiovascular system responsible for driving perfusion of nutrient‐rich oxygenated blood through the vasculature to support cell metabolism within most cell‐dense tissues. Since scaffold‐free biofabricated tissues being developed into clinical implants, research models, and pharmaceutical testing platforms should similarly exhibit perfused tissue‐like structures, we generated a generalizable biofabrication method resulting in self‐supporting perfused (SSuPer) tissue constructs incorporated with perfusible microchannels and integrated with the modular FABRICA perfusion bioreactor. As proof of concept, we perfused an MLO‐A5 osteoblast‐based SSuPer tissue in the FABRICA. Although our resulting SSuPer tissue replicated vascularization and perfusion observed in situ, supported its own weight, and stained positively for mineral using Von Kossa staining, our in vitro results indicated that computational fluid dynamics (CFD) should be used to drive future construct design and flow application before further tissue biofabrication and perfusion. We built a CFD model of the SSuPer tissue integrated in the FABRICA and analyzed flow characteristics (net force, pressure distribution, shear stress, and oxygen distribution) through five SSuPer tissue microchannel patterns in two flow directions and at increasing flow rates. Important flow parameters include flow direction, fully developed flow, and tissue microchannel diameters matched and aligned with bioreactor flow channels. We observed that the SSuPer tissue platform is capable of providing direct perfusion to tissue constructs and proper culture conditions (oxygenation, with controllable shear and flow rates), indicating that our approach can be used to biofabricate tissue representing primary tissues and that we can model the system in silico.

Clumps of Mesenchymal Stem Cell/Extracellular Matrix Complexes Generated with Xeno-Free Conditions Facilitate Bone Regeneration via Direct and Indirect Osteogenesis

Three-dimensional clumps of mesenchymal stem cell (MSC)/extracellular matrix (ECM) complexes (C-MSCs) consist of cells and self-produced ECM. We demonstrated previously that C-MSCs can be transplanted into bone defect regions with no artificial scaffold to induce bone regeneration. To apply C-MSCs in a clinical setting as a reliable bone regenerative therapy, the present study aimed to generate C-MSCs in xeno-free/serum-free conditions that can exert successful bone regenerative properties and to monitor interactions between grafted cells and host cells during bone healing processes. Human bone marrow-derived MSCs were cultured in xeno-free/serum-free medium. To obtain C-MSCs, confluent cells that had formed on the cellular sheet were scratched using a micropipette tip and then torn off. The sheet was rolled to make a round clump of cells. Then, C-MSCs were transplanted into an immunodeficient mouse calvarial defect model. Transplantation of C-MSCs induced bone regeneration in a time-dependent manner. Immunofluorescence staining showed that both donor human cells and host mice cells contributed to bone reconstruction. Decellularized C-MSCs implantation failed to induce bone regeneration, even though the host mice cells can infiltrate into the defect area. These findings suggested that C-MSCs generated in xeno-free/serum-free conditions can induce bone regeneration via direct and indirect osteogenesis.

The Efficacy of a Scaffold-free Bio 3D Conduit Developed from Autologous Dermal Fibroblasts on Peripheral Nerve Regeneration in a Canine Ulnar Nerve Injury Model: A Preclinical Proof-of-Concept Study

Autologous nerve grafting is widely accepted as the gold standard treatment for segmental nerve defects. To overcome the inevitable disadvantages of the original method, alternative methods such as the tubulization technique have been developed. Several studies have investigated the characteristics of an ideal nerve conduit in terms of supportive cells, scaffolds, growth factors, and vascularity. Previously, we confirmed that biological scaffold-free conduits fabricated from human dermal fibroblasts promote nerve regeneration in a rat sciatic nerve injury model. The purpose of this study is to evaluate the feasibility of biological scaffold-free conduits composed of autologous dermal fibroblasts using a large-animal model. Six male beagle dogs were used in this study. Eight weeks before surgery, dermal fibroblasts were harvested from their groin skin and grown in culture. Bio 3D conduits were assembled from proliferating dermal fibroblasts using a Bio 3D printer. The ulnar nerve in each dog’s forelimb was exposed under general anesthesia and sharply cut to create a 5 mm interstump gap, which was bridged by the prepared 8 mm Bio 3D conduit. Ten weeks after surgery, nerve regeneration was investigated. Electrophysiological studies detected compound muscle action potentials (CMAPs) of the hypothenar muscles and motor nerve conduction velocity (MNCV) in all animals. Macroscopic observation showed regenerated ulnar nerves. Low-level hypothenar muscle atrophy was confirmed. Immunohistochemical, histological, and morphometric studies confirmed the existence of many myelinated axons through the Bio 3D conduit. No severe adverse event was reported. Hypothenar muscles were re-innervated by regenerated nerve fibers through the Bio 3D conduit. The scaffold-free Bio 3D conduit fabricated from autologous dermal fibroblasts is effective for nerve regeneration in a canine ulnar nerve injury model. This technology was feasible as a treatment for peripheral nerve injury and segmental nerve defects in a preclinical setting.

Osteochondral regeneration using constructs of mesenchymal stem cells made by bio three-dimensional printing in mini-pigs

Osteoarthritis is a major joint disease that has been extensively investigated in humans and in model animals. In this study, we examined the regeneration of articular cartilage and subchondral bone using artificial scaffold-free constructs composed of adipose tissue-derived mesenchymal stem cells (AT-MSCs) created using bio three-dimensional (3D) printing with a needle-array. Printed constructs were implanted into osteochondral defects created in the right femoral trochlear groove of six mini-pigs, using femoral defects created in the left femurs as controls. Repair within the defects was evaluated at 3 and 6 months post-implantation using computed tomography (CT) and magnetic resonance (MR) imaging. The radiolucent volume (RV, mm³ ) in the defects was calculated using multi-planar reconstruction of CT images. MR images were evaluated based on a modified 2D- MOCART (magnetic resonance observation of cartilage repair tissue) grading system. Gross and microscopic pathology were scored according to the ICRS (International Cartilage Repair Society) scale at 6 months after implantation. The percentage RV at 3 months postoperation was significantly lower in the implanted defects than in the controls, whereas total scores based on the MOCART system were significantly higher in the implanted defects as compared with the controls. Although there were no statistical differences in the gross scores, the average histological scores were significantly higher in the implanted defects than in the controls. To our knowledge, this is the first report to suggest that artificial scaffold-free 3D-printed constructs of autologous AT-MSCs can be aid in the osteochondral regeneration in pigs. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1398-1408, 2019.

Replacement of Rat Tracheas by Layered, Trachea-Like, Scaffold-Free Structures of Human Cells Using a Bio-3D Printing System

Current scaffold-based tissue engineering approaches are subject to several limitations, such as design inflexibility, poor cytocompatibility, toxicity, and post-transplant degradation. Thus, scaffold-free tissue-engineered structures can be a promising solution to overcome the issues associated with classical scaffold-based materials in clinical transplantation. The present study seeks to optimize the culture conditions and cell combinations used to generate scaffold-free structures using a Bio-3D printing system. Human cartilage cells, human fibroblasts, human umbilical vein endothelial cells, and human mesenchymal stem cells from bone marrow are aggregated into spheroids and placed into a Bio-3D printing system with dedicated needles positioned according to 3D configuration data, to develop scaffold-free trachea-like tubes. Culturing the Bio-3D-printed structures with proper flow of specific medium in a bioreactor facilitates the rearrangement and self-organization of cells, improving physical strength and tissue function. The Bio-3D-printed tissue forms small-diameter trachea-like tubes that are implanted into rats with the support of catheters. It is confirmed that the tubes are viable in vivo and that the tracheal epithelium and capillaries proliferate. This tissue-engineered, scaffold-free, tubular structure can represent a significant step toward clinical application of bioengineered organs.

Recent Advances in Engineered Stem Cell-Derived Cell Sheets for Tissue Regeneration

The substantial progress made in the field of stem cell-based therapy has shown its significant potential applications for the regeneration of defective tissues and organs. Although previous studies have yielded promising results, several limitations remain and should be overcome for translating stem cell-based therapies to clinics. As a possible solution to current bottlenecks, cell sheet engineering (CSE) is an efficient scaffold-free method for harvesting intact cell sheets without the use of proteolytic enzymes, and may be able to accelerate the adoption of stem cell-based treatments for damaged tissues and organs regeneration. CSE uses a temperature-responsive polymer-immobilized surface to form unique, scaffold-free cell sheets composed of one or more cell layers maintained with important intercellular junctions, cell-secreted extracellular matrices, and other important cell surface proteins, which can be achieved by changing the surrounding temperature. These three-dimensional cell sheet-based tissues can be designed for use in clinical applications to target-specific tissue regeneration. This review will highlight the principles, progress, and clinical relevance of current approaches in the cell sheet-based technology, focusing on stem cell-based therapies for bone, periodontal, skin, and vascularized muscles.

Contraction Dynamics of Rod Microtissues of Gingiva-Derived and Periodontal Ligament-Derived Cells

Tissue engineering strategies using microtissues as "building blocks" have high potential in regenerative medicine. Cognition of contraction dynamics involved in the in vitro self-assembly of these microtissues can be conceived as the bedrock of an effective periodontal tissue regenerative therapy. Our study was directed at evaluating the shrinkage in the rod-shaped structure of a directed self-assembly of human gingiva-derived cells (GC) and periodontal ligament-derived cells (PDLC) and developing insights into the potential mechanisms responsible for the shrinkage. GC and PDLC were seeded in non-adherent agarose molds to form rod microtissues. Cells used for the experiments were characterized using fluorescence-activated cell sorting (FACS). To assess the viability, resazurin-based cytotoxicity assays, trypan blue dye exclusion assay, MTT and live/dead staining, and histological evaluation of rods based on hematoxylin and eosin staining were performed. Rod contraction was evaluated and measured at 0, 2, 6, and 24 h and compared to L-929 cells. The role of transforming growth factor (TGF)-β signaling, phosphoinositide 3-kinase (PI3K)/AKT, and mitogen activated protein kinase (MAPK) signaling was analyzed. Our results show that the rod microtissues were vital after 24 h. A reduction in the length of rods was seen in the 24 h period. While the recombinant TGF-β slightly reduced contraction, inhibition of TGF-β signaling did not interfere with the contraction of the rods. Interestingly, inhibition of phosphoinositide 3-kinase by LY294002 significantly delayed contraction in GC and PDLC rods. Overall, GC and PDLC have the ability to form rod microtissues which contract over time. Thus, approaches for application of these structures as "building blocks" for periodontal tissue regeneration should consider that rods have the capacity to contract substantially. Further investigation will be needed to unravel the mechanisms behind the dynamics of contraction.

The Maturation of Tissue-Engineered Skeletal Muscle Units following 28-Day Ectopic Implantation in a Rat

Volumetric muscle loss (VML) is a loss of skeletal muscle that results in a sustained impairment of function and is often accompanied by physical deformity. To address the need for more innovative repair options, our laboratory has developed scaffold-free, multiphasic tissue-engineered skeletal muscle units (SMUs) to treat VML injuries. In our previous work, using the concept of the "body as a bioreactor", we have shown that implantation promotes the maturation of our SMUs beyond what is possible in vitro. Thus, in this study we sought to better understand the effect of implantation on the maturation of our SMUs, including the effects of implantation on SMU force production and cellular remodeling. We used an ectopic implantation so that we could more easily dissect the implanted tissues post-recovery and measure the force contribution of the SMU alone and compare it to pre-implantation values. This study also aimed to scale up the size of our SMUs to enable the replacement of larger volumes of muscle in our future VML studies. Overall, implantation resulted in extensive maturation of the SMUs, as characterized by an increase in force production, substantial integration with native tissue, innervation, vascularization, and the development of structural organization similar to native tissue.

Dental Pulp Stem Cell-Derived, Scaffold-Free Constructs for Bone Regeneration

In the present study, a scaffold-free tissue construct was developed as an approach for the regeneration of tissue defects, which produced good outcomes. We fabricated a scaffold-free tissue construct from human dental pulp stem cells (hDPSCs construct), and examined the characteristics of the construct. For its fabrication, basal sheets prepared by 4-week hDPSCs culturing were subjected to 1-week three-dimensional culture, with or without osteogenic induction, whereas hDPSC sheets (control) were fabricated by 1-week culturing of basal sheets on monolayer culture. The hDPSC constructs formed a spherical structure and calcified matrix that are absent in the control. The expression levels for bone-related genes in the hDPSC constructs were significantly upregulated compared with those in the control. Moreover, the hDPSC constructs with osteogenic induction had a higher degree of calcified matrix formation, and higher expression levels for bone-related genes, than those for the hDPSC constructs without osteogenic induction. These results suggest that the hDPSC constructs with osteogenic induction are composed of cells and extracellular and calcified matrices, and that they can be a possible scaffold-free material for bone regeneration.

Dynamic Cultivation of Mesenchymal Stem Cell Aggregates

Mesenchymal stem cells (MSCs) are considered as primary candidates for cell-based therapies due to their multiple effects in regenerative medicine. Pre-conditioning of MSCs under physiological conditions—such as hypoxia, three-dimensional environments, and dynamic cultivation—prior to transplantation proved to optimize their therapeutic efficiency. When cultivated as three-dimensional aggregates or spheroids, MSCs display increased angiogenic, anti-inflammatory, and immunomodulatory effects as well as improved stemness and survival rates after transplantation, and cultivation under dynamic conditions can increase their viability, proliferation, and paracrine effects, alike. Only few studies reported to date, however, have utilized dynamic conditions for three-dimensional aggregate cultivation of MSCs. Still, the integration of dynamic bioreactor systems, such as spinner flasks or stirred tank reactors might pave the way for a robust, scalable bulk expansion of MSC aggregates or MSC-derived extracellular vesicles. This review summarizes recent insights into the therapeutic potential of MSC aggregate cultivation and focuses on dynamic generation and cultivation techniques of MSC aggregates.

Tissue-engineered tendon constructs for rotator cuff repair in sheep

Current rotator cuff repair commonly involves the use of single or double row suture techniques, and despite successful outcomes, failure rates continue to range from 20 to 95%. Failure to regenerate native biomechanical properties at the enthesis is thought to contribute to failure rates. Thus, the need for technologies that improve structural healing of the enthesis after rotator cuff repair is imperative. To address this issue, our lab has previously demonstrated enthesis regeneration using a tissue-engineered graft approach in a sheep anterior cruciate ligament (ACL) repair model. We hypothesized that our tissue-engineered graft designed for ACL repair also will be effective in rotator cuff repair. The goal of this study was to test the efficacy of our Engineered Tissue Graft for Rotator Cuff (ETG-RC) in a rotator cuff tear model in sheep and compare this novel graft technology to the commonly used double row suture repair technique. Following a 6-month recovery, the grafted and contralateral shoulders were removed, imaged using X-ray, and tested biomechanically. Additionally, the infraspinatus muscle, myotendinous junction, enthesis, and humeral head were preserved for histological analysis of muscle, tendon, and enthesis structure. Our results showed that our ETC-RCs reached 31% of the native tendon tangent modulus, which was a modest, non-significant, 11% increase over that of the suture-only repairs. However, the histological analysis showed the regeneration of a native-like enthesis in the ETG-RC-repaired animals. This advanced structural healing may improve over longer times and may diminish recurrence rates of rotator cuff tears and lead to better clinical outcomes. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:289-299, 2018.

Featured Article: In vitro development of personalized cartilage microtissues uncovers an individualized differentiation capacity of human chondrocytes

Personalized features in the treatment of knee injuries and articular replacement therapies play an important role in modern life with increasing demand. Therefore, cell-based therapeutic approaches for the regeneration of traumatic defects of cartilage tissue were developed. However, great variations in the quality of repair tissue or therapeutic outcome were observed. The aim of the study was to capture and visualize individual differentiation capacities of chondrocytes derived from different donors with regard to a possible personal regeneration capacity using a cell-based therapy. The redifferentiation potential of monolayer cultured cells was analyzed in a scaffold-free three-dimensional tissue model. Furthermore, stimulating options using cartilage maturation factors such as L-ascorbic acid and transforming growth factor beta 2 (TGF-β₂) on this process were of special interest. Cells and tissues were analyzed via histological and immunohistochemical methods. Gene expression was measured by quantitative real-time polymerase chain reaction. In monolayer culture, cells from all donors showed an almost identical differentiation profile. In contrast, the differentiation state of cartilage-like three-dimensional microtissues revealed clear differences with respect to individual donors. Analyses at the protein and mRNA levels showed high variations regarding cartilage-typical matrix components (e.g. proteoglycans, collagen type II) and intracellular proteins (e.g. S100). Interestingly, only donor chondrocytes with a basic tendency to re-differentiate in a three-dimensional environment were able to increase this tissue-specific maturation when exposed to L-ascorbic acid and/or TGF-β2. Our approach revealed clear-cut possibilities for classification of individual donors into responders or non-responders. On the basis of these results an in vitro platform could be designed to discriminate responders from non-responders. This in vitro three-dimensional test system may be a suitable basis to establish a "personalized diagnostic tool" with the opportunity to assess the capacity of expanded chondrocytes to respond to an autologous cell-based therapy. Impact statement A challenge in cell-based cartilage regeneration therapies is the identification of a "personalized diagnostic tool" to predict the chondrogenic potency of cells from patients who are going to be treated with autologous cells. Comparing the phenotype of isolated chondrocytes from different donors in vitro revealed an individual cartilage-specific differentiation capacity. These personalized features are not detectable in vitro until the monolayer cells have the possibility to rearrange in 3D tissues. Cells from articular cartilage in monolayer culture may not be a suitable basis to discriminate responders from non-responders with respect to a personalized cell-based therapy to treat cartilage defects. A more physiological 3D (micro-)environment enable the cells to present their individual differentiation capacity. The here described microtissue model might be the basis for an in vitro platform to predict the therapeutic outcome of autologous cell-based cartilage repair and/or a suitable tool to identify early biomarkers to classify the patients.

Physioxia Promotes the Articular Chondrocyte-Like Phenotype in Human Chondroprogenitor-Derived Self-Organized Tissue

INTRODUCTION

Biomaterial-based tissue engineering has not successfully reproduced the structural architecture or functional mechanical properties of native articular cartilage. In scaffold-free tissue engineering systems, cells secrete and organize the entire extracellular matrix over time in response to environmental signals such as oxygen level. In this study, we investigated the effect of oxygen on the formation of neocartilage from human-derived chondrogenic cells.

MATERIALS AND METHODS

Articular chondrocytes (ACs) and articular cartilage progenitor cells (ACPs) derived from healthy human adults were guided toward cell condensation by centrifugation onto plate inserts that were uncoated or coated with either agarose or fibronectin. Neocartilage discs were cultured at hyperoxic (20%) or physioxic (5%) oxygen levels, and biochemical, biomechanical, and molecular analyses were used to compare the cartilage produced by ACs versus ACPs.

RESULTS

Fibronectin-coated inserts proved optimal for growing cartilaginous discs from both cell types. In comparison with culture in hyperoxia, AC neocartilage cultured at physioxia exhibited a significant increase in chondrogenic gene expression, proteoglycan production, and mechanical properties with a concomitant decrease in collagen content. At both oxygen levels, ACP-derived neocartilage produced tissue with significantly enhanced mechanical properties and collagen content relative to AC-derived neocartilage. Both ACs and ACPs produced substantial collagen II and reduced levels of collagens I and X in physioxia relative to hyperoxia. Neocartilage from ACPs exhibited anisotropic organization characteristic of native cartilage with respect to collagen VI of the pericellular matrix when compared with AC-derived neocartilage; however, only ACs produced abundant surface-localized lubricin.

DISCUSSION AND CONCLUSIONS

Guiding human-derived cells toward condensation and subsequent culture in physioxia promoted the articular cartilage tissue phenotype for ACs and ACPs. Unlike ACs, ACPs are clonable and highly expandable while retaining chondrogenicity. The ability to generate large tissues utilizing a scaffold-free approach from a single autologous progenitor cell may represent a promising source of neocartilage destined for cartilage repair.

Scaffold-free tissue engineering of functional corneal stromal tissue

Blinding corneal scarring is predominately treated with allogeneic graft tissue; however, there is a worldwide shortage of donor tissue leaving millions in need of therapy. Human corneal stromal stem cells (CSSC) have been shown produce corneal tissue when cultured on nanofibre scaffolding, but this tissue cannot be readily separated from the scaffold. In this study, scaffold-free tissue engineering methods were used to generate biomimetic corneal stromal tissue constructs that can be transplanted in vivo without introducing the additional variables associated with exogenous scaffolding. CSSC were cultured on substrates with aligned microgrooves, which directed parallel cell alignment and matrix organization, similar to the organization of native corneal stromal lamella. CSSC produced sufficient matrix to allow manual separation of a tissue sheet from the grooved substrate. These constructs were cellular and collagenous tissue sheets, approximately 4 μm thick and contained extracellular matrix molecules typical of corneal tissue including collagen types I and V and keratocan. Similar to the native corneal stroma, the engineered corneal tissues contained long parallel collagen fibrils with uniform diameter. After being transplanted into mouse corneal stromal pockets, the engineered corneal stromal tissues became transparent, and the human CSSCs continued to express human corneal stromal matrix molecules. Both in vitro and in vivo, these scaffold-free engineered constructs emulated stromal lamellae of native corneal stromal tissues. Scaffold-free engineered corneal stromal constructs represent a novel, potentially autologous, cell-generated, biomaterial with the potential for treating corneal blindness. Copyright © 2016 John Wiley & Sons, Ltd.

Hypoxic Three-Dimensional Scaffold-Free Aggregate Cultivation of Mesenchymal Stem Cells in a Stirred Tank Reactor

Extensive expansion of mesenchymal stem cells (MSCs) for cell-based therapies remains challenging since long-term cultivation and excessive passaging in two-dimensional conditions result in a loss of essential stem cell properties. Indeed, low survival rate of cells, alteration of surface marker profiles, and reduced differentiation capacity are observed after in vitro expansion and reduce therapeutic success in clinical studies. Remarkably, cultivation of MSCs in three-dimensional aggregates preserve stem cell properties. Hence, the large scale formation and cultivation of MSC aggregates is highly desirable. Besides other effects, MSCs cultivated under hypoxic conditions are known to display increased proliferation and genetic stability. Therefore, in this study we demonstrate cultivation of adipose derived human MSC aggregates in a stirred tank reactor under hypoxic conditions. Although aggregates were exposed to comparatively high average shear stress of 0.2 Pa as estimated by computational fluid dynamics, MSCs displayed a viability of 78-86% and maintained their surface marker profile and differentiation potential after cultivation. We postulate that cultivation of 3D MSC aggregates in stirred tank reactors is valuable for large-scale production of MSCs or their secreted compounds after further optimization of cultivation parameters.