Water-borne synthesis of multi-responsive and biodegradable chitosan-crosslinked microgels: Towards self-assembled films with adaptable properties

The present study aims in the synthesis of new biodegradable stimuli-responsive microgels with controllable microstructure and with the ability to form cohesive films. Such self-assembled films by water evaporation at ambient conditions without any chemicals but just physical entanglements between soft colloid shell, present adaptable mechanical, adhesive and mechano-electrical properties. For that, oligo(ethylene glycol)-based stimuli-responsive microgels have been synthesized using biodegradable chitosan-methacrylates (Chi-MAs) with different degree of substitution (DS) as unique cross-linking agents by precipitation polymerization in water, for the first time. In all the cases, the microgels present thermo-responsiveness with hysteresis between heating and cooling cycles. However, this behavior is tuned and controlled using different types and amounts of Chi-MAs. In addition, the type of Chi-MA used can control microgels' microstructure as well as their enzymatic biodegradation. In addition, spontaneous cohesive films formation from colloidal aqueous dispersion with sol-gel transition is demonstrated. The films present tunable mechanical and adhesive properties through microgels' microstructure and enhanced mechano-electrical properties triggered by simple finger pressure (10-15 N). As self-supported films are able to encapsulate different types of active molecules, this study paves the way for suitable self-assembled microgel films for skincare applications as transdermal delivery systems.

Soft Self-Assembled Mechanoelectrical Transducer Films from Conductive Microgel Waterborne Dispersions

The present study aims in the developing of new soft transducers based on sophisticated stimuli-responsive microgels that exhibit spontaneous self-assembly forming cohesive films with conductive and mechanoelectrical properties. For that, oligo(ethylene glycol)-based stimuli-responsive microgels have been synthesized using bio-inspired catechol cross-linkers by one-step batch precipitation polymerization in aqueous media. Then, 3,4-ethylene dioxyyhiophene (EDOT) has been directly polymerized onto stimuli-responsive microgels using catechol groups as the unique dopant. PEDOT location is dependent on the cross-linking density of microgel particles and EDOT amount used. Moreover, the spontaneous cohesive film formation ability of the waterborne dispersion after evaporation at soft application temperature is demonstrated. The films obtained present conductivity and enhanced mechanoelectrical properties triggered by simple finger compression. Both properties are function of the cross-linking density of the microgel seed particles and PEDOT amount incorporated. In addition, to obtain maximum electrical potential generated and the possibility to amplify it, several films in series were demonstrated to be efficient. The present material can be a potential candidate for biomedical, cosmetic, and bioelectronic applications.

Recent advances in cellulose microgels: Preparations and functionalized applications

Microgels are soft, deformable, permeable, and stimuli-responsive microscopic polymeric particles that are now emerging as prospective multifunctional soft materials for delivery systems, interface stabilization, cell cultures and tissue engineering. Cellulose microgels are emerging biopolymeric microgels with unique characteristics such as abound hydroxyl structure, admirable designability, multiscale pore network and excellent biocompatibility. This review summarizes the fabrication strategies for microgel, then highlights the fabrication routes for cellulose microgels, and finally elaborates cellulose microgels' bright application prospects with unique characteristics in the fields of controlled release, interface stabilization, coating, purification, nutrition/drug delivery, and bio-fabrication. The challenges to be addressed for further applications and considerable scope for development in future of cellulose microgels are also discussed.

Translating Therapeutic Microgels into Clinical Applications

Microgels are crosslinked, water-swollen networks with a 10 nm to 100 µm diameter and can be modified chemically or biologically to render them biocompatible for advanced clinical applications. Depending on their intended use, microgels require different mechanical and structural properties, which can be engineered on demand by altering the biochemical composition, crosslink density of the polymer network, and the fabrication method. Here, the fundamental aspects of microgel research and development, as well as their specific applications for theranostics and therapy in the clinic, are discussed. A detailed overview of microgel fabrication techniques with regards to their intended clinical application is presented, while focusing on how microgels can be employed as local drug delivery materials, scavengers, and contrast agents. Moreover, microgels can act as scaffolds for tissue engineering and regeneration application. Finally, an overview of microgels is given, which already made it into pre-clinical and clinical trials, while future challenges and chances are discussed. This review presents an instructive guideline for chemists, material scientists, and researchers in the biomedical field to introduce them to the fundamental physicochemical properties of microgels and guide them from fabrication methods via characterization techniques and functionalization of microgels toward specific applications in the clinic.

Spontaneously Self-Assembled Microgel Film as Co-Delivery System for Skincare Applications

Nowadays, the design of innovative delivery systems is driving new product developments in the field of skincare. In this regard, serving as potential candidates for on-demand drug delivery and fulfilling advanced mechanical and optical properties together with surface protection, spontaneously self-assembled microgel films can be proposed as ideal smart skincare systems. Currently, the high encapsulation of more than one drug simultaneously in a film is a very challenging task. Herein, different ratios (1:1, 3:1, 9:1) of different mixtures of hydrophilic/hydrophobic UVA/UVB-absorbers working together in synergy and used for skin protection were encapsulated efficiently into spontaneously self-assembled microgel films. In addition, in vitro release profiles show a controlled release of the different active molecules regulated by the pH and temperature of the medium. The analysis of the release mechanisms by the Peppas-Sahlin model indicated a superposition of diffusion-controlled and swelling-controlled releases. Finally, the distribution of active molecule mixtures into the film was studied by confocal Raman microscopy imaging corroborating the release profiles obtained.

Temperature and pH Responsive Light-Harvesting System Based on AIE-Active Microgel for Cell Imaging

A highly emissive microgel is synthesized by polymerizing tetraphenylethene (TPE) based comonomers, acrylic acid, NIPAM, and permanent crosslinker ethylenebisacrylamide (BIS) (named as TPE microgel), which exhibited temperature responsive fluorescence emission. Rhodamine B (RhB), a positively charged molecule, is then inserted onto the surface of fabricated microgels through electrostatic interaction. As a result, a novel artificial light harvesting system with high energy transfer efficiency is constructed (named as TPE microgel-RhB light harvesting system), which is the first light harvesting system based on TPE microgels presenting dual response to pH and temperature. MTT assay indicates the fabricated TPE microgel and TPE microgel-RhB light harvesting system has good cytocompatibility. The strong fluorescence and good cytocompatibility make them perfect candidates for cell imaging. The prepared emissive microgel and light-harvesting system with desirable fluorescent property not only provide a new strategy for the fabrication of tunable luminescent nanomaterials, but also expand potential applications in the fields of stomach recognition, temperature sensors, and drug delivery.

Microgels self-assembly at liquid/liquid interface as stabilizers of emulsion: Past, present & future

The most recent developments on Pickering emulsions deal with the design of responsive emulsions able to undergo fast destabilization under the effect of an external stimulus. In this scenario, soft colloidal particles like microgels are considered novel class suitable emulsifiers. Microgels particles self-assemblies are highly deformable at interfaces covering higher surfaces than hard particles and their interfacial behavior strongly depends on external-stimuli. Microgels are very diverse owing to the large variety of them from the point of view of possible combinations of stimuli-responsiveness and different microstructures (crosslinking density and distribution). Herein, we illustrate the use of different types of responsive microgels not only from a structural point of view but also even from physical one. For that, the effect of different microgels parameters such as internal structure and charge density on mechanical properties of the interface will be discussed.

Microstructure-driven self-assembly and rheological properties of multi-responsive soft microgel suspensions

HYPOTHESES

The deformation and swelling ability of microgels is influenced by the crosslinking distribution. Varying microgels microstructure is expected to obtain suspensions with different flow behavior and thereby, different rheological properties.

EXPERIMENTS

Different multi-responsive microgels were synthesized using two different crosslinkers and varying their amounts: N,N-methylene bis-acrylamide (MBA) and oligo(ethylene glycol) diacrylate (OEGDA). The rheological results were obtained by zero-shear viscosity and long-time creep measurements on concentrated microgel suspensions Microgel microstructure was analyzed by ¹H nuclear magnetic resonance transverse relaxation measurements.

FINDINGS

At a constant crosslinking rate, we show that the viscosity of OEGDA-crosslinked microgels diverges at a higher concentration than MBA ones, suggesting a looser shell and less restricted dangling chains at the periphery for the later. By scaling with the effective volume fraction, the viscosity curves of the different microgel suspensions reduce into a single curve and closely follow hard sphere models up to ϕ_eff < 0.45. The results from creep tests revealed a much higher yield stress for MBA-crosslinked microgels, strengthening the hypothesis of a looser shell for the later. Finally, transverse relaxation (T₂) NMR measurements demonstrated that, although all microgels exhibit a core-shell microstructure, MBA samples present a less crosslinked shell corroborating with the rheological results.

Core@Corona Functional Nanoparticle-Driven Rod-Coil Diblock Copolymer Self-Assembly

Herein, a novel strategy to overcome the influence of π-π stacking on the rod-coil copolymer organization is reported. A diblock copolymer poly(3-hexylthiophene)-block-poly(ethylene glycol methyl ether methacrylate) (P3HT-b-PEGMA) was synthesized by the Huisgen cycloaddition, so-called "click chemistry", combining the PEGMA and P3HT blocks synthesized by atom transfer radical polymerization and Kumada catalyst transfer polymerization, respectively. Using a dip-coating process, we controlled the original film organization of the diblock copolymer by the crystallization of the P3HT block via π-π stacking. The morphology of the P3HT-b-PEGMA films was influenced by the incorporation of gold nanoparticles (GNPs) coated by poly(ethylene glycol) ligands. Indeed, the crystalline structuration of the P3HT sequence was counterbalanced by the addition in the film of gold nanoparticles finely localized within the copolymer PEGMA matrix. Transmission electron microscopy and time-of-flight secondary ion mass spectrometry analysis validated the GNP homogeneous localization into the compatible PEGMA phase. Differential scanning calorimetry showed the rod block crystallization disruption. A morphological transition of the self-assembly is observed by atomic force microscopy from P3HT fibrils into out-of-plane cylinders driven by the nanophase segregation.

Versatile oligo(ethylene glycol)-based biocompatible microgels for loading/release of active bio(macro)molecules

The present study aims in the understanding of the effect of oligo(ethylene glycol)-based biocompatible microgels inner structure on the encapsulation/release mechanisms of different types of cosmetic active molecules. For that, multi-responsive microgels were synthesized using three types of cross-linkers: ethylene glycol dimethacrylate (EGDMA), oligo(ethylene glycol) diacrylate (OEGDA) and N,N-methylenebisacrylamide (MBA). The inner morphology of the microgels synthesized was studied by ¹H-nuclear magnetic resonance (¹H NMR) and small-angle neutron scattering (SANS) techniques and no effect of cross-linker type on microgel microstructure was observed in the case of analysing purified microgel dispersions. Moreover, all the microgels synthesized presented conventional swelling/de-swelling behavior as a function of temperature and pH. Two hydrophobic, one hydrophilic, and one macromolecule as cosmetic active molecules were effectively loaded into different microgel particles via hydrophobic interactions and hydrogen-bonding interactions between -OH groups of active molecules and ether oxygens of different microgel particles. Their release profiles as a function of cross-linker type used and encapsulated amounts were studied by Peppas-Sahlin model. No effect of the cross-linker type was observed due to the similar inner structure of all the microgels synthesized.

Polyamidoamine Dendrimer Microgels: Hierarchical Arrangement of Dendrimers into Micrometer Domains with Expanded Structural Features for Programmable Drug Delivery and Release

In this paper, we report on the fabrication of micron-sized dendrimer hydrogels (μDHs) using the water-in-oil (w/o) inverse microemulsion method coupled with the highly efficient aza-Michael addition. EDA core polyamidoamine (PAMAM) dendrimer G5 (10 w%) and polyethylene glycol diacrylate (PEG-DA, M n = 575 g/mol) (the molar ratio of amine/acrylate = 1/1) were dissolved in the water phase and added to hexane in the presence of surfactants span 80/tween 80 (5/1, w/w) (volume ratio of hexane to surfactants: 70:1) to form w/o microemulsions, in which PAMAM G5 cross-links with PEG-DA via the aza-Michael addition reaction. The resulting microgels are within 3-5 μm with relatively narrow size distribution. μDHs are pH-responsive degradable. They show good cytocompatibility and do not cause acute toxicity in vivo. Furthermore, they can realize a high loading of the hydrophobic drug CPT and enter the cells in the form of particles. The CPT and CPT/dendrimer complex can be slowly released following the zero-order release kinetics. Taken together, μDHs possessing hierarchically ordered dendrimers in micron domains represent a new class of microparticles with expanded structural features for programmable drug delivery and release.

Tumor pH and intracellular reduction responsive polypeptide nanomedicine with a sheddable PEG corona and a disulfide-cross-linked core

To overcome the tumor and cellular barriers in cancer chemotherapy, a new class of polypeptide copolymer nanoparticles with a sheddable PEG corona and a dynamic disulfide bond cross-linked core was fabricated for intracellular pHe and reduction triggered cytotoxicity.

Smart self-assembled microgel films as encapsulating carriers for UV-absorbing molecules

This article reports the ability of oligo(ethylene glycol)-based stimuli-responsive microgels to spontaneously form self-assembled microgel films under different conditions such as the presence or absence of water soluble polymers (WSP, formed during the microgel synthesis) and the type and/or amount of salt in the microgel dispersion.