Biofabrication of living tissues and organs at the clinically-relevant volumes vitally depends on the integration of vascular network. Despite the great progress in traditional biofabrication approaches, building perfusable hierarchical vascular network is a major challenge. Bioprinting is an emerging technology to fabricate design-specific tissue constructs due to its ability to create complex, heterocellular structures with anatomical precision, which holds a great promise in fabrication of vascular or vascularized tissues for transplantation use. Although a great progress has recently been made on building perfusable tissues and branched vascular network, a comprehensive review on the state-of-the-art in vascular and vascularized tissue bioprinting has not reported so far. This contribution is thus significant because it discusses the use of three major bioprinting modalities in vascular tissue biofabrication for the first time in the literature and compares their strengths and limitations in details. Moreover, the use of scaffold-based and scaffold-free bioprinting is expounded within the domain of vascular tissue fabrication.

Three-dimensional (3D) bioprinting is an appealing approach for building tissues; however, bioprinting of mini-tissue blocks (i.e., spheroids) with precise control on their positioning in 3D space has been a major obstacle. Here, we unveil “aspiration-assisted bioprinting (AAB),” which enables picking and bioprinting biologics in 3D through harnessing the power of aspiration forces, and when coupled with microvalve bioprinting, it facilitated different biofabrication schemes including scaffold-based or scaffold-free bioprinting at an unprecedented placement precision, ~11% with respect to the spheroid size. We studied the underlying physical mechanism of AAB to understand interactions between aspirated viscoelastic spheroids and physical governing forces during aspiration and bioprinting. We bioprinted a wide range of biologics with dimensions in an order-of-magnitude range including tissue spheroids (80 to 600 μm), tissue strands (~800 μm), or single cells (electrocytes, ~400 μm), and as applications, we illustrated the patterning of angiogenic sprouting spheroids and self-assembly of osteogenic spheroids.

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Despite extensive use of polydimethylsiloxane (PDMS) in medical applications, such as lab-on-a-chip or tissue/organ-on-a-chip devices, point-of-care devices, and biological machines, the manufacturing of PDMS devices is limited to soft-lithography and its derivatives, which prohibits the fabrication of geometrically complex shapes. With the recent advances in three-dimensional (3D) printing, use of PDMS for fabrication of such complex shapes has gained considerable interest. This research presents a detailed investigation on printability of PDMS elastomers over three concentrations for mechanical and cell adhesion studies. The results demonstrate that 3D printing of PDMS improved the mechanical properties of fabricated samples up to three fold compared to that of cast ones because of the decreased porosity of bubble entrapment. Most importantly, 3D printing facilitates the adhesion of breast cancer cells, whereas cast samples do not allow cellular adhesion without the use of additional coatings such as extracellular matrix proteins. Cells are able to adhere and grow in the grooves along the printed filaments demonstrating that 3D printed devices can be engineered with superior cell adhesion qualities compared to traditionally manufactured PDMS devices.

Bioprinting of cellular aggregates, such as tissue spheroids, to form three-dimensional (3D) complex-shaped arrangements, has posed a major challenge due to lack of robust, reproducible and practical bioprinting techniques. Here, we demonstrate 3D aspiration-assisted freeform bioprinting of tissue spheroids by precisely positioning them in self-healing yield-stress gels, enabling the self-assembly of spheroids for fabrication of tissues. The presented approach enables the traverse of spheroids directly from the cell media to the gel and freeform positioning of the spheroids on demand. We study the underlying physical mechanism of the approach to elucidate the interactions between the aspirated spheroids and the gel’s yield-stress during the transfer of spheroids from cell media to the gel. We further demonstrate the application of the proposed approach in the realization of various freeform shapes and self-assembly of human mesenchymal stem cell spheroids for the construction of cartilage and bone tissues.

Micro- to nanosized droplets of liquid metals, such as eutectic gallium indium (EGaIn) and Galinstan, have been used for developing a variety of applications in flexible electronics, sensors, catalysts, and drug delivery systems. Currently used methods for producing micro- to nanosized droplets of such liquid metals possess one or several drawbacks, including the lack in ability to control the size of the produced droplets, mass produce droplets, produce smaller droplet sizes, and miniaturize the system. Here, a novel method is introduced using acoustic wave-induced forces for on-chip production of EGaIn liquid-metal microdroplets with controllable size. The size distribution of liquid metal microdroplets is tuned by controlling the interfacial tension of the metal using either electrochemistry or electrocapillarity in the acoustic field. The developed platform is then used for heavy metal ion detection utilizing the produced liquid metal microdroplets as the working electrode. It is also demonstrated that a significant enhancement of the sensing performance is achieved by introducing acoustic streaming during the electrochemical experiments. The demonstrated technique can be used for developing liquid-metal-based systems for a wide range of applications.

Extrusion-based bioprinting of hydrogels in a granular secondary gel enables the fabrication of cell-laden three-dimensional (3D) constructs in an anatomically accurate manner, which is challenging using conventional extrusion-based bioprinting processes. In this study, carbohydrazide-modified gelatin (Gel-CDH) was synthesized and deposited into a new multifunctional support bath consisting of gelatin microparticles suspended in an oxidized alginate (OAlg) solution. During extrusion, Gel-CDH and OAlg were rapidly cross-linked because of the Schiff base formation between aldehyde groups of OAlg and amino groups of Gel-CDH, which has not been demonstrated in the domain of 3D bioprinting before. Rheological results indicated that hydrogels with lower OAlg to Gel-CDH ratios possessed superior mechanical rigidity. Different 3D geometrically intricate constructs were successfully created upon the determination of optimal bioprinting parameters. Human mesenchymal stem cells and human umbilical vein endothelial cells were also bioprinted at physiologically relevant cell densities. The presented study has offered a novel strategy for bioprinting of natural polymer-based hydrogels into 3D complex-shaped biomimetic constructs, which eliminated the need for cytotoxic supplements as external cross-linkers or additional cross-linking processes, therefore expanding the availability of bioinks.

The heterogeneous and anisotropic articular cartilage is generally studied as a layered structure of "zones" with unique composition and architecture, which is difficult to recapitulate using current approaches. A novel hybrid bioprinting strategy is presented here to generate zonally stratified cartilage. Scaffold-free tissue strands (TSs) are made of human adipose-derived stem cells (ADSCs) or predifferentiated ADSCs. Cartilage TSs with predifferentiated ADSCs exhibit improved mechanical properties and upregulated expression of cartilage-specific markers at both transcription and protein levels as compared to TSs with ADSCs being differentiated in the form of strands and TSs of nontransfected ADSCs. Using the novel hybrid approach integrating new aspiration-assisted and extrusion-based bioprinting techniques, the bioprinting of zonally stratified cartilage with vertically aligned TSs at the bottom zone and horizontally aligned TSs at the superficial zone is demonstrated, in which collagen fibers are aligned with designated orientation in each zone imitating the anatomical regions and matrix orientation of native articular cartilage. In addition, mechanical testing study reveals a compression modulus of ≈1.1 MPa, which is similar to that of human articular cartilage. The prominent findings highlight the potential of this novel bioprinting approach for building biologically, mechanically, and histologically relevant cartilage for tissue engineering purposes.