Three-dimensional bioprinting of complex cell laden alginate hydrogel structures

Coaxial bioprinting of a stentable and endothelialized human coronary artery-sized in vitro model

Atherosclerosis accounts for two-thirds of deaths attributed to cardiovascular diseases, which continue to be the leading cause of mortality. Current clinical management strategies for atherosclerosis, such as angioplasty with stenting, face numerous challenges, including restenosis and late thrombosis. Smart stents, integrated with sensors that can monitor cardiovascular health in real-time, are being developed to overcome these limitations. This development necessitates rigorous preclinical trials on either animal models or in vitro models. Despite efforts being made, a suitable human-scale in vitro model compatible with a cardiovascular stent has remained elusive. To address this need, this study utilizes an in-bath bioprinting method to create a human-scale, freestanding in vitro model compatible with cardiovascular stents. Using a coaxial nozzle, a tubular structure of human coronary artery (HCA) size is bioprinted with a collagen-based bioink, ensuring good biocompatibility and suitable rheological properties for printing. We precisely replicated the dimensions of the HCA, including its internal diameter and wall thickness, and simulated the vascular barrier functionality. To simplify post-processing, a pumpless perfusion bioreactor is fabricated to culture a HCA-sized model, eliminating the need for a peristaltic pump and enabling scalability for high-throughput production. This model is expected to accelerate stent development in the future.

4D Printing for Biomedical Applications

4D (bio-)printing endows 3D printed (bio-)materials with multiple functionalities and dynamic properties. 4D printed materials have been recently used in biomedical engineering for the design and fabrication of biomedical devices, such as stents, occluders, microneedles, smart 3D-cell engineered microenvironments, drug delivery systems, wound closures, and implantable medical devices. However, the success of 4D printing relies on the rational design of 4D printed objects, the selection of smart materials, and the availability of appropriate types of external (multi-)stimuli. Here, this work first highlights the different types of smart materials, external stimuli, and design strategies used in 4D (bio-)printing. Then, it presents a critical review of the biomedical applications of 4D printing and discusses the future directions of biomedical research in this exciting area, including in vivo tissue regeneration studies, the implementation of multiple materials with reversible shape memory behaviors, the creation of fast shape-transformation responses, the ability to operate at the microscale, untethered activation and control, and the application of (machine learning-based) modeling approaches to predict the structure-property and design-shape transformation relationships of 4D (bio)printed constructs.

4D bioprinting of programmed dynamic tissues

Setting time as the fourth dimension, 4D printing allows us to construct dynamic structures that can change their shape, property, or functionality over time under stimuli, leading to a wave of innovations in various fields. Recently, 4D printing of smart biomaterials, biological components, and living cells into dynamic living 3D constructs with 4D effects has led to an exciting field of 4D bioprinting. 4D bioprinting has gained increasing attention and is being applied to create programmed and dynamic cell-laden constructs such as bone, cartilage, and vasculature. This review presents an overview on 4D bioprinting for engineering dynamic tissues and organs, followed by a discussion on the approaches, bioprinting technologies, smart biomaterials and smart design, bioink requirements, and applications. While much progress has been achieved, 4D bioprinting as a complex process is facing challenges that need to be addressed by transdisciplinary strategies to unleash the full potential of this advanced biofabrication technology. Finally, we present future perspectives on the rapidly evolving field of 4D bioprinting, in view of its potential, increasingly important roles in the development of advanced dynamic tissues for basic research, pharmaceutics, and regenerative medicine.

Applications of hydrogels in tissue-engineered repairing of temporomandibular joint diseases

Epidemiological studies reveal that symptoms of temporomandibular joint disorders (TMDs) occur in 60-70% of adults. The inflammatory damage caused by TMDs can easily lead to defects in the articular disc, condylar cartilage, subchondral bone and muscle of the temporomandibular joint (TMJ) and cause pain. Despite the availability of various methods for treating TMDs, few existing treatment schemes can achieve permanent recovery. This necessity drives the search for new approaches. Hydrogels, polymers with high water content, have found widespread use in tissue engineering and regeneration due to their excellent biocompatibility and mechanical properties, which resemble those of human tissues. In the context of TMD therapy, numerous experiments have demonstrated that hydrogels show favorable effects in aspects such as articular disc repair, cartilage regeneration, muscle repair, pain relief, and drug delivery. This review aims to summarize the application of hydrogels in the therapy of TMDs based on recent research findings. It also highlights deficiencies in current hydrogel research related to TMD therapy and outlines the broad potential of hydrogel applications in treating TMJ diseases in the future.

Three-dimensional bioprinting of tissue-engineered skin: Biomaterials, fabrication techniques, challenging difficulties, and future directions: A review

As an emerging new manufacturing technology, Three-dimensional (3D) bioprinting provides the potential for the biomimetic construction of multifaceted and intricate architectures of functional integument, particularly functional biomimetic dermal structures inclusive of cutaneous appendages. Although the tissue-engineered skin with complete biological activity and physiological functions is still cannot be manufactured, it is believed that with the advances in matrix materials, molding process, and biotechnology, a new generation of physiologically active skin will be born in the future. In pursuit of furnishing readers and researchers involved in relevant research to have a systematic and comprehensive understanding of 3D printed tissue-engineered skin, this paper furnishes an exegesis on the prevailing research landscape, formidable obstacles, and forthcoming trajectories within the sphere of tissue-engineered skin, including: (1) the prevalent biomaterials (collagen, chitosan, agarose, alginate, etc.) routinely employed in tissue-engineered skin, and a discerning analysis and comparison of their respective merits, demerits, and inherent characteristics; (2) the underlying principles and distinguishing attributes of various current printing methodologies utilized in tissue-engineered skin fabrication; (3) the present research status and progression in the realm of tissue-engineered biomimetic skin; (4) meticulous scrutiny and summation of the extant research underpinning tissue-engineered skin inform the identification of prevailing challenges and issues.

Gelatin Methacryloyl (GelMA)-Based Biomaterial Inks: Process Science for 3D/4D Printing and Current Status

Tissue engineering for injured tissue replacement and regeneration has been a subject of investigation over the last 30 years, and there has been considerable interest in using additive manufacturing to achieve these goals. Despite such efforts, many key questions remain unanswered, particularly in the area of biomaterial selection for these applications as well as quantitative understanding of the process science. The strategic utilization of biological macromolecules provides a versatile approach to meet diverse requirements in 3D printing, such as printability, buildability, and biocompatibility. These molecules play a pivotal role in both physical and chemical cross-linking processes throughout the biofabrication, contributing significantly to the overall success of the 3D printing process. Among the several bioprintable materials, gelatin methacryloyl (GelMA) has been widely utilized for diverse tissue engineering applications, with some degree of success. In this context, this review will discuss the key bioengineering approaches to identify the gelation and cross-linking strategies that are appropriate to control the rheology, printability, and buildability of biomaterial inks. This review will focus on the GelMA as the structural (scaffold) biomaterial for different tissues and as a potential carrier vehicle for the transport of living cells as well as their maintenance and viability in the physiological system. Recognizing the importance of printability toward shape fidelity and biophysical properties, a major focus in this review has been to discuss the qualitative and quantitative impact of the key factors, including microrheological, viscoelastic, gelation, shear thinning properties of biomaterial inks, and printing parameters, in particular, reference to 3D extrusion printing of GelMA-based biomaterial inks. Specifically, we emphasize the different possibilities to regulate mechanical, swelling, biodegradation, and cellular functionalities of GelMA-based bio(material) inks, by hybridization techniques, including different synthetic and natural biopolymers, inorganic nanofillers, and microcarriers. At the close, the potential possibility of the integration of experimental data sets and artificial intelligence/machine learning approaches is emphasized to predict the printability, shape fidelity, or biophysical properties of GelMA bio(material) inks for clinically relevant tissues.

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.

Three dimensional (bio)printing of blood vessels: from vascularized tissues to functional arteries

Tissue engineering has emerged as a strategy for producing functional tissues and organs to treat diseases and injuries. Many chronic conditions directly or indirectly affect normal blood vessel functioning, necessary for material exchange and transport through the body and within tissue-engineered constructs. The interest in vascular tissue engineering is due to two reasons: (1) functional grafts can be used to replace diseased blood vessels, and (2) engineering effective vasculature within other engineered tissues enables connection with the host's circulatory system, supporting their survival. Among various practices, (bio)printing has emerged as a powerful tool to engineer biomimetic constructs. This has been made possible with precise control of cell deposition and matrix environment along with the advancements in biomaterials. (Bio)printing has been used for both engineering stand-alone vascular grafts as well as vasculature within engineered tissues for regenerative applications. In this review article, we discuss various conditions associated with blood vessels, the need for artificial blood vessels, the anatomy and physiology of different blood vessels, available 3D (bio)printing techniques to fabricate tissue-engineered vascular grafts and vasculature in scaffolds, and the comparison among the different techniques. We conclude our review with a brief discussion about future opportunities in the area of blood vessel tissue engineering.

Differentiation of Human Adipose-derived Stem Cells to Exosome-affected Neural-like Cells Extracted from Human Cerebrospinal Fluid Using Bioprinting Process

BACKGROUND

Advancement in tissue engineering has provided novel solutions for creating scaffolds as well as applying induction factors in the differentiation of stem cells. The present research aimed to investigate the differentiation of human adipose-derived mesenchymal stem cells to neural-like cells using the novel bioprinting method, as well as the effect of cerebrospinal fluid exosomes.

METHODS

In the present study, the extent of neuronal proliferation and differentiation of adipose- derived stem cells were explored using the MTT method, immunocytochemistry, and real-- time PCR in the scaffolds created by the bioprinting process. Furthermore, in order to investigate the veracity of the identity of the CSF (Cerebrospinal fluid) derived exosomes, after the isolation of exosomes, dynamic light scattering (DLS), scanning electron microscopy (SEM), and atomic force microscopy (AFM) techniques were used.

RESULTS

MTT findings indicated survivability and proliferation of cells in the scaffolds created by the bioprinting process during a 14-day period. The results obtained from real-time PCR showed that the level of MAP2 gene (Microtubule Associated Protein 2) expression increased on days 7 and 14, while the expression of the Nestin gene (intermediate filament protein) significantly decreased compared to the control. The investigation to confirm the identity of exosomes indicated that the CSF-derived exosomes had a spherical shape with a 40-100 nm size.

CONCLUSION

CSF-derived exosomes can contribute to the neuronal differentiation of adipose- derived stem cells in alginate hydrogel scaffolds created by the bioprinting process.

A systematic approach to improve printability and cell viability of methylcellulose-based bioinks

Printability in 3D extrusion bioprinting encompasses extrudability, filament formation, and shape fidelity. Rheological properties can predict the shape fidelity of printed hydrogels. In particular, tan(δ), the ratio between loss (G'') and storage (G') modulus (G''/G'), is a powerful indicator of printability. This study explores the effect of different salt, sucrose, and MC concentrations on tan(δ), and therefore the printability of methylcellulose (MC) hydrogels. Salt and sucrose increased G', lowering tan(δ) and enabling printing of scaffolds with high shape fidelity. Conversely, MC concentration increased G'' and G', having a lesser effect on tan(δ). Shape fidelity of three formulations with similar G' but varying tan(δ) values were compared. Higher tan(δ) led to reduced height, while lower tan(δ) improved shape fidelity. Cell viability increased when reducing MC content, extrusion rate, and nozzle gauge. Higher MC concentration (G' > 1.5 kPa) increased the influence of needle size and extrusion rate on cell viability. Hydrogels with G' < 1 kPa could be extruded at high rates with small nozzles, minimally affecting cell viability. This work shows a direct relationship between tan(δ) and printability of MC-based hydrogels. Lowering the complex modulus of hydrogels, mitigates extrusion stress, thus improving cell survival.

Gelation of Uniform Interfacial Diffusant in Embedded 3D Printing

While the human body has many different examples of perfusable structures with complex geometries, biofabrication methods to replicate this complexity are still lacking. Specifically, the fabrication of self-supporting, branched networks with multiple channel diameters is particularly challenging. Here, we present the Gelation of Uniform Interfacial Diffusant in Embedded 3D Printing (GUIDE-3DP) approach for constructing perfusable networks of interconnected channels with precise control over branching geometries and vessel sizes. To achieve user-specified channel dimensions, this technique leverages the predictable diffusion of crosslinking reaction-initiators released from sacrificial inks printed within a hydrogel precursor. We demonstrate the versatility of GUIDE-3DP to be adapted for use with diverse physicochemical crosslinking mechanisms by designing seven printable material systems. Importantly, GUIDE-3DP allows for the independent tunability of both the inner and outer diameters of the printed channels and the ability to fabricate seamless junctions at branch points. This 3D bioprinting platform is uniquely suited for fabricating lumenized structures with complex shapes characteristic of multiple hollow vessels throughout the body. As an exemplary application, we demonstrate the fabrication of vasculature-like networks lined with endothelial cells. GUIDE-3DP represents an important advance toward the fabrication of self-supporting, physiologically relevant networks with intricate and perfusable geometries.

Controllable and biocompatible 3D bioprinting technology for microorganisms: Fundamental, environmental applications and challenges

3D bioprinting is a new 3D manufacturing technology, that can be used to accurately distribute and load microorganisms to form microbial active materials with multiple complex functions. Based on the 3D printing of human cells in tissue engineering, 3D bioprinting technology has been developed. Although 3D bioprinting technology is still immature, it shows great potential in the environmental field. Due to the precise programming control and multi-printing pathway, 3D bioprinting technology provides a high-throughput method based on micron-level patterning for a wide range of environmental microbiological engineering applications, which makes it an on-demand, multi-functional manufacturing technology. To date, 3D bioprinting technology has been employed in microbial fuel cells, biofilm material preparation, microbial catalysts and 4D bioprinting with time dimension functions. Nevertheless, current 3D bioprinting technology faces technical challenges in improving the mechanical properties of materials, developing specific bioinks to adapt to different strains, and exploring 4D bioprinting for intelligent applications. Hence, this review systematically analyzes the basic technical principles of 3D bioprinting, bioinks materials and their applications in the environmental field, and proposes the challenges and future prospects of 3D bioprinting in the environmental field. Combined with the current development of microbial enhancement technology in the environmental field, 3D bioprinting will be developed into an enabling platform for multifunctional microorganisms and facilitate greater control of in situ directional reactions.

High-Fidelity Extrusion Bioprinting of Low-Printability Polymers Using Carbopol as a Rheology Modifier

Extrusion three-dimensional (3D) bioprinting is a promising technology with many applications in the biomedical and tissue engineering fields. One of the key limitations for the widespread use of this technology is the narrow window of printability that results from the need to have bioinks with rheological properties that allow the extrusion of continuous filaments while maintaining high cell viability within the materials during and after printing. In this work, we use Carbopol (CBP) as rheology modifier for extrusion printing of biomaterials that are typically nonextrudable or present low printability. We show that low concentrations of CBP can introduce the desired rheological properties for a wide range of formulations, allowing the use of polymers with different cross-linking mechanisms and the introduction of additives and cells. To explore the opportunities and limitations of CBP as a rheology modifier, we used ink formulations based on poly(ethylene glycol)diacrylate with extrusion 3D printing to produce soft, yet stable, hydrogels with tunable mechanical properties. Cell-laden constructs made with such inks presented high viability for cells seeded on top of cross-linked materials and cells incorporated within the bioink during printing, showing that the materials are noncytotoxic and the printed structures do not degrade for up to 14 days. To our knowledge, this is the first report of the use of CBP-containing bioinks to 3D-print complex cell-laden structures that are stable for days and present high cell viability. The use of CBP to obtain highly printable inks can accelerate the evolution of extrusion 3D bioprinting by guaranteeing the required rheological properties and expanding the number of materials that can be successfully printed. This will allow researchers to develop and optimize new bioinks focusing on the biochemical, cellular, and mechanical requirements of the targeted applications rather than the rheology needed to achieve good printability.

Double-layered blood vessels over 3 mm in diameter extruded by the inverse-gravity technique

One of the most promising techniques for treating severe peripheral artery disease is the use of cellular tissue-engineered vascular grafts (TEVGs). This study proposes an inverse-gravity (IG) extrusion technique for creating long double-layered cellular TEVGs with diameters over 3 mm. A three-layered coaxial laminar hydrogel flow in an 8 mm-diameter pipe was realised simply by changing the extrusion direction of the hydrogel from being aligned with the direction of gravity to against it. This technique produced an extruded mixture of human aortic smooth muscle cells (HASMCs) and type-I collagen as a tubular structure with an inner diameter of 3.5 mm. After a 21 day maturation period, the maximal burst pressure, longitudinal breaking force, and circumferential breaking force of the HASMC TEVG were 416 mmHg, 0.69 N, and 0.89 N, respectively. The HASMC TEVG was endothelialised with human umbilical vein endothelial cells to form a tunica intima that simulated human vessels. Besides subcutaneous implantability on mice, the double-layered blood vessels showed a considerably lower adherence of platelets and red blood cells once exposed to heparinised mouse blood and were considered nonhaemolytic. The proposed IG extrusion technique can be applied in various fields requiring multilayered materials with large diameters.

Preparation of a scaffold for a vascular network channel with spatially varying diameter based on sucrose

In the past few decades, although tissue engineering has made significant progress and achieved many accomplishments, there are still some key problems that remain unsolved. One of the urgent research challenges in this field is how to prepare large-scale tissue engineering scaffolds with spatially complex structures. In this work, a sacrificial template process using sucrose as the sacrificial material and a gelatin/microbial transglutaminase mixed solution as the bio-scaffold material is proposed to fabricate a bio-scaffold with multi-level branching and spatially complex vascular network channels that mimic the structure and function of the human vascular network. To validate the feasibility of the fabrication process and the rationality of the process parameters, the morphological characteristics, connectivity of vascular network channels, shaping accuracy, and mechanical properties of the bio-scaffold were tested and analyzed. The results showed that the bio-scaffold fabricated using this process had a complete morphology and excellent connectivity. The diameter of the sucrose sacrificial template showed a linear relationship with the feeding speed, and the average diameter error rate between the sucrose sacrificial template and the vascular network channels inside the bio-scaffold was less than 8%. The mechanical properties of the bio-scaffold met the requirements for large-scale tissue defect repair. To evaluate the effect of the bio-scaffold on cell activity, human umbilical vein endothelial cells (HUVECs) were seeded into the vascular network channels of the bio-scaffold, and their attachment, growth, and proliferation on the surface of the vascular network channels were observed. To further assess the biocompatibility of the bio-scaffold, the bio-scaffold was implanted subcutaneously in the dorsal tissue of rats, and the tissue regeneration status was compared and analyzed through immunohistochemical analysis. The results showed that the vascular network channels within the bio-scaffold allowed uniform cell attachment, growth, with fewer dead cells and high cell viability. Moreover, clear cell attachment and growth were observed within the vascular network channels of the bio-scaffold after implantation in rats. These results indicate that the fabricated bio-scaffold meets the basic performance requirements for the repair and regeneration of large-scale tissue defects, providing a new approach for oxygen and nutrient transport in large-scale tissues and opening up new avenues for clinical applications.

Biomaterials-based additive manufacturing for customized bioengineering in management of otolaryngology: a comprehensive review

Three-dimensional (3D)/four-dimensional (4D) printing, also known as additive manufacturing or fast prototyping, is a manufacturing technique that uses a digital model to generate a 3D/4D solid product. The usage of biomaterials with 3D/4D printers in the pharma and healthcare industries is gaining significant popularity. 3D printing has mostly been employed in the domain of otolaryngology to build portable anatomical models, personalized patient-centric implants, biologic tissue scaffolds, surgical planning in individuals with challenging conditions, and surgical training. Although identical to 3D printing technology in this application, 4D printing technology comprises a fourth dimension of time. With the use of 4D printing, a printed structure may alter over time under various stimuli. Smart polymeric materials are also generally denoted as bioinks are frequently employed in tissue engineering applications of 3D/4D printing. In general, 4D printing could significantly improve the safety and efficacy of otolaryngology therapies. The use of bioprinting in otolaryngology has an opportunity to transform the treatment of diseases influencing the ear, nose, and throat as well as the field of tissue regeneration. The present review briefs on polymeric material including biomaterials and cells used in the manufacturing of patient centric 3D/4D bio-printed products utilized in management of otolaryngology.

In Situ Covalent Reinforcement of a Benzene-1,3,5-Tricarboxamide Supramolecular Polymer Enables Biomimetic, Tough, and Fibrous Hydrogels and Bioinks

Synthetic hydrogels often lack the load-bearing capacity and mechanical properties of native biopolymers found in tissue, such as cartilage. In natural tissues, toughness is often imparted via the combination of fibrous noncovalent self-assembly with key covalent bond formation. This controlled combination of supramolecular and covalent interactions remains difficult to engineer, yet can provide a clear strategy for advanced biomaterials. Here, a synthetic supramolecular/covalent strategy is investigated for creating a tough hydrogel that embodies the hierarchical fibrous architecture of the extracellular matrix (ECM). A benzene-1,3,5-tricarboxamide (BTA) hydrogelator is developed with synthetically addressable norbornene handles that self-assembles to form a and viscoelastic hydrogel. Inspired by collagen's covalent cross-linking of fibrils, the mechanical properties are reinforced by covalent intra- and interfiber cross-links. At over 90% water, the hydrogels withstand up to 550% tensile strain, 90% compressive strain, and dissipated energy with recoverable hysteresis. The hydrogels are shear-thinning, can be 3D bioprinted with good shape fidelity, and can be toughened via covalent cross-linking. These materials enable the bioprinting of human mesenchymal stromal cell (hMSC) spheroids and subsequent differentiation into chondrogenic tissue. Collectively, these findings highlight the power of covalent reinforcement of supramolecular fibers, offering a strategy for the bottom-up design of dynamic, yet tough, hydrogels and bioinks.

Bioprinted hASC-laden cell constructs with mechanically stable and cell alignment cue for tenogenic differentiation

3D bioprinting is a technology that enables the precise and controlled deposition of cells and an artificial extracellular matrix (ECM) to create functional tissue constructs. However, current 3D bioprinting methods still struggle to obtain mechanically stable and unique cell-morphological structures, such as fully aligned cells. In this study, we propose a new 3D bioprinting approach that utilizes a high concentration of bioink without cells to support mechanical properties and drag flow to fully align cells in a thin bath filled with cell-laden bioink, resulting in a hybrid cell-laden construct with a mechanical stable and fully aligned cell structure. To demonstrate the feasibility of this approach, we used it to fabricate a cell-laden construct using human adipose stem cells (hASCs) for tendon tissue engineering. To achieve appropriate processing conditions, various factors such as the bioink concentration, nozzle moving speed, and volume flow rate were considered. To enhance the biocompatibility of the cell-laden construct, we used porcine decellularized tendon ECM.In vitrocellular responses, including tenogenic differentiation of the fabricated hybrid cell structures with aligned or randomly distributed cells, were evaluated using hASCs. In addition, the mechanical properties of the hybrid cell-laden construct could be adjusted by controlling the concentration of the mechanically reinforcing strut using methacrylated tendon-decellularized extracellular matrix. Based on these results, the hybrid cell-laden structure has the potential to be a highly effective platform for the alignment of musculoskeletal tissues.

Machine Learning in Predicting Printable Biomaterial Formulations for Direct Ink Writing

Three-dimensional (3D) printing is emerging as a transformative technology for biomedical engineering. The 3D printed product can be patient-specific by allowing customizability and direct control of the architecture. The trial-and-error approach currently used for developing the composition of printable inks is time- and resource-consuming due to the increasing number of variables requiring expert knowledge. Artificial intelligence has the potential to reshape the ink development process by forming a predictive model for printability from experimental data. In this paper, we constructed machine learning (ML) algorithms including decision tree, random forest (RF), and deep learning (DL) to predict the printability of biomaterials. A total of 210 formulations including 16 different bioactive and smart materials and 4 solvents were 3D printed, and their printability was assessed. All ML methods were able to learn and predict the printability of a variety of inks based on their biomaterial formulations. In particular, the RF algorithm has achieved the highest accuracy (88.1%), precision (90.6%), and F1 score (87.0%), indicating the best overall performance out of the 3 algorithms, while DL has the highest recall (87.3%). Furthermore, the ML algorithms have predicted the printability window of biomaterials to guide the ink development. The printability map generated with DL has finer granularity than other algorithms. ML has proven to be an effective and novel strategy for developing biomaterial formulations with desired 3D printability for biomedical engineering applications.

Dual-Crosslinking of Gelatin-Based Hydrogels: Promising Compositions for a 3D Printed Organotypic Bone Model

Gelatin-based hydrogels have emerged as a popular scaffold material for tissue engineering applications. The introduction of variable crosslinking methods has shown promise for fabricating stable cell-laden scaffolds. In this work, we examine promising composite biopolymer-based inks for extrusion-based 3D bioprinting, using a dual crosslinking approach. A combination of carefully selected printable hydrogel ink compositions and the use of photoinduced covalent and ionic crosslinking mechanisms allows for the fabrication of scaffolds of high accuracy and low cytotoxicity, resulting in unimpeded cell proliferation, extracellular matrix deposition, and mineralization. Three selected bioink compositions were characterized and the respective cell-laden scaffolds were bioprinted. Temporal stability, morphology, swelling, and mechanical properties of the scaffolds were thoroughly studied and the biocompatibility of the constructs was assessed using rat mesenchymal stem cells while focusing on osteogenesis. Experimental results showed that the composition of 1% alginate, 4% gelatin, and 5% (w/v) gelatine methacrylate, was found to be optimal among the examined, with shape fidelity of 88%, large cell spreading area and cell viability at around 100% after 14 days. The large pore diameters that exceed 100 µm, and highly interconnected scaffold morphology, make these hydrogels extremely potent in bone tissue engineering and bone organoid fabrication.

Progress in the application of 3D-printed sodium alginate-based hydrogel scaffolds in bone tissue repair

In recent years, hydrogels have been widely used in the biomedical field as materials with excellent bionic structures and biological properties. Among them, the excellent comprehensive properties of natural polymer hydrogels represented by sodium alginate have attracted the great attention of researchers. At the same time, by physically blending sodium alginate with other materials, the problems of poor cell adhesion and mechanical properties of sodium alginate hydrogels were directly improved without chemical modification of sodium alginate. The composite blending of multiple materials can also improve the functionality of sodium alginate hydrogels, and the prepared composite hydrogel also has a larger application field. In addition, based on the adjustable viscosity of sodium alginate-based hydrogels, sodium alginate-based hydrogels can be loaded with cells to prepare biological ink, and the scaffold can be printed out by 3D printing technology for the repair of bone defects. This paper first summarizes the improvement of the properties of sodium alginate and other materials after physical blending. Then, it summarizes the application progress of sodium alginate-based hydrogel scaffolds for bone tissue repair based on 3D printing technology in recent years. Moreover, we provide relevant opinions and comments to provide a theoretical basis for follow-up research.

Polysaccharides and Structural Proteins as Components in Three-Dimensional Scaffolds for Breast Cancer Tissue Models: A Review

Breast cancer is the most common cancer among women, and even though treatments are available, efficiency varies with the patients. In vitro 2D models are commonly used to develop new treatments. However, 2D models overestimate drug efficiency, which increases the failure rate in later phase III clinical trials. New model systems that allow extensive and efficient drug screening are thus required. Three-dimensional printed hydrogels containing active components for cancer cell growth are interesting candidates for the preparation of next generation cancer cell models. Macromolecules, obtained from marine- and land-based resources, can form biopolymers (polysaccharides such as alginate, chitosan, hyaluronic acid, and cellulose) and bioactive components (structural proteins such as collagen, gelatin, and silk fibroin) in hydrogels with adequate physical properties in terms of porosity, rheology, and mechanical strength. Hence, in this study attention is given to biofabrication methods and to the modification with biological macromolecules to become bioactive and, thus, optimize 3D printed structures that better mimic the cancer cell microenvironment. Ink formulations combining polysaccharides for tuning the mechanical properties and bioactive polymers for controlling cell adhesion is key to optimizing the growth of the cancer cells.

3D-printed GelMA/CaSiO3 composite hydrogel scaffold for vascularized adipose tissue restoration

The increased number of mastectomies, combined with rising patient expectations for cosmetic and psychosocial outcomes, has necessitated the use of adipose tissue restoration techniques. However, the therapeutic effect of current clinical strategies is not satisfying due to the high demand of personalized customization and the timely vascularization in the process of adipose regeneration. Here, a composite hydrogel scaffold was prepared by three-dimensional (3D) printing technology, applying gelatin methacrylate anhydride (GelMA) as printing ink and calcium silicate (CS) bioceramic as an active ingredient for breast adipose tissue regeneration. The in vitro experiments showed that the composite hydrogel scaffolds could not only be customized with controllable architectures, but also significantly stimulated both 3T3-L1 preadipocytes and human umbilical vein endothelial cells in multiple cell behaviors, including cell adhesion, proliferation, migration and differentiation. Moreover, the composite scaffold promoted vascularized adipose tissue restoration under the skin of nude mice in vivo. These findings suggest that 3D-printed GelMA/CS composite scaffolds might be a good candidate for adipose tissue engineering.

Analytics in Extrusion-Based Bioprinting: Standardized Methods Improving Quantification and Comparability of the Performance of Bioinks

Three-dimensional bioprinting and especially extrusion-based printing as a most frequently employed method in this field is constantly evolving as a discipline in regenerative medicine and tissue engineering. However, the lack of relevant standardized analytics does not yet allow an easy comparison and transfer of knowledge between laboratories regarding newly developed bioinks and printing processes. This work revolves around the establishment of a standardized method, which enables the comparability of printed structures by controlling for the extrusion rate based on the specific flow behavior of each bioink. Furthermore, printing performance was evaluated by image-processing tools to verify the printing accuracy for lines, circles, and angles. In addition, and complementary to the accuracy metrics, a dead/live staining of embedded cells was performed to investigate the effect of the process on cell viability. Two bioinks, based on alginate and gelatin methacryloyl, which differed in 1% (w/v) alginate content, were tested for printing performance. The automated image processing tool reduced the analytical time while increasing reproducibility and objectivity during the identification of printed objects. During evaluation of the processing effect of the mixing of cell viability, NIH 3T3 fibroblasts were stained and analyzed after the mixing procedure and after the extrusion process using a flow cytometer, which evaluated a high number of cells. It could be observed that the small increase in alginate content made little difference in the printing accuracy but had a considerable strong effect on cell viability after both processing steps.

Gelation of Uniform Interfacial Diffusant in Embedded 3D Printing

While the human body has many different examples of perfusable structures with complex geometries, biofabrication methods to replicate this complexity are still lacking. Specifically, the fabrication of self-supporting, branched networks with multiple channel diameters is particularly challenging. Here, we present the Gelation of Uniform Interfacial Diffusant in Embedded 3D Printing (GUIDE-3DP) approach for constructing perfusable networks of interconnected channels with precise control over branching geometries and vessel sizes. To achieve user-specified channel dimensions, this technique leverages the predictable diffusion of crosslinking reaction-initiators released from sacrificial inks printed within a hydrogel precursor. We demonstrate the versatility of GUIDE-3DP to be adapted for use with diverse physiochemical crosslinking mechanisms by designing seven printable material systems. Importantly, GUIDE-3DP allows for the independent tunability of both the inner and outer diameters of the printed channels and the ability to fabricate seamless junctions at branch points. This 3D bioprinting platform is uniquely suited for fabricating lumenized structures with complex shapes characteristic of multiple hollow vessels throughout the body. As an exemplary application, we demonstrate the fabrication of vasculature-like networks lined with endothelial cells. GUIDE-3DP represents an important advance toward the fabrication of self-supporting, physiologically relevant networks with intricate and perfusable geometries.

Emerging 3D bioprinting applications in plastic surgery

Plastic surgery is a discipline that uses surgical methods or tissue transplantation to repair, reconstruct and beautify the defects and deformities of human tissues and organs. Three-dimensional (3D) bioprinting has gained widespread attention because it enables fine customization of the implants in the patient's surgical area preoperatively while avoiding some of the adverse reactions and complications of traditional surgical approaches. In this paper, we review the recent research advances in the application of 3D bioprinting in plastic surgery. We first introduce the printing process and basic principles of 3D bioprinting technology, revealing the advantages and disadvantages of different bioprinting technologies. Then, we describe the currently available bioprinting materials, and dissect the rationale for special dynamic 3D bioprinting (4D bioprinting) that is achieved by varying the combination strategy of bioprinting materials. Later, we focus on the viable clinical applications and effects of 3D bioprinting in plastic surgery. Finally, we summarize and discuss the challenges and prospects for the application of 3D bioprinting in plastic surgery. We believe that this review can contribute to further development of 3D bioprinting in plastic surgery and provide lessons for related research.

3D bioprinting of corneal models: A review of the current state and future outlook

The cornea is the outermost layer of the eye and serves to protect the eye and enable vision by refracting light. The need for cornea organ donors remains high, and the demand for an artificial alternative continues to grow. 3D bioprinting is a promising new method to create artificial organs and tissues. 3D bioprinting offers the precise spatial arrangement of biomaterials and cells to create 3D constructs. As the cornea is an avascular tissue which makes it more attractive for 3D bioprinting, it could be one of the first tissues to be made fully functional via 3D bioprinting. This review discusses the most common 3D bioprinting technologies and biomaterials used for 3D bioprinting corneal models. Additionally, the current state of 3D bioprinted corneal models, especially specific characteristics such as light transmission, biomechanics, and marker expression, and in vivo studies are discussed. Finally, the current challenges and future prospects are presented.

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.

Message in a Scaffold: Natural Biomaterials for Three-Dimensional (3D) Bioprinting of Human Brain Organoids

Brain organoids are invaluable tools for pathophysiological studies or drug screening, but there are still challenges to overcome in making them more reproducible and relevant. Recent advances in three-dimensional (3D) bioprinting of human neural organoids is an emerging approach that may overcome the limitations of self-organized organoids. It requires the development of optimal hydrogels, and a wealth of research has improved our knowledge about biomaterials both in terms of their intrinsic properties and their relevance on 3D culture of brain cells and tissue. Although biomaterials are rarely biologically neutral, few articles have reviewed their roles on neural cells. We here review the current knowledge on unmodified biomaterials amenable to support 3D bioprinting of neural organoids with a particular interest in their impact on cell homeostasis. Alginate is a particularly suitable bioink base for cell encapsulation. Gelatine is a valuable helper agent for 3D bioprinting due to its viscosity. Collagen, fibrin, hyaluronic acid and laminin provide biological support to adhesion, motility, differentiation or synaptogenesis and optimize the 3D culture of neural cells. Optimization of specialized hydrogels to direct differentiation of stem cells together with an increased resolution in phenotype analysis will further extend the spectrum of possible bioprinted brain disease models.

3D bioprinting optimization of human mesenchymal stromal cell laden gelatin-alginate-collagen bioink

3D bioprinting technology has gained increased attention in the regenerative medicine and tissue engineering communities over the past decade with their attempts to create functional living tissues and organsde novo. While tissues such as skin, bone, and cartilage have been successfully fabricated using 3D bioprinting, there are still many technical and process driven challenges that must be overcome before a complete tissue engineered solution is realized. Although there may never be a single adopted bioprinting process in the scientific community, adherence to optimized bioprinting protocols could reduce variability and improve precision with the goal of ensuring high quality printed constructs. Here, we report on the bioprinting of a gelatin-alginate-collagen bioink containing human mesenchymal stromal cells (hMSCs) which has been optimized to ensure printing consistency and reliability. The study consists of three phases: a pre-printing phase which focuses on bioink characterization; a printing phase which focuses on bioink extrudability/printability, construct stability, and printing accuracy; and a post-processing phase which focuses on the homogeneity and bioactivity of the encapsulated hMSC printed constructs. The results showed that eight identical constructs containing hMSCs could be reliably and accurately printed into stable cross-hatched structures with a single material preparation, and that batch-to-batch consistency was accurately maintained across all preparations. Analysis of the proliferation, morphology, and differentiation of encapsulated hMSCs within the printed constructs showed that cells were able to form large,interconnected colonies and were capable of robust adipogenic differentiation within 14 d of culturing.

Advances in the development of tubular structures using extrusion-based 3D cell-printing technology for vascular tissue regenerative applications

Until recent, there are no ideal small diameter vascular grafts available on the market. Most of the commercialized vascular grafts are used for medium to large-sized blood vessels. As a solution, vascular tissue engineering has been introduced and shown promising outcomes. Despite these optimistic results, there are limitations to commercialization. This review will cover the need for extrusion-based 3D cell-printing technique capable of mimicking the natural structure of the blood vessel. First, we will highlight the physiological structure of the blood vessel as well as the requirements for an ideal vascular graft. Then, the essential factors of 3D cell-printing including bioink, and cell-printing system will be discussed. Afterwards, we will mention their applications in the fabrication of tissue engineered vascular grafts. Finally, conclusions and future perspectives will be discussed.

Recent advances in organoid engineering: A comprehensive review

Organoid, a 3D structure derived from various cell sources including progenitor and differentiated cells that self-organize through cell-cell and cell-matrix interactions to recapitulate the tissue/organ-specific architecture and function in vitro. The advancement of stem cell culture and the development of hydrogel-based extracellular matrices (ECM) have made it possible to derive self-assembled 3D tissue constructs like organoids. The ability to mimic the actual physiological conditions is the main advantage of organoids, reducing the excessive use of animal models and variability between animal models and humans. However, the complex microenvironment and complex cellular structure of organoids cannot be easily developed only using traditional cell biology. Therefore, several bioengineering approaches, including microfluidics, bioreactors, 3D bioprinting, and organoids-on-a-chip techniques, are extensively used to generate more physiologically relevant organoids. In this review, apart from organoid formation and self-assembly basics, the available bioengineering technologies are extensively discussed as solutions for traditional cell biology-oriented problems in organoid cultures. Also, the natural and synthetic hydrogel systems used in organoid cultures are discussed when necessary to highlight the significance of the stem cell microenvironment. The selected organoid models and their therapeutic applications in drug discovery and disease modeling are also presented.

Toward 3D-Bioprinted Models of the Liver to Boost Drug Development

The main problems in drug development are connected to enormous costs related to the paltry success rate. The current situation empowered the development of high-throughput and reliable instruments, in addition to the current golden standards, able to predict the failures in the early preclinical phase. Being hepatotoxicity responsible for the failure of 30% of clinical trials, and the 21% of withdrawal of marketed drugs, the development of complex in vitro models (CIVMs) of liver is currently one of the hottest topics in the field. Among the different fabrication techniques, 3D-bioprinting is emerging as a powerful ally for their production, allowing the manufacture of three-dimensional constructs characterized by computer-controlled and customized geometry, and inter-batches reproducibility. Thanks to these, it is possible to rapidly produce tailored cell-laden constructs, to be cultured within static and dynamic systems, thus reaching a further degree of personalization when designing in vitro models. This review highlights and prioritizes the most recent advances related to the development of CIVMs of the hepatic environment to be specifically applied to pharmaceutical research, with a special focus on 3D-bioprinting, since the liver is primarily involved in the metabolism of drugs.

Systematic evaluation of agarose- and agar-based bioinks for extrusion-based bioprinting of enzymatically active hydrogels

Extrusion-based 3D bioprinting enables the production of customized hydrogel structures that can be employed in flow reactors when printing with enzyme-containing inks. The present study compares inks based on either low-melt agarose or agar at different concentrations (3-6%) and loaded with the thermostable enzyme esterase 2 from the thermophilic organism Alicyclobacillus acidocaldarius (AaEst2) with regard to their suitability for the fabrication of such enzymatically active hydrogels. A customized printer setup including a heatable nozzle and a cooled substrate was established to allow for clean and reproducible prints. The inks and printed hydrogel samples were characterized using rheological measurements and compression tests. All inks were found to be sufficiently printable to create lattices without overhangs, but printing quality was strongly enhanced at 4.5% polymer or more. The produced hydrogels were characterized regarding mechanical strength and diffusibility. For both properties, a strong correlation with polymer concentration was observed with highly concentrated hydrogels being more stable and less diffusible. Agar hydrogels were found to be more stable and show higher diffusion rates than comparable agarose hydrogels. Enzyme leaching was identified as a major drawback of agar hydrogels, while hardly any leaching from agarose hydrogels was detected. The poor ability of agar hydrogels to permanently immobilize enzymes indicates their limited suitability for their employment in perfused biocatalytic reactors. Batch-based activity assays showed that the enzymatic activity of agar hydrogels was roughly twice as high as the activity of agarose hydrogels which was mostly attributed to the increased amount of enzyme leaching. Agarose bioinks with at least 4.5% polymer were identified as the most suitable of the investigated inks for the printing of biocatalytic reactors with AaEst2. Drawbacks of these inks are limited mechanical and thermal stability, not allowing the operation of a reactor at the optimum temperature of AaEst2 which is above the melting point of the employed low-melt agarose.

Development of Biocomposite Alginate-Cuttlebone-Gelatin 3D Printing Inks Designed for Scaffolds with Bone Regeneration Potential

Fabrication of three-dimensional (3D) scaffolds using natural biomaterials introduces valuable opportunities in bone tissue reconstruction and regeneration. The current study aimed at the development of paste-like 3D printing inks with an extracellular matrix-inspired formulation based on marine materials: sodium alginate (SA), cuttlebone (CB), and fish gelatin (FG). Macroporous scaffolds with microporous biocomposite filaments were obtained by 3D printing combined with post-printing crosslinking. CB fragments were used for their potential to stimulate biomineralization. Alginate enhanced CB embedding within the polymer matrix as confirmed by scanning electron microscopy (ESEM) and micro-computer tomography (micro-CT) and improved the deformation under controlled compression as revealed by micro-CT. SA addition resulted in a modulation of the bulk and surface mechanical behavior, and lead to more elongated cell morphology as imaged by confocal microscopy and ESEM after the adhesion of MC3T3-E1 preosteoblasts at 48 h. Formation of a new mineral phase was detected on the scaffold's surface after cell cultures. All the results were correlated with the scaffolds' compositions. Overall, the study reveals the potential of the marine materials-containing inks to deliver 3D scaffolds with potential for bone regeneration applications.

Integrative In Situ Photodynamic Therapy-Induced Cell Death Measurement of 3D-Bioprinted MCF-7 Tumor Spheroids

The development of new in vitro models that closely mimic the tumor microenvironment (TME) to evaluate the efficacy of anticancer drugs has received great attention. In this study, a three-dimensional (3D) bioprinted Michigan Cancer Foundation-7 (MCF-7) cancer spheroid-embedded hydrogel model was suggested for integrative in situ determination of the half-maximal inhibitory concentration (IC₅₀) values of photosensitizers (PSs). The MCF-7 cell-laden alginate/gelatin hydrogel was printed for the fabrication of tumor spheroids. The hydrogel was used to mimic the extracellular matrix (ECM) surrounding the cancer cells in the TME. The fluorescence intensities corresponding to photodynamic therapy (PDT)-induced death of tumor spheroids probed by the laser showed a random distribution in the hydrogel, regardless of the focus of the laser and the vertical-axis direction in which the laser was passed. These results enabled integrative in situ measurement of all tumor spheroids probed by the laser without needing to separate the tumor spheroids in the hydrogel and measure them individually. When compared with two-dimensional (2D) monolayer cultures, very large IC₅₀ values of the PSs, chlorin e6 (Ce6) and sulfonated tetraphenyl porphyrin (sTPP), were achieved in MCF-7 spheroid-embedded hydrogels mainly due to the drug resistance of the tumor spheroids. Additionally, the heterogenic PDT response of single MCF-7 cancer cells in a single tumor spheroid was observed through 3D imaging of irregular apoptosis in a single spheroid since single tumor spheroids showed a heterogenic PDT response. Furthermore, the laser-power-dependent IC₅₀ values of PSs were obtained using the MCF-7 spheroid-embedded hydrogel model.

Hydrogel: A Potential Material for Bone Tissue Engineering Repairing the Segmental Mandibular Defect

Free flap surgery is currently the only successful method used by surgeons to reconstruct critical-sized defects of the jaw, and is commonly used in patients who have had bony lesions excised due to oral cancer, trauma, infection or necrosis. However, donor site morbidity remains a significant flaw of this strategy. Various biomaterials have been under investigation in search of a suitable alternative for segmental mandibular defect reconstruction. Hydrogels are group of biomaterials that have shown their potential in various tissue engineering applications, including bone regeneration, both through in vitro and in vivo pre-clinical animal trials. This review discusses different types of hydrogels, their fabrication techniques, 3D printing, their potential for bone regeneration, outcomes, and the limitations of various hydrogels in preclinical models for bone tissue engineering. This review also proposes a modified technique utilizing the potential of hydrogels combined with scaffolds and cells for efficient reconstruction of mandibular segmental defects.

Cytoprotection of Human Progenitor and Stem Cells through Encapsulation in Alginate Templated, Dual Crosslinked Silk and Silk-Gelatin Composite Hydrogel Microbeads

Susceptibility of mammalian cells against harsh processing conditions limit their use in cell transplantation and tissue engineering applications. Besides modulation of the cell microenvironment, encapsulation of mammalian cells within hydrogel microbeads attract attention for cytoprotection through physical isolation of the encapsulated cells. The hydrogel formulations used for cell microencapsulation are largely dominated by ionically crosslinked alginate (Alg), which suffer from low structural stability under physiological culture conditions and poor cell-matrix interactions. Here the fabrication of Alg templated silk and silk/gelatin composite hydrogel microspheres with permanent or on-demand cleavable enzymatic crosslinks using simple and cost-effective centrifugation-based droplet processing are demonstrated. The composite microbeads display structural stability under ion exchange conditions with improved mechanical properties compared to ionically crosslinked Alg microspheres. Human mesenchymal stem and neural progenitor cells are successfully encapsulated in the composite beads and protected against environmental factors, including exposure to polycations, extracellular acidosis, apoptotic cytokines, ultraviolet (UV) irradiation, anoikis, immune recognition, and particularly mechanical stress. The microbeads preserve viability, growth, and differentiation of encapsulated stem and progenitor cells after extrusion in viscous polyethylene oxide solution through a 27-gauge fine needle, suggesting potential applications in injection-based delivery and three-dimensional bioprinting of mammalian cells with higher success rates.

Biomimetic Vasculatures by 3D-Printed Porous Molds

Despite recent advances in biofabrication, recapitulating complex architectures of cell-laden vascular constructs remains challenging. To date, biofabricated vascular models have not yet realized four fundamental attributes of native vasculatures simultaneously: freestanding, branching, multilayered, and perfusable. In this work, a microfluidics-enabled molding technique combined with coaxial bioprinting to fabricate anatomically relevant, cell-laden vascular models consisting of hydrogels is developed. By using 3D porous molds of poly(ethylene glycol) diacrylate as casting templates that gradually release calcium ions as a crosslinking agent, freestanding, and perfusable vascular constructs of complex geometries are fabricated. The bioinks can be tailored to improve the compatibility with specific vascular cells and to tune the mechanical modulus mimicking native blood vessels. Crucially, the integration of relevant vascular cells (such as smooth muscle cells and endothelial cells) in a multilayer and biomimetic configuration is highlighted. It is also demonstrated that the fabricated freestanding vessels are amenable for testing percutaneous coronary interventions (i.e., drug-eluting balloons and stents) under physiological mechanical states such as stretching and bending. Overall, a versatile fabrication technique with multifaceted possibilities of generating biomimetic vascular models that can benefit future research in mechanistic understanding of cardiovascular diseases and the development of therapeutic interventions is introduced.

A Three-Dimensional Bioprinted Copolymer Scaffold with Biocompatibility and Structural Integrity for Potential Tissue Regeneration Applications

The present study was to investigate the rheological property, printability, and cell viability of alginate−gelatin composed hydrogels as a potential cell-laden bioink for three-dimensional (3D) bioprinting applications. The 2 g of sodium alginate dissolved in 50 mL of phosphate buffered saline solution was mixed with different concentrations (1% (0.5 g), 2% (1 g), 3% (1.5 g), and 4% (2 g)) of gelatin, denoted as GBH-1, GBH-2, GBH-3, and GBH-4, respectively. The properties of the investigated hydrogels were characterized by contact angle goniometer, rheometer, and bioprinter. In addition, the hydrogel with a proper concentration was adopted as a cell-laden bioink to conduct cell viability testing (before and after bioprinting) using Live/Dead assay and immunofluorescence staining with a human corneal fibroblast cell line. The analytical results indicated that the GBH-2 hydrogel exhibited the lowest loss rate of contact angle (28%) and similar rheological performance as compared with other investigated hydrogels and the control group. Printability results also showed that the average wire diameter of the GBH-2 bioink (0.84 ± 0.02 mm (*** p < 0.001)) post-printing was similar to that of the control group (0.79 ± 0.05 mm). Moreover, a cell scaffold could be fabricated from the GBH-2 bioink and retained its shape integrity for 24 h post-printing. For bioprinting evaluation, it demonstrated that the GBH-2 bioink possessed well viability (>70%) of the human corneal fibroblast cell after seven days of printing under an ideal printing parameter combination (0.4 mm of inner diameter needle, 0.8 bar of printing pressure, and 25 °C of printing temperature). Therefore, the present study suggests that the GBH-2 hydrogel could be developed as a potential cell-laden bioink to print a cell scaffold with biocompatibility and structural integrity for soft tissues such as skin, cornea, nerve, and blood vessel regeneration applications.

Non-Invasive Three-Dimensional Cell Analysis in Bioinks by Raman Imaging

3D bioprinting is an emerging biofabrication strategy using bioinks, comprising cells and biocompatible materials, to produce functional tissue models. Despite progress in building increasingly complex objects, biological analyses in printed constructs remain challenging. Especially, methods that allow non-invasive and non-destructive evaluation of embedded cells are largely missing. Here, we implemented Raman imaging for molecular-sensitive investigations on bioprinted objects. Different aspects such as culture formats (2D, 3D-cast, and 3D-printed), cell types (endothelial cells and fibroblasts), and the selection of the biopolymer (alginate, alginate/nanofibrillated cellulose, alginate/gelatin) were considered and evaluated. Raman imaging allowed for marker-independent identification and localization of subcellular components against the surrounding biomaterial background. Furthermore, single-cell analysis of spectral signatures, performed by multivariate analysis, demonstrated discrimination between endothelial cells and fibroblasts and identified cellular features influenced by the bioprinting process. In summary, Raman imaging was successfully established to analyze cells in 3D culture in situ and evaluate them with regard to the localization of different cell types and their molecular phenotype as a valuable tool for quality control of bioprinted objects.

Traction of 3D and 4D Printing in the Healthcare Industry: From Drug Delivery and Analysis to Regenerative Medicine

Three-dimensional (3D) printing and 3D bioprinting are promising technologies for a broad range of healthcare applications from frontier regenerative medicine and tissue engineering therapies to pharmaceutical advancements yet must overcome the challenges of biocompatibility and resolution. Through comparison of traditional biofabrication methods with 3D (bio)printing, this review highlights the promise of 3D printing for the production of on-demand, personalized, and complex products that enhance the accessibility, effectiveness, and safety of drug therapies and delivery systems. In addition, this review describes the capacity of 3D bioprinting to fabricate patient-specific tissues and living cell systems (e.g., vascular networks, organs, muscles, and skeletal systems) as well as its applications in the delivery of cells and genes, microfluidics, and organ-on-chip constructs. This review summarizes how tailoring selected parameters (i.e., accurately selecting the appropriate printing method, materials, and printing parameters based on the desired application and behavior) can better facilitate the development of optimized 3D-printed products and how dynamic 4D-printed strategies (printing materials designed to change with time or stimulus) may be deployed to overcome many of the inherent limitations of conventional 3D-printed technologies. Comprehensive insights into a critical perspective of the future of 4D bioprinting, crucial requirements for 4D printing including the programmability of a material, multimaterial printing methods, and precise designs for meticulous transformations or even clinical applications are also given.

Bioink Formulation and Machine Learning-Empowered Bioprinting Optimization

Bioprinting enables the fabrication of complex, heterogeneous tissues through robotically-controlled placement of cells and biomaterials. It has been rapidly developing into a powerful and versatile tool for tissue engineering. Recent advances in bioprinting modalities and biofabrication strategies as well as new materials and chemistries have led to improved mimicry and development of physiologically relevant tissue architectures constituted with multiple cell types and heterogeneous spatial material properties. Machine learning (ML) has been applied to accelerate these processes. It is a new paradigm for bioprinting. In this review, we explore current trends in bioink formulation and how ML has been used to accelerate optimization and enable real-time error detection as well as to reduce the iterative steps necessary for bioink formulation. We examined how rheometric properties, including shear storage, loss moduli, viscosity, shear-thinning property of biomaterials affect the printability of a bioink. Furthermore, we scrutinized the interplays between yield shear stress and the printability of a bioink. Moreover, we systematically surveyed the application of ML in precision in situ surgical site bioprinting, closed-loop AI printing, and post-printing optimization.

Optimization of the Rheological Properties of Self-Assembled Tripeptide/Alginate/Cellulose Hydrogels for 3D Printing

3D printing is an emerging and powerful technique to create shape-defined three-dimensional structures for tissue engineering applications. Herein, different alginate-cellulose formulations were optimized to be used as printable inks. Alginate (Alg) was chosen as the main component of the scaffold due to its tunable mechanical properties, rapid gelation, and non-toxicity, whereas microcrystalline cellulose (MCC) was added to the hydrogel to modulate its mechanical properties for printing. Additionally, Fmoc-FFY (Fmoc: 9-fluorenylmethoxycarbonyl; F: phenylalanine; Y: tyrosine), a self-assembled peptide that promotes cell adhesion was incorporated into the ink without modifying its rheological properties and shear-thinning behavior. Then, 3D-printed scaffolds made of Alg, 40% of MCC inks and Fmoc-FFY peptide were characterized by scanning electron microscopy and infrared spectroscopy, confirming the morphological microstructure of the hydrogel scaffolds with edged particles of MCC homogeneously distributed within the alginate matrix and the self-assembly of the peptide in a β-sheet conformation. Finally, the cytocompatibility of the scaffolds was tested in contact with the MG63 osteosarcoma cells, confirming the absence of cytotoxic components that may compromise their viability. Interestingly, MG63 cell growth was retarded in the scaffolds containing the peptide, but cells were more likely to promote adhesive interactions with the material rather than with the other cells, indicating the benefits of the peptide in promoting biological functionality to alginate-based biomaterials.

Hybprinting for musculoskeletal tissue engineering

This review presents bioprinting methods, biomaterials, and printing strategies that may be used for composite tissue constructs for musculoskeletal applications. The printing methods discussed include those that are suitable for acellular and cellular components, and the biomaterials include soft and rigid components that are suitable for soft and/or hard tissues. We also present strategies that focus on the integration of cell-laden soft and acellular rigid components under a single printing platform. Given the structural and functional complexity of native musculoskeletal tissue, we envision that hybrid bioprinting, referred to as hybprinting, could provide unprecedented potential by combining different materials and bioprinting techniques to engineer and assemble modular tissues.

Degradable, Photochemically Printable Poly(propylene fumarate)-Based ABA Triblock Elastomers

Additive manufacturing is rapidly advancing tissue engineering, but the scope of its clinical translation is limited by a lack of materials designed to meet specific mechanical properties and resorption timelines. Materials that are printable via photochemical cross-linking, fully degradable, and elastomeric have proven to be particularly challenging to develop. Herein, we report the synthesis of a series of poly(propylene fumarate-b-γ-methyl-ε-caprolactone-b-propylene fumarate) ABA triblock polymers using sequential ring-opening polymerization and ring-opening copolymerization. When cross-linked photochemically using a continuous liquid interface production digital light processing Carbon M2 printer, these ABA-type triblock copolymers are durable elastomers with tunable degradation and elastic properties. The polymers are shown to undergo slow, hydrolytic degradation in vitro with minimal loss of mechanical performance during degradation.

Hydrogels for Tissue Engineering: Addressing Key Design Needs Toward Clinical Translation

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Three-dimensional bioprinting of mesenchymal stem cells using an osteoinductive bioink containing alginate and BMP-2-loaded PLGA nanoparticles for bone tissue engineering

Hydrogels mimicking the physicochemical properties of the native extracellular matrix have attracted great attention as bioinks for three-dimensional (3D) bioprinting in tissue engineering applications. Alginate is a widely used bioink with beneficial properties of fast gelation and biocompatibility; however, bioprinting using alginate-based bioinks has several limitations, such as poor printability, structural instability, and limited biological activities. To address these issues, we formulated various bioinks using bone morphogenetic protein-2 (BMP-2)-loaded poly(lactic-co-glycolic acid) (PLGA) nanoparticles and alginate for mesenchymal stem cell (MSC) printing and induction of osteogenic differentiation. Incorporation of PLGA nanoparticles into alginate could enhance the mechanical properties and printability of the bioink. In particular, Alg/NP_N30 (30 mg/mL PLGA nanoparticles and 3% w/v alginate) was most suitable for 3D printing with respect to printability and stability. BMP-2-loaded PLGA nanoparticles (NP_BMP-2) displayed sustained in vitro release of BMP-2 for up to two weeks. Further in vitro studies indicated that bioinks composed of alginate and NP_BMP-2 significantly induced osteogenesis of the MSCs compared with other controls, evidenced by enhanced calcium deposition, alkaline phosphatase activity, and gene expression of osteogenic markers. Our novel bioink consisting of widely used biocompatible components displays good printability, stability, and osteogenic inductivity, and holds strong potential for cell printing and bone tissue engineering applications.