Advances in hydrogels for capturing and neutralizing inflammatory cytokines

Abstract

Inflammatory cytokines play a crucial role in the inflammatory response, and their aberrant expression and overproduction are closely associated with the development of many diseases. However, traditional inflammation treatment strategies are often accompanied by serious side effects, limiting their widespread use. In recent years, hydrogel, as a material with a three-dimensional network structure, good biocompatibility and modulability, has great potential for trapping and neutralizing inflammatory factors. Hydrogels can capture and neutralize inflammatory cytokines through various mechanisms such as electrostatic interactions, coupling with cytokine antibodies or binding nanoparticles. In addition, hydrogel microspheres, an important form of hydrogels, have excellent broad-spectrum binding of inflammatory cytokines in combination schemes with cell membranes. This article reviews recent research advances in hydrogel capture and neutralization of inflammatory cytokines, discussing the advantages of various mechanisms and their applications in different diseases. Overall, we believe that hydrogels are expected to further advance the development of therapeutic modalities for inflammatory diseases in the future.

Introduction

Cytokines are small-molecular-weight proteins synthesized and secreted by immune cells and some non-immune cells. Among them, inflammatory cytokines specifically refer to those cytokines involved in the inflammatory response, such as IL-1 and TNF-α. IL-1 binds to the IL-1 receptor, activating NF-κB and MAPK signaling pathways to promote NLRP3 inflammasome assembly and the and IL-6. ¹ TNF-α is mainly produced by activated macrophages. It binds to TNFR1/TNFR2 receptors and activates signaling pathways such as NF-κB, thereby regulating processes like apoptosis and the inflammatory cascade.^2,3 In the intricate network of life activities, they play numerous crucial roles and perform diverse functions such as regulating the inflammatory response and promoting tissue repair. However, an increasing body of evidence indicates that the imbalance of inflammatory cytokines is associated with a wide range of diseases.^4–6 For example, in rheumatoid arthritis, IL-1 and TNF-α can promote the differentiation and activation of osteoclasts, which in turn leads to bone loss. They can also stimulate chondrocytes and synovial fibroblasts to secrete degrading enzymes (such as cathepsins and matrix metalloproteinases), resulting in cartilage degradation.^7,8 During the occurrence of sepsis, when the body’s immune cells are stimulated by pathogen-associated molecular patterns and damage-associated molecular patterns, cytokines such as IL-1 and TNF-α will be released. These cytokines can activate transcription factors like nuclear factor-κB, inducing the release of a variety of inflammatory factors such as IL-6 and IL-8, thus triggering an inflammatory cascade reaction and exacerbating systemic inflammatory symptoms. At the same time, these cytokines will induce vascular endothelial cells to express adhesion molecules, affecting the balance between procoagulant and anticoagulant proteins. In severe cases, it can lead to disseminated intravascular coagulation and multiple organ failure. ⁹ Additionally, Inflammatory cytokines are crucial in the pathogenesis of chronic inflammatory diseases, such as cardiovascular diseases,^10,11 diabetes, ¹² and autoimmune disorders,^13,14 which are major global contributors to mortality. ¹⁵ This serious health challenge highlights the need and urgency of accelerating the development of therapeutic approaches targeting inflammatory cytokines.

Traditional methods for treating inflammation, such as nonsteroidal anti-inflammatory drugs (NSAIDs), glucocorticoids, and biologics, have many limitations.^16–18 First, non-specific effects are one of the main problems. NSAIDs and glucocorticoids lack selective inhibition of specific inflammatory cytokines, and their wide range of mechanisms of action makes it difficult to precisely target inflammatory cytokines, which may inhibit the inflammatory response while negatively affecting normal physiological functions. For example, in the treatment of rheumatic immune diseases such as rheumatoid arthritis and systemic lupus erythematosus, the long-term use of glucocorticoids can interfere with the body’s metabolism, giving rise to conditions like elevated blood pressure, abnormal blood lipid levels, increased blood glucose, and central obesity.^19,20 Second, these drugs have significant adverse effects. For instance, NSAIDs frequently result in gastrointestinal disorders, which encompass nausea, vomiting, ulcers, and even gastrointestinal bleeding. Moreover, in certain patients, NSAIDs can cause coagulation function abnormalities or hepatic and renal damage.^21,22 In the treatment of sepsis, while glucocorticoids can inhibit excessive inflammation, they may also suppress the immune response, causing the body to be in a state of “immune paralysis,” which in turn increases the risk of infection. ²³ Immunosuppressants are usually administered subcutaneously or intravenously, and their effects are difficult to be confined to a specific range, which inhibit normal immune function and increase the risk of infection and certain tumors. Although biological agents are more targeted, their rapid clearance in the body and limited maintenance of efficacy, coupled with high production costs, have limited their application. Consequently, this has led to the development of novel, precise, and effective anti-inflammatory therapeutic strategies aimed at surmounting the limitations of conventional approaches.

In recent years, the application of hydrogels in anti-inflammatory therapy has witnessed rapid development. Its core advantages lie in its highly tunable physicochemical properties and excellent biocompatibility. However, currently, the application of hydrogels in the field of anti-inflammation mainly focuses on three directions: Firstly, hydrogels serve as drug sustained-release carriers, loading anti-inflammatory drugs (such as celecoxib and dexamethasone) to achieve local precise controlled release.^24,25 Secondly, hydrogels load bioactive substances such as cells and exosomes to regulate macrophage polarization and promote the resolution of inflammation.^26,27 Thirdly, hydrogels act as physical barriers to isolate danger signals from damaged tissues and reduce stimulation.^28,29 In terms of capturing and neutralizing inflammatory cytokines, hydrogels exhibit unique advantages: (I) High-efficiency capture ability: by chemically coupling with antibodies, hydrogels can specifically capture inflammatory cytokines (such as IL-6), significantly increasing the action time and utilization rate of the antibodies. ³⁰ (II) Local precise action: for example, in-situ acting hydrogels can continuously capture inflammatory cytokines through electrostatic interactions, avoiding the risk of systemic suppression. ³¹ (III) Ability to adapt to complex inflammatory microenvironments: generally speaking, the inflammatory response involves multiple inflammatory cytokines, and anti-inflammation through a single pathway is often unsatisfactory. Whether hydrogels capture inflammatory cytokines through electrostatic interactions or bind to cell membranes to capture inflammatory cytokines, they have a stronger ability to adapt to complex inflammatory microenvironments.^32,33

In view of the important role of inflammatory cytokines in diseases, the mechanisms by which hydrogels capture inflammatory cytokines are mainly divided into four types (Figure 1). First, hydrogels can capture and neutralize inflammatory cytokines through their physical properties. Most typical hydrogels contain glycosaminoglycan (GAG) ingredients, which utilize electrostatic interactions with inflammatory cytokines to capture inflammatory cytokines.^31,32 Second, hydrogels can be conjugated with monoclonal antibodies to specifically capture and neutralize inflammatory cytokines, reducing the intensity of the inflammatory response, and improving the controllability, duration, and safety of biologics. ³⁴ Third, the combination of hydrogels and cell membranes allows for broad-spectrum capture and storage of inflammatory cytokines. In addition, hydrogels can also participate in the regulation of inflammatory cytokines by binding to nanoparticles (NPs). ³⁵ In summary, these strategies provide diverse approaches and potential clinical application prospects for the treatment of inflammation-related diseases.

Figure 1.

Four mechanisms of inflammatory cytokine capture by hydrogels. Image was drawn using BioRender.

However, despite the great potential of hydrogels in capturing and neutralizing inflammatory factors, there is still a lack of comprehensive reviews on their application in this regard. This paper aims to review recent progress in hydrogel-based inflammatory cytokine capture and neutralization, analyze the mechanisms and advantages of different strategies (Figure 2), explore the impact of various materials and structures, and discuss future applications in treating inflammatory diseases. Overall, we believe that hydrogels exhibit unique advantages and potential in capturing and neutralizing inflammatory cytokines. In the future, they are expected to become powerful tools for capturing inflammatory cytokines, providing more effective solutions for the treatment of diseases associated with inflammatory cytokines.

Figure 2.

Overview of hydrogel materials for capturing and neutralizing inflammatory cytokines. Image was drawn using BioRender.

Inflammation overview

Concepts and physiologic functions of inflammation

Inflammatory processes are similar to chemical equilibrium (Figure 3), and once this equilibrium is disturbed, the system will tend to recover or deviate further from homeostasis.^36,37 However, its underlying purpose is to enable the body to adapt to environmental changes and maintain physiological stability through integrated regulatory response.^38,39 Generally, the physiological functions of inflammation mainly include three aspects: defending against the invasion of pathogens, removing and repairing damaged tissues, and enabling the body to adapt to stress and restore the state of homeostasis. Inflammatory mediators are key drivers of the inflammatory process, including chemokines, vasoactive amines, cytokines (e.g. TNF-α, IL-6), and other chemicals. These mediators not only participate in the inflammatory response by activating immune cells, altering vascular permeability, and modulating the immune response in a variety of ways, but also interact to form a complex regulatory network that determines the nature and outcome of the inflammatory response.^38,40

Figure 3.

Inflammatory response and balance of inflammatory cytokines. Image was drawn using BioRender.

Inflammatory responses are usually divided into two categories: acute and chronic inflammation. Acute inflammation is the body’s rapid response to infection or injury, typically self-limiting, aimed at eliminating external stimuli and repairing damaged tissues. ⁴¹ During the early stages of acute inflammation, inflammatory cytokines such as TNF-α and IL-6 are rapidly released. These cytokines not only activate immune cells (e.g. neutrophils, macrophages) but also increase vascular permeability, facilitating immune cell infiltration and pathogen clearance. The role of cytokines is particularly critical at this stage, as they play a bridging role in initiating the immune response and driving tissue repair. ⁴² If the inflammatory response is not promptly resolved or persists for too long, it may progress to chronic inflammation. Chronic inflammation is characterized by an extended activation period of immune cells and the continuous release of pro-inflammatory cytokines. ⁴³ This ultimately leads to an imbalance in inflammatory cytokines, resulting in persistent tissue damage and pathological changes.

Mechanisms of inflammatory cytokines production and pathogenesis

Inflammatory cytokines serve as regulators of the immune response and key players in pathophysiological processes. These cytokines are typically secreted by immune cells, such as macrophages, T cells, and neutrophils, in response to exogenous stimuli (e.g. pathogens) and endogenous signals (e.g. danger-associated molecular patterns released by damaged tissues). Additionally, some studies have shown that probiotics and diet can also regulate the secretion of cytokines.^44–46 They work in conjunction with other inflammatory mediators to drive the initiation, progression, and resolution of inflammation. However, when the inflammatory cytokines in the body are unbalanced, they can become the pathogenic factors of various diseases. A large amount of evidence has shown that inflammatory cytokines are closely related to the occurrence and development of diseases (Table 1).

Table 1.

Properties of common inflammatory cytokines and their related diseases.

Name	Receptor	Related diseases	Characteristic
IL-1α	IL-1R1 IL-1R2	Rheumatoid arthritis11,13 Myocardial infarction 47 Leukaemia 48 Inflammatory bowel disease49–51	Proinflammatory
IL-1β	IL-1R1 IL-1R2	Rheumatoid arthritis52–54 Alzheimer’s disease55–57 Neoplasms58–60 Atherosclerotic61–63 Inflammatory bowel disease 64	Proinflammatory
IL-6	IL-6R gp130	Rheumatoid arthritis11,65,66 COVID-1967,68 ARDS69,70 Neonatal sepsis71,72 Myocardial infarction73–75	Proinflammatory
IL-10	IL-10R1 IL-10R2	Inflammatory bowel disease76,77 Non-small cell lung cancer78,79 Systemic lupus erythematosus*,80,81	Anti-inflammatory
IL-17	IL-17RA IL-17RB IL-17RC IL-17RD IL-17RE	Psoriasis82–84 Systemic lupus erythematosus14,85 Hypertension86,87 Inflammatory bowel disease88,89 Ankylosing spondylitis90,91	Proinflammatory
IL-18	IL-18Rα IL-18Rβ	Inflammatory bowel disease92–94 Rheumatoid arthritis95,96	Proinflammatory
TNF-α	TNFR1 TNFR2	Systemic lupus erythematosus97–99 Rheumatoid arthritis96,99,100 Aherosclerosis101–103 Inflammatory bowel disease99,104	Proinflammatory

“*” indicates that this inflammatory cytokine may have an opposite effect in this disease.

The secretion mechanism of inflammatory cytokines is complex and precisely regulated. When immune cells recognize pathogen-associated molecular patterns or danger-associated molecular patterns through their receptors (such as Toll-like receptors), a series of complex biochemical reactions are initiated. This enables precise regulation of the expression of pro-inflammatory cytokines, which ultimately play an important role in the immune system. ¹⁰⁵ For example, cytokines such as TNF-α, IL-1, and IL-6 are classic pro-inflammatory cytokines that promote inflammation by increasing vascular permeability, activating immune cells, and enhancing local immunity. Meanwhile, these cytokines also regulate the immune response through a feedback mechanism. As key regulators in the inflammatory response, inflammatory cytokines can be classified into two major categories: pro-inflammatory cytokines (e.g. TNF-α, IL-1β, IL-6) and anti-inflammatory cytokines (e.g. IL-10, TGF-β). These cytokines coordinate the activation, recruitment, and functional differentiation of immune cells through autocrine, paracrine, and endocrine mechanisms. Collectively, these cytokines give rise to a dynamic regulatory network that meticulously governs the initiation, evolution, and conclusion of the inflammatory process.

Cytokines, as core mediators of communication between the immune system and tissues, exert their functions not merely relying on single cytokines. ¹⁰⁶ More importantly, it is the combined effects resulting from the synergistic actions of multiple cytokines, the responses of body cells, and the changes in the microenvironment that play crucial roles.^107,108 The physiological functions of cytokines are principally demonstrated in aspects such as regulating inflammatory responses and facilitating tissue repair. Nevertheless, their functional imbalance, such as excessive inhibition or activation, can lead to a series of pathological manifestations, thus becoming a key factor in the occurrence and development of inflammation-related diseases. For instance, in rheumatoid arthritis, pro-inflammatory cytokines TNF-α and IL-6 are key regulatory factors. ⁹⁵ TNF-α is mainly secreted by macrophages, synovial fibroblasts, B cells, and T cells. It exerts its functions by binding to TNFR1/TNFR receptors on the cell surface. This binding activates signaling pathways such as NF-κB and induces the expression of downstream cytokines and matrix metalloproteinases. TNFR1 is thought to promote the progression of rheumatoid arthritis by activating inflammatory signals, while the activation of TNFR2 plays a protective role. ¹⁰⁹ IL-6 is mainly secreted by B cells and synovial fibroblasts. It functions by promoting osteoclastogenesis, the expression of VEGF, and recruiting Th17 cells.^110,111 It should be noted that TNF-α can also promote osteoclastogenesis.

Cytokines are also associated with the occurrence and development of tumors. For instance, IL-1β, IL-6, and TNF-α are involved. IL-1β can promote tumor growth, stimulate angiogenesis, and activate neutrophils.^112–114 IL-6 plays a crucial role in early-stage tumors and contributes to the induction of lymphoma and skin tumors by chemical carcinogens. IL-6 can bind to IL-6R and send signals to cancer cells, thereby promoting tumor growth.^115,116 It should be noted that blocking IL-6 helps alleviate the cytokine release syndrome caused by potent immunotherapies. ¹¹⁷ Some cancer cells can express TNF-α, which then secretes growth factors by activating the NF-κB signaling pathway within tumor cells. ¹¹⁸ In fact, certain cytokines can facilitate the progression of diseases from chronic infections to cancer. For example, IL-1 can cooperate with Helicobacter pylori to promote the occurrence of gastric cancer. ¹¹⁹

The role of hydrogels in treatment and the mechanisms of capturing and neutralizing inflammatory cytokines

Hydrogels, a class of polymeric materials with a three-dimensional cross-linked network structure, are widely applied in the field of disease treatment due to their tunable chemical and mechanical properties, excellent biocompatibility, and extracellular matrix (ECM)-like characteristics. ¹²⁰ In the area of drug delivery, hydrogels can encapsulate drugs through methods such as physical entrapment and chemical cross-linking. Moreover, responsive mechanisms (such as pH, enzyme, light, and temperature) can be designed according to environmental conditions, enabling the spatiotemporal and controlled release of drugs. ¹²¹ Hydrogels are used in wound dressings. They can not only absorb wound exudate but also provide a moist environment for wound growth and protect the wound surface, which contributes to wound healing and prevents infection.^122,123 In tissue engineering, because the three-dimensional network structure of hydrogels is similar to that of the ECM, it can provide an excellent microenvironment for cell migration and differentiation. ¹²⁴ Hydrogels can also function as biosensors, enabling the detection of Salmonella, human motion, and more.^125,126 In addition, hydrogels are applicable in bone repair, nerve repair, intestinal fistula occlusion, etc.

Physical adsorption mechanisms

Physical adsorption represents one of the core mechanisms by which hydrogels capture inflammatory cytokines. The porosity of hydrogels, which serves as the basis for physical adsorption, is not only a crucial parameter determining its adsorption efficiency but also a primary factor regulating the behavior of inflammatory cytokines. This characteristic directly influences whether inflammatory cytokines can successfully penetrate the internal structure of the hydrogel and determines the time scale of their capture or release. ³⁰ The three-dimensional network structure of hydrogels endows them with excellent porosity. This gives them a large specific surface area, providing more active sites and greater space for physical adsorption.^30,35,127 Therefore, inflammatory cytokines can not only enter the hydrogel but also enhance the efficiency of cytokine capture. In general, the pore size of hydrogels is influenced by factors such as polymer concentration, crosslinking density, polymerization conditions, and post-processing steps (e.g. freeze-drying, subsequent treatments). Larger pore sizes promote the entry of inflammatory cytokines and their complexes, whereas smaller pore sizes are effective in prolonging the escape of inflammatory cytokines by enhancing the barrier effect. This property provides significant support for the use of hydrogels in trapping and neutralizing inflammatory cytokines. With the advancement of hydrogel fabrication techniques, designing hydrogels with specific pore sizes has gradually become a reality.^128,129

Some hydrogels carry a positive charge on their backbone or side-chain groups, such as ε-polylysine hydrogels,^130,131 polyethyleneimine (PEI)-modified hydrogels, ³² and poly(2-(dimethylamino)ethylmethacrylate) hydrogels.^132,133 In addition, cationic polysaccharide-based hydrogels have been clinically approved for application as dressings for treatment of wounds. These types of hydrogels not only have excellent biocompatibility, but also possess good biodegradability, which lays the foundation for their wide application in regenerative medicine and wound repair. Interestingly, studies have shown that most pro-inflammatory cytokines (such as IL-1, IL-6, IL-12, HMGB-1, and TNF-α) carry a net negative charge (isoelectric point: 4.1–6), while most anti-inflammatory cytokines (such as IL-4, IL-10, IL-11, and TGF-β) carry a net positive charge (isoelectric point: 8.2–11.7; Figure 4(a)). ¹³⁴ It is this charge difference that makes it possible to capture and neutralize pro-inflammatory cytokines through electrostatic interactions. For example, an injectable hydrogel composed of oxidized chondroitin sulfate (OCS), cationic PEI, and tobramycin (Tob). ³² As we all know, Tob is an aminoglycoside antibiotic and a first-line treatment for many bacterial infections. The combined strategy of the primary amine groups of tobramycin and the cationic PEI provides the Tob/PEI/OCS hydrogel with a large number of positively charged groups, rendering the hydrogel cationic as a whole. Given that lipopolysaccharide (LPS) and pro-inflammatory factors, such as TNF-α, cell-free DNA (cfDNA), and HMGB1, all carry a net negative charge. Among them, LPS can trigger an inflammatory response by activating various cells, including macrophages, monocytes, and neutrophils, leading to the release of numerous inflammatory factors.^135–137 There will be interactions due to electrostatic attraction between the positively charged Tob/PEI/OCS hydrogel and the negatively charged cytokines. This enables the Tob/PEI/OCS hydrogel to bind to cytokines and other danger signals, thus achieving their capture. It is noteworthy that the concentration of Tob coupled to the Tob/PEI/OCS hydrogel was able to reach a very high level, a concentration value that far exceeded the minimum inhibitory concentration of Tob (Figure 4(b)). In addition, Zhang et al developed OCMC/Tob/PEI hydrogels using a combination of Tob, PEI, and oxidized carboxymethyl cellulose (OCMC). ¹³⁸ These hydrogels effectively capture negatively charged hazardous molecules such as LPS, TNF-α, and cfDNA (Figure 4(c)). Interestingly, the OCMC/Tob/PEI hydrogels also exhibit pH-responsive Tob release, enabling a degree of smart control over infection and inflammation.

Figure 4.

Charge characteristics of cytokines and physical adsorption mechanism of hydrogels. (a) Regarding the key cytokines in mice, their molecular weights and PIs have drawn attention. It turns out that there is a remarkable charge difference between proinflammatory cytokines, which typically carry a negative charge, and anti-inflammatory cytokines, which usually bear a positive charge. Reproduced with permission from Ref. ¹³⁴ Copyright © 2020, The Author(s). (b) Tob’s efficiency in inhibiting E. coli and S. aureus. Reproduced with permission from Ref. ³² Copyright © 2022 Wiley-VCH GmbH. (c) The schematic illustration of the development of the bioactive hydrogel composed of OCMC, Tob and PEI for realizing the “Pull–Push” approach to heal bacteria-infected diabetic wounds efficiently and safely. Reproduced with permission from Ref. ¹³⁸ Copyright © 2022 American Chemical Society. (d) The schematic representation of the suggested functionality of starPEG-GAG hydrogels for regulating chemokine gradients in chronic wounds over the course of time. Reproduced with permission from Ref. ³¹ Copyright © 2017, The American Association for the Advancement of Science.

It has been demonstrated that the peptide sequences of multiple cytokines, such as IL-1, ¹³⁹ IL-6, ¹⁴⁰ and TNF-α, ¹⁴¹ contain amino acids like arginine (Arg, pKa 12.48) and lysine (Lys, pKa 10.79). These amino acids endow certain peptide sequences of such cytokines with a positive charge, enabling them to undergo electrostatic interactions with the negatively charged sulfate groups of GAGs, such as heparan sulfate, in the ECM, and subsequently bind to them.^142,143 For instance, the starPEG-GAG hydrogel composed of star-shaped polyethylene glycol (starPEG) and GAG heparin(Hep) derivatives can utilize this characteristic of GAGs to effectively remove IL-8, MCP-1, MIP-1α, and MIP-1β from wounds (Figure 4(d)). ³¹ Interestingly, the binding ability of the starPEG-GAG hydrogel to cytokines can be altered by adjusting its degree of sulfate substitution. Specifically, the higher the degree of sulfation, the stronger the ability to bind inflammatory cytokines. This indicates that the capture and neutralization of inflammatory cytokines can be controlled and selectively regulated. Furthermore, starPEG-GAG hydrogels exhibit a weak ability to bind TNF-α, IL-1β, and IL-6, showing some specificity, which is consistent with findings from another study. ¹⁴⁴ However, it should be noted that this electrostatic interaction is not random but rather exhibits a certain degree of specificity. ¹⁴⁵ For instance, common Hep-binding sequences, such as the XBBXBX and XBBBXXBX motifs (where B represents a basic amino acid, typically Arg or Lys), ¹⁴⁶ these specific sequence patterns reflect the specific recognition mechanism during the interaction between proteins and GAGs. In addition to this, the binding specificity of GAGs also depends on the oligomerization ability of cytokines on GAGs and the oligomeric structures formed.^142,147

In a similar study, the researchers used the same principle to prepare a Cur/HH hydrogel (Figure 5(a)) by mixing hyaluronic acid (HA) and Hep hydrogels with Pluronic F127-encapsulated curcumin (Cur). Hep acts as the capture module for the Cur/HH hydrogel. ¹⁴³ The capture efficiency of Cur/HH hydrogels was higher than that of HH hydrogels, which was attributed to the higher cross-linking density of Cur/HH hydrogels, which enhanced the physical barrier effect of the hydrogels (Figure 5(b) and (c)). In fact, the physical barrier effect of hydrogel is more than that, for example, when hydrogel is applied to wound tissue, it can isolate danger signals such as bacteria in the outer layer. ²⁸ Moreover, snail GAG (the A.fulica GAG, AFG) demonstrates a stronger ability to capture inflammatory cytokines compared to Hep. In the double-network hydrogel composed of AFG and gelatin methacrylate (GelMA; Figure 5(d)), the AFG/GelMA hydrogel can significantly reduce a variety of inflammatory cytokines such as IL-1β, IL-4, IL-6, IL-10, and TNF-α in the diabetic wound tissue, and it did not trigger immune responses in animal experiments. ¹⁴⁸ More notably, the AFG/GelMA hydrogel attenuates the inflammatory response by modulating the NF-κB pathway and downregulating inflammation-related genes, while promoting wound healing by increasing the expression of genes involved in angiogenesis and collagen synthesis. These findings further confirm that the hydrogel not only effectively captures and neutralizes inflammatory cytokines, but also modulates the inflammatory response in multiple ways, playing a crucial role in the treatment of inflammation-related diseases.

Figure 5.

Applications of GAG-based hydrogels in wounds. (a) Schematic representation depicting the multifunctional Cur/HH gel and its progressive therapeutic impact on diabetic chronic wounds. (b and c) SEM images of HH gel and Cur/HH gel are presented in (b and c) respectively. (a–c) Reproduced with permission from Ref. ¹⁴³ Copyright © 2022 American Chemical Society. (d) The bioinspired hydrogel was fabricated by covalently linking polyanionic AFG and positively charged GelMA polymers, and its activity and mechanism in chronic wound healing were also investigated. Reproduced with permission from Ref. ¹⁴⁸ Copyright © 2023 Elsevier Ltd.

In contrast to the direct use of GAG materials mentioned above, some studies have extended this principle to other strategies. For example, Hao et al synthesized an antioxidant Hep-mimicking peptide hydrogel (K16, KYKYEYEYAGEGDSS-4Sa) for the repair of radiation-induced skin injury. ¹⁴⁹ Due to the 4-sulfobenzoic acid at the N-terminal end of the K16 peptide, the K16 hydrogel can adsorb inflammatory factors. In this study, although the K16 hydrogel can adsorb a large number of pro-inflammatory cytokines (such as IFN-γ, IL-1β, and TNF-α), effectively alleviating inflammation, it inevitably adsorbs some anti-inflammatory cytokines as well. Moreover, the K16 hydrogel reduces inflammation by enhancing the expression of anti-inflammatory cytokines and suppressing pro-inflammatory cytokines. It also shortens the coagulation time after blood recalcification, thereby enhancing its anti-inflammatory properties and accelerating wound hemostasis. This again demonstrates that hydrogels can modulate inflammation through multiple mechanisms.

In general, using charge differences to capture inflammatory factors is a promising strategy. However, cytokines and other signaling molecules have physiological concentrations, and this strategy will inevitably affect the normal physiological concentrations of other cytokines and signaling molecules, resulting in their normal physiological functions being affected. This issue primarily arises from the low specificity of the hydrogel, making it crucial to address this challenge. Nevertheless, when designing such hydrogels, we can optimize their charge characteristics (e.g. by adjusting the degree of substitution of charged groups or carefully selecting materials) to minimize interference with the normal physiological concentrations of cytokines and other signaling molecules.

Biometrics

Strategies for binding hydrogels to cell membranes

In recent years, the strategy of using cell membranes for biological neutralization has gained significant attention in the field of biomedicine.^150–152 To start with, the cell membrane is not merely able to identify and bind numerous inflammatory cytokines, but can also neutralize substances like bacteria and endotoxins, conferring selectivity and diversity upon the neutralization outcome. Furthermore, the biocompatibility of the cell membrane allows for low immune rejection when applied in vivo, and enables it to bind well to host tissues and function. In using this strategy it should be noted that the choice of cell membrane source is crucial for its function as well as for the application scenario, and cell membranes from different sources (e.g. macrophage membranes, erythrocyte membranes, neutrophil membranes, and specifically highly expressed cell membranes) can be precisely and optimally designed for specific diseases.^153–155 Hydrogel microspheres (HCM), as a significant form of hydrogels, have developed rapidly in many fields due to their unique physical and chemical properties. ¹⁵⁶ The combination of cell membranes and HCM can leverage their unique functional properties, offering significant potential for capturing and neutralizing inflammatory cytokines.

Macrophages, as one of the most important immune cells in the body, play multiple roles in immune responses, including antigen presentation, phagocytosis, cytokine secretion, and tissue repair. Its cell membrane expresses multiple receptors such as TNFR, IL-1R and IL-6R, making its cell membrane an excellent choice for neutralizing inflammatory cytokines. The combined strategy of hydrogel microspheres and macrophage membranes can not only inherit the responsiveness of the hydrogel but also retain the receptors on the macrophage membrane. For example, in a study, in order to mimic a bio-inspired hierarchical delivery system, researchers developed nanovesicles (NVs) encapsulating simvastatin (SIM) by fusing the macrophage membranes of the macrophage cell line J774 cells with synthetic lipid membranes. These NVs were then encapsulated into hyaluronidase (HAase)-responsive hydrogel microspheres (NV@SIM-MPs) for the treatment of diabetic wounds (Figure 6(a)). ³³ It is worth noting that the hydrogel microspheres can protect the NVs from being destroyed by enzymes, allowing the specific release of NVs in the diabetic wound environment. In this way, they can effectively bind and regulate inflammatory cytokines, terminate the inflammatory cascade reaction, and deliver the encapsulated SIM into cells through the membrane fusion or endocytosis pathway, significantly improving the delivery efficiency and pharmacological efficacy of SIM.

Figure 6.

The application of the combination of hydrogel microspheres and cell membranes. (a) When the NV@SIM-MPs with HAase responsiveness are applied to diabetic wounds, they can release NVs@SIM in a responsive manner, thereby neutralizing inflammatory cytokines and alleviating the inflammatory response. Reproduced with permission from Ref. ³³ Copyright © 2022 Elsevier B.V. (b) The changes of gene expression levels in the processes of glycolysis, tricarboxylic acid cycle and oxidative phosphorylation. Reproduced with permission from Ref. ¹⁵⁷ Copyright © 2023 The Authors. (c) The lung tissues of healthy, control, PMS, and iAE-PMS groups were subjected to histopathological analysis at 24 and 72 h after ALI induction. Scale bar: 100 μm. Reproduced with permission from Ref. ¹⁵⁸ Copyright © 2021 Elsevier Inc.

The cell membrane can not only be used for neutralizing inflammatory cytokines and drug delivery, but also can transform macrophages into anti-inflammatory macrophages by regulating macrophages in the body. For example, in the research on the treatment of osteoarthritis, the immune cell mobilizing HCM composed of HA methacrylate-grafted streptavidin and chondroitin sulfate methacrylate are used. Among them, HA methacrylate can closely bind to the CD44 antigen on the surface of engineered extracellular vesicles (sEVs), ensuring the stability of sEVs on HCM and providing a guarantee for the subsequent transformation of macrophages into anti-inflammatory macrophages. The sEVs are prepared from the cell membranes of synovial mesenchymal stem cells through the extrusion method. ¹⁵⁷ The role of the cell membrane in this study is mainly reflected in the following aspects: Firstly, the sEVs prepared from the cell membranes of synovial mesenchymal stem cells can be efficiently taken up by macrophages. Secondly, the sEVs can significantly improve the efficiency of the transformation of macrophages into an anti-inflammatory state. Thirdly, the sEVs prepared using the cell membrane can promote the transformation of pro-inflammatory macrophages into anti-inflammatory macrophages by regulating the mitochondrial energy metabolism of macrophages (Figure 6(b)). In another study, a similar principle was applied, significantly reducing the levels of pro-inflammatory cytokines and promoting the expression of anti-inflammatory genes. ¹⁵⁹ Unlike the direct use of exogenous macrophage membranes, immune cell-mobilizing hydrogel microspheres directly convert pro-inflammatory macrophages at the inflammation site into anti-inflammatory macrophages, thus exerting the neutralizing capacity of pro-inflammatory macrophage membranes.

The combination strategy of cell membranes and hydrogel microspheres demonstrated excellent synergistic effects, which can be used not only in wounds, joint cavities, but also applied in the treatment of lung infections with the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). For instance, inhaled microfluidic HA methacrylate hydrogel microspheres (iAE-PMS) by double-camouflaging with highly angiotensin-converting enzyme II (ACE2)-expressing HEK293-ACE2 cells and pro-inflammatory macrophage-derived bioactive membranes. ¹⁵⁸ Due to the addition of the cell membrane, iAE-PMS has shown remarkable efficacy against SARS-CoV-2 infection. This is because it can be widely distributed throughout the respiratory tract, including areas that are difficult to target with traditional treatment methods, such as the upper respiratory tract, thus effectively preventing the virus from invading. Studies have shown that iAE-PMS has been successfully enriched with ACE2 receptors, IL-6R, IL-1R, and TNFR. iAE-PMS can not only act as a decoy to bind to SARS-CoV-2, significantly reducing the infection efficiency of SARS-CoV-2, but also effectively adsorb and reduce the release of inflammatory mediators such as IL-1β, IL-6, and TNF-α, thereby alleviating the acute lung injury caused by coronavirus disease 2019 (Figure 6(c)). Compared to the physical adsorption strategy of inflammatory cytokines by hydrogels, the combination of hydrogels with cell membranes offers higher specificity and broad-spectrum capability, making it more suitable for complex diseases and diverse applications.

Although research on the combination of cell membranes and hydrogels is still limited, this strategy shows great potential. In the future, the neutralization technology using cell membranes is expected to play an increasingly important role in the treatment of various diseases. However, maintaining the functional integrity and bioactivity of cell membranes during production remains a critical technical challenge to be addressed. Moreover, despite the excellent biocompatibility of cell membrane neutralization materials, long-term use may pose potential risks such as nonspecific immune reactions. In practical applications, large-scale production and cost control still require further optimization and improvement. The clinical translation of cell membrane neutralization technology must undergo rigorous safety and efficacy evaluations, and the complex treatment mechanisms of diseases may further limit its application in the field of biological neutralization.

Cytokine antibody-coupled hydrogels

Cytokine antibody-conjugated hydrogels present a novel therapeutic strategy in biomedicine, capable of locally and systemically regulating inflammatory cytokines while inheriting the binding specificity of cytokine antibodies. This strategy reduces the systemic side effects caused by free cytokine antibodies and increases the duration and capture efficiency of cytokine antibodies. For instance, the IL-6 sponge (IL-6S) is obtained through the chemical coupling of poly(N-isopropylacrylamide-co-methacrylic acid) (P(NIPAAm-co-MAA)) and an IL-6 specific antibody. ³⁰ The mechanism of the antibody specificity of IL-6S is mainly based on the specific binding between the antibody and the antigen, as well as the characteristics of the hydrogel. In this study, compared with the administration of free IL-6 antibody, IL-6S does not affect the physiological concentration of IL-6 (Figure 7(a)). It only exerts a specific adsorption effect when the level of IL-6 is abnormally elevated, thereby significantly reducing systemic adverse reactions and remarkably inhibiting the cytokine release syndrome (CRS) caused by chimeric antigen receptor T cell immunotherapy. Interestingly, the adsorption threshold of IL-6S can be changed by altering the concentration of IL-6 antibodies in IL-6S and the distance of IL-6 antibodies in the hydrogel (Figure 7(b) and (c)). There exists a dynamic binding and restricting effect in IL-6S. IL-6 molecules can reversibly bind to the antibody or dissociate from it. However, due to the physical barrier of IL-6S, the escape time of free IL-6 in IL-6S is significantly prolonged, thereby enhancing the specific clearance effect of the antibody. Additionally, due to its thermosensitive properties, IL-6S can be extracted subcutaneously, offering new possibilities for the management of CRS, from monitoring to prevention. Regarding this strategy to improve the safety of cytokine antibodies, it has been well demonstrated in other studies, as exemplified by the HA-conjugated anti-TNF-α ((anti-TNF-α)-HA) hydrogel used in burn treatment. It significantly prolongs the local action time of cytokine-neutralizing antibodies and reduces inflammatory markers. Moreover, (anti-TNF-α)-HA can reduce the body’s innate immune response to therapeutic antibodies by spatially blocking the Fc region of the antibody. ¹⁶⁰

Figure 7.

Advantages of antibody-conjugated hydrogels for neutralizing inflammatory cytokines. (a) Healthy mice were given IL-6S or the same amount of IL-6 antibody for treatment (intravenously, 100 ng). Twenty-four hours later, the concentration of serum IL-6 was measured (n = 5). (b) The actual/theoretical adsorption ratio of IL-6S in the hydrogel with different amounts of antibody conjugation, where the initial molar amount of IL-6 is twice that of the conjugated antibody (n = 3). (c) The graphical representation in the form of a function curve depicts the correlation between the actual/theoretical absorption proportion and the mean distance of neighboring antibodies, with a sample quantity of n = 3. (a–c) Reproduced with permission from Ref. ³⁰ Copyright © 2023, The Author(s). (d) Fluorescence recovery after photobleaching (FRAP) of FITC-dextrans. Their respective FRAP curves have the diffusion coefficients (d) listed beside. Reproduced with permission from Ref. ¹⁶¹ Copyright © The Royal Society of Chemistry 2016. (e) A design of KLD2R/Hep hydrogel for dual-drug delivery aiming to boost renal repair is proposed. Through electrostatic interaction, the cationic KLD2R combines with the anionic Hep to form a hybrid hydrogel. Notably, Hep exhibits a strong affinity for HGF. Subsequently, the encapsulated anti-TNF -α and HGF are released from the hydrogel in a particular sequence, which jointly promotes tissue repair in a synergistic manner. Reproduced with permission from Ref. ¹⁶² Copyright © 2019 Acta Materialia Inc.

Currently, cytokine antibodies still face challenges such as being easily cleared in vivo and having an uncontrollable duration of action. Cytokine antibody-coupled hydrogels can effectively address these issues. In a study of (anti-TNF-α)-HA hydrogel, (anti-TNF-α)-HA hydrogel increased the time of antibody removal and duration of action compared to free HA, avoiding to a certain extent the side effects on the body due to uncontrollable levels of antibody. ³⁴ (anti-TNF-α)-HA hydrogels demonstrated superior performance in maintaining antibody concentrations around the wound, thus reducing the need for repeated administration. Due to the uneven structures and compositions of tissues, the diffusion of anti-TNF-α may be hindered. However, this unevenness has less impact on the diffusion of (anti-TNF-α)-HA, indicating that (anti-TNF-α)-HA hydrogels are more adaptable to various scenarios. The conjugation of cytokine antibodies with polymeric materials also increases the antibody’s molecular weight, which influences the diffusion rate and modifies its action duration, further demonstrating the advantages of this conjugation strategy. ¹⁶⁰ Studies indicate that the conjugation of cytokine antibodies with polysaccharides has minimal impact on the antibody’s binding affinity, ¹⁶³ consistent with findings from another study, ¹⁶⁴ further validating the feasibility of the antibody-polysaccharide conjugation strategy. In addition, antibodies can be conjugated with different polysaccharides to modulate their dissociation rates. ¹⁶³ Regarding the selection of cytokine antibodies, antibodies targeting upstream cytokines can enhance the efficiency of wound-healing. ¹⁶⁵

In another study, the authors investigated the diffusion coefficient of biomacromolecules in low-molecular-weight glycosyl-nucleoside-lipid amphiphiles (GNL) hydrogels and the relationship between biomacromolecule release and shear stress. ¹⁶¹ The diffusion coefficient of biomacromolecules in the hydrogel is closely related to their molecular weight, with lower molecular weights resulting in higher diffusion coefficients (Figure 7(d)). The release of TNF-α from GNL hydrogels significantly increased under shear stress, which is closely related to the properties of the hydrogel. In general, the shear thinning of hydrogels is negatively correlated with their strength, ¹⁶⁶ with high strength typically resulting from cross-linked polymer chain interactions within the hydrogel. This suggests that when designing cytokine antibody-conjugated hydrogels, optimizing the shear-thinning properties can alter their release profiles. It should also be noted that the introduction of nanomaterials can enhance the shear-thinning properties of hydrogels, effectively mitigating the decline in this performance caused by crosslinking agents. ¹⁶⁷ Compared to the “cytokine antibody-coupled hydrogel strategies” described above, some strategies avoid the involvement of chemical reactions and are able to maintain the biological activity of cytokine antibodies. For example, the KLD2R/Hep hydrogel, formed by the charge interaction between KLD2R (Ac-KLDLKLDLKLDLKLDLRR-CONH2) peptide and Hep, can physically encapsulate anti-TNF-α and HGF. ¹⁶² The KLD2R/Hep hydrogel enables sequential release of dual drugs: anti-TNF-α is released rapidly by the protein diffusion mechanism of the hydrogel nanofiber network, whereas HGF is released slowly by the synergistic effect of Hep binding affinity and molecular diffusion, and Hep protects the growth factor from thermal denaturation and protease degradation (Figure 7(e)).

Overall, the strategy of coupling hydrogels with cytokine antibodies can make up for the shortcomings of cytokine antibodies alone, and this synergistic effect, which is expected to break the limitations of the use of biologics, improve the efficacy, safety, and controllability of biologics, and thus promote the development of biologics. More importantly, it can reduce the number of drug administrations, improve patient compliance and lower the cost of treatment for patients, thus bringing more favorable protection for human health.

Other strategies

Hydrogel coatings for trapping inflammatory cytokines

Hydrogel has diverse application forms and scenarios, such as blending GelMA with negatively charged methacrylated Hep hydrogels, which are then sprayed onto electrospun polylactic acid fiber membranes to create an electrospun fiber tape with inflammation self-limiting properties (Figure 8(a)). ¹⁶⁸ The methacrylated Hep component imparts the tape with the ability to adsorb inflammatory cytokines.^142,143 The thermosensitive property of GelMA ensures the tape’s immediate applicability. ¹⁶⁹ Additionally, the tape demonstrates excellent mechanical properties, superior surface wettability, and the ability to promote cell adhesion and proliferation, which are crucial for emergency medical treatments.

Figure 8.

Other application forms of hydrogels for capturing inflammatory factors. (a) During the pre-tape preparation stage, the chemical constituents and the newly generated chemical bonds. Reproduced with permission from Ref. ¹⁶⁸ Copyright © 2022 Wiley-VCH GmbH. (b) At 2, 4, and 10 h after the treatment with PKH26-labeled Ce@LTA-NPs suspension and PKH26-labeled Ce@LTA-NPs-F127/chitosan hydrogel, the representative fluorescence images (superimposed with corresponding photographs) of the diabetic wound sites were obtained. The circular region was used to mark the diabetic wound site. All the data were presented as mean ± standard deviation (n = 3). Reproduced with permission from Ref. ³⁵ Copyright © 2021, The Author(s). (c) Schematic diagram of the microfluidic fabrication process for PDA@GM MSs. Reproduced with permission from Ref. ¹⁷⁰ Copyright © 2024 Wiley-VCH GmbH.

Joint strategy of hydrogels and nanomaterials

Hydrogels can be combined with NPs to provide various benefits, including prolonged local residence time, enhanced safety, and improved targeting of the NPs. For example, in order to adsorb the inflammatory cytokines continuously released in diabetic wounds, a temperature-sensitive hydrogel composed of Pluronic F127 and chitosan was used to dope and encapsulate Linde Type A (LTA) zeolite NPs loaded with cerium (Ce) (Ce@LTA-NPs), and it was named as Ce@LTA-NPs-F127/chitosan hydrogel. ³⁵ In this study, Ce@LTA-NPs have been proven to possess good adsorption capacity through Brunauer-Emmett-Teller surface area analysis. Its Brunauer-Emmett-Teller surface area is 54.2 m²/g and the pore volume is 0.1 cm³/g, which can provide sufficient adsorption sites for inflammatory cytokines. Thus, it endows the hydrogel with the ability to effectively absorb and neutralize harmful inflammatory factors (TNF-α, IL-6) in the wound. Compared with free Ce@LTA-NPs, Ce@LTA-NPs in the F127/chitosan hydrogel are slowly released from the hydrogel and remain at the wound site for a long time. This avoids multiple administrations and promotes long-term therapeutic effects (Figure 8(b)), achieving the goal of sustained release. In general, the two complement each other. The strategy of combining hydrogels with NPs helps overcome the instability of metal nanomaterials, thereby improving their safety. Ce@LTA-NPs accelerated the transition from inflammation to proliferation and reversed hyperglycemia-induced mitochondrial dysfunction by adsorbing a large amount of inflammatory cytokines from diabetic wounds and enhancing mitochondrial membrane potential.

In contrast to the NPs described above, which endow hydrogels with the capacity to capture inflammatory cytokines, certain studies have enhanced the hydrogel’s ability to capture inflammatory cytokines by incorporating conductive NPs, such as gold NPs, into the hydrogels. This addition increases the hydrogel’s surface area, electrical conductivity, and sensitivity to inflammatory cytokines. ¹⁷¹ However, it is important to note that this pertains specifically to hydrogels that capture inflammatory cytokines through electrostatic interactions. Certainly, research has also indicated that NPs can leverage the principle of electrostatic interaction to capture inflammatory mediators. For example, there exists a nanomedicine-hydrogel composite, which contains cationic NPs capable of delivering and eliminating cfDNA and cyclic guanosine monophosphate-adenosine monophosphate synthase inhibitors. ¹⁷² The charge of cationic NPs is a crucial determinant of their binding ability to cfDNA. The hydrogel can mitigate potential adverse effects stemming from non-specific electrostatic interactions between cationic NPs and biomolecules or cells, thereby enhancing the safety of the NPs. Additionally, the hydrogel can achieve the objective of controlled drug release.

Hydrogel microspheres, as an important form of hydrogels, also possess a hydrophilic three-dimensional network structure. However, compared to polymer network hydrogels, hydrogel microspheres with large pore structures have a larger surface area, meaning they provide more contact sites. ¹⁷³ PDA nanosheets (PDA NSs) were encapsulated in GelMA hydrogel microspheres to form PDA@GM MSs. ¹⁷⁰ PDA NSs and hydrogel microspheres respectively served as the capturing and storage components for cfDNA (Figure 8(c)). Although cfDNA is a DAMP in the host body, it acts as a “pro-inflammatory mediator” and plays an important role in the initiation and progression of inflammation.^174,175 It is worth noting that PDA NSs capture single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA) in different ways. PDA NSs primarily capture ssDNA through hydrogen bonding and π-π stacking with the base pairs exposed on the sides, while they capture dsDNA via π-π stacking between the terminal base pairs of dsDNA and PDA NSs. The polymer chains in GelMA can interact with the side-exposed bases of ssDNA by hydrogen bonding. Additionally, when used as a drug delivery platform, the porous structure of hydrogels may lead to rapid drug release. To improve control, the drug can be encapsulated in the core of NPs, which are then embedded into the hydrogel, creating a synergistic strategy that prevents the sudden release of the drug.^176–178

Cytokine detectors

Hydrogels can also be used as cytokine detectors, such as microcapsules consisting of a polyethylene glycol hydrogel shell together with cytokine-loaded antibody sensing beads for the detection of IFN-γ and TNF-α in the blood. ¹⁷⁹ The hydrogel shell layer effectively blocks leukocytes while allowing cytokines to pass through, protecting the internal antibodies from interference by immune cells to ensure the accuracy of detection. Currently, in the field of biological sample analysis methods, there are still challenges in achieving rapid and highly sensitive immunoassays. The hydrogel platform can help address this issue. For example, a hydrogel composite material is formed by covalently immobilizing specific antibodies of the target analyte on immunosensing polystyrene microspheres, which are then embedded in a poly(N-isopropylacrylamide) hydrogel. Due to the temperature-induced phase transition property of PNIPAM, the hydrogel shrinks, thereby enhancing the fluorescence signal and enabling the rapid detection of cytokines. ¹⁸⁰ Some studies have also shown that hydrogels can be used to detect cytokines by means of fast-response differential pulse voltammetry. Of course, this can be achieved by selecting conductive hydrogel materials such as polypyrrole. In this way, while improving the sensitivity, it can also reduce the detection cost, sample consumption, and operational difficulty. ¹⁷¹ This cytokine detector enriches the application of hydrogel to capture and neutralize inflammatory cytokines.

Material analysis

Glycosaminoglycan-based materials

GAGs are linear polysaccharides composed of repeating disaccharide units, widely distributed in animals, particularly in connective tissues such as cartilage, skin, and the cornea. GAGs not only play a critical structural role in the ECM but also exhibit diverse biological functions, including regulating cellular activities, participating in immune responses, and promoting tissue repair. ¹⁸¹ In recent years, GAG-based materials have been widely employed in hydrogel fabrication due to their excellent biocompatibility and biodegradability, showing great potential in wound healing, anti-inflammatory therapies, and drug delivery. Common natural GAGs include HA, Hep, and chondroitin sulfate.

Hyaluronic acid

Hyaluronic acid is a non-sulfated natural GAG. ¹⁸² The primary biological functions of HA include maintaining tissue stability and integrity, promoting cell migration, enhancing wound healing, and suppressing inflammatory responses.^183,184 It also exerts significant anti-inflammatory effects by binding to the CD44 receptor on immune cells. It should be noted that the molecular weight of HA is closely associated with its function.^185,186 Under normal conditions, HA exists predominantly in a high molecular weight form, serving as a major component of the ECM, exhibiting anti-inflammatory properties and a low affinity for CD44.^187–189 However, in response to danger signals such as pathogens, HA is degraded by HAase into low molecular weight forms, which exhibit increased affinity for CD44, thereby modulating the inflammatory response.^189,190

Due to its numerous advantages, HA is widely used in hydrogel fabrication, particularly in the fields of medicine, drug delivery, and tissue engineering. ¹⁹¹ One of its primary advantages is excellent biocompatibility, as HA exhibits strong affinity with human tissues, effectively reducing immune rejection and demonstrating great potential in wound repair and tissue regeneration.^192,193 Additionally, HA is biodegradable, being broken down by HAase in the body into low-molecular-weight products, thereby avoiding the risks associated with long-term accumulation. Its high hydration capacity enables it to maintain excellent stability in physiological environments, which is crucial for applications requiring high humidity or liquid conditions, particularly in drug delivery systems and wound healing materials. The tunable molecular structure of HA allows for modifications via chemical reactions and crosslinking, enabling adjustments to its molecular weight and physical properties to meet diverse application requirements. This customization makes it one of the ideal materials for tissue engineering and regenerative medicine.^191,194 Although HA cannot capture inflammatory cytokines through electrostatic interactions, it can be conjugated with antibodies (Figure 9(a)), significantly advancing antibody applications. This conjugation not only preserves antibody bioactivity but also greatly enhances their safety and controllability.^34,160,163

Figure 9.

The schematic diagram of the conjugation between antibody and hydrogel, and the Hep-mimicking peptides do not possess anticoagulant effects. (a) The use of N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC), N-hydroxysulfosuccinimide sodium salt (sulfo-NHS), and 4-(dimethylamino) pyridine (4-DMAP) enabled the activation of HA, facilitating the formation of an amide bond with the amine group present on the antibody. Reproduced with permission from Ref. ¹⁶⁰ Copyright © 2013 Society of Plastics Engineers. (b) Three minutes after the blood was collected, the blood in the test tube containing sodium Hep anticoagulant was still flowing normally, while the blood in the K16 hydrogel group had already clotted, indicating that the K16 hydrogel does not have the Hep-like anticoagulant effect. Reproduced with permission from Ref. ¹⁴⁹ Copyright © 2023 Wiley-VCH GmbH.

Despite its advantages, HA, as a primary component of hydrogels, also has certain limitations. First, the mechanical strength of HA-based hydrogels is relatively low, and their stability may be insufficient in environments requiring high mechanical resilience. Therefore, to enhance its mechanical properties, HA is often combined with other materials, such as polyvinyl alcohol or gelatin.^195,196 In addition, the degradation rate of HA is influenced by environmental factors, such as pH, temperature, and enzymatic activity, which may result in uncontrollable degradation, posing challenges in certain applications. However, its degradation rate can be regulated through chemical modifications.^190,197 Further, although HA is generally considered biologically safe, it may trigger allergic reactions in some cases, especially when applied over a long period of time or in high doses. Therefore, while HA-based hydrogels show great potential in the medical field, their properties still require further optimization to meet the demands of more practical applications.

Heparin

Heparin, a widely distributed sulfated polysaccharide in biological systems, is extensively used in drug delivery systems and tissue engineering due to its unique physical and chemical properties. ¹⁹⁸ Hep exhibits remarkable anticoagulant activity and excellent biocompatibility, effectively preventing thrombosis and undergoing rapid degradation in vivo, which underscores its significant value in medical applications. ¹⁹⁹ However, its biocompatibility is lower than that of HA, and its anticoagulant effects are less controllable compared to low molecular weight Hep. As a hydrogel component, Hep enhances the affinity of hydrogels for growth factors such as VEGF and HGF, ensuring their stability and sustained release.^162,200,201 Additionally, the negatively charged sulfate groups of Hep bind to inflammatory factors such as IL-8, MCP-1, and MIP-1α, playing a crucial role in anti-inflammatory processes.^31,101 More importantly, Hep can reversibly bind to IL-10, effectively treating idiopathic pulmonary fibrosis, further expanding its application scope. ²⁰² In recent years, the development of Hep-mimetic peptides has attracted growing attention. They can not only avoid the immune responses caused by exogenous Hep, but also do not have anticoagulant functions (Figure 9(b)), which greatly improves safety. ¹⁴⁹

While Hep materials offer significant advantages, they also have certain drawbacks. Hep molecules are prone to enzymatic degradation, which reduces the stability of Hep-based hydrogels and may lead to degradation during use. Although the anticoagulant properties of Hep are beneficial for certain medical applications, they may increase the risk of bleeding in other scenarios, particularly during surgeries or trauma repair requiring hemostasis. Additionally, Hep may trigger immune responses in vivo, leading to allergic reactions.^203–205

Chondroitin sulfate

Similar to Hep, chondroitin sulfate, a natural sulfated polysaccharide, holds significant potential in capturing inflammatory cytokines. Chondroitin sulfate is ubiquitous in the cell matrix and cell surface and has significant lubricating properties, anti-inflammatory effects, anti-tumor effects, and the ability to provide structural support and promote tissue repair and regeneration.^206–208 Studies have shown that conjugating chondroitin sulfate with drugs enhances drug delivery efficiency, therapeutic efficacy, solubility, and safety. ²⁰⁹ Its extensive biomedical applications are also attributed to its excellent biocompatibility and biodegradability. Despite its outstanding properties in various aspects, chondroitin sulfate faces certain limitations as a hydrogel component. ²⁰⁸ Its mechanical properties are relatively weak, with insufficient mechanical stability. Therefore, modification of chondroitin sulfate, cross-linking with other polymers or association with NPs is usually required to improve the mechanical properties of hydrogelsv.^210,211 Its relatively fast degradation rate may lead to sudden drug release, potentially compromising the sustained effectiveness in certain applications. Additionally, it is important to note that the use of chondroitin sulfate may trigger immune responses.^212,213

Chitosan

Chitosan, a positively charged polysaccharide derived from the deacetylation of chitin, is well-known for its excellent biocompatibility and biodegradability. ²¹⁴ It can gradually degrade into non-toxic small molecules within the body.^215,216 As we all know, bacterial resistance caused by the use of antibiotics has become a world health problem. Chitosan, as a natural antimicrobial material, can exert its antimicrobial effect through various mechanisms, for example, by binding its amino group (-NH2) to the negatively charged components on the bacterial surface, altering cell permeability and inhibiting bacterial growth, as well as inhibiting the synthesis of RNA by binding to DNA, and so on. ²¹⁷ Additionally, studies have demonstrated that positively charged chitosan can capture viruses through electrostatic interactions. ²¹⁸ Moreover, the advantages of chitosan are also reflected in its hemostatic properties. ²¹⁹ Chitosan can also be enhanced by combining it with self-assembling peptides. ²²⁰ These features form the foundation of its extensive applications in the field of biomaterials. Chitosan materials are less costly and more antimicrobial than compared with GAG materials. A temperature-sensitive hydrogel composed of Pluronic F127 and chitosan was used for the slow release of NPs, which not only improved the stability and safety of NPs, but also avoided the need for multiple drug administrations. ³⁵

However, chitosan is not without limitations. Its poor solubility restricts its applications, as it dissolves primarily in acidic solutions and exhibits low solubility under neutral and basic conditions. To address this issue, scientists have synthesized a range of chitosan derivatives, such as carboxymethyl chitosan and quaternized chitosan. It is worth noting that introducing specific functional groups not only preserves the inherent properties of chitosan but also imparts additional functionalities, significantly expanding its application potential. ²²¹

Gelatin

Similar to the materials mentioned above, gelatin is a natural polysaccharide with excellent biocompatibility and biodegradability. As a natural polysaccharide, gelatin can be enzymatically degraded in vivo by enzymes such as matrix metalloproteinases. It is non-toxic and fully meets the requirements for biomedical materials. ²²² Derivatives of gelatin, such as GelMA, exhibit photosensitivity, excellent biocompatibility, biodegradability, and rapid curing capabilities. ²²³ These properties make GelMA widely applicable in tissue repair and regeneration, wound healing, and drug delivery.^224,225 Hydrogel microspheres synthesized from GelMA have excellent mechanical properties and porous structure, which can not only serve as a storage part of inflammatory mediators, but also load growth factors through electrostatic interactions.^170,226 One major drawback of gelatin is its source limitation. It is typically extracted from animal tissues, making large-scale production challenging and restricting its availability to specific animal sources. Additionally, it may provoke religious controversies. ²²⁷ Gelatin’s stability, mechanical properties, and adhesive ability are relatively weak, limiting its practical applications. These drawbacks are typically addressed through chemical modifications or combination with other biomaterials. ²²⁸ However, it should be noted that gelatin is immunogenic and may trigger an immune response, which may not be suitable for some allergy-prone people. ²²⁹

Generally speaking, among various strategies for capturing and neutralizing inflammatory cytokines, most of the materials used belong to polysaccharides. Whether natural raw materials or modified materials are used, it reflects to some extent that polysaccharides are excellent materials for capturing and neutralizing inflammatory cytokines.^143,163 Although widely available in nature and characterized by good biocompatibility and biodegradability, polysaccharides also have notable drawbacks, such as susceptibility to contamination, complex purification processes, and challenges in large-scale production. Among synthetic materials, PEI, as a cationic polymer, can enhance the efficiency of hydrogels in capturing inflammatory cytokines by modulating charge intensity and crosslinking density. ³² The IL-6S, composed of P(NIPAAm-co-MAA) conjugated with IL-6 antibodies, has been shown to preserve antibody activity and exhibit no toxicity in mouse experiments. These findings demonstrate that synthetic materials also have unique advantages. ³⁰ However, research on synthetic materials in this particular area remains limited.

Challenges and future perspectives of hydrogels in capturing and neutralizing inflammatory cytokines

Current challenges

In-depth study of the mechanisms of inflammatory diseases

Despite significant progress in research on inflammatory diseases in recent years, a comprehensive understanding of their complex mechanisms remains insufficient. The occurrence and progression of inflammatory diseases typically involve the influences of multiple cytokines, signaling pathways, immune cells, and the microenvironment. This implies that clarifying the underlying mechanisms and connections among them is of great benefit for treatment. Moreover, these cytokines may exhibit different degrees of dominance in the context of various inflammatory diseases, or there may be synergistic effects among several cytokines. This is of vital importance for subsequent research, as it can provide researchers with a clear direction in designing hydrogels that target specific cytokines. However, current research still lacks an overall understanding of the cytokine network and the interactions among cytokines. This limitation has hindered the development of novel therapeutic strategies. To further advance the treatment of inflammatory diseases, it is essential to delve deeper into and describe these complex biological mechanisms, uncovering the multi-layered roles of cytokines and their receptors, in order to develop multi-target therapeutic strategies targeting inflammatory cytokines.

Advantages and disadvantages of various mechanisms of hydrogels in the direction of trapping inflammatory cytokines

Hydrogels hold great promise in capturing and neutralizing inflammatory cytokines, but several challenges remain to be addressed. First, the complex in vivo physiological environment and the diverse sites of inflammatory disease present significant challenges for the functional performance and application of hydrogels. Additionally, in the complex and dynamic physiological environment, hydrogels may lose their efficiency in capturing and neutralizing inflammatory cytokines due to structural changes or functional degradation. Finally, the diversity of inflammatory disease mechanisms and sites results in varying requirements for the use of hydrogels in different applications, further impacting their therapeutic efficacy.

These methods each have their own advantages and disadvantages (Table 2). Physical adsorption typically relies on the porous structure or surface characteristics of hydrogels to capture inflammatory cytokines, but this method lacks molecular recognition ability. Inflammatory cytokines (such as IL-6, IL-17, and TNF-α) have complex structures and distribution characteristics, and physical adsorption, based only on physical properties, is difficult to selectively capture specific cytokines. This may lead to non-specific adsorption of non-target molecules, which reduces therapeutic efficacy and may trigger serious side effects, making it difficult to competently perform in complex in vivo environments. A strategy combining hydrogels with cell membranes can achieve broad-spectrum and efficient capture and neutralization of inflammatory cytokines. However, this method still faces several challenges, such as optimizing the stability and biocompatibility of the combination strategy and adapting to the complex and variable pathological environment. Combining hydrogels with antibodies against inflammatory cytokines provides a highly specific and efficient approach for their capture and neutralization. Although this strategy can greatly improve the precision and effectiveness of treatment, the program still faces many problems in practical application that are worth pondering. For example, how can the stability and activity of antibodies be maintained in complex physiological environment? How can production costs be further reduced while ensuring clinical accessibility? In addition, all of these methods are deficient in clinical translation.

Table 2.

Advantages and disadvantages of hydrogels in capturing and neutralizing inflammatory cytokines.

Mechanism	Advantages	Disadvantages
Electrostatic interaction	Wide adaptability, adjustable charge amount, broad-spectrum capture	Poor specificity
Neutralization by antibody specificity	Strong specificity, high safety, adjustable antibody action time, controllable action range	High cost
Neutralization by cell membrane receptors	Broad-spectrum binding	Complex manufacturing process
Utilization of nanoparticles	Strong adsorption ability, adjustable charge amount	Poor specificity

Directions for future research

Innovation and optimization of hydrogel materials

Future research should focus on the innovation and optimization of hydrogel materials, particularly in meeting the demands of complex inflammatory pathological environments. Innovation in hydrogel materials should not only improve their biocompatibility and biodegradability but also enhance their ability to selectively capture various types of inflammatory cytokines. In complex inflammatory pathological environments, cytokine types and concentrations often vary significantly. Hydrogel materials must be capable of precisely recognizing and capturing specific inflammatory cytokines such as TNF-α, IL-6, and IL-1β. Therefore, the design of future hydrogels should focus on optimizing their physical properties, surface structure, and functionality. A multifunctional approach should integrate cell membranes, antibodies, or NPs into the hydrogel to enhance its selective adsorption and binding capacity for cytokines. In addition, in order to enhance the adaptability of hydrogels in complex pathological environments, research should also aim to develop responsive hydrogels, such as pH-responsive and temperature-responsive hydrogels, so as to regulate their structure and function under different physiological conditions and precisely regulate the capture and release of cytokines. These innovative hydrogels need to not only exhibit high selectivity but also provide sustained effects at the site of inflammation, avoiding excessive immune responses and side effects. The optimization of hydrogel materials should not only focus on their ability to capture and neutralize cytokines but also on their stability, degradation rate, and long-term controllability of therapeutic effects in vivo. Lastly, it is important to note that production efficiency and cost control directly affect the feasibility of hydrogels in clinical applications. Only by addressing these issues simultaneously can hydrogels transition from the laboratory to clinical applications, ultimately benefiting more patients.

Directions for joint strategies

First, the development of safer antibodies and cell membrane materials is essential for the success of treatment strategies. ²³⁰ Antibodies, as precise targeting tools, face issues such as immunogenicity, off-target effects, and long-term side effects. Therefore, future research should focus on engineering antibodies by optimizing their affinity, specificity, and immunogenicity to reduce potential side effects. Although cell membrane materials possess natural biocompatibility, their long-term biological stability and local immune responses still require further investigation. Strict control over the source of the cell membrane and surface modification can enhance its stability and reduce interactions with the host immune system, thereby improving the effectiveness of the treatment. In multi-antibody combination therapy, research should focus on resolving the synergistic effects and potential interference between different antibodies. By rationally designing the combination of antibodies (e.g. covalent ligation, multifunctionalization modification), it is possible to target and neutralize multiple inflammatory cytokines at the same time, thus providing a broader scope of action in inflammation therapy and effectively reducing the occurrence of drug resistance or rebound phenomenon. The strategy of combining multiple antibodies not only enhances the therapeutic effect, but also reduces the limitations of individual antibodies in single-target therapy.

Second, the safety of NPs remains one of the key bottlenecks for their widespread application. Despite their excellent targeting and drug delivery capabilities, the long-term biocompatibility, degradability, and potential immune responses of NPs in vivo still need further investigation. Future research should focus on optimizing the design of NPs to ensure that they can efficiently target and capture inflammatory cytokines without triggering a toxic or immune response.

Only by overcoming these problems simultaneously can the advantages and potential of a combined strategy be fully realized. Looking to the future, we believe that this field will evolve toward high precision, high efficiency, high safety and multi-functional integration. Through the efforts of global researchers and technological innovations, the application of hydrogels in capturing and neutralizing inflammatory factors will break through existing bottlenecks, becoming a powerful tool in combating inflammatory diseases, enabling more precise intervention in the pathogenesis of inflammation-related diseases and more efficient control of inflammation.

Conclusion

Addressing the key role of inflammatory cytokines in disease onset and progression, hydrogels have made significant progress in the field of capturing and neutralizing inflammatory cytokines. Its unique porous structure, biocompatibility, and tunable properties make it an ideal material for treating diseases related to inflammatory cytokines. Whether through physical adsorption of inflammatory cytokines, coupling with specific antibodies, synergistic effects with cell membranes, or integrating NPs, hydrogels demonstrate significant potential in efficiently capturing and neutralizing inflammatory cytokines. However, due to the complexity of the human microenvironment, these strategies still face significant challenges in terms of efficiency, safety, and stability. Future research should focus on material design and functional optimization to further enhance its adaptability to pathological environments and broaden its applications, providing more efficient and reliable solutions for precision medicine and chronic inflammation treatment.