Synthesis of 'Impossible' Rotaxanes

'Impossible' rotaxanes, which are constituted by interlocked components without obvious binding motifs, have attracted the interest of the mechanically interlocked molecules (MIMs) community. Within the synthetic efforts reported in the last decades towards the preparation of MIMs, some innovative protocols for accessing 'impossible' rotaxanes have been developed. This short review highlights different selected synthetic examples of 'impossible' rotaxanes, as well as suggests some future directions of this research area.

Supramolecular nanoarchitectonics of propionylated polyrotaxanes with bulky nitrobenzyl stoppers for light-triggered drug release

Cyclodextrin (CD)-based polyrotaxanes (PRXs) are supramolecular polymers comprising multiple CDs mechanically interlocked onto a linear polymer chain by capping the polymer ends with bulky stoppers. Among various PRX derivatives, propionylated PRXs (Pr-PRXs) composed of propionylated α-CD and high molecular-weight poly(ethylene glycol) (PEG) form self-assembled nanoparticles in aqueous solution through hydrophobic interactions. Although Pr-PRX nanoparticles can encapsulate hydrophobic drugs in their hydrophobic domains, their release rate is limited. To improve the efficiency of drug release from Pr-PRX nanoparticles, ultraviolet (UV) light-dissociable Pr-PRXs were designed using 4,5-dimethoxy 2-nitrobenzyl groups as UV-cleavable bulky stopper molecules to facilitate UV-induced drug release. Photodegradable Pr-PRX (Pr-PD-PRX) was synthesized, and its UV-induced dissociation was examined. Pr-PD-PRX was completely dissociated via UV irradiation (365 nm) for 30 min. Additionally, Pr-PD-PRX nanoparticles encapsulating hydrophobic drugs collapsed upon UV irradiation, which promoted the release of the encapsulated drugs compared to non-degradable Pr-PRX nanoparticles. UV irradiation of drug-loaded Pr-PD-PRX nanoparticles resulted in higher cytotoxicity than non-irradiated Pr-PD-PRX and non-degradable Pr-PRX. Consequently, designing photodegradable PRX-based nanoparticles provides new insights into developing photoresponsive drug carriers and smart biomedical materials.

Light-responsive rotaxane-based materials: inducing motion in the solid state

Light-responsive rotaxane-based solid-state materials are ideal scaffolds in order to develop smart materials due to the properties provided by the mechanical bond, such as control over the dynamics of the components upon application of external stimuli. This perspective aims to highlight the relevance of these materials, by pointing out recent examples of photoresponsive materials prepared from a rotaxanated architecture in which motion of the counterparts and/or macroscopic motion of the interlocked materials are achieved. Although further development is needed, these materials are envisioned as privileged scaffolds which will be used for different advanced applications in the area of molecular machinery.

Trends in mechanobiology guided tissue engineering and tools to study cell-substrate interactions: a brief review

Sensing the mechanical properties of the substrates or the matrix by the cells and the tissues, the subsequent downstream responses at the cellular, nuclear and epigenetic levels and the outcomes are beginning to get unraveled more recently. There have been various instances where researchers have established the underlying connection between the cellular mechanosignalling pathways and cellular physiology, cellular differentiation, and also tissue pathology. It has been now accepted that mechanosignalling, alone or in combination with classical pathways, could play a significant role in fate determination, development, and organization of cells and tissues. Furthermore, as mechanobiology is gaining traction, so do the various techniques to ponder and gain insights into the still unraveled pathways. This review would briefly discuss some of the interesting works wherein it has been shown that specific alteration of the mechanical properties of the substrates would lead to fate determination of stem cells into various differentiated cells such as osteoblasts, adipocytes, tenocytes, cardiomyocytes, and neurons, and how these properties are being utilized for the development of organoids. This review would also cover various techniques that have been developed and employed to explore the effects of mechanosignalling, including imaging of mechanosensing proteins, atomic force microscopy (AFM), quartz crystal microbalance with dissipation measurements (QCMD), traction force microscopy (TFM), microdevice arrays, Spatio-temporal image analysis, optical tweezer force measurements, mechanoscanning ion conductance microscopy (mSICM), acoustofluidic interferometric device (AID) and so forth. This review would provide insights to the researchers who work on exploiting various mechanical properties of substrates to control the cellular and tissue functions for tissue engineering and regenerative applications, and also will shed light on the advancements of various techniques that could be utilized to unravel the unknown in the field of cellular mechanobiology.

Chemorheological Monitoring of Cross-Linking in Slide-ring Gels Derived From α-cyclodextrin Polyrotaxanes

Despite hundreds of studies involving slide-ring gels derived from cyclodextrin (CD)-based polyrotaxanes (PRs), their covalent cross-linking kinetics are not well characterized. We employ chemorheology as a tool to measure the gelation kinetics of a model slide-ring organogel derived from α-cyclodextrin/poly (ethylene glycol) PRs cross-linked with hexamethylenediisocyanate (HMDI) in DMSO. The viscoelastic properties of the gels were monitored in situ by small-amplitude oscillatory shear (SAOS) rheology, enabling us to estimate the activation barrier and rate law for cross-linking while mapping experimental parameters to kinetics and mechanical properties. Gelation time, gel point, and final gel elasticity depend on cross-linker concentration, but polyrotaxane concentration only affects gelation time and elasticity (not gel point), while temperature only affects gelation time and gel point (not final elasticity). These measurements facilitate the rational design of slide-ring networks by simple parameter selection (temperature, cross-linker concentration, PR concentration, reaction time).

One stone, many birds: Recent advances in functional nanogels for cancer nanotheranostics

Inspired by the development of nanomedicine and nanotechnology, more and more possibilities in cancer theranostic have been provided in the last few years. Emerging therapeutic modalities like starvation therapy, chemodynamic therapy, and tumor oxygenation have been integrated with diagnosis, giving a plethora of theranostic nanoagents. Among all of them, nanogels (NGs) show superiority benefiting from their unique attributes: high stability, high water-absorption, large specific surface area, mechanical strength, controlled responsiveness, and high encapsulation capacity. There have been a vast number of investigations supporting various NGs combining drug delivery and multiple bioimaging techniques, encompassing photothermal imaging, photoacoustic imaging, fluorescent imaging, ultrasound imaging, magnetic resonance imaging, and computed tomography. This review summarizes recent advances in functional NGs for theranostic nanomedicine and discusses the challenges and future perspectives of this fast-growing field. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Diagnostic Tools > In Vivo Nanodiagnostics and Imaging.

Post-Cross-Linking of Collagen Hydrogels by Carboxymethylated Polyrotaxanes for Simultaneously Improving Mechanical Strength and Cell Proliferation

To improve the mechanical properties of collagen hydrogels, which are widely utilized as biomaterials, post-cross-linking of collagen hydrogels was performed using polyrotaxane (PRX) as a cross-linker. Herein, carboxymethyl group-modified PRXs (CMPRs) composed of carboxymethylated α-cyclodextrins (α-CDs) threaded along poly(ethylene glycol) (PEG) capped with bulky stoppers were used to cross-link via reaction with the amino groups in the collagen. Four series of CMPRs with different α-CD threading ratios and axle PEG molecular weights were used for the post-cross-linking of the collagen hydrogels to verify the optimal CMPR chemical compositions. The post-cross-linking of the collagen hydrogels with CMPRs improved the swelling ratios and mechanical properties, such as viscoelasticity and tensile strength. Among the tested CMPRs, CMPRs with an axle PEG molecular weight of 35,000 (PEG35k) resulted in better mechanical properties than CMPRs with a PEG10k axis. Additionally, the cell adhesion and proliferation were greatly improved on the surface of the collagen hydrogels post-cross-linked with CMPRs with the PEG35k axle. These findings suggest that the molecular weight of an axle polymer in CMPRs is a more important parameter than the α-CD threading ratios. Accordingly, the post-cross-linking of hydrogels with PRXs is promising for improving the mechanical properties and biomaterial functions of collagen hydrogels.