Architects memo no. 98: March 2010
next to godliness
In Architects memo No 85 (Strewth! Was
that really November 2006? It just seems like
yesterday!) we talked about the increasing use
of paints as bactericidal surfaces; especially in
Japan, but increasingly in areas of America and
There are several strategies used to make paint
films bactericidal, the simplest strategy being
to incorporate a bactericide into the paint films
which will concentrate at the paint-surface. On
contact with a bacterium, the bactericide will (by
molecular design) enter the cell wall and wreak
The trick with this approach is to be able to
select a bactericide that is lethal to bacteria but
harmless to humans!
Colloidal Silver, the technology that Resene
adopted, is a highly successful biocide with
virtually no mammalian toxicity. There is some
conjecture as to how the silver gets into the cell
of the bacterium, but one possibility is that the
bacterium actually does it to itself. It is known
that bacteria search for essential minerals, such
as iron, by going ‘fishing’ outside its cell. It
protrudes molecular arms, called siderophones,
outside its cell to grab atoms of iron. If it is fooled
into transporting silver atoms into its cell these
atoms then give it the slow ‘kiss of death’.
There has been, and continues to be, a lot of
effort put into developing paint binders that are
in themselves bactericidal. As these binders are
not soluble and are unable to enter cell walls
they have to adopt other strategies. Typically,
bactericidal polymer films look to accumulations
of electrical charge on their surfaces to ‘deal to’
bacteria. One such exciting polymer became very
close to commercialisation before being ‘pulled’
from the market.
The other exciting technology involves the use of
nano-sized particles of anatase titanium dioxide
which, under the irradiance of U.V. or near U.V.
light, produces short-range free radicals. These
free radicals blast everything organic, including
bacteria, within range. This, in turn, demands
special paint binders which are, in themselves,
resistant to these very active radicals.
Bactericidal coatings have proved to be effective
and are increasing in popularity but, as stated in
Memo 85 and re-emphasised here, they should
be seen as a useful adjuvant to normal hygiene
regimes. Typically, a number of the killed bacteria
will remain on the surface of the bactericidal
paint – eventually building up to the extent
that they will form a barrier, preventing contact
between new bacteria and the surface.
At this point the walls will need washing to
remove this build up and to rejuvenate the paint
surface. A sanitising cleaner is to be preferred; one
that will not leave a bacteria-friendly residue.
The silver bactericide will, naturally, be depleted
over time depending on the severity of the site
and the number of washes it has. Re-coating will
be required from time to time.
Download as a pdf. (You will need Acrobat Reader).