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Shuai Wang, Jia Min Lee and Wai Yee Yeong

erties of hydrogels. However, these tests might induce folds [106] .
hydrogel deformation that contributes to measurement
errors. In the compression of a hydrogel, mechanical 6. Conclusion
properties are calculated based on load and hydrogel Smart hydrogels are an interesting class of materials
displacement on the Z-axis [101] . As water was being that could be programmed to react to specific stimuli
compressed out of the hydrogel material during mea- such as pH, temperature, light, electric, and magnetic
surement, the mechanical properties of the hydrogel field. Successful printing of smart hydrogel materials
changed over the course of the test. To overcome the will call for a comprehensive understanding of the
limitations of destructive tests, non-destructive cha- materials, bioprinting systems, and bioprinting
racterization method such as indentation test was in- processes. Looking into the future, new characteriza-
troduced. Using an atomic force microscope, the hy- tion and modelling methods are needed to aid the sys-
drogel was indented at a single point. The applied tematic investigation of applying smart hydrogels in
force and depth can be monitored to calculate the hy- bioprinting. With interdisciplinary collaborative inves-
drogel’s mechanical properties [102] . The challenge of
this method is to optimize the contact of the probe tigation, it is predicted that smart hydrogels could re-
without unintentional adhesion to the hydrogel volutionize bioprinting technologies and bring ahead
surface [103] . 3D as well as 4D bioprinting in the future.

5.3 Formulation of Hydrogel Materials Blend for Conflict of Interest and Funding
Printability No conflict of interest was reported by the authors.
It is a challenge to formulate a specific bioprintable References
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5.4 Bio-origami or 4D Bioprinting
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