Engineering hybrid microgels: from rational design to next-generation smart materials

Abstract

Hybrid microgels represent advanced soft colloidal materials engineered by incorporating diverse functional components, such as metal nanoparticles, inorganic non-metals, organics, metal–organic frameworks (MOFs), and biomolecules, within crosslinked polymer networks. By synergistically combining the intrinsic stimuli-responsiveness and dispersibility of pristine microgels with the unique properties of incorporated functional units (e.g., photothermal effects, magnetic responsiveness, and bioactivity), hybrid materials effectively overcome the inherent limitations of conventional microgels, such as insufficient mechanical strength, restricted functionality, and poor stability in complex environments. This review systematically outlines key fabrication strategies (in situ hybridization, seed-mediated growth, and microfluidics), classifies hybrid microgels according to incorporated components (metal-hybrid, inorganic non-metallic hybrid, organic hybrid, MOF-hybrid, biohybrid, and multi-component hybrid), and elucidates their structure–property relationships. Leveraging recent advances in fabrication optimization and property regulation, we highlight emerging applications of hybrid microgels, including catalysis, drug delivery, cell therapy, environmental remediation, sensing, and emulsion stabilization. Furthermore, we discuss persistent challenges in the development of hybrid microgels, emphasizing the need for large-scale production, advanced characterization, biosafety evaluation, and multi-stimuli integration to accelerate the translation of hybrid microgels from fundamental research to practical applications.

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Hao Chang

Hao Chang received his BSc degree in Food Science and Engineering from Hainan University in 2024. He is currently a graduate student in the Department of Food Science and Engineering at Shenzhen University. His research interest focuses on the interfacial properties and structure–property relationships of hybrid microgels, aiming to design advanced functional materials.

Yue Zheng

Yue Zheng received her BSc degree in Food Science and Engineering from Beijing Forestry University in 2024. She is currently a graduate student in the Department of Food Science and Engineering at Shenzhen University. Her research interest focuses on the oil–water interfacial behavior of natural macromolecules and soft colloidal particles for developing food and industrial applications.

Mo Zhu

Mo Zhu obtained her Master's degree in 2012 under the supervision of Professor Meifang Zhu from Donghua University, and obtained her PhD in 2021 from the University of Science and Technology of China. She spent an additional three years for her postdoctoral research in the College of Chemistry and Environmental Engineering of Shenzhen University. Since 2025, Dr Zhu has continued her research as an associate research fellow at Shenzhen University, China. Her research interest focuses on the thermodynamics and dynamics of macromolecules in solutions.

Lianwei Li

Lianwei Li obtained his PhD in 2014 under the supervision of Professor Chi Wu from the University of Science and Technology of China. He spent an additional three years on his postdoctoral research in the Department of Chemistry at the University of Chicago under the supervision of Professor Luping Yu. From 2017 to 2019, he worked in the Department of Physical Chemistry at the University of Science and Technology of China. In 2019 he joined the College of Chemistry and Environmental Engineering at Shenzhen University as an associate professor and was promoted to a full professor in 2024. His primary research interest focuses on the fundamental problems of macromolecular solutions and characterization methodologies.

Xin Guan

Xin Guan obtained his PhD in 2023 under the supervision of Professor To Ngai from the Chinese University of Hong Kong. He spent an addition one and a half years on his postdoctoral research in the Department of Chemistry at the same university. In 2025 he joined the College of Chemistry and Environmental Engineering at Shenzhen University as an assistant professor. His primary research interest focuses on the development of functional soft matter (e.g., polymers, microgels, emulsions) and their applications.

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1. Introduction

Microgels are soft colloidal particles comprising three-dimensional crosslinked polymer networks, with sizes typically ranging from nanometers to micrometers.^1,2 These particles can be swollen and well-dispersed in compatible solvents.^3,4 To date, diverse synthetic strategies have been developed for microgels, including precipitation polymerization, emulsion polymerization, inverse emulsion templating, and microfluidics.^5–7 By selecting appropriate techniques and precisely regulating polymerization conditions, microgels can be synthesized with tailored physicochemical properties.^8–10 For example, poly(N-isopropylacrylamide) (PNIPAM)-based microgels exhibit thermal responsiveness, undergoing reversible swelling/shrinking transitions with environmental temperature changes.^11–14 Furthermore, the incorporation of functional comonomers imparts the microgels with adjustable hydrophobicity and surface charges.^15–18 Beyond PNIPAM-based systems, a diverse range of polymers have been developed to construct microgels, addressing the inherent limitations of PNIPAM and expanding the application boundaries of hybrid microgels. For instance, poly(N-vinylcaprolactam) (PVCL) and poly(oligo(ethylene glycol)methacrylate) systems feature excellent biocompatibility and tunable thermo-responsiveness, avoiding the potential cytotoxicity of residual PNIPAM monomers for biomedical applications.^19,20 Bio-based and biodegradable polymer networks, including gelatin, alginate, dextrin, and cellulose derivatives, enable the fabrication of fully degradable hybrid microgels, solving the challenge of long-term in vivo retention of non-degradable synthetic polymer matrices.^21–23 Polyelectrolyte networks such as poly(acrylic acid) (PAA) and poly(dimethylaminoethyl methacrylate) (PDMAEMA) endow hybrid microgels with robust pH and ionic strength responsiveness.^24,25 However, conventional microgels often suffer from intrinsic limitations, including relatively poor mechanical strength, limited functionality, and inadequate physicochemical stability in complex environments. These drawbacks significantly restrict their performance in specific applications, such as drug delivery, catalysis, and supramolecular assembly.^26,27

To overcome these limitations and expand the application scenarios of microgels, the development of hybrid microgels has emerged as a prominent research frontier in soft matter and colloid science. Typically, hybrid microgels are fabricated by incorporating various functional components, such as metal nanoparticles, inorganic non-metallic nanoparticles, organic materials, or biomolecules into microgels through physical encapsulation or chemical conjugation.^28,29 Such hybridization not only enables the unique functions of the incorporated components, but also retains the intrinsic softness, stimuli-responsiveness, and colloidal dispersibility of the pristine microgels.^30,31 For example, embedding gold (Au) nanoparticles into the microgel matrix imparts hybrid microgels with photothermal responsiveness and enhanced catalytic activity, enabling applications in droplet manipulation and pollutant degradation.^32–34 Similarly, magnetic nanoparticles (e.g., magnetite, Fe₃O₄) endow hybrid microgels with magnetic responsiveness for targeted delivery and facile recycling.^35–37 Moreover, enzyme immobilization allows the fabrication of biohybrid microgels for controllable and sustainable biocatalysis.^38,39 Thus, compared to conventional microgels, hybrid microgels exploit synergistic effects between distinct materials, integrating various functionalities, such as multi-stimulus responsiveness, self-healing properties, and specific bioactivities. This has dramatically expanded the scope of microgel research from fundamental studies to a variety of cutting-edge applications.

In recent years, the fabrication and application of hybrid microgels have garnered substantial attention due to their versatile properties. However, existing reviews have not clearly illustrated the correlation between the fabrication strategies, classification, properties, and derived applications of hybrid microgels. This review aims to provide a comprehensive and systematic overview of the development of hybrid microgels. First, we summarize the key fabrication strategies of hybrid microgels, followed by a detailed discussion of the classification and properties of hybrid microgels based on the incorporated functional components with special emphasis on elucidating their structure–property relationships to guide the rational design of advanced materials. Additionally, we showcase some emerging applications derived from hybrid microgels, including catalysis, drug delivery, cell therapy, environmental remediation, sensing, and emulsion stabilization. This review is expected to provide valuable insights for the innovative design and fabrication of hybrid microgels and facilitate the development of hybrid microgels in both fundamental research and practical applications.

2. Fabrication of hybrid microgels

The fabrication strategies for hybrid microgels can be categorized by the timing of functional component incorporation relative to microgel network formation. Hybridization during microgel synthesis involves incorporating functional units simultaneously with polymer network formation. This one-step strategy simplifies the workflow and improves the hybridization efficiency, while nanoparticle aggregation or component deactivation may occur under harsh polymerization conditions.⁴⁰ Additionally, achieving precise control over the spatial distribution of incorporated units within the microgel matrix remains challenging.⁴¹ In contrast, post-synthesis hybridization offers greater flexibility by leveraging the structural features, stimuli-responsiveness, and surface properties of pristine microgels to incorporate functional components through physical adsorption or covalent bonding.^34,42 This strategy can minimize potential adverse effects of polymerization conditions on sensitive components, preventing deactivation, and enables better regulation of the spatial distribution and loading amount of incorporated components.^43–45 The selection of appropriate strategies primarily depends on the specific requirements of target applications regarding the structural uniformity, stability, and functionalities of hybrid microgels.

2.1. Hybridization during microgel synthesis

2.1.1. In situ hybridization. In situ hybridization typically involves dissolving functional components, monomers (e.g., N-isopropylacrylamide), crosslinkers, and initiators in a reaction medium simultaneously. Upon polymerization initiation, three-dimensional polymer networks progressively form, while the functional components are in situ generated or encapsulated within the polymer network (Fig. 1a). This one-pot strategy offers several advantages, including operational simplicity, high loading capacity, and improved stability of the incorporated components due to the highly porous structure and protective polymeric barrier of the microgel matrix. For example, enzymes (e.g., α-chymotrypsin) can be immobilized in PNIPAM microgels during precipitation polymerization to fabricate biohybrid microgels, where the crosslinked network ensures high enzyme loading while preventing catalytic activity loss.³⁹

Fig. 1 Schematic illustration of representative fabrication strategies for hybrid microgels: (a) in situ hybridization, (b) seed-mediated polymerization, and (c) microfluidics.

In situ hybridization of microgels can also be achieved by non-covalent co-aggregation between functional components and microgel matrices.^46,47 Chen et al. developed an in situ hybridization strategy based on enhanced hydrophobic interactions between Au nanoparticles and thermo-responsive terpolymer above its lower critical solution temperature (LCST), achieving nearly 100% loading efficiency of Au nanoparticles in microgels (Fig. 2a).⁴⁸ However, harsh polymerization conditions (e.g., free radicals) may induce nanoparticle aggregation or surface modification, and nanoparticles tend to form localized clusters, making it difficult to achieve uniform spatial distribution within microgels.⁴⁹ This agglomeration is primarily driven by kinetic factors, such as diffusion-limited aggregation during polymerization, and thermodynamic considerations, including the minimization of interfacial energy between nanoparticles and the polymer matrix.⁵⁰ Additional forces, such as van der Waals attractions and hydrophobic interactions, contribute to cluster formation, particularly in systems where nanoparticles exhibit poor compatibility with the hydrophilic microgel network.^51,52

Fig. 2 Schematic illustration of representative interactions between functional components and microgel matrices: (a) hydrophobic interaction, (b) electrostatic interaction, (c) physical encapsulation (non-covalent), and (d) covalent binding.

2.1.2. Seed-mediated growth. Unlike the simultaneous incorporation in in situ strategies, seed-mediated growth is an effective strategy to fabricate core–shell hybrid microgels. Typically, pre-synthesized nanoparticles function as “seeds” to induce monomer polymerization on their surfaces to form microgel shells, thereby achieving the construction and functionalization of core–shell structures (Fig. 1b). By regulating the type and functionalities of nanoparticles and monomers, the unique optical, electrical, and magnetic properties of nanoparticles can be integrated with the intrinsic environmental responsiveness of microgels, enabling the fabrication of multifunctional hybrid materials.⁵³ Suzuki et al. emphasized that sufficient polymerizable sites on the seed surface are essential for uniform shell growth.⁵⁴ For example, Chen et al. used Fe₃O₄ nanoparticles with polymerizable surfaces as seeds, and fabricated recoverable thermosensitive hybrid microgels via self-initiated photopolymerization grafting of thermosensitive PDMAEMA polymer shells.⁵⁵ The loading of Fe₃O₄ enabled efficient separation and recovery of hybrid microgels from aqueous solutions.

This strategy not only enables the fabrication of single-core hybrid microgels but also provides a confined space within microgel matrices for subsequent formation of bimetallic nanostructures via electrosynthesis, thus realizing synergistic regulation of optical properties and catalytic activity of hybrid microgels. It is worth noting that insufficient affinity between monomers and inorganic seed surfaces may lead to incomplete/uneven shell coating, or even homogeneous nucleation, resulting in the formation of pure polymer particles.⁵⁶ It was also reported that insufficient or excessive monomers in the reaction solution may cause core particle aggregation or the formation of individual pristine microgels, impairing the dispersity and uniformity of hybrid microgels.⁴⁴

2.1.3. Microfluidics. Microfluidics has emerged as a promising tool for generating monodisperse droplets in customized channels. When monomers, crosslinkers, and functional components are introduced into the aqueous phase, the generated droplets serve as “microreactors” for subsequent polymerization, enabling precise control over the size, chemical composition, and internal structure of hybrid microgels (Fig. 1c).^57,58 For example, Carvalho et al. merged alginate precursors containing Ca²⁺-ethylenediaminetetraacetic acid and Zn²⁺-ethylenediamine-N,N'-diacetic acid complexes into microfluidic channels and achieved the controlled release of Ca²⁺ ions via competitive ligand exchange.⁵⁹ The released Ca²⁺ ions subsequently induced ionic crosslinking of the alginate polymer chains, enabling in situ fabrication of sulfate-modified polystyrene nanoparticle-embedded hybrid microgels with precisely regulated gelation kinetics. In another example, Kim et al. employed water-in-oil-in-water double emulsion droplets as confined microreactors, and concentrated Au nanoparticles uniformly in the gel precursor phase without aggregation via osmosis-driven outflow of the aqueous phase, followed by photopolymerization to fabricate Au-hybrid microgels.⁶⁰

Additionally, microfluidics enables versatile fabrication of microgels with complex topologies, such as core–shell, Janus, and multi-chamber structures, through multi-level flow channel design or multiphase fluid regulation, thereby advancing the development of functional microgels featuring precise heterogeneous architectures.^61,62 However, this technique imposes stringent requirements on equipment design and process control. Precise regulation of channel surface properties, fluid viscosity, and flow velocity is required to prevent droplet coalescence or channel blockage before polymerization, which is also the core bottleneck limiting its continuous large-scale production. For industrial scale-up, this technique faces four key challenges: (1) frequent channel clogging during long-term continuous production; (2) insufficient throughput of single-channel devices, and loss of droplet size uniformity in multi-channel parallel scale-up; (3) difficulty in ensuring batch-to-batch consistency due to the high sensitivity of gelation kinetics to process parameters; (4) high equipment and consumable costs that are prohibitive with low-cost industrial production.^63,64 Additionally, it is difficult to precisely control the size of microgels down to the submicron level due to the constraints of microfluidic channels, further restricting their potential application scenarios.⁶⁵

2.2. Post-synthesis hybridization

2.2.1. Non-covalent interaction. Following microgel synthesis, functional components can be incorporated and immobilized in pre-synthesized microgel matrices via non-covalent interactions, such as electrostatic interactions and hydrogen bonds.^66,67 Electrostatic interactions between oppositely charged microgels and functional components is the main driving force for component incorporation (Fig. 2b), which endows pristine microgels with non-intrinsic functionalities without complex chemical modifications during synthesis. For example, Singh et al. fabricated pH-responsive luminescent hybrid microgels via electrostatic attraction between positively charged PNIPAM microgels and negatively charged cadmium sulfide quantum dots.⁶⁸ Guan et al. prepared plasmonic hybrid microgels with photothermal responsiveness via electrostatic attraction between positively charged poly(N-isopropylacrylamide-co-allylamine) (PNIPAM-co-ALA) microgels and negatively charged Au nanoparticles, enabling light-controlled manipulation of Pickering emulsion droplets.³³ Besides electrostatic interactions, hydrogen bonds can also be used to fabricate metal-hybrid microgels with enhanced stability. For instance, hydrogen bonds between hydroxyl groups on γ-Fe₂O₃ nanoparticle surfaces and carbonyl groups on PNIPAM chains introduced additional physical crosslinking points in microgels, which enhanced network rigidity and restricted segment motion, thereby improving the thermal stability of microgels.⁶⁹

Physical encapsulation based on reversible swelling/shrinking behavior of microgels is another commonly used non-covalent hybridization strategy (Fig. 2c). By leveraging the thermal responsiveness of PNIPAM-based microgels, the microgels can transform from a shrunk to swollen state by lowering the temperature, allowing enzyme loading and spatial positioning in aqueous solution.⁷⁰ This strategy not only improves enzyme loading efficiency, but also enables the regulation of enzyme distribution in microgels or on their surfaces based on the relative size of the enzyme and the matrix pores. While non-covalent interactions enable the feasible fabrication of hybrid microgels, the inherent weakness of non-covalent bonds may compromise the long-term stability of hybrid microgels under extreme conditions, such as harsh pH levels, elevated ionic strength, or extreme temperatures. Optimizing the crosslinking density of microgels or introducing multiple non-covalent interactions between microgels and components is an effective strategy to enhance the stability of hybrid microgels in diverse environments.

2.2.2. Covalent binding. Compared to non-covalent interactions, covalent linkage of functional components to pre-synthesized microgels can fabricate hybrid microgels with superior structural stability (Fig. 2d).⁷¹ For example, Shi et al. synthesized thiol-functionalized PNIPAM microgels via amidation between 2-aminoethyl mercaptan and carboxyl groups in microgels, followed by in situ reduction of Au³⁺ ions to uniformly immobilize abundant Au nanoparticles in the microgel matrix.⁷² The strong Au–S coordination bond enabled higher Au nanoparticle loading and more robust adhesion compared to non-covalent physical adsorption. Consequently, the resulting Au-hybrid microgels maintained excellent structural stability and reversible optical properties throughout multiple freeze–thaw cycles. Similarly, the reduction of Au³⁺ ions can generate Au nanoparticles in amide-functionalized microgels via donor–acceptor interactions.⁷³ Besides metal nanoparticles, covalent binding is also applicable for biomolecule immobilization in microgels. It was reported that trypsin can be covalently immobilized onto carboxyl-functionalized microgel surfaces via EDC/NHS-mediated amide bond formation, achieving high enzyme loading capacity while maintaining enzymatic activity.⁷⁴

Covalently-bonded hybrid microgels demonstrate considerable advantages: the strong binding between microgels and incorporated components prevents the leaching of nanoparticles or biomolecules while enabling high loading capacities. This guarantees the consistent performance of hybrid microgels during recycling and under external stimuli. However, covalent hybridization typically involves multi-step chemical reactions, which are time-consuming and require complementary functional groups on both microgels and incorporated components.⁷⁵ Furthermore, the coupling process may impair the intrinsic stimuli-responsiveness of microgels or the activity of biomolecules.

3. Classification and properties of hybrid microgels

Benefiting from the development of diverse fabrication strategies for hybrid microgels, various functional components, such as metals, inorganic non-metals, organic materials, metal–organic frameworks (MOFs), and biological components, can be incorporated into microgels. Table 1 summarizes the fabrication strategies, incorporated components, size range, properties, and applications of representative hybrid microgels. Depending on the type of incorporated components, hybrid microgels can be endowed with light-responsiveness, magnetic responsiveness, catalytic activity, bioactivity, and other specific functions, breaking through the functional limitations of conventional microgels. The data in Table 1 clearly show that the performance and orientation of the applied hybrid microgels are co-determined by the incorporated functional components and the intrinsic properties of the microgel matrix. Functional components define the core functional attributes of the system, while the polymer network and stimuli-responsiveness of the microgel matrix directly regulate the stability and dynamic performance of the hybrid material. Meanwhile, the selection of fabrication strategies presents an inherent trade-off between the structural controllability and large-scale manufacture of hybrid microgels.

Table 1 Fabrication strategies, incorporated components, size range, properties, and applications of representative hybrid microgels

Pristine microgels	Fabrication strategies	Incorporated components	Size range	Properties	Applications	Ref.
Note: the size ranges of hybrid microgels were determined using dynamic light scattering (DLS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), confocal laser scanning microscopy (CLSM), or optical microscopy (OM). Notably, test principles and environmental conditions differ significantly across techniques, so results cannot be directly compared horizontally. DLS measures hydrodynamic diameters in solution, reflecting the real swelling state of microgels; SEM/TEM obtains fine dry-state morphology under vacuum conditions, but causes deformation of microgels; OM/CLSM enables liquid-phase characterization, with CLSM visualizing component spatial distribution, but is limited by microgel size and optical diffraction. Accurate characterization of microgel architecture and uniformity can be acquired using in situ techniques, such as cryo-electron microscopy, in situ atomic force microscopy, and small-angle neutron scattering.^2,76,77
PVCL; PNIPAM-co-ALA	In situ reduction	Au	∼149 nm (DLS); ∼130 nm (DLS)	Biocompatible; colloidally stable; thermo-responsive; photothermal conversion capability	Size/structure regulation of Au; controlled drug delivery	78 and 79
PNIPAM-co-ALA	Electrostatic attraction	Au	∼770 nm (DLS); ∼620 nm (DLS)	Thermo-responsive optical property; photothermal conversion capability	Surface-enhanced Raman scattering substrates; light-driven droplet manipulation	33 and 80
PNIPAM	Covalent binding via copolymerization	Au	∼400 nm (DLS)	Thermo-responsive; catalytic activity	Temperature-controlled catalysis	81
PNIPAM-co-AA	In situ reduction	Ag	∼320 nm (TEM)	Localized surface plasmon resonance; pH responsive	Probing pH values	82
PDMAEMA	Seed-mediated growth + in situ reduction	Fe3O4; Fe3O4-Au	∼250 nm (DLS)	Magnetic-responsive; thermo-responsive; catalytic activity	Sustainable catalysis	55
Thiol functionalized PNIPAM-co-AA	In situ reduction	Cu; Pd; Cu-Pd	300–400 nm (TEM)	Catalytic activity	Degradation of environmental pollutants	83
PNIPAM-co-AA; PNIPAM-co-ALA	Seed-mediated growth + in situ reduction	Au-Ag	∼910 nm (TEM); ∼342 nm (DLS)	Thermo-responsive; pH-responsive; catalytic activity	Temperature- and pH-controlled catalysis	84 and 85
PNIPAM	In situ reduction	Ag-Pd	∼155 nm (TEM)	Thermo-responsive; catalytic activity	Temperature-controlled catalysis	86
PNIPAM; methyl chloride quaternized PDMAEMA-co-AA	In situ growth	SiO2	∼1000 nm (DLS); 300–700 nm (SEM/TEM)	Thermo-responsive; tunable surface property	Emulsion stabilization; microbial cultivation and recovery	87–89
PEGDA	Physical encapsulation via microfluidics	TiO2	∼190 µm (SEM)	Catalytic activity; adsorption capability	Dye degradation	90
PNIPAM	In situ reduction	Se	∼360 nm (DLS)	Thermo-responsive; catalytic activity	Dye degradation	91
PEGMA	Electrostatic attraction	Carboxymethyl cellulose	∼121 nm (DLS)	Thermo-responsive; drug loading capacity; self-healing capability	Temperature-controlled drug release; chronic wound treatment	92
PNIPAM-co-FA	Seeded emulsion polymerization	Polystyrene	600–1000 nm (SEM)	Enhanced hydrophobicity	Emulsion stabilization	93
PNIPAM; PEG-graft-PNIPAM	Seed-mediated growth	Metal–organic frameworks	∼270 nm (TEM)	Biocompatible; thermo-responsive	Biolubrication; temperature-controlled drug delivery	94 and 95
PNIPAM; PDEAEMA	Physical encapsulation	Horseradish peroxidase; lipase	∼1762 nm (DLS); ∼800 nm (DLS);	Semi-permeability; catalytic activity	Interfacial biocatalysis; biocatalysis in polar organic solvents	96 and 97
GelMA	Emulsion templating	Multiple enzymes	<15 µm (CLSM)	Semi-permeability; catalytic activity	Enzymatic cascade reactions	98
GelMA	Physical encapsulation via microfluidics	Living cells	∼300 µm (OM); ∼165 µm (OM)	Biocompatible; porous structure	Diabetes treatment; bone tissue repair	99 and 100
PNIPAM-co-AA	In situ reduction + covalent binding + seed-mediated growth	Palygorskite, Au, folic acid	∼560 nm (DLS)	Biocompatible; thermo-responsive; photothermal conversion capability	Controlled drug delivery	101

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3.1. Metal-hybrid microgels

3.1.1. Noble metal-hybrid microgels. Among noble metals, Au and silver (Ag) nanoparticles are the most widely used metallic components for microgel hybridization due to their excellent biocompatibility, chemical stability, and unique functionalities (Fig. 3a and b). Embedding noble metal nanoparticles into microgel networks enables a synergistic combination of their unique properties with the stimuli-responsiveness of microgels, thus fabricating multifunctional hybrid materials with tunable properties. For example, temperature-induced swelling/shrinking of microgels can modulate the surface plasmon resonance of Au nanoparticles by changing the refractive index of their surrounding environment, making Au-hybrid microgels ideal materials for smart optical sensors and photonic devices.⁵⁶

Fig. 3 Morphological characterization of hybrid microgels embedded with various metal nanoparticles. (a) Au-hybrid microgels. Reproduced from ref. 102 with permission from Royal Society of Chemistry. (b) Ag-hybrid microgels. Reproduced from ref. 82 with permission from Royal Society of Chemistry. (c) Fe₃O₄-hybrid microgels. Reproduced from ref. 103 with permission from Royal Society of Chemistry. (d) Cu-hybrid microgels. Reproduced from ref. 83 with permission from Springer Nature. (e) Cu–Pt bimetallic hybrid microgels. Reproduced from ref. 83 with permission from Springer Nature. (f) Au–Ag bimetallic hybrid microgels. Reproduced from ref. 85 with permission from IOP Publishing.

Additionally, many noble metal nanoparticles (e.g., Au) exhibit superior catalytic activity owing to their high specific surface area and abundant active sites.^104,105 After incorporation into stimuli-responsive microgels, the dynamic swelling/shrinking transitions of the microgel matrix enable precise control of mass transport kinetics, thus regulating catalytic reaction rates.⁸⁵ The catalytic activity of Au nanoparticles can be further tuned by precisely controlling their size, morphology, and location in microgels through optimization of Au growth and microgel structures.^78,106 Moreover, the photothermal conversion property of Au nanoparticles enables localized thermal generation under near-infrared irradiation, triggering volume phase transitions of hybrid microgels.⁷⁹ As a result, Au-PNIPAM hybrid microgels have been used to achieve targeted drug delivery and photothermal therapy.¹⁰⁷ In comparison, Ag nanoparticles exhibit intrinsic broad-spectrum antibacterial properties, which can effectively inhibit the growth of various pathogens by releasing Ag⁺ ions to disrupt bacterial membranes. Consequently, Ag-hybrid microgels have emerged as promising antibacterial platforms for wound dressings and antimicrobial coatings.¹⁰⁸

3.1.2. Non-noble metal-hybrid microgels. Besides noble metals, non-noble metals such as iron (Fe) and copper (Cu) nanoparticles can also be incorporated into microgels (Fig. 3c and d). The as-fabricated hybrid microgels have been widely used in environmental treatment and catalysis owing to their low cost and high availability. The superparamagnetism of iron-based nanoparticles (e.g., Fe₃O₄, Fe₂O₃) endows hybrid microgels with remarkable magnetic responsiveness, which not only enables rapid separation and recovery of hybrid microgels using external magnetic fields, but also provides significant advantages for the adsorption and degradation of inorganic/organic pollutants in water.^56,109

Although non-noble metal nanoparticles typically exhibit lower intrinsic catalytic activity than noble metal nanoparticles, microgels can serve as versatile platforms to optimize nanoparticle synthesis, thus improving their stability and overall catalytic performance. For example, Cu-hybrid PNIPAM microgels displayed tunable catalytic activity in 4-nitrophenol (4-NP) reduction and methylene blue (MeB) degradation, with the efficiency regulated by the swollen state of the microgels.¹¹⁰ Although non-noble metal-hybrid microgels largely preserve the intrinsic stimuli-responsiveness of the matrix, the incorporation of metal nanoparticles may alter their responsive properties. It was reported that Cu nanoparticle incorporation enhanced the overall hydrophobicity of microgel matrices, thus reducing their volume phase transition temperature.⁸³ Furthermore, the temperature-controlled swelling/shrinking behavior of Cu-hybrid microgels can indirectly regulate their catalytic activity and adsorption capacity. It is worth noting that non-noble metal nanoparticles exhibit intrinsically lower stability than noble metal nanoparticles due to their higher surface energy, making them prone to oxidation and aggregation, leading to reduced catalytic activity.^56,111 Although microgel networks can alleviate nanoparticle aggregation and enhance stability via steric hindrance and electrostatic interactions, the structural integrity and catalytic stability of non-noble metal-hybrid microgels still need further optimization.

3.1.3. Bimetallic hybrid microgels. Bimetallic hybrid microgels are fabricated by co-loading two different types of metal nanoparticles into the microgel matrix, exhibiting synergistically enhanced performance. Building on monometallic systems, bimetallic systems integrate the properties of both metals, demonstrating superior catalytic performance mainly due to the interparticle electrostatic interactions between different metals.^44,112 Specifically, binary metal nanoparticles significantly improve catalytic efficiency by optimizing the adsorption energy of reaction intermediates and reducing the reaction energy barrier. For example, bimetallic copper–palladium (Cu–Pd) nanoparticle-embedded hybrid PNIPAM microgels exhibited higher activity in 4-NP reduction and MeB degradation than monometallic hybrid microgels (Fig. 3e).⁸³

Additionally, the catalytic activity of bimetallic hybrid microgels can be regulated by external stimuli, leveraging the interplay between metal–metal electronic effects and the swelling/shrinking behavior of microgels. Based on the intrinsic thermal responsiveness of PNIPAM, Au–Ag bimetallic hybrid microgels can realize temperature-controlled catalysis.⁸⁴ It was also reported that the catalytic rate of carboxyl-functionalized Au–Ag bimetallic hybrid microgels can be precisely tuned by environmental pH (Fig. 3f).⁸⁵ Similarly, Ag–Pd bimetallic hybrid microgels showed enhanced catalytic activity compared with monometallic hybrid microgels, with temperature-dependent catalytic activity in 4-NP reduction.⁸⁶ This dynamic regulation capability of bimetallic hybrid microgels enables rational design of intelligent and switchable catalytic systems. Moreover, the microgel matrix imparts excellent stability and recoverability to embedded binary metal nanoparticles, effectively suppressing nanoparticle leaching and aggregation, thus maintaining high catalytic performance during multiple reaction cycles.⁴⁴

3.2. Inorganic non-metallic hybrid microgels

Inorganic non-metallic materials typically possess high specific surface area, exceptional mechanical strength, and tunable optical/electrical properties, yet they often suffer from brittleness, poor processability, and limited biocompatibility.^113,114 Hybridization with microgels addresses these limitations via synergistic “rigid-soft” coupling: the inorganic component acts as a functional scaffold to enhance rigidity and functionality, while the polymer network confers flexibility, dispersibility, and interfacial activity. Silica (SiO₂) is one of the most extensively studied inorganic materials for microgel hybridization. By covalently growing SiO₂ nanoparticles onto PNIPAM microgel surfaces and varying the SiO₂ content, diverse microgel morphologies can be obtained.⁸⁷ As the SiO₂ content increased, dried hybrid microgels evolved from a “fried egg” morphology to a more spherical shape, enabling the customization of hydrophilicity and responsiveness of hybrid microgels. Additionally, SiO₂ hybridization can alter the surface roughness and isoelectric point of microgels.⁸⁸ Li et al. further demonstrated that the nanostructure, composition, and surface properties of SiO₂-hybrid microgels can be finely controlled by modulating the SiO₂ deposition times, cross-linking degrees, microgel composition, and media properties.⁸⁹

In addition to covalent bonding, physical encapsulation via microfluidics allowed the incorporation of titanium dioxide (TiO₂) into polyethylene glycol diacrylate (PEGDA) microgel matrices.⁹⁰ By leveraging the thermal responsiveness of PNIPAM, selenium (Se) nanoparticles can be incorporated into microgels to fabricate Se-hybrid microgels with superior catalytic activity, controllable size, reduced aggregation, high recyclability, and facile separation.⁹¹ Carbon dots (CDs), an emerging class of carbon-based inorganic nanomaterials with excellent optical properties, photoluminescence, and electronic tunability, can endow microgels with optical properties and enhanced responsiveness to various external stimuli.¹¹⁵ CD-embedded microgels also demonstrated improved mechanical strength and biocompatibility, making them suitable for biomedical applications.¹¹⁶

3.3. Organic hybrid microgels

Incorporating organic materials with distinct properties into microgel matrices achieves synergistic “soft–soft” coupling, which modifies the surface properties and endows pristine microgels with additional functionalities while preserving their intrinsic stimuli-responsiveness.¹¹⁷ For example, Liu et al. synthesized poly(ethylene glycol) methacrylate (PEGMA) microgels via precipitation polymerization, then modified them with carboxymethyl cellulose (CMC) through electrostatic interactions to fabricate CMC-hybrid microgels with improved thermo-responsive properties.⁹² Compared to PEGMA microgels, CMC-hybrid microgels exhibited enhanced hydrophilicity, temperature-dependent optical properties, a more compact microstructure, superior self-healing capability, and improved drug loading capacity, thus achieving temperature-controlled drug release. Conversely, Watanabe et al. enhanced microgel hydrophobicity by grafting hydrophobic polystyrene (PS) onto hydrophilic poly(N-isopropylacrylamide-co-fumaric acid) (PNIPAM-co-FA) microgels. The fabricated PS-hybrid microgels overcame the intrinsic hydrophilicity of PNIPAM-co-FA microgels, facilitating the formation of inverse water-in-oil (w/o) Pickering emulsions.⁹³ To impart microgels with additional stimuli-responsiveness, various organic moieties have been incorporated. One example involves complexation between anionic poly(N-isopropylacrylamide-co-allylacetic acid) microgels and cationic surfactants featuring azobenzene-containing tails via electrostatic interactions.¹¹⁸ These surfactants underwent reversible trans-to-cis photoisomerization under light irradiation, altering their hydrophobicity and thereby modulating the swelling/shrinking behavior of surfactant-hybrid microgels to confer dynamic light-responsiveness.

3.4. MOF-hybrid microgels

Compared to conventional inorganic and pure organic materials, MOFs are an emerging class of organic–inorganic hybrid materials with crystalline porous structures formed by the self-assembly of inorganic metal nodes and organic bridging ligands, combining inorganic rigidity with organic flexibility.^119,120 The integration of MOFs into microgels provides distinct advantages, including controlled MOF growth, improved colloidal stability, and enhanced aqueous dispersibility.¹²¹ For example, Neha et al. synthesized Ce-MOF-hybrid microgels by coordinating cerium ions with carboxylic acid ligands within the microgel matrix. These MOF-hybrid microgels integrated the high porosity of Ce-MOFs with the thermo-responsiveness of PNIPAM-based microgels, exhibiting excellent biocompatibility and high sensitivity as fluorescent sensors for detecting trace amounts of antibiotics.¹²² MOFs are also widely exploited as nanocarriers for drug delivery due to their exceptional loading capacity and structural versatility. It was reported that the integration of MOF with microgels created a soft-hard coupled structure with synergistic treatment of osteoarthritis.⁹⁴ Additionally, Wu et al. fabricated core–shell nanoMOFs@PNIPAM microgels for aqueous lubrication and controlled drug release, where the PNIPAM shell thickness can be precisely tuned by varying monomer and crosslinker concentrations.⁹⁵ The results indicate that MOF-hybrid microgels significantly enhanced the lubricity of the aqueous dispersions, while conferring good biocompatibility, sustained drug-release profiles, and thermo-controllable behavior.

3.5. Biohybrid microgels

The biocompatibility, porous microstructure, and stimuli-responsiveness enable microgels to serve as promising carriers for loading biological components, such as enzymes and living cells, to form biohybrid microgels.^125,126 Microgels as enzyme carriers can effectively preserve enzymatic activity and stability. For example, pH-responsive poly(2-(diethylamino)ethyl methacrylate) (PDEAEMA) microgels were utilized to load lipase by exploiting pH-induced reversible swelling/shrinking transitions (Fig. 4a).⁹⁷ Lipase can diffuse into the microgel matrix at low pH, while being stably encapsulated at high pH due to matrix shrinkage. Besides physical trapping, enzymes can be immobilized on microgel surfaces via electrostatic attraction when microgels carry opposite charges. It was reported that positively charged PVCL microgels can adsorb negatively charged P450 monooxygenases. The fabricated biohybrid microgels exhibited enhanced resistance to organic solvents and allowed reversible enzyme release and recapture through simple ionic strength adjustments.¹²⁷ In another approach, Gawlitza et al. immobilized horseradish peroxidase (HRP) within PNIPAM microgels using a solvent exchange method (Fig. 4b). By replacing water with isopropanol, HRP was entrapped within the collapsed microgel matrix, which provided a protective microenvironment to preserve HRP catalytic activity.⁹⁶

Fig. 4 Schematic illustration of representative strategies for fabricating enzyme-loaded biohybrid microgels. (a) Lipase loading in PDEAEMA microgels via pH-driven swelling/shrinking transitions. Reproduced from ref. 97 with permission from American Chemical Society. (b) HRP loading in PNIPAM microgels via solvent exchange. Reproduced from ref. 96 with permission from American Chemical Society. (c) Lipase loading in alginate microgels via the coalescence of w/o emulsion droplets. Reproduced from ref. 123 with permission from Royal Society of Chemistry. (d) Multiple enzymes loading into GelMA microgels via w/w emulsion templating. Reproduced from ref. 98 with permission from Royal Society of Chemistry. (e) Cell encapsulation in microgels using droplet-based microfluidics. Reproduced from ref. 124 with permission from Wiley.

In contrast to enzyme loading via microgel swelling/shrinking transitions, emulsion templating enables in situ enzyme encapsulation during microgel formation. For example, w/o Pickering emulsions were used to prepare lipase-loaded alginate-TiO₂ biohybrid microgels, where Ca²⁺-induced gelation entrapped the enzyme, followed by surface modification to tune wettability (Fig. 4c).¹²³ Similarly, water-in-water (w/w) emulsions allowed mild encapsulation of multiple enzymes in crosslinked methacrylic fish-gelatin (GelMA) microgels (Fig. 4d).⁹⁸ Although emulsion templating generally offers higher encapsulation efficiency and loading capacity than swelling/shrinking-based methods, it is challenging to precisely control the size and uniformity of enzyme-loaded biohybrid microgels.

Extending beyond enzymes, biohybrid microgels can also incorporate living cells, marking a critical step toward the development of “living materials”. The soft, porous polymer network not only protects cellular integrity but also facilitates efficient diffusion of nutrients and oxygen.¹²⁸ The inherently high surface-area-to-volume ratio of microgels promotes rapid cell proliferation, while tunable physicochemical properties, including matrix elasticity and surface roughness, provide valuable platforms for mechanistic investigations of cell–matrix and microgel–organ interactions.^129,130 Employing techniques such as microfluidics, cell–laden biohybrid microgels can be fabricated via either free radical cross-linking or physical crosslinking (Fig. 4e), and have been applied for stem cell expansion, therapeutic delivery, and tissue engineering.^124,131

3.6. Multi-component hybrid microgels

Multi-component hybrid microgels incorporate multiple distinct components, such as metal, inorganic non-metallics, organic functional molecules, and biological entities, to achieve synergistic enhancements in structure and functionality.⁴² However, the fabrication of multi-component microgels is inherently complex, requiring precise control over their composition, morphology, and responsive properties. For example, George et al. developed multi-component hybrid microgels by incorporating Ag nanoparticles into chitosan-acrylic acid (AA) hybrid microgel matrices.¹³² The chitosan-based microgel matrix provided protective steric hindrance, effectively preventing Ag nanoparticle aggregation. As a result, such multi-component microgels enabled rapid, stable, and cost-effective H₂O₂ sensing, as well as potent antibacterial activity towards waterborne bacteria via the synergistic combination of the microgel's high surface area and Ag's intrinsic antibacterial properties. In another work, multi-component hybrid microgels were fabricated by grafting folic acid (FA) onto a PNIPAM-based microgel co-incorporating functionalized palygorskite-Au and AA, resulting in multi-stimuli responsiveness to temperature, pH, and light.¹⁰¹ The reversible swelling/shrinking behavior of microgels allowed controlled drug loading and release. The embedded Au nanoparticles introduced photo-responsiveness, while FA confers receptor-mediated targeting. The combination of organic and inorganic non-metallic materials can also enhance microgel functionality. Li et al. demonstrated that incorporating tetra(4-sulfonatophenyl)porphine molecules and SiO₂ into microgels induced the formation of multi-component hybrid microgels with controllable porous structures and intense fluorescence emission, showing potential for applications in encapsulation, controlled release, labeling, and imaging.⁸⁹

3.7. Structure–property relationships of hybrid microgels

The rational design of high-performance hybrid microgels fundamentally relies on the precise elucidation of structure–property relationships, which serve as the core bridge connecting the fabrication methodology, component regulation, and application performance of these soft colloidal materials. The above classification and discussion based on incorporated components have demonstrated that the integration of diverse functional units can endow microgels with versatile and tunable properties, including multi-stimuli responsiveness, catalytic activity, interfacial activity, and biological activity, through the synergistic coupling between the polymer matrix and functional components.

However, it should be emphasized that most existing studies in this field are still limited to performance descriptions for specific material systems. Due to the extreme diversity of hybrid microgels, the establishment of systematic, universal, and quantitative structure–property relationships for hybrid microgels still remains a critical challenge in the field. Even for the most widely studied PNIPAM-based hybrid microgels, the structure–property correlations show significant differences with changes in crosslinker type, functional component surface chemistry, and hybridization strategy. For other systems, such as bio-based degradable microgels, polyelectrolyte microgels, and non-covalently crosslinked microgels, the relevant structure–property investigations are even more fragmented and inadequate. The systematic and in-depth exploration of structure–property relationships will be the core priority for future research on hybrid microgels, which pave the way for the rational design of high-performance hybrid microgels for specific applications.

4. Emerging applications

Hybrid microgels, owing to their tailored physicochemical properties, such as stimuli-responsiveness, catalytic activity, biocompatibility, and interfacial activity, present significant potential for emerging applications across diverse fields, including catalysis, drug delivery, cell therapy, environmental remediation, sensing, and emulsion stabilization. Optimizing component selection and synthesis strategies is crucial to engineering hybrid microgels with tailored properties to meet the requirements of specific applications.

4.1. Catalysis

Hybrid microgels incorporating metal nanoparticles or enzymes have emerged as innovative catalysts, offering tunable catalytic activity, superior stability, enhanced mass transfer, and facile recycling compared to traditional catalysts.¹³³ For example, Au@PNIPAM hybrid microgels exhibited temperature-dependent catalytic behavior in 4-NP reduction (Fig. 5a).⁸¹ Below the LCST, expanded PNIPAM chains facilitated substrate access to the Au nanoparticle surface, enabling efficient 4-NP reduction. Above the LCST, chain collapse restricted diffusion, significantly suppressing the catalytic activity of Au nanoparticles. This thermo-responsive modulation offers precise control over the catalytic reaction that is not attainable with bare Au nanoparticles. In a more complex design, Tzounis et al. developed core–shell-satellite AuAg@PNIPAM@Ag hybrid microgels, featuring an Au core coated by a thermo-sensitive PNIPAM-co-ALA shell, followed by deposition of Ag satellite nanostructures.⁸⁴ These fabricated bimetallic hybrid microgels functioned as a highly efficient catalyst for 4-NP reduction, with reaction rates finely modulated by temperature-induced swelling/shrinking transitions of microgels. A similar environmental control of catalytic performance can be achieved by incorporating pH-responsive comonomers, such as AA. Bhol et al. synthesized PNIPAM-co-AA@AgAu hybrid microgels, which exhibited pH-dependent catalytic activity in 4-NP reduction, with slower reaction kinetics at higher pH values (Fig. 5b).⁸⁵

Fig. 5 Schematic illustration of the catalytic activity regulation by using hybrid microgels. (a) Temperature-dependent regulation of Au@PNIPAM hybrid microgels below and above the LCST. Reproduced from ref. 81 with permission from American Chemical Society. (b) pH-dependent regulation of PNIPAM-co-AA@AgAu hybrid microgels in 4-NP reduction. Reproduced from ref. 85 with permission from IOP Publishing. (c) Versatile enzymatic cascade reactions enabled by multi-enzyme-loaded biohybrid microgels. Reproduced from ref. 98 with permission from Royal Society of Chemistry.

Beyond metal nanoparticles, enzymes exhibit excellent selectivity in biocatalysis and can be immobilized in microgel matrices through either covalent or non-covalent interactions.¹³⁴ Compared to covalent immobilization, non-covalent immobilization offers milder conditions that better prevent enzyme conformation changes and enable enzyme recovery after catalysis. Emulsion templating and droplet microfluidics provide versatile platforms for in situ enzyme loading in aqueous droplets prior to gelation.¹³⁵ For example, Okuno et al. employed w/w emulsion droplets to independently or simultaneously encapsulate multiple enzymes (e.g., horseradish peroxidase (HRP), glucose oxidase (GOx), and β-galactosidase), followed by photo-crosslinking to form enzyme-loaded biohybrid microgels with high loading capabilities, which served as microreactors for efficient enzymatic cascade reactions (Fig. 5c).⁹⁸ Alternatively, post-formation physical trapping is also feasible for enzyme loading. It was reported that lipase can diffuse into the matrix of PDEAEMA microgels in their swollen state and become entrapped by pH-induced shrinkage.⁹⁷ The well-preserved enzymatic activity, combined with the interfacial activity of microgels, allowed cooperation with hydrophobic SiO₂ nanoparticles to stabilize w/o Pickering emulsions, enabling recyclable interfacial biocatalysis.

4.2. Drug delivery

Owing to the customizable size, tunable softness, excellent biocompatibility, and stimuli-responsiveness, hybrid microgels provide powerful platforms for drug delivery. Precise spatiotemporal control over drug release profiles can be achieved through various stimuli, preventing undesired drug release in non-target tissues, minimizing adverse effects, and enriching drug accumulation at target tissues through specific modifications.^136,137 For example, Liu et al. engineered multi-component hybrid microgels incorporating palygorskite, Au nanoparticles, NIPAM, AA, and FA, which exhibited good biocompatibility, enhanced cellular uptake, and reduced cytotoxicity.¹⁰¹ The synergistic effect among different components enabled high drug loading efficacy and multi-stimuli-triggered release of doxorubicin to the target region. Magnetic responsiveness has also been harnessed for non-invasive control of drug delivery. Almeida et al. developed pectin maleate@PNIPAM@Fe₃O₄ hybrid microgels using emulsion templating, which can serve as drug carriers for efficient curcumin encapsulation, demonstrating good cytocompatibility towards healthy Vero cells while effectively inhibiting the growth of diseased Caco-2 colon cancer cells.¹³⁸ Additionally, the release profiles of curcumin can be precisely modulated by the external magnetic field, temperature, and pH, offering multifaceted control for enhanced anticancer therapy.

Hybrid microgels are also promising candidates for antimicrobial materials to combat bacterial infections. It is well known that Ag nanoparticles exert bactericidal effects through sustained release of Ag⁺ ions, which disrupt bacterial membranes, induce cellular content leakage and generate reactive oxygen species that damage DNA.^139,140 By incorporating Ag nanoparticles into microgels, the porous structure facilitated controlled Ag⁺ ion discharge into the aqueous medium, yielding superior sustained antimicrobial properties compared to free Ag nanoparticles.^141,142 Besides metal nanoparticle antimicrobial agents, organic antimicrobial drugs can also be loaded in hybrid microgels. For example, chitosan-polyaniline hybrid microgels allowed vancomycin (VM) loading, which resisted strong gastric acidity and prevented premature VM leakage in healthy gastrointestinal tracts, while achieving rapid VM release in the simulated inflamed intestine microenvironment via lysozyme-triggered chitosan biodegradation.¹⁴³ This system exhibited potent antimicrobial activity against Staphylococcus aureus, demonstrating potential for inflammatory bowel disease treatment. Notably, metal-hybrid microgels for in vivo drug delivery may lead to long-term retention or release of metal components. The long-term retention risk of metal components in hybrid microgels is primarily dependent on nanoparticle size and surface chemistry. Specifically, ultrasmall nanoparticles (<5.5 nm) are mainly cleared via renal filtration, resulting in minimized toxicity, while larger nanoparticles tend to accumulate in the liver, lung, and spleen, making them difficult to eliminate from the body.¹⁴⁴ Meanwhile, the surface chemistry of metal nanoparticles, including surface charge, wettability, and ligand modification, is the core factor regulating the in vivo protein corona formation, cellular phagocytosis, and final metabolic clearance pathways, which directly affects their long-term retention risk.¹⁴⁵

4.3. Cell therapy

In cell therapy, biohybrid microgels function as “cell carriers” designed to replace or repair damaged tissues and promote organ regeneration.^147,148 Conventional cell therapy involves direct injection of therapeutic cells into the affected sites. However, the inherent fragility of these cells often leads to poor retention, low survival rates, and limited therapeutic efficacy. Biohybrid microgels provide a protective, biomimetic microenvironment that significantly improves cell engraftment, viability, and functionality post-implantation.¹⁴⁹ For example, Sun et al. employed microfluidics to directly co-encapsulate β-cells and mesenchymal stem cells (MSCs) within droplets containing GelMA and poly(ethylene oxide) (PEO), followed by photo-crosslinking and PEO removal to fabricate porous cell–laden biohybrid microgels (Fig. 6a). The favorable biocompatibility and interconnected porous structures of biohybrid microgels facilitated cell proliferation and insulin outflux, highlighting their potential for diabetes treatment.⁹⁹ For bone tissue repair, Zhao et al. developed injectable osteogenic scaffolds by encapsulating bone marrow-derived mesenchymal stem cells (BMSCs) and growth factors within GelMA microgels fabricated via microfluidics (Fig. 6b).¹⁰⁰ The microgel matrix not only protected BMSCs from mechanical stresses, enhanced their viability, proliferation, migration, and osteogenic differentiation, but also enabled the prolonged growth factor release, significantly promoting bone formation and ossification. Very recently, double-responsive biohybrid microgels loaded with Salmonella-targeting phage cocktail were developed through electrohydrodynamic spraying for improved oral phage delivery (Fig. 6c).¹²⁶ These biohybrid microgels effectively shield phages from harsh gastric environments, extend intestine residence time, and trigger targeted colon release, ultimately promoting gut microbiota balance and alleviating colitis symptoms. To further illustrate the versatility of biohybrid microgels in cell therapy, Tian et al. developed magnetically actuated micro-robots by encapsulating MSCs and magnetic nanoparticles within sodium alginate microgels fabricated via emulsification (Fig. 6d).¹⁴⁶ Under electromagnetic control, the micro-robot enabled targeted MSC delivery with high precision, achieving improved cell retention and rapid healing of in vitro scratch wounds.

Fig. 6 Schematic illustration of biohybrid microgels in cell therapy applications. (a) Porous biohybrid microgels co-encapsulating β cells and MSCs for diabetes treatment. Reproduced from ref. 99 with permission from Wiley. (b) BMSC-laden GelMA biohybrid microgels for osteogenesis and bone generation. Reproduced from ref. 100 with permission from Wiley. (c) Salmonella phage-loaded biohybrid microgels for restoring gut microbiota balance and alleviating colitis symptoms. Reproduced from ref. 126 with permission from Springer Nature. (d) Magnetic biohybrid microgels co-encapsulating magnetic nanoparticles and MSCs for targeted cell delivery and regenerative healing. Reproduced from ref. 146 with permission from Wiley.

4.4. Environmental remediation

Due to their porous structure, high surface area, and stimuli-responsiveness, hybrid microgels enable efficient molecule capture and catalytic conversion, offering unique advantages for wastewater treatment, particularly in heavy metal ion adsorption and organic dye degradation. For example, Pd-hybrid microgels enabled the adsorption and catalytic degradation of 4-NP.^150,151 Additionally, Au- or Ag-embedded hybrid microgels have been extensively reported to catalyze the reduction of organic dyes, such as Congo red and MeB.^152–154 Besides metal nanoparticles, certain inorganic non-metallic nanoparticles also exhibit remarkable catalytic properties for pollutant treatment. Garg et al. developed Se-embedded PNIPAM hybrid microgels as visible-light photocatalysts for dye degradation.⁹¹ The PNIPAM layer coated on Se nanoparticles effectively prevented agglomeration and recombination, thus enhancing catalytic performance, while the stimuli-responsiveness of microgels enabled controllable photocatalytic efficiency and easy catalyst recycling. Similarly, Chen et al. developed TiO₂-PEGDA hybrid microgels via microfluidics, which achieved efficient adsorption and removal of MeB from solutions under low UV irradiation, with no photocatalytic efficiency loss over 10 cycles.⁹⁰

Recent advances have extended hybrid microgels to heavy metal ion adsorption. Wi et al. developed physically crosslinked Fe-aminoclay/polyvinyl alcohol hybrid microgels via microfluidics, achieving high adsorption capacity for various metal ions, including Cr⁶⁺, Au³⁺, and Pd²⁺, with excellent reusability and magnetic separability.¹⁵⁵ Beyond direct adsorption, dual-function adsorption-catalysis can be realized in hybrid microgels. Arif et al. synthesized core–shell SiO₂–chitosan hybrid microgels for Pd²⁺ ion extraction from water, followed by in situ reduction to Pd nanoparticles on the shell. These Pd-embedded hybrid microgels were further utilized for catalytic reduction of organic pollutants, such as 4-NP, MeB, and Rhodamine B, from aqueous medium, demonstrating integrated pollutant treatment in a single system.¹⁵⁶

4.5. Sensing

By leveraging the stimuli-responsiveness to convert target recognition into detectable signals, hybrid microgels enabled sensitive detection in complex media, such as biological fluids or microenvironments.¹⁵⁷ Krajczewski developed pH-responsive Ag-hybrid PAA microgels, where pH-induced volume changes of microgels modulated the surface-enhanced Raman scattering (SERS) response of Ag nanoparticles.¹⁵⁸ This pH-dependent SERS response demonstrated the potential of Au-hybrid microgels for pH-sensitive detection of dye molecules, such as Nile blue. Another innovative study utilized Au-hybrid PEGDA microgels for SERS detection of tricyclazole in milk.⁶⁰ The microgel's selective permeability excluded adhesive proteins, keeping Au nanoparticles uncontaminated and enabling direct SERS detection of small target molecules in protein solutions without pretreatment. Furthermore, enzyme-based sensing was demonstrated in core–shell Fe₃O₄-hybrid microgels with covalently immobilized GOx and HRP in the shell.¹⁵⁹ In the presence of GOx, glucose oxidation yielded hydrogen peroxide, which further reacted with acetylacetone under HRP catalysis to form a fluorescent product, enabling sensitive colorimetric glucose determination. The Fe₃O₄ core further enhanced the thermal stability and reusability of hybrid microgels for glucose sensing.

4.6. Emulsion stabilization

Microgels, with their tunable amphiphilicity and stimuli-responsiveness, can serve as stabilizers for the preparation of smart Pickering emulsions.^5,161–163 By incorporating functional components, hybrid microgels enable dynamic control over emulsion properties, thus expanding their application scenarios. For example, Jiang et al. grafted hydrophobic SiO₂ nanoparticles onto PNIPAM microgel surfaces via either emulsion templating or in situ growth.^87,164 The fabricated SiO₂-hybrid microgels exhibited tunable surface properties, enabling the stabilization of stimuli-responsive Pickering emulsions with temperature-dependent stability for anaerobic microbial cultivation, which significantly enhanced bacterial survival rates and culture efficiency (Fig. 7a). By leveraging the electrostatic attraction between oppositely charged PNIPAM-co-ALA microgels and Au nanoparticles, Guan et al. fabricated Au-hybrid microgels with photothermal responsiveness.³³ These hybrid microgels not only stabilized o/w Pickering emulsions, but also enabled the light-driven spatiotemporal manipulation of emulsion droplets relying on the “cascade effect” between Au nanoparticles and microgels (Fig. 7b). The plasmonic photothermal effect of Au nanoparticles converted light to heat, inducing localized heating and microgel desorption from the interface, thus realizing precise regulation of demulsification, droplet merging, bubble generation, and inter-droplet exchange. More recently, Wang et al. fabricated Janus hybrid microgels with appropriate hydrophobicity, stimuli-responsiveness, and catalytic activity, which served as emulsion stabilizers for one-pot removal of organic pollutants from aqueous medium (Fig. 7c).¹⁶⁰ Au nanoparticle loading enabled efficient conversion of water-soluble dyes (e.g., methyl orange) into insoluble products that transferred to the oil phase, while the thermal responsiveness of PNIPAM chains enabled rapid emulsion demulsification by a light-induced temperature increase above the LCST, achieving separation and recovery of hybrid microgels and the pollutants.

Fig. 7 Schematic illustration of hybrid microgel-stabilized Pickering emulsions for advanced applications. (a) SiO₂@PNIPAM hybrid microgel-stabilized w/o Pickering emulsions for recyclable anaerobic fermentation and bacteria separation via temperature-induced phase separation. Reproduced from ref. 87 with permission from American Chemical Society. (b) Plasmonic Au-hybrid microgel-stabilized o/w Pickering emulsions for light-driven droplet manipulation. Reproduced from ref. 33 with permission from Wiley. (c) Janus hybrid microgel-stabilized o/w Pickering emulsions for the removal of water-soluble organic pollutants via temperature-induced phase separation. Reproduced from ref. 160 with permission from American Chemical Society.

5. Conclusions and perspectives

This review presents a comprehensive overview of hybrid microgels, including their fabrication strategies, classification based on incorporated components, structure-dependent properties, and emerging applications. Due to their tunable physicochemical properties, hybrid microgels serve as versatile platforms that have received considerable attention in different fields. In particular, the integration of diverse components endows microgels with synergistic functionalities, such as enhanced catalytic efficiency, multi-stimuli responsiveness, protective cell microenvironments, and dynamic emulsion control, making them highly adaptable to complex demands in bio-manufacturing, biomedicine, environmental protection, and materials engineering.

Nevertheless, despite significant advances in hybrid microgel fabrication, several key challenges remain in this field. First, achieving a precise spatial distribution of functional components within microgels is difficult. Second, the large-scale production of hybrid microgels still faces a trade-off between structural controllability and manufacturing scalability. Microfluidics enables precise control over the size, morphology, and component distribution of hybrid microgels and can construct complex topological structures, but its scale-up is limited by frequent channel clogging during continuous production, loss of droplet uniformity in multi-channel parallel manufacturing, poor batch-to-batch consistency, and high production costs. In comparison, conventional fabrication strategies, such as in situ hybridization and seed-mediated growth, feature simple operation, low equipment costs, and easy batch amplification, but frequently require time-consuming multi-step reactions and lack precise control over the internal structure of hybrid microgels. Third, for multi-component hybrid microgels, precise characterization and control of their internal structures remain challenging, and the heterogeneity of multiple components may lead to inconsistent performance, such as uneven distribution of active sites or impaired stimuli-responsiveness. For biomedical applications, systematic long-term in vivo biosafety evaluation of hybrid microgels, especially the metabolic clearance and potential retention of embedded nano-metal components, remains to be investigated to advance clinical translation. Meanwhile, the on-demand precision of stimuli-triggered drug delivery still needs further optimization. Moreover, studies on hybrid microgels as emulsion stabilizers remain relatively scarce. To expand the applications of hybrid microgel-stabilized emulsions, further investigations into the interfacial behavior of hybrid microgels and the stabilization mechanisms of emulsions are required.

Overall, leveraging the versatile integration of diverse components, we envisage that rationally designed hybrid microgels have significant promise as next-generation smart materials. Continued innovation in fabrication strategies, characterization techniques, multi-stimuli responsive designs, and biosafety improvements will address existing challenges and drive the translation of hybrid microgels from fundamental research to practical applications.

Author contributions

H. C. and Y. Z contributed to conducting the literature review, organizing the figures, and writing the original draft. M. Z. and L. W. L. contributed to reviewing and editing. X. G. contributed to conceptualization, writing – review and editing, project administration, and supervision.

Conflicts of interest

There are no conflicts to declare.

Data availability

No primary research results, software or code have been included and no new data were generated or analyzed as part of this review.

Acknowledgements

The authors gratefully acknowledge the financial support from the Guangdong Basic and Applied Basic Research Foundation (2026A1515012441), the National Natural Science Foundation of China (22322303), the Natural Science Foundation for Distinguished Young Scholars of Guangdong Province (2023B1515020001), the Shenzhen Science and Technology Program (JCYJ20250604182413018), and the Scientific Foundation for Youth Scholars of Shenzhen University (868-000001033347).

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Footnote

† Hao Chang and Yue Zheng contributed equally.

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