A Self-Focusing Acoustic Transducer That Exploits Cytoskeletal Differences for Selective Cytolysis of Cancer Cells
Biophysical effects of ultrasonic energy in tissue include changes induced by heat, cavitation, and body force (radiation energy). Conventional acoustic devices generate low-frequency (1-4 MHz) high-intensity acoustic waves (> 103 W/cm2), which cause tissue destruction primarily through thermal or cavitation effects. However, these effects may be difficult to precisely control and not specific for cancerous cells over normal tissue. Here, we describe the design, fabrication, and therapeutic potential of high-frequency (18-MHz) acoustic irradiation with a self-focusing acoustic transducer (SFAT). A surface micromachining technique was used on a piezoelectric substrate to produce a SFAT device capable of focusing acoustic energy within an area of 100 μm in diameter at the 800-μm focal length. As we sought to minimize potentially nonspecific heat or cavitation effects by acoustic irradiation, operational parameters were chosen to study bioeffects of the device in the absence of tissue heating or biological effects due to cavitation. By varying the acoustic energy, we identified an acoustic intensity threshold (AIT) of 0.15 W/cm2 at 17.3 MHz, sufficient to cause this cytolysis effect in human prostate cancer cells 22RV1 without heat or cavitation. Next, we compared the AIT in various cell lines representative of benign and malignant prostate, breast, and skin cells and observed lower AITs in cancer cells over nonmalignant variants. As decreased stiffness (increased compliance) is a biomechanical characteristic, which differs between malignant and nonmalignant cell lines, we hypothesized that a less organized actin cytoskeletal pattern, which is known to be associated with decreased cell stiffness, would correlate with changes in the AIT. Actin staining of cytoskeletal structures confirmed an association between a pattern of diffuse and less organized actin filaments with decreased AIT. Moreover, the same trend of decreased actin organization and decreased AIT was observed following treatments that changed actin patterns in the MCF-10A breast epithelial cell line. These results suggest that biomechanical properties make malignant cells specifically sensitive to cytolysis caused by this form of acoustic energy. In summary, we describe a miniaturized acoustic transducer capable of producing a heatless and cavitation-free, cancer-specific focused cytolysis by direct body force (radiation pressure) effects alone. Ultimately, this device may lead to a miniaturized cancer-treatment system that can be used to focally and specifically ablate cancerous tissue with microscopic precision.