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Three-Dimensional Printing Technologies for Drug Delivery Applications
Table 4. Characteristics and challenges of syringe extrusion
Characteristics Challenges
Method This process prints semisolid or semi-molten materials, The extrusion forces depend on the viscosity of
such as gels and pastes, through orifice by syringe plunger . the material .
[16]
[10]
After printing, the drying step is required [10,16] .
HME is the main method for creating good quality gels or
pastes containing APIs at room or elevated temperature .
[10]
Material Polymers with crosslinking mechanism or shear thinning APIs are required to be uniformly dispersed in
properties are preferred . the printing material .
[16]
[16]
Syringe extrusion is the main printing technique for many
biocompatible materials .
[16]
Quality The nozzle size of the syringe defines the print resolution . The rheology of raw materials can produce
[16]
To improve the properties, a post-treatment can be used, such inconsistent extrusion patterns [10,11,16] .
as crosslinking . The mechanical strength and durability are low .
[16]
[16]
bottom surface of the vat resin . The use of the DMD
[12]
means that the UV light can cure larger areas of resin
per unit time than that seen in traditional SLA, while
maintaining its high dimensional accuracy (Table 7) [12,14] .
(4) Continuous liquid interface production
Similarly, to DLP, continuous liquid interface production
(CLIP) utilizes a DMD to project digital light into the
polymer vat through an O window, which inhibits the
2
cross-linking of the layer of resin closest to the window,
called the dead-zone, allowing the solidified resin not to
adhere to the window .
[12]
Figure 4. Vat photopolymerization.
2.4. PBF

(1) Stereolithography PBF is defined as the process in which thermal energy
selectively fuses regions of a powder bed , (Figure 5).
[9]
Stereolithography (SLA) uses a vat of UV-cross linkable Similarly, to the binder jetting processes detailed in
polymer resin paired with a UV light source which scans section 2.1.2, once a print layer is completed, the print bed
along the X and Y axes of the surface of the resin in a is lowered by a specified layer thickness, another layer
defined geometry . A single layer of resin is cross-linked, of powder deposited and spread through a roller, and the
[10]
and the build plate lowered a specified layer thickness next layer fused to the previous. PBF comes in two main
between each curing layer to allow for the next layer to forms: selective laser sintering (SLS) and selective laser
be cured on top (Table 5) [12,16] . melting (SLM) [10,16] .

(2) 2PP (1) Selective laser sintering
2PP follows a similar technique. 2PP is a non-linear Selective laser sintering (SLS) technique uses a focused
near infrared (NIR) light process in which two photons layer to selectively scan polymer powder material
are simultaneously absorbed with short laser pulses in a slightly below its melting temperature, while selective
photosensitive material. 2PP, along with DLP and CLIP laser melting (SLM) uses a laser, which fully melts the
in sections 2.3.3 and 2.3.4, may allow for the pre-loading powder, fusing it to the layer below [10,14] . As SLS is mainly
of APIs directly into the liquid prepolymer solution, but used for polymers, it has a wide range of applications for
may suffer from a loss of drug loaded and precision on DDDs purposes; conversely, since SLM is mainly used
printing (Table 6) . for metals, it is not applied for DDDs. These similar
[12]
techniques have comparable properties with respect
(3) DLP
to quality and macroscale resolution; however, SLS
DLP projects UV light onto a digital micro-mirror device techniques are capable of producing parts with lower
(DMD) which projects the light waves onto the top or layer thicknesses and higher flexibility (Table 8) [10,16] .

326 International Journal of Bioprinting (2022)–Volume 8, Issue 4