Laser Cleaning Of Bacterially Infected Surfaces

Traditionally, in the food industry, cleaning and disinfection are performed using detergents and liquid water. Bacteria on surfaces form biofilms and adhere very strongly to the surface [Reference 1].

Studies have shown that these biofilms are not easily removed with detergent solutions because they are sticky. Bacteria are easier to remove from liquid solutions than from solid surfaces such as stainless steel.

Biofilms are often difficult to remove by mechanical and chemical means. Alternative disinfection methods need to be explored to replace mechanical streaking and chemical methods. Figure 1 shows a bacterially infected stainless steel surface.

Laser cleaning of bacterially infected surfaces is a very effective method because the heat produced by the laser is used to disinfect the surface. Figure 2 shows the laser cleaning of a contaminated pallet.

Research has been conducted to understand the effectiveness of using different types of lasers to clean bacterially infected surfaces. The lasers commonly used are pulsed lasers, but the choice of wavelength, pulse energy and repetition rate is very important. In one study [Reference 2], seven different types of lasers, ranging from ultraviolet (355 nm) to far infrared (118 μm ), were used to study their effectiveness in killing Escherichia coli (E-Coli) bacteria. Among these lasers, a 10.6 μm CO2 pulsed laser and multiple Nd:YAG lasers operating at nominal, second and third harmonic wavelengths were used.

Research shows that above a certain energy density, CO2 pulse laser is the most effective in removing bacteria, followed by Nd:YAG laser. The effectiveness of UV rays in killing bacteria is well known, and a third effective laser is the triple frequency Nd:YAG laser (355 nm emission). For testing purposes, E. coli was grown on several plates and the plates were exposed to laser light. After exposure, the plates were incubated at 37° for 24 h. If laser sterilization works, bacteria-free areas will be observed after growth. Table 1 shows the different laser parameters and the bacteria-free areas observed after exposure to such lasers.

As can be seen from the above table, energy density of the Lumonics Nd:YAG laser (10 msec pulses with 10 Joules of energy at 20 Hz) was 246 times that of the CO2 laser and the exposure time was 533 times longer as well. This difference could be attributed to the fact that water absorbs the mid-IR radiation (at 10.6 μm) much stronger than the near IR (1.06 μm) and since the E-Coli bacteria reside in water, they get killed more easily.

The UV wavelengths at 355 nm also responded well to sterilization as shown in the table. The frequency tripled lasers operated at 10 Hz repetition rate and had a pulse duration of about 5 nsec. Comparing the Surlite frequency tripled laser with the Lumonics Nd:YAG laser, one can see that with a mean power 200 time smaller and an exposure time of nearly 5 time shorter (Energy density was almost 20 times less), the Surlite laser achieved the same order of magnitude cleaning area as compared to Nd: YAG (0.123 cm2 as compared to 0.715 cm2).

Aside from lasers which were effective in killing the E-Coli Bacteria, there were other lasers that were ineffective. Some of these lasers included a far IR laser at 118 μm, diode laser at 0.81 μm and Argon ion laser at 0.488 μm. Several different energy densities were used for these lasers but they were found to be ineffective in killing the bacteria on the surfaces.

Allied Scientific Pro has developed a fiber based laser cleaning system that has already been used and proved itself working in many different fields such as ruch removal in aviation industry, cleaning historical monuments and decontamination of a nuclear facility. These laser cleaning systems have a laser head, optics and galvo mirrors that can make different shaped beams. Normally, a linear beam is used but to extend the application of these laser cleaning systems to cleaning surfaces infected with bacteria, and to increase the energy density of the beam, a circular spot size can be generated. As far as the repetition rate and mean power are concerned, the specifications are compatible with the parameters of Nd:YAG laser from Lumonics, mentioned in table 1.

Figure 4 shows the laser cleaning system by Allied Scientific pro. This is 100 Watt system operating at 1030 nm wavelength called Laser Blast 100.

Cleaning operations on contaminated metal surfaces in the food industry can greatly benefit from the laser cleaning systems described above. It is faster and more effective than traditional methods using mechanical and chemical means.