1 Introduction
Condensed-phase chemistry takes as its object of study the condensed states of matter formed by the stable bonding relationships among elementary particles, which possess specific compositions, multi-level structures, and distinct properties and functions. It encompasses a wealth of conceptual content and a broad scope of application, serving as a powerful tool for explaining various chemical reaction phenomena and investigating chemical reaction mechanisms
［1-3］. One of the primary research focuses of condensed-phase chemistry is to analyze the composition and structure of materials using condensed-phase chemical principles, clarify the differences among various structural levels, establish correlations between structure and function, and thereby further promote the rational design of functional material systems
［4］. Selecting different material or structural systems and, based on their intrinsic characteristics, exploring the condensed-phase chemical concepts embedded within them constitutes an important approach for expanding the scope of condensed-phase chemical research
［5］. Although condensed-phase chemistry has found extensive applications across multiple fields of chemical research
［6-8］, choosing appropriate research systems and deeply integrating condensed-phase chemical concepts into them remains crucial for achieving a profound understanding of these systems and for further advancing their exploration
［9-13］.
As a typical class of material systems, gels generally exhibit a material state between liquid and solid and possess multi-level network structures that can vary with environmental conditions. These characteristics provide diverse reaction spaces and unique chemical microenvironments for chemical reactions. By adjusting their composition and structure, gels can be applied to various material systems
［14］. The aforementioned features make gels an ideal research platform for condensed-phase chemistry, enabling the conceptual framework of condensed-phase chemistry to be manifested in all relevant aspects of gel research. In this paper, taking the principles of condensed-phase chemistry as the core, we focus on discussing the application of condensed-phase chemistry in gel preparation strategies, the relationship between the multi-level network structure of gels and condensed-phase chemistry, and innovative methods and techniques for characterizing gel structures. Through these discussions, we aim to further elucidate the physicochemical properties of gels and their associated chemical reactions. Finally, by examining the typical applications of gel systems across different fields, we highlight the significance and value of condensed-phase chemistry. It should be noted that, due to the vast diversity of gel types, significant differences exist among them in terms of preparation, structure, and properties. Therefore, the understanding of gels from the perspective of condensed-phase chemistry presented herein is limited to certain representative gel systems and does not fully encompass all gel types. Moreover, although there are numerous reviews in the field of gel research
［15-16］that have elaborated in detail on the current state of development and the existing scientific challenges from various perspectives, no reports have yet appeared specifically addressing the intersection of gels and condensed-phase chemistry.
2 Gel Preparation Strategies and Condensed Matter Chemistry
In a gel system, the components are interconnected through forces of appropriate strength, which increases the viscosity and causes the loss of fluidity, resulting in a substance that exists between liquid and solid states. During this process, precise control of the intercomponent forces plays a decisive role in the successful construction of the gel system
［17］. When the interaction forces among the constituent particles are too strong, the components bind tightly, leading to the loss of the porous structure and elasticity, and transforming the system into a dense solid material. Conversely, if the forces are too weak, they cannot provide sufficient support to maintain the structural stability, causing the system to exist in a state close to that of a fluid. Therefore, developing simple methods to regulate these forces is a key factor in gel formation. Considering the impact of such forces on the system’s structure, introducing interactions of varying strengths into the system represents one effective approach to addressing this challenge. Drawing on the principles of condensed matter chemistry, basic particles such as atoms, ions, or molecules can be assembled into building blocks with diverse structures and functionalities through strong interactions like covalent and coordination bonds. Subsequently, by controlling the strength of the interactions among these building blocks, it becomes possible to prepare gel systems. For example, hydrogen bonding interactions can be introduced between the building blocks, and their number, strength, and spatial arrangement can be regulated via external stimuli, thereby enabling the fine-tuning of the gel’s structure and morphology
［18］. Within the gel system, the components inside each building block exhibit stable connections, ensuring that their internal structure remains largely unchanged during the preparation process. Consequently, only the interbuilding-block interactions need to be adjusted to fabricate gels tailored to specific requirements, significantly simplifying the fabrication procedure and enhancing its versatility. By adopting this strategy, the gel preparation process is divided into multiple stages; although this increases the number of synthetic steps, the staged regulation approach facilitates more precise control over both the synthesis pathway and the final product structure, laying a foundation for further functionalization of the gel. Moreover, the various types of building blocks prepared possess a certain degree of interchangeability: they can serve as intermediate products in reactions and be recombined with other building blocks to yield novel gel materials. For instance, the preparation of metal aerogels can proceed along two distinct pathways: a one-step method directly converting metal salts into a gel (Pathway 1), or a two-step approach involving the prior synthesis of metal nanoparticles followed by gel formation using an initiator (Pathway 2) (
Figure 1)
［19］. Although the two-step method entails additional synthetic steps, the intermediate metal nanoparticles produced can be further modified to alter the morphology of the resulting gel (Paths 3 and 4).
Depending on the type of gel, the structures of the gel building blocks also differ. For polymeric gels, polymer chains with different degrees of polymerization can serve as building blocks; they are connected internally via covalent bonds, resulting in a stable structure. The interactions among these building blocks are typically mediated by intermolecular forces such as hydrogen bonding and hydrophobic/hydrophilic interactions, which confer a certain degree of stimulus responsiveness. For example, a hydrogel formed by polyacrylamide and dodecyl glyceryl itaconate through hydrogen bonding between the building blocks exhibits strong self-healing properties and fatigue resistance
［20-21］. In contrast, for supramolecular gels, although the building blocks are also interconnected through noncovalent bonds, multiple strong cooperative interactions generally work together to maintain the stability of each building block, whereas the interactions among the building blocks themselves are relatively weak. This makes them more susceptible to changes under external stimuli, thereby endowing them with responsiveness. In the design and preparation of such gels, it is often necessary to prioritize the structural units within the building blocks. Once their stable existence is ensured, attention can then be turned to designing the interactions among the building blocks. By adopting this approach based on molecular-level assembly principles, the gel preparation process can be decomposed, enabling the facile fabrication of gels with specific functionalities.
3 Multiscale Networks of Gels and Their Characterization Methods and Techniques
3.1 Structure of the Gel and Condensed Matter Chemistry
The rationale behind the stepwise preparation strategy for gels stems from their multi-level structure. Although there are certain differences among various types of gel structures, in general, a gel’s structure can be likened to that of proteins, which is divided into primary, secondary, and tertiary structures. The structural dimensions span from angstroms and nanometers to micrometers (
Figure 2)
［22-23］. In the primary structure, the focus is on molecular-level bonding. The fundamental constituent particles of the gel—such as molecules, ions, and even atoms—are linked together via strong interactions like covalent bonds to form primary building blocks with specific structures, shapes, and sizes. These building blocks exhibit high internal structural stability and robust interparticle forces, enabling them to remain stable even under significant changes in external conditions. In the secondary structure, the primary building blocks assemble through relatively weaker intermolecular forces, giving rise to specific architectures such as fibers, micelles, vesicles, and layers. Compared with the primary structure, the secondary structure is less stable; its architecture can typically be disrupted or transformed by adjusting the temperature or the pH of the solution. Moreover, some secondary structures are concentration-dependent. For example, amphiphilic organic molecules aggregate in water, and the micelles formed near the critical micelle concentration undergo structural transitions to other secondary structures as the concentration increases
［24］. Under even milder environmental conditions, such as solvent adjustments, these secondary structures may entangle and cross-link with one another, leading to an increase in structural size from the nanoscale to the micrometer scale. This process endows the system with enhanced solvent-encapsulation capacity, ultimately resulting in gel formation. The type and strength of interactions among the secondary structures determine whether they will transition into a tertiary structure to form a gel, or whether excessive interparticle forces will cause the structure to become too compact, thereby losing its ability to accommodate solvent and directly precipitating out.The multi-level structural model of gels aligns well with the characteristics of condensed-phase materials in condensed matter chemistry, which possess specific compositions and hierarchical structures. This further underscores the application of condensed matter chemical principles within gel systems.
Although gels can be prepared through a variety of strategies and using numerous material systems, the resulting gel structures invariably exhibit several common characteristics. As a widely accepted concept, gels are regarded as possessing a permanent continuous structure and solid-like rheological behavior over analytical time scales
［25-26］. As an intrinsic feature of gels, whether they are hydrogels, organogels, ionic gels, or multiphase gels, they generally possess a three-dimensional cross-linked network structure that enables them to accommodate solvents. The presence of such network structures not only endows gels with enhanced extensibility and responsiveness but also improves their mechanical properties, allowing them to rapidly recover to their initial state even after being subjected to substantial loads. For most gel structures, the three-dimensional cross-linked network is hierarchical and multiscale, meaning that synergistic reinforcement mechanisms operate across various structural levels—from the molecular scale and microfibers to the macroscopic gel level. This hierarchical network architecture facilitates energy dissipation during loading across multiple dimensions, thereby effectively enhancing the load-bearing capacity of the gel. On the other hand, gel networks often exist in an interpenetrating form, further augmenting their strength. Within the cross-linked network system of a gel, cross-linking agents play a crucial role: they not only significantly influence the cross-linking density but also determine the interaction strength among the building blocks of the gel network, making them one of the decisive factors governing gel performance. In general, the rupture of covalent bonds results in permanent damage to the gel, whereas disruptions of intermolecular forces or dynamic covalent bonds can be repaired under specific conditions, thereby conferring self-healing capabilities on the gel.
3.2 Methods and Techniques for Structural Characterization of Gels
Due to the multilevel network structure of gels, with structural dimensions spanning from the angstrom scale to the micrometer and even millimeter scales, the characterization of gels encompasses the entire length scale range—from the microscopic to the mesoscopic and then to the macroscopic levels (
Figure 3). Given the broad size range and relatively disordered nature of gel structures, accurate characterization often requires the integration of multiple techniques. The structural information provided by each technique must be comprehensively analyzed and interpreted to ultimately achieve a clear understanding of the gel’s architecture
［27］. Regarding the intramolecular chemical bonds and intermolecular interactions within the basic building blocks of gel structures—such as covalent bonds, hydrogen bonds, and electrostatic interactions—these can be characterized and confirmed through shifts in the corresponding peak positions observed in Fourier transform infrared spectroscopy (FTIR) or Raman spectroscopy. For testing the relatively ordered assembled structures within the gel, several experimental methods are available. These include wide-angle and small-angle X-ray diffraction (collectively referred to as XRD), as well as wide-angle and small-angle X-ray scattering (W/SAXS), depending on the sample conditions and experimental setup. Additionally, electron microscopy serves as an important tool for characterizing gel morphology. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) can provide precise visualization of nanoscale morphologies within the gel, which can further be cross-validated with X-ray diffraction and scattering results. Cryo-transmission electron microscopy also offers technical support for in situ observation of the gel’s state. In contrast to TEM, scanning electron microscopy (SEM) provides a larger field of view, making it more suitable for elucidating the overall morphological features of the gel. It should be noted that the preparation of samples for electron microscopy typically involves a drying process, which may to some extent alter the original gel structure and pore size; this factor must be appropriately considered during structural analysis.At a more macroscopic scale, optical microscopy (OM) can be used to observe gels, particularly the swelling behavior of hydrogels and organic gels, thereby providing insights into the degree of crosslinking and other relevant parameters. Such observations can serve as a reference for the further materialization and functional applications of gels.
In addition to traditional characterization methods, more sophisticated techniques for characterizing subtle structural changes within gel systems have also been gradually developed. For example, to dynamically capture the kinetic processes of rapid gelation and the anisotropic behavior of colloidal local structures, a near-interface rheological characterization method based on micrometer-scale magnetic probes can be employed. This approach enables observation, at smaller scales and with higher speed and precision, of how locally formed cluster structures in the early stages of gelation influence the viscoelastic properties of the gel
［28］. Furthermore, the relationship between particle Brownian motion velocity and particle size established through dynamic light scattering is another important means of elucidating gel structure. Recently, this technique has been further utilized to reveal the dynamic association and dissociation of weak bonds within complex gel systems, thereby establishing universal multi-level microscopic kinetic laws and developing methods for characterizing the strength of weak bonds and crosslinking density in gels
［29］.
4 Physical and Chemical Properties and Chemical Reactions of Gels
4.1 Physical and Chemical Properties of the Gel
Gels possess a multi-level network structure and an intermediate state between solids and liquids, enabling the integration of multiple incompatible components into a single phase and imparting novel chemical characteristics. These features also determine the thermodynamic properties and mechanical performance of gels. In terms of thermodynamic properties, the composition of a gel—including the types of building blocks and the cross-linking density—is closely related to its thermal stability, which in turn dictates the temperatures at which the gel undergoes decomposition, shrinkage, or swelling. Specifically, above the glass transition temperature, gels generally exhibit softness and elasticity, whereas below this temperature they assume a hard and brittle glassy state. From the perspective of condensed matter chemistry, this behavior primarily arises from the cohesive forces among the building blocks assembled through stable adhesive interactions, rather than from the direct contributions of fundamental units such as molecules or ions. Furthermore, the glass transition temperature of a gel is associated with its heat capacity: when the heat capacity is high, greater amounts of heat are required for heating or cooling, thereby further influencing the phase transition behavior of the gel. From an energetic standpoint, both entropy and enthalpy undergo concomitant changes during the phase transition of a gel. Quantitative assessment of the entropy and enthalpy changes across different stages of the gel's transformation can facilitate a more comprehensive elucidation of the underlying mechanisms governing these transitions.
The mechanical properties of gels are fundamental to their functionalization. These properties are determined by the gel’s compositional structure and exhibit a mutually corroborative relationship with that structure
［30］. For instance, for a gel with a specific composition, its mechanical performance can be controlled by cross-linking density, which can further be enhanced through thermal polymerization, photopolymerization, or catalytic polymerization. Rheological testing is an important method for characterizing gel mechanical properties; stress–strain curves enable the quantitative determination of viscoelastic parameters such as the storage modulus and loss modulus, as well as key indicators like viscosity and yield stress. By utilizing these parameters, one can further elucidate the balance between energy storage and dissipation, shear behavior, and stimulus responsiveness of the gel, thereby facilitating a deeper understanding of the relationship between structure and performance. As an important mechanical property of gels, viscoelasticity can also be characterized by parameters including Young’s modulus, fracture stress, and fracture strain, which can be obtained through uniaxial tensile testing. Meanwhile, the compressibility of a gel can be represented by the compression modulus, while fatigue resistance can be assessed via tensile–relaxation cycles. If the gel is too thin to undergo tensile or compressive testing, its toughness and fatigue resistance can be evaluated using puncture tests or nanoindentation. In summary, the physicochemical properties of a gel originate from the types and strengths of interactions among its constituent components—that is, the modes and intensities of bonding between structural building blocks in condensed-phase chemistry. Consequently, studying the physicochemical properties of a gel can, to a certain extent, reveal its structural characteristics.
As an important complement to experimental methods, theoretical computational simulations also occupy a significant position in gel research. Commonly used methods include density functional theory based on quantum mechanical principles, Monte Carlo simulations that obtain the statistical laws of a system through random sampling, and molecular dynamics simulations based on Newton’s laws of motion. Theoretical computational simulation methods can be applied throughout all stages of gel research, including formulating synthesis strategies, completing structural analysis, achieving force regulation, and predicting mechanical properties. Through computational simulations, the composition of gels can be optimized, the lowest-energy-state structural forms can be determined, and the gel synthesis process can be guided. Furthermore, computational simulations can be used to investigate the interaction strengths among different chain segments within a gel and the relative contributions of various forces to maintaining the gel structure. In the computational simulation of gels, the use of coarse-grained models not only greatly reduces the computational workload but also preserves the interaction characteristics among particles and key structural features. Regarding the study of gel properties, computational simulations can provide insights into the responsiveness of gels under different external forces, thereby enabling the calculation of mechanical parameters such as the elastic modulus, hardness, and flexibility. During the computational simulation process, fully considering the differences between liquid and solid phases within the gel system, as well as the characteristics of interfacial forces between them, is expected to further enhance the accuracy of the simulations.
4.2 Chemical Reactions in the Gel System
Compared with chemical reactions conducted under liquid, solid, and gaseous conditions, those in gel systems exhibit unique characteristic properties. Due to the much lower fluidity of gel systems relative to liquids, the mobility of reactant molecules is significantly restricted, resulting in a marked decrease in diffusion rates within the system (
Figure 4a). Furthermore, influenced by the pore size of the gel network, there are certain limitations on the dimensions of reaction substrates; however, this also enhances the selectivity of specific reactions by suppressing the formation of large-sized products. Another distinctive feature of chemical reactions in gels is that, owing to the restricted movement of reactants, local concentration gradients can develop within the gel, thereby affecting reaction kinetics and even altering the reaction pathway. Although environmental factors cause chemical reactions in gel systems to proceed relatively slowly, this slower pace provides an opportunity to observe the reaction process in greater detail, potentially enabling the detection of more reaction intermediates and facilitating a deeper understanding of the reaction mechanism. Despite these advantages, products formed in gels may become entrapped within the cross-linked network, making their release from the system difficult and thereby hindering product recovery and purification. In summary, the unique characteristics of gel systems offer a distinct reaction environment for chemical processes, and through careful design of the reaction system, it may be possible to gain new insights into chemical reactivity. The features and applications of gel systems in chemical reactions underscore the importance of the system itself in condensed-phase chemistry, as such systems can give rise to properties that individual constituent building blocks do not possess.
Another typical approach to applying gel systems in chemical reactions is to use the gel either as a catalyst support or as the catalyst material itself to catalyze chemical reactions (
Figure 4 b)
[31–32]. Due to the inherent characteristics of gels—low density, large specific surface area, high porosity, low cost, high light transmittance, and facile thermal deformation—they are more suitable for photo-driven reactions than thermally driven ones. Employing photocatalysts in gel form not only effectively prevents catalyst aggregation, thereby enhancing catalyst stability and utilization efficiency, but also facilitates catalyst recovery and reuse.
5 Application of Gels
The gel system possesses a rich composition, unique structure, and distinctive physicochemical characteristics, which endow it with excellent application potential as a functional material across multiple domains. Expanding the application scope of these materials represents one of the important frontier scientific issues in condensed matter chemistry. This section will showcase the applications of gel materials in four areas: tissue engineering, drug delivery, human–machine interfaces, and environmental engineering. It is worth noting that the application fields of gels are extremely broad, far beyond the aforementioned examples. For instance, aerogels prepared via cross-linking of hollow glass microspheres exhibit outstanding mechanical strength and high thermal insulation performance
［33］, making them widely used in pipeline insulation; gels constructed through the coordination interaction between anions and pyridine demonstrate rapid responsiveness to anions, pH values, and gases, classifying them as important smart soft materials
［34］; furthermore, hydrogels assembled by intermolecular forces between polyoxometalates and polysaccharides exhibit pronounced Keton effect and multi-responsive behavior, enabling their use in chiral gel materials
［35］. Here, only a few typical examples are presented to illustrate the applications of gels and their relationship with condensed matter chemistry.
5.1 Applications of Gels in Tissue Engineering
Using tissue engineering approaches to address tissue damage and facilitate tissue repair is an important emerging modality in the field of therapy
［36］. Through this regenerative medicine strategy, biomaterials and cells are employed to create favorable conditions for tissue regeneration, which more closely mimics the body's natural healing process and effectively reduces the risk of immune rejection
［37-38］. As an important class of biomaterials, hydrogels exhibit excellent biocompatibility. The water confined within their structure exists in a liquid condensed state with low fluidity, which facilitates the transport of substances and information among cells embedded within the gel, thereby providing a suitable environment for cell growth and proliferation
［39］. More importantly, hydrogels possess good water-absorbing properties, enabling them to mimic the hydration characteristics of in vivo tissues and absorb and retain large amounts of water. By injecting liquid hydrogel into the body and allowing it to gradually solidify into a stable structure, damaged tissues can be effectively repaired via a minimally invasive approach
［40］.
Taking rat skull injury repair as an example, a glycol–peptide copolymer (Blk-Gel) was used as the gel material (
Figure 5a). Its aqueous solution is a free-flowing sol at low temperatures; upon increasing the temperature, a sol–gel phase transition occurs, and it forms a non-flowing hydrogel at body temperature (
Figure 5b)
［41］. Leveraging the phase-transition properties of the gel material, normal extracellular vesicles (Norm-EVs) and hypoxic extracellular vesicles (Hypo-EVs) were loaded into the gel, respectively, and then injected into the injured skull site of rats. As shown in
Figure 5c, after 12 weeks of treatment, skull tissues were collected for computed tomography analysis. The results indicated that, compared with the control group and the group treated with gel alone, the gels loaded with Norm-EVs and Hypo-EVs exhibited superior therapeutic effects, with Hypo-EVs demonstrating the strongest bone-repair capability. The enhanced bone-tissue-repair ability of Hypo-EVs stems from their hypoxic characteristics, which closely resemble the actual bone-marrow microenvironment, thereby promoting more effective bone repair. Throughout the entire repair process, the hydrogel, as a condensed state situated between liquid and solid, not only improved cellular stability but also enhanced its fluidity to better cover the damaged bone tissue, thus accelerating the bone-repair process.
5.2 Applications of Gels in Drug Delivery
The sol-gel transition characteristics of gels have led to their widespread application in the field of tissue engineering, while another typical feature of gels—stimuli responsiveness—further facilitates their use in drug delivery
［42］. Gels possess a strong loading capacity; regardless of the molecular weight, solubility, or origin (synthetic or natural) of the drug molecules, they can be loaded into gels via physical encapsulation or chemical bonding
［43］. More importantly, through rational design, different types of drugs can be loaded into gel systems without mutual interference, thereby achieving dual drug loading. Once the drug-loaded gel reaches a specific site in the body and is subjected to stimulation, it releases the drugs, thus realizing precise drug delivery. The stimuli responsiveness of gels originates from the differences in intermolecular forces within the condensed phase. Compared with the strong interactions among building blocks, the weaker interactions between building blocks can change under certain conditions, thereby affecting the solvent-encapsulation ability of the gel and releasing the loaded substances. The hierarchical structure of gels enables them to control the release of drug molecules at different rates, allowing for regulation under multiple stimuli or through multistage feedback mechanisms
［44］. In addition to stimuli responsiveness, other properties of hydrogels also play an important role in the process of drug delivery and release and are crucial considerations in the preparation of gel-based drug carriers
［45］. For example, the hydrophilicity and swelling behavior of gels determine the possible routes by which they can enter the body. High hydrophilicity and swelling capacity help reduce the adsorption of gels onto proteins or platelets in vivo, thereby prolonging their circulation time within the body
［46］.
Due to the advantages of gels in drug delivery systems, they have been employed for the treatment of foot ulcers caused by diabetes. Researchers prepared a hyaluronic acid-based hydrogel dressing with photothermal/photodynamic synergistic antibacterial activity and dual pH/glucose responsiveness
［47］. This dressing was fabricated via a double dynamic cross-linking mechanism involving Schiff base and phenylboronic acid linkages, and it was loaded with a polydopamine-reduced graphene oxide–glycine-modified fullerene composite (GPC) that exhibits excellent photothermal/photodynamic synergistic antibacterial efficacy, as well as pirfenidone (PFD), a drug known to promote angiogenesis and suppress inflammation. As shown in the inset of
Figure 6a, under low-pH and high-glucose conditions, the hydrogel rapidly dissociated, and the release of the dye resulted in a darker solution color. Under these conditions, the release rate of PFD from the hydrogel was higher than that observed under normal physiological conditions (pH = 7.4) (
Figure 6a). The pH- and glucose-responsive behavior of the gel originates from the facile dissociation of the phenylboronate ester and Schiff base structures under glucose-rich and acidic conditions, thereby facilitating the accelerated release of PFD. Upon near-infrared light irradiation, GPC endows the hydrogel with photothermal/photodynamic synergistic antibacterial properties; meanwhile, the tissue-adhesive and antioxidant characteristics provided by the catechol moiety within GPC also play an important role in the treatment of affected tissues. Evaluations of wound closure (
Figure 6b) and inflammatory responses (
Figure 6c) indicate that the hydrogel significantly promoted the healing of infected areas by reducing inflammation and enhancing angiogenesis.
The above studies have shown that hydrogels, as carriers with excellent biocompatibility, can load various functional components including drugs. Their good mechanical strength and self-healing properties ensure efficient drug delivery, while their stimulus-responsive characteristics enable targeted drug release at specific sites. This significantly reduces the impact of drugs on non-diseased tissues and enhances therapeutic efficacy.
5.3 Applications of Gels in the Field of Human–Machine Interfaces
The diverse properties of hydrogels determine their potential applications across different fields. Compared with the high biocompatibility and stimulus responsiveness required in tissue engineering and drug delivery, applications in human–machine interfaces place even higher demands on hydrogel properties
［48-49］. A human–machine interface is an interactive channel between the human body and machines established based on a series of communication protocols, and it requires personalized design tailored to specific applications and biological tissues
［50］. According to the interaction mechanisms between humans and machines, the hydrogel interface in human–machine interfaces should possess at least the following functional modes: mechanical, acoustic, electrical, optical, chemical, and biological. Among these, mechanical flexibility, strong adhesion, and durability facilitate seamless integration between humans and machines; acoustic and electrical characteristics ensure efficient energy transmission and enhanced signal stability; optical responsiveness enables controlled light responses; chemical selectivity promotes improved molecular recognition capabilities; and high biocompatibility guarantees long-term stability of the interface as well as interactions with cells. The aforementioned functionalities of hydrogels originate from their condensed-state properties that lie between those of solids and liquids, allowing hydrogels to maintain structural integrity while integrating multiple functional elements, thereby enabling their application in the field of human–machine interfaces
［51］.
Due to the influence of material impedance on human–machine interaction performance, smaller-sized neural electrodes are typically employed to facilitate better integration with surrounding tissues and to more closely mimic brain tissue behavior. To achieve this goal, axial-aligned nanofibers were fabricated using a molecular peptide (betaVhex) as a building block, which were then combined with carbon nanotubes (CNTs) to form a helical composite (
Figure 7a)
［52］. This composite creates a uniform three-dimensional electrical network within the hydrogel, thereby reducing signal loss and enhancing conduction. Moreover, the multilayered complexity of neural networks is well reflected in the hierarchical structure of the designed hydrogel, providing an effective interface for amplifying neural oscillations across all frequency bands. The prepared hydrogel increases the contact area between the electrode and the tissue, enabling tight tissue–hydrogel coupling; during epileptic seizures in patients, both signal amplitude and frequency are enhanced (
Figure 7b). The impedance of the hydrogel is determined by the concentration of carbon nanotubes in the system and is independent of its dimensions, making it suitable for fabricating multifunctional interfaces with broad contact areas, particularly for high-frequency brain signal recording. Thanks to the hydrogel’s high mechanical strength and its viscoelastic properties similar to those of brain tissue, neuronal and vascular structures can maintain their structural integrity even after cortical injection. Furthermore, the biocompatible hydrogel remains stably positioned on the cortical surface and within the brain tissue for several months without eliciting any significant inflammatory response, demonstrating the long-term reliability of the hydrogel-based neural interface and supporting its applicability for monitoring neural signals under various brain states (
Figure 7c).
5.4 Application of Gels in Water Treatment Field
Hydrogels possess a high water content, a three-dimensional network structure, and solid-like properties, making them ideal flexible substrates for photothermal materials
［53］. Compared with other substrate materials, the tunable physicochemical properties and diverse structures of hydrogels not only enrich but also simplify their functionalization process. By rationally designing the pore size, pore distribution, and hydrophilicity/hydrophobicity of hydrogels, the state of water molecules within the gel can be effectively regulated, reducing or eliminating hydrogen bonding among large water clusters, thereby decreasing the enthalpy change of water evaporation and accelerating the phase transition from liquid to gas
［54］. As a matrix material for water evaporation, the porous network structure of hydrogels can enhance their ability to continuously transport water from the bottom layer to the top
［55］. Furthermore, through meticulous design of polymer chains, layered structures, or specific sites within the gel, it is possible to fabricate stable internal building-block structures with reversible inter-building-block connections, resulting in robust mechanical strength and self-healing properties that ensure the sustained stability of hydrogels during the evaporation process
［56］.
In traditional water evaporators, the anti-fouling performance of water evaporation membranes is often enhanced by increasing the flow rate. However, this strategy leads to substantial heat loss and reduces the material's evaporation efficiency. To address this issue, researchers have designed a dual-network hydrogel (DNH) with an alternating pore-size layered structure. By employing a multi-mechanism synergistic crosslinking process, water evaporation membranes with dimensions up to 10 cm × 10 cm can be fabricated (
Figure 8a)
［57］. TEM images reveal uniformly distributed hierarchical network channels within the gel, indicating homogeneous gelation of the hydrogel (
Figure 8b). By fine-tuning the amide groups in the hydrogel network, the state of water can be altered, effectively activating it and significantly reducing the enthalpy of vaporization. This innovative design not only enhances light absorption but also optimizes the continuous water supply pathway to the evaporation surface. As a result, the membrane can maintain a high evaporation rate even after 12 hours of continuous operation in a high-salinity solution with a mass fraction of 25% (
Figure 8c). After evaporation, the concentrations of various ions in the water are markedly reduced, and all concentration indicators meet or exceed the relevant standards set by the World Health Organization (WHO) (
Figure 8d).
Despite the obvious advantages and improved performance of hydrogel-based interfacial solar-driven evaporation, further fundamental research is still needed to fully elucidate the roles of hydrogels in energy, mass, and momentum transfer during this process. Such studies will deepen our understanding of the underlying mechanisms and enhance evaporation efficiency. From the perspective of condensed matter chemistry, the water evaporation process involves a transition from the liquid condensed state to the gaseous state. By modulating the interaction strength between water molecules and the gel interface, it becomes more favorable to weaken the hydrogen bonding within water molecules, reduce the enthalpy change, and thereby improve evaporation efficiency.
6 Conclusion and Outlook
Gels are materials characterized by multi-level network structures formed through the synergistic interplay of multiple types of interactions among building blocks with diverse architectures and functionalities. This highly aligns with the research focus of condensed matter chemistry, which centers on condensed states—materials composed of fundamental particles that coalesce via stable bonding interactions to form specific compositions, multi-level architectures, and distinct properties and functions. Consequently, gels represent one of the ideal systems for studying condensed matter chemistry. By applying the principles of condensed matter chemistry, it is possible to standardize gel preparation strategies, thereby facilitating the precise synthesis of gels with tailored structures and functionalities. Furthermore, the tertiary network architecture of gels reflects the differences in interaction strengths among various building blocks, as well as the resulting variations in the robustness of their interconnections. These disparities in intermolecular forces between different components not only determine the functional characteristics of gels but also necessitate a diverse array of characterization methods and techniques. The rich structural composition, fine-tuned force regulation, and intricate network topology of gels have enabled their widespread application as multifunctional materials in fields such as tissue engineering, drug delivery, and environmental science. Adopting the conceptual framework of condensed matter chemistry to understand gel systems across their preparation, structure, characterization, properties, and applications has not only advanced the development of gel systems themselves but has also further expanded and deepened the scope and implications of condensed matter chemistry.
Despite some encouraging advances in the application of condensed matter chemistry to gel systems, there remains substantial room for further research in this field. In designing and preparing gel materials with specific functionalities, precise control over sensitive experimental conditions—such as temperature, concentration, and pH—is often required. This process not only heavily relies on researchers' expertise but is also prone to experimental errors, thereby reducing the reproducibility of material property data. By contrast, artificial intelligence methods, particularly machine learning techniques like neural networks, can rapidly extract valuable insights from existing datasets, effectively enhancing the accuracy of synthesis and the predictability of material performance. Drawing on the principles of condensed matter chemistry and accelerating the development of efficient synthetic strategies, AI-driven approaches can significantly reduce the cost of gel material preparation while promoting the rational design and optimization of novel gel materials. In terms of processing applications, gel materials fabricated through conventional methods typically require multiple steps for customization, which not only increases material application costs but also results in lower processing accuracy and difficulty in achieving rapid, high-quality fabrication. Leveraging the inherent fluidity of gels, 3D printing technology enables fast, efficient, and precise shaping of gel materials, substantially expanding their application scope while simultaneously lowering production costs.
the National Natural Science Foundation of China(22271117)
the Natural Science Foundation of Jilin Province(20250102065JC)
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