Page 90 - IJB-6-1

Hydrolytic expansion accelerates Fe biodegradation
Fe/Mg Si composites were caused by the rapid SiH + 2H O → SiO + 4H ↑ (9)
2
2
2
2
4
degradation of the Fe matrix. This also aggravated the damage to degradation
As biodegradable composites, the degradation product layers and hindered the accumulation of
rate needs to be consistent with the healing rate of the degradation product layers, further enlarging
the defect bones to allow a gradual transfer of load the access of corrosive solution to the matrix.
to the new bone and to avoid the long-term negative Meanwhile, the produced silicon dioxide (SiO )
2
effects of permanent implants. After exposure acted as a cathode site, and then galvanic corrosion
to physiological environment, Fe degraded in a occurred between SiO and Fe matrix due to the
2
manner of oxygen absorption corrosion mode, different corrosion potentials.
producing degradation products as follows: Based on this corrosion mode, the hydrolysis
2Fe → 2Fe + 4e − (1) of Mg Si would, on the one hand, create vacancies
2+
2
in the matrix, exposing more surface of Fe to the
O + 2H O + 4e → 4OH − (2) solution during the initial immersion. On the other
−
2 2
hand, the produced gases by the hydrolysis had an
2Fe + 2H O + O → 2Fe(OH) ↓ (3) expansive effect, which cracked the degradation
2 2 2
product layers as well as Fe matrix, and brought
Fe → Fe + e − (4) about the breakdown of degradation product layers,
3+
2+
thereby contributing to the corrosion propagation
Fe + 3OH → Fe(OH) ↓ (5) toward the interior of the matrix. As a result, the
3+
−
3
degradation product layers became porous, loose
Fe(OH) + 2Fe(OH) → Fe O ↓ + 4H O (6) and easily detached from the matrix, resulting
2 3 3 4 2
Reactions between Fe from anodic oxidation in peeling off of corrosion products or even Fe
2+
and other anions in physiological environment matrix, as evidenced in Figure 7D,E. Moreover,
could simultaneously occur [39,48] : since Mg Si was homogeneously distributed in
2
the Fe matrix, fast and macroscopical corrosion
3Fe + 2PO + 8H O → Fe (PO ) ·8H O↓ (7) would occur throughout the matrix. This corrosion
3−
2+
4 2 3 4 2 2 mechanism was first proposed and verified in this
Degradation products, such as Fe(OH) , study, which fundamentally solved the problem
2
Fe(OH) , Fe O , and Fe (PO ) ·8H O, were almost that corrosion product accumulation hindered the
2
3
3
4 2
3
4
insoluble in the physiological environment [49,50] . As degradation. It should be noted that the generated
a result, the degradation products would deposit gases by the hydrolysis of Mg Si would be carried
2
on Fe matrix and form dense product layers, as away by the circulating blood in the human body,
shown in Figure 7A, significantly reducing the which would not produce obvious side effects.
degradation rates. In this study, Mg Si with high
2
chemical activity was introduced into the Fe matrix 3.6 Cytocompatibility
and it would be readily hydrolyzed according to Cytocompatibility tests using MG-63 cells
Equation (8): were taken to evaluate the biological safety of
Fe/0.9Mg Si composite. Fluorescent images in
Mg Si + 4H O → 2Mg(OH) + SiH ↑ (8) extracts were taken to investigate the growth of
2
2
4
2
2
The generated gas (SiH ) would escape from MG-63 cells, as exhibited in Figure 9A-D. It could
4
the matrix and then diffuse into the solution, which be found that there was no obvious difference
destroyed the accumulated degradation product in cell morphologies after 72 h of exposure to
layers. Once the protective layers were broken different extracts. The average cell number was
down, the corrosive solution would quickly infiltrate estimated by ImageJ software according to the
and induce Fe corrosion. More importantly, the fluorescent images in Figure 9E, which showed no
generated SiH underwent further hydrolysis: significant difference. Besides, cell viability was
4
86 International Journal of Bioprinting (2020)–Volume 6, Issue 1