Background and Purpose: Sonophoresis is a noninvasive ultrasound-based method for the transdermal delivery of drugs. However, the importance of the thermal effects in sonophoresis via therapeutic ultrasound remained unclear. In this study, an animal study was used to evaluate the contribution of thermal effects in 1MHz ultrasound mediated transdermal drug delivery. Method: Rats in the ultrasound-treatment, heat-treatment, and control groups received ultrasound treatments (1MHz, 2W/cm^2, 100% duty cycle, 5 min), heat treatment (tail in 50℃ water-bath for 10 min), and no treatment, respectively, before applying eutectic mixture of local anesthetics (EMLA) cream. Sensory conduction in the tail nerve was investigated before and every 5 min after treatment for 60 min. Results: There was no significant difference between the control and heat-treatment groups in the latency to the reduction in sensory nerve conduction velocity (SNCV) or in the sensory nerve action potential (SNAP) amplitude. The latency for the SNCV to decrease by 20% was significantly lower in the ultrasound-treatment group than in the control and thermal-treatment groups. Compared with control group and thermal treatment group, using ultrasound could reduce the latency for an EMLA reaction from 60 min to less than 20 min. Conclusion: In this study, the thermal effect seemed less important than the mechanical effects in sonophoresis via therapeutic ultrasound.
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