Page 64 - MSAM-3-4

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

after 4 and 24 h to determine the cell viability using
Scan300 (InterScience, France). The materials on the
vertical axis are: (1) copper disk (positive standard), (2) PA
12 disks made by HSS, (3) PA 12 disks made by HSS and
dip-coated with Mg(OH) , (4) sample (3) after washing
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and re-use, (5) IM PA 12 disks, and (6) IM disk made by
melt compounding of 5 wt.% Mg(OH) with PA 12. The
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log (CFU/mL) is plotted on the horizontal axis, where
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CFU is the number of colony-forming units.
Figure 7A displays the SEM image of the surface of
the IM PA 12 disk with 5 wt.% Mg(OH) . Most of the
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crystals are imaged faintly, as they lie below the surface
under a polymer skin. The EDX spectrum of various areas
on the surface of the disk did not detect Mg, suggesting
that most of the nanocrystals were not protruding over
Figure 4. Melt-compounded and injection molded PA 12 disks. Neat PA
12 (left) and PA 12-loaded with 5 wt.% Mg(OH) (right) nanoplatelets. the disk’s surface. Table 2 indicates that the detected
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Disk diameter: 25 mm; thickness: 1.55 mm. elements were principally C and N, which arise from the
Abbreviation: PA: polyamide PA 12. The O arises from both the amide group and the
Mg(OH) . Only ~1.3% of Mg was detected on the surface
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significantly reduce the mechanical properties. Further, as of the disk.
the Mg(OH) has to be in contact with the bacteria to be Figure 7B features the cross-section when the IM disk
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effective, it is not useful to have it distributed throughout produced with PA 12 and melt-compounded with 5 wt.%
the article. Hence, for HSS fabrication, surface coating by Mg(OH) was cut in half across its diameter; an increased
dip immersion of the HSS PA 12 article in a suspension presence of Mg was observed in the EDX spectrum (~19.2%
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of Mg(OH) was performed. We anticipated that this on average; Table 2). At 5 wt.% Mg(OH) , there were few
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approach would also be challenging, as the crystals on NPs on the surface of the molded disk (Figure 7A), and the
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the surface of the manufactured article might be removed bulk was in the interior (Figure 7B).
during handling and washing. Hence, it was thought that
a thermo-fixing method using heat and pressure would be The cross-sectional cut of the IM disk also revealed areas
necessary, similar to the technique used by Alkarri et al. with Mg(OH) crystal aggregates (arrows in Figure 7B).
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2
with flat polypropylene (PP) sheets coated with Mg(OH) 2 This highlights the challenges associated with dispersing
NPs. However, their method of thermo-fixing cannot NPs from a dry powder into the polymer melt. Nanocrystals
be easily applied to non-sheet substrates (i.e., non-flat have an agglomeration tendency, and even a 5 wt.% loading
objects). is considered very high. In contrast, micron-sized powders
can typically be added at 25–30 vol.% to the polymer
Figure 5A displays a PA 12 disk that was printed by HSS; during melt compounding before agglomeration becomes
Figure 5B features the HSS-printed PA 12 disks placed in too severe. However, with nanoparticles, agglomeration
the pods for immersion in the bacterial broth. The PA 12 becomes insurmountable at ~5 vol.% .
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disks made by IM and the copper control disk were also
placed in similar pods. Figure 5C presents the tensile Figure 7C displays a higher magnification of the
bars made by HSS, uncoated and coated with Mg(OH) agglomerated region (arrows in Figure 7B). The Mg(OH) 2
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suspension, for testing whether the coating significantly NPs are visible, but they are coated with polymer compared
decreases mechanical properties. The disk and bars from with the bare Mg(OH) NPs in Figure 3.
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HSS have a different appearance from the IM parts; they From Figure 7A and the elemental composition at
appear greyish due to the inkjet printing of every layer with the surface of the IM disk (Table 2), we can deduce why
black ink, which serves as an infrared radiation absorber nanocomposites made by melt compounding and IM
during the HSS process (Section S4). are ineffective anti-bacterial agents. At 5 wt.% loading of
Mg(OH) NPs, there are not many exposed crystals on the
3.3. Anti-bacterial evaluation of PA 12 disks made surface of the disk. Since the anti-microbial mechanism
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by injection molding
of Mg(OH) is based on direct contact with microbes and
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The bar chart (Figure 6) displays the anti-bacterial activity does not involve ionic leaching, the disk does not function
of various substrates. The bacterial count was measured as an anti-microbial nanocomposite. Thus, in a PA 12

Volume 3 Issue 4 (2024) 7 doi: 10.36922/msam.4970