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3D-printed Bolus in Radiotherapy
A B C

D E F

Figure 4. Schematic illustrations of different 3D printing technologies. (A) Fused deposition modeling printing. (B) Direct ink writing
printing. (C) Inkjet printing. (D) Stereolithography printing. (E) Digital projection lithography printing. (F) Two-photon polymerization
printing.

is deposited to build the target entity. The extruded materials to ensure the printing fidelity and continuity,
filament is deposited at the designed position through the including yield point and shear thinning . The extrusion
[55]
movement of the XY axis to complete the pattern of one of viscoelastic material is considered a transient shear
layer. After finishing one layer, the build platform moves process, which allows the extrusion to be smooth and
down to deposit the next layer. These steps are repeated ensures the deposited filaments retain their shapes. When
until the designed objects are completed . the inks are extruded from the nozzle, such viscoelastic
[51]
At present, the extrusion printing mainly includes liquids can recover their initial storage modulus and
FDM and direct ink writing (DIW). As for the FDM, viscosity. Compared to other printing methods, the DIW
thermoplastic filaments are liquefied into their semi- process can accept a larger variety of materials that
molten state using a heated nozzle. The liquefied materials possess such rheological properties. Therefore, a large
are then solidified when they are cooled below their glass number of polymer materials can be printed using the
transition temperature. Due to the high viscosity and the DIW, including silicone elastomer, PU, and hydrogels .
[56]
swelling of the melted polymer, the resolution of printed The main disadvantages of the DIW technology are its
structures is greatly reduced to a few 100 µ . In addition slow building speed and low resolution. At present, it
[52]
to the basic thermoplasticity, the printed materials must has been reported that the resolution of DIW can just
maintain a balance between the mechanical performance reach a few 100 µ and the highest printing speed of
and the rheological properties, which greatly limits DIW is only 100 mm/s, which is greatly less than that of
the variety of printable materials. Up to now, several photopolymerization printing .
[57]
thermoplastic polymers have been used in the FDM,
including PCL, ABS, PLA, and other thermoplastics . 3.3. Material jetting
[53]
However, inhomogeneity associated with an FDM printed
bolus may also impact the use in radiotherapy. Inkjet printing is an additive manufacturing technology
The DIW method is mainly used to print through which innumerable droplets of printed materials
[58]
viscoelastic materials. Compared with the FDM using are deposited layer-by-layer to form the target entity .
a heated nozzle, DIW utilizes a common needle to The typical setup of inkjet printing is generally composed
extrude the printed materials . After deposition, of a motion platform with jetting heads, an X-Y-Z three
[54]
the curing processes, including cooling curing and axis, and auxiliary curing devices. Low-viscosity liquids
photopolymerization, are used to solidify the printed are ejected from the jetting heads to form the droplets,
structures. The viscoelasticity of material is critical for which are deposited on the building platform and then
DIW, which requires the rheological property of printed solidified . The inkjet printing needs to meet two basic
[59]
32 International Journal of Bioprinting (2021)–Volume 7, Issue 4