# Multivalent Aptamer Approach: Designs, Strategies, and Applications
## 4 Biosensing and Biomedical Applications  
### Biosensors  
Appamer-target interaction is an important recognition mechanism in biosensing applications. In particular, multivalent aptamers allow elevated local concentrations of aptamers and increased binding avidities between the capturing motif and the target. As a result, the multivalent strategy has been widely used in many large-sized target detections,  
Figure 5: A schematic representing the inhibitory nanostructures for dengue virus with their corresponding mean EC50 (half maximal effective concentration) values. The hexagonal spacing resulted in reduced detection compared to the star-shaped spacing, despite its being the correct spacing. The schematic for the star and hexagon shows an unzipped hairpin region because potent pattern matching occurs. Other scaffolds represent the hairpins as stem–loop structures to indicate a lack of potent pattern matching ([5]). such as human cells [6; 18; 42; 67; 99] and bacterial cells [87; 100; 101; 102] in whole-cell assays. In addition, the multivalent strategy has also been used to detect smaller sized targets, such as extracellular vehicles [54; 103] and small molecules [104]. In comparison with antibodies, aptamers allow readily and reversible capturing of targets for further analysis and characterizations [105]. Linear aptamers and spherical aptamers (see Figure 1) are the most frequently used aptamer forms in detection applications; to further increase multivalency, the combinations of the two forms have also been reported [24; 100]. For instance, [PERSON] et al. [24] proposed a NanoOctopus to capture CTCs in blood samples. To achieve this, the researchers conjugated multivalent aptamer chains onto the surfaces of magnetic nanoparticles (i.e., NanoOctopus), and they showed that the multivalent NanoOctopus significantly increased binding avidity to their targets. Subsequently, the captured cells were released for further analysis. Similarly, DNA nanochains that contain multiple copies of aptamers have also been used [55; 62; 100]. Inspired by climbing plants, [PERSON] et al. [53] developed a multivalent aptamer nanoclimber (MANC)-functionalized PDMS microfluidic device for minimal residual disease testing. This work demonstrated a 57.5-fold increase in binding affinity in comparison with a mono-aptamer approach when the MANCs was used to capture target cells in human plasma samples, suggesting that the MANCs are clinically applicable to assay minimal residual disease in peripheral blood. The captured cells were then released by using complementary DNA displacement or nuclease digestion for downstream analysis. Spherical multivalent aptamers have also been used in microfluidic devices [6; 19; 23; 106] to increase detection performances of the microfluidic devices. Inspired by octopus tentacles, [PERSON] et al. [23] conjugated SYL3C aptamers onto AuNPs that were subsequently attached to the inner surfaces of a micropillar-decorated microfluidic channel. The researchers showed that approximately 250 SYL3C aptamers were attached onto each AuNP and that the detection platform was able to detect CTCs directly in blood samples without any sample pretreatments. In addition, the authors further demonstrated that the multivalent aptamer approach improved the binding efficiency by 100-fold and that the capturing efficiency was enhanced by more than 300% in blood samples when compared with aptamer-monomer-modified microfluidic devices. The captured target cells were released by using a thiol-exchange reaction that was shown to be of high efficiency and maintained high cell viability. Alternately, polymeric dendrimers, such as PAMAM, have also been used to construct spherical multivalent aptamers structures [101]. [PERSON] and coworkers [100] developed a fluorescent microfluidic sensing platform for detecting _E. coli_ O157:H7, using rolling circle amplification (RCA) chain-conjugated PAMAM dendrimers as an inner-channel surface modification.