# Multivalent Aptamer Approach: Designs, Strategies, and Applications
## 1 Introduction  
The concept of multivalency is intrinsic to nature. For instance, marine mussels use multiplebyssal threads to ensure attachment to rock surfaces, and octopus appendages make use of numerous suckers to efficiently capture preys. Likewise, multivalent interactions play a vital role in biological systems, down to the cellular level. Examples of biological processes utilizing multivalence include viral entry, cell-cell interaction, host-pathogen interaction, and immune synapse formation [1]. In each of these processes, multiple functional units work synergistically to improve overall performance. By taking advantage of the working principle of multivalency, researchers have applied this strategy in many molecular-recognition-based applications, such as targeted drug delivery [2; 3], pathogen inhibition [4; 5], cell capture and downstream analysis [6; 7], detection [8; 9], and in vivo imaging [10; 11]. To this end, multimeric nucleic acids [12], antibodies [13; 14; 15], and aptamers have all been used to enhance functional performance in comparison to their monomeric counterparts.  
Among the multivalency-based strategies mentioned above, the multivalent aptamer approach has attracted the most attention in recent years. Aptamers are short single-stranded DNA (ssDNA) or RNA molecules that bind to targets via specific structures [16] and possess advantages over other binding mechanisms, such as antibody-antigen binding. These short aptamer oligonucleotides are artificially selected from a process called Systematic Evolution of Ligands by Exponential Enrichment (SELEX) [17]. Aptamers can be selected to target ligands that are extremely small, highly toxic, or non-immunogenic,characteristics that are not readily achievable with conventional antibodies. In addition, thanks to large-scale oligonucleotide synthesis, the production of aptamers is significantly more cost-effective and accessible than that of antibodies.  
In general, multivalent aptamer approaches aim to achieve enhanced binding avidities toward their target ligands. The avidity of an aptamer describes the overall affinity of the interactions between an aptamer and its target; by linking two or more aptamer units, multivalent aptamers have been shown to work cooperatively to increase the avidity of interactions [18; 19]. Higher local aptamer concentration leads to a higher chance of interaction between aptamers and cell-surface ligands. In addition, after the initial binding of an aptamer with its ligand, nearby aptamers can be readily recruited to facilitate additional binding, thereby leading to a higher avidity. Physiological conditions present another challenge for aptamers; in their monomeric form, aptamers have been shown to be rapidly degraded by nuclease, a major drawback limiting their in vivo applications [20]. However, studies have demonstrated that conjugating multiple aptamers onto nanoparticles can successfully provide resistance to enzymatic degradation and increase nucleic acid stabilities due to increased local salt concentrations on the surfaces of the nanoparticles [21; 22].  
The current review focuses on the structures and design principles of multivalent aptamer approaches developed in recent years. To this end, multivalent aptamers are first divided into several categories based on their structures. This will set the stage for detailed discussions on the design principles and strategy highlights of multivalent aptamers. Subsequently, applications based on recently published studies related to multivalent aptamers are reviewed, followed by a brief discussion of current challenges and future research directions.