In phonophoresis, what is typically driven into the skin using ultrasound?

Phonophoresis is a therapeutic technique that utilizes ultrasound waves to enhance the delivery of medication through the skin. The primary aim of this method is to facilitate the absorption of certain types of medication, particularly those that are effective in reducing inflammation, such as steroid compounds.

Steroid compounds are often employed in phonophoresis because they are anti-inflammatory and can target conditions like arthritis or soft tissue injuries. The ultrasonic waves create a cavitation effect that drives these molecules into the deeper layers of the skin, improving their local effects without the need for injections.

In contrast, the other options do not fit the specific context of phonophoresis. While liquids can be used in various therapeutic modalities, the focus here is primarily on the absorption of active pharmacological compounds—specifically, steroids. Gases and solid particles are not typically utilized for penetration through the skin via ultrasound in this modality.