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Deng Y, et al.

4.3 The Effect of β-TCP on Degradation of ZK60/8β-TCP was believed to be closely related to
Behavior the enhanced corrosion resistance. As shown in Figure
6A, the pH value significantly increased during the
It was well known that Mg had a lively chemical immersion due to the rapid degradation of Mg-based
property. As Mg based alloys were exposed to aqueous composites. In general, a weak alkaline environment
solution, Mg would degrade according to the following with pH value ranging from 7.4 to 7.8 was more suitable
[26]
chemical reactions : for cell survival. An increased pH would significantly
2+
Anodic: Mg → Mg + 2e − (4.1) impair the enzyme activity, thus affecting the transport
of substance for cell membrane. Meanwhile, a high pH
−
2+
Cathodic: 2H 2 O + 2e → H 2 ↑ + 2OH (4.2) value with high Mg and OH concentration led to a too
−
−
high osmotic pressure in culture medium [29] . Thus, the
−
2+
Product formation: Mg + 2OH → Mg(OH) 2 ↓ (4.3) significantly increased pH had obvious inhibitory effects
on cell proliferation. For ZK60/8β-TCP, the pH value
Hence a heavy layer of Mg(OH) , which exhibited a 2+ −
2

porous structure, was observed on the surface of ZK60 and ion concentration (Mg , OH ) in culture medium
−
(Figure 7A). Meanwhile, the Cl ions contained in SBF were considerably reduced compared with ZK60,
could transformed Mg(OH) into resoluble MgCl as resulting in more appropriate pH value and osmolality
2
2+
2
follows : for cell proliferation. Besides, the released Ca caused
[16]
by the degradation of β-TCP should also be responsible
−
−
Mg(OH) 2 + 2Cl → MgCl 2 + 2OH (4.4) for the improvement of cytocompatibility for ZK60/8β-
Thus, the coated Mg(OH) layer could not provide an TCP. Ca, as a nutrient element for the human body, is
2
[30]
effective protection for the inner fresh Mg matrix from essential in chemical signaling with cells . Li et al. also
2+
further degradation. Compared with ZK60, ZK60/8β- reported that Ca enhanced the proliferation of bone-
2+
[31]
TCP had an enhanced corrosion resistance. This could derived cell . Thus, the release of Ca from ZK60/8β-
be confirmed by the reduced pH value of SBF after TCP might exert a positive effect on the cell growth and
immersion of ZK60/8β-TCP composite. The enhanced proliferation.
corrosion resistance of ZK60/8β-TCP was due to a more 5. Conclusions
protective film formed on the corrosion surface. The
β-TCP distributed in Mg matrix would dissolve in the In this study, β-TCP was composited with ZK60 via
[27]
aqueous solution as follows : laser rapid solidification with an aim to improve the
−
2-
2+
3−
3Ca 3 (PO 4 ) 2 + H 2 O→HPO 4 + OH +9Ca + 5PO 4 degradation behavior. It was revealed that laser rapid
solidification restrained the segregation of β-TCP
(4.5) particles in Mg matrix. In addition, β-TCP particles
The released calcium ion and phosphate ion favored uniformly distributed along grains boundaries in
for the formation of apatite layer on surface. On the ZK60/8β-TCP. The results showed that the laser rapidly
other hand, the β-TCP usually acted as the apatite nuclei solidified ZK60/8β-TCP had an enhanced corrosion
leading to the deposition of apatite spontaneously [28] . resistance and mechanical properties as compared
Therefore, a large amount of apatite deposited on with ZK60. Besides, the results of in vitro cell culture
the surface and filled the pores of Mg(OH) layer. assays also revealed that ZK60/8β-TCP composite
2
As a result, a more compact surface film formed and had an improved cytocompatibility to MG-63 cells.
effectively retarded the further degradation. It was suggested that ZK60/8β-TCP was a potential
It should be noted that the corrosion rate accelerated biodegradable implant.
as the β-TCP content increased up to 12 wt. %. This
was because too much β-TCP aggregated at the grain Conflict of Interest and Funding
boundaries and reduced the relative density of the No conflict of interest was reported by the authors. The
matrix. As a consequence, the strongly corrosive Cl ion authors gratefully acknowledge the following projects
−
easily invaded into the matrix, which accelerated the and funds for the financial support: (1) The Natural
degradation. Science Foundation of China (51575537, 81572577,
4.4 In Vitro Cell Response of ZK60/xβ-TCP 51705540, 81472058); (2) Hunan Provincial Natural
Science Foundation of China (2016JJ1027); (3) The
In the presented study, the extracts of ZK60/xβ-TCP were Project of Innovation-driven Plan of Central South
utilized to mimic the environment during implantation. University (2016CX023); (4) The Open-End Fund for
In vitro cell culture experiments indicated that MG-63 the Valuable and Precision Instruments of Central South
cells exhibited better growth in extracts of ZK60/8β-TCP University; (5) The fund of the State Key Laboratory of
than in that of ZK60. The improved cytocompatibility Solidification Processing at NWPU (SKLSP201605);
International Journal of Bioprinting (2018)–Volume 4, Issue 1 9