Page 49 - IJB-2-2

Huijun Li, Sijun Liu and Lin Li

7
properties such as poor mechanical strength and lack to > 6 × 10 mPa.s) is suitable for an extrusion-based
[3]
of structural integrity, which limits its applications. printer . Thus, rheological properties of potential
Recently, much effort has been made for improving hydrogels for an extrusion based printer are consi-
the performance of alginate based scaffolds. Com- dered to be a key factor in controlling the printability
pounding of alginate with other polymers such as pec- of the hydrogels and the fidelity of the printed 3D
tin [13] or chitosan [14] has been found to provide just a structures. However, the relationship between rheo-
marginal effect. Meanwhile, graphene oxide (GO) has logical properties of hydrogels and 3D printability has
attracted a great attention in various fields including not been systematically studied in the literature.
tissue engineering because of its novel properties such A printable hydrogel needs to be optimized to have
as good electric conductivity, thermal conductivity, low viscosity during printing and sufficient mechani-
mechanical stiffness, and biocompatible properties [15–18] . cal strength after printed. So, it is ideal for a printable
GO sheets have been reported to be ultra-strong and hydrogel to have a thixotropic property and fast re-
biocompatible; therefore, GO is a desirable nanoma- covery ability. However, most of the materials could
terial for modifying scaffolds used in tissue engineer- not recover their properties immediately and the re-
ing [19] . The functional groups on GO sheets may im- covery time depends on the materials and shear rate.
prove interfacial interaction between GO and a poly- To improve the quality of printing, it is important to
mer matrix, which paves the pathways for developing choose a material with excellent thixotropic properties
GO based nanocomposites with remarkable mechani- and print the material with a reasonable recovery time.
cal properties [20] . For example, Fan et al. reported a For a non-Newtonian fluid, viscosity is a function of
significant increase of elastic modulus of chitosan by shear rate in a printing syringe. Finding the relation-
about 200% with the addition of small amount of ship between piston speed and shear rate for an extru-
graphene oxide [17] . Mariana et al. reported that GO/ sion based printing process is fundamentally impor-
alginate films showed superior thermal and mechani- tant. Furthermore, to study the thixotropic property
cal properties compared with alginate [21] . However, one should know the shear rate that is generated by the
the effect of GO on the alginate hydrogel has not been piston speed. It is also important to know whether the
extensively studied in the literature. breakdown of crosslinks by shearing is reversible after
In recent years, many researchers and scientists have removing the shear force.
investigated the printability of materials for extru- In this study, alginate-based hydrogels were chosen
sion-based printing. The word, printability, not only as a basic printing material for an extrusion-based
means how easy a hydrogel can be printed out by a 3D printer. Rheological studies were performed for the
printer, but also implies how stable a printed 3D con- samples with different formulas. The range of shear
struct or scaffold is. The latter is more important in rate that the hydrogels suffered during printing was
real applications of bio-fabrication. For example, deduced. This range of shear rates helped us to select
Chung’s group found that the alginate-gelatin hydro- a proper shear rate to investigate the thixotropic prop-
gel had to be printed at low temperatures as the gela- erties of the hydrogels. Graphene oxide (GO) was
tion temperature for alginate-gelatin hydrogels is aro- added to modify the rheological properties and 3D
und 11°C [22] . Jia et al. evaluated the printability of printability of the hydrogels. Furthermore, some mea-
hydrogel by using a point-to-point strategy to print sureable parameters were defined for quantifying the
several dots and found that the plot of dots areas and quality of 3D printing.
viscosity can directly shows the relationship between
printability and viscosity of different samples. Mean- 2. Materials and Methods
while, the results indicated that the optimal range of 2.1 Materials and Sample Preparation
kinematic viscosity for a piston driven system is from
2
[6]
about 400 to 3000 mm /s . Murphy et al. concluded Sodium alginate was purchased from Sigma-Aldrich,
that there are many factors influencing the printability Singapore. According to the supplier, the molecular
of materials, such as pressure, flow rate, viscosity, etc. weight of the sodium alginate ranged from 100,000 to
They summarized a range of viscosities for different 150,000 g/mol and the G block content was 50%–60%.
3D bio-printers (inkjet, extrusion-based and laser as- Calcium chloride with 99% ACS grade was obtained
sisted) based on the previous studies and concluded from Sigma-Aldrich, Singapore. Graphene oxide (GO)
that the sample with the range of viscosity (30 mPa.s was a product of XF NANO (Nanjing, China). All

International Journal of Bioprinting (2016)–Volume 2, Issue 2 55