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

Figure 2. Preparation of Mg(OH) anti-microbial suspension for dip coating of PA 12 disks (made through injection molding and HSS) and tensile bars
2
(made through HSS). Adapted from 34
Abbreviation: HSS: High-speed sintering

2.6. Dip-coating and washing of test disks plates (BBL/Difco, Sparks, MD, USA). After incubation at
37°C for 24 h, a single colony was selected and transferred
The dip-coating and washing process of the tested articles
was conducted as previously described . to 5 mL of Tryptic Soy Broth (BBL/Difco, Sparks, MD,
35
USA) and incubated at 37°C for 18 h. After incubation,
2.7. Characterization of coated and uncoated disks 1 mL of culture was centrifuged at 13,000 × g for 5 min
made by HSS and IM using a Fisher Scientific accuSpin micro 17 R centrifuge,
and the supernatant was discarded. The E. coli cells were
The dip-coated PA 12 disks were characterized for the suspended in 1 mL of phosphate-buffered saline (PBS, from
deposition and adherence of the Mg(OH) nanocrystals Crystalgen, Innovation for Science, Commack, NY, USA)
2
by scanning electron microscopy (SEM) using a JEOL by vortexing. The cell suspension was then transferred to
7500F field emission emitter (JEOL Ltd., Japan) and by
energy dispersive X-ray (EDX) analysis using an Oxford a 15 mL tube and 11.5 mL of PBS was added. Aliquots of
Instruments Aztec system (UK). Samples for SEM analysis this suspension were then exposed to the different types
of disks (HSS or injection molded). Each disk was placed
were prepared by mounting the disks on aluminum stubs separately in a plastic pod.
using glue and subsequently coated with ~2.7 nm-thick
layers of iridium. The pods were contact lens cases from Bosch + Lomb
(Germany). For each pod containing a single disk, 1 mL of
The surface of the IM disks with melt-compounded
5 wt.% Mg(OH) was imaged with SEM to identify the bacterial suspension was added to submerge the disk into
the culture broth and the pods were closed. The pods were
2
presence of the NPs. These disks were also cut across the attached to a mini rotator (Benchmark Scientific, Roto
diameter, and the cross-section was imaged to determine
the distribution of the NPs through the thickness. The dip- Mini Plus R 2024, Sayreville, NJ, USA) and rotated at 20
coated IM disks were imaged on the surface to assess the RPM (Section 6) around the machine’s horizontal axis to
agitate continuously the broth and cause liquid renewal
quality of the adherence of the Mg(OH) NPs. The dip- on the surface of the disks. The agitation also removed
2
coated HSS PA 12 disks were imaged only on the circular
surface, as the Mg(OH) NPs were only expected on this dead bacteria from forming a biofilm on the surface of the
2
surface. plastic disk. At 0, 4, and 24 h intervals, a 100 L sample of
the bacterial suspension after a 1:10 dilution, was plated
2.8. Anti-microbial evaluation of the plastic disks in TSA, and then incubated at 37°C overnight to grow
made by HSS and IM bacterial colonies.
The ISO and other techniques of testing of anti-microbial The colony forming unit (CFU) number was counted
activity of plastics were not followed; instead, the following to determine the cell viability (using a Scan300 from
method was employed, which gives repeatable results with InterScience). The bacterial culturing was also done with
an internal consistency that fits the response expected a neat PA 12 disk (made by IM and HSS) as a “negative”
from a contact biocide . The stock culture of E. coli was control sample, and with a copper disk (of the same
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stored at −80°C and streaked on Tryptic Soy Agar (TSA) dimensions) as a “positive” control sample. Copper is well

Volume 3 Issue 4 (2024) 5 doi: 10.36922/msam.4970