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3D-printed Bolus in Radiotherapy
requirements, namely, the fluid suitable for ink jetting which stimulates the polymerization . Compared to
[67]
and the ability to solidify into an entity. As a key process the SLA possessing a focal plane, the TPP gets a focal
of inkjet printing, the generation of droplets is mainly spot. Thus, the accuracy of TPP is generally higher than
dependent on both the fluid performance and the 3D SLA. It is reported that the optimal printing resolution
printing parameters . The former includes dynamic of the TPP so far is around 100 nm . TPP is normally
[60]
[68]
viscosity (η), fluid density (ρ) and surface tension (γ), used to fabricate 3D sophisticated objects within several
and the latter encompasses the velocity of the ejected micrometers, but the printing dimensions are very
fluid (ν), the droplet length (l), and the nozzle diameter small, usually limited in 1 cm . For example, using this
3
(d). The inkjet printing can be achieved only when these method, PEGDA gels were processed into 3D helix-
parameters are controlled in an appropriate processing shaped constructs in micron grade . In summary, these
[69]
condition . two laser-based prototyping techniques possess the
[61]
The dimensionless Z parameter is generally used to characteristic of high precision and resolution. However,
determine whether the liquids can be ejected stably. As the processing speed of SLA and TPP is relatively slow,
for the high values of Z, the ejected droplets are easily which may make 3D printing bolus challenging.
to splash into multiple satellite droplets, whereas at To increase the manufacturing speed of
low values of Z, the droplet ejection will be prevented photopolymerization, researchers have developed the
by viscous dissipation. In general, stable jetting can be DLP technology , which makes an entire pattern on
[70]
generated when Z is controlled between 1 and 10 . the focal plane to be crosslinked simultaneously. In this
[62]
Due to such stringent requirements, few polymers method, a dynamic pattern can be projected by using a
can be printed by inkjet printing. Among these polymers, liquid crystal display or a digital micro-mirror device [43,71] .
because many hydrogels have excellent inkjet printing Therefore, the printing speed of DLP is significantly
performance, inkjet printing hydrogels have become a improved in comparison to the point light technology.
very hot topic . Nowadays, the advanced inkjet printing Recently, continuous liquid interface production (CLIP)
[60]
technology possesses a powerful ability to rapidly technology, which is an advanced DLP technology with
construct complex 3D structures using multi-nozzle higher printing speed has been developed. The CLIP
arrays, which can eject more than 100 million droplets can achieve the continuity of printing by forming a
per second . In fact, it is difficult to eject complex fluids polymerization “dead zone,” which allows the 3D objects
[42]
without clogging, limiting the diversity of printable to be printed in minutes instead of hours .
[42]
polymers. Recently, a novel photocuring printing technology
named CAL was developed. By irradiating a rotating
3.4. Vat polymerization volume of photosensitive polymers with a dynamically
As a kind of additive manufacturing process with changing light pattern, CAL can obtain the concurrent
[71]
great potential, photocuring printing uses ultraviolet printing of all points within a 3D object . The
(UV) light to selectively solidify liquid photosensitive conventional photocuring printing, such as DLP, prints
polymers by photocuring layer-by-layer to obtain the objects layer-by-layer, whereas the CAL delivers light
target object [63,64] . Up to now, the photopolymerization energy to the material volume in the form of a series of 2D
printing technology mainly includes stereolithography images. Each image projection is propagated through the
(SLA), digital projection lithography (DLP), two-photon material volume at a different angle. The superposition of
polymerization (TPP), and computed axial lithography light energy causes the whole entity to solidify at 1 time
(CAL). in accordance with the designed geometry. It has been
SLA is the first photocuring printing developed reported that the printing time for a centimeter-scale
in the early 1980s. The SLA uses a point light source object just needed about 30 – 120 s, which is greatly
(e.g. laser) to irradiate only one voxel at a time and faster than the layer-by-layer printing method. However,
prints patterns with the movement of the beam. After the resolution of CAL is limited to sub-millimeters .
[72]
one layer is completed, the building plate moves by one- In summary, polymers suitable for most
layer thickness and a new layer of liquid photosensitive photopolymerization printing should obtain two basic
[73]
polymers turns into the printing regions to print the characteristics: good fluidity and photocurable ability .
next layer. These procedures are repeated layer-by- The photopolymers should be liquid with low viscosity
layer until the printing of the desired object entity is so that the polymers can be evenly spread within the
accomplished [65,66] . In general, the photocuring resin is printable area. In general, the photopolymers have a
mainly used as the printed material of SLA to fabricate specific group, which can be induced to undergo photo
3D objects with large size in the manufacturing industry. crosslinking reactions triggered by the initiators. In
In TPP, two laser pulses with high wavelengths addition to the initiators and the photopolymers, other
intersect to generate a single pulse with a low wavelength, additives are also used to improve the printing quality,

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