Three-dimensional bioprinting of gelatin methacryloyl (GelMA)

An overview of nasal cartilage bioprinting: from bench to bedside

Nasal cartilage diseases and injuries are known as significant challenges in reconstructive medicine, affecting a substantial number of individuals worldwide. In recent years, the advent of three-dimensional (3D) bioprinting has emerged as a promising approach for nasal cartilage reconstruction, offering potential breakthroughs in the field of regenerative medicine. This paper provides an overview of the methods and challenges associated with 3D bioprinting technologies in the procedure of reconstructing nasal cartilage tissue. The process of 3D bioprinting entails generating a digital 3D model using biomedical imaging techniques and computer-aided design to integrate both internal and external scaffold features. Then, bioinks which consist of biomaterials, cell types, and bioactive chemicals, are applied to facilitate the precise layer-by-layer bioprinting of tissue-engineered scaffolds. After undergoing in vitro and in vivo experiments, this process results in the development of the physiologically functional integrity of the tissue. The advantages of 3D bioprinting encompass the ability to customize scaffold design, enabling the precise incorporation of pore shape, size, and porosity, as well as the utilization of patient-specific cells to enhance compatibility. However, various challenges should be considered, including the optimization of biomaterials, ensuring adequate cell viability and differentiation, achieving seamless integration with the host tissue, and navigating regulatory attention. Although numerous studies have demonstrated the potential of 3D bioprinting in the rebuilding of such soft tissues, this paper covers various aspects of the bioprinted tissues to provide insights for the future development of repair techniques appropriate for clinical use.

Enhancing bone regeneration and immunomodulation via gelatin methacryloyl hydrogel-encapsulated exosomes from osteogenic pre-differentiated mesenchymal stem cells

Mesenchymal stem cell-derived exosomes (MSC-Exos) have emerged as promising candidates for cell-free therapy in tissue regeneration. However, the native osteogenic and angiogenic capacities of MSC-Exos are often insufficient to repair critical-sized bone defects, and the underlying immune mechanisms remain elusive. Furthermore, achieving sustained delivery and stable activity of MSC-Exos at the defect site is essential for optimal therapeutic outcomes. Here, we extracted exosomes from osteogenically pre-differentiated human bone marrow mesenchymal stem cells (hBMSCs) by ultracentrifugation and encapsulated them in gelatin methacryloyl (GelMA) hydrogel to construct a composite scaffold. The resulting exosome-encapsulated hydrogel exhibited excellent mechanical properties and biocompatibility, facilitating sustained delivery of MSC-Exos. Osteogenic pre-differentiation significantly enhanced the osteogenic and angiogenic properties of MSC-Exos, promoting osteogenic differentiation of hBMSCs and angiogenesis of human umbilical vein endothelial cells (HUVECs). Furthermore, MSC-Exos induced polarization of Raw264.7 cells from a pro-inflammatory phenotype to an anti-inflammatory phenotype under simulated inflammatory conditions, thereby creating an immune microenvironment conducive to osteogenesis. RNA sequencing and bioinformatics analysis revealed that MSC-Exos activate the p53 pathway through targeted delivery of internal microRNAs and regulate macrophage polarization by reducing DNA oxidative damage. Our study highlights the potential of osteogenic exosome-encapsulated composite hydrogels for the development of cell-free scaffolds in bone tissue engineering.

Collagenase motors in gelatine-based hydrogels

Nano/micromotors outperform Brownian motion due to their self-propulsive capabilities and hold promise as carriers for drug delivery across biological barriers such as the extracellular matrix. This study employs poly(2-(diethylamino)ethyl methacrylate) polymer brushes to enhance the collagenase-loading capacity of silica particle-based motors with the aim to systematically investigate the impact of gelatine viscosity, motors' size, and morphology on their propulsion velocity. Notably, 500 nm and 1 μm motors achieve similar speeds as high as ∼15 μm s-1 in stiff gelatine-based hydrogels when triggered with calcium. Taken together, our findings highlight the potential of collagenase-based motors for navigating the extracellular matrix, positioning them as promising candidates for efficient drug delivery.

Nanofibrous polyelectrolyte complex incorporated BSA-alginate composite bioink for 3D bioprinting of bone mimicking constructs

Although several bioinks have been developed for 3D bioprinting applications, the lack of optimal printability, mechanical properties, and adequate cell response has limited their practical applicability. Therefore, this work reports the development of a composite bioink consisting of bovine serum albumin (BSA), alginate, and self-assembled nanofibrous polyelectrolyte complex aggregates of gelatin and chitosan (PEC-GC). The nanofibrous PEC-GC aggregates were prepared and incorporated into the bioink in varying concentrations (0 % to 3 %). The bioink samples were bioprinted and crosslinked post-printing by calcium chloride. The average nanofiber diameter of PEC-GC was 62 ± 15 nm. It was demonstrated that PEC-GC improves the printability and cellular adhesion of the developed bioink and modulates the swelling ratio, degradation rate, and mechanical properties of the fabricated scaffold. The in vitro results revealed that the bioink with 2 % PEC-GC had the best post-printing cell viability of the encapsulated MG63 osteosarcoma cells and well oragnized stress fibers, indicating enhanced cell adhesion. The cell viability was >90 %, as observed from the MTT assay. The composite bioink also showed osteogenic potential, as confirmed by the estimation of alkaline phosphatase activity and collagen synthesis assay. This study successfully fabricated a high-shape fidelity bioink with potential in bone tissue engineering.

Bioprinting of Perfusable, Biocompatible Vessel-like Channels with dECM-Based Bioinks and Living Cells

There is a growing interest in the production of bioinks that on the one hand, are biocompatible and, on the other hand, have mechanical properties that allow for the production of stable constructs that can survive for a long time after transplantation. While the selection of the right material is crucial for bioprinting, there is another equally important issue that is currently being extensively researched-the incorporation of the vascular system into the fabricated scaffolds. Therefore, in the following manuscript, we present the results of research on bioink with unique physico-chemical and biological properties. In this article, two methods of seeding cells were tested using bioink B and seeding after bioprinting the whole model. After 2, 5, 8, or 24 h of incubation, the flow medium was used in the tested systems. At the end of the experimental trial, for each time variant, the canals were stored in formaldehyde, and immunohistochemical staining was performed to examine the presence of cells on the canal walls and roof. Cells adhered to both ways of fiber arrangement; however, a parallel bioprint with the 5 h incubation and the intermediate plating of cells resulted in better adhesion efficiency. For this test variant, the percentage of cells that adhered was at least 20% higher than in the other analyzed variants. In addition, it was for this variant that the lowest percentage of viable cells was found that were washed out of the tested model. Importantly, hematoxylin and eosin staining showed that after 8 days of culture, the cells were evenly distributed throughout the canal roof. Our study clearly shows that neovascularization-promoting cells effectively adhere to ECM-based pancreatic bioink. Summarizing the presented results, it was demonstrated that the proposed bioink compositions can be used for bioprinting bionic organs with a vascular system formed by endothelial cells and fibroblasts.

Leveraging the Recent Advancements in GelMA Scaffolds for Bone Tissue Engineering: An Assessment of Challenges and Opportunities

The field of bone tissue engineering has seen significant advancements in recent years. Each year, over two million bone transplants are performed globally, and conventional treatments, such as bone grafts and metallic implants, have their limitations. Tissue engineering offers a new level of treatment, allowing for the creation of living tissue within a biomaterial framework. Recent advances in biomaterials have provided innovative approaches to rebuilding bone tissue function after damage. Among them, gelatin methacryloyl (GelMA) hydrogel is emerging as a promising biomaterial for supporting cell proliferation and tissue regeneration, and GelMA has exhibited exceptional physicochemical and biological properties, making it a viable option for clinical translation. Various methods and classes of additives have been used in the application of GelMA for bone regeneration, with the incorporation of nanofillers or other polymers enhancing its resilience and functional performance. Despite promising results, the fabrication of complex structures that mimic the bone architecture and the provision of balanced physical properties for both cell and vasculature growth and proper stiffness for load bearing remain as challenges. In terms of utilizing osteogenic additives, the priority should be on versatile components that promote angiogenesis and osteogenesis while reinforcing the structure for bone tissue engineering applications. This review focuses on recent efforts and advantages of GelMA-based composite biomaterials for bone tissue engineering, covering the literature from the last five years.

Advances in 3D bioprinting for regenerative medicine applications

Biofabrication techniques allow for the construction of biocompatible and biofunctional structures composed from biomaterials, cells and biomolecules. Bioprinting is an emerging 3D printing method which utilizes biomaterial-based mixtures with cells and other biological constituents into printable suspensions known as bioinks. Coupled with automated design protocols and based on different modes for droplet deposition, 3D bioprinters are able to fabricate hydrogel-based objects with specific architecture and geometrical properties, providing the necessary environment that promotes cell growth and directs cell differentiation towards application-related lineages. For the preparation of such bioinks, various water-soluble biomaterials have been employed, including natural and synthetic biopolymers, and inorganic materials. Bioprinted constructs are considered to be one of the most promising avenues in regenerative medicine due to their native organ biomimicry. For a successful application, the bioprinted constructs should meet particular criteria such as optimal biological response, mechanical properties similar to the target tissue, high levels of reproducibility and printing fidelity, but also increased upscaling capability. In this review, we highlight the most recent advances in bioprinting, focusing on the regeneration of various tissues including bone, cartilage, cardiovascular, neural, skin and other organs such as liver, kidney, pancreas and lungs. We discuss the rapidly developing co-culture bioprinting systems used to resemble the complexity of tissues and organs and the crosstalk between various cell populations towards regeneration. Moreover, we report on the basic physical principles governing 3D bioprinting, and the ideal bioink properties based on the biomaterials' regenerative potential. We examine and critically discuss the present status of 3D bioprinting regarding its applicability and current limitations that need to be overcome to establish it at the forefront of artificial organ production and transplantation.

Investigation of the temperature gradient control in the printing space for the material extrusion of medical biodegradable hydrogel

Material extrusion has shown promise in the fabrication of biocompatible scaffolds for tissue engineering using medical biodegradable hydrogel materials. However, the uncontrollable shape of prepared 3D architecture decelerates the development of large-size complex hydrogel models for the fabrication of human-scale tissue or organs. A primary cause of the collapse as well as shrinkage of prepared architectures is the uncontrollable ambient temperature distribution during the extruding process for hydrogel materials. Therefore, there is a need to accurately control the temperature gradient in the printing space during the material extrusion. The study proposed a novel temperature-controlled substrate configuration with a multilayered enclosure, by which the temperature gradient in the printing space can be regulated by varying the height as well as the internal diameter of the enclosure. Subsequently, a finite element simulation model, as well as a self-developed temperature measuring device, was established to numerically and experimentally investigate the temperature distribution in the printing space. Furthermore, printing trials were implemented on the novel substrate. The collapse of 3D architectures was successfully avoided, and the height of scaffolds was improved obviously from 2.21 mm to 13.24 mm.

An Adhesive Bioink toward Biofabrication under Wet Conditions

Three-dimensional (3D) bioprinting is driving significant innovations in biomedicine over recent years. Under certain scenarios such as in intraoperative bioprinting, the bioinks used should exhibit not only cyto/biocompatibility but also adhesiveness in wet conditions. Herein, an adhesive bioink composed of gelatin methacryloyl, gelatin, methacrylated hyaluronic acid, and skin secretion of Andrias davidianus is designed. The bioink exhibits favorable cohesion to allow faithful extrusion bioprinting in wet conditions, while simultaneously showing good adhesion to a variety of surfaces of different chemical properties, possibly achieved through the diverse bonds presented in the bioink formulation. As such, this bioink is able to fabricate sophisticated planar and volumetric constructs using extrusion bioprinting, where the dexterity is further enhanced using ergonomic handheld bioprinters to realize in situ bioprinting. In vitro experiments reveal that cells maintain high viability; further in vivo studies demonstrate good integration and immediate injury sealing. The characteristics of the bioink indicate its potential widespread utility in extrusion bioprinting and will likely broaden the applications of bioprinting toward situations such as in situ dressing and minimally invasive tissue regeneration.

Mending a broken heart by biomimetic 3D printed natural biomaterial-based cardiac patches: a review

Myocardial infarction is one of the major causes of mortality as well as morbidity around the world. Currently available treatment options face a number of drawbacks, hence cardiac tissue engineering, which aims to bioengineer functional cardiac tissue, for application in tissue repair, patient specific drug screening and disease modeling, is being explored as a viable alternative. To achieve this, an appropriate combination of cells, biomimetic scaffolds mimicking the structure and function of the native tissue, and signals, is necessary. Among scaffold fabrication techniques, three-dimensional printing, which is an additive manufacturing technique that enables to translate computer-aided designs into 3D objects, has emerged as a promising technique to develop cardiac patches with a highly defined architecture. As a further step toward the replication of complex tissues, such as cardiac tissue, more recently 3D bioprinting has emerged as a cutting-edge technology to print not only biomaterials, but also multiple cell types simultaneously. In terms of bioinks, biomaterials isolated from natural sources are advantageous, as they can provide exceptional biocompatibility and bioactivity, thus promoting desired cell responses. An ideal biomimetic cardiac patch should incorporate additional functional properties, which can be achieved by means of appropriate functionalization strategies. These are essential to replicate the native tissue, such as the release of biochemical signals, immunomodulatory properties, conductivity, enhanced vascularization and shape memory effects. The aim of the review is to present an overview of the current state of the art regarding the development of biomimetic 3D printed natural biomaterial-based cardiac patches, describing the 3D printing fabrication methods, the natural-biomaterial based bioinks, the functionalization strategies, as well as the in vitro and in vivo applications.

The role of microfluidics and 3D-bioprinting in the future of exosome therapy

Exosome-based strategies constitute a promising tool for therapeutics, avoiding potential immunogenic and tumorigenic side-effects of cell therapies. However, the collection of a suitable exosome pool, and the need for high doses with conventional administration approaches, hamper their clinical translation. To overcome these challenges, versatile exosome collection strategies together with advanced delivery platforms may represent major progress in this field. Microfluidics enables large-scale gathering of both natural and synthetic exosomes for their implementation into bioinks, while 3D-bioprinting holds great promise in regenerative medicine with the use of exosome-loaded scaffolds that mimic the target tissue with controlled pharmacokinetics and pharmacodynamics. Hence, the combination of both strategies might become the key for the translation of exosome therapies to clinical practice.

A 3D-Printed Dual Driving Forces Scaffold with Self-Promoted Cell Absorption for Spinal Cord Injury Repair

Stem cells play critical roles in cell therapies and tissue engineering for nerve repair. However, achieving effective delivery of high cell density remains a challenge. Here, a novel cell delivery platform termed the hyper expansion scaffold (HES) is developed to enable high cell loading. HES facilitated self-promoted and efficient cell absorption via a dual driving force model. In vitro tests revealed that the HES rapidly expanded 80-fold in size upon absorbing 2.6 million human amniotic epithelial stem cells (hAESCs) within 2 min, representing over a 400% increase in loading capacity versus controls. This enhanced uptake benefited from macroscopic swelling forces as well as microscale capillary action. In spinal cord injury (SCI) rats, HES-hAESCs promoted functional recovery and axonal projection by reducing neuroinflammation and improving the neurotrophic microenvironment surrounding the lesions. In summary, the dual driving forces model provides a new rationale for engineering hydrogel scaffolds to facilitate self-promoted cell absorption. The HES platform demonstrates great potential as a powerful and efficient vehicle for delivering high densities of hAESCs to promote clinical treatment and repair of SCI.

A Biomimetic Adhesive and Robust Janus Patch with Anti-Oxidative, Anti-Inflammatory, and Anti-Bacterial Activities for Tendon Repair

Early stage oxidative stress, inflammatory response, and infection after tendon surgery are highly associated with the subsequent peritendinous adhesion formation, which may diminish the quality and function of the repaired tendon. Although various anti-inflammatory and/or antibacterial grafts have been proposed to turn the scale, most of them suffer from the uncertainty of drug-induced adverse effects, low mechanical strength, and tissue adhesiveness. Here, inspired by the tendon anatomy and pathophysiology of adhesion development, an adhesive and robust dual-layer Janus patch is developed, whose inner layer facing the operated tendon is a multifunctional electrospun hydrogel patch (MEHP), encircled further by a poly-l-lactic acid (PLLA) fibrous outer layer facing the surrounding tissue. Specifically, MEHP is prepared by gelatin methacryloyl (GelMA) and zinc oxide (ZnO) nanoparticles, which are co-electrospun first and then treated by tannic acid (TA). The inner MEHP exhibits superior mechanical performance, adhesion strength, and outstanding antioxidation, anti-inflammation, and antibacterial properties, and it can adhere to the injury site offering a favorable microenvironment for tendon regeneration. Meanwhile, the outer PLLA acts as a physical barrier that prevents extrinsic cells and tissues from invading the defect site, reducing peritendinous adhesion formation. This work presents a proof-of-concept of a drug-free graft with anisotropic adhesive and biological functions to concert the healing phases of injured tendon by alleviating incipient inflammation and oxidative damage but supporting tissue regeneration and reducing tendon adhesion in the later phase of repair and remodeling. It is envisioned that this Janus patch could offer a promising strategy for safe and efficient tendon therapy.

Construction of 3D-Bioprinted cartilage-mimicking substitute based on photo-crosslinkable Wharton's jelly bioinks for full-thickness articular cartilage defect repair

Three-dimensional (3D) bioprinted cartilage-mimicking substitutes for full-thickness articular cartilage defect repair have emerged as alternatives to in situ defect repair models. However, there has been very limited breakthrough in cartilage regeneration based on 3D bioprinting owing to the lack of ideal bioinks with printability, biocompatibility, bioactivity, and suitable physicochemical properties. In contrast to animal-derived natural polymers or acellular matrices, human-derived Wharton's jelly is biocompatible and hypoimmunogenic with an abundant source. Although acellular Wharton's jelly can mimic the chondrogenic microenvironment, it remains challenging to prepare both printable and biologically active bioinks from this material. Here, we firstly prepared methacryloyl-modified acellular Wharton's jelly (AWJMA) using a previously established photo-crosslinking strategy. Subsequently, we combined methacryloyl-modified gelatin with AWJMA to obtain a hybrid hydrogel that exhibited both physicochemical properties and biological activities that were suitable for 3D bioprinting. Moreover, bone marrow mesenchymal stem cell-loaded 3D-bioprinted cartilage-mimicking substitutes had superior advantages for the survival, proliferation, spreading, and chondrogenic differentiation of bone marrow mesenchymal stem cells, which enabled satisfactory repair of a model of full-thickness articular cartilage defect in the rabbit knee joint. The current study provides a novel strategy based on 3D bioprinting of cartilage-mimicking substitutes for full-thickness articular cartilage defect repair.

In vitro high-content tissue models to address precision medicine challenges

The field of precision medicine allows for tailor-made treatments specific to a patient and thereby improve the efficiency and accuracy of disease prevention, diagnosis, and treatment and at the same time would reduce the cost, redundant treatment, and side effects of current treatments. Here, the combination of organ-on-a-chip and bioprinting into engineering high-content in vitro tissue models is envisioned to address some precision medicine challenges. This strategy could be employed to tackle the current coronavirus disease 2019 (COVID-19), which has made a significant impact and paradigm shift in our society. Nevertheless, despite that vaccines against COVID-19 have been successfully developed and vaccination programs are already being deployed worldwide, it will likely require some time before it is available to everyone. Furthermore, there are still some uncertainties and lack of a full understanding of the virus as demonstrated in the high number new mutations arising worldwide and reinfections of already vaccinated individuals. To this end, efficient diagnostic tools and treatments are still urgently needed. In this context, the convergence of bioprinting and organ-on-a-chip technologies, either used alone or in combination, could possibly function as a prominent tool in addressing the current pandemic. This could enable facile advances of important tools, diagnostics, and better physiologically representative in vitro models specific to individuals allowing for faster and more accurate screening of therapeutics evaluating their efficacy and toxicity. This review will cover such technological advances and highlight what is needed for the field to mature for tackling the various needs for current and future pandemics as well as their relevancy towards precision medicine.

3D Printing Approaches to Engineer Cardiac Tissue

PURPOSE OF REVIEW

Bioengineering of functional cardiac tissue composed of primary cardiomyocytes has great potential for myocardial regeneration and in vitro tissue modeling. 3D bioprinting was developed to create cardiac tissue in hydrogels that can mimic the structural, physiological, and functional features of native myocardium. Through a detailed review of the 3D printing technologies and bioink materials used in the creation of a heart tissue, this article discusses the potential of engineered heart tissues in biomedical applications.

RECENT FINDINGS

In this review, we discussed the recent progress in 3D bioprinting strategies for cardiac tissue engineering, including bioink and 3D bioprinting methods as well as examples of engineered cardiac tissue such as in vitro cardiac models and vascular channels. 3D printing is a powerful tool for creating in vitro cardiac tissues that are structurally and functionally similar to real tissues. The use of human-induced pluripotent stem cell-derived cardiomyocytes (iPSC-CM) enables the generation of patient-specific tissues. These tissues have the potential to be used for regenerative therapies, disease modeling, and drug testing.

Advancements in Research on Constructing Physiological and Pathological Liver Models and Their Applications Utilizing Bioprinting Technology

In recent decades, significant progress has been made in liver tissue engineering through the use of 3D bioprinting technology. This technology offers the ability to create personalized biological structures with precise geometric design capabilities. The complex and multifaceted nature of liver diseases underscores the need for advanced technologies to accurately mimic the physiological and mechanical characteristics, as well as organ-level functions, of liver tissue in vitro. Bioprinting stands out as a superior option over traditional two-dimensional cell culture models and animal models due to its stronger biomimetic advantages. Through the use of bioprinting, it is possible to create liver tissue with a level of structural and functional complexity that more closely resembles the real organ, allowing for more accurate disease modeling and drug testing. As a result, it is a promising tool for restoring and replacing damaged tissue and organs in the field of liver tissue engineering and drug research. This article aims to present a comprehensive overview of the progress made in liver tissue engineering using bioprinting technology to provide valuable insights for researchers. The paper provides a detailed account of the history of liver tissue engineering, highlights the current 3D bioprinting methods and bioinks that are widely used, and accentuates the importance of existing in vitro liver tissue models based on 3D bioprinting and their biomedical applications. Additionally, the article explores the challenges faced by 3D bioprinting and predicts future trends in the field. The progress of 3D bioprinting technology is poised to bring new approaches to printing liver tissue in vitro, while offering powerful tools for drug development, testing, liver disease modeling, transplantation, and regeneration, which hold great academic and practical significance.

Current Strategies for Engineered Vascular Grafts and Vascularized Tissue Engineering

Blood vessels not only transport oxygen and nutrients to each organ, but also play an important role in the regulation of tissue regeneration. Impaired or occluded vessels can result in ischemia, tissue necrosis, or even life-threatening events. Bioengineered vascular grafts have become a promising alternative treatment for damaged or occlusive vessels. Large-scale tubular grafts, which can match arteries, arterioles, and venules, as well as meso- and microscale vasculature to alleviate ischemia or prevascularized engineered tissues, have been developed. In this review, materials and techniques for engineering tubular scaffolds and vasculature at all levels are discussed. Examples of vascularized tissue engineering in bone, peripheral nerves, and the heart are also provided. Finally, the current challenges are discussed and the perspectives on future developments in biofunctional engineered vessels are delineated.

Gelatin methacrylate hydrogel with drug-loaded polymer microspheres as a new bioink for 3D bioprinting

3D bioprinted hydrogel constructs are advanced systems of a great drug delivery application potential. One of the bioinks that has recently gained a lot of attention is gelatin methacrylate (GelMA) hydrogel exhibiting specific properties, including UV cross-linking possibility. The present study aimed to develop a new bioink composed of GelMA and gelatin modified by addition of polymer (polycaprolactone or polyethersulfone) microspheres serving as bioactive substance carriers. The prepared microspheres suspension in GelMA/gelatin bioink was successfully bioprinted and subjected to various tests, which showed that the addition of microspheres and their type affects the physicochemical properties of the printouts. The hydrogel stability and structure was examined using scanning electron and optical microscopy, its thermal properties with differential scanning calorimetry and thermogravimetric analysis and its biocompatibility on HaCaT cells using viability assay and electron microscopy. Analyses also included tests of hydrogel equilibrium swelling ratio and release of marker substance. Subsequently, the matrices were loaded with ampicillin and the antibiotic release was validated by monitoring the antibacterial activity on Staphylococcus aureus and Escherichia coli. It was concluded that GelMA/gelatin bioink is a good and satisfying material for potential medical use. Depending on the polymer used, the addition of microspheres improves its structure, thermal and drug delivery properties.

Tailoring Hydrogel Composition and Stiffness to Control Smooth Muscle Cell Differentiation in Bioprinted Constructs

BACKGROUND

Reliable in vitro cellular models are needed to study the phenotypic modulation of smooth muscle cells (SMCs) in health and disease. The aim of this study was to optimize gelatin methacrylate (GelMA)/alginate hydrogels for bioprinting three-dimensional (3D) SMC constructs.

METHODS

Four different hydrogel groups were prepared by mixing different concentrations (% w/v) of GelMA and alginate: G1 (5/1.5), G2 (5/3), G3 (7.5/1.5), and G4 (7.5/3). GelMA 10% was used as control (G5). A circular structure containing human bladder SMCs was fabricated by using an extrusion-based bioprinter. The effects of the mixing ratios on printability, viability, proliferation, and differentiation of the cells were investigated.

RESULTS

Rheological analysis showed that the addition of alginate significantly stabilized the change in mechanical properties with temperature variations. The group with the highest GelMA and alginate concentrations (G4) exhibited the highest viscosity, resulting in better stability of the 3D construct after crosslinking. Compared to other hydrogel compositions, cells in G4 maintained high viability (> 80%), exhibited spindle-shaped morphology, and showed a significantly higher proliferation rate within an 8-day period. More importantly, G4 provided an optimal environment for the induction of a SMC contractile phenotype, as evidenced by significant changes in the expression of marker proteins and morphological parameters.

CONCLUSION

Adjusting the composition of GelMA/alginate hydrogels is an effective means of controlling the SMC phenotype. These hydrogels support bioprinting of 3D models to study phenotypic smooth muscle adaptation, with the prospect of using the constructs in the study of therapies for the treatment of urethral strictures.

A vertical additive-lathe printing system for the fabrication of tubular constructs using gelatin methacryloyl hydrogel

Reproducing both the mechanical and biological performance of native blood vessels remains an ongoing challenge in vascular tissue engineering. Additive-lathe printing offers an attractive method of fabricating long tubular constructs as a potential vascular graft for the treatment of cardiovascular diseases. Printing hydrogels onto rotating horizontal mandrels often leads to sagging, resulting in poor and variable mechanical properties. In this study, an additive-lathe printing system with a vertical mandrel to fabricate tubular constructs is presented. Various concentrations of gelatin methacryloyl (gelMA) hydrogel were used to print grafts on the rotating mandrel in a helical pattern. The printing parameters were selected to achieve the bonding of consecutive gelMA filaments to improve the quality of the printed graft. The hydrogel filaments were fused properly under the action of gravity on the vertical mandrel. Thus, the vertical additive-lathe printing system was used to print uniform wall thickness grafts, eliminating the hydrogel sagging problem. Tensile testing performed in both circumferential and longitudinal direction revealed that the anisotropic properties of printed gelMA constructs were similar to those observed in the native blood vessels. In addition, no leakage was detected through the walls of the gelMA grafts during burst pressure measurement. Therefore, the current printing setup could be utilized to print vascular grafts for the treatment of cardiovascular diseases.

The Next Frontier of 3D Bioprinting: Bioactive Materials Functionalized by Bacteria

3D bioprinting has become a flexible technical means used in many fields. Currently, research on 3D bioprinting is mainly focused on the use of mammalian cells to print organ and tissue models, which has greatly promoted progress in the fields of tissue engineering, regenerative medicine, and pharmaceuticals. In recent years, bacterial bioprinting has gradually become a rapidly developing research fields, with a wide range of potential applications in basic research, biomedicine, bioremediation, and other field. Here, this works reviews new research on bacterial bioprinting, and discuss its future research direction.

Tomato based gelatin methacryloyl hydrogel as an effective natural and low-cost scaffold for accelerative wound healing

Oxidative stress and infection are the main reasons for postponement of wound healing rate. They can potentially lead to serious inflammation and eventually lead to a longer and more painful recovery phase. Although wound dressings based on synthetic materials with antioxidative property have been proved to exhibit remarkable effect in controlling ROS level and improving wound healing, issues, such as high cost in raw materials, complicated procedures, usage of various toxic additives, and potential allergies, have significantly confined further clinical applications. In this study, a novel type of tissue engineering scaffold, based on tomatoes (Solanum lycopersicon) and gelatin methacryloyl (GelMA), was prepared via facile lyophilization and photo cross-link method (SL/GelMA). By taking advantages of various antioxidative components, such as carotenoids, flavonoids, phenolic acids, vitamin E, and vitamin C in tomatoes, SL/GelMA can effectively regulate ROS level, relieve the oxidative stress in wound bed, promote cell migration and angiogenesis, contribute to collagen deposition, and thus accelerate the rate of wound enclosure. Along with its high biocompatibility and low allergic potential, we believe that the food-derived wound dressing with facile preparation method, easy accessibility, and high cost-effectiveness can be translated for clinical treatments of various chronic wounds.

3D bioprinting and the revolution in experimental cancer model systems-A review of developing new models and experiences with in vitro 3D bioprinted breast cancer tissue-mimetic structures

Growing evidence propagates those alternative technologies (relevant human cell-based-e.g., organ-on-chips or biofabricated models-or artificial intelligence-combined technologies) that could help in vitro test and predict human response and toxicity in medical research more accurately. In vitro disease model developments have great efforts to create and serve the need of reducing and replacing animal experiments and establishing human cell-based in vitro test systems for research use, innovations, and drug tests. We need human cell-based test systems for disease models and experimental cancer research; therefore, in vitro three-dimensional (3D) models have a renaissance, and the rediscovery and development of these technologies are growing ever faster. This recent paper summarises the early history of cell biology/cellular pathology, cell-, tissue culturing, and cancer research models. In addition, we highlight the results of the increasing use of 3D model systems and the 3D bioprinted/biofabricated model developments. Moreover, we present our newly established 3D bioprinted luminal B type breast cancer model system, and the advantages of in vitro 3D models, especially the bioprinted ones. Based on our results and the reviewed developments of in vitro breast cancer models, the heterogeneity and the real in vivo situation of cancer tissues can be represented better by using 3D bioprinted, biofabricated models. However, standardising the 3D bioprinting methods is necessary for future applications in different high-throughput drug tests and patient-derived tumour models. Applying these standardised new models can lead to the point that cancer drug developments will be more successful, efficient, and consequently cost-effective in the near future.

Stereolithography apparatus and digital light processing-based 3D bioprinting for tissue fabrication

Three-dimensional (3D) bioprinting has emerged as a class of promising techniques in biomedical research for a wide range of related applications. Specifically, stereolithography apparatus (SLA) and digital light processing (DLP)-based vat-polymerization techniques are highly effective methods of bioprinting, which can be used to produce high-resolution and architecturally sophisticated structures. Our review aims to provide an overview of SLA- and DLP-based 3D bioprinting strategies, starting from factors that affect these bioprinting processes. In addition, we summarize the advances in bioinks used in SLA and DLP, including naturally derived and synthetic bioinks. Finally, the biomedical applications of both SLA- and DLP-based bioprinting are discussed, primarily centered on regenerative medicine and tissue modeling engineering.

Fabrication of a Gelatin-Based Microdevice for Vascular Cell Culture

This study presents a novel technique for fabricating microfluidic devices with microbial transglutaminase-gelatin gels instead of polydimethylsiloxane (PDMS), in which flow culture simulates blood flow and a capillary network is incorporated for assays of vascular permeability or angiogenesis. We developed a gelatin-based device with a coverslip as the bottom, which allows the use of high-magnification lenses with short working distances, and we observed the differences in cell dynamics on gelatin, glass, and PDMS surfaces. The tubes of the gelatin microfluidic channel are designed to be difficult to pull out of the inlet hole, making sample introduction easy, and the gelatin channel can be manipulated from the cell introduction to the flow culture steps in a manner comparable to that of a typical PDMS channel. Human umbilical vein endothelial cells (HUVECs) and normal human dermal fibroblasts (NHDFs) were successfully co-cultured, resulting in structures that mimicked blood vessels with inner diameters ranging from 10 µm to 500 µm. Immunostaining and scanning electron microscopy results showed that the affinity of fibronectin for gelatin was stronger than that for glass or PDMS, making gelatin a suitable substrate for cell adhesion. The ability for microscopic observation at high magnification and the ease of sample introduction make this device easier to use than conventional gelatin microfluidics, and the above-mentioned small modifications in the device structure are important points that improve its convenience as a cell assay device.

Cell Infiltrative Inner Connected Porous Hydrogel Improves Neural Stem Cell Migration and Differentiation for Functional Repair of Spinal Cord Injury

The disadvantages of cell-adaptive microenvironments and cellular diffusion out of the lesion have limited hydrogel-based scaffold transplantation treatment for neural connectivity, leading to permanent neurological disability from spinal cord injury. Herein, porous GelMA scaffold was prepared, in which the inner porous structure was optimized. The average pore size was 168 ± 71 μm with a porosity of 77.1%. The modulus of porous hydrogel was 593 ± 4 Pa compared to 1535 ± 85 Pa of bulk GelMA. The inner connected porous structure provided a cell-infiltrative matrix for neural stem cell migration and differentiation in vitro and eventually enhanced neuron differentiation and hindlimb strength and movement of animals in in vivo experiments. Furthermore, inflammation response and apoptosis were also alleviated after implantation. This work demonstrated that the porous hydrogel with appropriately connected micropores exhibit favorable cellular responses compared with traditional non-porous GelMA hydrogel. Taken together, our findings suggest that porous hydrogel is a promising scaffold for future delivery of stem cells and has prospects in material design for the treatment of spinal cord injury.

Knowledge domain and hotspots concerning photosensitive hydrogels for tissue engineering applications: A bibliometric and visualized analysis (1996-2022)

Objective: The aim of tissue engineering (TE) is to replace the damaged tissues or failed organs, or restore their missing functions. The important means to achieve this aim is to integrate biomaterials and life elements. Hydrogels are very attractive biomaterials in the field of TE. In particular, engineering extracellular matrices (ECMs) formed by photosensitive hydrogels have captivated much attention, because photopolymerization has many advantages over traditional polymerization approaches, such as rapidity of reaction, spatiotemporal controllability of polymerization process, and operability at physiological temperature, especially it can realize the fabrications of engineering ECMs in the presence of living cells. There have been many excellent reviews on the applications of photosensitive hydrogels in TE in recent years, however, it is inevitable that researchers may have left out many important facts due to exploring the literature from one or a few aspects. It is also a great challenge for researchers to explore the internal relationships among countries, institutions, authors, and references from a large number of literatures in related fields. Therefore, bibliometrics may be a powerful tool to solve the above problems. A bibliometric and visualized analysis of publications concerning the photosensitive hydrogels for TE applications was performed, and the knowledge domain, research hotspots and frontiers in this topic were identified according to the analysis results.

Methods: We identified and retrieved the publications regarding the photosensitive hydrogels for TE applications between 1996 and 2022 from Web of Science Core Collection (WoSCC). Bibliometric and visualized analysis employing CiteSpace software and R-language package Bibliometrix were performed in this study.

Results: 778 publications meeting the eligibility criteria were identified and retrieved from WoSCC. Among those, 2844 authors worldwide participated in the studies in this field, accompanied by an average annual article growth rate of 15.35%. The articles were co-authored by 800 institutions from 46 countries/regions, and the United States published the most, followed by China and South Korea. As the two countries that published the most papers, the United States and China could further strengthen cooperation in this field. Univ Colorado published the most articles (n = 150), accounting for 19.28% of the total. The articles were distributed in 112 journals, among which Biomaterials (n = 66) published the most articles, followed by Acta Biomaterialia (n = 54) and Journal of Biomedical Materials Research Part A (n = 42). The top 10 journals published 47.8% of the 778 articles. The most prolific author was Anseth K (n = 33), followed by Khademhosseini A (n = 29) and Bryant S (n = 22). A total of 1443 keywords were extracted from the 778 articles and the keyword with the highest centrality was “extracellular matrix” (centrality: 0.12). The keywords appeared recently with strong citation bursts were “gelatin”, “3d printing” and “3d bioprinting”, representing the current research hotspots in this field. “Gelma”, “3d printing” and “thiol-ene” were the research frontiers in recent years.

Conclusion: This bibliometric and visualized study offered a comprehensive understanding of publications regarding the photosensitive hydrogels for TE applications from 1996 to 2022, including the knowledge domain, research hotspots and frontiers in this filed. The outcome of this study would provide insights for scholars in the related research filed.

Formulation and Characterization of Gelatin Methacrylamide – Hydroxypropyl Methacrylate Based Bioink for Bioprinting Applications

Three-dimensional (3D) bioprinting has emerged as a revolutionary technology for constructing functional tissue equivalents/scaffolds for tissue engineering applications. Bioink design is a crucial element in 3D bioprinting, which typically comprises a mixture of biomaterials, biological molecules or cells followed by its printing and tissue maturation. An ideal bioink should possess suitable physicochemical, mechanical, rheological, and biological features of the target tissue. However, mimicking multifaceted compositions similar to native extracellular matrix (ECM) with bioactive milieu of soluble and non-soluble factors is challenging. Herein, we report the formulation and characterization of a bioink system, comprising methacrylamide modified gelatin (GelMA) and 2-hydroxylpropyl methacrylate (HPMA) with a cost-effective redox initiators based cross-linking. GelMA was synthesized by reacting gelatin with methacrylic anhydride (MA) and subsequently, copolymerized with HPMA at room temperature by redox mechanism. Various hydrogel formulations by varying GelMA: HPMA w/v% ratios (G:HP) were studied as 10:0 (G100HP0), 9.5:0.5 (G95HP05), 9:1 (G90HP10), 8:2 (G80HP20), and 6:4 (G60HP40), to identify the best bioink composition. The formulations were characterized for its opacity, chemical, rheological, mechanical, porosity and swelling properties and cytocompatibility as per ISO-10993 standards. Cell encapsulation studies using live/dead assay analyzed cell viability inside the handprinted and 3D printed constructs. The preliminary results indicate successful formulation of cytocompatible bioink for potential 3D bioprinting and biofabrication applications.

Latest Advances in 3D Bioprinting of Cardiac Tissues

Cardiovascular diseases (CVDs) are known as the major cause of death worldwide. In spite of tremendous advancements in medical therapy, the gold standard for CVD treatment is still transplantation. Tissue engineering, on the other hand, has emerged as a pioneering field of study with promising results in tissue regeneration using cells, biological cues, and scaffolds. Three-dimensional (3D) bioprinting is a rapidly growing technique in tissue engineering because of its ability to create complex scaffold structures, encapsulate cells, and perform these tasks with precision. More recently, 3D bioprinting has made its debut in cardiac tissue engineering, and scientists are investigating this technique for development of new strategies for cardiac tissue regeneration. In this review, the fundamentals of cardiac tissue biology, available 3D bioprinting techniques and bioinks, and cells implemented for cardiac regeneration are briefly summarized and presented. Afterwards, the pioneering and state-of-the-art works that have utilized 3D bioprinting for cardiac tissue engineering are thoroughly reviewed. Finally, regulatory pathways and their contemporary limitations and challenges for clinical translation are discussed.

Silylated biomolecules: Versatile components for bioinks

Physical hydrogels prepared from natural biopolymers are the most popular components for bioinks. However, to improve the mechanical properties of the network, in particular its durability for long-lasting tissue engineering applications or its stiffness for bone/cartilage applications, covalent chemical hydrogels have to be considered. For that purpose, biorthogonal reactions are required to allow the inclusion of living cells within the bioink reservoir before the 3D printing procedure. Interestingly, such reactions also unlock the possibility to further multifunctionalize the network, adding bioactive moieties to tune the biological properties of the resulting printed biomaterial. Surprisingly, compared to the huge number of studies disclosing novel bioink compositions, no extensive efforts have been made by the scientific community to develop new chemical reactions meeting the requirements of both cell encapsulation, chemical orthogonality and versatile enough to be applied to a wide range of molecular components, including fragile biomolecules. That could be explained by the domination of acrylate photocrosslinking in the bioprinting field. On the other hand, proceeding chemoselectively and allowing the polymerization of any type of silylated molecules, the sol-gel inorganic polymerization was used as a crosslinking reaction to prepare hydrogels. Recent development of this strategy includes the optimization of biocompatible catalytic conditions and the silylation of highly attractive biomolecules such as amino acids, bioactive peptides, proteins and oligosaccharides. When one combines the simplicity and the versatility of the process, with the ease of functionalization of any type of relevant silylated molecules that can be combined in an infinite manner, it was obvious that a family of bioinks could emerge quickly. This review presents the sol-gel process in biocompatible conditions and the various classes of relevant silylated molecules that can be used as bioink components. The preparation of hydrogels and the kinetic considerations of the sol-gel chemistry which at least allowed cell encapsulation and extrusion-based bioprinting are discussed.

Regulable Supporting Baths for Embedded Printing of Soft Biomaterials with Variable Stiffness

Three-dimensional (3D) embedded printing is emerging as a potential solution for the fabrication of complex biological structures and with ultrasoft biomaterials. For the supporting medium, bulk gels can support a wide range of bioinks with higher printing resolution as well as better finishing surfaces than granular microgel baths. However, the difficulties of regulating the physical properties of existing bulk gel supporting baths limit the further development of this method. This work has developed a bulk gel supporting bath with easily regulable physical properties to facilitate soft-material fabrication. The proposed bath is composed based on the hydrophobic association between a hydrophobically modified hydroxypropylmethyl cellulose (H-HPMC) and Pluronic F-127 (PF-127). Its rheological properties can be easily regulated; in the preprinting stage by varying the relative concentration of components, during printing by changing the temperature, and postprinting by adding additives with strong hydrophobicity or hydrophilicity. This has made the supporting bath not only available for various bioinks with a range of printing windows but also easy to be removed. Also, the removal strategy is independent of printing conditions like temperature and ions, which empowers the bath to hold great potential for the embedded printing of commonly used biomaterials. The adjustable rheological properties of the bath were leveraged to characterize the embedded printing quantitatively, involving the disturbance during the printing, filament cross-sectional shape, printing resolution, continuity, and the coalescence between adjacent filaments. The match between the bioink and the bath was also explored. Furthermore, low-viscosity bioinks (with 0.008-2.4 Pa s viscosity) were patterned into various 3D complex delicate soft structures (with a 0.5-5 kPa compressive modulus). It is believed that such an easily regulable assembled bath could serve as an available tool to support the complex biological structure fabrication and open unique prospects for personalized medicine.

Human gelatin-based composite hydrogels for osteochondral tissue engineering and their adaptation into bioinks for extrusion, inkjet, and digital light processing bioprinting

The investigation of novel hydrogel systems allows for the study of relationships between biomaterials, cells, and other factors within osteochondral tissue engineering. Three-dimensional (3D) printing is a popular research method that can allow for further interrogation of these questions via the fabrication of 3D hydrogel environments that mimic tissue-specific, complex architectures. However, the adaptation of promising hydrogel biomaterial systems into 3D-printable bioinks remains a challenge. Here, we delineated an approach to that process. First, we characterized a novel methacryloylated gelatin composite hydrogel system and assessed how calcium phosphate and glycosaminoglycan additives upregulated bone- and cartilage-like matrix deposition and certain genetic markers of differentiation within human mesenchymal stem cells (hMSCs), such as RUNX2 and SOX9. Then, new assays were developed and utilized to study the effects of xanthan gum and nanofibrillated cellulose, which allowed for cohesive fiber deposition, reliable droplet formation, and non-fracturing digital light processing (DLP)-printed constructs within extrusion, inkjet, and DLP techniques, respectively. Finally, these bioinks were used to 3D print constructs containing viable encapsulated hMSCs over a 7 d period, where DLP printed constructs facilitated the highest observed increase in cell number over 7 d (∼2.4×). The results presented here describe the promotion of osteochondral phenotypes via these novel composite hydrogel formulations, establish their ability to bioprint viable, cell-encapsulating constructs using three different 3D printing methods on multiple bioprinters, and document how a library of modular bioink additives affected those physicochemical properties important to printability.

3D Printed Solutions for Spheroid Engineering and Cancer Research

In multicellular organisms, cells are organized in a 3-dimensional framework and this is essential for organogenesis and tissue morphogenesis. Systems to recapitulate 3D cell growth are therefore vital for understanding development and cancer biology. Cells organized in 3D environments can evolve certain phenotypic traits valuable to physiologically relevant models that cannot be accessed in 2D culture. Cellular spheroids constitute an important aspect of in vitro tumor biology and they are usually prepared using the hanging drop method. Here a 3D printed approach is demonstrated to fabricate bespoke hanging drop devices for the culture of tumor cells. The design attributes of the hanging drop device take into account the need for high-throughput, high efficacy in spheroid formation, and automation. Specifically, in this study, custom-fit, modularized hanging drop devices comprising of inserts (Q-serts) were designed and fabricated using fused filament deposition (FFD). The utility of the Q-serts in the engineering of unicellular and multicellular spheroids-synthetic tumor microenvironment mimics (STEMs)—was established using human (cancer) cells. The culture of spheroids was automated using a pipetting robot and bioprinted using a custom bioink based on carboxylated agarose to simulate a tumor microenvironment (TME). The spheroids were characterized using light microscopy and histology. They showed good morphological and structural integrity and had high viability throughout the entire workflow. The systems and workflow presented here represent a user-focused 3D printing-driven spheroid culture platform which can be reliably reproduced in any research environment and scaled to- and on-demand. The standardization of spheroid preparation, handling, and culture should eliminate user-dependent variables, and have a positive impact on translational research to enable direct comparison of scientific findings.

Improving printability of hydrogel-based bio-inks for thermal inkjet bioprinting applications via saponification and heat treatment processes

Material jetting bioprinting is a highly promising three-dimensional (3D) bioprinting technique that facilitates drop-on-demand (DOD) deposition of biomaterials and cells at pre-defined positions with high precision and resolution. A major challenge that hinders the prevalent use of the material jetting bioprinting technique is due to its limited range of printable hydrogel-based bio-inks. As a proof-of-concept, further modifications were made to gelatin methacrylate (GelMA), a gold-standard bio-ink, to improve its printability in a thermal inkjet bioprinter (HP Inc. D300e Digital Dispenser). A two-step modification process comprising saponification and heat treatment was performed; the GelMA bio-ink was first modified via a saponification process under highly alkali conditions to obtain saponified GelMA (SP-GelMA), followed by heat treatment via an autoclaving process to obtain heat-treated SP-GelMA (HSP-GelMA). The bio-ink modification process was optimized by evaluating the material properties of the GelMA bio-inks via rheological characterization, the bio-ink crosslinking test, nuclear magnetic resonance (NMR) spectroscopy and the material swelling ratio after different numbers of heat treatment cycles (0, 1, 2 and 3 cycles). Lastly, size-exclusion chromatography with multi-angle light scattering (SEC-MALS) was performed to determine the effect of heat treatment on the molecular weight of the bio-inks. In this work, the 4% H2SP-GelMA bio-inks (after 2 heat treatment cycles) demonstrated good printability and biocompatibility (in terms of cell viability and proliferation profile). Furthermore, thermal inkjet bioprinting of the modified hydrogel-based bio-ink (a two-step modification process comprising saponification and heat treatment) via direct/indirect cell patterning is a facile approach for potential fundamental cell-cell and cell-material interaction studies.

3D printing of skin equivalents with hair follicle structures and epidermal-papillary-dermal layers using gelatin/hyaluronic acid hydrogels

Recent advances in three-dimensional (3D) bioprinting technologies enabled the fabrication of sophisticated live 3D tissue analogs. Although various hydrogel-based bioink has been reported, the development of advanced bioink materials that can reproduce the composition of native extracellular matrix (ECM) accurately and mimic the intrinsic property of laden cells is still challenging. In this work, 3D printed skin equivalents incorporating hair follicle structures and epidermal-papillary-dermal layers are fabricated with gelatin methacryloyl (GelMA)/hyaluronic acid (HA) MA (HAMA) hydrogel (GelMA/HAMA) bioink. The composition of collagen and glycosaminoglycan (GAG) of native skin was recapitulated by adjusting the combination of GelMA and HAMA. The GelMA/HAMA bioink was proven to have excellent viscoelastic and physicochemical properties, 3D printability, cytocompatibility, and functionality to maintain the hair inductive potency and facilitated spontaneous hair pore development. Overall, we suggest that the GelMA/HAMA hydrogels can be promising candidates as bioinks for the 3D printing of skin equivalents with epidermal-papillary-dermal multi-layers and hair follicle structures, and they might serve as a useful model in skin tissue engineering and regeneration.

Gelatin Methacryloyl Hydrogels for Musculoskeletal Tissue Regeneration

Musculoskeletal disorders are a significant burden on the global economy and public health. Hydrogels have significant potential for enhancing the repair of damaged and injured musculoskeletal tissues as cell or drug delivery systems. Hydrogels have unique physicochemical properties which make them promising platforms for controlling cell functions. Gelatin methacryloyl (GelMA) hydrogel in particular has been extensively investigated as a promising biomaterial due to its tuneable and beneficial properties and has been widely used in different biomedical applications. In this review, a detailed overview of GelMA synthesis, hydrogel design and applications in regenerative medicine is provided. After summarising recent progress in hydrogels more broadly, we highlight recent advances of GelMA hydrogels in the emerging fields of musculoskeletal drug delivery, involving therapeutic drugs (e.g., growth factors, antimicrobial molecules, immunomodulatory drugs and cells), delivery approaches (e.g., single-, dual-release system), and material design (e.g., addition of organic or inorganic materials, 3D printing). The review concludes with future perspectives and associated challenges for developing local drug delivery for musculoskeletal applications.

Bioprinting Decellularized Breast Tissue for the Development of Three-Dimensional Breast Cancer Models

The tumor extracellular matrix (ECM) plays a vital role in tumor progression and drug resistance. Previous studies have shown that breast tissue-derived matrices could be an important biomaterial to recreate the complexity of the tumor ECM. We have developed a method for decellularizing and delipidating a porcine breast tissue (TDM) compatible with hydrogel formation. The addition of gelatin methacrylamide and alginate allows this TDM to be bioprinted by itself with good printability, shape fidelity, and cytocompatibility. Furthermore, this bioink has been tuned to more closely recreate the breast tumor by incorporating collagen type I (Col1). Breast cancer cells (BCCs) proliferate in both TDM bioinks forming cell clusters and spheroids. The addition of Col1 improves the printability of the bioink as well as increases BCC proliferation and reduces doxorubicin sensitivity due to a downregulation of HSP90. TDM bioinks also allow a precise three-dimensional printing of scaffolds containing BCCs and stromal cells and could be used to fabricate artificial tumors. Taken together, we have proven that these novel bioinks are good candidates for biofabricating breast cancer models.

Tissue-engineered heart chambers as a platform technology for drug discovery and disease modeling

Current drug screening approaches are incapable of fully detecting and characterizing drug effectiveness and toxicity of human cardiomyocytes. The pharmaceutical industry uses mathematical models, cell lines, and in vivo models. Many promising drugs are abandoned early in development, and some cardiotoxic drugs reach humans leading to drug recalls. Therefore, there is an unmet need to have more reliable and predictive tools for drug discovery and screening applications. Biofabrication of functional cardiac tissues holds great promise for developing a faithful 3D in vitro disease model, optimizing drug screening efficiencies enabling precision medicine. Different fabrication techniques including molding, pull spinning and 3D bioprinting were used to develop tissue-engineered heart chambers. The big challenge is to effectively organize cells into tissue with structural and physiological features resembling native tissues. Some advancements have been made in engineering miniaturized heart chambers that resemble a living pump for drug screening and disease modeling applications. Here, we review the currently developed tissue-engineered heart chambers and discuss challenges and prospects.

3D Printed Hydrogels for Ocular Wound Healing

Corneal disease is one of the most significant causes of blindness around the world. Presently, corneal transplantation is the only way to treat cornea blindness. It should be noted that the amount of cornea that people donate is so much less than that required (1:70). Therefore, scientists have tried to resolve this problem with tissue engineering and regenerative medicine. Fabricating cornea with traditional methods is difficult due to their unique properties, such as transparency and geometry. Bioprinting is a technology based on additive manufacturing that can use different biomaterials as bioink for tissue engineering, and the emergence of 3D bioprinting presents a clear possibility to overcome this problem. This new technology requires special materials for printing scaffolds with acceptable biocompatibility. Hydrogels have received significant attention in the past 50 years, and they have been distinguished from other materials because of their unique and outstanding properties. Therefore, hydrogels could be a good bioink for the bioprinting of different scaffolds for corneal tissue engineering. In this review, we discuss the use of different types of hydrogel for bioink for corneal tissue engineering and various methods that have been used for bioprinting. Furthermore, the properties of hydrogels and different types of hydrogels are described.

Infiltration from Suspension Systems Enables Effective Modulation of 3D Scaffold Properties in Suspension Bioprinting

Bioprinting is a biofabrication technology which allows efficient and large-scale manufacture of 3D cell culture systems. However, the available biomaterials for bioinks used in bioprinting are limited by their printability and biological functionality. Fabricated constructs are often homogeneous and have limited complexity in terms of current 3D cell culture systems comprising multiple cell types. Inspired by the phenomenon that hydrogels can exchange liquids under the infiltration action, infiltration-induced suspension bioprinting (IISBP), a novel printing technique based on a hyaluronic acid (HA) suspension system to modulate the properties of the printed scaffolds by infiltration action, was described in this study. HA served as a suspension system due to its shear-thinning and self-healing rheological properties, simplicity of preparation, reusability, and ease of adjustment to osmotic pressure. Changes in osmotic pressure were able to direct the swelling or shrinkage of 3D printed gelatin methacryloyl (GelMA)-based bioinks, enabling the regulation of physical properties such as fiber diameter, micromorphology, mechanical strength, and water absorption of 3D printed scaffolds. Human umbilical vein endothelial cells (HUVEC) were applied as a cell culture model and printed within cell-laden scaffolds at high resolution and cell viability with the IISBP technique. Herein, the IISBP technique had been realized as a reliable hydrogel-based bioprinting technique, which enabled facile modulation of 3D printed hydrogel scaffolds properties, being expected to meet the scaffolds requirements of a wide range of cell culture conditions to be utilized in bioprinting applications.

A hierarchical vascularized engineered bone inspired by intramembranous ossification for mandibular regeneration

Mandibular defects caused by injuries, tumors, and infections are common and can severely affect mandibular function and the patient’s appearance. However, mandible reconstruction with a mandibular bionic structure remains challenging. Inspired by the process of intramembranous ossification in mandibular development, a hierarchical vascularized engineered bone consisting of angiogenesis and osteogenesis modules has been produced. Moreover, the hierarchical vascular network and bone structure generated by these hierarchical vascularized engineered bone modules match the particular anatomical structure of the mandible. The ultra-tough polyion complex has been used as the basic scaffold for hierarchical vascularized engineered bone for ensuring better reconstruction of mandible function. According to the results of in vivo experiments, the bone regenerated using hierarchical vascularized engineered bone is similar to the natural mandibular bone in terms of morphology and genomics. The sonic hedgehog signaling pathway is specifically activated in hierarchical vascularized engineered bone, indicating that the new bone in hierarchical vascularized engineered bone underwent a process of intramembranous ossification identical to that of mandible development. Thus, hierarchical vascularized engineered bone has a high potential for clinical application in mandibular defect reconstruction. Moreover, the concept based on developmental processes and bionic structures provides an effective strategy for tissue regeneration.

Stabilizing gelatin-based bioinks under physiological conditions by incorporation of ethylene-glycol-conjugated Fmoc-FF peptides

Over the last decade, three-dimensional (3D) printing technologies have attracted the interest of researchers due to the possibility of fabricating tissue- and organ-like structures with similarities to the organ of interest. One of the most widely used materials for the fabrication of bioinks is gelatin (Gel) due to its excellent biocompatibility properties. However, in order to fabricate stable scaffolds under physiological conditions, the most common approach is to use gelatin methacrylate (GelMA) that allows the crosslinking and therefore the stabilization of the hydrogel through UV crosslinking. The crosslinking process can be harmful to cells thus decreasing total cell viability. To overcome the need for post-printing crosslinking, a new approach of bioink formulation was studied, incorporating the Fluorenylmethoxycarbonyl diphenylalanine (Fmoc-FF) peptide into the Gel bioink. However, although Fmoc-FF possesses excellent mechanical properties, the lack of elasticity and viscosity makes it unsuitable for 3D-printing. Here, we demonstrate that covalent conjugation of two different ethylene glycol (EG) motifs to the Fmoc-FF peptide increases the hydrophilicity and elasticity properties, which are essential for 3D-printing. This new approach for bioink formulation avoids the need for any post-printing manufacturing processes, such as chemical or UV crosslinking.

Molecularly cleavable bioinks facilitate high-performance digital light processing-based bioprinting of functional volumetric soft tissues

Digital light processing bioprinting favors biofabrication of tissues with improved structural complexity. However, soft-tissue fabrication with this method remains a challenge to balance the physical performances of the bioinks for high-fidelity bioprinting and suitable microenvironments for the encapsulated cells to thrive. Here, we propose a molecular cleavage approach, where hyaluronic acid methacrylate (HAMA) is mixed with gelatin methacryloyl to achieve high-performance bioprinting, followed by selectively enzymatic digestion of HAMA, resulting in tissue-matching mechanical properties without losing the structural complexity and fidelity. Our method allows cellular morphological and functional improvements across multiple bioprinted tissue types featuring a wide range of mechanical stiffness, from the muscles to the brain, the softest organ of the human body. This platform endows us to biofabricate mechanically precisely tunable constructs to meet the biological function requirements of target tissues, potentially paving the way for broad applications in tissue and tissue model engineering.

Micropore-Forming Gelatin Methacryloyl (GelMA) Bioink Toolbox 2.0: Designable Tunability and Adaptability for 3D Bioprinting Applications

It is well-known that tissue engineering scaffolds that feature highly interconnected and size-adjustable micropores are oftentimes desired to promote cellular viability, motility, and functions. Unfortunately, the ability of precise control over the microporous structures within bioinks in a cytocompatible manner for applications in 3D bioprinting is generally lacking, until a method of micropore-forming bioink based on gelatin methacryloyl (GelMA) was reported recently. This bioink took advantage of the unique aqueous two-phase emulsion (ATPE) system, where poly(ethylene oxide) (PEO) droplets are utilized as the porogen. Considering the limitations associated with this very initial demonstration, this article has furthered the understanding of the micropore-forming GelMA bioinks by conducting a systematic investigation into the additional GelMA types (porcine and fish, different methacryloyl-modification degrees) and porogen types (PEO, poly(vinyl alcohol), and dextran), as well as the effects of the porogen concentrations and molecular weights on the properties of the GelMA-based ATPE bioink system. This article exemplifies not only the significantly wider range of micropore sizes achievable and better emulsion stability, but also the improved suitability for both extrusion and digital light processing bioprinting with favorable cellular responses.

Uniaxial and Coaxial Vertical Embedded Extrusion Bioprinting

The 3D bioprinting technologies have attracted increasing attention due to their flexibility in producing architecturally relevant tissue constructs. Here, a vertical embedded extrusion bioprinting strategy using uniaxial or coaxial nozzles is presented, which allows formation of vertical structures of homogeneous or heterogeneous properties. By adjusting the bioprinting parameters, the characteristics of the bioprinted vertical patterns can be precisely controlled. Using this strategy, two proof-of-concept applications in tissue biofabrication are demonstrated. Specifically, intestinal villi and hair follicles, two liner-shaped tissues in the human body, are successfully generated with the vertical embedded bioprinting method, reconstructing some of their key structures as well as restoring partial functions in vitro. Caco-2 cells in the bioprinted intestinal villus constructs proliferated and aggregated properly, also showing functional biomarker expressions such as ZO-1 and villin. Moreover, preliminary hair follicle structures featuring keratinized human keratinocytes and spheroid-shaped human dermal papilla cells are formed after vertical bioprinting and culturing. In summary, this vertical embedded extrusion bioprinting technique harnessing a uniaxial or coaxial format will likely bring further improvements in the reconstruction of certain human tissues and organs, especially those with a linear structure, potentially leading to wide utilities in tissue engineering, tissue model engineering, and drug discovery.