Page 39 - IJB-4-1

3D Printing of hydrogel composite systems: Recent advances in technology for tissue engineering

microdots. The thickness of microstrand can be controlled spreading during extrusion.
by the viscosity of hydrogel, deposition rate, diameter 2.2.3 Pressure-assisted Microsyringe (PAM)
of nozzle, and applied pressure. The material dispensing
head generally moves in x, y, z directions, while the build The PAM technique, similar to FDM without heating
[46]
platform is kept in place. Liquid flow is generated by system is first proposed by Vozzi in 2002 . In the initial
working stepper motor (volume-driven injection nozzle) stage, pneumatic driven glass capillary microsyringe which
or filtered air pressure (pneumatic nozzle). The key point moves in the vertical plane was used to deposit materials
of this technique is to plot hydrogel into a liquid solution on a substrate. Vozzi and his research group modified PAM
[47]
with a carefully designed density that matches that of the systems for hydrogel microfabrication . Compressed air
hydrogel. and pneumatic driven microsyringe were replaced with a
Various materials can be used in 3D plotting system such stepper motor and piston assisted microsyringe, respectively.
as hydrogels, nanocomposite hydrogels, polymer sol, and Moreover,a temperature controlled syringe module with an
bioactive polymers such as proteins [39–42] . Materials with aluminum jacket was added to control the temperature of
low viscosity can also be used in plotting since deposition deposit materials.
takes place in a liquid medium with a matching density. 2.2.4 Low-temperature Deposition and Manufacturing
Moreover, thermal sensitive biocomponents such as growth (LDM)
factor and even cells can be incorportated into hydrogels
because heating is not required. Extruded hydrogels can be Xiong et al. designed LDM systems to overcome heating
[48]
cured by plotting in a reactive solution or by using mixing process . In this technique, temperature is decreased
nozzles with multiple dispensing component. However, to solidify materials. Materials such as hydrogels are
the microstrands of scaffolds constructed by 3D plotting embedded in feeder connected to a screw pump nozzle
technique normally possess smooth surfaces, which cause and injected from the nozzle that can move along
unfavorable environment for initial cell adhesion. Thus the XY axis onto a building stage at a temperature
additional surface treatment has been researched for below 0 °C. Printed scaffolds are necessary to undergo
fabricating the favorable surface with modified initial cell freeze-drying process to remove the solvent. Modified
[43]
adhesion . LDM technique, called multinozzle low-temperature
deposition and manufacturing (M-LDM) was developed
2.2.2 Direct Ink Writing (DIW) by incorporating multiple nozzles with different types
Direct ink writing (DIW) or direct write assembly (DWA) [49] . They are used for fabrication of scaffolds with
was first investigated by Lewis et al. [44] A variety of inks gradient structures and materials by the incorporation of
such as hydrogels, nanoparticle filled inks, colloidal additional jetting nozzles into the LDM process.
suspensions and gels, and organic inks can be printed in 2.2.5 Robocasting
2D and 3D patterns with feature sizes ranging from 100
nm to 1000 μm. Components of DIW apparatus are the Robocasting is also a nozzle based process which was
three-axis translation platform, compressed air supply, originally adapted to produce ceramic scaffold using
[50]
cylindrical nozzle, and optical microscope for real-time highly loaded ceramic slurries . The system is composed
monitoring. The hydrogels are stored in each orifice of stationary dispensing head and movable platform that
mounted on the Z direction motion stage and printed can move in X, Y and Z axis. The slurry injected layer by
through a nozzle onto a moving XY stage. The pressure layer from a syringe has to sustain their weight and the
of orifice and printing speed depend on nozzle diameter weight of next layers to sustain the printed features. Thus,
and rheology of hydrogel, respectively. low viscous slurry or hydrogel alone are inadequate for
There are two important considerations regarding robocasting technique. Recently, hydrogels were applied
the hydrogels used in DIW technique. First, they must as carriers for ceramic powder in this system. Although the
obtain self-supporting ability and spanning shapes final product is a ceramic scaffold that is formed through
with controlled viscoelastic characteristics. Therefore, burning out hydrogels, this result indicates the great
extruded hydrogels should set instantly to enable potential of robocasting process in fabrication of hydrogel-
based composites.
feature retention of the printed structures. Second, high
concentration of nanoparticle or colloid in hydrogels is 2.2.6 Other Apparatus
preferred to reduce shrinkage during the drying process Nozzle-based 3D printing process is a promising technique
of the completed assembly. Generally, 70–85 wt% of to fabricate hydrogel-based composite scaffolds due to its
solid loadings in hydrogels are preferred for assembling versatility in various printing conditions. This technique is
planar and spanning filaments [45] . The nanoparticle or capable of printing large porous structures for infiltrating
colloid network in hydrogels is able to resist compression body fluid and controlling mechanical and biological
stress caused by capillary tension, thereby preventing properties, which cannot be carried out by other hydrogel

6 International Journal of Bioprinting (2018)–Volume 4, Issue 1