Polymer gel as thermally responsive attenuator for ultrasonic waves

The ultrasonic attenuation in N-isopropylacrylamide (NIPA) gel has been mea sured as a function of temperature at various frequencies. It is found that at room temperature, the attenuation of a longitudinal ultrasonic wave in the gel is small and close to that of pure water. However, as the temperatu re increases above the spinodal phase transition point of the gel, the atte nuation increases drastically. This change of the attenuation is completely reversible and due to the microdomains formed in the NIPA gel above its ph ase transition temperature. The increase of attenuation at 15 MHz from 26 t o 45 degrees C is about 26 dB/cm. The attenuation varies within 5 min after the temperature change is made for a sample with diameter 1 cm and height 0.8 cm. The results reported here may have potential applications from ultr asonic assisted drug release to a switch for ultrasonic signals. (C) 1999 A merican Institute of Physics. [S0003-6951(99)00115-1].