Ultrasound-Responsive Systems
The intensity of ultrasound energy can be tailored and applied over a small area of the body. The mechanism of ultrasound involves a medium that focuses, reflects, and refracts the ultrasonic waves. Ultrasound-sensitive vehicles have the potential to treat cancers because of their invasive character, ability to penetrate deeply into the human body, and ease of control. However, clinical use of this system may still require repeated use, with many related disadvantages such as patient noncompliance, higher risk ofsonication side effects, and the associated cost [43].
Enzyme-Responsive Systems
Enzymes play a central role in cell regulation. These proteins are not only important targets in therapeutics, but also essential for formulation development. Drug delivery systems, especially nanocarriers can be monitored to deliver or release drugs via enzymatic conversion of the vehicles. For instance, if the enzyme level is higher at a target site or the enzymatic activity is higher in diseased tissue, this imbalance can trigger higher drug release at the target site [17,42,44,52].
