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Shuai Wang, Jia Min Lee and Wai Yee Yeong

to displace the materials via jetting. The material re- (1) Printablility: The hydrogels must be suitable
quirements for LIFT are similar to the inkjet-based for printer deposition with suitable viscosity, shear-
bioprinting, as discussed above. For vat polymeriza- thinning property, short response, transition time, and
tion, the optical property of the material is important suitable sol-gel transition stimulus. Extrusion-based
as it will affect the degree of crosslink in the printing techniques usually require higher viscosity and
process. shear-thinning property [46,47] ; while inkjet-based tech-
niques require low viscosity material with very short
3.4 Requirements of Hydrogel for Bioprinting sol-gel response and transition time [48] .

A successful bioprinted hydrogel construct should (2) Biocompatibility: For tissue engineering, the
possess the following properties (Table 1): (i) printa- hydrogels must have suitable degradability, be able to
bility, (ii) biocompatibility, (iii) mechanical properties, support cell attachments, and do not cause a serious
and (iv) shape and structure. adverse immune response or toxicity.
(3) Mechanical properties: The hydrogels should
Table 1. Ideal bioprinting hydrogel properties match the mechanical properties of targeting tissues in

Ideal bioprinting hydrogel properties terms of stiffness, elasticity, and strength.
(4) Shape and structure: The printed construct
Viscosity should represent adequate similarity to the natural
Shear-thinning property [5,49]
Printability tissue in terms of shape and structure .
Response and transition time
Sol-gel transition stimulus 4. Smart Hydrogels in Bioprinting

Degradability Smart hydrogels are hydrogels that change their net-
Cell binding motifs
Biocompatibility work structures, mechanical strengths, permeability,
Non-toxic and swelling behavior in response to environmental
Non-immunogenic [50,51]
stimuli . As shown in Table 2, chemical stimuli,
Stiffness such as pH can change the interactions between poly-
Mechanical properties Elasticity mer chains or interactions between polymer chain and
Strength the solvent. Physical stimuli, such as temperature and
light can alter critical point interactions. Hydrogels
Pore size
Shape and structure that react to electrical and magnetic influence are also
Micro/Nano structure discussed.

Table 2. Evaluation of hydrogels for bioprinting
Biocompatibility/ Mechanical prop-
Example Printability Shape and structure
Degradability erties
Cell suffers in
pH Responsive Collagen non-physiological pH envi- Soft and weak
Hydrogel Keratin Slow gelation ronment before or during the without additional Porous
crosslinking
gelation process
Need to optimize
Temperature Res- Pluronic F127 temperature thre- Biocompatible Soft Porous
ponsive Hydrogel
shold
Gelatin methacrylate
hyaluronic acid Photoinitiator and free radi-
Photocrosslinkable methacrylate Relatively fast cals generated before and Tunable Good printing shape
Hydrogel curing time during gelation are harmful to fidelity
gellan gum metha- cells
crylate
poly(acrylic acid) Lack of cell-binding motif. Scaffolds’ swelling
Electric Field Res- sodium Polymer charge Cell suffers in a properties can be
ponsive Hydrogel salt-modified dependent non-physiological pH envi- Tunable modulated through
pluronic ronment electrical signals
Magnetic nanopar- Scaffolds’ shapes are
Magnetic Hydrogels ticles combined with Polymer depen- Lack of cell-binding motif Tunable responsive to mag-
dent
the polymers netic field

International Journal of Bioprinting (2015)–Volume 1, Issue 1 5