###### Abstract
**(c)** Sketch of a potential scheme of drug delivery with gas bubbles and subsequent drug photoactivation with sonoluminescence UV spectral range light. **(d)** Illustration of an idealised fluorescence-based fibre-optic probe that can be used for imaging, sensing, and drug photoactivation inside a living body. sonoluminescence [38], where the brightness of sonoluminescence is adversely affected by interactions between neighbouring bubbles. In fact, while the light emitted by a single air bubble oscillating in a water-filled glass flask can be seen by the naked eye in a darkened room when using low-power 25-150 kHz ultrasound pressure waves [35], visible multibubble sonoluminescence is possible mostly using high-frequency high-intensity ultrasound waves and bubbles that contain noble gases such as xenon [34]. Moreover, since sonoluminescence light has a significant spectral ultraviolet (UV) component [34; 35; 39], but bubble oscillations resulting in sonoluminescence can be triggered by ultrasound waves, which can pass through the body without noticeable attenuation, it has been argued [40; 41] (Figure 1c) that sonoluminescence light can be exploited to noninvasively photoactivate drugs inside a living body in hard-to-reach organs such as the brain. Such ideas are of immediate relevance to the current research efforts in the field of photopharmacology, which aims to develop smart drugs, which, through the incorporation of molecular photoswitches, enable the remote spatial and temporal control of bioactivity by light [42; 43]. This concept is particularly beneficial in the treatment of bacterial infections, where there exists the need to control the side effects of antibiotics. However, its practical implementation faces several problems since modern light-responsive drugs mostly rely on UV light, which is a problem because UV radiation is strongly attenuated by bodily fluids and tissues, but increasing its intensity results in adverse phototoxic effects on biological cells. These challenges motivate the research on novel drugs that could be activated using red light, which falls within the so-called therapeutic window [43], where light has its maximum depth of penetration in tissue [44]. However, using sonoluminescence, one might generate UV light directly inside the body, thus resolving the problem of light delivery to the body interior from an external source of UV radiation and also enabling one to use the already- discovered UV-sensitive drug compositions. Yet, it has been theoretically shown that sonoluminescence UV light generated inside an internal body organ may have an additional antibacterial effect [39], which can improve the overall performance of UV-activated drugs. More broadly speaking, using sonoluminescence in the field of biomedicine may be advantageous compared with well-established optical sensing, imaging, and drug activation techniques [45], including fluorescence-based methods [46; 47], where the use of laser beams and optical fibre probes (Figure 1d) implies that a surgical intervention is needed to access an internal organ such as the brain. On the contrary, a sonoluminescence-based approach may be fully noninvasive since it could rely on advances in the well-established field of conventional medical and transcranial ultrasound imaging methods, where imaging of internal organs and noninvasive functional brain studies are enabled by microbubble-based contrast agents [48]. Fluorescence-based techniques rely on fluorophores--molecules or nanoparticles--that can be applied directly to live specimens to investigate the interior of a cell in situ or an internal body organ in vivo [45; 47]. The light absorption by a fluorophore is followed by re-emission of a photon with a lower energy and a longer wavelength. Hence, fluorescence-based techniques separate the re-emitted lower intensity light from its excitation counterpart, creating an image or enabling one to detect chemical composition [49] and temperature [45; 50].