## 3 Microfluidics as a Diagnostic Platform
### Enzyme Activity and Biomarker Detection  
Measuring enzyme activity using an MF platform is not necessarily a contemporary idea, as many MF designs are centered around this [71,72]. However, due to recent advances in neighboring fields, namely additive manufacturing (AM) and nano-detection, the capacity for MFs to act as an area-leading platform has been propelled. Complex MF structures can be printed with high resolution to allow for precise control and monitoring of the environment within the MF chip. Recent enzyme studies using MFs [73] incorporate hydrodynamic traps within the MF chip to monitor an enzyme's activity, like Granzyme B, to determine an individual's response to therapeutic treatment and improve prognosis [74]. This type of assessment offers a high standard for determining clinical response as it focuses on single-cell activity, using a life (rather than fixed) cell [73]. By exploiting this fact, it is possible to measure a cell's response over a prolonged duration of time, giving a clearer image of a patient's response to therapy. If this technique were scaled up to other monitoring practices, personalized medicine optimization would be a far more achievable goal than currently possible, which could help relieve the economic burden on healthcare systems.  
In a similar principle to the hydrodynamic traps used by [PERSON] et al., peptide antibodies within micro-wells or via specialized surfaces on the MF chip can be used to isolate specific cells during identification assays [75,76]. Complementary antibodies to target receptors are immobilized onto the surfaces of the chip. Then blood samples are passed through the chip to perform the evaluation. This process is an effective tool used to act as an early detection test for diseases. In the case of studies performed by [PERSON] et al., circulating tumor cells (CTCs), which are the root cause of metastatic cancer, and cancer stem-like cells (CSCs) were targeted within the system. Owing to the adaptability of the chip, multiple antibodies were present within the chip, allowing detection of multiple different CTC biomarkers, for example, epithelial cell adhesion molecule (EpCAM) and a CSC biomarker, CD133. Current testing methods, such as EpCAM-based isolation, are used as a standard test for these molecules. However, false-negative results are observed occasionally with this method. The microfluidic isolation method appeared to give a higher level of accuracy than its testing counterparts, although more research must be performed before it can conclusively be said that this method produces more reliable results.  
Once again, owing to the MFs high throughput capacity, it would be simple to run multiple assays within a limited timeframe. Relating to vessel isolation, exosome isolation has been shown to be an efficient process using MFs [77]. Exosomes are vesicles ranging from 30-150 nm in size [78] that occur naturally within the body and act as a transport medium for substances like miRNA, complex proteins, lipids and unwanted waste products. Exosomes can be exploited to assist with cancer diagnosis owing to unique markers being present on cancer-related exosomes. To list a few examples, melanoma is associated with CD63, and caveolin-1 biomarkers and acute myeloid leukemia is linked to TGF-\(\upbeta_{1}\) markers [79]. More examples can be found in Table 3 that could be exploited via an MF system. The isolation process is similar to that performed in studies by [PERSON] et al. and boasts a far less invasive procedure for the patient to obtain a diagnosis compared to a biopsy.