Biocidal Mechanisms of Metallic Copper Surfaces - Use of Biocidal Surfaces for Reduction of Healthcare Acquired Infections - page 116

6.3.2 Additional Physical and Physiological Factors
Modulating the Contact-Killing Process
There are several factors which influence killing rates by metallic copper surfaces:
alloy copper content, temperature, moisture, copper chelators, osmotic stress,
reactive oxygen species, cellular physiology, copper detoxifying systems, and
pre-adaptation to copper, but not anaerobiosis (oxygen-free environment).
6.3.2.1 Copper Alloy Content and Roughness
Copper alloy content influences cell survival exposed to these alloys. This seems to
be a logic feature of copper alloy surfaces, where the higher the copper content of
the alloy the higher the killing efficiency. For the majority of the alloys this is true,
but there are exceptions. The metallurgy industry mixes copper with other metals in
order to modify intrinsic alloy properties for a defined purpose. Nordic Gold is a
gold-colored alloy used in coinage which contains 89 % copper, 5 % aluminium,
5 % zinc, and 1 % tin; it was developed to be of low allergenic without compromis-
ing the resistance to tarnishing. This is not as efficient in bacterial killing as other
alloys [ 72 ]. For example, a lower copper containing alloy, 18 % Nickel Silver alloy,
which contains 65 % copper, 18 % nickel, 17 % zinc, and does not contain silver,
has a higher killing efficiency than Nordic Gold [ 72 ]. These results can be explained
by different copper release rates or to the presence of other metals that can aid in
copper toxicity, however there is no experimental data to support such claim. A
great challenge for metallurgic industry would be the development of an alloy that
compromises a fast killing efficiency and tarnish resistance (with good esthetics),
together with the capacity to remain “bioactive” in the long term.
In the experimental studies pure copper (99 % Cu) is often chosen to investigate
effect of copper on contact killing with different bacteria. Additionally, this has also
facilitated the comparison of results between experiments and groups.
The degree of surface homogeneity was also shown to modulate the killing
efficiency [ 37 ]. A rougher surface is characterized by larger area of contact with
bacteria cells; hence cells are exposed to more corrosion products, thus more
toxicity. As described before, contact is required for killing by metallic copper
surfaces, which corroborates these findings [ 53 ].
6.3.2.2 Temperature and Moisture
Temperature was one of the first identified factors influencing the killing by
metallic copper surfaces. Typical metallic copper experiments were performed at
controlled room temperature, between 18 and 24 C. The rate of killing is in direct
correlation with temperature changes, where higher temperature corresponds to