In situ formation of thermosensitive PNIPAAm-based hydrogels by Michael-type addition reaction

A Spontaneous In Situ Thiol-Ene Crosslinking Hydrogel with Thermo-Responsive Mechanical Properties

The thermo-responsive behavior of Poly(N-isopropylacrylamide) makes it an ideal candidate to easily embed cells and allows the polymer mixture to be injected. However, P(NiPAAm) hydrogels possess minor mechanical properties. To increase the mechanical properties, a covalent bond is introduced into the P(NIPAAm) network through a biocompatible thiol-ene click-reaction by mixing two polymer solutions. Co-polymers with variable thiol or acrylate groups to thermo-responsive co-monomer ratios, ranging from 1% to 10%, were synthesized. Precise control of the crosslink density allowed customization of the hydrogel's mechanical properties to match different tissue stiffness levels. Increasing the temperature of the hydrogel above its transition temperature of 31 °C induced the formation of additional physical interactions. These additional interactions both further increased the stiffness of the material and impacted its relaxation behavior. The developed optimized hydrogels reach stiffnesses more than ten times higher compared to the state of the art using similar polymers. Furthermore, when adding cells to the precursor polymer solutions, homogeneous thermo-responsive hydrogels with good cell viability were created upon mixing. In future work, the influence of the mechanical micro-environment on the cell's behavior can be studied in vitro in a continuous manner by changing the incubation temperature.

Metal-Free Click-Chemistry: A Powerful Tool for Fabricating Hydrogels for Biomedical Applications

Increasing interest in the utilization of hydrogels in various areas of biomedical sciences ranging from biosensing and drug delivery to tissue engineering has necessitated the synthesis of these materials using efficient and benign chemical transformations. In this regard, the advent of "click" chemistry revolutionized the design of hydrogels and a range of efficient reactions was utilized to obtain hydrogels with increased control over their physicochemical properties. The ability to apply the "click" chemistry paradigm to both synthetic and natural polymers as hydrogel precursors further expanded the utility of this chemistry in network formation. In particular, the ability to integrate clickable handles at predetermined locations in polymeric components enables the formation of well-defined networks. Although, in the early years of "click" chemistry, the copper-catalyzed azide-alkyne cycloaddition was widely employed, recent years have focused on the use of metal-free "click" transformations, since residual metal impurities may interfere with or compromise the biological function of such materials. Furthermore, many of the non-metal-catalyzed "click" transformations enable the fabrication of injectable hydrogels, as well as the fabrication of microstructured gels using spatial and temporal control. This review article summarizes the recent advances in the fabrication of hydrogels using various metal-free "click" reactions and highlights the applications of thus obtained materials. One could envision that the use of these versatile metal-free "click" reactions would continue to revolutionize the design of functional hydrogels geared to address unmet needs in biomedical sciences.

Plasmonic nanomaterials with responsive polymer hydrogels for sensing and actuation

Plasmonic nanomaterials have become an integral part of numerous technologies, where they provide important functionalities spanning from extraction and harvesting of light in thin film optical devices to probing of molecular species and their interactions on biochip surfaces. More recently, we witness increasing research efforts devoted to a new class of plasmonic nanomaterials that allow for on-demand tuning of their properties by combining metallic nanostructures and responsive hydrogels. This review addresses this recently emerged vibrant field, which holds potential to expand the spectrum of possible applications and deliver functions that cannot be achieved by separate research in each of the respective fields. It aims at providing an overview of key principles, design rules, and current implementations of both responsive hydrogels and metallic nanostructures. We discuss important aspects that capitalize on the combination of responsive polymer networks with plasmonic nanostructures to perform rapid mechanical actuation and actively controlled nanoscale confinement of light associated with resonant amplification of its intensity. The latest advances towards the implementation of such responsive plasmonic nanomaterials are presented, particularly covering the field of plasmonic biosensing that utilizes refractometric measurements as well as plasmon-enhanced optical spectroscopy readout, optically driven miniature soft actuators, and light-fueled micromachines operating in an environment resembling biological systems.

Click Chemistry Hydrogels for Extrusion Bioprinting: Progress, Challenges, and Opportunities

The emergence of 3D bioprinting has allowed a variety of hydrogel-based "bioinks" to be printed in the presence of cells to create precisely defined cell-loaded 3D scaffolds in a single step for advancing tissue engineering and/or regenerative medicine. While existing bioinks based primarily on ionic cross-linking, photo-cross-linking, or thermogelation have significantly advanced the field, they offer technical limitations in terms of the mechanics, degradation rates, and the cell viabilities achievable with the printed scaffolds, particularly in terms of aiming to match the wide range of mechanics and cellular microenvironments. Click chemistry offers an appealing solution to this challenge given that proper selection of the chemistry can enable precise tuning of both the gelation rate and the degradation rate, both key to successful tissue regeneration; simultaneously, the often bio-orthogonal nature of click chemistry is beneficial to maintain high cell viabilities within the scaffolds. However, to date, relatively few examples of 3D-printed click chemistry hydrogels have been reported, mostly due to the technical challenges of controlling mixing during the printing process to generate high-fidelity prints without clogging the printer. This review aims to showcase existing cross-linking modalities, characterize the advantages and disadvantages of different click chemistries reported, highlight current examples of click chemistry hydrogel bioinks, and discuss the design of mixing strategies to enable effective 3D extrusion bioprinting of click hydrogels.

Structurally Tunable pH-responsive Phosphine Oxide Based Gels by Facile Synthesis Strategy

Design and synthesis of nanostructured responsive gels have attracted increasing attention, particularly in the biomedical domain. Polymer chain configurations and nanodomain sizes within the network can be used to steer their functions as drug carriers. Here, a catalyst-free facile one-step synthesis strategy is reported for the design of pH-responsive gels and controlled structures in nanoscale. Transparent and impurity free gels were directly synthesized from trivinylphosphine oxide (TVPO) and cyclic secondary diamine monomers via Michael addition polymerization under mild conditions. NMR analysis confirmed the consumption of all TVPO and the absence of side products, thereby eliminating post purification steps. The small-angle X-ray scattering (SAXS) elucidates the nanoscale structural features in gels, that is, it demonstrates the presence of collapsed nanodomains within gel networks and it was possible to tune the size of these domains by varying the amine monomers and the nature of the solvent. The fabricated gels demonstrate structure tunability via solvent-polymer interactions and pH specific drug release behavior. Three different anionic dyes (acid blue 80, acid blue 90, and fluorescein) of varying size and chemistry were incorporated into the hydrogel as model drugs and their release behavior was studied. Compared to acidic pH, a higher and faster release of acid blue 80 and fluorescein was observed at pH 10, possibly because of their increased solubility in alkaline pH. In addition, their release in phosphate buffered saline (PBS) and simulated body fluid (SBF) matrix was positively influenced by the ionic interaction with positively charged metal ions. In the case of hydrogel containing acid blue 90 a very low drug release (<1%) was observed, which is due to the reaction of its accessible free amino group with the vinyl groups of the TVPO. In vitro evaluation of the prepared hydrogel using human dermal fibroblasts indicates no cytotoxic effects, warranting further research for biomedical applications. Our strategy of such gel synthesis lays the basis for the design of other gel-based functional materials.

Thiolated polymers: Bioinspired polymers utilizing one of the most important bridging structures in nature

Thiolated polymers designated "thiomers" are obtained by covalent attachment of thiol functionalities on the polymeric backbone of polymers. In 1998 these polymers were first described as mucoadhesive and in situ gelling compounds forming disulfide bonds with cysteine-rich substructures of mucus glycoproteins and crosslinking through inter- and intrachain disulfide bond formation. In the following, it was shown that thiomers are able to form disulfides with keratins and membrane-associated proteins exhibiting also cysteine-rich substructures. Furthermore, permeation enhancing, enzyme inhibiting and efflux pump inhibiting properties were demonstrated. Because of these capabilities thiomers are promising tools for drug delivery guaranteeing a strongly prolonged residence time as well as sustained release on mucosal membranes. Apart from that, thiomers are used as drugs per se. In particular, for treatment of dry eye syndrome various thiolated polymers are in development and a first product has already reached the market. Within this review an overview about the thiomer-technology and its potential for different applications is provided discussing especially the outcome of studies in non-rodent animal models and that of numerous clinical trials. Moreover, an overview on product developments is given.

Reversible Thermoresponsive Hydrogel Fabricated from Natural Biopolymer for the Improvement of Critical Limb Ischemia by Controlling Release of Stem Cells

Stem cells therapy is an effective treatment for critical limb ischemia diseases (CLI), but is limited to low cells retention and poor target release in severe ischemia tissues. Due to the notable feature of CLI, namely, the temperature of ischemia tissues decreases with the severity of the lesions, a thermoresponsive and reversible hydrogel based on methylcellulose-salt system encapsulating stem cells is facilely prepared and successfully achieved the goal of releasing stem cells in lower temperature areas. The investigations show that the thermogel presents notable biocompatibility, thermoresponsiveness, and cytoprotection. Furthermore, the combined transplantation of hydrogel and stem cells system effectively inhibits the fibrosis and muscular atrophy of lower limb ischemia, accelerates the recovery of lower limb blood flow, and promotes angiogenesis, indicating that the reversible thermogel can promote vascular repair by controlling the release of loaded stem cells in the treatment of CLI.

Injectable and Thermosensitive Hydrogel Containing Liraglutide as a Long-Acting Antidiabetic System

Diabetes, a global epidemic, has become a serious threat to public health. The present study is aimed at constructing an injectable thermosensitive PEG-polyester hydrogel formulation of liraglutide (Lira), a "smart" antidiabetic polypeptide, in the long-acting treatment of type 2 diabetes mellitus. A total of three thermosensitive poly(ε-caprolactone-co-glycolic acid)-poly(ethylene glycol)-poly(ε-caprolactone-co-glycolic acid) (PCGA-PEG-PCGA) triblock copolymers with similar molecular weights but different ε-caprolactone-to-glycolide (CL-to-GA) ratios were synthesized. The polymer aqueous solutions exhibited free-flowing sols at room temperature and formed in situ hydrogels at body temperature. While the different bulk morphologies, stabilities of aqueous solutions, and the varying in vivo persistence time of hydrogels in ICR mice were found among the three copolymers, all of the Lira-loaded gel formulations exhibited a sustained drug release manner in vitro regardless of CL-to-GA ratios. The specimen with a powder form in the bulk state, a stable aqueous solution before heating, and an appropriate degradation rate in vivo was selected as the optimal carrier to evaluate the in vivo efficacy. A single injection of the optimal gel formulation showed a remarkable hypoglycemic efficacy up to 1 week in diabetic db/db mice. Furthermore, three successive administrations of this gel formulation within one month significantly lowered glycosylated hemoglobin and protected islets of db/db mice. As a result, a promising once-weekly delivery system of Lira was developed, which not only afforded long-term glycemic control but also significantly improved patient compliance.

An in situ forming tissue adhesive based on poly(ethylene glycol)-dimethacrylate and thiolated chitosan through the Michael reaction

A novel tissue adhesive composed of CSS and PEGDMA based on the Michael addition reaction.

Mesenchymal stem cell and gelatin microparticle encapsulation in thermally and chemically gelling injectable hydrogels for tissue engineering

In this work, we investigated the viability and osteogenic differentiation of mesenchymal stem cells encapsulated with gelatin microparticles (GMPs) in an injectable, chemically and thermally gelling hydrogel system combining poly(N-isopropylacrylamide)-based thermogelling macromers containing pendant epoxy rings with polyamidoamine-based hydrophilic and degradable diamine crosslinking macromers. Specifically, we studied how the parameters of GMP size and loading ratio affected the viability and differentiation of cells encapsulated within the hydrogel. We also examined the effects of cell and GMP co-encapsulation on hydrogel mineralization. Cells demonstrated long-term viability within the hydrogels, which was shown to depend on GMP size and loading ratio. In particular, increased interaction of cells and GMPs through greater available GMP surface area, use of an epoxy-based chemical gelation mechanism, and the tunable high water content of the thermogelled hydrogels led to favorable long-term cell viability. Compared with cellular hydrogels without GMPs, hydrogels co-encapsulating cells and GMPs demonstrated greater production of alkaline phosphatase by cells at all time-points and a transient early enhancement of hydrogel mineralization for larger GMPs at higher loading ratios. Such injectable, in situ forming hydrogels capable of delivering and maintaining populations of encapsulated mesenchymal stem cells and promoting mineralization in vitro offer promise as novel therapies for applications in tissue engineering and regenerative medicine.

Strain-Promoted Crosslinking of PEG-based Hydrogels via Copper-Free Cycloaddition

The synthesis of a 4-dibenzocyclooctynol (DIBO) functionalized polyethylene glycol (PEG) and fabrication of hydrogels via strain-promoted, metal-free, azide-alkyne cycloaddition is reported. The resulting hydrogel materials provide a versatile alternative in which to encapsulate cells that are sensitive to photochemical or chemical crosslinking mechanisms.

Synthesis and characterization of thermally and chemically gelling injectable hydrogels for tissue engineering

Novel, injectable hydrogels were developed that solidify through a physical and chemical dual-gelation mechanism upon preparation and elevation of temperature to 37 °C. A thermogelling, poly(N-isopropylacrylamide)-based macromer with pendant epoxy rings and a hydrolytically degradable polyamidoamine-based diamine cross-linker were synthesized, characterized, and combined to produce in situ forming hydrogel constructs. Network formation through the epoxy-amine reaction was shown to be rapid and facile, and the progressive incorporation of the hydrophilic polyamidoamine cross-linker into the hydrogel was shown to mitigate the often problematic tendency of thermogelling materials to undergo significant postformation gel syneresis. The results suggest that this novel class of injectable hydrogels may be attractive substrates for tissue engineering applications due to the synthetic versatility of the component materials and beneficial hydrogel gelation kinetics and stability.

Ultrathin, flexible, and transparent polymer multilayer composites for the protection of silver surfaces

Silver coatings at the nanoscale became of high interest for the integration of electronic functionalities on all kinds of objects for daily use. In these thin coatings, corrosion is a big problem as it destroys these thin layers and leads to a loss of conductivity due to missing bulk material. For protection of thin silver coatings against H(2)S induced corrosion, we developed nanocoatings based on the covalent layer-by-layer technique. We prepared composites by subsequent deposition of polyamines like polyethylenimine (PEI) or polyallylamine (PAAm) and polyanhydrides like poly(maleic anhydride-alt-methyl vinyl ether) (Gantrez) or poly(styrene-co-maleic anhydride) (PSMA). For the tuning of the hydrophobicity, the layers were terminated by reaction with palmitoylic acid derivatives. Reflectivity measurements, contact angle measurements, and AFM measurements were made to investigate how the coatings affect the surface properties. All coatings show a lower reflectivity below 450 nm compared to pure silver, depending on the number of layers deposited. The addition of a palmitoylic derivative to the surface increases the hydrophobicity, but only in case of the Gantrez-PVAm-composite, this approach leads to real hydrophobicity, reaching contact angles above 90°. AFM measurements show a decrease of the roughness of the polymer coated surfaces compared to the pure metal surfaces. Corrosion tests in a H(2)S atmosphere show a good protective effect of the palmitoyl-terminated composites. Martindale abrasion tests on coated textiles reveal a good stability of the prepared polymer composites.

Fabrication and functionalization of hydrogels through "click" chemistry

Hydrogels are crosslinked polymeric materials that play a vital role in many biomedical areas such as drug delivery, sensor technology, and tissue engineering. Increasing demand of these materials for such advanced applications has necessitated the development of hydrogels with complex chemical compositions such as incorporating small molecules and biomolecules that provide the functional attributes. This Focus Review highlights the tremendous impact of click chemistry on the design, synthesis, and functionalization of hydrogels. The high efficiency and fidelity of the click reactions have enabled rapid and modular synthesis of hydrogels with near-ideal network structures. Efficient incorporation of biomolecular building blocks, such as peptide sequences either during or after the fabrication of hydrogels, have been achieved through various click reactions. Utilization of these efficient reactions has led to the fabrication of many stimuli-responsive or 'smart' hydrogels in recent years.

In situ forming poly(ethylene glycol)-based hydrogels via thiol-maleimide Michael-type addition

The incorporation of cells and sensitive compounds can be better facilitated without the presence of UV or other energy sources that are common in the formation of biomedical hydrogels such as poly(ethylene glycol) hydrogels. The formation of hydrogels by the step-growth polymerization of maleimide- and thiol-terminated poly(ethylene glycol) macromers via Michael-type addition is described. The effects of macromer concentration, pH, temperature, and the presence of biomolecule gelatin on gel formation were investigated. Reaction kinetics between maleimide and thiol functional groups were found to be rapid. Molecular weight increase over time was characterized via gel permeation chromatography during step-growth polymerization. Swelling and degradation results showed incorporating gelatin enhanced swelling and accelerated degradation. Increasing gelatin content resulted in the decreased storage modulus (G'). The in vitro release kinetics of fluorescein isothiocyanate (FITC)-labeled dextran from the resulting matrices demonstrated the potential in the development of novel in situ gel-forming drug delivery systems. Moreover, the resulting networks were minimally adhesive to primary human monocytes, fibroblasts, and keratinocytes thus providing an ideal platform for further biofunctionalizations to direct specific biological response.