Advances in the field of supramolecular chemistry have made it possible, in many situations, to reliably engineer soft materials to address a specific technological problem. Particularly exciting are “smart” gels that undergo reversible physical changes on exposure to remote, non-invasive environmental stimuli. This review explores the development of gels which are transformed by heat, light and ultrasound, as well as other mechanical inputs, applied voltages and magnetic fields. Focusing on small-molecule gelators, but with reference to organic polymers and metal–organic systems, we examine how the structures of gelator assemblies influence the physical and chemical mechanisms leading to thermo-, photo- and mechano-switchable behaviour. In addition, we evaluate how the unique and versatile properties of smart materials may be exploited in a wide range of applications, including catalysis, crystal growth, ion sensing, drug delivery, data storage and biomaterial replacement.

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Gels with sense: supramolecular materials that respond to heat, light and sound

Nanogels and microgels are soft, deformable, and penetrable objects with an internal gel-like structure that is swollen by the dispersing solvent. Their softness and the potential to respond to external stimuli like temperature, pressure, pH, ionic strength, and different analytes make them interesting as soft model systems in fundamental research as well as for a broad range of applications, in particular in the field of biological applications. Recent tremendous developments in their synthesis open access to systems with complex architectures and compositions allowing for tailoring microgels with specific properties. At the same time state-of-the-art theoretical and simulation approaches offer deeper understanding of the behavior and structure of nano- and microgels under external influences and confinement at interfaces or at high volume fractions. Developments in the experimental analysis of nano- and microgels have become particularly important for structural investigations covering a broad range of length scales relevant to the internal structure, the overall size and shape, and interparticle interactions in concentrated samples. Here we provide an overview of the state-of-the-art, recent developments as well as emerging trends in the field of nano- and microgels. The following aspects build the focus of our discussion: tailoring (multi)functionality through synthesis; the role in biological and biomedical applications; the structure and properties as a model system, e.g., for densely packed arrangements in bulk and at interfaces; as well as the theory and computer simulation.

Nanogels and Microgels: From Model Colloids to Applications, Recent Developments, and Future Trends

Encapsulation, protection, and delivery of bioactive proteins and peptides using nanoparticle and microparticle systems: A review.

Designing biopolymer microgels to encapsulate, protect and deliver bioactive components: Physicochemical aspects.

Nanostructural heterogeneity in polymer networks and gels

In this work, the classical theory of polymer/polyelectrolyte gel swelling is reviewed. This formalism is easy to understand and has been widely applied to gels and microgel particles. Nevertheless, its limitations and obscure aspects should be known before use. The case of temperature-sensitive gels is discussed in some detail because it deserves particular clarification. The application to experimental swelling data (of both gels and microgels) is also reviewed. In this way, strengths and weaknesses of this approach can be elucidated. Moreover, other formalisms are also outlined. Many of them are inspired by the classical one. Their improvements are briefly commented in this case. Others are based on different grounds.

Gel swelling theories: the classical formalism and recent approaches

In this work, the effect of the counterion valence on the behavior of thermo-sensitive gels and microgels is studied through computer simulations. The polyelectrolyte gel is described within a bead–spring model and ions are explicitly considered. The thermo-shrinking behavior is simulated with the help of a phenomenological solvent-mediated polymer–polymer interaction that captures the essential features of experimental swelling data. Our results show that the swelling effect of multivalent species is smaller than in the case of monovalent counterions and therefore lower temperatures are required for the hydrophobic collapse of the polymer network. The reduction of the number of counterions when their valence is increased is responsible to a great extent for the smaller swelling effect. However, our simulations also reveal other mechanisms involved but completely ignored by most of the Flory–Rhener-inspired theories. For instance, the strong electrostatic interaction between multivalent counterions and th...

Effect of the Counterion Valence on the Behavior of Thermo-Sensitive Gels and Microgels: A Monte Carlo Simulation Study

Charge reversal in real colloids: Experiments, theory and simulations

The interaction between nanogels is a central question in the field of soft matter which has been scarcely studied. In fact, effective potentials for nanogels are less advanced than for other colloidal particles. The soft-sphere potential and the Hertz potential are two theoretical formalisms used until now by some authors to quantify forces between nano- and microgels. Accordingly, in this work we have performed explicit coarse-grained Monte Carlo simulations to find out if these models can capture the interactions between overlapping neutral nanogels. To this end, pairs of nanogels with different number of monomers per chain have been simulated, and the corresponding effective interaction potentials have been calculated as a function of the distance between their respective centers of mass. Then, our simulation results have been used to analyze the functional form of the soft-sphere and Hertz potentials. Both are too simple to describe nanogel interactions when these nanoparticles overlap to a significa...

Interaction between Ideal Neutral Nanogels: A Monte Carlo Simulation Study

In this work, the size-exclusion partitioning of neutral solutes in crosslinked polymer networks has been studied through Monte Carlo simulations. Two models that provide user-friendly expressions to predict the partition coefficient have been tested over a wide range of volume fractions: Ogston's model (especially devised for fibrous media) and the pore model. The effects of crosslinking and bond stiffness have also been analyzed. Our results suggest that the fiber model can acceptably account for size-exclusion effects in crosslinked gels. Its predictions are good for large solutes if the fiber diameter is assumed to be the effective monomer diameter. For solutes sizes comparable to the monomer dimensions, a smaller fiber diameter must be used. Regarding the pore model, the partition coefficient is poorly predicted when the pore diameter is estimated as the distance between adjacent crosslinker molecules. On the other hand, our results prove that the pore sizes obtained from the pore model by fitting partitioning data of swollen gels are overestimated.

José Alberto Maroto-Centeno

Papers

Gel swelling theories: the classical formalism and recent approaches

Effect of the Counterion Valence on the Behavior of Thermo-Sensitive Gels and Microgels: A Monte Carlo Simulation Study

Charge reversal in real colloids: Experiments, theory and simulations

Interaction between Ideal Neutral Nanogels: A Monte Carlo Simulation Study

Size-exclusion partitioning of neutral solutes in crosslinked polymer networks: a Monte Carlo simulation study.