Multiresponsive Polymer Hydrogels by Orthogonal Supramolecular Chain Cross-Linking

Fluorescence color change of supramolecular polymer networks controlled by crown ether-cation recognition

We report a fluorescent supramolecular polymer networks (SPNs) system based on crown ether-cation recognition. The polymer side chains bear ammonium cations, which can be recognized by host molecules with a B15C5 unit and a quinoline group at each end. The quinoline group makes the host molecule exhibit blue fluorescence. After the formation of SPNs, the recognition of the crown ether-cation transforms the blue fluorescence into yellow fluorescence. The accompanying fluorescence color change during the formation of SPNs makes it with potential applications in the fields of display, printing, information storage, and bioimaging.

Hydrogels with Reversible Crosslinks for Improved Localised Stem Cell Retention: A Review

Successful stem cell applications could have a significant impact on the medical field, where many lives are at stake. However, the translation of stem cells to the clinic could be improved by overcoming challenges in stem cell transplantation and in vivo retention at the site of tissue damage. This review aims to showcase the most recent insights into developing hydrogels that can deliver, retain, and accommodate stem cells for tissue repair. Hydrogels can be used for tissue engineering, as their flexibility and water content makes them excellent substitutes for the native extracellular matrix. Moreover, the mechanical properties of hydrogels are highly tuneable, and recognition moieties to control cell behaviour and fate can quickly be introduced. This review covers the parameters necessary for the physicochemical design of adaptable hydrogels, the variety of (bio)materials that can be used in such hydrogels, their application in stem cell delivery and some recently developed chemistries for reversible crosslinking. Implementing physical and dynamic covalent chemistry has resulted in adaptable hydrogels that can mimic the dynamic nature of the extracellular matrix.

Orthogonally crosslinked alginate conjugate thermogels with potential for cell encapsulation

Hydrogels with more than one mode of crosslinking have gained interest due to improved control over hydrogel properties such as mechanical strength using multiple stimuli. In this work, sodium alginate was covalently conjugated onto thermoresponsive polyurethanes to prepare hybrid polymers (EPC-Alg) that are responsive to both temperature and Ca²⁺, forming orthogonally crosslinked hydrogels which are non-toxic to cells. Notably, the crosslinks are fully reversible, allowing for gel strength to be modulated via selective removal of either stimulus, or complete deconstruction of the hydrogel network by removing both stimuli. Higher alginate fractions increased the hydrophilicity and Ca²⁺ response of the EPC-Alg hydrogel, enabling tunable modulation of the thermal stability, stiffness and gelation temperatures. The EPC-Alg hydrogel could sustain protein release for a month and encapsulate neural spheroids with high cell viability after 7-day culture, demonstrating feasibility towards 3D cell encapsulation in cell-based biomedical applications such as cell encapsulation and cell therapy.

Probing and Manipulating Noncovalent Interactions in Functional Polymeric Systems

Noncovalent interactions, which usually feature tunable strength, reversibility, and environmental adaptability, have been recognized as driving forces in a variety of biological and chemical processes, contributing to the recognition between molecules, the formation of molecule clusters, and the establishment of complex structures of macromolecules. The marriage of noncovalent interactions and conventional covalent polymers offers the systems novel mechanical, physicochemical, and biological properties, which are highly dependent on the binding mechanisms of the noncovalent interactions that can be illuminated via quantification. This review systematically discusses the nanomechanical characterization of typical noncovalent interactions in polymeric systems, mainly through direct force measurements at microscopic, nanoscopic, and molecular levels, which provide quantitative information (e.g., ranges, strengths, and dynamics) on the binding behaviors. The fundamental understandings of intermolecular and interfacial interactions are then correlated to the macroscopic performances of a series of noncovalently bonded polymers, whose functions (e.g., stimuli-responsiveness, self-healing capacity, universal adhesiveness) can be customized through the manipulation of the noncovalent interactions, providing insights into the rational design of advanced materials with applications in biomedical, energy, environmental, and other engineering fields.

Dually Crosslinked Polymer Networks Incorporating Dynamic Covalent Bonds

Covalent adaptable networks (CANs) are polymeric networks containing covalent crosslinks that are dynamic under specific conditions. In addition to possessing the malleability of thermoplastics and the dimensional stability of thermosets, CANs exhibit a unique combination of physical properties, including adaptability, self-healing, shape-memory, stimuli-responsiveness, and enhanced recyclability. The physical properties and the service conditions (such as temperature, pH, and humidity) of CANs are defined by the nature of their constituent dynamic covalent bonds (DCBs). In response to the increasing demand for more sophisticated and adaptable materials, the scientific community has identified dual dynamic networks (DDNs) as a promising new class of polymeric materials. By combining two (or more) distinct crosslinkers in one system, a material with tailored thermal, rheological, and mechanical properties can be designed. One remarkable ability of DDNs is their capacity to combine dimensional stability, bond dynamicity, and multi-responsiveness. This review aims to give an overview of the advances in the emerging field of DDNs with a special emphasis on their design, structure-property relationships, and applications. This review illustrates how DDNs offer many prospects that single (dynamic) networks cannot provide and highlights the challenges associated with their synthesis and characterization.

A Survey of Strain-Promoted Azide-Alkyne Cycloaddition in Polymer Chemistry

Highly efficient reactions that enable the assembly of molecules into complex structures have driven extensive progress in synthetic chemistry. In particular, reactions that occur under mild conditions and in benign solvents, while producing no by-products and rapidly reach completion are attracting significant attention. Amongst these, the strain-promoted azide-alkyne cycloaddition, involving various cyclooctyne derivatives reacting with azide-bearing molecules, has gained extensive popularity in organic synthesis and bioorthogonal chemistry. This reaction has also recently gained momentum in polymer chemistry, where it has been used to decorate, link, crosslink, and even prepare polymer chains. This survey highlights key achievements in the use of this reaction to produce a variety of polymeric constructs for disparate applications.

Strained alkyne polymers capable of SPAAC via ring-opening metathesis polymerization

We present a strategy that combines the attractive traits of chain-growth polymerization and strain-promoted azide–alkyne cycloaddition chemistry for the production of functional polymers.

Bone Scaffolds Based on Degradable Vaterite/PEG-Composite Microgels

Vaterite, a metastable modification of calcium carbonate, embedded in a flexible microgel packaging with adjustable mechanical properties, functionality, and biocompatibility, provides a powerful scaffolding for bone tissue regeneration, as it is easily convertible to bone-like hydroxyapatite (HA). In this study, the synthesis and physical analysis of a packaging material to encapsulate vaterite particles and osteoblast cells into monodisperse, sub-millimeter-sized microgels, is described whereby a systematic approach is used to tailor the microgel properties. The size and shape of the microgels is controlled via droplet-based microfluidics. Key requirements for the polymer system, such as absence of cytotoxicity as well as biocompatibility and biodegradability, are accomplished with functionalized poly(ethylene glycol) (PEG), which reacts in a cytocompatible thiol-ene Michael addition. On a mesoscopic level, the microgel stiffness and gelation times are adjusted to obtain high cellular viabilities. The co-encapsulation of living cells provides i) an in vitro platform for the study of cellular metabolic processes which can be applied to bone formation and ii) an in vitro foundation for novel tissue-regenerative therapies. Finally, the degradability of the microgels at physiological conditions caused by hydrolysis-sensitive ester groups in the polymer network is examined.

Thermodynamic control over energy dissipation modes in dual-network hydrogels based on metal-ligand coordination

Modern polymeric hydrogels use reversible bonds to mimic biological functionalities. However, true biological materials benefit from several supramolecular elements and deliver multiple functions at the same time. To approach similar creation and control of multiple different functional elements in a synthetic soft material, we develop a model dual-network hydrogel in which multiple energy dissipating modes are formed by metal-ligand coordination and regulated by their association thermodynamics. This idea is realized by using linear and tetra-arm poly(ethylene glycol) (PEG) precursors with complementary reactive end groups. The former also carries terpyridine ligands on both ends, which form metallo-supramolecular bonds upon addition of metal ions. Multiple relaxation modes are provided by a combination of different metal ions. The timescale and amplitude of energy dissipating elements are characterized by oscillatory shear deformation. These studies suggest that the composition of metal ions controls the contribution of the corresponding relaxation modes in a linear fashion. A molecular-level confirmation is provided by following the UV-vis absorbance of the linear precursor in combination with mixtures of metal ions, accompanied by a theoretical study on the kinetics of the reversible association process. These results show that the linearity of the aforementioned dependence holds for such systems in which the utilized combination of metal ions and ligands exclusively form stable bis-complexes. By contrast, in many other cases, especially when the ions may compete to form mono-, bis-, or tris-complexes with the ligand, deviation from linearity is expected.

Design, synthesis, and characterization of novel organic–inorganic hybrid polymeric materials for electroluminescence applications

Design and syntheses of two electroluminescent metallopolymers from a novel, multidentate, redox active, and di-anionic organic building block.

Syntheses, characterization, multi-acid fluorescence sensing and electroluminescence properties of Cr(ii)-based metallopolymers

Cr(ii)-Based multifunctional fluorescent metallopolymers with different degrees of backbone rigidity were synthesized and applied as multi-acid sensors and electroluminescent ON–OFF switches.

Supramolecular Gel Based on Crown-Ether-Appended Dynamic Covalent Macrocycles

A new type of dynamic covalent macrocycle with self-promoted supramolecular gelation behavior is developed. Under oxidative conditions, the dithiol compound containing a diamide alkyl linker with an odd number (7) of carbon chain and an appended crown ether shows a remarkable gelation ability in acetonitrile, without any template molecules. Due to the existence of crown ethers and disulfide bonds, the obtained gel shows a multiple stimuli-responsiveness behavior. The mechanical properties and reversibility of the gel are investigated. Computational modeling suggests that the peripheral chain for diamide hydrogen bonding is responsible for the gelation process.

An ATP/ATPase responsive supramolecular fluorescent hydrogel constructedviaelectrostatic interactions between poly(sodiump-styrenesulfonate) and a tetraphenylethene derivative

We report a supramolecular fluorescent hydrogel based on poly(sodiump-styrenesulfonate) and a tetraphenylethene derivative.

Fluorescent Supramolecular Polymeric Materials

Fluorescent supramolecular polymeric materials are rising stars in the field of fluorescent materials not only because of the inherent optoelectronic properties originating from their chromophores, but also due to the fascinating stimuli-responsiveness and reversibility coming from their noncovalent connections. Especially, these noncovalent connections influence the fluorescence properties of the chromophores because their state of aggregation and energy transfer can be regulated by the assembly-disassembly process. Considering these unique properties, fluorescent supramolecular polymeric materials have facilitated the evolution of new materials useful for applications in fluorescent sensors, probes, as imaging agents in biological systems, light-emitting diodes, and organic electronic devices. In this Review, fluorescent supramolecular polymeric materials are classified depending on the types of main driving forces for supramolecular polymerization, including multiple hydrogen bonding, electrostatic interactions, π-π stacking interactions, metal-coordination, van der Waals interactions and host-guest interactions. Through the summary of the studies about fluorescent supramolecular polymeric materials, the status quo of this research field is assessed. Based on existing challenges, directions for the future development of this field are furnished.

Thermoresponsive functional polymers based on 2,6-diaminopyridine motif with tunable UCST behaviour in water/alcohol mixtures

Two thermoresponsive polyacrylamides based on the 2,6-diaminopyridine motif were synthesized and their UCST-type reversible thermoresponsive behaviour was studied in water/alcohol mixtures.

Control over the assembly and rheology of supramolecular networks via multi-responsive double hydrophilic copolymers

An orthogonal control over network formation and dynamics is achieved in metallo-supramolecular micellar gels via multi-responsive double hydrophilic copolymers.

Orthogonal Control of the Dynamics of Supramolecular Gels from Heterotelechelic Associating Polymers

One of the first examples of supramolecular gels presenting independent dual dynamics is built through a combination of hydrophobic and metal-ligand interactions. The associating building block consists in a water-soluble linear polymer terminated by a short hydrophobic sticker at one end, and a coordinating moiety at the other end. The distinct supramolecular nature of these noninterfering binding motifs allows the dynamics of the hydrogels to be finely tuned in an orthogonal fashion by the application of specific stimuli. Precisely, the solvent-induced plasticization of the hydrophobic associations and the acid-promoted dissociation of the metal-ligand complexes are used to control the network dynamics. By opposition to classically encountered binary gel-sol responses, we demonstrate that the stimuli-induced transition in material properties can be gradual, provided that the material structure is well designed and strong enough.

Kinetically stable metal ligand charge transfer complexes as crosslinks in nanogels/hydrogels: Physical properties and cytotoxicity

A terpyridine end-functionalized 8-arm poly(ethylene glycol) was prepared using the reaction of a 4'-aminopentanoxy substituted terpyridine with a p-nitrophenyl chloroformate activated PEG-(OH)8. Supramolecular complexation of the polymer terpyridine moieties by Fe(2+) ions was investigated using NMR, UV-Vis and dynamic light scattering experiments. At low concentrations addition of Fe(2+) ions to an aqueous solution of the polymer conjugate afforded nanogels with a single size distribution around 250 nm. At concentrations above 3 wt%, and at a 1:2 metal to ligand molar ratio, hydrogels were formed with increasing mechanical properties at increasing polymer concentrations. Using bovine chondrocytes, the biocompatibility and potential cytotoxicity of the polymer conjugate, nanogels and hydrogels were studied. The polymer conjugate with free ligands was toxic to the cells likely due to depletion of essential metal ions. When the terpyridine groups were complexed with Fe(2+) ions, both nanogel suspensions and hydrogels showed no cytotoxicity in direct contact with chondrocytes. Indirect contact of gels with chondrocytes using transwells revealed the absence of toxic components by leaching. A Live-Dead assay on chondrocytes encapsulated in the hydrogels indicated that the hydrogels are cytocompatible, revealing the potential use of these materials for biomedical and pharmaceutical applications.

STATEMENT OF SIGNIFICANCE

The binding between transition metal ions and ligands with multiple binding sites can be almost as strong as covalent bonds. This metal-ligand charge transfer (MLCT) complexation was used to crosslink water soluble polymers into hydrogels. This approach to novel materials may find applications in the biomedical and pharmaceutical fields. Transition metal ions are essential trace elements present in tissue but up to now no cytotoxicity data of free ligands are available. Data presented show that free ligands are toxic to cells likely by depletion of trace metal ions, whereas kinetically stable complexes are not cytotoxic even when embedded in hydrogels. These results provide fundamental issues to be considered in the design of hydrogels crosslinked through metal ligand complexation.

Hybrid Microgels with Thermo-Tunable Elasticity for Controllable Cell Confinement

Stimuli-responsive hydrogels are able to change their physical properties such as their elastic moduli in response to changes in their environment. If biocompatible polymers are used to prepare such materials and if living cells are encapsulated within these networks, their switchability allows the cell-matrix interactions to be investigated with unprecedented consistency. In this paper, thermo-responsive macro- and microscopic hydrogels are presented based on azide-functionalized copolymers of poly(N-(2-hydroxypropyl)-methacrylamide) and poly(hydroxyethyl methacrylate) grafted with poly(N-isopropylacrylamide) side chains. Crosslinking of these comb polymers is realized by bio-orthogonal strain-promoted azide-alkyne cycloaddition with cyclooctyne-functionalized poly(ethylene glycol). The resulting hybrid hydrogels exhibit thermo-tunable elasticity tailored by the polymer chain length and grafting density. This bio-orthogonal polymer crosslinking strategy is combined with droplet-based microfluidics to encapsulate living cells into stimuli-responsive microgels, proving them to be a suitable platform for future systematic stem-cell research.

Revealing the supramolecular nature of side-chain terpyridine-functionalized polymer networks

Nowadays, finely controlling the mechanical properties of polymeric materials is possible by incorporating supramolecular motifs into their architecture. In this context, the synthesis of a side-chain terpyridine-functionalized poly(2-(dimethylamino)ethyl methacrylate) is reported via reversible addition-fragmentation chain transfer polymerization. By addition of transition metal ions, concentrated aqueous solutions of this polymer turn into metallo-supramolecular hydrogels whose dynamic mechanical properties are investigated by rotational rheometry. Hence, the possibility for the material to relax mechanical constrains via dissociation of transient cross-links is brought into light. In addition, the complex phenomena occurring under large oscillatory shear are interpreted in the context of transient networks.

Micro- and nanogels with labile crosslinks – from synthesis to biomedical applications

We emphasize the synthetic strategies to produce micro-/nanogels and the importance of degradable linkers incorporated in the gel network.

A thermally stable pH-responsive “supramolecular buckle” based on the encapsulation of 4-(4-aminophenyl)-N-methylpyridinium by cucurbit[8]uril

A CB[8]-based host-guest system forms a thermally stable 1 : 2 complex under neutral conditions and transforms into a 1 : 1 complex upon acidification, which makes it a “supramolecular buckle” to fabricate pH-responsive materials at elevated temperatures.

A double supramolecular crosslinked polymer gel exhibiting macroscale expansion and contraction behavior and multistimuli responsiveness

Here we report a novel supramolecular polymer gel containing two types of physical crosslinks that are orthogonal to each other. It shows macroscale expansion and contraction behavior and multistimuli responsiveness.