Bioprinting and Tissue Engineering: Recent Advances and Future Perspectives

Development of Biocompatible 3D-Printed Artificial Blood Vessels through Multidimensional Approaches

Within the human body, the intricate network of blood vessels plays a pivotal role in transporting nutrients and oxygen and maintaining homeostasis. Bioprinting is an innovative technology with the potential to revolutionize this field by constructing complex multicellular structures. This technique offers the advantage of depositing individual cells, growth factors, and biochemical signals, thereby facilitating the growth of functional blood vessels. Despite the challenges in fabricating vascularized constructs, bioprinting has emerged as an advance in organ engineering. The continuous evolution of bioprinting technology and biomaterial knowledge provides an avenue to overcome the hurdles associated with vascularized tissue fabrication. This article provides an overview of the biofabrication process used to create vascular and vascularized constructs. It delves into the various techniques used in vascular engineering, including extrusion-, droplet-, and laser-based bioprinting methods. Integrating these techniques offers the prospect of crafting artificial blood vessels with remarkable precision and functionality. Therefore, the potential impact of bioprinting in vascular engineering is significant. With technological advances, it holds promise in revolutionizing organ transplantation, tissue engineering, and regenerative medicine. By mimicking the natural complexity of blood vessels, bioprinting brings us one step closer to engineering organs with functional vasculature, ushering in a new era of medical advancement.

Modulus-dependent effects on neurogenic, myogenic, and chondrogenic differentiation of human mesenchymal stem cells in three-dimensional hydrogel cultures

Human mesenchymal stromal cells (hMSCs) are of significant interest as a renewable source of therapeutically useful cells. In tissue engineering, hMSCs are implanted within a scaffold to provide enhanced capacity for tissue repair. The present study evaluates how mechanical properties of that scaffold can alter the phenotype and genotype of the cells, with the aim of augmenting hMSC differentiation along the myogenic, neurogenic or chondrogenic linages. The hMSCs were grown three-dimensionally (3D) in a hydrogel comprised of poly(ethylene glycol) (PEG)-conjugated to fibrinogen. The hydrogel's shear storage modulus (G'), which was controlled by increasing the amount of PEG-diacrylate cross-linker in the matrix, was varied in the range of 100-2000 Pascal (Pa). The differentiation into each lineage was initiated by a defined culture medium, and the hMSCs grown in the different modulus hydrogels were characterized using gene and protein expression. Materials having lower storage moduli (G' = 100 Pa) exhibited more hMSCs differentiating to neurogenic lineages. Myogenesis was favored in materials having intermediate modulus values (G' = 500 Pa), whereas chondrogenesis was favored in materials with a higher modulus (G' = 1000 Pa). Enhancing the differentiation pathway of hMSCs in 3D hydrogel scaffolds using simple modifications to mechanical properties represents an important achievement toward the effective application of these cells in tissue engineering.

Photo-Polymerization Damage Protection by Hydrogen Sulfide Donors for 3D-Cell Culture Systems Optimization

Photo-polymerized hydrogels are ideally suited for stem-cell based tissue regeneration and three dimensional (3D) bioprinting because they can be highly biocompatible, injectable, easy to use, and their mechanical and physical properties can be controlled. However, photo-polymerization involves the use of potentially toxic photo-initiators, exposure to ultraviolet light radiation, formation of free radicals that trigger the cross-linking reaction, and other events whose effects on cells are not yet fully understood. The purpose of this study was to examine the effects of hydrogen sulfide (H₂S) in mitigating cellular toxicity of photo-polymerization caused to resident cells during the process of hydrogel formation. H₂S, which is the latest discovered member of the gasotransmitter family of gaseous signalling molecules, has a number of established beneficial properties, including cell protection from oxidative damage both directly (by acting as a scavenger molecule) and indirectly (by inducing the expression of anti-oxidant proteins in the cell). Cells were exposed to slow release H₂S treatment using pre-conditioning with glutathione-conjugated-garlic extract in order to mitigate toxicity during the photo-polymerization process of hydrogel formation. The protective effects of the H₂S treatment were evaluated in both an enzymatic model and a 3D cell culture system using cell viability as a quantitative indicator. The protective effect of H₂S treatment of cells is a promising approach to enhance cell survival in tissue engineering applications requiring photo-polymerized hydrogel scaffolds.

4D Printing: A Review on Recent Progresses

Since the late 1980s, additive manufacturing (AM), commonly known as three-dimensional (3D) printing, has been gradually popularized. However, the microstructures fabricated using 3D printing is static. To overcome this challenge, four-dimensional (4D) printing which defined as fabricating a complex spontaneous structure that changes with time respond in an intended manner to external stimuli. 4D printing originates in 3D printing, but beyond 3D printing. Although 4D printing is mainly based on 3D printing and become an branch of additive manufacturing, the fabricated objects are no longer static and can be transformed into complex structures by changing the size, shape, property and functionality under external stimuli, which makes 3D printing alive. Herein, recent major progresses in 4D printing are reviewed, including AM technologies for 4D printing, stimulation method, materials and applications. In addition, the current challenges and future prospects of 4D printing were highlighted.

3D Coaxial Bioprinting of Vasculature

Development of a suitable vascular network for an efficient mass exchange is crucial to generate three-dimensional (3D) viable and functional thick construct in tissue engineering. Different technologies have been reported for the fabrication of vasculature conduits, such as decellularized tissues and biomaterial-based blood vessels. Recently, bioprinting has also been considered as a promising method in vascular tissue engineering. In this work, human umbilical vein smooth muscle cells (HUVSMCs) were encapsulated in sodium alginate and printed in the form of vasculature conduits using a coaxial nozzle deposition system. Protocols for cell encapsulation and 3D bioprinting are presented. Investigations including dehydration, swelling, degradation characteristics, and patency, permeability, and mechanical properties were also performed and presented to the reader. In addition, in vitro studies such as cell viability and evaluation of extra cellular matrix deposition were performed.

Effect of Material and Process Specific Factors on the Strength of Printed Parts in Fused Filament Fabrication: A Review of Recent Developments

Additive manufacturing (AM) is rapidly evolving as the most comprehensive tool to manufacture products ranging from prototypes to various end-user applications. Fused filament fabrication (FFF) is the most widely used AM technique due to its ability to manufacture complex and relatively high strength parts from many low-cost materials. Generally, the high strength of the printed parts in FFF is attributed to the research in materials and respective process factors (process variables, physical setup, and ambient temperature). However, these factors have not been rigorously reviewed for analyzing their effects on the strength and ductility of different classes of materials. This review systematically elaborates the relationship between materials and the corresponding process factors. The main focus is on the strength and ductility. A hierarchical approach is used to analyze the materials, process parameters, and void control before identifying existing research gaps and future research directions.

From Microscale Devices to 3D Printing: Advances in Fabrication of 3D Cardiovascular Tissues

Current strategies for engineering cardiovascular cells and tissues have yielded a variety of sophisticated tools for studying disease mechanisms, for development of drug therapies, and for fabrication of tissue equivalents that may have application in future clinical use. These efforts are motivated by the need to extend traditional 2-dimensional (2D) cell culture systems into 3D to more accurately replicate in vivo cell and tissue function of cardiovascular structures. Developments in microscale devices and bioprinted 3D tissues are beginning to supplant traditional 2D cell cultures and preclinical animal studies that have historically been the standard for drug and tissue development. These new approaches lend themselves to patient-specific diagnostics, therapeutics, and tissue regeneration. The emergence of these technologies also carries technical challenges to be met before traditional cell culture and animal testing become obsolete. Successful development and validation of 3D human tissue constructs will provide powerful new paradigms for more cost effective and timely translation of cardiovascular tissue equivalents.

In Vitro Study of Directly Bioprinted Perfusable Vasculature Conduits

The ability to create three dimensional (3D) thick tissues is still a major tissue engineering challenge. It requires the development of a suitable vascular supply for an efficient media exchange. An integrated vasculature network is particularly needed when building thick functional tissues and/or organs with high metabolic activities, such as the heart, liver and pancreas. In this work, human umbilical vein smooth muscle cells (HUVSMCs) were encapsulated in sodium alginate and printed in the form of vasculature conduits using a coaxial deposition system. Detailed investigations were performed to understand the dehydration, swelling and degradation characteristics of printed conduits. In addition, because perfusional, permeable and mechanical properties are unique characteristics of natural blood vessels, for printed conduits these properties were also explored in this work. The results show that cells encapsulated in conduits had good proliferation activities and that their viability increased during prolonged in vitro culture. Deposition of smooth muscle matrix and collagen was observed around the peripheral and luminal surface in long-term cultured cellular vascular conduit through histology studies.

Tubular organ epithelialisation

Hollow, tubular organs including oesophagus, trachea, stomach, intestine, bladder and urethra may require repair or replacement due to disease. Current treatment is considered an unmet clinical need, and tissue engineering strategies aim to overcome these by fabricating synthetic constructs as tissue replacements. Smart, functionalised synthetic materials can act as a scaffold base of an organ and multiple cell types, including stem cells can be used to repopulate these scaffolds to replace or repair the damaged or diseased organs. Epithelial cells have not yet completely shown to have efficacious cell-scaffold interactions or good functionality in artificial organs, thus limiting the success of tissue-engineered grafts. Epithelial cells play an essential part of respective organs to maintain their function. Without successful epithelialisation, hollow organs are liable to stenosis, collapse, extensive fibrosis and infection that limit patency. It is clear that the source of cells and physicochemical properties of scaffolds determine the successful epithelialisation. This article presents a review of tissue engineering studies on oesophagus, trachea, stomach, small intestine, bladder and urethral constructs conducted to actualise epithelialised grafts.

Engineering alginate as bioink for bioprinting

Recent advances in three-dimensional (3-D) printing offer an excellent opportunity to address critical challenges faced by current tissue engineering approaches. Alginate hydrogels have been used extensively as bioinks for 3-D bioprinting. However, most previous research has focused on native alginates with limited degradation. The application of oxidized alginates with controlled degradation in bioprinting has not been explored. Here, a collection of 30 different alginate hydrogels with varied oxidation percentages and concentrations was prepared to develop a bioink platform that can be applied to a multitude of tissue engineering applications. The authors systematically investigated the effects of two key material properties (i.e. viscosity and density) of alginate solutions on their printabilities to identify a suitable range of material properties of alginates to be applied to bioprinting. Further, four alginate solutions with varied biodegradability were printed with human adipose-derived stem cells (hADSCs) into lattice-structured, cell-laden hydrogels with high accuracy. Notably, these alginate-based bioinks were shown to be capable of modulating proliferation and spreading of hADSCs without affecting the structure integrity of the lattice structures (except the highly degradable one) after 8days in culture. This research lays a foundation for the development of alginate-based bioink for tissue-specific tissue engineering applications.