Nanostructured silicon surfaces and silicon nanoparticles slow down the division of stem and cancer cells, and the combination of nanoparticles and ultrasound kill the cells. The research has been conducted at the Physics department of Moscow State University, the Institute of Theoretical and Experimental Biophysics, Russian Academy of Sciences, the I.M. Sechenov First Moscow State Medical University, the Kulakov Research Centre for Obstetrics, Gynecology and Perinatology, and the Kurchatov Institute Russian Research Centre. The researchers believe that the results of the study can be useful for medicine, in particular oncology. The research was conducted with partial support by the Ministry of Education and Science of the Russian Federation.

Silicon nanoparticles have several valuable properties: biocompatibility, biodegradation capability, and high penetration power. They can increase the permeability of cellular membranes making it possible to take high doses of the pharmaceutical substances bound with the particles. Also, silicon particles are planned to be used as a toothpaste additive as they can softly remove bacterial dental plaque without damaging the enamel. The nanoparticles can serve as a fine preservative because silicon particles inhibit the development pathogenic microorganisms in food products. That creates multiple reasons for introducing silicon nanoparticles into the organism. It is not surprising that their properties and their interaction with the cells are subject of active research in many laboratories including Russian ones.

The researchers used nanostructured silicon surfaces and silicon nanoparticles for their study. The nanostructured silicon were obtained by electrochemical etching of crystal silicon plates. As a result, a 15-micrometer-thick layer was formed on the plates which consisted of protuberances and cavities varying from 10 to 100 nanometers. The plates were then placed into Petri dishes containing nutrient medium in order to cultivate human spinal cord stem cells on their surfaces. The cells maintained their viability for ten days but performed practically no division, however, whereas they managed to propagate well in the similar medium but without the plates. The researchers believe that the mechanism of this effect can be associated with both local electric fields on the structured silicon surfaces and purely chemical action of orthosilicic acid which is generated due to partial dissolution of the plate in the water. As silicon films slow down the growth of stem cells it is theoretically possible to use them for cell preservation.

The second part of the experiment was associated with the properties of polycrystalline or porous silicon nanoparticles. These were added into the nutrient medium where the cells of human larynx tumour or mouse fibroblast cells were being cultivated.

It was not accidental that cancer cells were used in the experiment. Foreign researchers are attempting to purposefully introduce silicon nanoparticles bound with medicinal substances into tumour cells. In the experiments conducted by Russian researchers, the nanoparticles in concentrations over three mg per ml hindered the growth of cells of both types — normal and tumour ones.

Ultrasound is known to be able to increase the damaging action of nanoparticles. High-energy radiation (2 watt/cm²) in itself can destroy up to 30 per cent of the cells; the presence of nanoparticles decreases the amount of living cells by four times as compared to the reference sample (ultrasound with no nanoparticles) within half an hour. Porous silicon particles are more effective than those of polycrystalline silicon as in this case almost all cells are destroyed. Under the action of low-intensity ultrasound (0.2 watt/cm²) the cells containing nanoparticles did not die but their division ceased completely within 80 hours.

Nanoparticles might serve as centres of emergence of cavitation bubbles leading to cell destruction. It is possible that their oscillation in the ultrasound wave results in mechanical damage to the cells. Depending on the ultrasound power, the damage results in complete destruction of tumour cells or to the emergence of defects inhibiting their division. The researchers believe that the findings of the study can be used in oncology; however, it will take further explorations to identify full capabilities of silicon nanoparticles.

Natalia Reznik