Polyrotaxanes (PRXs) are a class of supramolecular threaded macromolecules, in which cyclic molecules are threaded onto the main- or side-chain of polymers. To date, various studies have been conducted on the synthesis of PRXs, and various combinations of cyclic molecules and polymers that can form a PRX have been discovered. Among these combinations, PRXs composed of cyclodextrins (CDs) and a linear polymer have attracted much attention and have been investigated by many researchers. Because of the non-covalently associated characteristic of PRXs, these supermolecules exhibit unique properties, such as the dynamic motion of the threaded cyclic molecules along a polymer axle and complete dissociation of the supramolecular structure, that are never observed in other synthetic polymers. These inherent properties of PRXs are of interest in the design of novel biomaterials, such as hydrogels, scaffolds in tissue engineering, drug delivery carriers, and polymer–drug conjugates. Thus, various studies have been conducted to utilize PRXs as a framework for biomaterials. In this review, we describe the recent progress in biomaterial application of PRXs such as drug delivery and gene delivery.

RNA interference is a post-transcriptional gene-silencing pathway induced by double-stranded small interfering RNA (siRNA). The potential use of siRNA as a therapeutic agent has attracted great attention as a novel approach to the treatment of several intractable diseases. Despite the rapid progress in the therapeutic use of siRNA, systemic application is still controversial due to the limitations of siRNA, such as low enzymatic tolerability, cellular internalization and body distribution after systemic administration. This review describes the recent progress and strategies of siRNA delivery systems based on polyion complexes. Numerous siRNA-containing polyion complex systems bound together through electrostatic interactions between the negatively charged siRNA and positively charged components, including synthetic polymers, biopolymers and nanoparticles, have been developed for the therapeutic application of siRNA. Additionally, stimulus-sensitive smart siRNA carrier systems, including bioreducible polycations and hydrophilic polymer-siRNA conjugates, have been developed to enhance the gene-silencing efficacy of siRNAs.