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Lu, et al.
total scalp irradiation . In addition, Munoz et al. used results in a slow printing speed, which will greatly limit
[9]
SLA method to create a customized nose bolus composed the manufacturing efficiency of personalized bolus. In
of elastomeric materials, which showed compliant, elastic, contrast, photocuring printing can selectively solidify
and water equivalent properties . Compared to the the polymers from a liquid tank, which can perform the
[84]
rigid bolus, the soft bolus created by these two methods patterning process and curing process at the same time.
greatly reduces the patient discomfort and unwanted air Especially for the DLP- and CLIP-based layer-by-layer
gaps. It suggests that the customization of bolus is not printing mode, these photocuring printing methods
only related with the selection of 3D printing methods, enable the direct and rapid construction of bolus. As
but also dependent on the properties of printed materials. an emerging photocuring printing technology, the CAL
Direct printing is a promising approach to rapidly shows a faster patterning speed in comparison to the
achieve the customization of bolus, but 3D printing of DLP method. However, this technology is premature,
bolus using soft polymers is still in the early stage of costly, and the printing precision is still relatively low.
development. An ideal bolus should not only provide Therefore, among different 3D printing techniques, the
a close contact with the patients’ irregular surface, but DLP and CLIP methods based on layer photocuring are
also possess excellent properties, such as flexibility, probably the most ideal technology to print personalized
biocompatibility, and adhesion. The 3D printing methods bolus at present.
determine the printing speed and printing accuracy of Up to now, the printable materials suitable
bolus, while the printed materials mainly influence the for DLP-based photocuring printing are mainly
performance of bolus. With the current 3D printing photosensitive resins, which have the basic characteristic
technology, the bottleneck restricting the application of of photocurability . Photosensitive resins are a class
[86]
3D printed bolus is mainly the deficient development of relatively mature photocurable prepolymers with
of printable polymer materials. The advances in low molecular weights, which mainly include esterified
materials science and 3D printing technology lead to the acrylate epoxy resin, unsaturated polyester, PU, and
development of more printable polymers, which will be polymercaptan/polyene photocurable resin systems .
[87]
used for 3D printing of bolus. Although the photo-resins (Young modulus, ~GPa) have
been widely used in the manufacturing industry, they are
5. Summary and outlook not suitable for constructing bolus by DLP-based printing

Various materials that are able to be processed into bolus due to their higher hardness compared to patients’ skin
have been used in radiation therapy, but the development tissues (~KPa). The mismatching in hardness between
of customized bolus prepared by 3D printing of polymers bolus and skin tissues will inevitably lead to the failure of
is still in its infancy. To achieve the personalized radiotherapy. Besides, the potential toxicity of photo-resin
customization of bolus, this review aims at providing makes it even less likely to be used in clinic. Therefore,
comprehensive insights into the 3D printing bolus. How it is urgent to develop a kind of material that can not only
to choose an appropriate 3D printing technique and to be used for DLP printing, but also has an elastic modulus
design a suitable printable polymer is an urgent issue to equivalent to skin tissues.
be solved in customizing bolus. We focus on three points: Hydrogels and silicon gels have an elastic modulus
(i) polymer materials used for fabricating bolus, (ii) close to that of human skin tissue, while most of these
3D printing techniques suitable for processing polymer materials cannot be directly used for photocuring printing
materials, and (iii) personalized customization of bolus due to a lack of photosensitive properties. To endow the
through 3D printing technology. gels with photocurable ability, some photopolymerizable
Various 3D printing techniques utilizing different functional groups have been grafted onto the molecular
principle to pattern are suitable for different materials. chain of the gels. The modified gels with both
The choice of 3D printing methods is mainly determined photocurable ability and bionic hardness will be a kind of
by the actual requirements of application . A 3D ideal materials for constructing bolus. In addition, it has
[85]
printing technique suitable for processing bolus should been recognized that the bolus-assisted radiotherapy can
have the following three characteristics: rapid printing efficiently control the recurrence of subcutaneous tumor,
speed (<1 h), medium printing scale (X-Y-Z three axis but it also causes some side effects. For example, it is likely
≤30 cm), and general printing accuracy (≤200 µm). to damage the normal tissues around tumors and cause
The extrusion printing and the inkjet printing utilize a dermatitis in the exposed areas. More severely, inflamed
nozzle to deliver the polymers to the designed position areas are susceptible to bacterial infection, which may
and solidify the polymers by a curing process. As for prevent irradiated skin from healing and even aggravate
the nozzle-based printing techniques, most polymers skin necrosis. Therefore, to improve the curative effect of
can be printed as long as they have suitable rheological radiotherapy, it is urgent and necessary to design a new-
properties. However, the point-to-point printing mode type bolus with combined features, including printability,

International Journal of Bioprinting (2021)–Volume 7, Issue 4 37