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Lu, et al.
of composite hydrogels, including slide-ring hydrogels, have been developed . Especially for polymer materials
[45]
double network hydrogels, and nanocomposite hydrogels, with various polymerization characteristics, four
have been developed by introducing an effective printing techniques are mainly used, including powder
energy dissipation mechanism [39,40] . For example, a bed fusion, material extrusion, material jetting, and vat
nanocomposite hydrogel composed of hectorite clay polymerization (Figure 4). The performance of different
and N-isopropylacrylamide has an elongation of up to printing technologies is listed in Table 2.
1300%. A tough and stretchable hydrogel with double
networks was created by mixing covalently crosslinked 3.1. Powder bed fusion
PAM and ionically crosslinked alginate. The resulting As a kind of additive manufacturing process, power bed
composite hydrogel can be stretched more than 20 times fusion makes use of a laser to sinter powdered materials.
of its original length. It also showed excellent notch- It is also called selective laser sintering (SLS) according
insensitive and self-recovery performance . Hou to the phase states of powder bonding . With the
[41]
[46]
et al. have reported a composite hydrogel (PU/PAM) help of lasers that automatically aim at points in space
composed of PU and PAM as bolus. This novel hydrogel manipulated by a 3D control system, the powdered
with excellent mechanical, self-healing and adhesive materials are bonded together to form a solid structure.
properties, can provide an optimal dose distribution for The manufacturing process of SLS consists of three
radiotherapy . Over the past decade, major breakthroughs repeated steps . First, the powdered materials are
[7]
[47]
have been made in the research of composite hydrogels uniformly distributed as a printing layer by scraping or
with strengthened mechanics. We believe that these rolling. Second, the powder is selectively fused to form
advances will lead to a framework that helps construct a solid structure by scanning the laser. Third, to print the
an ideal bolus in radiotherapy through rational design of next layer, the build platform descends one layer. These
hydrogels. Therefore, with the development of hydrogel three steps are repeated until the SLS process is finished.
research, more and more hydrogels will be developed to Compared to other 3D printing, SLS does not require
construct boluses used for radiotherapy in the future. additional supporting materials. In addition, the powder
In this section, we mainly review the current in the non-molten region can be recycled after printing,
soft polymers used to prepare boluses and analyze the resulting in a material utilization rate of close to 100%.
physicochemical properties of these materials as boluses. The resolution of SLS printed parts is largely
Compared with plastics, elastomers, and hydrogels with dependent on the particle size: the larger the particle and
excellent flexibility and tissue equivalence are more the lower the spatial resolution. However, for the sake
suitable to construct boluses. The properties of the of safety, cost efficiency and process ability, the size of
material play an important role in radiotherapy, but to powder particles is usually limited to a range of 10 –
form full contact with the irregular surface of patients’ 100 µ. Up to now, the resolution of SLS can reach 100
skin, the structure of the bolus needs to be customized. µm under optimal conditions .
[48]
In general, powdered materials used in SLS should
3. 3D printing technology suitable for possess several properties, such as compactness, good
processing polymers fluidity, and thermal stability. During the first step of SLS

As a promising additive manufacturing, 3D printing process, good fluidity and compactness are the key factors
has become a versatile technology for manufacturing to ensure proper coalescence in the subsequent sintering.
3D structures composed of different materials, such as In addition, the most crucial requirement for SLS powders
ceramics, metals, and polymers . According to the is the thermal properties that allow the powders to
[42]
[49]
digital data of 3D models, the designed 3D objects can solidify uniformly during melting and sintering . As for
be processed layer-by-layer. Different from traditional soft polymer materials used for SLS, these requirements
manufacturing methods, 3D printing can rapidly turn are extraordinarily harsh. At present, few soft polymer
digital-aided designs into 3D complex objects without powders are processed using SLS, such as polycarbonate
wasting any materials. According to the printing (PC), PCL, and thermoplastic elastomers (TPEs). There
materials and the principles, print heads or laser optics is a popular view that the biomedical fields are the most
[50]
are generally used to deposit one layer of 3D objects. active areas for SLS using soft polymer materials ;
During the process of printing, the deposited regions are however, SLS is not well suited for processing soft bolus
crosslinked or solidified to yield entities . In addition, with good softness and tissue equivalence due to its strict
[43]
the ability of 3D printing to quickly produce products on requirements for materials.
demand has greatly boosted the academic research and 3.2. Material extrusion
the industrial production . Up to now, based on different
[44]
principles and materials, over dozen types of 3D printing In extrusion printing, the polymer materials are extruded
technologies that meet the nature of different materials through a nozzle to form a continuous filament, which

International Journal of Bioprinting (2021)–Volume 7, Issue 4 31