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

3.4 Anisotropic Filler-Reinforced Hydrogel
Composites 3D Printing
Nanoclay is a nanoparticle which is composed of layered
hydrous silicate. It has been used in a wide range of
applications such as pharmacy, paints, cosmetics as well as
catalysis owing to their good surface properties and excellent
rheology controllability. Depending on the type of clay, each
layer consists of two or more sheets of either (AlO (OH) )
3 6
3
octahedra or (SiO ) -tetrahedra. Nanoclays are classified into
4 4
several classes such as Laponite, montmorillonite, bentonite,
nontronite, saponite, kaolinite, hectorite, and halloysite by
their geometrical shape and chemical composition which
can affect biocompatibility. Rawat et al. investigated the
cytotoxicity and antimicrobial properties of various shape Figure 8. (A) The structure and composition of Laponite nanoclay
and concentration of nanoclays [100] . They prepared Laponite as an anisotropic filler and (B) “house-of-cards” mechanism of
with an aspect ratio of 25:1 and montmorillonite (MMT) with self-assembling printed hydrogels including Laponite. (reproduced
with permission from [96]. Copyright 2017, ACS Publications).
an aspect ratio of 300:1, The cytotoxicity and antimicrobial
properties of both nanoclays with various concentration from modified four-armed poly(ethylene glycol) (PEG-D4) Liu
0.00005 ug/mL to 0.0125 ug/mL were assessed by eukaryotes- et al. [103] The introduction of Laponite did not change the
human embryonic kidney (HEK), and cervical cancer SiHa degradability and cytocompatibility of PEG-D4. However,
cell and Kirbey-Bauer protocol method, respectively. Laponite the curing time, mechanical and adhesive properties were
exhibited good antimicrobial properties, while MMT showed significantly increased. Consequently, PEG-D4/Laponite
better cytotoxicity. Their explanation behind this finding is hydrogel nanocomposites minimized inflammatory
due to the difference in charge density and anisotropy of the response and improved cellular infiltration in vivo as
clays. Modification of nanoclays as organic-inorganic hybrid compared to Laponite-free specimens.
nanomaterials have potentials for use as rheological modifiers, Recently, a variety of Laponite incorporated hydrogel
gas absorbents and drug delivery carriers in customizing composites are 3D-printed for hard tissue engineering.
polymer composites. Jin et al. proposed a direct hydrogel printing approach
Laponite, a synthetic magnesium silicate, is well without any supporting bath by using self-supporting
known as a nano biofiller in cosmetics. The potential use nanoclay [96] . Laponite RD and XLG were mixed with
of Laponite as tissue engineering constructs has been three types of hydrogels including poly(ethylene
discussed because they enhance cell spreading and promote glycol) diacrylate (PEGDA), alginate, and gelatin.
osteogenesis. The crystal structure of Laponite is a disc- Each prepared composites were extruded by direct
shaped layered magnesium silicate with a particle size of ink writing with appropriate crosslinking methods,
approximately 25 nm in diameter and 1 nm in thickness respectively. Laponite-incorporated hydrogels were
as shown in (Figure 8A). These ultrathin structures with a readily printed through a nozzle and solidified after
high degree of anisotropy and functionality enhance their extrusion in the air. The addition of Laponite improved
surface interactions. In addition, Laponite is negatively the mechanical properties of extruded scaffolds and
charged on its face and positively on the rim thus undergoes also adjusted the degradation rates. The elastic modulus
self-assembly through electrostatic interactions to form a of PEGDA-Laponite, alginate-Laponite, and gelatin-
shear thinning gel state with a “house-of-cards structure” Laponite scaffolds increased 1.9, 7.4 and 3.3-fold than
(Figure 8B) [101] . Therefore, many studies have been carried each pure hydrogels without Laponite, respectively. The
out on blending Laponite with polymers for improving cytocompatibility of PEGDA-Laponite was confirmed
mechanical and biological properties. Hydrogels with by fibroblast cell adhesion and proliferation.
poor mechanical properties can yield these properties by Zhai et al. reported that the physical crosslinking of
forming strong interaction between chains of hydrogel hydrogel chain-clay coupled with hydrogen bonding
and monodispersed Laponite. Su et al. fabricated silk remarkably increased the mechanical performance of
[97]
fibroin hydrogel composites with Laponite for bone defect hydrogel scaffolds . N-acryloyl glycinamide (NAGA) was
repair application [102] . As the concentration of Laponite dissolved in deionized water with varying concentrations
increased from 0 to 5%, rheological properties of hydrogel from 10 to 30%, and then mixed with different quantities
composites increased from 30 to 200 kPa. Osteoblasts cell of Laponite XLG. The mixed solutions were extruded by
proliferation and differentiation also increased with the 3D plotting method and printed specimens were cured in a
addition of Laponite. Injectable hydrogel naoncomposite cross-link oven. Fabricated PNAGA-Clay scaffolds showed
was investigated by combining Laponite and dopamine- homogeneous structures and the mechanical properties of

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