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

Figure 3. The schematic images of (A) I3DP-P and (B) I3DP-L printer based inkjet 3D printing system.(adapted with permission from
[139]. Copyright 2016, John Wiley & Sons, Ltd).

accuracy, but the cost of I3DP-L is higher. stimuli-dependent viscosity to be used in various printing
A wide range of materials can be used with both methods which may involve changes in temperature and
Inkjet-based 3D Printer with powder and liquid as these shear thinning to prevent the nozzle from clogging and to
processes are done in room temperature. Moreover, these maintain the intended shape after printing. Research has
techniques offer more options for soft tissue engineering reported the addition of ceramic or metal based nano or
and bioprinting because incorporated biocomponents microparticles as rheology modifiers often interrupt the
are not subjected to deleterious effects of laser-mediated crosslinking of hydrogels, thus decreasing the printability of
[69]
fusion or force by extrusion. However, post processing materials . In addition, the incorporation of these additives
is required as water soluble liquid binders often remain may lead to a decrease in the accuracy of printed scaffolds
after 3D printing. In addition, it is difficult to remove due to an increase in nozzle size or even make the resulting
internal unbound powder or liquid which were trapped material completely unusable. Therefore, many studies have
in the negative spaces such as hollow structures. tried to print hydrogel scaffolds by incorporating additional
hydrogels, soft polymers or inorganic second phases.
3. 3D Printing of Hydrogel Composites 3D printing techniques for the fabrication of hydrogel
Hydrogels provide many advantages for tissue engineering composites can be categorized into (i) polymer or other
and cell delivery applications owing to their tunable hydrogel reinforced composite (ii) particle-reinforced
degradability, biocompatibility, and capacity to be modified. composite (iii) anisotropic filler-reinforced composite, and
However, their inherently poor mechanical properties make (iv) fiber-reinforced composite hydrogel printing systems,
them unsuitable for applications requiring strength such as represented in Table 1. It should be noted that for the
as load bearing components. The rapid biodegradation category (i), hydrogel-reinforced composites, matrix and
behavior of hydrogels also has greatly limited their further reinforcement materials were defined based on the volume
application in the tissue engineering. In addition, in the case fraction of hydrogels in the composites according to our
of biodegradable synthetic hydrogels with polyester chains, framework. For instance, if gelatin has a higher volume
acidic by-products during the hydrolysis degradation fraction than alginate does in their composite, we assume
process of ester bonds were found to induce the side effects that gelatin is the matrix and alginate is the reinforcement
[68]
to the cells . Therefore, the addition of materials including for this gelatin-alginate composite. These categories
metals, ceramics and polymers were essential to improve will provide a platform for designing an appropriate
some of the limitation of hydrogels. combination of materials and 3D printing technique for
Printability is one of the most important criteria to achieving the desired properties. Each system involves an
consider for 3D-printing of hydrogel based composites. It innovative combination of reinforcement and hydrogel
plays a critical role in determining the degree of accuracy matrix that generate not only mechanical strengthening but
and precision relative to the computed spatial and temporal also a plurality of property enhancements such as biological
design. The printability of hydrogel composites requires activity, degradation tunability, and enzyme sensitivity.

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