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Bottom-Up Microvessel Engineering
based fluidic assembly method . The method allows the could naturally generate cellular traction as a contractile
[65]
assembly of ring-shaped modules in an open environment force, which is generated by actin-globin interactions and
using microbubble-excited microflow and automates the actin polymerization, pulling toward the center of the cell
assembly process by a multi-micromanipulator system body. Thus, various 3D microstructures can be generated
with the assistance of computer vision techniques. This by changing the geometric design of the 2D template.
non-contact assembly method does not involve any Figure 3E shows a schematic diagram of the microvascular
picking and releasing operations, thus minimizing the structure generation based on such a principle. It is a
time spent in precise micromanipulation. The operations highly biocompatible, simple, and efficient technique for
in the open environment and the robotic system also encapsulating cells into microstructures with just one step.
allow the flexible assembly of the microvessels with It is particularly suitable for the efficient production of
varied sizes. The outer diameter of the assemble simple microvessels. The outer diameter of the microvessels
microvessels ranged 150-500 μm. The reported longest fabricated by this method can be as small as 50 μm.
microvessels adopting this method were longer than
3 mm. This research suggests that the combination of 4.6. Attaching on rod
fluidic assembly and robotic assembly can be a potential Based on the research on the electrochemical
solution for micro-assembly where high throughput and detachment and patterning of the self-assembled cellular
flexibility are required. monolayers [74,75] , Seto et al. proposed a microvessel

4.4. Spinning fibers fabrication method combining the self-assembly of
the cells based on chemical bond and electrochemical
In the fabrication of artificial microvessels, spinning detachment (Figure 3F) [22,76] . Human umbilical vein
the cell-laden microfibers into tubular structures is endothelial cells are attached to the gold surface by
also an important assembly approach. As shown in an oligopeptide. In this research, the oligopeptide
Figure 3D, Sun et al. successfully constructed a spring- CCRRGDWLC chemically adhered to the gold surface
like microstructure for promoting the formation of the of the rod driven by the gold–thiolate bond. Then, the
microvascular structures in a 3D environment . By cells were automatically assembled on the rod and
[66]
including magnetic materials inside the microfibers, Sun formed a tubular cellular monolayer. Finally, to separate
et al. set a magnetic tweezer system to operate and assemble the microvascular structure from the rod, the cells are
these magnetic microfibers by direct mechanical contact. detached by applying a negative potential to the rod
The magnetic microfibers can be guided to move around for reductively splitting the gold–thiolate bond. This
a rod to form microvascular structures directly [67-69] . This technique can detach more than 90% of the attached cells
method required high-precision manipulation, and the within minutes of applying a negative potential.
efficiency was extremely low. In another research work,
magnetic alginate microfibers were used to fabricate the 5. Devices for direct bioprinting
ring-shaped microstructures by spinning first. Then, ring- 5.1. Inkjet, extrusion, and laser-assisted
shaped modules were picked by the magnetic tweezers
in a non-contact way and subsequently stacked along bioprinting
the micropillars [70,71] . This method has higher operational The inkjet, extrusion, and laser-assisted bioprinting all adopt
efficiency than the direct fiber spinning by the magnetic a point-by-point printing approach, featuring excellent
tweezer and no damage caused by mechanical contact. The flexibility in constructing large high-resolution tissues with
diameter of the microvessels assembled by microfibers complex microarchitectures. Figure 4A shows the general
can be controlled to smaller than 1 mm. concept of using these bioprinting methods for fabricating
microvascular structures . The inkjet bioprinting is
[77]
4.5. Self-folding origami of microplates
developed by modifying the conventional 2D inkjet
The traditional art of origami has always been popular printer [78-81] . The common ink is replaced by the bioink,
because it allows the production of various 3D sculptures and a Z motorized stage is employed to expand the 2D
by folding only 2D pages. In recent years, engineers printing to 3D. Due to the well-developed 2D inkjet printing
inspired by have applied the art of origami in a variety of devices, inkjet bioprinting costs the least among the existing
ways, including the spatial deployment of solar panels for bioprinting methods. It is capable of printing microvessels
manufacturing, flexible medical scaffolds, microrobots, with outer diameter smaller than 300 μm [80,81] . According to
and DNA objects [29,72,73] . Kuribayashi-Shigetomi et al. the working mechanism of the inkjet, the inkjet bioprinting
introduced a technique called cellular origami using can be divided into two types categorized by the use of
living cells as the driving force for self-folding to create a either the piezoelectric actuator or the thermal actuator.
variety of different 3D cellular microstructures, including With multiple inkjets and varied inks containing different
microvascular structures . They mentioned that cells cell sources, inkjet bioprinting is capable of composite
[39]
10 International Journal of Bioprinting (2021)–Volume 7, Issue 3