Biological effects of low frequency ultrasound on the skin

Several studies have demonstrated that low frequency ultrasound (20 kHz) can increase skin permeability leading to the idea that the phenomena called phonophoresis could improve the efficiency of patches. This requires one to investigate skin tolerance to ultrasonic energy. Firstly, fresh human skin samples were submitted to various intensities at 20 kHz, and then subjected to histological analysis. Intensities up to 2.5 W/cm² (continuous or pulsed mode) produced no observable lesions. However, epidermal and dermal necrosis was observed with higher intensities. Secondly, the thermal effect of ultrasound was investigated in vivo on 2 groups of anesthetized hairless rats. The A group was treated with the previous ultrasound protocol while surface temperature being monitored. The maximum recorded was 45°C. The B group was treated with no ultrasound but the thermal profile was mimicked by heating through an electrical resistance. In both groups, a slight and transient erythema was observed. Two days later, a total absence of lesions was observed in the B group, while the A group displayed epidermal and dermal necrosis. The authors´ results show that ultrasound could cause delayed lesions in hairless rat skin at low intensity, while human skin seems to be less sensitive in vitro. Furthermore, thermal effects seem not to be involved in ultrasound induced lesions which then may result from mechanical effects such as cavitation