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Shuai, et al.
compression modulus also had a similar trend brittle fracture . When 0.3% GO was introduced,
[53]
(Figure 5B). To investigate the cause of the change the fracture surface began to become rough and
in compressive properties, the surface morphologies uneven (Figure 6D). GO sheets were embedded in
of the samples were observed by SEM, as shown the PLLA matrix when the content was increased
in Figure 5C-G. The surface of PLLA was smooth to 0.6%, which was indicated by the red arrow
and flat (Figure 5C), some flakes began to appear (Figure 6D). When the GO content was further
on the surface of the PLLA/0.3 GO sample (Figure increased, more GO was embedded on the matrix,
5D), and as the GO content continued to increase, and the embedded GO could effectively transfer
more and more flakes appeared (Figure 5E and F). and consume external force, thereby increasing the
A large number of flakes were stacked together to tensile strength of the scaffolds. However, when the
form continuous agglomerates when 1.2% GO was GO content was further increased, some of the GO
introduced (Figure 5G). Red line scan 1 was used to sheets accumulated to form agglomeration, which
distinguish the difference by penetrating the matrix weakened the enhancement of GO to PLLA, and
and flakes (Figure 5H). In the direction of the line thus the tensile strength and modulus decreased
scan, the content of the C element first increased (Figure 6G).
and then decreased. On the contrary, the content of
the O element first decreased and then increased. 3.6 Bioactivity
The ratio of C to O elements proved that the flakes Good bioactivity is one of the required properties for
were GO. When the line scan 1 was moved from bone scaffolds [54,55] . PLLA and PLLA/GO scaffolds
PLLA to GO, since the ratio of C to O in GO was with 0.3%, 0.6%, 0.9%, and 1.2 wt% GO were
higher than that of PLLA, so the C element rises. immersed in SBF for 4 weeks to assess bioactivity.
Then, when the line scan moved from GO to There did not have a calcium-phosphorus
PLLA, the C element began to drop. The EDS of layer appearing on the surface of PLLA after
the powders in Figure 1 also confirmed that the immersing in SBF, indicating that PLLA lacked
ratio of C to O in GO was larger than that in PLLA.
The trend of the elements in the line scan 1 also good biological activity (Figure 7A). Zhou also
[56]
confirmed that the flakes were GO. GO was a sheet- has similar reports . Calcium-phosphorus layer
like structure with excellent mechanical strength, it appeared on the surface of the sample when 0.3%
could be used as a reinforcing phase to improve the GO was introduced into PLLA (Figure 7B). As the
compression properties of the scaffolds. However, GO content continued to increase, there was more
when an excessive amount of GO was introduced, calcium-phosphorus layer on the surface of the
it was difficult to uniformly disperse and formed sample, and the particle size was getting larger and
agglomerates, which formed defects in the matrix, larger (Figure 7C-E). Point 1 in Figure 7E was
resulting in a decrease in compressive strength. used to measure the elements of white particles by
The tensile strength and modulus of samples EDS, as shown in Figure 7F. There were Ca and P
containing different GO ratios are measured and elements in the EDS map, which could prove that
presented in Figure 6A and B. The tensile strength of the white particles were the calcium-phosphorus
PLLA was 16.90 MPa. As the GO content increased, layer. The surface of GO contained a large number
the tensile strength also increased, but the tensile of functional groups, such as –COOH and –OH [57-
strength decreased when 1.2% GO was introduced. 59] . These functional groups could adsorb Ca ions
The compressive modulus was also increased first and in SBF, and Ca ions could continue to adsorb
then decreased. To explore the mechanism of tensile PO ions to nucleate and grow, thereby forming a
4
properties change, the fracture surface morphologies calcium-phosphorus layer [60,61] .
were analyzed by SEM. The fracture surface of PLLA 3.7 Cytocompatibility
was smooth with no obvious wrinkles, indicating that
PLLA was brittle material (Figure 6C). Todo et al. Fluorescence staining experiment and CCK8
also confirmed that the fracture mode of PLLA was experiments were used to assess cell compatibility

International Journal of Bioprinting (2020)–Volume 6, Issue 1 99