Page 10 - IJB-4-2

3D bioprinting processes: A perspective on classification and terminology

deposition of cells. printing strategy can also be observed in hybrid printing
of construct with thermoplastic filament adjacent to
2.2 Material Extrusion cell-laden hydrogel filament. Hybrid bioprinting, that
In material extrusion techniques, biomaterials integrates other fabrication methods such as melt-
are extruded from the nozzle producing defined extrusion and electrospinning with bioprinting, fabricates
structures [26–28] . Extrusion-based printing can be divided construct with enhanced structural fidelity due to the
into three categories based on the mechanism for additional scaffolding material [45–51] .
extruding materials. Mechanical extrusion uses motor Resolution from material extrusion technology is
to drive piston downwardly, where material extrusion is determined by variables such as nozzle size, applied
determined by rate of motor’s displacement; Pneumatic pressure on material, printing speed, substrate wettability,
dispensing system supplies air pressure into the syringe and printing temperature [44,52,53] . Highest resolution of
cartridge, where the difference between supplied 15–400 µm has been reported [30,37,39–41,45,49,54–59] .
air pressure and ambient pressure drives the flow of 2.3 Vat Polymerization Printing
material; the third setup uses a rotary-screw, where
the angular turn of the screw affects the amount of In vat polymerization printing (VPP), a container
material extruded. In extrusion-based bioprinting, both filled with cell-hydrogel suspension is subjected to
pneumatic [29–38] and mechanical systems [39,40] have been selective curing of polymer to form 3D structures. The
commonly used for dispensing cell-hydrogel (Figure components of vat polymerization printing system used
4). The use of rotary screw extrusion in bioprinting for bioprinting resembles closely of those from additive
has been limited only when high viscosity material manufacturing counterparts. VPP systems comprise of an
such as PCL melt is used (Figure 4) [40] . Alternatively, energy source that selectively initiates the polymerization
valves can be placed at the nozzle to regulate the process within the entire vat containing photosensitive
flow of the hydrogel [41] . Such printing method is also polymer. Three dimensional constructs are formed point
termed as microvalve bioprinting [42] . Droplets or by point through laser curing in stereolithography (SLA)
strands of hydrogel can be dispensed using microvalve (Figure 5). Alternatively, UV light can be area-projected
[43]
bioprinting . in digital light processing (DLP) (Figure 5) into the vat
With adjustable pressure setting, extrusion-based of photopolymer using digital micro-mirror device [60–62] .
bioprinting can process material with wide range of Hydrogels used in this printing system are light
7
[44]
viscosity 30–6 × 10 mPa.s . A printing method termed sensitive photopolymers such as polyethylene glycol–
as conformal printing involves deposition of Agarose diacrylate (PEGDA) and gelatin-methylacryloyl
hydrogel filament that act as support structures for (GelMA). Notably, there has been increase research
subsequent cell-hydrogel filament [39] . This conformal interest in using non-UV based systems for

Figure 4. Illustration of Material Extrusion with/without cells.

4 International Journal of Bioprinting (2018)–Volume 4, Issue 2