Exploring the Future of Hydrogels in Rapid Prototyping: A Review on Current Trends and Limitations - Biologically Responsive Biomaterials for Tissue Engineering - page 208

For example, Schade et al. [ 160 ] developed hydrogel-like scaffolds possessing
well-defined 3D structures using a methacrylated polyurethane and PEGDA as
starting materials. Ovsianikov et al. [ 161 ] also selected PEGDA as starting material
for 2PP scaffolds. More recently, they evaluated the feasibility to produce porous
scaffolds using methacrylamide-modified gelatin developed in our research group
[ 162 ]. The results of their study will be published in due course [ 163 ] .
Table 9.5 summarizes more technical details on the implementation of hydrogels
in the different RP technologies discussed.

9.3.2 Nozzle-Based Systems

9.3.2.1 Working Principles and Recent Trends of Nozzle-Based Systems

The class of nozzle-based systems is characterized by a wide diversification (Fig. 9.2 ).
Fused deposition modelling (FDM), 3D fibre deposition, precision extrusion deposi-
tion (PED), precise extrusion manufacturing (PEM) and multiphase jet solidification
(MJS) are techniques based on a melting process. Generally, the melt process involves
elevated temperatures which are undesirable from the perspective of scaffold bioac-
tivity [ 17 ]. Researchers have therefore tried to bring forth several other techniques
that overcome this limitation by applying a dissolution process, which is attractive
for the processing of hydrogels. Four major nozzle designs have been described in
literature: pressure-actuated, solenoid-actuated, piezoelectric and volume-actuated
nozzles [ 164 , 165 ]. These nozzle types can be found in the following systems.

Pressure-assisted microsyringe (PAM). A technique that resembles FDM without the
need for heat is the PAM technique, developed by Vozzi et al. [ 166 ] . The set-up con-
sists of a 5-20 mm pneumatic-driven glass capillary syringe that can move in the
vertical plane and deposits material on a substrate. The substrate proceeds in the
planar field relative to the syringe. Transforming jpeg or bitmap images into a sequen-
tial list of linear coordinates easily allows to deposit practically any type of structure
in subsequent layers [ 167 ]. Material viscosities, deposition speed, tip diameter and
applied pressure correlate with the final deposited strand dimensions. The PAM sys-
tem has been described in several publications [ 168, 169 ] . Recently, the fabrication
of hydrogel scaffolds was successful with the PAM method [ 170, 171 ] .

Low-temperature deposition modelling (LDM). Proposed by Xiong et al. [ 172 ] in
2002, LDM found its way as an RP system with biomedical applications. The key
feature of this technique is a non-heating liquefying processing of materials [ 172 ] .
Using temperatures below 0 °C, the material solution is solidified when deposited
on the fabrication platform [ 173 ]. The material gets extruded out of a nozzle capable
of moving in the xy -plane onto a built platform movable in the z -direction.
Incorporating multiple nozzles with different designs into the LDM technique gave
existence to multi-nozzle (low-temperature) deposition modelling (MDM, M-LDM)
[165, 165, 174, 175 ]. A multinozzle low-temperature deposition and manufacture