Self-Healing Polymeric Soft Actuators.

Natural evolution has provided multicellular organisms with sophisticated functionalities and repair mechanisms for surviving and preserve their functions after an injury and/or infection. In this context, biological systems have inspired material scientists over decades to design and fabricate both self-healing polymeric materials and soft actuators with remarkable performance. The latter are capable of modifying their shape in response to environmental changes, such as temperature, pH, light, electrical/magnetic field, chemical additives, etc. In this review, we focus on the fusion of both types of materials, affording new systems with the potential to revolutionize almost every aspect of our modern life, from healthcare to environmental remediation and energy. The integration of stimuli-triggered self-healing properties into polymeric soft actuators endow environmental friendliness, cost-saving, enhanced safety, and lifespan of functional materials. We discuss the details of the most remarkable examples of self-healing soft actuators that display a macroscopic movement under specific stimuli. The discussion includes key experimental data, potential limitations, and mechanistic insights. Finally, we include a general table providing at first glance information about the nature of the external stimuli, conditions for self-healing and actuation, key information about the driving forces behind both phenomena, and the most important features of the achieved movement.

An air-tolerant polymer gel-immobilized iridium photocatalyst with pumping recyclability properties.

A novel methacrylate-based cross-linked polymer gel bearing an iridium photocatalyst showed air tolerance and pumping recyclability features through its tunable swelling and deswelling ability. The photocatalytic activity of the polymer gel was demonstrated through an E-to-Z isomerisation reaction and in an azide-alkene [2+3] cycloaddition.

A pH-Triggered Polymer Degradation or Drug Delivery System by Light-Mediated Cis/Trans Isomerization of o-Hydroxy Cinnamates.

A new methodology for the pH-triggered degradation of polymers or for the release of drugs under visible light irradiation based on the cyclization of ortho-hydroxy-cinnamates (oHC) to coumarins is described. The key oHC structural motif can be readily incorporated into the rational design of novel photocleavable polymers via click chemistry. This main-chain moiety undergoes a fast photocleavage when irradiated with 455 nm light provided that a suitable base is added. A series of polyethylene glycol-alt-ortho-hydroxy cinnamate (polyethylene glycol (PEG)n -alt-oHC)-based polymers are synthesized and the time-dependent visible-light initiated cleavage of the photoactive monomer and polymer is investigated in solution by a variety of spectroscopic and chromatographic techniques. The photo-degradation behavior of the water-soluble poly(PEG2000 -alt-oHC) is investigated within a broad pH range (pH = 2.1-11.8), demonstrating fast degradation at pH 11.8, while the stability of the polymer is greatly enhanced at pH 2.1. Moreover, the neat polymer shows long-term stability under daylight conditions, thus allowing its storage without special precautions. In addition, two water-soluble PEG-based drug-carrier molecules (mPEG2000 -oHC-benzhydrol/phenol) are synthesized and used for drug delivery studies, monitoring the process by UV-vis spectroscopy in an ON/OFF intermittent manner.

Aerobic Visible-Light-Driven Borylation of Heteroarenes in a Gel Nanoreactor.

Heteroarene boronate esters constitute valuable intermediates in modern organic synthesis. As building blocks, they can be further applied to the synthesis of new materials, since they can be easily transformed into any other functional group. Efforts toward novel and efficient strategies for their preparation are clearly desirable. Here, we have achieved the borylation of commercially available heteroarene halides under very mild conditions in an easy-to-use gel nanoreactor. Its use of visible light as the energy source at room temperature in photocatalyst-free and aerobic conditions makes this protocol very attractive. The gel network provides an adequate stabilizing microenvironment to support wide substrate scope, including furan, thiophene, selenophene, and pyrrole boronate esters.

Effect of Reaction Media on Photosensitized [2+2]-Cycloaddition of Cinnamates.

The outcome of photosensitized [2+2]-cycloaddition reactions of various cinnamates has been compared in different reaction media, including homogeneous organic solutions under inert conditions, degassed water, and aerated physical gels. The reactions were performed under LED blue light (λmax=455 nm) irradiation and [Ir{dF(CF3)ppy}2(dtb-bpy)]PF6 (1.0 mol%) as photocatalyst. The processes were optimized taking into consideration solvent, gelator, and substrate. Comparative kinetics analyses, as well as the effect of the reaction media on the diastereoselectivity of the process, were evaluated during this investigation. In a number of cases, carrying out the reaction in a less polar solvent, like toluene or highly polar solvent, like water had a tremendous impact on the diastereoselectivity of the process, pointing towards an effect on the stabilization of the putative diradical intermediate in this medium. Moreover, while for reactions run in homogeneous solution oxygen needs to be excluded, no erosion in yield is observed when the photoadditions were run in aerated gel media.

The Prospect of Photochemical Reactions in Confined Gel Media.

Nature is intrinsically able to control kinetics, conversion, and selectivity of biochemical processes by means of confined reaction environments such as enzyme pockets, bilayer membranes, micelles, vesicles, cells, or bioorganic frameworks. The main reason for this fact is the optimal molecular alignment and restricted motion of reactant molecules compared to those found in bulk solution. Under this inspiration, a number of synthetic photo-nanoreactors based on supramolecular self-assembled systems have been developed during the last decades, including mesoporous inorganic materials, microemulsions, micelles, vesicles, lipid bilayer foams, polyelectrolyte nanoparticles, etc. In a broader sense, nanoreactor technology constitutes nowadays a promising tool to enhance organic synthesis under sustainable reaction conditions. In general, nanoreactors change the essential properties of the molecules within them, thus affecting their chemical reactivity. Among the nanoreactor-like systems described in the literature to facilitate photochemical processes, the more recent use of viscoelastic supramolecular gels, typically made of low-molecular-weight (LMW) compounds self-assembled through noncovalent interactions, as compartmentalized reaction media is particularly appealing due to the versatility of these materials in terms of fabrication, properties, and processability. Furthermore, the high specific surface areas found in supramolecular gels, their stimuli-responsive reversibility, good diffusion properties enhancing the interactions between reactants and the three-dimensional (3D) porous network, functional tunability, and blocking effect of external oxygen are some of the most important features that can benefit photoinduced processes carried out in confined gel media. Not surprisingly, the efficiency of photochemical processes inside gel media is largely dependent on the type of reaction, characteristics of the gel network, solvent nature, reactant properties, and reaction conditions. Thus, the main focus of this Account is to provide a concise overview of the most relevant examples reported by us and others in order to illustrate the main advantages associated with the emerging use of gel-based materials as nonconventional reaction media to facilitate and control photochemical reactions. In particular, photodimerization, triplet-triplet annihilation upconversion (TTA-UC) coupled to single electron transfer (SET), photooxidation, photoreduction, and trifluoromethylation reactions will be illustrated during the discussion. These examples suggest that gel-based media can provide a versatile platform for the discovery of new reaction pathways and facilitate the way that photochemical reactions are traditionally carried out in academia and industry in terms of reaction conditions and required infrastructure. In addition, the use of physical or chemical gels as reaction systems may also accelerate high-throughput screening of photocatalysts. Overall, a judicious choice of gelators, reactants, solvent, and reaction conditions for the assembly of these gelators is crucial for controlling conversion, kinetics, and selectivity of intragel photoinduced processes.

Metal- and Oxidant-Free Photoinduced Aromatic Trifluoromethylation Performed in Aerated Gel Media: Determining the Effects on Yield and Selectivity.

In this work we have investigated the potential benefits of using supramolecular gel networks as reaction media to carry out air-sensitive metal-free light-induced trifluoromethylation of six-membered (hetero)arenes under aerobic conditions. This reaction was performed at room temperature (RT) using sodium triflinate (CF3SO2Na, Langlois' reagent) as a source of radicals and diacetyl as electron donor. The effects of confinement in gel media, concentration of reactants, and type of light source on yield and product distribution were evaluated and compared to the results obtained in homogeneous solution. Four different low molecular weight (LMW) gelators were employed in this study. The results confirmed the blocking effect of the gel medium against reaction quenching by external oxygen, as well as a certain control on the kinetics and selectivity.

Synthesis and supramolecular self-assembly of glutamic acid-based squaramides.

We describe the preparation and characterization of two new unsymmetrical squaramide-based organogelators. The synthesis of the compounds was carried out by subsequent amine condensations starting from dimethyl squarate. The design of the gelators involved a squaramide core connected on one side to a long aliphatic chain and on the other side to a glutamic acid residue. The gelator bearing the free carboxylic groups showed a lower gelation capacity than its precursor diester derivative. Some selected gels were further studied by infrared spectroscopy, rheology and electron microscopy. Critical gelation concentrations and gel-to-sol transition temperatures were also determined for each case. In addition, the superior squaramide diester gelator was compared with an analogue triazole-based gelator in terms of critical gelation concentration, gelation kinetics and thermal phase transition.

Air-Sensitive Photoredox Catalysis Performed under Aerobic Conditions in Gel Networks.

In this work, we demonstrate that useful C-C bond-forming photoredox catalysis can be performed in air using easily prepared gel networks as reaction media to give similar results as are obtained under inert atmosphere conditions. These reactions are completely inhibited in homogeneous solution in air. However, the supramolecular fibrillar gel networks confine the reactants and block oxygen diffusion, allowing air-sensitive catalytic activity under ambient conditions. We investigate the mechanism of this remarkable protection, focusing on the boundary effect in the self-assembled supramolecular gels that enhances the rates of productive reactions over diffusion-controlled quenching of excited states. Our observations suggest the occurrence of triplet-sensitized chemical reactions in the gel networks within the compartmentalized solvent pools held between the nanofibers. The combination of enhanced viscosity and added interfaces in supramolecular gel media seems to be a key factor in facilitating the reactions under aerobic conditions.