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3D Printed and Ion Controllable Release
modulus of native cancellous bones was in the range of scaffolds were determined using ICP-AES. As exhibited
50 – 500 MPa . It could be seen that the modulus of the in Figure 5A, the cumulative release concentration of Zn
[36]
composite scaffold was close to that of native cancellous ions for all samples increased with prolonging soaking
bone. The probable reasons for strengthening were mainly time. The cumulative release concentration of Zn ions
attributed to two aspects. On the one hand, appropriate also enhanced with increasing amount of Zn-MS. The
amount of Zn-MS evenly distributed in the PLLA matrix, Zn release rates for the 4Zn-MS/PLLA and 6Zn-MS/
which played a dispersion-strengthening effect . On PLLA samples were relatively high at the beginning
[35]
the other hand, rosette-like morphology and mesoporous of immersion (<6 days), then reached a steady phase
structure of Zn-MS were beneficial to the penetration of during the rest of the time. After soaking for 14 days,
the PLLA melts into the interstices between petals and the cumulative release concentrations of Zn ions for
mesopores on the petals, thus forming micromechanical 2Zn-MS/PLLA, 4Zn-MS/PLLA, and 6Zn-MS/PLLA
interlocking effect [13,37] . However, excessive Zn-MS in the samples were 1.25, 1.76, and 2.36 mg/L. Meanwhile, the
PLLA matrix had an adverse effect on the laser sintering average release concentrations of Zn ions during 2 days
property, thus generating more defects and decreasing for the 4Zn-MS/PLLA and 6Zn-MS/PLLA samples
compactness of the composite scaffolds, which reduced gradually decreased at the initial stage (<6 days), and
their mechanical properties [38,39] . then reached stability, suggesting that the composite
Zn ions play an important role in the key enzymes scaffolds containing different amount of Zn-MS could
related activity with bone metabolism, mainly including achieve sustained release of Zn ions (Figure 5B). The
alkaline phosphatase, collagenase, and carbonic probable reasons for sustained release of Zn ions were
anhydrase . Meanwhile, they also directly participated the shield effect of the PLLA matrix under Van der Waals
[40]
in the metabolism of bone-related cells . In addition, Zn forces and strong bonding of Si-O-Zn [18,44,45] . When the
[41]
ions also could promote adhesion, spread, proliferation, soaking time was between 12 and 14 days, the average
and osteogenic differentiation of stem cells . However, concentrations of Zn ion released from 2Zn-MS/PLLA,
[42]
high concentration of Zn ions can induce cytotoxicity 4Zn-MS/PLLA and 6Zn-MS/PLLA were 0.15, 0.18, and
and cell apoptosis . Therefore, the release of Zn 0.21 mg/L, respectively.
[43]
from biomaterials needs to be precisely controlled, The variation of cumulative release concentration
thus ensuring their biosafety and osteogenesis. The of Si ions for different composite scaffolds after being
concentrations of Zn released from the composite soaked in PBS is exhibited in Figure 5C. The Si ion

A B

C D

Figure 5. The cumulative release concentration (A, C) and the average release concentration (B, D) of Zn (A, B) and Si (C, D) ions of the poly-L-
lactic acid composite scaffolds with different amount of zinc-doped mesoporous silica after soaking in phosphate buffer saline for different times.

98 International Journal of Bioprinting (2021)–Volume 7, Issue 2