Acrylic acid controlled reusable temperature-sensitive hydrogel phantoms for thermal ablation therapy

Polymerization of N-isopropylacrylamide (NIPAM) with acrylic acid (AAc) has been adopted to fabricate reusable tissue-mimicking hydrogel phantoms designed for the real-time visualization and examination of thermal lesion formation in ablation and hyperthermia therapies. It is shown that the cloud point temperature of the NIPAM-based hydrogel phantoms can be adjusted by the concentration of AAc to represent the threshold temperature of pain (42 °C) or tissue damage (52 °C). The mechanical, thermal and acoustic properties of the developed phantoms are similar to those of human soft tissues. The ability of the phantoms to provide visualization of thermal lesions produced by either microwave or high-intensity focused ultrasound (HIFU) ablation was examined. Evolution of the optical transparency of the phantoms with temperature was found to be a stable hysteretic behavior and reproducible in consecutive heating–cooling cycles, demonstrating the reusability of the phantoms. By processing the optical images of the phantoms at different stages of the heating process, a thermal lesion can be considered formed (i.e., threshold temperature reached) when the grayscale value reaches the half-saturation point. The image processing method proposed for the NIPAM-based hydrogel phantoms is shown to be independent on the type of heating device used.