## 3 Microfluidics as a Diagnostic Platform
### Microfluidics for Peptide Imaging  
The formation of microbubbles to assist ultrasound imaging has traditionally been performed via various methods, including membrane emulsification, mechanical agitation or laser cavitation [63]. However, MFs have been found to produce microbubbles of a higher quality than the aforementioned techniques. Bubbles with consistent sizes (approximately 10 \(\upmu\)m diameter) can be produced by combining a continuous aqueous phase with a gas phase (e.g., perfluorobutane (C\({}_{4}\)F\({}_{10}\))) within the micron-sized channels of the MF chip [64]. Depending upon the chip's design, two major methods for microbubble synthesis can be performed using MFs. First, a standard T-junction method can be used, which combines two separate phase streams within the chip at a \"chaos point\". The microbubbles produced via this method appeared to be less consistently sized than using the second method; flow focusing. Flow focusing exploits the geometric properties of the chip to produce uniform and smaller microbubbles from a pre-mixed stream of phases. A diagrammatic representation of these processes can be seen in Figure 5.  
The microbubbles are used to create contrasting areas of imaging within the body to optimize the functionality of ultrasound scanning. It is also possible to modify the process by simple modification of the chip design to incorporate multiple inlets, affecting the final properties of the bubble, such as attaching additional imaging modalities to the microbubbles or encapsulating APIs within the structure. Microbubbles have been functionalized with fluorescent and magnetic resonance to ameliorate the imaging capacity of the formulation, as well as allowing the addition of peptide moieties onto the shell structure to increase the target specificity of the imaging medium [65]. It was concluded that varying the capillary size within the microfluidic chip had a dramatic effect upon the final size of the microbubble produced. As this area of research expands alongside the general use of peptide formulations within the system, future microbubble formulations will gain exponential levels of adaptability to help achieve their optimized imaging potential. When processes are time-dependent, for example, the synthesis of radiolabelled atoms, MFs offer an extremely effective route to establish quick synthesis whilst maintaining product quality. Prior to MFs implementation, synthesis of the peptide radiotracer [18]F F-Py-YGGFL from anhydrous 18F, a two-step reaction, required a process that took over an hour, which led to decreased yields as the half-life of 18F is only 110 min [67]. Using an automated MF system, which employs two separate microfluidic reactor environments in series, within the same system, it is possible to reduce this time to just 8 min, as shown in  
Figure 5: Microbubble formulation methods using MFs, as suggested by [PERSON] et al. using (**a**) t-junction incorporation and (**b**) flow focusing; (**c**) radiolabelled peptide production, adapted from [PERSON] et al. [66]. Figure 5 [66]. As the system operated in two separate reactors, it is possible to isolate the environmental conditions from each reactor in case different temperatures are optimal for individual steps. The MF process ensured minimal time-based degradation, which meant that yields for the process were higher (28%) compared to traditional methods. The applications of radiolabelled peptides exist in the field of oncology in vivo imaging (e.g., PET scans), as they are non-toxic, effectively renally excreted due to their small size and possess low levels of immunogenicity. The premise of using multiple MF chips/reactors in series is an effective way that the system design can be manipulated to perform complex tasks, including synthesizing peptide conjugates, as seen above.