A laser-produced gradient field can capture and align biological cells. Images show cells before (a), during (b-d), and after (e-f) interferometric trapping; the pattern period is 8.2 µm. Courtesy STEPANOV INSTITUTE OF PHYSICS
Laser coherence has generally not been considered an important factor in laser therapeutics. Now, researchers at the Stepanov Institute of Physics in the National Academy of Sciences (Minsk, Belarus) have demonstrated that laser coherence may be a critical factor in controlling biological processes. "You could conclude from our experimental studies that a well-chosen coherent therapy can stimulate the repair of cells and increase their viability," says team leader A. N. Rubinov.
The theory is based on the non-resonant dipole interaction of gradient laser fields with biological matter. A gradient field in the tissues and cells can be generated through laser interference patterns or through speckle formation caused by scattering. The effects are dependent on the laser's coherence and not the wavelength; wavelengths are chosen to avoid unwanted absorption. Linearly polarized light produces sharper contrast of speckles than non-polarized, and consequently provides larger gradient forces.
The experiments showed that these gradient fields could cause spatial modulation of the particles, allowing them to be trapped or moved, and improving the rate of damage repair. The process appears to be size-dependent. Sweeping the field can move selected particles, and particles in the gradient field can self-organize in a crystal-like structure. In other words, the therapies have the potential to be custom designed for particular cell types or conditions.
In the experiments, output from a singlemode, 40-mW HeNe laser was split into two separate parallel beams of equal intensity using identical 20° Fresnel biprisms. The beams were brought together with a spherical lens at an angle in the plane of the cells. The period of the resultant interference pattern generated in the focal plane of the lens was defined by the angle of the beams, which was in turn set by the separation of the two parallel beams. The distance between the biprisms controls this separation, allowing a range of interference-pattern periods to be tested. The cells were contained in a 200-µm-thick glass vessel and the diameter of the laser pattern was 50 to 100 µm, depending on the lens used; a digital camera captured the images.
The group studied the basic properties using micro-scale polymeric balls with optical properties similar to those of blood cells; experiments repeated with biological cells demonstrated good correlation to those for the polymeric balls. The team found that after a few seconds of exposure to the gradient field, the cells moved from a random distribution to becoming localized in the interference maxima (see figure). They stayed there until a few seconds after the gradient field was removed and then began randomly dispersing.
By substituting an argon laser operating at 514 nm for the HeNe laser, the group was able to study red-blood-cell aggregation index, a measure of clotting fundamental to the cells. They demonstrated that aggregation index could be directly affected by applying, changing, and then removing the coherence-based gradient field. This is a clear demonstration that the gradient field produces a measurable biological effect in the cells. Further work with white blood cells showed that gradient fields of particular periodicity could enhance the repair mechanisms of the cells. Exposure times of 15 to 21 minutes were found to have the most pronounced effect. The gradient forces produced both rough and delicate biological effects depending on the incident power.
These experiments were designed to look for measurable effects from coherent therapy, and now the researchers want to further understand the mechanisms behind the effects. The application of interference fields opens up new possibilities for controlling fundamental biological processes for low-intensity laser therapy.