Cardiovascular diseases are the leading cause of deaths throughout the world. Vascular diseases are mostly treated with autografts and blood vessel transplantations. However, traditional grafting methods have several problems including lack of suitable harvest sites, additional surgical costs for harvesting procedure, pain, infection, lack of donors, and even no substitutes at all. Recently, tissue engineering and regenerative medicine approaches are used to regenerate damaged or diseased tissues. Most of the tissue engineering investigations have been based on the cell seeding into scaffolds by providing a suitable environment for cell attachment, proliferation, and differentiation. Because of the challenges such as difficulties in seeding cells spatially, rejection, and inflammation of biomaterials used, the recent tissue engineering studies focus on scaffold-free techniques. In this paper, the development of novel computer aided algorithms and methods are developed for 3D bioprinting of scaffold-free biomimetic macrovascular structures. Computer model mimicking a real human aorta is generated using imaging techniques and the proposed computational algorithms. An optimized three-dimensional bioprinting path planning are developed with the proposed self-supported model. Mouse embryonic fibroblast (MEF) cell aggregates and support structures (hydrogels) are 3D bioprinted layer-by-layer according to the proposed self-supported method to form an aortic tissue construct.

Using decellularized extracellular matrix (dECM) hydrogels as bioinks has been an important step forward for bioprinting of functional tissue constructs, considering their rich microenvironment and their high degree of biomimicry. However, directly using dECM hydrogels as bioinks may not be suitable for bioprinting processes because of the loss of shape fidelity and geometrical precision of bioprinted structure due to their slow gelation kinetics. In this article, the development and direct bioprinting of dECM hydrogel bioink from bovine Achilles tendon were presented. The developed bioink is used for a microcapillary-based bioprinting process without any support structure and/or any additional cross-linker components. The reported decellularization and solubilization methods yield dECM pre-gels which turn into stable hydrogels in a short time at physiological conditions. The gelation kinetics and mechanical strength of bioinks with different concentrations and digestion times are characterized. A support structure-free 3D bioprinting of the developed bioink is shown by aspirating dECM bioinks and then in situ gelation and extrusion through a fine microcapillary nozzle. The viability assays indicate that the developed dECM bioink has no cytotoxic effect on encapsulated NIH 3T3 cells and the cells show lineage-specific morphology in the early days of culture as well.

Three-dimensional bioprinting of cell-laden hydrogels in a sacrificial support-bath has recently emerged as a potential solution for fabricating complex biological structures. Physical properties of the support-bath strongly influence the bioprinting process and the outcome of the fabricated constructs. In this study, we reported the application of a composite Pluronic-nanoclay support-bath including calcium ions as the crosslinking agent for bioprinting of cell-laden alginate-based hydrogels. By tuning the rheological properties, a shear-thinning composite support-bath with fast self-recovery behavior was yielded, which allowed continuous printing of complex and large-scale structures. The printed structures were easily and efficiently harvested from the support-bath without disturbing their shape fidelity. Moreover, the results showed that support-bath assisted bioprinting process did not influence the viability of cells encapsulated within hydrogel. This study demonstrates that Pluronic-nanoclay support-bath can be utilized for bioprinting of complex, cell-laden constructs for vascular and other tissue engineering applications.

Three-dimensional (3D) bioprinting enables the controlled fabrication of complex constructs for tissue engineering applications and has been actively explored in recent years. However, its progress has been limited by the existing difficulties in the development of bioinks with suitable biocompatibility and mechanical properties and at the same time adaptability to the process. Herein, we describe the engineering of a nanocomposite agarose bioink with tailored properties using 2D nanosilicate additives. The suitability of agarose for 3D bioprinting has been debated due to its bioinert nature and compatibility with extrusion-based bioprinting. Nanosilicates were used to tailor the flow behavior of agarose solutions, and detailed rheological characterization of different bioink formulations was performed to elucidate the effect of nanosilicates on the flow behavior and gelation of agarose bioinks. The proper selection of nanosilicate concentration resulted in extrusion 3D printed structures with high shape fidelity and structural integrity. Moreover, the influence of addition of nanosilicates on the bioactivity of agarose was studied, and nanocomposite bioinks showed significant improvement in metabolic activity of encapsulated cells. The bioactivity of the nanocomposite bioinks was also evaluated through a cell spreading assay; the charged surfaces of nanosilicates resulted in full spreading and elongation of fibroblasts, and the extent of change in morphology of cells was found to be directly correlated with the nanosilicate concentration. Our findings suggested that engineered agarose-nanosilicate bioinks can be exploited as a new generation of hydrogel bioinks for extrusion 3D bioprinting with tunable flow properties and bioactivity.

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A single additive, a grafted copolymer, is designed to ensure the stability of suspensions of highly loaded iron oxide nanoparticles (IOPs) and to facilitate three-dimensional (3D) printing of these suspensions in the filament form. This poly (ethylene glycol)-grafted copolymer of N-[3(dimethylamino)propyl]methacrylamide and acrylic acid harnesses both electrostatic and steric repulsion to realize an optimum formulation for 3D printing. When used at 1.15 wt % (by the weight of IOPs), the suspension attains ∼81 wt % solid loading-96% of the theoretical limit as calculated by the Krieger-Dougherty equation. Rectangular, thick-walled toroidal, and thin-walled toroidal magnetic cores and a porous lattice structure are fabricated to demonstrate the utilization of this suspension as an ink for 3D printing. The electrical and magnetic properties of the magnetic cores are characterized through impedance spectroscopy (IS) and vibrating sample magnetometry (VSM), respectively. The IS indicates the possibility of utilizing wire-wound 3D printed cores as the inductive coils. The VSM verifies that the magnetic properties of IOPs before and after the ink formulation are kept almost unchanged because of the low dosage of the additive. This particle-targeted approach for the formulation of 3D printing inks allows embodiment of a fully aqueous system with utmost target material content.

Scaffold-based tissue engineering approaches have been commonly used for skin regeneration or wound healings caused by diseases or trauma. For an ideal complete healing process, scaffold structures need to meet the criteria of biocompatibility, biodegradability, and antimicrobial properties, as well as to provide geometrical necessities for the regeneration of damaged tissue. In this study, design, synthesis and characterization of a three dimensional (3D) printable copolymer based on polycaprolactone-block-poly(1,3-propylene succinate) (PCL-PPSu) including anti-microbial silver particles is presented. 3D printing of PCL-PPSu copolymers provided a lower processing temperature compared to neat PCL, hence, inclusion of temperature-sensitive bioactive reagents into the developed copolymer could be realized. In addition, 3D printed block copolymer showed an enhanced hydrolytic and enzymatic degradation behavior. Cell viability and cytotoxicity of the developed copolymer were evaluated by using human dermal fibroblast (HDF) cells. The addition of silver nitrate within the polymer matrix resulted in a significant decrease in the adhesion of different types of microorganisms on the scaffold without inducing any cytotoxicity on HDF cells in vitro. The results suggested that 3D printed PCL-PPSu scaffolds containing anti-microbial silver particles could be considered as a promising biomaterial for emerging skin regenerative therapies, in the light of its adaptability to 3D printing technology, low-processing temperature, enhanced degradation behavior and antimicrobial properties.

Bacteria are one of the significant causes of infection in the body after scaffold implantation. Effective use of nanotechnology to overcome this problem is an exciting and practical solution. Nanoparticles can cause bacterial degradation by the electrostatic interaction with receptors and cell walls. Simultaneously, the incorporation of antibacterial materials such as zinc and graphene in nanoparticles can further enhance bacterial degradation. In the present study, zinc-doped hydroxyapatite/graphene was synthesized and characterized as a nanocomposite material possessing both antibacterial and bioactive properties for bone tissue engineering. After synthesizing the zinc-doped hydroxyapatite nanoparticles using a mechanochemical process, they were composited with reduced graphene oxide. The nanoparticles and nanocomposite samples were extensively investigated by transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. Their antibacterial behaviors against Escherichia coli and Staphylococcus aureus were studied. The antibacterial properties of hydroxyapatite nanoparticles were found to be improved more than 2.7 and 3.4 times after zinc doping and further compositing with graphene, respectively. In vitro cell assessment was investigated by a cell viability test and alkaline phosphatase activity using mesenchymal stem cells, and the results showed that hydroxyapatite nanoparticles in the culture medium, in addition to non-toxicity, led to enhanced proliferation of bone marrow stem cells. Furthermore, zinc doping in combination with graphene significantly increased alkaline phosphatase activity and proliferation of mesenchymal stem cells. The antibacterial activity along with cell biocompatibility/bioactivity of zinc-doped hydroxyapatite/graphene nanocomposite are the highly desirable and suitable biological properties for bone tissue engineering successfully achieved in this work.