Thermoreversible sol–gel transition of an aqueous solution of polyrotaxane composed of highly methylated α-cyclodextrin and polyethylene glycol

Injectable Dual Fenton/Enzymatically Cross-Linked Double-Network Hydrogels Based on Acrylic/Phenolic Polymers with Highly Reinforced and Tunable Mechanical Properties

Injectable hydrogels have been extensively used as promising therapeutic scaffolds for a wide range of biomedical applications, such as tissue regeneration and drug delivery. However, their low fracture toughness and brittleness often limit their scope of application. Double-network (DN) hydrogel, which is composed of independently cross-linked rigid and ductile polymer networks, has been proposed as an alternative technique to compensate for the weak mechanical properties of hydrogels. Nevertheless, some challenges still remain, such as the complicated and time-consuming process for DN formation, and the difficulty in controlling the mechanical properties of DN hydrogels. In this study, we introduce a simple, rapid, and controllable method to prepare in situ cross-linkable injectable DN hydrogels composed of acrylamide (AAm) and 4-arm-PPO-PEO-tyramine (TTA) via dual Fenton- and enzyme-mediated reactions. By varying the concentration of Fenton's reagent, the DN hydrogels were rapidly formed with controllable gelation rate. Importantly, the DN hydrogels showed a 13-fold increase in compressive strength and a 14-fold increase in tensile strength, compared to the single network hydrogels. The mechanical properties, elasticity, and plasticity of DN hydrogels could also be modulated by simply varying the preparation conditions, including the cross-linking density and reagent concentrations. At low cross-linker concentration (<0.05 wt %), the plastic DN hydrogel stretched to over 6,500%, whereas high cross-linker concentration (≥0.05 wt %) induced fully elastic hydrogels, without hysteresis. Besides, DN hydrogels were endowed with rapid self-recovery and highly enhanced adhesion, which can be further applied to wearable devices. Moreover, human dermal fibroblasts treated with DN hydrogels retained viability, demonstrating the biocompatibility of the cross-linking system. Therefore, we expect that the dual Fenton-/enzyme-mediated cross-linkable DN hydrogels offer great potential as advanced biomaterials applied for hard tissue regeneration and replacement.

Advanced supramolecular design for direct ink writing of soft materials

The exciting advancements in 3D-printing of soft materials are changing the landscape of materials development and fabrication. Among various 3D-printers that are designed for soft materials fabrication, the direct ink writing (DIW) system is particularly attractive for chemists and materials scientists due to the mild fabrication conditions, compatibility with a wide range of organic and inorganic materials, and the ease of multi-materials 3D-printing. Inks for DIW need to possess suitable viscoelastic properties to allow for smooth extrusion and be self-supportive after printing, but molecularly facilitating 3D printability to functional materials remains nontrivial. While supramolecular binding motifs have been increasingly used for 3D-printing, these inks are largely optimized empirically for DIW. Hence, this review aims to establish a clear connection between the molecular understanding of the supramolecularly bound motifs and their viscoelastic properties at bulk. Herein, extrudable (but not self-supportive) and 3D-printable (self-supportive) polymeric materials that utilize noncovalent interactions, including hydrogen bonding, host-guest inclusion, metal-ligand coordination, micro-crystallization, and van der Waals interaction, have been discussed in detail. In particular, the rheological distinctions between extrudable and 3D-printable inks have been discussed from a supramolecular design perspective. Examples shown in this review also highlight the exciting macroscale functions amplified from the molecular design. Challenges associated with the hierarchical control and characterization of supramolecularly designed DIW inks are also outlined. The perspective of utilizing supramolecular binding motifs in soft materials DIW printing has been discussed. This review serves to connect researchers across disciplines to develop innovative solutions that connect top-down 3D-printing and bottom-up supramolecular design to accelerate the development of 3D-print soft materials for a sustainable future.

Supramolecular Surface Coatings with Acetylated Polyrotaxane-Based Triblock Copolymers for Thermal Regulation of Cell Adhesion and Fabrication of Cell Sheets

Polyrotaxanes (PRXs) containing acetylated α-cyclodextrins exhibit a temperature-dependent phase transition in aqueous solutions across their lower critical solution temperature (LCST) of approximately 26.6 °C. To gain insights into the interactions of acetylated PRXs (Ac-PRXs) with biological components, thermoresponsive supramolecular surfaces were prepared by coating tissue culture polystyrene (TCPS) surfaces with Ac-PRX triblock copolymers, and their surface properties across the LCST were evaluated. The wettability and protein adsorption of Ac-PRX-coated surfaces changed significantly between 10 and 37 °C, whereas the uncoated TCPS and unmodified PRX-coated surfaces did not alter the wettability and protein adsorption at 10 and 37 °C. The adhesion, proliferation, morphology, and adhesion strength of NIH/3T3 cells on Ac-PRX-coated surfaces were found to be similar to those of the uncoated and unmodified PRX-coated surfaces. However, the adhesion strength of NIH/3T3 cells on Ac-PRX-coated surfaces decreased drastically at 10 °C. Consequently, the cells spontaneously detached from the Ac-PRX-coated surfaces without enzymatic treatment. Additionally, when incubating confluent cells at 10 °C, the cells detached from Ac-PRX-coated surfaces as cell sheets while retaining extracellular matrix proteins. The findings of this study provide new directions for the design of thermoresponsive supramolecular biointerfaces for applications in bioseparation and cell manipulation.

Constitutive Theory for Direct Coupling of Molecular Frictions and the Viscoelasticity of Soft Materials

How to directly relate frictions at the level of a single molecular chain to the viscoelasticity of soft materials is intriguing. Here, we choose to investigate classical elastomers, where molecular frictions are known to be generated when dangling chains move relatively to the surrounding polymer chain network. With explicit forms employed for the relationship between friction and velocity at the molecular scale, a constitutive theory is then developed for the coupling of molecular frictions and the macroscopic viscoelasticity of elastomers. With the utilization of this theory, viscoelastic behaviors of varied elastomeric materials are predicted, which agree well with existing experiments at both low and high strain rates under different loading conditions. The theory also reproduces the time-temperature equivalent principle of elastomers. We suggest that this work might have provided a modeling framework that directly couples frictions at the level of a single molecular chain to the viscoelasticity of soft materials.

Tissue Adhesion-Anisotropic Polyrotaxane Hydrogels Bilayered with Collagen

Hydrogels are promising materials in tissue engineering scaffolds for healing and regenerating damaged biological tissues. Previously, we developed supramolecular hydrogels using polyrotaxane (PRX), consisting of multiple cyclic molecules threaded by an axis polymer for modulating cellular responses. However, since hydrogels generally have a large amount of water, their adhesion to tissues is extremely weak. Herein, we designed a bilayered hydrogel with a PRX layer and a collagen layer (PRX/collagen hydrogel) to achieve rapid and strong adhesion to the target tissue. The PRX/collagen hydrogel was fabricated by polymerizing PRX crosslinkers in water with placement of a collagen sponge. The differences in components between the PRX and collagen layers were analyzed using Fourier transform infrared spectroscopy (FT-IR). After confirming that the fibroblasts adhered to both layers of the PRX/collagen hydrogels, the hydrogels were implanted subcutaneously in mice. The PRX hydrogel without collagen moved out of its placement site 24 h after implantation, whereas the bilayer hydrogel was perfectly adherent at the site. Together, these findings indicate that the bilayer structure generated using PRX and collagen may be a rational design for performing anisotropic adhesion.

Post-synthesis modification of slide-ring gels for thermal and mechanical reconfiguration

Ring-sliding behavior in polyrotaxanes imbues gels, elastomers, and glasses with remarkable stress-dissipation and actuation properties. Since these properties can be modulated and tuned by structural parameters, many efforts have been devoted to developing synthetic protocols that define the structures and properties of slide-ring materials. We introduce post-synthetic modifications of slide-ring gels derived from unmodified α-cyclodextrin and poly(ethylene glycol) polyrotaxanes that enable (i) actuation and control of the thermo-responsive lower critical solution temperature (LCST) behavior of ring-modified slide-ring hydrogels, and (ii) chemically bonding separate gels into hybrid or shape-reconfigured macro-structures with a slide-ring adhesive solution. The mechanical properties of the post-modified gels have been characterized by shear rheology and uniaxial tensile tests, while the corresponding xerogels were characterized by wide-angle X-ray scattering. These demonstrations show that post-synthetic modification offers a practical solution for re-configuring the properties and shapes of slide-ring gels.

Dual effect of molecular mobility and functional groups of polyrotaxane surfaces on the fate of mesenchymal stem cells

Polyrotaxanes are supramolecular assemblies consisting of cyclic molecules (e.g., α-cyclodextrins) and linear polymer chains (e.g., poly[ethylene glycol]), in which cyclic molecules can move along the polymer chain. Here, we examined the effect of functional groups introduced into the α-cyclodextrins of polyrotaxane on cell responses such as adhesion, proliferation, and differentiation. Polyrotaxane-based triblock copolymers modified with methyl (CH3, hydrophobic, and nonionic), hydroxy (OH, hydrophilic and nonionic), amino (NH2, cationic), and sulfo (SO3H, anionic) groups were coated on the surface of the culture plate to fabricate polyrotaxane surfaces with different surface chemistries. The chemical compositions of each surface were determined via time-of-flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy. The contact angle hysteresis reflecting the molecular mobility and zeta potential of each polyrotaxane surface changed depending on the functional groups. When osteoblast and adipocyte differentiation was induced in human mesenchymal stem cells cultured on each polyrotaxane surface, the cells adhered to the SO3H-modified polyrotaxane surfaces exhibited osteoblast differentiation, whereas the cells adhered to the OH-, NH2-, and SO3H-modified polyrotaxane surfaces preferentially underwent adipocyte differentiation compared with those on the unmodified and CH3-modified polyrotaxane surfaces. Interestingly, the SO3H-modified polyrotaxane surfaces promoted both osteoblast and adipocyte differentiation. High molecular mobility and negative charge on the SO3H-modified polyrotaxane surfaces are expected to contribute to the facilitation of both osteoblast and adipocyte differentiation.

Intranasal co-delivery of berberine and evodiamine by self-assembled thermosensitive in-situ hydrogels for improving depressive disorder

The orally administrated antidepressants not only caused side effects such as dizziness, diarrhea, and drug resistance, but also worked slowly. Therefore, new antidepressants and preparations derived from natural medicines play an important role in the study of antidepressant drugs. It was reported that the two components of Zuojin pill, berberine (BBR) and evodiamine (EVO), were used in combination to improve depressive disorder. In our study, a self-assembled thermosensitive in-situ hydrogel was prepared to achieve sustained co-delivery of BBR and EVO. The preparation process of hydrogel consists of two steps, namely, the inclusion of the drugs and thermosensitive self-assembly of the hydrogel. In vitro experimental results indicated that the prepared hydrogel showed a good thermosensitive property under physiological temperature. The hydrogel had a slow and controlled release behavior for BBR and EVO, according with first-order equation. In vivo experimental results indicated that compared to intragastric administration of drug solution, the intranasal administration of hydrogel increased bioavailability of BBR and EVO, approximately 135 and 112 folds, respectively. The hydrogel at a low dose significantly reversed behavioral despair of the mice, improved depressive symptom of rats, and treated depressive disorder by regulating the abnormal levels of monoamine neurotransmitters (including 5-hydroxytryptamine, noradrenalin and dopamine) metabolism and related metabolic pathways such as purine, citrate cycle, scorbate and aldarate, butanoate, vitamin B6, and pyrimidine metabolism. Therefore, as a drug co-delivery system, the intranasally administrated hydrogels with a good release and high bioavailability provides a non-invasive therapeutic strategy for the clinical treatment of depression, which attains antidepressant effects by regulation of the monoamine neurotransmitters metabolism and related metabolic pathways.

Cyclodextrin-Based Supramolecular Complexes of Osteoinductive Agents for Dental Tissue Regeneration

Oral tissue regeneration has received growing attention for improving the quality of life of patients. Regeneration of oral tissues such as alveolar bone and widely defected bone has been extensively investigated, including regenerative treatment of oral tissues using therapeutic cells and growth factors. Additionally, small-molecule drugs that promote bone formation have been identified and tested as new regenerative treatment. However, treatments need to progress to realize successful regeneration of oral functions. In this review, we describe recent progress in development of regenerative treatment of oral tissues. In particular, we focus on cyclodextrin (CD)-based pharmaceutics and polyelectrolyte complexation of growth factors to enhance their solubility, stability, and bioactivity. CDs can encapsulate hydrophobic small-molecule drugs into their cavities, resulting in inclusion complexes. The inclusion complexation of osteoinductive small-molecule drugs improves solubility of the drugs in aqueous solutions and increases in vitro osteogenic differentiation efficiency. Additionally, various anionic polymers such as heparin and its mimetic polymers have been developed to improve stability and bioactivity of growth factors. These polymers protect growth factors from deactivation and degradation by complex formation through electrostatic interaction, leading to potentiation of bone formation ability. These approaches using an inclusion complex and polyelectrolyte complexes have great potential in the regeneration of oral tissues.

Thermo-responsive 3D-printed polyrotaxane monolith

Thermo-responsive 3D-printed hydrogels that are composed of methylated α-cyclodextrin polyrotaxanes have been synthesized through post-3D-printing methylation.

Emerging Nanoassembly of Polyrotaxanes Comprising Acetylated α-Cyclodextrins and High-Molecular-Weight Axle Polymer

Acetylated α-cyclodextrin (α-CD)/poly(ethylene glycol) (PEG)-based polyrotaxanes (Ac-PRXs) with varying degrees of acetylation (DA) and molecular weight of axle PEG were synthesized and their solubility in aqueous solutions was investigated. Ac-PRXs with low DA (less than 35%) were dissolved in aqueous solutions without considering the molecular weight of axle PEG, whereas Ac-PRXs with high DA (more than 40%) and low molecular weight of axle PEG (less than 35000) were precipitated into the solutions. Interestingly, Ac-PRXs with high DA and high molecular weight of axle PEG (100000) exhibited a colloidal dispersion in aqueous solutions. It is considered that the threaded acetylated α-CDs formed hydrophobic microenvironments via hydrophobic interactions and the noncovered segments of axle PEGs provided colloidal stability. Furthermore, the potential application of Ac-PRX_100k as a drug carrier was examined and it was established that Ac-PRX_100k can encapsulate a hydrophobic drug. Accordingly, acetylation of PRXs is a viable approach to promote solubility in aqueous solutions and prepare self-assembled nanoparticles.

Polyrotaxane-based biointerfaces with dynamic biomaterial functions

The molecular mobility of cyclic molecules (e.g.α-cyclodextrins) threaded along a linear polymer chain (e.g. poly(ethylene glycol)) in polyrotaxanes is a unique feature for biomaterials with dynamic functionality. Surfaces with molecular mobility can be obtained by introducing polyrotaxanes. The molecular mobility of polyrotaxane-based surfaces can be modulated by changing the number of threaded cyclic molecules and modifying their functional groups. Biological ligands modified with α-cyclodextrins exhibit increased multivalent interactions with their receptors due to the molecular mobility of the latter. Furthermore, polyrotaxane-based surfaces not only improve the initial response of cells via multivalent interactions, but also affect cytoskeleton formation and the inherent quality of cells, including differentiation. Such polyrotaxane surfaces can emerge as new biointerfaces that can adapt to the dynamic biological nature.

Supramolecular Self-Assembly of Methylated Rotaxanes for Solid Polymer Electrolyte Application

Li⁺-conducting solid polymer electrolytes (SPEs) obtained from supramolecular self-assembly of trimethylated cyclodextrin (TMCD), poly(ethylene oxide) (PEO), and lithium salt are investigated for application in lithium-metal batteries (LMBs) and lithium-ion batteries (LIBs). The considered electrolytes comprise nanochannels for fast lithium-ion transport formed by CD threaded on PEO chains. It is demonstrated that tailored modification of CD beneficially influences the structure and transport properties of solid polymer electrolytes, thereby enabling their application in LMBs. Molecular dynamics (MD) simulation and experimental data reveal that modification of CDs shifts the steady state between lithium ions inside and outside the channels, in this way improving the achievable ionic conductivity. Notably, the designed SPEs facilitated galvanostatic cycling in LMBs at fast charging and discharging rates for more than 200 cycles and high Coulombic efficiency.

Hydroxypropyl-β-CD vs. its α-homologue for a 3D modified polyrotaxane network formation and properties: the relationship between modified CD and polymer revealed through comparison

The threading mechanism of the hydroxypropyl-cyclodextrin (Hy-CD)/tetrahedron-like poly(ethylene glycol) (tetra-PEG) based host-guest complex and the relationship between Hy-CD and poly(ethylene oxide) (PEO) in the three-dimensional modified polyrotaxane (PR) formed by the complex were revealed through the comparison between Hy-β-CD/tetra-PEG and Hy-α-CD/tetra-PEG based systems from the macroscopic material view to the microscopic molecular view. The complexation between Hy-CD and tetra-PEG in water experiences a threading-dethreading-rethreading process which is controlled by the intermolecular interaction intensity or molecular hindrance depending on the feed ratio of Hy-CD to tetra-PEG. In the 3D modified PR, the methyl group of the Hy part on one Hy-CD can insert into the cavity of the adjacent Hy-CD and interacts with both the interior surface of the cavity and the PEO segment within the cavity if the cavity of Hy-CD is large enough. The threaded Hy-CD in the PR straightens the chain of PEO and suppresses the segment motion of the PEO. With the decrease of the cavity size of Hy-CD, the degree of suppression on the segment motion of PEO increases. Hy-CD threaded on the PEO chain can also deform when the 3D modified PR is compressed, and the degree of deformation increases with the increase of the cavity size of Hy-CD. These results of the modified CD/PEG based complex system set it apart from the unmodified CD/PEG based one, and reveal the structure-property relationship of this new type of Hy-CD/tetra-PEG based 3D modified PR material.

Acid-Sensitive Polypseudorotaxanes Based on Ortho Ester-Modified Cyclodextrin and Pluronic F-127

We demonstrate a new type of acid-sensitive amphiphilic polypseudorotaxanes (PPRs) formed via inclusion complexation between Pluronic F127 and the hydrophobic β-cyclodextrin (CD) derivative in alcoholic solvents. The 6-OH ortho ester-substituted hydrophobic β-CD derivative (EMD-CD) was prepared by "click" reaction of β-CD with 2-ethylidene-4-methyl-1,3-dioxalane under mild conditions. The water-insoluble EMD-CD (host) is capable of forming PPRs with F127 (guest) in ethanol or methanol but not in water, which is confirmed by ¹H NMR, wide-angle X-ray diffraction, small-angle X-ray scattering, and the time-dependent threading kinetics. Depending on the host/guest ratio, the PPRs self-assembled into sheet-like structure or vesicular nanoparticles with different sizes in water. These PPR assemblies were stable at pH 8.4 but quickly dissociated into biocompatible products in neutral or in acidic buffers due to the hydrolysis of the ortho ester groups. Good biocompatibility, ease of fabrication, and extremely pH-sensitive character make the PPRs promising carriers for anticancer drug delivery. Moreover, the present work provides an alternative method for the preparation of PPRs composed of water-insoluble CD derivatives.

Supramolecular polymers based on cyclodextrins for drug and gene delivery

Supramolecular polymers based on cyclodextrins (CDs) have inspired interesting and rapid developments as novel biomaterials in a broad range of drug and gene delivery applications, due to their low cytotoxicity, controllable size, and unique architecture. This review will summarize the potential applications of polyrotaxanes in the field of drug delivery and gene delivery. Generally, cyclodextrin-based biodegradable polypseudorotaxane hydrogels could be used as a promising injectable drug delivery system for sustained and controlled drug release. Temperature-responsive, pH-sensitive, and controllable hydrolyzable polyrotaxane hydrogels have attracted much attention because of their controllable properties, and the self-assembly micelles formed by amphiphilic copolymer threaded with CDs could be used as a carrier for controlled and sustained drug release. Polyrotaxanes with drug or ligand conjugated CDs threaded on a polymer chain with a biodegradable end group could be useful for controlled and multivalent targeted delivery. In the field of gene delivery, cationic polyrotaxanes consisting of multiple OEI-grafted CDs threaded on a block copolymer chain are attractive non-viral gene carries due to the strong DNA-binding ability, low cytotoxicity, and high gene delivery capability. Furthermore, cytocleavable end-caps were introduced in the polyrotaxane systems in order to ensure efficient endosomal escape for intracellular trafficking of DNA. The development of the supramolecular approach using CD-containing polyrotaxanes is expected to provide a new paradigm for biomaterials.

Polyrotaxanes for applications in life science and biotechnology

Due to their low cytotoxicity, controllable size, and unique architecture, cyclodextrin (CD)-based polyrotaxanes and polypseudorotaxanes have inspired interesting exploitation as novel biomaterials. This review will update the recent progress in the studies on the structures of polyrotaxanes and polypseudorotaxanes based on different CDs and polymers, followed by summarizing their potential applications in life science and biotechnology, such as drug delivery, gene delivery, and tissue engineering. CD-based biodegradable polypseudorotaxane hydrogels could be used as promising injectable drug delivery systems for sustained and controlled drug release. Polyrotaxanes with drug or ligand-conjugated CDs threaded on polymer chain with biodegradable end group could be useful for controlled and multivalent targeting delivery. Cationic polyrotaxanes consisting of multiple oligoethylenimine-grafted CDs threaded on a block copolymer chain were attractive non-viral gene carries due to the strong DNA-binding ability, low cytotoxicity, and high gene transfection efficiency. Cytocleavable end caps were also introduced in the polyrotaxane systems in order to ensure efficient endosomal escape for intracellular trafficking of DNA. Finally, hydrolyzable polyrotaxane hydrogels with cross-linked α-CDs could be a desirable scaffold for cartilage and bone tissue engineering.

Thermogelling aqueous fluids containing low concentrations of Pluronic F127 and laponite nanoparticles

The triblock copolymer Pluronic F127 (PF127) is frequently used in colloidal and pharmaceutical formulations. Concentrated aqueous solutions of PF127 (>15 wt %) are known to undergo thermogelling (i.e., a sol-to-gel transition upon heating), which is attributed to the formation of a volume-filling cubic array of micelles. Here, we report that thermogelling can occur at much lower PF127 concentrations (1.2 to 8 wt %) if nanoparticles of laponite (25-nm-diameter disks) are also present in the formulation. Thermogelling in laponite/PF127 mixtures requires each component to be present above a minimum level. The gels have moduli around 100 Pa, and they can be reversibly liquefied to sols upon cooling. Rheological techniques, small-angle neutron scattering (SANS), and transmission electron microscopy (TEM) are used to characterize the thermogels. We attribute the onset of thermogelling to depletion flocculation of the laponite particles into a network by spherical micelles of PF127.

Thermothickening in solutions of telechelic associating polymers and cyclodextrins

Telechelic associating polymers (hydrophilic ethoxylated backbone, hydrophobic n-alkyl end-groups) form viscous solutions in water due to associations between the hydrophobes. The addition of alpha-, beta-, or gamma-cyclodextrin (CD) substantially reduces the solution viscosity because the CD molecules envelop and sequester the hydrophobes in their hydrophobic cavities. The present paper explores the variation in polymer-CD solution viscosity with temperature. We find that, in the case of alpha-CD alone, the solutions show "thermothickening", i.e., the viscosity increases from 25 to ca. 60 degrees C whereupon it reaches a peak value and then drops. In contrast, solutions with beta- and gamma-CD show monotonic drops in viscosity upon heating. At a fixed polymer content, the thermothickening is higher for higher alpha-CD concentrations. We have also studied how surfactants and lipids impact the thermothickening. Addition of single-tailed micelle-forming surfactants causes the viscosity to revert to the more typical decreasing trend with temperature. However, addition of double-tailed lipids to a polymer/alpha-CD solution accentuates the thermothickening behavior. The thermothickening is explained by the propensity of alpha-CDs to unbind from the hydrophobes and form inclusion complexes with the polymer backbone as the temperature is raised.