Multimaterial bioprinting and combination of processing techniques towards the fabrication of biomimetic tissues and organs

Targeted Cancer Therapy-on-A-Chip

Targeted cancer therapy (TCT) is gaining increased interest because it reduces the risks of adverse side effects by specifically treating tumor cells. TCT testing has traditionally been performed using two-dimensional (2D) cell culture and animal studies. Organ-on-a-chip (OoC) platforms have been developed to recapitulate cancer in vitro, as cancer-on-a-chip (CoC), and used for chemotherapeutics development and testing. This review explores the use of CoCs to both develop and test TCTs, with a focus on three main aspects, the use of CoCs to identify target biomarkers for TCT development, the use of CoCs to test free, un-encapsulated TCTs, and the use of CoCs to test encapsulated TCTs. Despite current challenges such as system scaling, and testing externally triggered TCTs, TCToC shows a promising future to serve as a supportive, pre-clinical platform to expedite TCT development and bench-to-bedside translation.

Multimaterial 3D and 4D Bioprinting of Heterogenous Constructs for Tissue Engineering

Additive manufacturing (AM), which is based on the principle of layer-by-layer shaping and stacking of discrete materials, has shown significant benefits in the fabrication of complicated implants for tissue engineering (TE). However, many native tissues exhibit anisotropic heterogenous constructs with diverse components and functions. Consequently, the replication of complicated biomimetic constructs using conventional AM processes based on a single material is challenging. Multimaterial 3D and 4D bioprinting (with time as the fourth dimension) has emerged as a promising solution for constructing multifunctional implants with heterogenous constructs that can mimic the host microenvironment better than single-material alternatives. Notably, 4D-printed multimaterial implants with biomimetic heterogenous architectures can provide a time-dependent programmable dynamic microenvironment that can promote cell activity and tissue regeneration in response to external stimuli. This paper first presents the typical design strategies of biomimetic heterogenous constructs in TE applications. Subsequently, the latest processes in the multimaterial 3D and 4D bioprinting of heterogenous tissue constructs are discussed, along with their advantages and challenges. In particular, the potential of multimaterial 4D bioprinting of smart multifunctional tissue constructs is highlighted. Furthermore, this review provides insights into how multimaterial 3D and 4D bioprinting can facilitate the realization of next-generation TE applications.

Biomimetic gradient scaffolds for the tissue engineering and regeneration of rotator cuff enthesis

Rotator cuff tear is one of the most common musculoskeletal disorders, which often results in recurrent shoulder pain and limited movement. Enthesis is a structurally complex and functionally critical interface connecting tendon and bone that plays an essential role in maintaining integrity of the shoulder joint. Despite the availability of advanced surgical procedures for rotator cuff repair, there is a high rate of failure following surgery due to suboptimal enthesis healing and regeneration. Novel strategies based on tissue engineering are gaining popularity in improving tendon-bone interface (TBI) regeneration. Through incorporating physical and biochemical cues into scaffold design which mimics the structure and composition of native enthesis is advantageous to guide specific differentiation of seeding cells and facilitate the formation of functional tissues. In this review, we summarize the current state of research in enthesis tissue engineering highlighting the development and application of biomimetic scaffolds that replicate the gradient TBI. We also discuss the latest techniques for fabricating potential translatable scaffolds such as 3D bioprinting and microfluidic device. While preclinical studies have demonstrated encouraging results of biomimetic gradient scaffolds, the translation of these findings into clinical applications necessitates a comprehensive understanding of their safety and long-term efficacy.

3D bioprinted small extracellular vesicles from periodontal cells enhance mesenchymal stromal cell function

Recent research indicates that combining 3D bioprinting and small extracellular vesicles (sEVs) offers a promising 'cell-free' regenerative medicine approach for various tissue engineering applications. Nonetheless, the majority of existing research has focused on bioprinting of sEVs sourced from cell lines. There remains a notable gap in research regarding the bioprinting of sEVs derived from primary human periodontal cells and their potential impact on ligamentous and osteogenic differentiation. Here, we investigated the effect of 3D bioprinted periodontal cell sEVs constructs on the differentiation potential of human buccal fat pad-derived mesenchymal stromal cells (hBFP-MSCs). Periodontal cell-derived sEVs were enriched by size exclusion chromatography (SEC) with particle-shaped morphology, and characterized by being smaller than 200 nm in size and CD9/CD63/CD81 positive, from primary human periodontal ligament cells (hPDLCs) and human gingival fibroblasts (hGFs). The sEVs were then 3D bioprinted in 10 % gelatin methacryloyl (GelMA) via microextrusion bioprinting. Release of sEVs from bioprinted constructs was determined by DiO-labelling and confocal imaging, and CD9 ELISA. Attachment and ligament/osteogenic/cementogenic differentiation of hBFP-MSCs was assessed on bioprinted GelMA, without and with sEVs (GelMA/hPDLCs-sEVs and GelMA/hGFs-sEVs), scaffolds. hBFP-MSCs seeded on the bioprinted sEVs constructs spread well with significantly enhanced focal adhesion, mechanotransduction associated gene expression, and ligament and osteogenesis/cementogenesis differentiation markers in GelMA/hPDLCs-sEVs, compared to GelMA/hGFs-sEVs and GelMA groups. A 2-week osteogenic and ligamentous differentiation showed enhanced ALP staining, calcium formation and toluidine blue stained cells in hBFP-MSCs on bioprinted GelMA/hPDLCs-sEVs constructs compared to the other two groups. The proof-of-concept data from this study supports the notion that 3D bioprinted GelMA/hPDLCs-sEVs scaffolds promote cell attachment, as well as ligamentous, osteogenic and cementogenic differentiation, of hBFP-MSCs in vitro.

Integrating bioprinting, cell therapies and drug delivery towards in vivo regeneration of cartilage, bone and osteochondral tissue

The biological and biomechanical functions of cartilage, bone and osteochondral tissue are naturally orchestrated by a complex crosstalk between zonally dependent cells and extracellular matrix components. In fact, this crosstalk involves biomechanical signals and the release of biochemical cues that direct cell fate and regulate tissue morphogenesis and remodelling in vivo. Three-dimensional bioprinting introduced a paradigm shift in tissue engineering and regenerative medicine, since it allows to mimic native tissue anisotropy introducing compositional and architectural gradients. Moreover, the growing synergy between bioprinting and drug delivery may enable to replicate cell/extracellular matrix reciprocity and dynamics by the careful control of the spatial and temporal patterning of bioactive cues. Although significant advances have been made in this direction, unmet challenges and open research questions persist. These include, among others, the optimization of scaffold zonality and architectural features; the preservation of the bioactivity of loaded active molecules, as well as their spatio-temporal release; the in vitro scaffold maturation prior to implantation; the pros and cons of each animal model and the graft-defect mismatch; and the in vivo non-invasive monitoring of new tissue formation. This work critically reviews these aspects and reveals the state of the art of using three-dimensional bioprinting, and its synergy with drug delivery technologies, to pattern the distribution of cells and/or active molecules in cartilage, bone and osteochondral engineered tissues. Most notably, this work focuses on approaches, technologies and biomaterials that are currently under in vivo investigations, as these give important insights on scaffold performance at the implantation site and its interaction/integration with surrounding tissues.

Perfusable Tissue Bioprinted into a 3D-Printed Tailored Bioreactor System

Bioprinting provides a powerful tool for regenerative medicine, as it allows tissue construction with a patient's specific geometry. However, tissue culture and maturation, commonly supported by dynamic bioreactors, are needed. We designed a workflow that creates an implant-specific bioreactor system, which is easily producible and customizable and supports cell cultivation and tissue maturation. First, a bioreactor was designed and different tissue geometries were simulated regarding shear stress and nutrient distribution to match cell culture requirements. These tissues were then directly bioprinted into the 3D-printed bioreactor. To prove the ability of cell maintenance, C2C12 cells in two bioinks were printed into the system and successfully cultured for two weeks. Next, human mesenchymal stem cells (hMSCs) were successfully differentiated toward an adipocyte lineage. As the last step of the presented strategy, we developed a prototype of an automated mobile docking station for the bioreactor. Overall, we present an open-source bioreactor system that is adaptable to a wound-specific geometry and allows cell culture and differentiation. This interdisciplinary roadmap is intended to close the gap between the lab and clinic and to integrate novel 3D-printing technologies for regenerative medicine.

Advances in volumetric bioprinting

The three-dimensional (3D) bioprinting technologies are suitable for biomedical applications owing to their ability to manufacture complex and high-precision tissue constructs. However, the slow printing speed of current layer-by-layer (bio)printing modality is the major limitation in biofabrication field. To overcome this issue, volumetric bioprinting (VBP) is developed. VBP changes the layer-wise operation of conventional devices, permitting the creation of geometrically complex, centimeter-scale constructs in tens of seconds. VBP is the next step onward from sequential biofabrication methods, opening new avenues for fast additive manufacturing in the fields of tissue engineering, regenerative medicine, personalized drug testing, and soft robotics, etc. Therefore, this review introduces the printing principles and hardware designs of VBP-based techniques; then focuses on the recent advances in VBP-based (bio)inks and their biomedical applications. Lastly, the current limitations of VBP are discussed together with future direction of research.

Is it possible to 3D bioprint load-bearing bone implants? A critical review

Rehabilitative capabilities of any tissue engineered scaffold rely primarily on the triad of (i) biomechanical properties such as mechanical properties and architecture, (ii) chemical behavior such as regulation of cytokine expression, and (iii) cellular response modulation (including their recruitment and differentiation). The closer the implant can mimic the native tissue, the better it can rehabilitate the damage therein. Among the available fabrication techniques, only 3D bioprinting (3DBP) can satisfactorily replicate the inherent heterogeneity of the host tissue. However, 3DBP scaffolds typically suffer from poor mechanical properties, thereby, driving the increased research interest in development of load-bearing 3DBP orthopedic scaffolds in recent years. Typically, these scaffolds involve multi-material 3D printing, comprising of at-least one bioink and a load-bearing ink; such that mechanical and biological requirements of the biomaterials are decoupled. Ensuring high cellular survivability and good mechanical properties are of key concerns in all these studies. 3DBP of such scaffolds is in early developmental stages, and research data from only a handful of preliminary animal studies are available, owing to limitations in print-capabilities and restrictive materials library. This article presents a topically focused review of the state-of-the-art, while highlighting aspects like available 3DBP techniques; biomaterials' printability; mechanical and degradation behavior; and their overall bone-tissue rehabilitative efficacy. This collection amalgamates and critically analyses the research aimed at 3DBP of load-bearing scaffolds for fulfilling demands of personalized-medicine. We highlight the recent-advances in 3DBP techniques employing thermoplastics and phosphate-cements for load-bearing applications. Finally, we provide an outlook for possible future perspectives of 3DBP for load-bearing orthopedic applications. Overall, the article creates ample foundation for future research, as it gathers the latest and ongoing research that scientists could utilize.

Volumetric Printing Across Melt Electrowritten Scaffolds Fabricates Multi-Material Living Constructs with Tunable Architecture and Mechanics

Major challenges in biofabrication revolve around capturing the complex, hierarchical composition of native tissues. However, individual 3D printing techniques have limited capacity to produce composite biomaterials with multi-scale resolution. Volumetric bioprinting recently emerged as a paradigm-shift in biofabrication. This ultrafast, light-based technique sculpts cell-laden hydrogel bioresins into 3D structures in a layerless fashion, providing enhanced design freedom over conventional bioprinting. However, it yields prints with low mechanical stability, since soft, cell-friendly hydrogels are used. Herein, the possibility to converge volumetric bioprinting with melt electrowriting, which excels at patterning microfibers, is shown for the fabrication of tubular hydrogel-based composites with enhanced mechanical behavior. Despite including non-transparent melt electrowritten scaffolds in the volumetric printing process, high-resolution bioprinted structures are successfully achieved. Tensile, burst, and bending mechanical properties of printed tubes are tuned altering the electrowritten mesh design, resulting in complex, multi-material tubular constructs with customizable, anisotropic geometries that better mimic intricate biological tubular structures. As a proof-of-concept, engineered tubular structures are obtained by building trilayered cell-laden vessels, and features (valves, branches, fenestrations) that can be rapidly printed using this hybrid approach. This multi-technology convergence offers a new toolbox for manufacturing hierarchical and mechanically tunable multi-material living structures.

Out of Box Thinking to Tangible Science: A Benchmark History of 3D Bio-Printing in Regenerative Medicine and Tissues Engineering

Advancements and developments in the 3D bioprinting have been promising and have met the needs of organ transplantation. Current improvements in tissue engineering constructs have enhanced their applications in regenerative medicines and other medical fields. The synergistic effects of 3D bioprinting have brought technologies such as tissue engineering, microfluidics, integrated tissue organ printing, in vivo bioprinted tissue implants, artificial intelligence and machine learning approaches together. These have greatly impacted interventions in medical fields, such as medical implants, multi-organ-on-chip models, prosthetics, drug testing tissue constructs and much more. This technological leap has offered promising personalized solutions for patients with chronic diseases, and neurodegenerative disorders, and who have been in severe accidents. This review discussed the various standing printing methods, such as inkjet, extrusion, laser-assisted, digital light processing, and stereolithographic 3D bioprinter models, adopted for tissue constructs. Additionally, the properties of natural, synthetic, cell-laden, dECM-based, short peptides, nanocomposite and bioactive bioinks are briefly discussed. Sequels of several tissue-laden constructs such as skin, bone and cartilage, liver, kidney, smooth muscles, cardiac and neural tissues are briefly analyzed. Challenges, future perspectives and the impact of microfluidics in resolving the limitations in the field, along with 3D bioprinting, are discussed. Certainly, a technology gap still exists in the scaling up, industrialization and commercialization of this technology for the benefit of stakeholders.

Strategies for Improving Vascularization in Kidney Organoids: A Review of Current Trends

Kidney organoids possess the potential to revolutionize the treatment of renal diseases. However, their growth and maturation are impeded by insufficient growth of blood vessels. Through a PubMed search, we have identified 34 studies that attempted to address this challenge. Researchers are exploring various approaches including animal transplantation, organ-on-chips, and extracellular matrices (ECMs). The most prevalent method to promote the maturation and vascularization of organoids involves transplanting them into animals for in vivo culture, creating an optimal environment for organoid growth and the development of a chimeric vessel network between the host and organoids. Organ-on-chip technology permits the in vitro culture of organoids, enabling researchers to manipulate the microenvironment and investigate the key factors that influence organoid development. Lastly, ECMs have been discovered to aid the formation of blood vessels during organoid differentiation. ECMs from animal tissue have been particularly successful, although the underlying mechanisms require further research. Future research building upon these recent studies may enable the generation of functional kidney tissues for replacement therapies.

A Psychrophilic GelMA: Breaking Technical and Immunological Barriers for Multimaterial High-Resolution 3D Bioprinting

The increasing demand for tissue replacement has encouraged scientists worldwide to focus on developing new biofabrication technologies. Multimaterials/cells printed with stringent resolutions are necessary to address the high complexity of tissues. Advanced inkjet 3D printing can use multimaterials and attain high resolution and complexity of printed structures. However, a decisive yet limiting aspect of translational 3D bioprinting is selecting the befitting material to be used as bioink; there is a complete lack of cytoactive bioinks with adequate rheological, mechanical, and reactive properties. This work strives to achieve the right balance between resolution and cell support through methacrylamide functionalization of a psychrophilic gelatin and new fluorosurfactants used to engineer a photo-cross-linkable and immunoevasive bioink. The syntonized parameters following optimal formulation conditions allow proficient printability in a PolyJet 3D printer comparable in resolution to a commercial synthetic ink (∼150 μm). The bioink formulation achieved the desired viability (∼80%) and proliferation of co-printed cells while demonstrating in vivo immune tolerance of printed structures. The practical usage of existing high-resolution 3D printing systems using a novel bioink is shown here, allowing 3D bioprinted structures with potentially unprecedented complexity.

3D Bioprinting tissue analogs: Current development and translational implications

Three-dimensional (3D) bioprinting is a promising and rapidly evolving technology in the field of additive manufacturing. It enables the fabrication of living cellular constructs with complex architectures that are suitable for various biomedical applications, such as tissue engineering, disease modeling, drug screening, and precision regenerative medicine. The ultimate goal of bioprinting is to produce stable, anatomically-shaped, human-scale functional organs or tissue substitutes that can be implanted. Although various bioprinting techniques have emerged to develop customized tissue-engineering substitutes over the past decade, several challenges remain in fabricating volumetric tissue constructs with complex shapes and sizes and translating the printed products into clinical practice. Thus, it is crucial to develop a successful strategy for translating research outputs into clinical practice to address the current organ and tissue crises and improve patients' quality of life. This review article discusses the challenges of the existing bioprinting processes in preparing clinically relevant tissue substitutes. It further reviews various strategies and technical feasibility to overcome the challenges that limit the fabrication of volumetric biological constructs and their translational implications. Additionally, the article highlights exciting technological advances in the 3D bioprinting of anatomically shaped tissue substitutes and suggests future research and development directions. This review aims to provide readers with insight into the state-of-the-art 3D bioprinting techniques as powerful tools in engineering functional tissues and organs.

The effect of multi-material architecture on the ex vivo osteochondral integration of bioprinted constructs

Extrusion bioprinted constructs for osteochondral tissue engineering were fabricated to study the effect of multi-material architecture on encapsulated human mesenchymal stem cells' tissue-specific matrix deposition and integration into an ex vivo porcine osteochondral explant model. Two extrusion fiber architecture groups with differing transition regions and degrees of bone- and cartilage-like bioink mixing were employed. The gradient fiber (G-Fib) architecture group showed an increase in chondral integration over time, 18.5 ± 0.7 kPa on Day 21 compared to 9.6 ± 1.6 kPa on Day 1 for the required peak push-out force, and the segmented fiber (S-Fib) architecture group did not, which corresponded to the increase in sulfated glycosaminoglycan deposition noted only in the G-Fib group and the staining for cellularity and tissue-specific matrix deposition at the fiber-defect boundary. Conversely, the S-Fib architecture was associated with significant mineralization over time, but the G-Fib architecture was not. Notably, both fiber groups also had similar chondral integration as a re-inserted osteochondral tissue control. While architecture did dictate differences in the cells' responses to their environment, architecture was not shown to distinguish a statistically significant difference in tissue integration via fiber push-out testing within a given time point or explant region. Use of this three-week osteochondral model demonstrates that these bioink formulations support the fabrication of cell-laden constructs that integrate into explanted tissue as capably as natural tissue and encapsulate osteochondral matrix-producing cells, and it also highlights the important role that spatial architecture plays in the engineering of multi-phasic tissue environments. STATEMENT OF SIGNIFICANCE: Here, an ex vivo model was used to interrogate fundamental questions about the effect of multi-material scaffold architectural choices on osteochondral tissue integration. Cell-encapsulating constructs resembling stratified osteochondral tissue were 3D printed with architecture consisting of either gradient transitions or segmented transitions between the bone-like and cartilage-like bioink regions. The printed constructs were assessed alongside re-inserted natural tissue plugs via mechanical tissue integration push-out testing, biochemical assays, and histology. Differences in osteochondral matrix deposition were observed based on architecture, and both printed groups demonstrated cartilage integration similar to the native tissue plug group. As 3D printing becomes commonplace within biomaterials and tissue engineering, this work illustrates critical 3D co-culture interactions and demonstrates the importance of considering architecture when interpreting the results of studies utilizing spatially complex, multi-material scaffolds.

An open-source bioink database for microextrusion 3D printing

3D printing has rapidly become a critical enabling technology in tissue engineering and regenerative medicine for the fabrication of complex engineered tissues. 3D bioprinting, in particular, has advanced greatly to facilitate the incorporation of a broad spectrum of biomaterials along with cells and biomolecules of interest forin vitrotissue generation. The increasing complexity of novel bioink formulations and application-dependent printing conditions poses a significant challenge for replicating or innovating new bioprinting strategies. As the field continues to grow, it is imperative to establish a cohesive, open-source database that enables users to search through existing 3D printing formulations rapidly and efficiently. Through the efforts of the NIH/NIBIB Center for Engineering Complex Tissues, we have developed, to our knowledge, the first bioink database for extrusion-based 3D printing. The database is publicly available and allows users to search through and easily access information on biomaterials and cells specifically used in 3D printing. In order to enable a community-driven database growth, we have established an open-source portal for researchers to enter their publication information for addition into the database. Although the database has a broad range of capabilities, we demonstrate its utility by performing a comprehensive analysis of the printability domains of two well-established biomaterials in the printing world, namely poly(ϵ-caprolactone) and gelatin methacrylate. The database allowed us to rapidly identify combinations of extrusion pressure, temperature, and speed that have been used to print these biomaterials and more importantly, identify domains within which printing was not possible. The data also enabled correlation analysis between all the printing parameters, including needle size and type, that exhibited compatibility for cell-based 3D printing. Overall, this database is an extremely useful tool for the 3D printing and bioprinting community to advance their research and is an important step towards standardization in the field.

Indirect 3D Bioprinting of a Robust Trilobular Hepatic Construct with Decellularized Liver Matrix Hydrogel

The liver exhibits complex geometrical morphologies of hepatic cells arranged in a hexagonal lobule with an extracellular matrix (ECM) organized in a specific pattern on a multi-scale level. Previous studies have utilized 3D bioprinting and microfluidic perfusion systems with various biomaterials to develop lobule-like constructs. However, they all lack anatomical relevance with weak control over the size and shape of the fabricated structures. Moreover, most biomaterials lack liver-specific ECM components partially or entirely, which might limit their biomimetic mechanical properties and biological functions. Here, we report 3D bioprinting of a sacrificial PVA framework to impart its trilobular hepatic structure to the decellularized liver extracellular matrix (dLM) hydrogel with polyethylene glycol-based crosslinker and tyrosinase to fabricate a robust multi-scale 3D liver construct. The 3D trilobular construct exhibits higher crosslinking, viscosity (182.7 ± 1.6 Pa·s), and storage modulus (2554 ± 82.1 Pa) than non-crosslinked dLM. The co-culture of HepG2 liver cells and NIH 3T3 fibroblast cells exhibited the influence of fibroblasts on liver-specific activity over time (7 days) to show higher viability (90-91.5%), albumin secretion, and increasing activity of four liver-specific genes as compared to the HepG2 monoculture. This technique offers high lumen patency for the perfusion of media to fabricate a densely populated scaled-up liver model, which can also be extended to other tissue types with different biomaterials and multiple cells to support the creation of a large functional complex tissue.

The Forefront of Dentistry-Promising Tech-Innovations and New Treatments

KNOWLEDGE TRANSFER STATEMENT

This article discusses innovations in technology and treatments that have enormous potential to revolutionize our dental care, including novel concepts in electronic health records, communication between dentists and patients, biologics around diagnosis and treatment, digital dentistry, and, finally, the real-time optimization of information technology. The early implementation and validation of these innovations can drive down their costs and provide better dental and medical services to all members of our society.

Implants with Sensing Capabilities

Because of the aging human population and increased numbers of surgical procedures being performed, there is a growing number of biomedical devices being implanted each year. Although the benefits of implants are significant, there are risks to having foreign materials in the body that may lead to complications that may remain undetectable until a time at which the damage done becomes irreversible. To address this challenge, advances in implantable sensors may enable early detection of even minor changes in the implants or the surrounding tissues and provide early cues for intervention. Therefore, integrating sensors with implants will enable real-time monitoring and lead to improvements in implant function. Sensor integration has been mostly applied to cardiovascular, neural, and orthopedic implants, and advances in combined implant-sensor devices have been significant, yet there are needs still to be addressed. Sensor-integrating implants are still in their infancy; however, some have already made it to the clinic. With an interdisciplinary approach, these sensor-integrating devices will become more efficient, providing clear paths to clinical translation in the future.

Self-Healing Injectable Hydrogels for Tissue Regeneration

Biomaterials with the ability to self-heal and recover their structural integrity offer many advantages for applications in biomedicine. The past decade has witnessed the rapid emergence of a new class of self-healing biomaterials commonly termed injectable, or printable in the context of 3D printing. These self-healing injectable biomaterials, mostly hydrogels and other soft condensed matter based on reversible chemistry, are able to temporarily fluidize under shear stress and subsequently recover their original mechanical properties. Self-healing injectable hydrogels offer distinct advantages compared to traditional biomaterials. Most notably, they can be administered in a locally targeted and minimally invasive manner through a narrow syringe without the need for invasive surgery. Their moldability allows for a patient-specific intervention and shows great prospects for personalized medicine. Injected hydrogels can facilitate tissue regeneration in multiple ways owing to their viscoelastic and diffusive nature, ranging from simple mechanical support, spatiotemporally controlled delivery of cells or therapeutics, to local recruitment and modulation of host cells to promote tissue regeneration. Consequently, self-healing injectable hydrogels have been at the forefront of many cutting-edge tissue regeneration strategies. This study provides a critical review of the current state of self-healing injectable hydrogels for tissue regeneration. As key challenges toward further maturation of this exciting research field, we identify (i) the trade-off between the self-healing and injectability of hydrogels vs their physical stability, (ii) the lack of consensus on rheological characterization and quantitative benchmarks for self-healing injectable hydrogels, particularly regarding the capillary flow in syringes, and (iii) practical limitations regarding translation toward therapeutically effective formulations for regeneration of specific tissues. Hence, here we (i) review chemical and physical design strategies for self-healing injectable hydrogels, (ii) provide a practical guide for their rheological analysis, and (iii) showcase their applicability for regeneration of various tissues and 3D printing of complex tissues and organoids.

Hyaluronic acid/gelatin microcapsule functionalized with carbon nanotube through laccase-catalyzed crosslinking for fabrication of cardiac microtissue

Carbon nanotube (CNT) and gelatin (Gela) molecules are effective substrates in promoting engineered cardiac tissue functions. This study developed a microfluidic-based encapsulation process for biomimetic hydrogel microcapsule fabrication. The hydrogel microcapsule was produced through a coaxial double orifice microfluidic technique and a water-in-oil emulsion system in two sequential processes. The phenol (Ph) substituted Gela (Gela-Ph) and CNT (CNT-Ph), respectively as cell-adhesive and electrically conductive substrates were incorporated in hyaluronic acid (HA)-based hydrogel through laccase-mediated crosslinking. The Cardiomyocyte-enclosing microcapsule fabricated and cellular survival, function, and possible difference in the biological activity of encapsulated cells within micro vehicles were investigated. The coaxial microfluidic method and Lac-mediated crosslinking reaction resulted in spherical vehicle production in 183 μm diameter at 500 capsules/min speed. The encapsulation process did not affect cellular viability and harvested cells from microcapsule proliferated well likewise subcultured cells in tissue culture plate. The biophysical properties of the designed hydrogel, including mechanical strength, swelling, biodegradability and electroconductivity upregulated significantly for hydrogels decorated covalently with Gela-Ph and CNT-Ph. The tendency of the microcapsule for the spheroid formation of cardiomyocytes inside the proposed microcapsule occurred 3 days after encapsulation. Interestingly, immobilized Gela-Ph and CNT-Ph promote cellular growth and specific cardiac markers. Overall, the microfluidic-based encapsulation technology and synthesized biomimetic substrates with electroconductive properties demonstrate desirable cellular adhesion, proliferation, and cardiac functions for engineering cardiac tissue.

Bone Marrow Niches of Hematopoietic Stem and Progenitor Cells

The mammalian hematopoietic system is remarkably efficient in meeting an organism’s vital needs, yet is highly sensitive and exquisitely regulated. Much of the organismal control over hematopoiesis comes from the regulation of hematopoietic stem cells (HSCs) by specific microenvironments called niches in bone marrow (BM), where HSCs reside. The experimental studies of the last two decades using the most sophisticated and advanced techniques have provided important data on the identity of the niche cells controlling HSCs functions and some mechanisms underlying niche-HSC interactions. In this review we discuss various aspects of organization and functioning of the HSC cell niche in bone marrow. In particular, we review the anatomy of BM niches, various cell types composing the niche, niches for more differentiated cells, metabolism of HSCs in relation to the niche, niche aging, leukemic transformation of the niche, and the current state of HSC niche modeling in vitro.

Natural Hydrogel-Based Bio-Inks for 3D Bioprinting in Tissue Engineering: A Review

Three-dimensional (3D) printing is well acknowledged to constitute an important technology in tissue engineering, largely due to the increasing global demand for organ replacement and tissue regeneration. In 3D bioprinting, which is a step ahead of 3D biomaterial printing, the ink employed is impregnated with cells, without compromising ink printability. This allows for immediate scaffold cellularization and generation of complex structures. The use of cell-laden inks or bio-inks provides the opportunity for enhanced cell differentiation for organ fabrication and regeneration. Recognizing the importance of such bio-inks, the current study comprehensively explores the state of the art of the utilization of bio-inks based on natural polymers (biopolymers), such as cellulose, agarose, alginate, decellularized matrix, in 3D bioprinting. Discussions regarding progress in bioprinting, techniques and approaches employed in the bioprinting of natural polymers, and limitations and prospects concerning future trends in human-scale tissue and organ fabrication are also presented.

3D Tissue-Engineered Vascular Drug Screening Platforms: Promise and Considerations

Despite the efforts devoted to drug discovery and development, the number of new drug approvals have been decreasing. Specifically, cardiovascular developments have been showing amongst the lowest levels of approvals. In addition, concerns over the adverse effects of drugs to the cardiovascular system have been increasing and resulting in failure at the preclinical level as well as withdrawal of drugs post-marketing. Besides factors such as the increased cost of clinical trials and increases in the requirements and the complexity of the regulatory processes, there is also a gap between the currently existing pre-clinical screening methods and the clinical studies in humans. This gap is mainly caused by the lack of complexity in the currently used 2D cell culture-based screening systems, which do not accurately reflect human physiological conditions. Cell-based drug screening is widely accepted and extensively used and can provide an initial indication of the drugs' therapeutic efficacy and potential cytotoxicity. However, in vitro cell-based evaluation could in many instances provide contradictory findings to the in vivo testing in animal models and clinical trials. This drawback is related to the failure of these 2D cell culture systems to recapitulate the human physiological microenvironment in which the cells reside. In the body, cells reside within a complex physiological setting, where they interact with and respond to neighboring cells, extracellular matrix, mechanical stress, blood shear stress, and many other factors. These factors in sum affect the cellular response and the specific pathways that regulate variable vital functions such as proliferation, apoptosis, and differentiation. Although pre-clinical in vivo animal models provide this level of complexity, cross species differences can also cause contradictory results from that seen when the drug enters clinical trials. Thus, there is a need to better mimic human physiological conditions in pre-clinical studies to improve the efficiency of drug screening. A novel approach is to develop 3D tissue engineered miniaturized constructs in vitro that are based on human cells. In this review, we discuss the factors that should be considered to produce a successful vascular construct that is derived from human cells and is both reliable and reproducible.

Applications of 3D Bioprinting Technology in Induced Pluripotent Stem Cells-Based Tissue Engineering

Induced pluripotent stem cells (iPSCs) are essentially produced by the genetic reprogramming of adult cells. Moreover, iPSC technology prevents the genetic manipulation of embryos. Hence, with the ensured element of safety, they rarely cause ethical concerns when utilized in tissue engineering. Several cumulative outcomes have demonstrated the functional superiority and potency of iPSCs in advanced regenerative medicine. Recently, an emerging trend in 3D bioprinting technology has been a more comprehensive approach to iPSC-based tissue engineering. The principal aim of this review is to provide an understanding of the applications of 3D bioprinting in iPSC-based tissue engineering. This review discusses the generation of iPSCs based on their distinct purpose, divided into two categories: (1) undifferentiated iPSCs applied with 3D bioprinting; (2) differentiated iPSCs applied with 3D bioprinting. Their significant potential is analyzed. Lastly, various applications for engineering tissues and organs have been introduced and discussed in detail.

Use of electroconductive biomaterials for engineering tissues by 3D printing and 3D bioprinting

Existing methods of engineering alternatives to restore or replace damaged or lost tissues are not satisfactory due to the lack of suitable constructs that can fit precisely, function properly and integrate into host tissues. Recently, three-dimensional (3D) bioprinting approaches have been developed to enable the fabrication of pre-programmed synthetic tissue constructs that have precise geometries and controlled cellular composition and spatial distribution. New bioinks with electroconductive properties have the potential to influence cellular fates and function for directed healing of different tissue types including bone, heart and nervous tissue with the possibility of improved outcomes. In the present paper, we review the use of electroconductive biomaterials for the engineering of tissues via 3D printing and 3D bioprinting. Despite significant advances, there remain challenges to effective tissue replacement and we address these challenges and describe new approaches to advanced tissue engineering.