# Advanced Microbubbles as a Multifunctional Platform Combining Imaging and Therapy
## 2 Advanced Microbubbles for Imaging and Therapy
### Blood-Brain Barrier Disruption  
The blood-brain barrier (BBB) is a special capillary endothelial system with tight junctions and metabolic barriers. It can keep the internal central nervous system (CNS) stabilized by preventing the macromolecular agents from transporting freely, but can also hinder neuro-active substances and drugs (such as nervous peptides, proteins and chemotherapeutic drugs) from curing brain diseases by BBB opening. Since 2001, [PERSON] et al. found that the intravenous injection of MBs before focused ultrasound (FUS) irradiation can significantly enhance the opening of BBB, and the BBB could remain intact within 24 h without causing any neuronal injuries [68]. Therefore, the study of focused ultrasound on opening BBB as a novel therapeutic method associated with functional MBs has implied a new era in this regard [69] (Figure 4). [PERSON] determined that transcranial FUS techniques could be used to transiently open the BBB and stimulate hippocampal neurogenesis [70]. They also found that FUS at a pressure of \(\sim\)0.78 MPa along with MBs could promote hippocampal neurogenesis and efficiently increase the BBB permeability. The ultrasound-induced BBB disruption plays an increasingly key role in the drug/gene delivery to the CNS. US in the presence of MBs transiently and reversibly opens the BBB in rodents and humans, thereby providing a time window for boosting drug delivery into the brain. [PERSON] investigated the feasibility of FUS in combination with MBs to improve the response by temporary disruption of the BBB. They prepared rat models inoculated with HER2-positive cells derived from a brain metastasis of a breast cancer patient (MDA-MB-361) and received trastuzumab and perutzumab combined with six weekly sessions of BBB disruption using focused ultrasound. Finally, the results demonstrated that the application of FUS to disrupt BBB combined with antibody therapy could inhibit the brain metastasis [71]. [PERSON] et al. analyzed the mechanisms that govern ultrasound-mediated opening of the BBB, and disclosed that porcine brain endothelial cells (PBEC) have low permeability for both small molecules and proteins. Their studies indicated that stable cavitations in presence of SonoVue MBs might have a more pronounced impact on cell permeability as compared to inertial cavitation [72]. Figure 4: BBB (blood-brain barrier) disruption in MRI (magnetic resonance imaging). (**A**) Four spots in the dorsal hippocampus were chosen marked with \(\times\) in T2-weighted images. (**B**) Following sonication, in T1-weighted images, four hyperintense spots (marked with red arrow) illustrated BBB disruption with injection of gadolinium contrast agents (0.2 mL/kg; Omniscan, GE Healthcare, Milwaukee, WI, USA). **Notes:** Copyright 2013, American Chemical Society. Abbreviation: BBB: blood-brain barrier. [PERSON] proposed the use of folate-conjugated DNA-loaded cationic MBs (FCMBs) to increase the efficiency of gene delivery to brain tumors and optimized the FUS parameters for FCMB-enhanced gene delivery by cell experiments. It indicated that FCMBs show better gene transfection efficiency than MBs without folate conjugation [73]. Numerous experiments have confirmed that the BBB disruption has an expansibility significance in the CNS. [PERSON] found that the use of FUS combined with MBs can promote the nerve growth factor into the substantia nigra striatum with peripheral intravenous administration [74]. At 1 h after administration, the biological activity of the nerve growth factor in the substantia nigra striatum was significantly higher than the direct administration without FUS, and the activation of the downstream signal pathway of nerve growth factor was detected in the neurons. [PERSON] further developed a new delivery protocol by conjugating gene-loaded liposomes with MBs. They successfully improved gene-delivery efficiency and applied it to Parkinson's disease (PD) treatment.