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Bottom-Up Microvessel Engineering
researchers commonly printed the microvascular However, 3D stereolithography has incomparable
structures together with other microarchitectures efficiency. Moreover, bioprinting devices specifically
simultaneously. However, stereolithography originally designed for printing microvessels are expected to achieve
does not allow the printing of multiple materials. Shanjani more attractive efficiency [88-90] . The extrusion-based
and Yan developed special devices to print more than one bioprinting has relatively low printing speed and poor
kind of material [93,94] . The printed artificial tissue including cell viability [82-85] . Except the extrusion-based bioprinting,
microvascular structure is shown in Figure 4C. However, engineering microvessels using bioprinting can achieve
patterned light utilized in bioprinting based on optical satisfied cell viability [98,99] . When the high mechanical
stereolithography requires the bioink to be transparent, property is required, the extrusion-based bioprinting
and the system is extremely complex and expensive. strategies are recommended. A disadvantage of using
the inkjet-based bioprinting to engineer microvessels is
5.4. Thermal stereolithography the weak mechanical property of the fabricated artificial
Thermally responsive gels have the potential to build microvessels. The optical stereolithography can guarantee
large artificial tissues with complex architectures in the resolution of printing the microvessels to as low as 1
the presence of precise local heating [95-97] . As shown in μm, but the system setup is much more complex than the
Figure 4D, to realize layer-by-layer curing and printing bioprinting based on inkjet and extrusion. Laser-assisted
of microvessels, a microheater array is fabricated to shape bioprinting also needs a complex system setup.
the thermally responsive gel in 2D plane. Specifically, Although bio-assembling and bioprinting techniques
a microheater with a 2D arrangement structure is used for engineering microvessels are all based on the “bottom-
[21-32,98,99,101]
as a DMD that allows area-selective heating at any up” concept , they still have significant
position . Since the temperature distribution on the differences in the following aspects: resolution, fabrication
[79]
glass substrate can be digitally processed, when combined efficiency, mechanical property, and complexity of
with a thermally responsive polymer, its temperature fabricated microvessels. Limited by the speed of
variation can be controlled, and high-speed graphing can integrating the most recent advanced microfabrication
be achieved. This system allows reversible pasting and techniques into the bioprinting devices, the bio-
liquefaction at arbitrary locations. The relatively simpler assembled microvessels still hold a higher resolution
than the bioprinted structure, but the gap will be bridged
system and low cost make it an extremely promising by the effort in developing the bioprinting devices over
solution for building microvascular structures.
time [30,34-44,77-97,100] . At present, bioprinting has much higher
6. Discussion and prospects fabrication efficiency in engineering the microvessels
from the bottom up; however, it is not absolute. Recent
Emerging microfabrication techniques have significantly advances in micromanipulation keep improving the
advanced the bio-assembling for engineering efficiency of the bio-assembling [61-71] . The self-assembly
microvessels [28,29,33,39,54-76] . Since the smaller and more precise and on-chip assembly achieved satisfied fabrication
fabricated micro modular tissues are now available, we can efficiency but inevitably sacrificed the complexity of
build larger artificial tissues without sacrificing the necessary the engineered microvessels. Fully automated robotic
microarchitectural feature [30,34-44] . The newly developed microassembly provides a solution to the problem
micromanipulation methods are necessary for the building caused by the conflict of the fabrication efficiency and
of microvessels by modules with varied geometries. Fully the complexity of the engineered microvessels. Although
automated assembly utilizing the robotic micromanipulation, the fabricated micromodules have excellent mechanical
self-assembly, and on-chip assembly by microfluidic properties, the overall mechanical property highly relies
devices significantly improve the efficiency of fabricating on the secondary crosslinking quantity. Moreover,
the microvessels by bio-assembling approaches [61-71] . smaller gaps between the assembled micromodules
Engineering microvessels by bioprinting highly achieved by the precise assembly contribute to better
relies on the overall development of bioprinting [77-100] . mechanical property [61-67] . The mechanical property of
With the advanced bioinks featuring short curing the printed microvessels depends on the utilized bioinks
time and better mechanical property, and efficient and the respective solidification mechanisms [51-53,98,99] .
respective bioprinting devices, we can now achieve Sometimes, researchers are in a dilemma of either
fabrication of the microvessels with high throughput [98,99] . choosing higher efficiency to reduce the curing time or
Conventional inkjet-based bioprinting holds advantages opting for better mechanical property in the products at
for printing microvessels together with the other tissues the expense of longer curing time. Both bio-assembling
simultaneously with composite bioprinting techniques and bioprinting hold the potential in constructing complex
while the 3D stereolithography can only deal with one 3D microvascular networks. Bio-assembling can construct
bioink containing the same cell source [77-81,91-94,100] . complex microvessels by fabricating the complex 2D

12 International Journal of Bioprinting (2021)–Volume 7, Issue 3