Antibacterial Biocidal Steels
Much work has already been done into the potential for certain alloying elements to perform effectively as antibacterial agents in the prevention of harmful biofilms on the materials surface.
Certain elements are specifically effective against certain forms of bacteria for example it is now well know and effectively applied that copper and zinc are both effective in inhibiting the growth of both E. coli and S. aureus bacteria strains.
Most metals used in industry are alloys of several elements. For example, stainless steels contain Cr, Ni, Mo, Cu etc. in order to improve corrosion resistance, strength and stiffness. There is now solid investigation and studies into how alloying elements can also function as antibacterial agents. Such elements can be used to develop antimicrobial steels with slight modification of current products. For example, Cu or Ag alloyed antibacterial stainless steels are already under production and the effects of Ag alloying was already described above. Molybdenum, which was alloyed in type 316 and 316L stainless steels in order to increase toughness, inhibits growth of biofilm on stainless steels. If Mo is precipitated there surfaces by any methods, including heat treatments, surface conditioning, and/or increase Mo contents, such stainless steels can have the bactericidal function. Moreover, combination of different toxic metals can increase there bactericidal effects effectively.
Also, alloying steels with nitrogen is very promising to give then special functional properties for example corrosion resistance in bioactive environments, bactericidal activity or disinfecting ability, high resistance to special impacts, lightweight steels with lower density and high strength and plasticity.
Antibacterial materials and products have been widely and rapidly accepted by general consumers as fulfilling a relatively new function, which is distinguishable from the more traditional function of material protection.
Antibacterial products created by incorporating an antibacterial agent (biocide) can suppress the growth of bacteria on the surfaces of products when conditions exist where growth can occur. They can keep surfaces clean and sanitary and can also have an advantage in minimizing the impact on the environment by minimizing diffusion of the agent. This technology is significant for the quality of life, not only in developed countries but also in developing countries.
Antibacterial products have been widely used in plastics, coating materials, ceramics, natural and artificial leather, stainless steel, rubber, etc. The products involved cover a variety of categories, such as electrical appliances, personal items, household goods, nursing-care articles, pet accessories and aircraft-interior fittings.
The major object of the study of H.Kawakami et al. is to evaluate antibacterial activity of metallic elements (Al, Si, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Zr, Nb, Mo, Pd, Ag, Sn, Ta, W, Pb, Au, Pt) with application of JIS Z 2081. Escherichia coli (Gram-negative bacteria) and Staphylococcus aureus subsp. aureus (Gram-positive bacteria) were used as model bacteria.
In JIS Z 2801, the value of antimicrobial activity R is defined as positive if the R value is equal to or larger than 2.0. The results are summarized in Table 1 and shown in Figure 1.
Such data should be useful for the development of antibacterial alloys.
Table 1: The values of antimicrobial activity, R
Figure 1: Antibacterial activity of metals evaluated by application of JIS Z 2801
The conclusions are following:
+: Al, Co, Ni, Cu, Zn, Mo, Pd, Ag and W
+: only for E. coli: Pt and Pb
+: only for S. aureus: V and Zr
-: Si, Ti Cr, Mn, Nb, Sn, Ta and Au
Silver and Cu were strong biocide and Al, Co, Ni were moderately toxic. Following them are Mo, Pd, Zn, W and Pb. The antimicrobial effects of Mo might be archived by reducing pH.