Developing powerful, simple and low-cost DNA amplification techniques is of great significance to bioanalysis and biomedical research. Thus far, many signal amplification strategies have been developed, such as polymerase chain reaction (PCR), rolling circle amplification (RCA), and DNA strand displacement amplification (SDA). In particular, hybridization chain reaction (HCR), a type of toehold-mediated strand displacement (TMSD) reaction, has attracted great interest because of its enzyme-free nature, isothermal conditions, simple protocols, and excellent amplification efficiency. In a typical HCR, an analyte initiates the cross-opening of two DNA hairpins, yielding nicked double helices that are analogous to alternating copolymers. As an efficient amplification platform, HCR has been utilized for the sensitive detection of a wide variety of analytes, including nucleic acids, proteins, small molecules, and cells. In recent years, more complicated sets of monomers have been designed to develop nonlinear HCR, such as branched HCR and even dendritic systems, achieving quadratic and exponential growth mechanisms. In addition, HCR has attracted enormous attention in the fields of bioimaging and biomedicine, including applications in fluorescence in situ hybridization (FISH) imaging, live cell imaging, and targeted drug delivery. In this review, we introduce the fundamentals of HCR and examine the visualization and analysis techniques for HCR products in detail. The most recent HCR developments in biosensing, bioimaging, and biomedicine are subsequently discussed with selected examples. Finally, the review provides insight into the challenges and future perspectives of HCR.

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Branched catalytic hairpin assembly is proposed for dynamic self-assembly of DNA dendrimers for signal amplification and in situ imaging of microRNAs in live cells with high sensitivity and selectivity.

A hyper-branched hybridization chain reaction (HB-HCR) is presented herein, which consists of only six species that can metastably coexist until the introduction of an initiator DNA to trigger a cascade of hybridization events, leading to the self-sustained assembly of hyper-branched and nicked double-stranded DNA structures. The system can readily achieve ultrasensitive detection of target DNA. Moreover, the HB-HCR principle is successfully applied to construct three-input concatenated logic circuits with excellent specificity and extended to design a security-mimicking keypad lock system. Significantly, the HB-HCR-based keypad lock can alarm immediately if the "password" is incorrect. Overall, the proposed HB-HCR with high amplification efficiency is simple, homogeneous, fast, robust, and low-cost, and holds great promise in the development of biosensing, in the programmable assembly of DNA architectures, and in molecular logic operations.