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SaniUV: Antimicrobial Coating

Tue, 06/19/2012 - 1:00pm
Huu-Tuan Nguyen

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Subsidiary of Surfaces Synergie Holding,Ionitec has developed a new antimicrobial and self-cleaning coating (TiO2) called SaniUV. This coating is deposited by PVD (Physical Vapor Deposition) and the process parameters for obtaining such efficiency are the subject of a patent.

Photo-SaniUV

Photocatalysis is a natural phenomenon in which a substance, called photocatalyst, accelerates the speed of a chemical reaction under the action of light (natural or artificial).

Using light energy, the photocatalyst generates the transformation of water and oxygen from the ambient air, highly reactive molecules (called free radicals), able to break down all organic substances, sometimes harmful, in completely harmless compounds by a redox reaction.

TiO2 is a semiconductor material; electrons in it are therefore able to change their energy state. If we give them enough energy they will move from the valence band to the conduction band across an energy gap called "band gap" (Theory of band / solid state physics).

This change of state generates a "hole", a deficit of electrons in the valence band which will be called "h+", and an excess electron in the conduction layer which will be called "e-".

Holes "h+" react with water to form hydroxyl radicals OH°, and the electrons react with oxygen to form superoxide 02-. These are two powerful oxidants that can destroy organic molecules such as bacteria, viruses, germs, odors, volatile organic compounds, mold, algae, fungi, pollen, dust mites...

The basic reactions:

  • TiO2 + UV -> h+ + e- = pair creation: hole / electron
  • H2O + h+-> °OH + H+= oxidation of water, hydroxyl formation
  • O2 + e- >-> O2- = reduction of oxygen, superoxide formation

Energy can be supplied in several ways, in our case it is provided by light radiation. This light radiation need to be higher than the band gap’s energy so that the change of state can happen.

Condition: Light energy > 3.2 eV (electron volt) minimum, which corresponds to light of wavelength shorter than 388 nm è UV-A.

E.coli

Our TiO2 has been optimized to reduce the value of "Band Gap" energy; therefore it is also active with visible light.

Effectiveness of the layer:
The photocatalytic ability of SaniUV is characterized by measuring an efficiency coefficient called CIF: Catalytic Improvement Factor.

This coefficient corresponds to a specific degradation rate of methylene blue (MB), similar to the one mentioned in ISO 10678(1). It involves measuring the difference in concentration of a solution of MB in contact with a coated surface before and after 24 hours exposure under UV-A.

To date, SaniUV has a CIF of 9.82 on a scale from 0 to 10, which corresponds to a complete discoloration of the solution of MB within 24 hours, clearly showing its high photocatalytic activity.

In addition, SaniUV has a CIF of 2.5 when it is exposed to visible light. For comparison, coated windows of building have a CIF of 1.8 under UV-A.

Because of its crystalline structure, the effectiveness of the layer is proportional to its thickness.

The photocatalytic ability of the coating gives therefore an antimicrobial effect. The antimicrobial ability, characterized by the ISO 27447(2), consist of comparing the evolution of two cultured E. Coli populations under UV, one on a coated surface, the other on a neutral surface.

This method measures a factor called "Log Reduction" corresponding to the number of decades in which the initial population of bacteria was divided 8H after UV exposure.

Example: a surface capable of dividing by 10 000 (104) the amount of bacteria in 8H has a "Log Reduction" of 4.

There is another unit of measurement known as "decimal reduction time" being the time necessary for a surface to divide by 10 a population of bacteria.

The French pharmacopoeia speaks of sterilization when it remains one bacteria on a million present before the sterilization operation, so sterilization is obtained with a reduction of bacteria in the order of 106 (Log Reduction = 6) .

To date, we have not measured accurately the "Log Reduction" of our SaniUV coating. However, the antimicrobial effect is directly related to the photocatalytic effect, our CIF of 9.82 suggests a particularly high antimicrobial potency.

(1)ISO 10678: Fine ceramics — Determination of photocatalytic activity of surfaces in aqueous medium by degradation of methylene blue.

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(2)ISO 27447: Fine ceramics — Test method for antibacterial activity of semiconducting photocatalytic materials.

Enabling SaniUV:
Through optimization of the coating, the photocatalytic reaction is now possible with visible light and SaniUV is active with the majority of lighting (neon lights, compact fluorescent bulbs, LED lamp). However, it is best to expose the layer to a light having a wavelength shorter than 388 nm (UV-A, UV-B, UV-C) in order to have an optimal effectiveness which is four time stronger. The more the wavelength becomes shorter, the more the reaction will be swift.

The coating has a memory effect, in fact, after UV exposure, the coating remains active for a while preventing the proliferation of new contaminants.

SaniUV is biocompatible; it perfectly fits to medical devices from a simple surgical instrument to a medical endoscope. With a CIF of 9.82, SaniUV distinctly demonstrate its industrial advance ahead others photocatalytic technologies.

Uses are many and refer to every objects handled or touched by numerous people in the daily life.

Huu-Tuan Nguyen, 24 years old, is a business manager at IONITEC, subsidiary of Surface Synergie Holding. He is in charge of the commercial development of SaniUV: an innovative antimicrobial coating. Young and motivated about his job, he is convinced about the coating’s potential and all.

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