Cargo shuttling by electrochemical switching of core-shell microgels obtained by a facile one-shot polymerization

Thermo-Electro-Responsive Redox-Copolymers for Amplified Solvation, Morphological Control, and Tunable Ion Interactions

Electro-responsive metallopolymers can possess highly specific and tunable ion interactions, and have been explored extensively as electrode materials for ion-selective separations. However, there remains a limited understanding of the role of solvation and polymer-solvent interactions in ion binding and selectivity. The elucidation of ion-solvent-polymer interactions, in combination with the rational design of tailored copolymers, can lead to new pathways for modulating ion selectivity and morphology. Here, we present thermo-electrochemical-responsive copolymer electrodes of N-isopropylacrylamide (NIPAM) and ferrocenylpropyl methacrylamide (FPMAm) with tunable polymer-solvent interactions through copolymer ratio, temperature, and electrochemical potential. As compared to the homopolymer PFPMAm, the P(NIPAM0.9-co-FPMAm0.1) copolymer ingressed 2 orders of magnitude more water molecules per doping ion when electrochemically oxidized, as measured by electrochemical quartz crystal microbalance. P(NIPAM0.9-co-FPMAm0.1) exhibited a unique thermo-electrochemically reversible response and swelled up to 83% after electrochemical oxidation, then deswelled below its original size upon raising the temperature from 20 to 40 °C, as measured through spectroscopic ellipsometry. Reduced P(NIPAM0.9-co-FPMAm0.1) had an inhomogeneous depth profile, with layers of low solvation. In contrast, oxidized P(NIPAM0.9-co-FPMAm0.1) displayed a more uniform and highly solvated depth profile, as measured through neutron reflectometry. P(NIPAM0.9-co-FPMAm0.1) and PFPMAm showed almost a fivefold difference in selectivity for target ions, evidence that polymer hydrophilicity plays a key role in determining ion partitioning between solvent and the polymer interface. Our work points to new macromolecular engineering strategies for tuning ion selectivity in stimuli-responsive materials.

Electrochemically Controlled Release from a Thin Hydrogel Layer

In this study, we present a thermoresponsive thin hydrogel layer based on poly(N-isopropylacrylamide), functionalized with β-cyclodextrin groups (p(NIPA-βCD)), as a novel electrochemically controlled release system. This thin hydrogel layer was synthesized and simultaneously attached to the surface of a Au quartz crystal microbalance (QCM) electrode using electrochemically induced free radical polymerization. The process was induced and monitored using cyclic voltammetry and a quartz crystal microbalance with dissipation monitoring (QCM-D), respectively. The properties of the thin layer were investigated by using QCM-D and scanning electron microscopy (SEM). The incorporation of β-cyclodextrin moieties within the polymer network allowed rhodamine B dye modified with ferrocene (RdFc), serving as a model metallodrug, to accumulate in the p(NIPA-βCD) layer through host-guest inclusion complex formation. The redox properties of the electroactive p(NIPA-βCD/RdFc) layer and the dissociation of the host-guest complex triggered by changes in the oxidation state of the ferrocene groups were investigated. It was found that oxidation of the ferrocene moieties led to the release of RdFc. It was crucial to achieve precise control over the release of RdFc by applying the appropriate electrochemical signal, specifically, by applying the appropriate potential to the electrode. Importantly, the electrochemically controlled RdFc release process was performed at a temperature similar to that of the human body and monitored using a spectrofluorimetric technique. The presented system appears to be particularly suitable for transdermal delivery and delivery from intrabody implants.

Temperature-Modulated Changes in Thin Gel Layer Thickness Triggered by Electrochemical Stimuli

A series of thermoresponsive hydrogels containing positively charged groups in the polymeric network were synthesized and modified with the electroactive compound 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS). ABTS, which forms a dianion in aqueous solutions, acts as an additional physical cross-linker and strongly affects the swelling ratio of the gels. The influence of the amount of positively charged groups and ABTS oxidation state on the volume phase transition temperature was investigated. A hydrogel that possesses a relatively wide and well-defined temperature window (the temperature range where changes in the ABTS oxidation state affects the swelling ratio significantly) was found. The influence of the presence and oxidation state of ABTS on mechanical properties was investigated using a tensile machine and a rheometer. Then, a very thin layer of the gel was deposited on an Au electrochemical quartz crystal microbalance with dissipation (EQCM-D) electrode using the electrochemically induced free radical polymerization method. Next, chronoamperometry combined with quartz crystal microbalance measurements, obtained with an Au EQCM-D electrode modified by the gel, showed that the size of the thin layer could be controlled by an electrochemical trigger. Furthermore, it was found that the electrosensitivity could be modulated by the temperature. Such properties are desired from the point of view construction of electrochemical actuators.

Photo- and thermo-responsive microgels with supramolecular crosslinks for wavelength tunability of the volume phase transition temperature

Functional microgels have powerful applications, especially due to their quick responsiveness to different external stimuli such as temperature, pH, ionic strength, solvent composition and light. Here, we describe the synthesis of novel dual-responsive poly(N-isopropylacrylamide) (PNIPAM) microgels and demonstrate that, in addition to temperature, light changes their properties. The crosslinks inside the microgels were achieved by the host-guest interactions between the trans azobenzene (transAzo) and β-cyclodextrin (βCD) units. transAzo can be photoisomerized to cisAzo which exhibits significant lower binding affinity to βCD. As a consequence, the crosslink density, and thus several microgel properties, can be controlled by light irradiation. Surprisingly, this irradiation with light can significantly change the volume phase transition temperature (VPTT) by several degrees centigrade, presumably due to the fact that the polar βCD shields the transAzo bound to it, whereas the unbound cisAzo is rather apolar. As a result, continuous irradiation with specific wavelengths until reaching the respective photostationary state allows for a full control over the VPTT within the physiologically relevant range between 32 °C and 38 °C.

Microgels and Nanogels for the Delivery of Poorly Water-Soluble Drugs

While microgels and nanogels are most commonly used for the delivery of hydrophilic therapeutics, the water-swollen structure, size, deformability, colloidal stability, functionality, and physicochemical tunability of microgels can also offer benefits for addressing many of the barriers of conventional vehicles for the delivery of hydrophobic therapeutics. In this review, we describe approaches for designing microgels with the potential to load and subsequently deliver hydrophobic drugs by creating compartmentalized microgels (e.g., core-shell structures), introducing hydrophobic domains in microgels, leveraging host-guest interactions, and/or applying "smart" environmentally responsive materials with switchable hydrophobicity. In particular, the challenge of promoting hydrophobic drug loading without compromising the inherent advantages of microgels as delivery vehicles and ensuring practically relevant release kinetics from such structures is highlighted, with an eye toward the practical translation of such vehicles to the clinic.

Nanogels: A novel approach in antimicrobial delivery systems and antimicrobial coatings

The implementation of nanotechnology to develop efficient antimicrobial systems has a significant impact on the prospects of the biomedical field. Nanogels are soft polymeric particles with an internally cross-linked structure, which behave as hydrogels and can be reversibly hydrated/dehydrated (swollen/shrunken) by the dispersing solvent and external stimuli. Their excellent properties, such as biocompatibility, colloidal stability, high water content, desirable mechanical properties, tunable chemical functionalities, and interior gel-like network for the incorporation of biomolecules, make them fascinating in the field of biological/biomedical applications. In this review, various approaches will be discussed and compared to the newly developed nanogel technology in terms of efficiency and applicability for determining their potential role in combating infections in the biomedical area including implant-associated infections.

3D-Printable Hierarchical Nanogel-GelMA Composite Hydrogel System

The strength of the extracellular matrix (ECM) is that it is hierarchical in terms of matrix built-up, matrix density and fiber structure, which allows for hormones, cytokines, and other small biomolecules to be stored within its network. The ECM-like hydrogels that are currently used do not possess this ability, and long-term storage, along with the need for free diffusion of small molecules, are generally incompatible requirements. Nanogels are able to fulfill the additional requirements upon successful integration. Herein, a stable hierarchical nanogel–gelatin methacryloyl (GelMA) composite hydrogel system is provided by covalently embedding nanogels inside the micropore network of GelMA hydrogel to allow a controlled local functionality that is not found in a homogenous GelMA hydrogel. Nanogels have emerged as a powerful tool in nanomedicine and are highly versatile, due to their simplicity of chemical control and biological compatibility. In this study, an N-isopropylacrylamide-based nanogel with primary amine groups on the surface was modified with methacryloyl groups to obtain a photo-cross-linking ability similar to GelMA. The nanogel-GelMA composite hydrogel was formed by mixing the GelMA and the photo-initiator within the nanogel solution through UV irradiation. The morphology of the composite hydrogel was observed by scanning electron microscopy, which clearly showed the nanogel wrapped within the GelMA network and covering the surface of the pore wall. A release experiment was conducted to prove covalent bonding and the stability of the nanogel inside the GelMA hydrogel. In addition, 3D printability studies showed that the nanogel-GelMA composite ink is printable. Therefore, the suggested stable hierarchical nanogel-GelMA composite hydrogel system has great potential to achieve the in situ delivery and controllable release of bioactive molecules in 3D cell culture systems.

Electrochemical Stimulation of Water-Oil Interfaces by Nonionic-Cationic Block Copolymer Systems

Variable interfacial tension could be desirable for many applications. Beyond classical stimuli like temperature, we introduce an electrochemical approach employing polymers. Hence, aqueous solutions of the nonionic-cationic block copolymer poly(ethylene oxide)₁₁₄-b-poly{[2-(methacryloyloxy)ethyl]diisopropylmethylammonium chloride}₁₇₁ (i.e., PEO₁₁₄-b-PDPAEMA₁₇₁ with a quaternized poly(diisopropylaminoethyl methacrylate) block) were investigated by emerging drop measurements and dynamic light scattering, analyzing the PEO₁₁₄-b-qPDPAEMA₁₇₁ impact on the interfacial tension between water and n-decane and its micellar formation in the aqueous bulk phase. Potassium hexacyanoferrates (HCFs) were used as electroactive complexants for the charged block, which convert the bishydrophilic copolymer into amphiphilic species. Interestingly, ferricyanides ([Fe(CN)₆]^3-) act as stronger complexants than ferrocyanides ([Fe(CN)₆]^4-), leading to an insoluble qPDPAEMA block in the presence of ferricyanides. Hence, bulk micellization was demonstrated by light scattering. Due to their addressability, in situ redox experiments were performed to trace the interfacial tension under electrochemical control, directly utilizing a drop shape analyzer. Here, the open-circuit potential (OCP) was changed by electrolysis to vary the ratio between ferricyanides and ferrocyanides in the aqueous solution. While a chemical oxidation/reduction is feasible, also an electrochemical oxidation leads to a significant change in the interfacial tension properties. In contrast, a corresponding electrochemical reduction showed only a slight response after converting ferricyanides to ferrocyanides. Atomic force microscopy (AFM) images of the liquid/liquid interface transferred to a solid substrate showed particles that are in accordance with the diameter from light scattering experiments of the bulk phase. In conclusion, the present results could be an important step toward economic switching of interfaces suitable, e.g., for emulsion breakage.

Highly Efficient Antimicrobial and Antifouling Surface Coatings with Triclosan-Loaded Nanogels

Multifunctional nanogel coatings provide a promising antimicrobial strategy against biomedical implant-associated infections. Nanogels can create a hydrated surface layer to promote antifouling properties effectively. Further modification of nanogels with quaternary ammonium compounds (QACs) potentiates antimicrobial activity owing to their positive charges along with the presence of a membrane-intercalating alkyl chain. This study effectively demonstrates that poly(N-isopropylacrylamide-co-N-[3(dimethylamino)propyl]methacrylamide) (P(NIPAM-co-DMAPMA)-based nanogel coatings possess antifouling behavior against S. aureus ATCC 12600, a Gram-positive bacterium. Through the tertiary amine in the DMAPMA comonomer, nanogels are quaternized with a 1-bromo-dodecane chain via an N-alkylation reaction. The alkylation introduces the antibacterial activity due to the bacterial membrane binding and the intercalating ability of the aliphatic QAC. Subsequently, the quaternized nanogels enable the formation of intraparticle hydrophobic domains because of intraparticle hydrophobic interactions of the aliphatic chains allowing for Triclosan incorporation. The coating with Triclosan-loaded nanogels shows a killing efficacy of up to 99.99% of adhering bacteria on the surface compared to nonquaternized nanogel coatings while still possessing an antifouling activity. This powerful multifunctional coating for combating biomaterial-associated infection is envisioned to greatly impact the design approaches for future clinically applied coatings.

Antimicrobial Nanogels with Nanoinjection Capabilities for Delivery of the Hydrophobic Antibacterial Agent Triclosan

With the ever-growing problem of antibiotic resistance, developing antimicrobial strategies is urgently needed. Herein, a hydrophobic drug delivery nanocarrier is developed for combating planktonic bacteria that enhances the efficiency of the hydrophobic antimicrobial agent, Triclosan, up to a 1000 times. The poly(N-isopropylacrylamide-co-N-[3-(dimethylamino)propyl]methacrylamide), p(NIPAM-co-DMAPMA), based nanogel is prepared via a one-pot precipitation polymerization, followed by quaternization with 1-bromododecane to form hydrophobic domains inside the nanogel network through intraparticle self-assembly of the aliphatic chains (C12). Triclosan, as the model hydrophobic antimicrobial drug, is loaded within the hydrophobic domains inside the nanogel. The nanogel can adhere to the bacterial cell wall via electrostatic interactions and induce membrane destruction via the insertion of the aliphatic chains into the cell membrane. The hydrophobic antimicrobial Triclosan can be actively injected into the cell through the destroyed membrane. This approach dramatically increases the effective concentration of Triclosan at the bacterial site. Both the minimal inhibitory concentration and minimal bactericidal concentration against the Gram-positive bacteria S. aureus and S. epidermidis decreased 3 orders of magnitude, compared to free Triclosan. The synergy of physical destruction and active nanoinjection significantly enhances the antimicrobial efficacy, and the designed nanoinjection delivery system holds great promise for combating antimicrobial resistance as well as the applications of hydrophobic drugs delivery for many other possible applications.

Multiple applications of polymers containing electron-reservoir metal-sandwich complexes

Ferrocene-containing polymers have been investigated for more than six decades, and more recently modern synthetic methods have allowed the fabrication of precise polymers that contain a variety of transition-metal complexes. Trends are now oriented towards applications, such as optics, energy conversion and storage, electrochemistry, magnetics, electric conductors and biomedicine. Metal-sandwich complexes such as those of ferrocene type and other related complexes that present redox-robust groups in polymers, i.e. that are isolable in both their oxidized and reduced forms, are of particular interest, because it is possible to address them using electronic or photonic redox stimuli for application. Our research groups have called such complexes Electron-Reservoirs and introduced them in the main chain or in the side chains of well-defined polymers. For instance, polymers with ferrocene in the main chain or in the side chain are oxidized to stable polycationic polyelectrolytes only if ferrocene is part of a biferrocene unit, because biferrocene oxidation leads to the biferrocenium cation that is stabilized by the mixed valency. Then a group of several redox-robust iron sandwich complexes were fabricated and incorporated in precise polymers including multi-block copolymers whose controlled synthesis and block incorporation was achieved for instance using ring-opening-metathesis polymerization. Applications of this family of Electron-Reservoir-containing polymers includes electrochemically induced derivatization of electrodes by decorating them with these polymers, molecular recognition and redox sensing, electrochromics with multiple colours, generation of gold and silver nanoparticles of various size by reduction of gold(iii) and silver(i) precursors and their use for nanocatalysis towards depollution and biomedicine.

Computational Design of Nanostructured Soft Interfaces: Focus on Shape Changes and Spreading of Cubic Nanogels

Understanding the dynamics of gels at soft interfaces is vital for a range of applications, from biocatalysis and drug delivery to enhanced oil recovery applications. Herein, we use dissipative particle dynamics simulations to focus on the shape changes of a cubic nanogel as it adsorbs from the aqueous phase onto the oil-water interface, effectively acting as a compatibilizer. Upon adsorption at the interface, the hydrogel spreads over the interface, adopting various shapes depending on its size and cross-link density. We characterize these shapes by the shape anisotropy and an effective extent of spreading. We highlight the differences between these characteristics for cubic and spherical nanogels and show that the choice of the cubic shape over the spherical one results in a wider range of topographies that could be dynamically prescribed onto the soft interface due to the gels' adsorption. We first validate our model parameters with respect to the known experimental values for polyacrylamide (PAAm) gels and focus on spreading and shape changes of PAAm nanogels onto the oil-water interfaces. We then probe the behavior of active gels by changing an affinity of the polymer matrix for the solvent, which can be caused by the application of an external stimulus (light, temperature, or change in the chemical composition of solvent). Furthermore, we focus on the interactions between multiple gels placed at the liquid-liquid interface. We show that controlling the shapes and the clustering of the gels at the interfaces via variations in solvent quality result in tailoring the dynamics and topography of soft nanostructured interfaces. Hence, our findings provide insights into the design of soft active nanostructured interfaces with topographies controlled externally via solvent quality.

Redox-active polyamine-salt aggregates as multistimuli-responsive soft nanoparticles

Polyamine-salt aggregates have become promising soft materials in nanotechnology due to their easy preparation process and pH-responsiveness. Here, we report the use of hexacyanoferrate(ii) and hexacyanoferrate(iii) as electroactive crosslinking agents for the formation of nanometer-sized redox-active polyamine-redox-salt aggregates (rPSA) in bulk suspension. This nanoplatform can be selectively assembled or disassembled under different stimuli such as redox environment, pH and ionic strength. By changing the charge of the building blocks, external triggers allow switching the system between two phase states: aggregate-free solution or colloidal rPSA dispersion. The stimuli-activated modulation of the assembly/disassembly processes opens a path to exploit rPSA in technologies based on smart nanomaterials.

Multiresponsive Microgels with Phase-Separated Nanodomains and Self-Regulating Properties via Incorporation of Anthraquinone Moieties

Of the multitude of stimuli-responsive microgels, it is still a challenge to achieve multiple responsivenesses to one single stimulus, which can even revert to the corresponding original state autonomously after stimulus. In this work, we reported a series of anthraquinone functionalized microgels (PNI-xVAQ) with thermosensitivity and redox-actuated self-regulating color, size, and fluorescent properties, which were easily synthesized via surfactant-free emulsion copolymerization (SFEP) with N-isopropylacrylamide (NIPAm) as the monomer, 2-vinylanthraquinone (VAQ) as the comonomer, and N,N'-methylenebis(acrylamide) (BIS) as the cross-linker in an aqueous solution at 70 °C. The hydrophobic interactions of comonomer VAQ also led to the formation of internal phase-separated hydrophobic nanodomains in the obtained PNI-xVAQ microgels. The self-regulating color, size, and fluorescence changes of the PNI-xVAQ microgels were reliant on the nonequilibrium redox process of anthraquinone moieties by the addition of sodium dithionite as the chemical fuel to activate the positive feedback that was the reduction of anthraquinone to transient anthraquinone radical anions, following the slow oxidation of anthraquinone radical anions by autonomous "breathing" oxygen in air as the delayed negative feedback. These autonomous self-regulating properties of the PNI-xVAQ microgel were recyclable to a certain extent by repeated feeding of sodium dithionite.

Microgel PAINT - nanoscopic polarity imaging of adaptive microgels without covalent labelling

Polymer nanostructures have enormous potential for various applications in materials and life sciences. In order to exploit and understand their full capabilities, a detailed analysis of their structures and the environmental conditions in them is essential on the nanoscopic scale. With a super-resolution fluorescence microscopy technique known as PAINT (Points Accumulation for Imaging in Nanoscale Topography), we imaged colloidal hydrogel networks, so-called microgels, having a hydrodynamic radius smaller than the diffraction limit, gaining unprecedented insight into their full 3D structure which is not accessible in this much detail with any other experimental method. In addition to imaging of the microgel structure, the use of Nile Red as the solvatochromic fluorophore allowed us to resolve the polarity conditions within the investigated microgels, thus providing nanoscopic information on the x,y,z-position of labels including their polarity without the need of covalent labelling. With this imaging approach, we give a detailed insight into adapting structural and polarity properties of temperature-responsive microgels when changing the temperature beyond the volume phase transition.

Recent advances in metallopolymer-based drug delivery systems

Metallopolymers (MPs) or metal-containing polymers have shown great potential as new drug delivery systems (DDSs) due to their unique properties, including universal architectures, composition, properties and surface chemistry. Over the past few decades, the exponential growth of many new classes of MPs that deal with these issues has been demonstrated. This review presents and assesses the recent advances and challenges associated with using MPs as DDSs. Among the most widely used MPs for these purposes, metal complexes based on synthetic and natural polymers, coordination polymers, metal-organic frameworks, and metallodendrimers are distinguished. Particular attention is paid to the stimulus- and multistimuli-responsive metallopolymer-based DDSs. Of considerable interest is the use of MPs for combination therapy and multimodal systems. Finally, the problems and future prospects of using metallopolymer-based DDSs are outlined. The bibliography includes articles published over the past five years.

Electroactive and degradable supramolecular microgels

In this work, we synthesized electroactive and degradable microgels based on biomacromolecular building blocks, which enable the controlled release of therapeutic drugs. Functional chitosan-poly(hydroquinone) (Ch:PHQ) microgels exhibiting redox-active and pH-sensitive properties were synthesized by an oxidative polymerization in an inverse miniemulsion system. Physically crosslinked microgels were formed by polymerization of hydroquinone in the presence of chitosan through the formation of hydrogen bonds between PHQ and Ch. A series of microgel samples with variable Ch : PHQ ratios were synthesized. These obtained microgels exhibit pH-responsive properties due to the protonation/deprotonation of amino-groups of chitosan in the microgel system. Poly(hydroquinone) is a redox-active polymer exhibiting a two-electron/proton-transfer behavior and conveys this property to the microgels as confirmed by cyclic voltammetry. In addition, the microgels can be switched by electrochemical means: they swell in the oxidized state or shrink in the reduced state. In the presence of urea or lysozyme, the microgels undergo a fast degradation due to the disruption of hydrogen bonds acting as physical crosslinks in the microgel networks or due to the cleavage of glucosidic linkages of the incorporated chitosan scaffold, respectively. Doxorubicin (DOX), an anticancer drug, could be effectively encapsulated into the microgels and released in the presence of an enzyme, indicating that these biodegradable microgels could be used as drug delivery vehicles for tumor cells.

Effect of Divalent Cation on Swelling Behavior of Anionic Microgels: Quantification and Dynamics of Ion Uptake and Release

Poly(N-vinylcaprolactam-co-itaconate) (P(VCL-co-IADME) microgels were synthesized varying the molar ratio between VCL and IADME via free radical precipitation polymerization in the presence of quaternary ammonium surfactant. In order to determine the effect of the divalent metal ions on the structure and the swelling behavior of the microgel systems, both neutral and charged forms of the hydrogels after hydrolysis were investigated. The triggered gel collapse caused by the divalent metal ion together with the quantification of the metal ion uptake was studied in detail by titration and ion chromatography methods and revealed the minimum concentration around 0.1 mM to trigger gel collapse on the treated gels. Uptake and release dynamics of the gels were followed by turbidity measurements and were in the time-range of 2 and 17 s, depending on the composition and the concentrations.

Degradable and Thermosensitive Microgels Synthesized via Simultaneous Quaternization and Siloxane Condensation

Degradable and thermosensitive microgels were successfully prepared via simultaneous quaternization and siloxane condensation during surfactant-free emulsion polymerization, with N-vinylcaprolactam as the main monomer and 1-vinylimidazole (VIM) as the comonomer, in the presence of (3-bromopropyl)trimethoxysilane (BPTMOS). The formation mechanism of cross-linking network was attributed to the hydrolysis and condensation of the methoxysilyl groups of BPTMOS and the quaternization of imidazole moiety of VIM by the bromine group of BPTMOS, leading to the microgels. The microgels were spherical in shape with a narrow size distribution, stable in an acidic buffer solution, but degradable in neutral and alkaline solutions. The presence of quaternized imidazolium in the same chain segment of Si-O-Si cross-linking points promoted the decomposition of Si-O-Si bonds and hence the degradation of the microgels. The obtained microgels could load and release the model drug, doxorubicin. The size, thermosensitivity, stability, degradation rate, and drug release behavior of the resultant microgels could be tuned by controlling the cross-linking degree, chemical composition, and degradation medium.