# Advanced Microbubbles as a Multifunctional Platform Combining Imaging and Therapy
## 3 Challenges and Perspectives  
Ultrasound (US) imaging is one of the most widely used medical imaging modalities in clinics, due to its advantages of non-invasiveness, ionization-free radiation, real-time availability and commercially low costs. These unique advantages and apparent potential of ultrasound differentiates it from other techniques. The introduction of multifunctional microbubbles (MBs) rejuvenates resolution with improved sensitivity and specificity. By conjugating specific ligands to the MBs to construct targeted \"missiles,\" these MBs can be oriented to the diseased lesions with unprecedented precision. Moreover, this targeted therapy can be enhanced by carrying drugs/genes on the engineered MBs so that a combinatory strategy with both synergistic chemo-physical treatments can be implemented.  
However, the development of multifunctional MBs and the MBs + US technique are still in its infancy, and several bottleneck issues exist. For example, it is mostly used in intravascular diseases due to the micron sizes, but it is difficult to realize the investigation of a single cell or isolated tissues. Presently, nano-scaled bubbles are not well studied, and it is difficult to meet the actual requirements in this respect. Secondly, multiple targeting may offer better diagnosis than single or dual targeting. Nevertheless, it is still challenging to fabricate multifunctional MBs with better stability and prolonged circulation profiles. The limitation of real practical use of MBs in the work of [PERSON] et al. was that sonoporation efficiency is relatively low [24]. [PERSON] et al. analyzed 26 recent published papers and found that only a small part of the data illustrated sonoporation efficiency beyond 50% [103]. Another common limitation was the small number of samples; more detailed studies are thus needed in future research [104].  
In addition, safety evaluation is always an issue regarding the successful translation of the multifunctional MBs to humans. Future focus could emphasize the improvement of their biocompatibility and biodegradability. Potential toxicity and side effects for clinics should also be taken into consideration. In addition, secondary risks induced by US technology such as microcirculation vascular damage effect, ecchymosis and hypotension are also worthy of systematic investigations.  
Although there are still a lot of areas in need of improvement, contrast agents, such as SonoVue, have been approved for human use by the FDA and are widely used for clinical examinations in China. Compared to other US, SonoVue reflects its safety in myocardial perfusion assessment, abdominal applications and transcranial ultrasound diagnostics [105; 106; 107]. The four-dimensional hysterosalpingo contrast sonography, combined with SonoVue, achieves a real-time, intuitive examination [108]. Additionally, SonoVue has recently been used to expand the yield of gene delivery. US-mediated gene delivery combined with MBs was evidenced to be significantly safer than other local gene-delivery methods [109]. The combination of US-mediated MB destruction and shRNA could enhance antitumor efficacy [110]. One study conducted by [PERSON] also found that SonoVue enhanced the efficiency of rAAV transfection to RPE-J cells [111].  
In conclusion, the development of advanced MBs as a multifunctional platform for both imaging and therapy against diseases has shown bright perspectives in various biomedical areas. It can be expected that with the vast and portable application of ultrasound technology, the integration of \"bubbles for US\" will play a more active role in the future, and will serve the individualized treatments of human diseases in a more precise manner.  
This study was supported by Wenzhou City Science & Technology Bureau, China (No.Y20150084, No.Y20150085), and Health and Family Planning Commission of Zhejiang Province, China (No.2015 KYA159, No.2016 KYA143).  
The authors declare no conflict of interest.