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

equipment and reusable parts. Hence, the development of of combining Mg(OH) , a non-leaching, non-toxic
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anti-microbial plastic parts is an endeavor in research . nanocrystal with anti-microbial properties 18,19 , with inert
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It has been reported that fungi and bacteria can attack thermoplastics. Usually, such nanocomposites would
plastic articles, leading to unwanted degradation, especially be developed by melt compounding the nanocrystal
in hot, tropical climates . There is therefore another line of with a thermoplastic polymer, followed by injection
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research to protect plastic articles from microbe-induced molding (IM). With Mg(OH) , which requires physical

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degradation. In addition, microbes are investigated for contact with microbes for anti-microbial activity, Alkarri
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their ability to synthesize biodegradable polymers. Further, et al. found that when Mg(OH) nanoplatelets (NPs)
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microbial decomposition of polymers is being investigated were melt-compounded with a polymer and cast into a
as a potential solution to the environmental problem sheet, the nanocomposite did not exhibit anti-microbial
of plastic waste . In contrast, this work is specifically activity because the Mg(OH) crystals were embedded
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devoted to reducing the risk of plastic surfaces transmitting inside the plastic. Alkarri et al. then coated PP sheets
infectious microbes to humans. Hence, the term “anti- with a Mg(OH) NP suspension; after drying, the NPs
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microbial polymer part” is used in this context. were thermo-fixed by applying heat and pressure below
the sheet’s melting point. The thermo-fixed Mg(OH)
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One approach to anti-microbial polymers is to attach NPs could not be removed from the sheet’s surface by
anti-microbial functional groups, such as quaternary handling or washing, and its anti-microbial activity was
ammonium salt or N-halamine, to the chain 8-10 . Cationic validated by the reduction in bacterial count after the PP
polymers , such as polyethylene imines 12,13 , exhibit sheet with surface-embedded Mg(OH) NP was placed in a
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strong anti-microbial activity. However, such polymers broth containing the bacteria . The sheet retained its anti-
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are usually water-soluble or swellable, and can mainly be microbial activity after repeated use. However, for a shaped
used as coatings. They cannot be used to construct shaped (non-flat) plastic object made by IM, the challenge with
articles. In contrast, traditional moldable thermoplastics using Mg(OH) NPs is how to fix them at the surface of
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(polyethylene, polypropylene (PP), polyamides (PA), the article, as it is difficult to apply pressure on a non-flat
polyesters, polycarbonate, etc.), which are used in everyday article without distorting the shape.
applications and for medical devices, lack any intrinsic
capacity to kill microbes and may even be susceptible to The challenge, therefore, is how to create shaped plastic
microbe attack. articles with an anti-microbial surface using a non-toxic
“contact biocide” like Mg(OH) . IM is the traditional process
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Anti-microbial polymer nanocomposites combine of manufacturing 3D shapes from plastics. However, it
an inert commodity thermoplastic with an anti- was not obvious how the surface of a non-flat IM article
microbial biocide to produce extrudable and moldable could be embedded with Mg(OH) and with the crystals
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compositions. One can use an inert polymer with biocidal fixed to the surface. In this work, additive manufacturing
nanoparticles, or blends of a polymeric biocide and (AM), also known colloquially as 3D printing, was used to
an inert polymer . Examples of biocidal nanoparticles investigate the production of anti-microbial plastic articles
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include silver and copper nanoparticles 14-16 , as well as with Mg(OH) coated on their surface. There are seven
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nanocrystals of CuCl , MgO, Mg(OH) , and ZnO, all of categories of AM methods listed by standards institutes,
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which can be incorporated into standard polymers by melt such as ASTM and the International Organization for
compounding . The anti-microbial mechanisms may be Standardization (ISO) . One category is called powder
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different and can sometimes be unknown. For example, bed fusion (PBF), of which there are two main variants:
with silver nanoparticles, the leaching of silver ions acts as selective laser sintering (SLS) and high-speed sintering
an anti-microbial mechanism . With other nanocrystals, (HSS). In all AM processes, a computer-aided design
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such as MgO and Mg(OH) the reactive oxygen species (CAD) model of the part is created and digitally sliced into
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(ROS) they generate are responsible for the destruction layers; the part is built layer-by-layer. In SLS and HSS, the
of microbes 17,18 . With Mg(OH) , direct contact between raw material used to build the part is a polymer powder.
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nanoparticle and microbe is required to destroy the cell The polymer powder is held in a heated bed at 5 – 10°C
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membrane; conversely, direct contact is not necessary for below the melting point of the polymer. In SLS, an infrared
silver nanoparticles. Multiple anti-microbial mechanisms laser scans the area corresponding to the cross-section of
are also possible with certain biocides . the desired object, causing selective sintering/fusion; the
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While plastics embedded with silver nanoparticles are process is repeated layer-by-layer 22-25 . In HSS, the powder
anti-microbial, there is concern that nanometal biocides bed is initially inkjet-printed with black ink; a traveling
can leach metal ions that may be harmful to humans infrared lamp then moves across the bed, resulting in
and microbes. In this work, we explored the possibility sintering/fusion of the blackened area; this process is
Volume 3 Issue 4 (2024) 2 doi: 10.36922/msam.4970