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.

Antimicrobial activity of gamma-poly (glutamic acid), a preservative coating for cherries

We investigated the minimum inhibitory concentration (MIC), antibacterial activity, and preservation ability of four molar masses of γ-polyglutamic acid (PGA) against Escherichia coli, Bacillus subtilis, and yeast. The antibacterial mechanism was determined based on the cell structure, membrane permeability, and microscopic morphology of the microorganisms. We then measured the weight loss, decay rate, total acid, catalase activity, peroxidase activity, and malondialdehyde content toward the possible use of PGA as a preservative coating for cherries. When the molar mass was greater than 700 kDa, the MIC for Escherichia coli and Bacillus subtilis was less than 2.5 mg/mL. The mechanism of action of the four molar masses of PGA was different with respect to the three microbial species, but a higher molar mass of PGA corresponded to stronger inhibition against the microbes. PGA of 2000 kDa molar mass damaged the microbial cellular structure, resulting in excretion of alkaline phosphatase, but PGA of 1.5 kDa molar mass affected the membrane permeability and the amount of soluble sugar. Scanning electron microscopy indicated the inhibitory effect of PGA. The antibacterial mechanism of PGA was related to the molar mass of PGA and the microbial membrane structure. Compared with the control, a PGA coating effectively inhibit the spoilage rate, delay the ripening, and prolong the shelf life of cherries.

Hydrophobic Modification of Poly(γ-glutamic acid) by Grafting 4-Phenyl-butyl Side Groups for the Encapsulation and Release of Doxorubicin

The delivery of drugs is a great challenge, since most of active pharmaceutical ingredients developed today are hydrophobic and poorly water soluble. From this perspective, drug encapsulation on biodegradable and biocompatible polymers can surpass this problem. Poly(γ-glutamic acid) (PGGA), a bioedible and biocompatible polymer has been chosen for this purpose. Carboxylic side groups of PGGA have been partially esterified with 4-phenyl-butyl bromide, producing a series of aliphatic-aromatic ester derivatives with different hydrophilic-lipophilic balances. Using nanoprecipitation or emulsion/evaporation methods, these copolymers were self-assembled in a water solution, forming nanoparticles with average diameters between 89 and 374 nm and zeta potential values between -13.1 and -49.5 mV. The hydrophobic core containing 4-phenyl-butyl side groups was used for the encapsulation of an anticancer drug, such as Doxorubicin (DOX). The highest encapsulation efficiency was reached for a copolymer derived from PGGA, with a 46 mol% degree of esterification. Drug release studies carried out for 5 days at different pHs (4.2 and 7.4) indicated that DOX was released faster at pH 4.2, revealing the potential of these nanoparticles as chemotherapy agents.

Polylactic-Containing Hyperbranched Polymers through the CuAAC Polymerization of Aromatic AB2 Monomers

We report on the synthesis and characterization of a novel class of hyperbranched polymers, in which a copper(I)-catalyzed alkyne azide cycloaddition (CuAAC) reaction (the prototypical "click" reaction) is used as the polymerization step. The AB₂ monomers bear two azide functionalities and one alkyne functionality, which have been installed onto a 1,3,5 trisubstituted benzene aromatic skeleton. This synthesis has been optimized in terms of its purification strategies, with an eye on its scalability for the potential industrial applications of hyperbranched polymers as viscosity modifiers. By taking advantage of the modularity of the synthesis, we have been able to install short polylactic acid fragments as the spacing units between the complementary reactive azide and alkyne functionalities, aiming to introduce elements of biodegradability into the final products. The hyperbranched polymers have been obtained with good molecular weights and degrees of polymerization and branching, testifying to the effectiveness of the synthetic design. Simple experiments on glass surfaces have highlighted the possibility of conducting the polymerizations and the formation of the hyperbranched polymers directly in thin films at room temperature.

A strategy for nitrogen conversion in aquaculture water based on poly-γ-glutamic acid synthesis

Ammonia and nitrite are nitrogenous pollutants in aquaculture effluents, which pose a major threat to the health of aquatic animals. In this study, we developed a nitrogen conversion strategy based on synthesis of poly-γ-glutamic acid (γ-PGA) by Bacillus subtilis NX-2. The nitrogen removal efficiency of NX-2 was closely related to synthesizing γ-PGA, and was positively correlated with the inoculum level. The degradation rates of ammonia nitrogen and nitrite at 10⁴ CFU/mL were 84.42 % and 62.56 %, respectively. Through adaptive laboratory evolution (ALE) experiment, we obtained a strain named ALE 5 M with ammonia degradation rate of 98.03 % and nitrite of 93.62 % at the inoculum level of 10⁴ CFU/mL. Transcriptome analysis showed that the strain was more likely to produce γ-PGA after ALE. By enzyme activity and qPCR analysis, we confirmed that ALE 5 M degraded ammonia nitrogen through γ-PGA synthesis, which provided a new way for nitrogen removal in aquaculture water.

“Clickable” Polymer Brush Interfaces: Tailoring Monovalent to Multivalent Ligand Display for Protein Immobilization and Sensing

Facile and effective functionalization of the interface of polymer-coated surfaces allows one to dictate the interaction of the underlying material with the chemical and biological analytes in its environment. Herein, we outline a modular approach that would enable installing a variety of “clickable” handles onto the surface of polymer brushes, enabling facile conjugation of various ligands to obtain functional interfaces. To this end, hydrophilic anti-biofouling poly(ethylene glycol)-based polymer brushes are fabricated on glass-like silicon oxide surfaces using reversible addition–fragmentation chain transfer (RAFT) polymerization. The dithioester group at the chain-end of the polymer brushes enabled the installation of azide, maleimide, and terminal alkene functional groups, using a post-polymerization radical exchange reaction with appropriately functionalized azo-containing molecules. Thus, modified polymer brushes underwent facile conjugation of alkyne or thiol-containing dyes and ligands using alkyne–azide cycloaddition, Michael addition, and radical thiol–ene conjugation, respectively. Moreover, we demonstrate that the radical exchange approach also enables the installation of multivalent motifs using dendritic azo-containing molecules. Terminal alkene groups containing dendrons amenable to functionalization with thiol-containing molecules using the radical thiol–ene reaction were installed at the interface and subsequently functionalized with mannose ligands to enable sensing of the Concanavalin A lectin.

An Overview of the Main Trends in the Creation of Biodegradable Polymer Materials

Plastic is one of the most demanded materials on the planet, and the increasing consumption of which contributes to the accumulation of significant amounts of waste based on it. For this reason, a new approach to the development of these materials has been formed: the production of polymers with constant operational characteristics during the period of consumption and capable of then being destroyed under the influence of environmental factors and being involved in the metabolic processes of natural biosystems. The paper outlines the prerequisites for the development of the field of creating biodegradable composite materials, as well as the main technical solutions for obtaining such polymeric materials. The main current solutions for reducing and regulating the degradation time of polymer materials are presented. The most promising ways of further development of the field of bioplastics production are described. Common types of polymers based on renewable raw materials, composites with their use, and modified materials from natural and synthetic polymers are considered.

A rapid and rapid method to quantify poly (γ-glutamic acid) content via copper ion complexation

Presently, there have been some limitations in most of methods to determine poly (γ-glutamic acid) (γ-PGA) content because of many impurities in test specimens. It is necessary to establish a rapid and accurate method to quantify γ-PGA content. In this work, γ-PGA and some impurities commonly seen in fermented broth like glucose, glutamic acid and proteins were used to complex with copper ions. The results show that only γ-PGA can make copper ion precipitated, which content linearly correlates with the precipitate amount. From the study on the validity of the method, it is found that the accuracy and precision are 95.82% and 99.29%, much higher than the ones of method UV and weighing. Therefore, the method via the complexation of copper ion will be popularized to determine γ-PGA content in crude biological samples.

Blue light driven free-radical polymerization using arylazo sulfones as initiators

The polymerization of a broad range of electron-poor olefins has been achieved under free-radical conditions by using arylazo sulfones as visible light photoinitiators.

Biocompatible graft copolymers from bacterial poly(γ-glutamic acid) and poly(lactic acid)

Biocompatible graft copolymers from bacterial poly(γ-glutamic acid) and poly(lactic acid) are realized using a “grafting to” approach combined with click chemistry.

Recognition and selective extraction of poly-γ-glutamic acid based on molecular imprinting technology

Poly-γ-glutamic acid (γ-PGA) is one of the few bacterial polymers in nature with high added value of biodegradability. Especially, the traditional method of extracting γ-PGA is organic solvent extraction, etc., which has the disadvantages of low extraction rate and serious environmental pollution. With the expansion of γ-PGA industrial fermentation, an efficient and environmentally friendly method is required to be adopted. In this contribution, we report a novel method of separation of γ-PGA from fermentation broth based on molecular imprinting technology. The molecular imprinted polymer (MIP) was synthesized from chitosan (CS) and glutaraldehyde in the presence of γ-PGA. A nonimprinted polymer (NIP) was also synthesized by the same procedure in the absence of γ-PGA. The chemical structures and morphological structures of both MIP and NIP were examined by FTIR spectroscopy and scanning electron microscopy. The adsorption isotherms showed that the maximum adsorption capacity of MIP was 137.85 mg/g. The maximum adsorption capacity in the adsorption of NIP was 68.92 mg/g, which indicates that MIP shows specific selectivity for γ-PGA. A high saturated absorption capacity (Qmax=140.90 mg/g) was calculated from Freundlich isotherm equation. The imprinting factor of MIP was 4.76, indicating that MIP possess good recognition ability and selectivity for γ-PGA. The adsorption capacity decreased slightly (17.0%), which suggests the satisfactory reusability of γ-PGA after 5 cycles of reuse. Our study indicates that molecularly imprinted polymers present development prospects in the effective and selective separation of γ-PGA from fermentation broth compared with organic solvent precipitation.