Multifunctional
zinc oxide (ZnO) has been generated as nanoparticles or nanorods and
applied to various medical purposes since it exhibits several biological
actions including anticancer activity. Especially, due to antibacterial
activity and effects on bone regeneration, ZnO is widely used in implants
and scaffolds in the orthopedic and dental fields. However, concerns
over side effects have been raised recently in the clinical use of
ZnO, and it is necessary to assess the safety of ZnO regarding its
inflammatory potential in the bone environment. This made us hypothesize
that the inflammatory activity of zinc ions released from ZnO NPs
could be harmful to induce bone resorption but that their cytotoxicity
would be beneficial to kill osteosarcoma. To clarify this hypothesis,
in the present work, the effects of ZnO on bone matrix and abnormal
bone environments were investigated quantitatively using ZnO chips,
filter paper, or glass slides coated with thin films of ZnO grown
via atomic layer deposition (ALD). ALD-grown ZnO thin films exhibit
thickness with atomic precision, which enables the quantitative analysis
of the effects of ZnO. <i>In vivo</i> application of ZnO
chips to mouse calvarial bone induced bone resorption, presumably
due to the activation of osteoclasts by zinc ion-induced TNF-α
release. However, <i>in vitro</i> application of ZnO chips
to osteosarcoma cells induced caspase-dependent apoptosis and oxidative
stress. Taken together, the results showed two sides of ZnO as our
hypothesis. Therefore, careful design and multiple evaluations for
the safety and efficacy of ZnO materials are necessary for its successful
clinical application
