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Shuai C et al.

by many other studies. For example, Park et al. used micropores obtained by chemical etching. Moreover,
chemical etching to create surface pores on the PLGA the macropore structure of the scaffolds was determined
[34]
scaffolds . Gautier et al. constructed surface pores on by the AM parameters, including laser spot size, hatch
[35]
silicon layers by chemical treatment . Yerokhov et al. spacing, etc. Therefore, at the same AM parameters,
developed macroporous crater-like surface on silicon by there was no obvious differences in porosity between
chemical and electrochemical etching [36] . As revealed the scaffolds with and without GO. In this study, GO/
by the FE-SEM analysis in our study, the hydrolysis PLLA scaffolds exhibited a much higher compressive
reactions could be promoted with prolonged etching strength, which might be ascribed to the reinforcing
[43]
time, leading to increased pore number and pore size in mechanisms by GO, such as crack bridging and pull-
[44]
both depth and width. But on the other hand, excessive out , enabling the efficient absorption and dissipation
etching resulted in the damage of original porous of stress in the scaffolds. More importantly, the addition
network or even shrinkage-induced cracks, thereby of GO provided more active sites for apatite nucleation,
deteriorating the properties of the polymer scaffolds. thereby further improving the bioactivity of PLLA
Once the scaffolds were immersed in SBF solution, scaffolds. The biocompatibility of PLLA and GO/PLLA
the micro pores on scaffolds surface allowed more scaffolds has been studied using CCK 8 assays in our
contact of the scaffolds with the molecules in SBF, previous study. And the results reveled that PLLA/GO
resulting in accelerated degradation rate. Moreover, the scaffold had better biocompatibility than PLLA scaffold.
slopes of the degradation curves changed over time, The stimulatory effect of GO on cell behavior was
indicating a variation in the degradation mechanism attributed to the oxygen-containing functional groups on
from an initial low ion diffusion at early stage to a faster GO surface that could promote cell proliferation [45–47] .
network degradation at prolonged immersing times. 5. Conclusions
In addition, the formed polar hydroxyl and carboxyl
groups on scaffolds surface could attract the calcium A combined process of AM and chemical etching was
cations in SBF solution and acted as nucleation sites for developed to fabricate multi-scale porous scaffolds. By
initial apatite formation. Thus it was easy to understand modulating the parameters of AM and chemical etching,
that both the degradation rate and bioactivity of the the GO/PLLA scaffolds possessed: (i) controllable multi-
scaffolds were controllable by varying the chemical scale porous structure, that was interconnected macro
etching time. It should be noted that the micropores pores and surface micro pores; (ii) tunable degradation
obtained by chemical etching improved the surface rate ranging from a few months to a year; (iii) favorable
activity via accelerated apatite deposition, which bioactivity resulting from the formed polar groups and
contributed to forming good bone/scaffold interface GO; (iv) adjustable mechanical strength in a range of
bonding [37–39] . Meanwhile, the micropores increased the 15.5~24.5 MPa. The combined process in this study
surface roughness and thus could significantly facilitate could be extended to other polymer-based scaffolds and
cell adhesion and resultantly bone tissue growth [40–42] . was expected to provide a new strategy for developing
Moreover, the formed micropores accelerated the porous scaffolds.
degradation rate of the scaffold. There was no doubt
that timely degradation is important to the healing of Conflict of Interest and Funding
bone defects. Therefore, these surface micropores were
certainly beneficial for bone tissue regeneration. No conflict of interest was reported by the authors.
However, the mechanical properties of PLLA scaffolds The authors gratefully acknowledge the following
were significantly decreased after the alkaline treatment projects and funds for the financial support: (1) The
due to the increased porosity. Thus, GO was incorporated Natural Science Foundation of China (51575537,
to enhance the mechanical properties of PLLA scaffolds. 81572577, 51705540); (2) Hunan Provincial Natural
It was well accepted that the microstructure and porosity Science Foundation of China (2016JJ1027); (3) The
had a significant effect on the mechanical properties Project of Innovation-driven Plan of Central South
of scaffolds. Nevertheless, little differences in the University (2016CX023); (4) The Open-End Fund for
microstructure and porosity were observed between the the Valuable and Precision Instruments of Central South
etched scaffolds with and without GO, as presented in University; (5) The fund of the State Key Laboratory of
Figures 2 and 4. In fact, the size of surface micropores Solidification Processing at NWPU (SKLSP201605); (6)
obtained by chemical etching was about 1-2 microns, The Project of State Key Laboratory of High Performance
which was 2-3 orders of magnitude different from the Complex Manufacturing, Central South University; (7)
macropores (hundreds of microns) constructed by AM. National Postdoctoral Program for Innovative Talents
Therefore, the porosity of the scaffold mainly depended (BX201700291); (8) The Project of Hunan Provincial
on the macropores constructed by AM, instead of the Science and Technology Plan (2017RS3008) and (9)

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