On human pluripotent stem cell control: The rise of 3D bioengineering and mechanobiology

Current multi-scale biomaterials for tissue regeneration following spinal cord injury

Spinal cord injury (SCI) may cause loss of motor and sensory function, autonomic dysfunction, and thus disrupt the quality of life of patients, leading to severe disability and significant psychological, social, and economic burden. At present, existing therapy for SCI have limited ability to promote neural function recovery, and there is an urgent need to develop innovative regenerative approaches to repair SCI. Biomaterials have become a promising strategy to promote the regeneration and repair of damaged nerve tissue after SCI. Biomaterials can provide support for nerve tissue by filling cavities, and improve local inflammatory responses and reshape extracellular matrix structures through unique biochemical properties to create the optimal microenvironment at the SCI site, thereby promoting neurogenesis and reconnecting damaged spinal cord tissue. Considering the importance of biomaterials in repairing SCI, this article reviews the latest progress of multi-scale biomaterials in SCI treatment and tissue regeneration, and evaluates the relevant technologies for manufacturing biomaterials.

Spatial cell fate manipulation of human pluripotent stem cells by controlling the microenvironment using photocurable hydrogel

Human pluripotent stem cells (hPSCs) dynamically respond to their chemical and physical microenvironment, dictating their behavior. However, conventional in vitro studies predominantly employ plastic culture wares, which offer a simplified representation of the in vivo microenvironment. Emerging evidence underscores the pivotal role of mechanical and topological cues in hPSC differentiation and maintenance. In this study, we cultured hPSCs on hydrogel substrates with spatially controlled stiffness. The use of culture substrates that enable precise manipulation of spatial mechanical properties holds promise for better mimicking in vivo conditions and advancing tissue engineering techniques. We designed a photocurable polyethylene glycol-polyvinyl alcohol (PVA-PEG) hydrogel, allowing the spatial control of surface stiffness and geometry at a micrometer scale. This versatile hydrogel can be functionalized with various extracellular matrix proteins. Laminin 511-functionalized PVA-PEG gel effectively supports the growth and differentiation of hPSCs. Moreover, by spatially modulating the stiffness of the patterned gel, we achieved spatially selective cell differentiation, resulting in the generation of intricate patterned structures.

Current advances in biomaterials for inner ear cell regeneration

Inner ear cell regeneration from stem/progenitor cells provides potential therapeutic strategies for the restoration of sensorineural hearing loss (SNHL), however, the efficiency of regeneration is low and the functions of differentiated cells are not yet mature. Biomaterials have been used in inner ear cell regeneration to construct a more physiologically relevant 3D culture system which mimics the stem cell microenvironment and facilitates cellular interactions. Currently, these biomaterials include hydrogel, conductive materials, magneto-responsive materials, photo-responsive materials, etc. We analyzed the characteristics and described the advantages and limitations of these materials. Furthermore, we reviewed the mechanisms by which biomaterials with different physicochemical properties act on the inner ear cell regeneration and depicted the current status of the material selection based on their characteristics to achieve the reconstruction of the auditory circuits. The application of biomaterials in inner ear cell regeneration offers promising opportunities for the reconstruction of the auditory circuits and the restoration of hearing, yet biomaterials should be strategically explored and combined according to the obstacles to be solved in the inner ear cell regeneration research.

Cell-extracellular matrix mechanotransduction in 3D

Mechanical properties of extracellular matrices (ECMs) regulate essential cell behaviours, including differentiation, migration and proliferation, through mechanotransduction. Studies of cell-ECM mechanotransduction have largely focused on cells cultured in 2D, on top of elastic substrates with a range of stiffnesses. However, cells often interact with ECMs in vivo in a 3D context, and cell-ECM interactions and mechanisms of mechanotransduction in 3D can differ from those in 2D. The ECM exhibits various structural features as well as complex mechanical properties. In 3D, mechanical confinement by the surrounding ECM restricts changes in cell volume and cell shape but allows cells to generate force on the matrix by extending protrusions and regulating cell volume as well as through actomyosin-based contractility. Furthermore, cell-matrix interactions are dynamic owing to matrix remodelling. Accordingly, ECM stiffness, viscoelasticity and degradability often play a critical role in regulating cell behaviours in 3D. Mechanisms of 3D mechanotransduction include traditional integrin-mediated pathways that sense mechanical properties and more recently described mechanosensitive ion channel-mediated pathways that sense 3D confinement, with both converging on the nucleus for downstream control of transcription and phenotype. Mechanotransduction is involved in tissues from development to cancer and is being increasingly harnessed towards mechanotherapy. Here we discuss recent progress in our understanding of cell-ECM mechanotransduction in 3D.

The Synergistic Effect of Cyclic Tensile Force and Periodontal Ligament Cell-Laden Calcium Silicate/Gelatin Methacrylate Auxetic Hydrogel Scaffolds for Bone Regeneration

The development of 3D printing technologies has allowed us to fabricate complex novel scaffolds for bone regeneration. In this study, we reported the incorporation of different concentrations of calcium silicate (CS) powder into fish gelatin methacrylate (FGelMa) for the fabrication of CS/FGelMa auxetic bio-scaffolds using 3D printing technology. Our results showed that CS could be successfully incorporated into FGelMa without influencing the original structural components of FGelMa. Furthermore, it conveyed that CS modifications both the mechanical properties and degradation rates of the scaffolds were improved in accordance with the concentrations of CS upon modifications of CS. In addition, the presence of CS enhanced the adhesion and proliferation of human periodontal ligament cells (hPDLs) cultured in the scaffold. Further osteogenic evaluation also confirmed that CS was able to enhance the osteogenic capabilities via activation of downstream intracellular factors such as pFAK/FAK and pERK/ERK. More interestingly, it was noted that the application of extrinsic biomechanical stimulation to the auxetic scaffolds further enhanced the proliferation and differentiation of hPDLs cells and secretion of osteogenic-related markers when compared to CS/FGelMa hydrogels without tensile stimulation. This prompted us to explore the related mechanism behind this interesting phenomenon. Subsequent studies showed that biomechanical stimulation works via YAP, which is a biomechanical cue. Taken together, our results showed that novel auxetic scaffolds could be fabricated by combining different aspects of science and technology, in order to improve the future chances of clinical applications for bone regeneration.

Effects of Biomimetic Micropatterned Surfaces on the Adhesion and Morphology of Cervical Cancer Cells

It has been demonstrated that micropatterned surfaces have an important influence on modulating cellular behavior. In recent years, with the rapid development of microfabrication techniques and in-depth study of nature, an increasing number of patterned structures imitating natural organisms have been successfully fabricated and widely evaluated. However, there are only a few reports about biomimetic patterned microstructures in biologically related fields. In our work, micropatterned polydimethylsiloxane (PDMS) was fabricated by mimicking the surface microstructures of natural Trifolium and Parthenocissus tricuspidata leaves using the template duplication method. The interactions between the two types of biomimetic micro-PDMS surfaces and two kinds of human cervical cancer cells (HeLa and SiHa) were investigated. HeLa and SiHa cells cultured on the two micropatterned PDMS samples exhibited more stretchable morphology, higher diffusion, and a much lower nuclear/cytoplasmic ratio than those cultured on flat PDMS surfaces, indicating a higher adhesion area of the cells. Both of the micro-PDMS substrates were found to induce significantly different morphological changes between HeLa and SiHa cells. This suggests that the micropatterned structure affects cell adhesion and morphology correlated with their surface geometric structure and roughness. The results reveal that biomimetic micropatterned surfaces from natural leaves significantly regulate the morphology and adhesion behavior of cervical cancer cells and are believed to be the new platforms for investigating the interaction between cells and substrates.

Engineering multiscale structural orders for high-fidelity embryoids and organoids

Embryoids and organoids hold great promise for human biology and medicine. Herein, we discuss conceptual and technological frameworks useful for developing high-fidelity embryoids and organoids that display tissue- and organ-level phenotypes and functions, which are critically needed for decoding developmental programs and improving translational applications. Through dissecting the layers of inputs controlling mammalian embryogenesis, we review recent progress in reconstructing multiscale structural orders in embryoids and organoids. Bioengineering tools useful for multiscale, multimodal structural engineering of tissue- and organ-level cellular organization and microenvironment are also discussed to present integrative, bioengineering-directed approaches to achieve next-generation, high-fidelity embryoids and organoids.

Blood-Spinal Cord Barrier: Its Role in Spinal Disorders and Emerging Therapeutic Strategies

The blood-spinal cord barrier (BSCB) has been long thought of as a functional equivalent to the blood-brain barrier (BBB), restricting blood flow into the spinal cord. The spinal cord is supported by various disc tissues that provide agility and has different local immune responses compared to the brain. Though physiologically, structural components of the BSCB and BBB share many similarities, the clinical landscape significantly differs. Thus, it is crucial to understand the composition of BSCB and also to establish the cause–effect relationship with aberrations and spinal cord dysfunctions. Here, we provide a descriptive analysis of the anatomy, current techniques to assess the impairment of BSCB, associated risk factors and impact of spinal disorders such as spinal cord injury (SCI), amyotrophic lateral sclerosis (ALS), peripheral nerve injury (PNI), ischemia reperfusion injury (IRI), degenerative cervical myelopathy (DCM), multiple sclerosis (MS), spinal cavernous malformations (SCM) and cancer on BSCB dysfunction. Along with diagnostic and mechanistic analyses, we also provide an up-to-date account of available therapeutic options for BSCB repair. We emphasize the need to address BSCB as an individual entity and direct future research towards it.

Micro/nanoengineered technologies for human pluripotent stem cells maintenance and differentiation

Human pluripotent stem cells (hPSCs) are a promising source of cells for cell replacement-based therapies as well as modeling human development and diseases in vitro. However, achieving fate control of hPSC with a high yield and specificity remains challenging. The fate specification of hPSCs is regulated by biochemical and biomechanical cues in their environment. Driven by this knowledge, recent exciting advances in micro/nanoengineering have been leveraged to develop a broad range of tools for the generation of extracellular biomechanical and biochemical signals that determine the behavior of hPSCs. In this review, we summarize such micro/nanoengineered technologies for controlling hPSC fate and highlight the role of biochemical and biomechanical cues such as substrate rigidity, surface topography, and cellular confinement in the hPSC-based technologies that are on the horizon.

Viscoelasticity and Adhesion Signaling in Biomaterials Control Human Pluripotent Stem Cell Morphogenesis in 3D Culture

Organoids are lumen-containing multicellular structures that recapitulate key features of the organs, and are increasingly used in models of disease, drug testing, and regenerative medicine. Recent work has used 3D culture models to form organoids from human induced pluripotent stem cells (hiPSCs) in reconstituted basement membrane (rBM) matrices. However, rBM matrices offer little control over the microenvironment. More generally, the role of matrix viscoelasticity in directing lumen formation remains unknown. Here, viscoelastic alginate hydrogels with independently tunable stress relaxation (viscoelasticity), stiffness, and arginine-glycine-aspartate (RGD) ligand density are used to study hiPSC morphogenesis in 3D culture. A phase diagram that shows how these properties control hiPSC morphogenesis is reported. Higher RGD density and fast stress relaxation promote hiPSC viability, proliferation, apicobasal polarization, and lumen formation, while slow stress relaxation at low RGD densities triggers hiPSC apoptosis. Notably, hiPSCs maintain pluripotency in alginate hydrogels for much longer times than is reported in rBM matrices. Lumen formation is regulated by actomyosin contractility and is accompanied by translocation of Yes-associated protein (YAP) from the nucleus to the cytoplasm. The results reveal matrix viscoelasticity as a potent factor regulating stem cell morphogenesis and provide new insights into how engineered biomaterials may be leveraged to build organoids.

Generation of 2.5D lung bud organoids from human induced pluripotent stem cells

Human induced pluripotent stem cells (hiPSCs) are a promising cell source to generate the patient-specific lung organoid given their superior differentiation potential. However, the current 3D cell culture approach is tedious and time-consuming with a low success rate and high batch-to-batch variability. Here, we explored the establishment of lung bud organoids by systematically adjusting the initial confluence levels and homogeneity of cell distribution. The efficiency of single cell seeding and clump seeding was compared. Instead of the traditional 3D culture, we established a 2.5D organoid culture to enable the direct monitoring of the internal structure via microscopy. It was found that the cell confluence and distribution prior to induction were two key parameters, which strongly affected hiPSC differentiation trajectories. Lung bud organoids with positive expression of NKX 2.1, in a single-cell seeding group with homogeneously distributed hiPSCs at 70% confluence (SC_70%_hom) or a clump seeding group with heterogeneously distributed cells at 90% confluence (CL_90%_het), can be observed as early as 9 days post induction. These results suggest that a successful lung bud organoid formation with single-cell seeding of hiPSCs requires a moderate confluence and homogeneous distribution of cells, while high confluence would be a prominent factor to promote the lung organoid formation when seeding hiPSCs as clumps. 2.5D organoids generated with defined culture conditions could become a simple, efficient, and valuable tool facilitating drug screening, disease modeling and personalized medicine.

Three-Dimensional Printability of an ECM-Based Gelatin Methacryloyl (GelMA) Biomaterial for Potential Neuroregeneration

The current study introduces two novel, smart polymer three-dimensional (3D)-printable interpenetrating polymer network (IPN) hydrogel biomaterials with favorable chemical, mechanical, and morphological properties for potential applications in traumatic brain injury (TBI) such as potentially assisting in the restoration of neurological function through closure of the wound deficit and neural tissue regeneration. Additionally, removal of injury matter to allow for the appropriate scaffold grafting may assist in providing a TBI treatment. Furthermore, due to the 3D printability of the IPN biomaterials, complex structures can be designed and fabricated to mimic the native shape and structure of the injury sight, which can potentially assist with neural tissue regeneration after TBI. In this study, a peptide-only approach was employed, wherein collagen and elastin in a blend with gelatin methacryloyl were prepared and crosslinked using either Irgacure or Irgacure and Genipin to form either a semi or full IPN hydrogel 3D-printable neuromimicking platform system, respectively. The scaffolds displayed favorable thermal stability and were amorphous in nature with high full width at half-maximum values. Furthermore, no alteration to the peptide secondary structure was noted using Fourier transform infrared spectroscopy. The IPN biomaterials have a stiffness of around 600 Pa and are suitable for softer tissue engineering applications-that is, the brain. Scanning electron micrographs indicated that the IPN biomaterials had a morphological structure with a significant resemblance to the native rat cortex. Both biomaterial scaffolds were shown to support the growth of PC12 cells over a 72 h period. Furthermore, the increased nuclear eccentricity and nuclear area were shown to support the postulation that the IPN biomaterials maintain the cells in a healthy state encouraging cellular mitosis and proliferation. The Genipin component of the full IPN was further shown to exhibit antimicrobial properties and this suggests that Genipin can prevent the growth of pathogens associated with postsurgical brain infections. In addition to these findings, the study presents an anomaly, wherein the full IPN is found to be more brittle than the semi IPN, a finding that is in contradiction with the literature. This research, therefore, contributes to the collection of potential biomaterials for TBI applications coupled with 3D printing and can assist in the progression of neural treatments toward patient-specific scaffolds through the development of custom scaffolds.

Engineered cardiac tissue microsphere production through direct differentiation of hydrogel-encapsulated human pluripotent stem cells

Engineered cardiac tissues that can be directly produced from human induced pluripotent stem cells (hiPSCs) in scalable, suspension culture systems are needed to meet the demands of cardiac regenerative medicine. Here, we demonstrate successful production of functional cardiac tissue microspheres through direct differentiation of hydrogel encapsulated hiPSCs. To form the microspheres, hiPSCs were suspended within the photocrosslinkable biomaterial, PEG-fibrinogen (25 million cells/mL), and encapsulated at a rate of 420,000 cells/minute using a custom microfluidic system. Even at this high cell density and rapid production rate, high intra-batch and batch-to-batch reproducibility was achieved. Following microsphere formation, hiPSCs maintained high cell viability and continued to grow within and beyond the original PEG-fibrinogen matrix. These initially soft microspheres (<250 Pa) supported efficient cardiac differentiation; spontaneous contractions initiated by differentiation day 8, and the microspheres contained >75% cardiomyocytes (CMs). CMs responded appropriately to pharmacological stimuli and exhibited 1:1 capture up to 6.0 Hz when electrically paced. Over time, cells formed cell-cell junctions and aligned myofibril fibers; engineered cardiac microspheres were maintained in culture over 3 years. The capability to rapidly generate uniform cardiac microsphere tissues is critical for advancing downstream applications including biomanufacturing, multi-well plate drug screening, and injection-based regenerative therapies.

Mechano-regulation of vascular network formation without branches in 3D bioprinted cell-laden hydrogel constructs

Restoration of a wound is a common surgical procedure in clinic. Currently, the skin required for clinical use is taken from the patient's own body. However, it can be difficult to obtain enough skin sources for large-sized wounds and thus surgeons have started using commercial skin substitutes. The current commercial skin, which includes epidermis substitute, dermis substitute, and bilateral skin substitute, has been popularized in clinic. However, the application is limited by the occurrence of ischemia necrosis after transplantation. Recent studies suggest the use of pre-vascularized skin substitutes for wound healing is a promising area in the research field of skin tissue engineering. Pre-vascularization can be induced by changes in cultivation periods, exertion of mechanical stimuli, or coculture with endothelial cells and various factors. However, few methods could control the formation of vascular branches in engineering tissue in a self-assembly way. In this study, we use three-dimensional (3D) printing technology to confirm that a mechanical force can control the growth of blood vessels in the direction of mechanical stimulation with no branches, and that Yes-associated protein activity is involved in the regulatory progress. In vivo experiments verified that the blood vessels successfully function for blood circulation, and maintain the same direction. Results provide a theoretical basis for products of pre-vascularized skin tissues and other organs created by 3D bioprinting.

Anisotropically Functionalized Aptamer-DNA Nanostructures for Enhanced Cell Proliferation and Target-Specific Adhesion in 3D Cell Cultures

The development of supramolecular hydrogel scaffolds for the precise positioning of biochemical cues is paramount for applications such as tissue engineering. Nucleic acid engineering allows fabrication of three-dimensional (3D) nanostructures with high variability and nanoscale precision. In this study, aptamers were anisotropically functionalized onto branched DNA nanostructures to control their cell adhesion capability, and their efficiency as biological signal inducers for 3D cell cultivation was investigated. Each arm of the X-shaped DNA nanostructure (X-DNA) was functionalized with photo-cross-linkable or cell adhesion moieties, and the steric hindrance of the 3D DNA nanostructures on a cell was optimized. X-DNA nanostructures with cell-positioning parameters were rapidly photopolymerized to form hybrid hydrogels, and their effects on cell behaviors and positions were investigated. We observed that aptamer-functionalized X-DNA nanostructures exhibited significantly enhanced cell proliferation and provided homogeneous distribution and target-specific adhesion of encapsulated cells within hydrogel matrices. Overall, the anisotropic functionalization of DNA nanostructures provides a controllable function for the advancement of conventional 3D culture platforms.

Avoiding tensional equilibrium in cells migrating on a matrix with cell-scale stiffness-heterogeneity

Intracellular stresses affect various cell functions, including proliferation, differentiation and movement, which are dynamically modulated in migrating cells through continuous cell-shaping and remodeling of the cytoskeletal architecture induced by spatiotemporal interactions with extracellular matrix stiffness. When cells migrate on a matrix with cell-scale stiffness-heterogeneity, which is a common situation in living tissues, what intracellular stress dynamics (ISD) emerge? In this study, to explore this issue, finite element method-based traction force microscopy was applied to cells migrating on microelastically patterned gels. Two model systems of microelastically patterned gels (stiff/soft stripe and stiff triangular patterns) were designed to characterize the effects of a spatial constraint on cell-shaping and of the presence of different types of cues to induce competing cellular taxis (usual and reverse durotaxis) on the ISD, respectively. As the main result, the prolonged fluctuation of traction stress on a whole-cell scale was markedly enhanced on single cell-size triangular stiff patterns compared with homogeneous gels. Such ISD enhancement was found to be derived from the interplay between the nomadic migration of cells to regions with different degrees of stiffness and domain shape-dependent traction force dynamics, which should be an essential factor for keeping cells far from tensional equilibrium.

Bioengineering approaches to mature induced pluripotent stem cell-derived atrial cardiomyocytes to model atrial fibrillation

Induced pluripotent stem cells (iPSCs) serve as a robust platform to model several human arrhythmia syndromes including atrial fibrillation (AF). However, the structural, molecular, functional, and electrophysiological parameters of patient-specific iPSC-derived atrial cardiomyocytes (iPSC-aCMs) do not fully recapitulate the mature phenotype of their human adult counterparts. The use of physiologically inspired microenvironmental cues, such as postnatal factors, metabolic conditioning, extracellular matrix (ECM) modulation, electrical and mechanical stimulation, co-culture with non-parenchymal cells, and 3D culture techniques can help mimic natural atrial development and induce a more mature adult phenotype in iPSC-aCMs. Such advances will not only elucidate the underlying pathophysiological mechanisms of AF, but also identify and assess novel mechanism-based therapies towards supporting a more 'personalized' (i.e. patient-specific) approach to pharmacologic therapy of AF.

Engineering spatial-organized cardiac organoids for developmental toxicity testing

Emerging technologies in stem cell engineering have produced sophisticated organoid platforms by controlling stem cell fate via biomaterial instructive cues. By micropatterning and differentiating human induced pluripotent stem cells (hiPSCs), we have engineered spatially organized cardiac organoids with contracting cardiomyocytes in the center surrounded by stromal cells distributed along the pattern perimeter. We investigated how geometric confinement directed the structural morphology and contractile functions of the cardiac organoids and tailored the pattern geometry to optimize organoid production. Using modern data-mining techniques, we found that pattern sizes significantly affected contraction functions, particularly in the parameters related to contraction duration and diastolic functions. We applied cardiac organoids generated from 600 μm diameter circles as a developmental toxicity screening assay and quantified the embryotoxic potential of nine pharmaceutical compounds. These cardiac organoids have potential use as an in vitro platform for studying organoid structure-function relationships, developmental processes, and drug-induced cardiac developmental toxicity.

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Micropattern-based geometric confinement directs cardiac organoid development

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Cardiac organoid structure-function relationships are guided by organoid size

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Cardiac organoids can be used as an in vitro embryotoxicity assessment tool

Biomaterials Regulate Mechanosensors YAP/TAZ in Stem Cell Growth and Differentiation

Tissue-resident stem cells are surrounded by a microenvironment known as 'stem cell niche' which is specific for each stem cell type. This niche comprises of cell-intrinsic and -extrinsic factors like biochemical and biophysical signals, which regulate stem cell characteristics and differentiation. Biochemical signals have been thoroughly studied however, the effect of biophysical signals on stem cell regulation is yet to be completely understood. Biomaterials have aided in addressing this issue since they can provide a defined and tuneable microenvironment resembling in vivo conditions. We review various biomaterials used in many studies which have shown a connection between biomaterial-generated mechanical signals and alteration in stem cell behaviour. Researchers probed to understand the mechanism of mechanotransduction and reported that the signals from the extracellular matrix regulate a transcription factor yes-associated protein (YAP)/transcriptional coactivator with PDZ-binding motif (TAZ), which is a downstream-regulator of the Hippo pathway and it transduces the mechanical signals inside the nucleus. We highlight the role of the YAP/TAZ as mechanotransducers in stem cell self-renewal and differentiation in response to substrate stiffness, also the possibility of mechanobiology as the emerging field of regenerative medicines and three-dimensional tissue printing.

A matrigel-free method to generate matured human cerebral organoids using 3D-Printed microwell arrays

The current methods of generating human cerebral organoids rely excessively on the use of Matrigel or other external extracellular matrices (ECM) for cell micro-environmental modulation. Matrigel embedding is problematic for long-term culture and clinical applications due to high inconsistency and other limitations. In this study, we developed a novel microwell culture platform based on 3D printing. This platform, without using Matrigel or external signaling molecules (i.e., SMAD and Wnt inhibitors), successfully generated matured human cerebral organoids with robust formation of high-level features (i.e., wrinkling/folding, lumens, neuronal layers). The formation and timing were comparable or superior to the current Matrigel methods, yet with improved consistency. The effect of microwell geometries (curvature and resolution) and coating materials (i.e., mPEG, Lipidure, BSA) was studied, showing that mPEG outperformed all other coating materials, while curved-bottom microwells outperformed flat-bottom ones. In addition, high-resolution printing outperformed low-resolution printing by creating faithful, isotropically-shaped microwells. The trend of these effects was consistent across all developmental characteristics, including EB formation efficiency and sphericity, organoid size, wrinkling index, lumen size and thickness, and neuronal layer thickness. Overall, the microwell device that was mPEG-coated, high-resolution printed, and bottom curved demonstrated the highest efficacy in promoting organoid development. This platform provided a promising strategy for generating uniform and mature human cerebral organoids as an alternative to Matrigel/ECM-embedding methods.

Printability and bio-functionality of a shear thinning methacrylated xanthan - gelatin composite bioink

3D bioprinting is a recent technique that can create complex cell seeded scaffolds and therefore holds great promise to revolutionize the biomedical sector by combining materials and structures that more closely mimic the 3D cell environment in tissues. The most commonly used biomaterials for printing are hydrogels, however, many of the hydrogels used still present issues of printability, stability, or poor cell-material interactions. We propose that bio-inks with intrinsic self-assembling and shear thinning properties, such as xanthan gum, can be methacrylated (XGMA) and combined with a bio-functional material such as gelatin methacryloyl (GelMa) to create a stable, cell-interactive bio-ink with improved properties for 3D bioprinting. These biomaterials have reduced viscosity under high shear and recover their viscosity rapidly after the shear is removed, retaining their shape, which translates to easier extrusion whilst maintaining good fidelity after printing. This was confirmed in printing studies, with measured normalized strand widths of 1.2 obtained for high gel concentrations (5+5 % XGMA-GelMA). Furthermore, the introduction of a secondary photo-cross-linking method allowed tuning of the mechanical properties of the hydrogel with stiffness between 15 and 30 kPa, as well as improving the stability of the hydrogel with retention of 75 % of its mass after 90 days. The hydrogel was shown to be biocompatible and bio-active with 97 % cell viability, and cell spreading after 7 days of culture for low gel concentrations (3+3 % XGMA-GelMA). Shear stresses were relatively low while printing (1 kPa) as a result of the shear thinning property of the material, which supported cell viability during extrusion. Finally, printed hydrogels retained high cell viability for lower gel concentrations, and showed improved cell viability for more concentrated hydrogels when compared to cells cultured in bulk hydrogels, presumably due to improved nutrient/oxygen diffusion and cell migration. In conclusion, stability and formulation of a XGMA-GelMA shear thinning composite hydrogel has been optimized to create a bio-functional bio-ink, with improved printability, and in vitro culture stability via secondary photo-induced cross-linking, making this composite a promising bio-ink for 3D bioprinting.

Fabrication of Thick and Anisotropic Cardiac Tissue on Nanofibrous Substrate for Repairing Infarcted Myocardium

In this chapter, we introduce the method for fabricating thick and anisotropic cardiac tissue for heart regeneration. Aligned and biodegradable nanofiber can be prepared by electrospinning Food and Drug Administration-approved poly (lactic-co-glycolic acid) on a rotating drum. After the nanofibers are transferred on to a polydimethylsiloxane frame, the cardiomyocytes could be plated on the nanofiber to form thick and anisotropic cardiac tissue rapidly. Cardiac tissue-like construct could be easily created by one-step method, and transplanted onto the hearts of myocardium infarction models and lead to their functional recovery.

Probing the subcellular nanostructure of engineered human cardiomyocytes in 3D tissue

The structural and functional maturation of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) is essential for pharmaceutical testing, disease modeling, and ultimately therapeutic use. Multicellular 3D-tissue platforms have improved the functional maturation of hiPSC-CMs, but probing cardiac contractile properties in a 3D environment remains challenging, especially at depth and in live tissues. Using small-angle X-ray scattering (SAXS) imaging, we show that hiPSC-CMs matured and examined in a 3D environment exhibit a periodic spatial arrangement of the myofilament lattice, which has not been previously detected in hiPSC-CMs. The contractile force is found to correlate with both the scattering intensity (R ² = 0.44) and lattice spacing (R ² = 0.46). The scattering intensity also correlates with lattice spacing (R ² = 0.81), suggestive of lower noise in our structural measurement than in the functional measurement. Notably, we observed decreased myofilament ordering in tissues with a myofilament mutation known to lead to hypertrophic cardiomyopathy (HCM). Our results highlight the progress of human cardiac tissue engineering and enable unprecedented study of structural maturation in hiPSC-CMs.

Stem Cell Mechanobiology and the Role of Biomaterials in Governing Mechanotransduction and Matrix Production for Tissue Regeneration

Mechanobiology has underpinned many scientific advances in understanding how biophysical and biomechanical cues regulate cell behavior by identifying mechanosensitive proteins and specific signaling pathways within the cell that govern the production of proteins necessary for cell-based tissue regeneration. It is now evident that biophysical and biomechanical stimuli are as crucial for regulating stem cell behavior as biochemical stimuli. Despite this, the influence of the biophysical and biomechanical environment presented by biomaterials is less widely accounted for in stem cell-based tissue regeneration studies. This Review focuses on key studies in the field of stem cell mechanobiology, which have uncovered how matrix properties of biomaterial substrates and 3D scaffolds regulate stem cell migration, self-renewal, proliferation and differentiation, and activation of specific biological responses. First, we provide a primer of stem cell biology and mechanobiology in isolation. This is followed by a critical review of key experimental and computational studies, which have unveiled critical information regarding the importance of the biophysical and biomechanical cues for stem cell biology. This review aims to provide an informed understanding of the intrinsic role that physical and mechanical stimulation play in regulating stem cell behavior so that researchers may design strategies that recapitulate the critical cues and develop effective regenerative medicine approaches.

Chitosan 3D cell culture system promotes naïve-like features of human induced pluripotent stem cells: A novel tool to sustain pluripotency and facilitate differentiation

A simplified and cost-effective culture system for maintaining the pluripotency of human induced pluripotent stem cells (hiPSCs) is crucial for stem cell applications. Although recombinant protein-based feeder-free hiPSC culture systems have been developed, their manufacturing processes are expensive and complicated, which hinders hiPSC technology progress. Chitosan, a versatile biocompatible polysaccharide, has been reported as a biomaterial for three-dimensional (3D) cell culture system that promotes the physiological activities of mesenchymal stem cells and cancer cells. In the current study, we demonstrated that chitosan membranes sustained proliferation and pluripotency of hiPSCs in long-term culture (up to 365 days). Moreover, using vitronectin as the comparison group, the pluripotency of hiPSCs grown on the membranes was altered into a naïve-like state, which, for pluripotent stem cells, is an earlier developmental stage with higher stemness. On the chitosan membranes, hiPSCs self-assembled into 3D spheroids with an average diameter of ~100 μm. These hiPSC spheroids could be directly differentiated into lineage-specific cells from the three germ layers with 3D structures. Collectively, chitosan membranes not only promoted the naïve pluripotent features of hiPSCs but also provided a novel 3D differentiation platform. This convenient biomaterial-based culture system may enable the effective expansion and accessibility of hiPSCs for regenerative medicine, disease modeling, and drug screening.

Recent trends in the development of peptide and protein-based hydrogel therapeutics for the healing of CNS injury

Traumatic brain injury (TBI) and spinal cord injury (SCI) cause millions of deaths and permanent or prolonged physical disabilities around the globe every year. It generally happens due to various incidents, such as accidents during sports, war, physical assault, and strokes which result in severe damage to brain and spinal cord. If this remains untreated, traumatic CNS injuries may lead to early development of several neurodegenerative diseases like Alzheimer's, Parkinson, multiple sclerosis, and other mental illnesses. The initial physical reaction, which is also termed as the primary phase, includes swelling, followed by inflammation as a result of internal haemorrhage causing damage to indigenous tissue, i.e., axonal shear injury, rupture of blood vessels, and partial impaired supply of oxygen and essential nutrients in the neurons, thereby initiating a cascade of events causing secondary injuries such as hypoxia, hypotension, cognitive impairment, seizures, imbalanced calcium homeostasis and glutamate-induced excitotoxicity resulting in concomitant neuronal cell death and cumulative permanent tissue damage. In the modern era of advanced biomedical technology, we are still living with scarcity of the clinically applicable comparative non-invasive therapeutic strategies for regeneration or functional recovery of neurons or neural networks after a massive CNS injury. One of the key reasons for this scarcity is the limited regenerative ability of neurons in CNS. Growth-impermissive glial scar and the lack of a synthetic biocompatible platform for proper neural tissue engineering and controlled supply of drugs further retard the healing process. Injectable or implantable hydrogel materials, consisting majorly of water in its porous three-dimensional (3D) structure, can serve as an excellent drug delivery platform as well as a transplanted cell-supporting scaffold medium. Among the various neuro-compatible bioinspired materials, we are limiting our discussion to the recent advancement of engineered biomaterials comprising mainly of peptides and proteins due to their growing demand, low immunogenicity and versatility in the fabrication of neuro regenerative medicine. In this article, we try to explore all the recent scientific avenues that are developing gradually to make peptide and peptide-conjugated biomaterial hydrogels as a therapeutic and supporting scaffold for treating CNS injuries.

Use of a PTFE Micro-Bioreactor to Promote 3D Cell Rearrangement and Maintain High Plasticity in Epigenetically Erased Fibroblasts

Phenotype definition is driven by epigenetic mechanisms as well as directly influenced by the cell microenvironment and by biophysical signals deriving from the extracellular matrix. The possibility to interact with the epigenetic signature of an adult mature cell, reversing its differentiated state and inducing a short transient high plasticity window, was previously demonstrated. In parallel, in vitro studies have shown that 3D culture systems, mimicking cell native tissue, exert significant effects on cell behavior and functions. Here we report the production of "PTFE micro-bioreactors" for long-term culture of epigenetically derived high plasticity cells. The system promotes 3D cell rearrangement, global DNA demethylation and elevated transcription of pluripotency markers, that is dependent on WW domain containing transcription regulator 1 (TAZ) nuclear accumulation and SMAD family member 2 (SMAD2) co-shuttling. Our findings demonstrate that the use of 3D culture strategies greatly improves the induction and maintenance of a high plasticity state.

Three-dimensional imaging of cell and extracellular matrix elasticity using quantitative micro-elastography

Recent studies in mechanobiology have revealed the importance of cellular and extracellular mechanical properties in regulating cellular function in normal and disease states. Although it is established that cells should be investigated in a three-dimensional (3-D) environment, most techniques available to study mechanical properties on the microscopic scale are unable to do so. In this study, for the first time, we present volumetric images of cellular and extracellular elasticity in 3-D biomaterials using quantitative micro-elastography (QME). We achieve this by developing a novel strain estimation algorithm based on 3-D linear regression to improve QME system resolution. We show that QME can reveal elevated elasticity surrounding human adipose-derived stem cells (ASCs) embedded in soft hydrogels. We observe, for the first time in 3-D, further elevation of extracellular elasticity around ASCs with overexpressed TAZ; a mechanosensitive transcription factor which regulates cell volume. Our results demonstrate that QME has the potential to study the effects of extracellular mechanical properties on cellular functions in a 3-D micro-environment.

Bioengineering Considerations for a Nurturing Way to Enhance Scalable Expansion of Human Pluripotent Stem Cells

Understanding how defects in mechanotransduction affect cell-to-cell variability will add to the fundamental knowledge of human pluripotent stem cell (hPSC) culture, and may suggest new approaches for achieving a robust, reproducible, and scalable process that result in consistent product quality and yields. Here, the current state of the understanding of the fundamental mechanisms that govern the growth kinetics of hPSCs between static and dynamic cultures is reviewed, the factors causing fluctuations are identified, and culture strategies that might eliminate or minimize the occurrence of cell-to-cell variability arising from these fluctuations are discussed. The existing challenges in the development of hPSC expansion methods for enabling the transition from process development to large-scale production are addressed, a mandatory step for industrial and clinical applications of hPSCs.

Differentiation of Induced Pluripotent Stem Cells towards Mesenchymal Stromal Cells is Hampered by Culture in 3D Hydrogels

Directed differentiation of induced pluripotent stem cells (iPSCs) towards specific lineages remains a major challenge in regenerative medicine, while there is a growing perception that this process can be influenced by the three-dimensional environment. In this study, we investigated whether iPSCs can differentiate towards mesenchymal stromal cells (MSCs) when embedded into fibrin hydrogels to enable a one-step differentiation procedure within a scaffold. Differentiation of iPSCs on tissue culture plastic or on top of fibrin hydrogels resulted in a typical MSC-like phenotype. In contrast, iPSCs embedded into fibrin gel gave rise to much smaller cells with heterogeneous growth patterns, absence of fibronectin, faint expression of CD73 and CD105, and reduced differentiation potential towards osteogenic and adipogenic lineage. Transcriptomic analysis demonstrated that characteristic genes for MSCs and extracellular matrix were upregulated on flat substrates, whereas genes of neural development were upregulated in 3D culture. Furthermore, the 3D culture had major effects on DNA methylation profiles, particularly within genes for neuronal and cardiovascular development, while there was no evidence for epigenetic maturation towards MSCs. Taken together, iPSCs could be differentiated towards MSCs on tissue culture plastic or on a flat fibrin hydrogel. In contrast, the differentiation process was heterogeneous and not directed towards MSCs when iPSCs were embedded into the hydrogel.

Mimicking megakaryopoiesis in vitro using biomaterials: Recent advances and future opportunities

Presently donor-derived platelets used in the clinic are associated with concerns about adequate availability, expense, risk of bacterial contamination and complications due to immunological reaction. To prevail over our dependence on transfusion of donor-derived platelets, efforts are being made to generate them in vitro. Development of biomaterials that support or mimic bone marrow niche micro-environmental cues could improve the in vitro production of platelets from megakaryocytes (MKs) derived from various stem cell sources. In spite of significant advances in the production of MKs from various stem cell sources using 2D as well as 3D culture approaches in vitro and the development of biomaterials-based platelet systems, yield and quality of these platelets remains unsuitable for clinical use. Thus, in vitro production of clinically useful platelets on a large scale remains an unmet target to date. This review summarizes the most frequently used 2D and 3D approaches to generate MKs and platelets in vitro, emphasizing the importance of mimicking in vivo micro-environment. Further, this review proposes the use of interpenetrating network (IPN) biomaterial-based approach as a promising strategy for improving the generation of MK and platelets in sufficient numbers in vitro. STATEMENT OF SIGNIFICANCE: Thrombocytopenia is one of the major global health and socio-economic problems. Transfusion with donor-derived platelets (PLTs) is the only effective treatment for this condition. However, this approach is limited by factors like short shelf-life of PLTs, PLT activation, alloimmunization, risk of bacterial contamination, infection etc. In vitro generated MKs and PLTs derived from non-donor-dependent sources may help to overcome the platelet transfusion concerns. Here we have reviewed various 2D and 3D strategies used for in vitro generation of MKs and PLTs, with special emphasis on various biomaterial platforms and different physico/chemical cues being used for the purpose. We have also proposed a biomaterial-based approach of using interpenetrating network (IPN) for generating clinically relevant numbers of MKs and PLTs.

Microengineered human amniotic ectoderm tissue array for high-content developmental phenotyping

During early post-implantation human embryogenesis, the epiblast (EPI) within the blastocyst polarizes to generate a cyst with a central lumen. Cells at the uterine pole of the EPI cyst then undergo differentiation to form the amniotic ectoderm (AM), a tissue essential for further embryonic development. While the causes of early pregnancy failure are complex, improper lumenogenesis or amniogenesis of the EPI represent possible contributing factors. Here we report a novel AM microtissue array platform that allows quantitative phenotyping of lumenogenesis and amniogenesis of the EPI and demonstrate its potential application for embryonic toxicity profiling. Specifically, a human pluripotent stem cell (hPSC)-based amniogenic differentiation protocol was developed using a two-step micropatterning technique to generate a regular AM microtissue array with defined tissue sizes. A computer-assisted analysis pipeline was developed to automatically process imaging data and quantify morphological and biological features of AM microtissues. Analysis of the effects of cell density, cyst size and culture conditions revealed a clear connection between cyst size and amniogenesis of hPSC. Using this platform, we demonstrated that pharmacological inhibition of ROCK signaling, an essential mechanotransductive pathway, suppressed lumenogenesis but did not perturb amniogenic differentiation of hPSC, suggesting uncoupled regulatory mechanisms for AM morphogenesis vs. cytodifferentiation. The AM microtissue array was further applied to screen a panel of clinically relevant drugs, which successfully detected their differential teratogenecity. This work provides a technological platform for toxicological screening of clinically relevant drugs for their effects on lumenogenesis and amniogenesis during early human peri-implantation development, processes that have been previously inaccessible to study.

Multi-foci laser microfabrication of 3D polymeric scaffolds for stem cell expansion in regenerative medicine

High quality large scale fabrication of cellular scaffolds, with three-dimensional resolution comparable to cell size, is an important task to enable regenerative medicine applications with stem cells. We are using two-photon polymerization to produce our stem cell culture substrate called Nichoid, which we already demonstrated capable of stimulating cell proliferation while maintaining their stemness, without the need of dangerous additives. Parallelization of this technique can be achieved with the use of a spatial light modulator: here we show the results obtained combining this device with fast linear stages to produce Nichoid-covered substrates by two-photon polymerization. The well-polymerized structures confirm that this approach is particularly convenient for porous structures, and allows a significant time saving by a factor of almost five, with minor design adjustments. A Live & Dead assay was performed on mesenchymal stem cells cultured into the Nichoid microstructures in order to verify that no difference in cell viability is present, compared to microstructures fabricated by a single focus. This parallel setup opens the possibility to obtain a much larger number of microstructured substrates, that are essential to test new stem cell-based therapies. This approach can be also used for the fast fabrication of other kinds of cell culture devices.

Microfluidic preparation, shrinkage, and surface modification of monodispersed alginate microbeads for 3D cell culture

Functionalized alginate microbeads (MB) have been widely used for three-dimensional (3D) culture of cells and creating biomimetic tissue models. However, conventional methods for preparing these MB suffer from poor polydispersity, due to coalescence of droplets during the gelation process and post-aggregation. It remains an immense challenge to prepare alginate MB with narrow size distribution and uniform shape, especially when their diameters are similar to the size of cells. In this work, we developed a simple method to produce monodispersed, cell-size alginate MB through microfluidic emulsification, followed by a controlled shrinkage process and gelation in mineral oil with low concentration of calcium ion (Ca²⁺). During the gelation process caused by the diffusion of Ca²⁺ from the oil to water phase, a large amount of satellite droplets with sub-micrometer sizes was formed at the water/oil interface. As a result, each original droplet was transformed to one shrunken-MB with much smaller size and numerous submicron-size satellites. To explore the feasibility of the shrunken-MB for culturing with cells, we have successfully modified a variety of polymer nanofilms on MB surfaces using a layer-by-layer assembly approach. Finally, the nanofilm-modified MB was applied to a 3D culture of GFP-expressing fibroblast cells and demonstrated good biocompatibility.

Cell-size alginate microbeads for 3D cell culture were prepared by microfluidic emulsification and controlled shrinkage, followed by nanofilm modification.

Analysis of spatial resolution in phase-sensitive compression optical coherence elastography

Optical coherence elastography (OCE) is emerging as a method to image the mechanical properties of tissue on the microscale. However, the spatial resolution, a main advantage of OCE, has not been investigated and is not trivial to evaluate. To address this, we present a framework to analyze resolution in phase-sensitive compression OCE that incorporates the three main determinants of resolution: mechanical deformation of the sample, detection of this deformation using optical coherence tomography (OCT), and signal processing to estimate local axial strain. We demonstrate for the first time, through close correspondence between experiment and simulation of structured phantoms, that resolution in compression OCE is both spatially varying and sample dependent, which we link to the discrepancies between the model of elasticity and the mechanical deformation of the sample. We demonstrate that resolution is dependent on factors such as feature size and mechanical contrast. We believe that the analysis of image formation provided by our framework can expedite the development of compression OCE.

Two decades of embryonic stem cells: a historical overview

STUDY QUESTION

How did the field of stem cell research develop in the years following the derivation of the first human embryonic stem cell (hESC) line?

SUMMARY ANSWER

Supported by the increasing number of clinical trials to date, significant technological advances in the past two decades have brought us ever closer to clinical therapies derived from pluripotent cells.

WHAT IS KNOWN ALREADY

Since their discovery 20 years ago, the use of human pluripotent stem cells has progressed tremendously from bench to bedside. Here, we provide a concise review of the main keystones of this journey and focus on ongoing clinical trials, while indicating the most relevant future research directions.

STUDY DESIGN, SIZE, DURATION

This is a historical narrative, including relevant publications in the field of pluripotent stem cells (PSC) derivation and differentiation, recounted both through scholarly research of published evidence and interviews of six pioneers who participated in some of the most relevant discoveries in the field.

PARTICIPANTS/MATERIALS, SETTING, METHODS

The authors all contributed by researching the literature and agreed upon body of works. Portions of the interviews of the field pioneers have been integrated into the review and have also been included in full for advanced reader interest.

MAIN RESULTS AND THE ROLE OF CHANCE

The stem cell field is ever expanding. We find that in the 20 years since the derivation of the first hESC lines, several relevant developments have shaped the pluripotent cell field, from the discovery of different states of pluripotency, the derivation of induced PSC, the refinement of differentiation protocols with several clinical trials underway, as well as the recent development of organoids. The challenge for the years to come will be to validate and refine PSCs for clinical use, from the production of highly defined cell populations in clinical grade conditions to the possibility of creating replacement organoids for functional, if not anatomical, function restoration.

LIMITATIONS, REASONS FOR CAUTION

This is a non-systematic review of current literature. Some references may have escaped the experts’ analysis due to the exceedingly diverse nature of the field. As the field of regenerative medicine is rapidly advancing, some of the most recent developments may have not been captured entirely.

WIDER IMPLICATIONS OF THE FINDINGS

The multi-disciplinary nature and tremendous potential of the stem cell field has important implications for basic as well as translational research. Recounting these activities will serve to provide an in-depth overview of the field, fostering a further understanding of human stem cell and developmental biology. The comprehensive overview of clinical trials and expert opinions included in this narrative may serve as a valuable scientific resource, supporting future efforts in translational approaches.

STUDY FUNDING/COMPETING INTEREST(S)

ESHRE provided funding for the authors’ on-site meeting and discussion during the preparation of this manuscript. S.M.C.S.L. is funded by the European Research Council Consolidator (ERC-CoG-725722-OVOGROWTH). M.P. is supported by the Special Research Fund, Bijzonder Onderzoeksfonds (BOF01D08114). M.G. is supported by the Methusalem grant of Vrije Universiteit Brussel, in the name of Prof. Karen Sermon and by Innovation by Science and Technology in Flanders (IWT, Project Number: 150042). A.V. and B.A. are supported by the Plataforma de Proteomica, Genotipado y Líneas Celulares (PT1770019/0015) (PRB3), Instituto de Salud Carlos III. Research grant to B.H. by the Research Foundation—Flanders (FWO) (FWO.KAN.2016.0005.01 and FWO.Project G051516N). There are no conflicts of interest to declare.

TRIAL REGISTRATION NUMBER

Not applicable.

ESHRE Pages are not externally peer reviewed. This article has been approved by the Executive Committee of ESHRE.

Biomaterials and Gene Manipulation in Stem Cell-Based Therapies for Spinal Cord Injury

Spinal cord injury (SCI), a prominent health issue, represents a substantial portion of the global health care burden. Stem cell-based therapies provide novel solutions for SCI treatment, yet obstacles remain in the form of low survival rate, uncontrolled differentiation, and functional recovery. The application of engineered biomaterials in stem cell therapy provides a physicochemical microenvironment that mimics the stem cell niche, facilitating self-renewal, stem cell differentiation, and tissue reorganization. Nonetheless, external microenvironment support is inadequate, and some obstacles persist, for example, limited sources, gradual aging, and immunogenicity of stem cells. Targeted stem cell gene manipulation could eliminate many of these drawbacks, allowing safer, more effective use under regulation of intrinsic mechanisms. Additionally, through genetic labeling of stem cells, their role in tissue engineering may be elucidated. Therefore, combining stem cell therapy, materials science, and genetic modification technologies may shed light on SCI treatment. Herein, recent advances and advantages of biomaterials and gene manipulation, especially with respect to stem cell-based therapies, are highlighted, and their joint performance in SCI is evaluated. Current technological limitations and perspectives on future directions are then discussed. Although this combination is still in the early stages of development, it is highly likely to substantially contribute to stem cell-based therapies in the foreseeable future.

Laminin as a Potent Substrate for Large-Scale Expansion of Human Induced Pluripotent Stem Cells in a Closed Cell Expansion System

The number of high-quality cells required for engineering an adult human-sized bioartificial organ is greater than one billion. Until the emergence of induced pluripotent stem cells (iPSCs), autologous cell sources of this magnitude and with the required complexity were not available. Growing this number of cells in a traditional 2D cell culture system requires extensive time, resources, and effort and does not always meet clinical requirements. The use of a closed cell culture system is an efficient and clinically applicable method that can be used to expand cells under controlled conditions. We aimed to use the Quantum Cell Expansion System (QES) as an iPSC monolayer-based expansion system. Human iPSCs were expanded (up to 14-fold) using the QES on two different coatings (laminin 521 (LN521) and vitronectin (VN)), and a karyotype analysis was performed. The cells were characterized for spontaneous differentiation and pluripotency by RT-PCR and flow cytometry. Our results demonstrated that the QES provides the necessary environment for exponential iPSC growth, reaching 689.75 × 10⁶ ± 86.88 × 10⁶ in less than 7 days using the LN521 coating with a population doubling level of 3.80 ± 0.19. The same result was not observed when VN was used as a coating. The cells maintained normal karyotype (46-XX), expressed pluripotency markers (OCT4, NANOG, LIN28, SOX2, REX1, DPPA4, NODAL, TDGFb, TERT3, and GDF), and expressed high levels of OCT4, SOX2, NANOG, SSEA4, TRA1-60, and TRA1-81. Spontaneous differentiation into ectoderm (NESTIN, TUBB3, and NEFH), mesoderm (MSX1, BMP4, and T), and endoderm (GATA6, AFP, and SOX17) lineages was detected by RT-PCR with both coating systems. We conclude that the QES maintains the stemness of iPSCs and is a promising platform to provide the number of cells necessary to recellularize small human-sized organ scaffolds for clinical purposes.

Programming Niche Accessibility and In Vitro Stemness with Intercellular DNA Reactions

Stem cells generally exist in low abundance and tend to lose stemness in the absence of self-renewal signals. While extracellular-matrix-mimicking techniques have been developed to support stem cell proliferation, the lack of niche cells in these synthetic systems often hampers continuous stem cell expansion and maintenance of pluripotency, which are indispensable for regenerative medicine. Here, an intercellular DNA-reaction-programmed ESPN (expansion of stem cells with pairing niches) strategy is developed for 3D culture of mammary stem cells (MaSCs). Boolean logic operations are implemented to confer DNA-programmed mechanical signaling and genetically engineered morphogen signaling by niche cells, resulting in sustained expansion of MaSCs in vitro. The creation of stem cell niches improves the proliferation of pluripotent cells by four times during one-week culture. This method thus provides a novel approach for logical regulation of stemness and proliferation of stem cells for biomedicine.

Why the impact of mechanical stimuli on stem cells remains a challenge

Mechanical stimulation affects growth and differentiation of stem cells. This may be used to guide lineage-specific cell fate decisions and therefore opens fascinating opportunities for stem cell biology and regenerative medicine. Several studies demonstrated functional and molecular effects of mechanical stimulation but on first sight these results often appear to be inconsistent. Comparison of such studies is hampered by a multitude of relevant parameters that act in concert. There are notorious differences between species, cell types, and culture conditions. Furthermore, the utilized culture substrates have complex features, such as surface chemistry, elasticity, and topography. Cell culture substrates can vary from simple, flat materials to complex 3D scaffolds. Last but not least, mechanical forces can be applied with different frequency, amplitude, and strength. It is therefore a prerequisite to take all these parameters into consideration when ascribing their specific functional relevance-and to only modulate one parameter at the time if the relevance of this parameter is addressed. Such research questions can only be investigated by interdisciplinary cooperation. In this review, we focus particularly on mesenchymal stem cells and pluripotent stem cells to discuss relevant parameters that contribute to the kaleidoscope of mechanical stimulation of stem cells.

Scalable Generation of Mesenchymal Stem Cells from Human Embryonic Stem Cells in 3D

Human embryonic stem cell (hESC) derived mesenchymal stem cells (EMSC) are efficacious in treating a series of autoimmune, inflammatory, and degenerative diseases in animal models. However, all the EMSC derivation methods reported so far rely on two-dimensional (2D) culture systems, which are inefficient, costive and difficult for large-scale production. HESC, as an unlimited source, can be successively propagated in spheroids. Here, we demonstrate that hESC spheroids can directly differentiate into MSC spheroids (EMSC_Sp) within 20 days in one vessel without passaging and the system is scalable to any desired size. EMSC_Sp can further differentiate into osteocytes and chondrocytes in spheres or demineralized bone matrix. EMSC_Sp also retains immune-modulatory effects in vitro and therapeutic effects on two mouse models of colitis after dissociation. Compared to EMSC differentiated in monolayer, EMSC_Sp-derived cells have faster proliferation and higher yield and develop less apoptosis and slower senescence. Thus, the 3D differentiation system allows simple, cost-effective, and scalable production of high-quality EMSC and subsequently bone and cartilage tissues for therapeutic application.

3D heterogeneous islet organoid generation from human embryonic stem cells using a novel engineered hydrogel platform

Organoids, which exhibit spontaneous organ specific organization, function, and multi-cellular complexity, are in essence the in vitro reproduction of specific in vivo organ systems. Recent work has demonstrated human pluripotent stem cells (hPSCs) as a viable regenerative cell source for tissue-specific organoid engineering. This is especially relevant for engineering islet organoids, due to the recent advances in generating functional beta-like cells from human pluripotent stem cells. In this study, we report specific engineering of regenerative islet organoids of precise size and cellular heterogeneity, using a novel hydrogel system, Amikagel. Amikagel facilitated controlled and spontaneous aggregation of human embryonic stem cell derived pancreatic progenitor cells (hESC-PP) into robust homogeneous spheroids. This platform further allowed fine control over the integration of multiple cell populations to produce heterogeneous spheroids, which is a necessity for complex organoid engineering. Amikagel induced hESC-PP spheroid formation enhanced pancreatic islet-specific Pdx-1 and NKX6.1 gene and protein expression, while also increasing the percentage of committed population. hESC-PP spheroids were further induced towards mature beta-like cells which demonstrated increased Beta-cell specific INS1 gene and C-peptide protein expression along with functional insulin production in response to in vitro glucose challenge. Further integration of hESC-PP with biologically relevant supporting endothelial cells resulted in multicellular organoids which demonstrated spontaneous maturation towards islet-specific INS1 gene and C-peptide protein expression along with a significantly developed extracellular matrix support system. These findings establish Amikagel -facilitated platform ideal for islet organoid engineering.

Looking into the Future: Toward Advanced 3D Biomaterials for Stem-Cell-Based Regenerative Medicine

Stem-cell-based therapies have the potential to provide novel solutions for the treatment of a variety of diseases, but the main obstacles to such therapies lie in the uncontrolled differentiation and functional engraftment of implanted tissues. The physicochemical microenvironment controls the self-renewal and differentiation of stem cells, and the key step in mimicking the stem cell microenvironment is to construct a more physiologically relevant 3D culture system. Material-based 3D assemblies of stem cells facilitate the cellular interactions that promote morphogenesis and tissue organization in a similar manner to that which occurs during embryogenesis. Both natural and artificial materials can be used to create 3D scaffolds, and synthetic organic and inorganic porous materials are the two main kinds of artificial materials. Nanotechnology provides new opportunities to design novel advanced materials with special physicochemical properties for 3D stem cell culture and transplantation. Herein, the advances and advantages of 3D scaffold materials, especially with respect to stem-cell-based therapies, are first outlined. Second, the stem cell biology in 3D scaffold materials is reviewed. Third, the progress and basic principles of developing 3D scaffold materials for clinical applications in tissue engineering and regenerative medicine are reviewed.

3-D Bioprinting of Neural Tissue for Applications in Cell Therapy and Drug Screening

Neurodegenerative diseases affect millions of individuals in North America and cost the health-care industry billions of dollars for treatment. Current treatment options for degenerative diseases focus on physical rehabilitation or drug therapies, which temporarily mask the effects of cell damage, but quickly lose their efficacy. Cell therapies for the central nervous system remain an untapped market due to the complexity involved in growing neural tissues, controlling their differentiation, and protecting them from the hostile environment they meet upon implantation. Designing tissue constructs for the discovery of better drug treatments are also limited due to the resolution needed for an accurate cellular representation of the brain, in addition to being expensive and difficult to translate to biocompatible materials. 3-D printing offers a streamlined solution for engineering brain tissue for drug discovery or, in the future, for implantation. New microfluidic and bioplotting devices offer increased resolution, little impact on cell viability and have been tested with several bioink materials including fibrin, collagen, hyaluronic acid, poly(caprolactone), and poly(ethylene glycol). This review details current efforts at bioprinting neural tissue and highlights promising avenues for future work.

Human Pluripotent Stem Cell-Derived Cardiac Tissue-like Constructs for Repairing the Infarcted Myocardium

High-purity cardiomyocytes (CMs) derived from human induced pluripotent stem cells (hiPSCs) are promising for drug development and myocardial regeneration. However, most hiPSC-derived CMs morphologically and functionally resemble immature rather than adult CMs, which could hamper their application. Here, we obtained high-quality cardiac tissue-like constructs (CTLCs) by cultivating hiPSC-CMs on low-thickness aligned nanofibers made of biodegradable poly(D,L-lactic-co-glycolic acid) polymer. We show that multilayered and elongated CMs could be organized at high density along aligned nanofibers in a simple one-step seeding process, resulting in upregulated cardiac biomarkers and enhanced cardiac functions. When used for drug assessment, CTLCs were much more robust than the 2D conventional control. We also demonstrated the potential of CTLCs for modeling engraftments in vitro and treating myocardial infarction in vivo. Thus, we established a handy framework for cardiac tissue engineering, which holds high potential for pharmaceutical and clinical applications.

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hiPSC-CMs are seeded on aligned nanofibers to obtain 3D cardiac tissue-like constructs

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Drug assessment using CTLCs can be more robust than conventional cultures

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CTLCs can be used to model in vitro cardiac tissue engraftment

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CTLCs improve the function of rat hearts with myocardial infarction

Engineering cell aggregates through incorporated polymeric microparticles

Ex vivo cell aggregates must overcome significant limitations in the transport of nutrients, drugs, and signaling proteins compared to vascularized native tissue. Further, engineered extracellular environments often fail to sufficiently replicate tethered signaling cues and the complex architecture of native tissue. Co-cultures of cells with microparticles (MPs) is a growing field directed towards overcoming many of these challenges by providing local and controlled presentation of both soluble and tethered proteins and small molecules. Further, co-cultured MPs offer a mechanism to better control aggregate architecture and even to report key characteristics of the local microenvironment such as pH or oxygen levels. Herein, we provide a brief introduction to established and developing strategies for MP production including the choice of MP materials, fabrication techniques, and techniques for incorporating additional functionality. In all cases, we emphasize the specific utility of each approach to form MPs useful for applications in cell aggregate co-culture. We review established techniques to integrate cells and MPs. We highlight those strategies that promote targeted heterogeneity or homogeneity, and we describe approaches to engineer cell-particle and particle-particle interactions that enhance aggregate stability and biological response. Finally, we review advances in key application areas of MP aggregates and future areas of development.

STATEMENT OF SIGNIFICANT

Cell-scaled polymer microparticles (MPs) integrated into cellular aggregates have been shown to be a powerful tool to direct cell response. MPs have supported the development of healthy cartilage, islets, nerves, and vasculature by the maintenance of soluble gradients as well as by the local presentation of tethered cues and diffusing proteins and small molecules. MPs integrated with pluripotent stem cells have directed in vivo expansion and differentiation. Looking forward, MPs are expected to support both the characterization and development of in vitro tissue systems for applications such as drug testing platforms. However, useful co-cultures must be designed keeping in mind the limitations and attributes of each material strategy within the context of the overall tissue biology. The present review integrates prospectives from materials development, drug delivery, and tissue engineering to provide a toolbox for the development and application of MPs useful for long-term co-culture within cell aggregates.

Three-Dimensional Laminin Mimetic Peptide Nanofiber Gels for In Vitro Neural Differentiation

The extracellular matrix (ECM) provides biochemical signals and structural support for cells, and its functional imitation is a fundamental aspect of biomaterial design for regenerative medicine applications. The stimulation of neural differentiation by a laminin protein-derived epitope in two-dimensional (2D) and three-dimensional (3D) environments is investigated. The 3D gel system is found to be superior to its 2D counterpart for the induction of neural differentiation, even in the absence of a crucial biological inducer in nerve growth factor (NGF). In addition, cells cultured in 3D gels exhibits a spherical morphology that is consistent with their form under in vivo conditions. Overall, the present study underlines the impact of bioactivity, dimension, and NGF addition, as well as the cooperative effects thereof, on the neural differentiation of PC-12 cells. These results underline the significance of 3D culture systems in the development of scaffolds that closely replicate in vivo environments for the formation of cellular organoid models in vitro.