Due to the wide range of applicable materials, extrusion-type biological 3D printing has become the most widely used printing method at present, but in the construction of large-size and high-precision tissue structures, traditional extrusion printing is slightly insufficient. In order to overcome the limitations of the existing extrusion printing, the application of suspension printing was born, marking the new era of biological 3D printing based on extrusion. So what exactly is suspended printing? Recently, the “3D Printing in Suspension Baths: Keeping the Promises of Bioprinting Afloat” review paper published on the “Trends in Biotechnology” by the Melchels team of Heriot-Watt University in Edinburgh, UK, introduced the technical principles and advantages of suspension printing in detail Several typical applications of this technology are discussed.
- What is suspended printing?
Suspension printing is based on the traditional extrusion printing, which adds a suspension medium, and the nozzle prints in the suspension medium. In detail, the suspension medium exhibits solid characteristics when no external force is applied or when the external force is very small, so as to realize the self-support of the printing structure; when the printing nozzle moves, the yield stress generated causes the flow of the suspension medium, showing liquid After the printing nozzle passes, due to the self-healing nature of the suspended medium, its microstructure is restored spontaneously to ensure the realization of printing (Figure 1). Therefore, the success of printing has a lot to do with the characteristics of suspended media.
- Forming performance advantage of suspension printing
Suspension printing is quick to get started and has a lower threshold. At present, laboratories using extrusion 3D printers only need to add an additional suspension medium to achieve suspension printing. The advantages of floating printing are: the printing structure is not easy to collapse, the printing speed is greatly improved, the printing material has no dehydration problems, and omnidirectional printing can be realized (not just limited to bottom-up layer-by-layer deposition). Suspended printing has two purposes: the first is used as an auxiliary during the printing process, and the printed structure is extracted after the end. The methods commonly used to extract the printed structure include heating the melting suspension medium, diluting the suspension medium, changing the PH value to degrade the suspension medium, and enzyme The degradation method decomposes the suspension medium; the second is to remove the printing structure and retain the suspension medium, which can be used for the construction of vascularized tissue structures.
- The biological performance advantages of suspension printing
Traditional 3D printing biomimetic structures often use high-viscosity bio-ink to ensure stable deposition, but using this method will damage cell activity. With suspension printing, low-viscosity biological ink can be used, and only the suspension medium is used to support the printing structure without other supports, thereby ensuring cell survival rate. Suspension printing can realize collagen, fibrin, and pure cells without biological materials that cannot be printed by traditional extrusion 3D printing, as well as the self-supporting structural features of many organs and tissues.
In terms of biological performance, the suspension medium can also be used as an extracellular matrix (ECM). The suspension medium shows solid characteristics when no external force is applied or when the external force is very small, so that it can provide a matrix with greater mechanical properties for cell culture. . Many tissues are layered in structure, which means that the proliferation of cells in this structure requires structures with different resolutions. However, the printing resolution limits the printing time, which results in cell performance being affected. However, suspension printing can provide different resolutions in all directions Print rate so as to solve this problem. The material properties of the suspension medium are conducive to the omnidirectional printing of biological ink in three-dimensional space, which provides a good way to solve the problem of constructing vascularization.
Figure 2 Development of omnidirectional 3D printing technology: from yielding stress fluid to suspension. (A) Schematic diagram of 3D printing blood vessel network using red bio-ink; (B) Suspended printing miniature matryoshka; (C) Fluorescent labeling ink Green) Surrounded by a continuous spiral structure (red); (D) Print a hollow tubular rope knot in suspension medium with fluorescent microspheres; (E, F) Human brain model printed in alginate suspension medium
Figure 3(A) outlines two ways of suspending printing: curing the printed structure after printing is completed and removing the suspended medium (above); curing the suspended medium after printing is completed and removing the printed structure (below); (B1) suspending in the embryoid The ink after the printing is sacrificed in the medium. (B2) (upper picture) organ unit block; (lower picture) the cross section of the embryonic body suspension medium structure after removing the printing structure; (C1) in the carboxyvinyl polymer suspension medium Printed structure of highly branched tubular network (C2) after removing suspended media
Figure 4 Suspension printing for human heart bio 3D printing. (AC) Bio 3D printing of a miniaturized, cell-rich human heart; (D) 3D printed heart removed from the culture medium and then infused with red and blue dyes , To demonstrate the printed hollow chamber; (EF) two suspension media composed of gelatin particles; GH) collagen heart valve structure printed in (F)