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Materials Science in Additive Manufacturing 3D printing of anti-microbial parts

Figure 11. Summary of how biocidal nanoparticles damage bacterial cells, with one or multiple mechanisms operating for different nanoparticle materials.
Adapted with permission from 43

nanoparticles, are effective due to multiple modes of 3.5. Validity and relevance of the anti-microbial test
action, including ion leaching 1,15,16 . However, the leaching used
of ions can be toxic to humans 41,42 . In contrast, Mg(OH) There is more than one protocol for testing the anti-
2
has very low solubility and ion-leaching tendency. Its microbial activity of plastics (Sections S6 and S7). Campos
solubility constant is 5.6 × 10 ; i.e., 64 ppb of ions can be 45
−12
found if dissolved in water at 25°C. While the requirement et al. have noted that the activity of anti-microbial
for direct contact with the microbe is a limitation, as surfaces varies with the testing protocol utilized. In this
surface attachment is required for the NPs, the biosafety work, we used a protocol developed by Aqua Inc. (USA),
of Mg(OH) , due to its low solubility, confers a major the supplier of the Mg(OH) NPs (Section S6). Considering
2
2
advantage over silver and copper nanoparticle-based the studies that demonstrate Mg(OH) NPs function by
2
37
biocides, where leaching of ions can be considered a contact killing , along with the observed activity of HSS
toxicity issue. Wang et al. demonstrated the biosafety of disks coated with Mg(OH) NPs and the lack of activity
42
2
Mg(OH) NPs in humans. Thus, plastics with Mg(OH) in IM disks with Mg(OH) NPs embedded in an inert
2
2
2
can be preferable to silver nanoparticles to avoid side polymer, it would appear that the anti-microbial effect
effects/toxicity to humans. of HSS disks coated with Mg(OH) NPs is legitimate and
2
not an artifact of the testing method. Hence, the test used
There is some ambiguity on the exact bacterial here is valid as it demonstrates repeatability when there is
destruction mechanism of Mg(OH) NPs. Dong et al. a significant difference in anti-microbial activity between
18
2
have indicated that the Mg and OH ions do not kill the sample and the control, and the inactive IM sample
2+
−
the bacteria. It is hypothesized that the anti-microbial where the Mg(OH) is buried inside the polymer.
mechanism of Mg(OH) NPs is due to ROS generation that 2
2
kills the microbe. Figure 12 illustrates how semiconductor Several studies have also pointed out that the testing
materials, such as metal oxides and hydroxides can generate conditions used for detecting anti-microbial activity may
ROS (further details in Section S7). In addition to ROS, it not be applicable in real-world situations. This raises the
is suggested here that due to the ~10 nm thickness of the question of what occurs in real-world situations, such
Mg(OH) NPs (Figures 3 and 10), they may act like nano as on a door handle, where dead bacteria may remain
2
knives when in an edge-wise orientation, puncturing the on the surface. Unlike in the test, where dead bacteria
1
cell wall of bacteria that impinge on them. are continually removed by fluid agitation in the pod
Volume 3 Issue 4 (2024) 11 doi: 10.36922/msam.4970