Fig. 2

Graphic description of the HIFU-triggered drug release of a wax-coated capsule. Conceptually, once the wax-coated capsule is in the body (e.g. in the GI tract), it can be tracked due to its hypointensity on T2-weighted MRI (Fig. 5a). While the wax coating confers resistance to a premature release of the capsule content, its melting point can be designed to be low enough (in this study 43 °C) for a mild non-invasive external heat trigger (e.g. by HIFU) to melt a cavity into the coating. Water influx from the gut lumen [59] into the capsule core dissolves the lyophilised GBCA, leading to a well-visible hyperintense signal (white cloud) on T1-weighted MRI. GBCA: Gadolinium-based contrast agent; GI: Gastrointestinal; HIFU: High-intensity focused ultrasound; MRI: Magnetic resonance imaging