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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2024 , Vol. 36 >Issue 10: 1594 - 1606

DOI: https://doi.org/10.7536/PC240305

综述

Research Progress of Nanofiber Composite Hydrogels

  • Yvqing Ma 1 ,
  • Zheng Li , 1, * ,
  • Guobao Zheng 2 ,
  • Songnan Zhang 1 ,
  • Jixian Gong 1 ,
  • Changsheng Qiao 3
Expand
  • 1 State Key Laboratory of Separation Membrane and Membrane Processes/International Joint Research Center for Separation Membrane Science and Technology/Key Laboratory of Advanced Textile Composites, Ministry of Education, Tianjin Polytechnic University, Tianjin 300387, China
  • 2 Agricultural Biotechnology Research Center, Ningxia Academy of Agricultural and Forestry Sciences, Yinchuan 750002, China
  • 3 School of Bioengineering, Tianjin University of Science and Technology, Tianjin 300457, China
* ;

Received date: 2024-03-04

  Revised date: 2024-03-28

  Online published: 2024-04-16

Supported by

Ningxia Key Research and Development Project(2022BEG02006)

Tianjin Key Research and Development Project(20YFZCSN00130)

Central Government of Heilongjiang Province Guides Funds(ZY23CG35)

Ningxia Autonomous Region Flexible Introduction of Science and Technology Innovation team(2021RXTDLX08)

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Abstract

Hydrogels have become one of the most widely researched materials across disciplines due to their excellent softness, wettability, responsiveness and biocompatibility. However, the mechanical properties of hydrogels are poor and cannot meet the use of some special materials. Nanofibers have been used to prepare nanofiber composite hydrogels with nano-size, porous structure and tunable mechanical properties due to their high aspect ratio, uniform fiber morphology and easy functionalization. Nanofiber composite hydrogels have suitable mechanical properties, ductility, adhesion, and the ability to mimic the microstructure of the extracellular matrix (ECM) and the microenvironment of the cell, which makes them widely used in many fields. This paper summarizes the classification of nanofiber composite hydrogels, their preparation methods and their development and application in the fields of multifunctional wound dressings, tissue engineering, sensors, and filter absorption materials future development.

Contents

1 Introduction

2 Nanofiber composite hydrogel classification

2.1 Organic nanofiber composite hydrogel l

2.2 Inorganic nanofiber composite hydrogel

2.3 Organic-inorganic hybrid nanofiber composite hydrogels

3 Preparation method of nanofiber composite hydrogel

3.1 Doping method

3.2 lamination method

3.3 Other methods

4 Nanofiber composite hydrogel application

4.1 Multifunctional wound dressing

4.2 Tissue engineering

4.3 Conductive sensors

4.4 Absorbent filter material for dye and metal ion removal

5 Conclusions and outlook

Key words： nanofiber; hydrogel; composite material; mechanical property

Cite this article

Yvqing Ma , Zheng Li , Guobao Zheng , Songnan Zhang , Jixian Gong , Changsheng Qiao . Research Progress of Nanofiber Composite Hydrogels[J]. Progress in Chemistry, 2024 , 36(10) : 1594 -1606 . DOI: 10.7536/PC240305

1 Introduction

Hydrogels are composed of polymer networks swollen in water or other liquids[1]. Due to their cross-linked polymer network, hydrogels exhibit solid-state (elastic) properties, featuring deformability and softness[2]. On the other hand, the high water content in hydrogels gives them liquid-like (viscous) characteristics, including permeability to a variety of chemical and biological molecules[3]. This combination of solid and liquid states endows hydrogels with other unique properties, such as swelling and responsiveness[4]. Today, due to their excellent softness, wetness, responsiveness, biocompatibility, and bioactivity, hydrogels have become one of the most widely studied materials in interdisciplinary research fields[3,5,6]. They are extensively used in wound dressings[7⇓~9], tissue engineering[10,11], wearable electronics[12⇓~14], and absorbent materials[15⇓~17]. However, traditional hydrogels with loose cross-linking and homogeneous structure are weak and brittle, which greatly limits their applications[18⇓~20]. Researchers have explored various methods to improve the mechanical strength of hydrogels, such as double-network hydrogels, topological hydrogels with sliding cross-linkers, and chemically cross-linked hydrogels[21,22]. Although the mechanical strength of hydrogels has been improved to some extent, issues such as residual chemical cross-linking agents, cytotoxicity, and limited mechanical strength still restrict their practical applications[23,24].
Natural materials such as fibrous tissues (such as skin, muscle, and ligaments) exhibit strong mechanical properties, which are attributed to the nanofibers (such as collagen fibrils) embedded in the soft matrix and their interactions[25]. For example, for tendon tissue, the tightly arranged collagen fibers give the tendon high tensile strength, allowing it to firmly connect muscles and bones[10]. Therefore, introducing nanofibers into a hydrogel matrix can significantly improve the mechanical properties of the hydrogel[26]. This is because when a load is applied, the nanofibers can share the load, thereby minimizing damage to the matrix and enhancing the mechanical strength of the composite hydrogel[27,28]. In addition, nanofibers can mimic the structure of the ECM[29], thus giving the composite hydrogels additional biological functions (mimicking the ECM structure and cellular microenvironment of tissues)[30]. Nanofiber composite hydrogels can be classified into: organic nanofiber composite hydrogels, inorganic nanofiber composite hydrogels, and organic-inorganic hybrid nanofiber composite hydrogels. Common organic nanofiber composite hydrogels include: cellulose nanofiber (CNF) composite hydrogels, bacterial cellulose nanofiber (BC-f) composite hydrogels, polylactic acid (PLA) nanofiber composite hydrogels, aramid nanofiber (ANF) composite hydrogels, and polycaprolactone (PCL) composite hydrogels. Common inorganic nanofiber composite hydrogels include silica (SiO2) nanofiber composite hydrogels, carbon nanofiber powder (CFP) composite hydrogels, and hydroxyapatite nanofiber (HANF) composite hydrogels. Currently, methods such as blending, multilayer lamination, and polymerization have been developed to prepare nanofiber composite hydrogels[31].
This paper aims to introduce the classification, preparation methods, and application fields of nanofiber composite hydrogels, and to provide an outlook on the future research and development of nanofiber composite hydrogels.

2 Classification of Nanofiber Composite Hydrogels

In recent years, nanofibers have been considered ideal reinforcing materials for preparing composite hydrogels with excellent mechanical properties due to their extremely high aspect ratio, good flexibility, and wide range of raw materials[25]. The fibers used in nanofiber-hydrogel composites can come from different sources, including organic nanofibers, inorganic nanofibers, and organic-inorganic hybrid nanofibers (Table 1).
表1 纳米纤维复合水凝胶分类及应用

Table 1 Nanofiber composite hydrogel classification and applications

Nanofiber Hydrogel material Features & Applications Ref
Cellulose nanofibers
(CNF)		 Gelatine (Gel); Gelatinized methacrylate (GelMA) 3D bioprinting of biomedical scaffolds.
Modulation of gelatin gel properties with potential application in gelatin food additives		 32,33
Bacterial cellulose nanofiber (BC-f) Polyacrylamide (PAM) composite hydrogels; Poly(γ-glutamic acid) (γ-PGA) hydrogel High mechanical properties with great potential for wound dressings and biomedical applications.
High mechanical strength, on-demand drug release, smart drug delivery patch		 34,35
Polylactic acid (PLA) nanofiber GelMA Alginate (Alg)-hyaluronic acid (HA) hydrogel (Alg-HA) Providing a suitable microenvironment for bone tissue engineering repair.
Meet the mechanical properties of cartilage tissue, no cytotoxicity, suitable for chondrocyte migration.		 36,37
Polycaprolactone (PCL) nanofiber Gelatin-based hydrogel;
Composite hydrogel		 3D hybrid scaffolds are mimicking natural heart tissue structures.
Bone regeneration in bone repair in clinical therapy		 38,39
Aramid nanofiber (ANF) polyvinyl alcohol (PVA) Implantable tissue prosthetics, tendon-mimetic hydrogels, flexible Electronic Sensors 10,40
Silicon dioxide (SiO2) nanofiber Vinylsilane/Sodium/Alginate/
Polyacrylamide composite hydrogel; Siloxane-derived hydroxy propyl methyl cellulose composite hydrogel		 Bionic electronic sensors with high mechanical strength.
Injectable materials for minimally invasive cartilage surgery		 25,41
Carbon nanofiber powder (CFP) Alginate (Alg) based hydrogels New biomaterial for bone tissue engineering scaffolds,
topical drug-delivery carriers		 42,43
Hydroxyapatite nanofiber (HANF) GelMA Great potential for application in tissue engineering. 44
Organic-inorganic hybrid nanofiber
 Chitosan (CS) hydrogel; GelMA Good anti-scaling properties, filtration of dyes in wastewater.
Self-assembly of nanofibers within hydrogels for applications in biomimetic scaffolds		 45,46

2.1 Organic Nanofiber Composite Hydrogel

2.1.1 Natural Organic Nanofiber Composite Hydrogels

Natural organic nanofibers exhibit excellent cell behavior[47] and good mechanical properties. When combined with hydrogels, they not only can produce nanofiber composite hydrogels with high mechanical performance but also absorb wound exudates to create a favorable healing environment for wounds[48].
The high surface area and porosity of CNF are used to develop nanofiber composite hydrogels with high mechanical properties, which can create an ideal environment for cell adhesion, proliferation, migration, and cell regeneration[48,49]. BC-f, due to its high degree of polymerization, high crystallinity, and good water retention, is considered an ideal material for enhancing the mechanical properties of hydrogels[50,51].
Zhong et al.[52]significantly enhanced the mechanical properties (303% elongation at break) of polyvinyl alcohol (PVA) based hydrogels through CNF, and introduced neomycin as an antibacterial agent into the hydrogel network. This allowed the composite hydrogel to effectively resist a broad spectrum of bacteria and provide stable antibiotic delivery to the wound site. Das et al.[53]prepared CNF-reinforced cellulose-based hydrogel composites. With the increase in CNF content, the tensile strength of the composites was greatly improved. When the CNF content reached 0.7%, the tensile strength of the composite was 4 times that of pure cellulose-based hydrogels. Dou et al.[34]covalently crosslinked poly-γ-glutamic acid (γ-PGA) with bacterial cellulose nanofibers to obtain γ-PGA/BC composite hydrogels (see Figure 1). Due to more effective energy dissipation through hydrogen bonding, the γ-PGA/BC composite hydrogel exhibited a high compressive strength of 5.72 MPa, 8.16 times that of pure γ-PGA hydrogels. More importantly, BC-f could be uniformly dispersed in the γ-PGA hydrogel, giving the γ-PGA/BC composite hydrogel good interfacial compatibility. Park et al.[35]incorporated BC-f into polyacrylamide (PAM) hydrogels. Under the reinforcement of BC-f, the compressive strength of the composite hydrogel was 3 times that of pure PAM hydrogels. Additionally, the composite hydrogel could delay drug release by twice as long, while also enabling on-demand drug release according to the applied stress, creating a favorable environment for wound healing.
图1 γ-PGA/BC纳米纤维复合水凝胶机理图[34]

Fig. 1 Mechanism diagram of γ-PGA/BC nanofiber composite hydrogel[34]

2.1.2 Synthesis of Organic Nanofiber Composite Hydrogels

In addition to natural nanofibers, synthetic organic nanofibers are used to prepare biomimetic (such as tendons and articular cartilage) materials with high mechanical properties[55] due to their excellent mechanical properties and suitable degradation characteristics[54]. ANF, because of its extremely high strength, is used to prepare nanofiber composite hydrogels with excellent strength and stiffness. Polylactic acid (PLA) nanofibers, which have good mechanical properties and excellent biological properties, are used to prepare high-strength biobased composite hydrogels[36,56]. Polycaprolactone (PCL) nanofibers can be used to prepare nanofiber composite hydrogels with tunable mechanical and biological properties[48].
Sun et al.[10] constructed anisotropic composite hydrogels (ACH) through ANF and PVA. The highly oriented network within ACH can simulate the microstructure between aligned collagen fibers and proteoglycans (Figure 2a), giving it high mechanical properties that match those of natural tendons (elastic modulus 1.1 GPa, tensile strength 72 MPa). Zhou et al.[40] prepared a sandwich-structured hybrid hydrogel with ANF-reinforced PVA, where the reinforcement by ANF and the heterogeneous structure endowed the hydrogel with a high tensile strength of 5.5 MPa and a high elastic modulus of 15.4 MPa.
图2 （a）ACH中ANF的增强作用和高度定向的网络[10]；（b）APA/CMCS/KGN@PGF/GM制备示意图，具有优异的生物相容性[11]；（c）在PCL/Gel纳米纤维上涂覆GelMA凝胶用于组织工程支架[57]

Fig. 2 (a) Enhancement of ANF in ACH and highly oriented networks[10]. (b) Schematic diagram of APA/CMCS/KGN@PGF/GM with excellent biocompatibility[11]. (c) GelMA gel coated on PCL/Gel nanofibers for tissue engineering scaffolds[57]

Liu et al[11]prepared PGF nanofibers with PLA nanofibers as the shell and gelatin (Gelatine, Gel) as the core, and combined them with aldehydized polyethylene glycol/carboxymethyl chitosan hydrogel (APA/CMCS) to prepare injectable hydrogels (APA/CMCS/KGN@PGF/GM) with nanofiber composite microchannels (Figure 2b). The PGF nanofibers endowed the hydrogel with a microchannel structure, which promoted the proliferation and inward growth of bone marrow mesenchymal stem cells (Bone marrow mesenchymal stem cells, BMSC). Yue et al[36]incorporated silk fibroin (Silk fibroin, SF)/PLA nanofibers into methacrylated gelatin (gelatin methacrylate, GelMA) hydrogels, and the compressive modulus (2.65±0.31) MPa of the nanofiber composite hydrogel reached a level comparable to that of natural cartilage. Mohabatpour et al[37]incorporated PLA nanofibers into alginate-hyaluronic acid (alginate- hyaluronic acid, Alg-HA) hydrogels, increasing the compressive modulus by about 81% and providing a suitable ECM microenvironment for chondrocytes.
Hong et al.[57] incorporated PCL/Gel nanofibers into GelMA to prepare tissue engineering scaffolds with good mechanical strength (the stiffness of the GelMA hydrogel was increased by 3 times) and bioactivity (Figure 2c). Zare et al.[47] enhanced the mechanical properties of Alg-based hydrogels through PCL/Gel nanofiber layers, while making the composite hydrogels exhibit an environment suitable for cell growth. The composite hydrogels are considered promising candidates for cartilage regeneration.

2.2 Inorganic Nanofiber Composite Hydrogels

Inspired by natural bone tissue, integrating inorganic nanofibers into hydrogels not only improves the mechanical strength of the composite hydrogels but also mimics the natural bone tissue[58,59]. For example, CFP supports the adhesion and proliferation of osteoblasts[60], and the incorporation of CFP can combine the properties of hydrogels with the biological and mechanical properties of CFP to prepare composite hydrogels with high mechanical strength and biological performance[42]. HANF, characterized by high hardness and good biocompatibility, is used as a reinforcing material to improve the mechanical properties of hydrogels[27,59]. Silica (SiO2) nanofibers and hydrogel composites form a more compact microstructure and higher mechanical strength of nanofiber-reinforced hydrogels[61].
Dibazar et al[42]incorporated CFP into Ca2+crosslinked polyglucuronic acid (polyglucuronic acid, PGU)/sodium alginate (sodium alginate, Alg) to prepare PGU/Alg/CFP nanofiber composite hydrogels. The incorporation of CFP induced the formation of interconnected porous structures within the hydrogel, providing space for the migration of bone cells in PGU/Alg/CFP and enhancing the success rate of bone regeneration. Llorens-Gámez et al[43]introduced CFP-reinforced Alg-based hydrogels and characterized their physical and biological properties. The addition of 2% w/w CFP increased the tensile modulus and compressive modulus of the hydrogel by 3 times and 6 times respectively, and exhibited excellent biological properties in human keratinocytes.
Li et al[44]prepared HANF/GelMA composite hydrogel by combining HANF with GelMA. The entanglement of HANF allowed HANF/GelMA to form a tighter continuous structure, enhancing its mechanical properties. HANF/GelMA composite hydrogel can provide a good three-dimensional growth environment for bone tissue cells. Wang et al[59]incorporated HANF into GelMA to mimic the structure and composition of natural bone tissue, improving the biocompatibility, mechanical properties, and bone regeneration performance of GelMA.
Lu et al.[25]prepared a SiO2nanofiber reinforced hydrogel (SFRH) by integrating flexible SiO2nanofibers (SNF) and vinyl silane/sodium alginate/PAM (Figure 3). Under the strong interfacial chemical bonding between SNF nanofibers and PAM chains, SFRH has a tensile strength of 0.3 MPa and a Young's modulus of 0.11 MPa at a strain of 1400%, without undergoing plastic deformation after 1000 cycles of stretching. Mi et al.[62]added SiO2nanofibers to cellulose-based gels to prepare a superhydrophobic composite gel. The continuous SiO2nanofibers provided mechanical stiffness and structural complexity to the gel, giving it high compressive performance and mechanical recoverability. Buchtová et al.[41]adjusted the rigidity of the composite hydrogels through the chemical affinity between SiO2nanofibers and silanol groups of the side chains of silane-derived hydroxypropyl methylcellulose, thereby affecting the morphology and function of chondrocytes under load.
图3 仿生化学集成SiO2纳米纤维增强水凝胶的制备[25]

Fig. 3 Preparation of biomimetic chemically integrated SiO2 nanofiber reinforced hydrogel[25]

2.3 Organic-Inorganic Hybrid Nanofiber Composite Hydrogel

Nanofiber membranes made of pure polymer materials have the characteristics of poor strength and toughness, and lack functional factors, which limits their application range[63]. Researchers have attempted to prepare organic-inorganic hybrid nanofibers, so that such hybrid nanofiber membranes possess both the good flexibility and plasticity of organic polymers, as well as the excellent thermal stability, chemical stability, and high strength of inorganic materials[64].
Cai et al.[45]prepared hybrid nanofibers of carboxyl multi-walled carbon nanotubes (carboxyl multi-walled carbon nanotubes, cMWCNTs) grafted BC (BC-g-cMWCNTs), which were coated with chitosan hydrogel to produce a chitosan-coated BC-g-cMWCNTs nanofiber composite gel (Figure 4a). The tensile strength and Young's modulus of the composite hydrogel were more than twice those of pure chitosan hydrogel, attributed to the reinforcing effect of the BC-g-cMWCNTs hybrid nanofibers. Joshi et al.[46]prepared fluffy PLA nanofibers doped with β-tricalcium phosphate (β-TCP) (PLA)-co-(β-TCP), dispersing the fluffy nanofibers in GelMA. Under the synergistic action of β-TCP and PLA, (PLA)-co-(β-TCP) could improve the pore structure in the hydrogel, making the hydrogel structure more complete and enhancing the compressive strength and modulus of the hydrogel.
图4 （a）BC-g-cMWCNTs纳米纤维复合凝胶制备流程[45]；（b）PAHL复合水凝胶优异的机械性能[65]

Fig.4 (a) BC-g-cMWCNTs nanofiber composite gel preparation process[45]. (b) Excellent mechanical properties of PAHL composite hydrogels[65]

Yu et al.[65]introduced ANF as an organic enhancer and one-dimensional ultra-fine hydroxyapatite nanofibers (HL) synthesized through in-situ polymerization as the inorganic reinforcement into a PVA hydrogel matrix, thereby preparing PAHL composite hydrogels. Under the synergistic toughening mechanism of organic-inorganic nanofibers, the mechanical strength and toughness of PAHL reached up to (24.15±1.12) MPa and (15.68±1.78) MJ·m−3, surpassing most toughened hydrogels (Figure 4b).

3 Preparation Methods of Nanofiber Composite Hydrogels

The preparation of fiber-reinforced hydrogels includes: blending method, lamination method, and other methods[26,66](Figure 5).
图5 纳米纤维/水凝胶复合材料制备策略[66]

Fig. 5 Preparation strategy of nanofiber/hydrogel composites[66]

3.1 Doping Method

Simply incorporating nanofibers into hydrogels is a simple, reliable, and widely adopted strategy for preparing nanofiber composite hydrogels[29]. Directly mixing nanofibers into hydrogels can result in non-uniform distribution[67,68]. Typically, grinding[37] or ultrasonic dispersion[69] of nanofibers is used to incorporate them into hydrogels to achieve nanofiber composite hydrogels with uniform properties[70,71].
Cheng et al.[72]prepared PVA/CFP composite hydrogels through a simple and controllable method. CFP was incorporated into the PVA solution, and the CFP and PVA were homogenized by water bath heating and magnetic stirring, followed by freeze-thaw cycles to prepare the PVA/CFP composite hydrogel (Figure 6a). Huang et al.[73]ultrasonically dispersed (polyvinyl alcohol-co-ethylene) (PVA-co-PE) nanofibers in water, which showed good dispersibility, and used them as reinforcing materials added to Ca2+-crosslinked alginate hydrogels.
图6 PVA/CFP复合水凝胶的制备流程[72]；（b）CPH水凝胶的三网络结构和优异的可拉伸和自修复性能[75]

Fig. 6 (a) Preparation process of PVA/CFP composite hydrogel[72]. (b) Tri-network structure and excellent stretchability and self-healing properties of CPH hydrogels[75].

The control over fiber length using grinding decomposition for nanofibers is limited. For nanofibers of less brittle polymers, the surface becomes rougher with more pits after ultrasonic treatment[37]. Therefore, to alter the morphology of the nanofibers, Mohabatpour et al.[37] prepared fragmented PLA nanofibers via aminolysis reaction and incorporated them into Alg-HA hydrogel precursor solution, obtaining PLA/Alg-HA composite hydrogels through Ca2+ crosslinking. The cleaved PLA nanofibers had a more uniform discrete morphology, making the compression modulus of PLA/Alg-HA 45% higher than that of pure Alg-HA hydrogels.
In addition to cutting and degrading nanofibers, researchers have incorporated nanofibers formed by in-situ polymerization of monomers within hydrogels[49,74]. Chen et al.[75] used in-situ oxidatively polymerized polypyrrole (PPy) nanofibers as a conductive network, integrating them into 2,2,6,6-tetramethyl-1-piperidine oxyl cellulose nanofiber (TOCNF)/PAA hydrogel matrix to prepare CPH hydrogel. Under the synergistic effect of a hierarchical three-network structure, CPH exhibits high stretchability (a fracture strain of about 890%), conductivity (3.9 S·m-1), and self-healing properties (healing efficiency for conductivity and mechanical properties is 99.4% and 98.3%, respectively) (Figure 6b). Tie et al.[49] deposited pyrrole (Py) monomer on the surface of CNFs to form PPy nanofibers through polymerization, preparing PPy@CNF-PAM composite hydrogels. The in-situ formed PPy nanofibers are tightly bound with CNFs and PAM chains, endowing the composite hydrogel with good transparency.
Compared with the simple blending method, the in-situ formation of nanofibers within hydrogels can effectively avoid the aggregation problem of polymers and endow the composite hydrogels with more excellent overall performance.

3.2 Lamination Method

3.2.1 Layer-by-Layer Stacking Lamination

The preparation of nanofiber composite hydrogels with a 3D hierarchical structure through layer-by-layer stacking to mimic tissue engineering scaffolds has attracted considerable attention from researchers[29,66,76].
Yang et al[77]prepared poly(L,D-lactic acid) nanofiber layers by electrospinning. The cell-seeded nanofiber mesh was placed on the surface of a type I collagen hydrogel, and then a square filter paper frame was placed on top as a spacer between the nanofiber layers, which was fixed after the hydrogel was cured. This process was repeated to create 3D nanofiber-cell-hydrogel composites (see Figure 7). The nanofibers were integrated into the hydrogel through layer-by-layer assembly, forming macroscopic and highly organized composite hydrogel scaffolds. Xu et al[78]electrospun L-lactide (L-PLA) nanofiber layers, coated them with HA-based hydrogel precursor solution, and then placed another L-PLA nanofiber layer, repeating this process to prepare a four-layer laminated composite. The addition of the nanofiber layers formed a laminated composite hydrogel with robust mechanical properties, which has the potential to provide structural support for load-bearing orthopedic applications in regenerative areas.
图7 3D复合材料组装示意图及纳米纤维排列方向对细胞方向的影响[77]

Fig. 7 Schematic of 3D composite assembly and the effect of nanofiber alignment direction on cell orientation[77]

3.2.2 Wrapper Method

Similar to stacked lamination, nanofibers can be wrapped in a hydrogel precursor solution and further crosslinked to form nanofiber composite hydrogels[29,79]. Nanofibers can serve as a structural support and dimensionally stable framework for preparing nanofiber composite hydrogels of various shapes and complex structures[66].
MCMAHON et al.[67]cut the electrospun poly(ε-caprolactone-co-polyurethane) urea nanofibers into rectangular segments, placed them in a hollow polytetrafluoroethylene cylinder, forming two concentric mesh layers. A thrombin/fibrinogen cell suspension was added to encapsulate and penetrate the fiber layer, followed by immersion in a poly(ethylene glycol)-fibrin solution for UV polymerization, ultimately yielding a nanofiber composite hydrogel (see Figure 8). The low stiffness behavior of the composite hydrogel is dominated by the hydrogel, while the high stiffness behavior is governed by the nanofibers. Deepthi et al.[80]laid chitosan-collagen (Chitosan-collagen, Ch-Col) hydrogel flat on poly(L-lactic acid) (Poly(L-lactic acid), PLLA) nanofibers, rolled it up to obtain a composite nanofiber membrane, and then immersed the composite nanofiber membrane in Ca2+alginate (Alg) hydrogel, preparing a Ch-Col/PLLA/Alg composite hydrogel.
图8 具有复杂结构纳米纤维复合水凝胶制备过程[67]

Fig. 8 Preparation of nanofiber composite hydrogels with complex structure[67]

3.2.3 Coating Method

To maintain and improve the mechanical integrity of processed composite materials, the precursor solution of hydrogels is directly coated onto nanofiber membranes or the nanofiber membranes are soaked in the solution, and then nanofiber composite hydrogels are prepared through photo-crosslinking, physical crosslinking, or chemical crosslinking, which is an easy-to-operate and reliable preparation method[29,66].
Wen et al.[81]prepared a sandwich-structured nanofiber composite gel. First, a polyvinyl pyrrolidone (PVP)/silver/polyurethane (PU) (PVP/Ag@PU) nanofiber membrane was prepared, then the gel precursor solution was evenly coated on both sides of the nanofiber membrane and combined with a freeze-thaw cycle strategy to prepare the NCRO. The nanofiber membrane endowed the composite gel with excellent mechanical properties, such as tensile strength (7.38±0.24) MPa, toughness (31.59± 1.53) MJ·m−3, and fracture energy (5.41±0.63) kJ·m−2. Shen et al.[82]coated a chitosan-based hydrogel precursor solution onto a polyurethane/polydimethylsiloxane (PDMS) (PU/PDMS) nanofiber membrane and prepared a bilayer composite dressing through photocrosslinking (Figure 9). The PU/PDMS nanofiber membrane served as a protective and supportive upper layer, enabling the bilayer composite dressing to exhibit excellent mechanical properties and self-healing capability.
图9 双层复合敷料结构性能示意图[82]

Fig. 9 Schematic diagram of structural properties of double-layer composite dressing[82]

3.3 Other Methods

Nanofibers and hydrogels can also be integrated at the microscale. The dual electrospinning/electrospray method deposits hydrogel solutions onto the collector of nanofibers, thus forming nanofiber composite hydrogels at the microscale[66], which is conducive to addressing the issue of cell infiltration in nanofiber membranes for regenerative medicine[83].
Ekaputra et al.[84] performed dual electrospinning of a mixture solution of PCL and collagen (collagen, COl), while electrospraying a hydrogel solution of glycosaminoglycans to form a co-deposited PCL/COl-Hep composite hydrogel (Figure 10a). The PCL/COL-Hep showed better cell permeability, with cells penetrating through the PCL/COL-Hep at a depth greater than 200 μm, much larger than the 50 μm for the PCL/COL fiber scaffold. Hong et al.[85] carried out dual electrospinning of poly(ester urethane)urea (PEUU) and simultaneously electrosprayed gels containing porcine skin tissue ECM, both collected on a rotating metal mandrel, ultimately preparing an ECM/PEUU composite scaffold. Compared to the electrospun PEUU nanofiber membrane, the composite scaffold had characteristics more similar to the natural abdominal wall, allowing a large number of cells to infiltrate it.
图10 （a）在双静电纺丝电喷射协同作用下制备PCL/COl-Hep复合水凝胶[84]；（b）PVA/SF/SA/GelMA水凝胶纳米纤维的制备示意图[87]

Fig. 10 (a) Preparation of PCL/COl-Hep composite hydrogels under the synergistic action of dual electrostatic spinning electrospraying[84]. (b) Schematic of the preparation of PVA/SF/SA/GelMA hydrogel nanofibers[87]

Another approach is to directly electrospin hydrogel precursor solutions to prepare nanofiber hydrogels for mimicking bionic ECM[29], and it can provide a three-dimensional cell culture model, gradually replacing traditional two-dimensional cell culture substrates[86].
Liu et al.[87]constructed aqueous solvent-based PVA/SF/SA/GelMA nanofibers using a handheld electrospinning device, which formed ionically crosslinked hydrogels after absorbing wound exudates. The ionic crosslinked hydrogel and PVA/SF/SA/GelMA nanofibers formed a hydrogel-nanofiber composite structure, maintaining a stable structure at the wound site after photopolymerization (Figure 10b). Gilotra et al.[88]prepared PVA-SS nanofiber hydrogels with antibacterial activity, swelling capacity, and excellent biocompatibility by electrospinning PVA hydrogel precursor solutions doped with silk sericin (SS). Specifically, mouse fibroblasts and human keratinocytes cultured on PVA-SS showed higher proliferation capacity.

4 Applications of Nanofiber Composite Hydrogels

4.1 Multifunctional Wound Dressing

Hydrogel wound dressings have the advantages of providing a moist healing environment for wounds, hemostasis, and low cost[89]. The high porosity, extracellular matrix-like structure, flexible components, and extensible morphology of nanofibers have shown unique advantages in improving the mechanical properties of hydrogels and tissue repair[89]. Therefore, nanofiber composite hydrogels as nanoscale building blocks for multifunctional wound dressings have broad prospects.
Li et al.[90] utilized the high mechanical strength of ANF combined with the antibacterial drug rhein to prepare an ANF/rhein composite hydrogel, which can continuously release rhein to inhibit the growth of Staphylococcus aureus, thus being used for the healing of infected wounds (Figure 11). Additionally, the ANF/rhein composite hydrogel has appropriate mechanical strength, high water retention capacity (>99%), and good biocompatibility (non-irritating, non-hemolytic, and non-cytotoxic). Chen et al.[91] constructed a tannic acid (TH)-loaded cellulose-based nanofiber composite PAA hydrogel dressing. It exhibits a high adhesive strength of 17.5 kPa on moist biological tissues and can sustain the release of TH for 120 h, providing a long-term antibacterial environment for wounds and promoting the healing process. Romero et al.[92] prepared a clindamycin-loaded PCL-based nanofiber composite cellulose hydrogel to inhibit Staphylococcus aureus in wounds and promote wound healing.
图11 ANF/大黄酸复合水凝胶合成和生物性能[90]

Fig. 11 Synthesis and biological properties of ANF/rhubaric acid composite hydrogels[90]

4.2 Tissue Engineering

Hydrogels, due to their 3D structure and elastic properties similar to soft tissues, are designed for use as tissue engineering scaffold matrices[93,94]. Nanofiber meshes, with their microstructures resembling those of the ECM, are used as tissue engineering substrates[95]. Adding nanofiber components to hydrogels can improve the mechanical properties of composite hydrogels[96]. In recent years, nanofiber composite hydrogels have been extensively studied for use as natural tissue substitutes.
Li et al.[95]covalently connected surface-functionalized PCL nanofibers and HA-based hydrogels to prepare a composite hydrogel that can match the mechanical properties of natural adipose tissue (shear storage modulus 150~600 Pa). Surface-functionalized PCL nanofibers can promote angiogenesis within the composite hydrogel, thereby laying the foundation for tissue repair. Wu et al.[38]encapsulated conductive nanofiber networks (NFYs-NET) in UV-crosslinked GelMA hydrogels for use as 3D hybrid scaffolds mimicking the structure of natural cardiac tissue. NFYs-NET can induce cardiomyocyte (CMs) alignment, elongation, and enhance the maturation and functionality of CMs, while the GelMA hydrogel shell provides a suitable 3D environment for CMs endothelialization. Wu et al.[97]prepared a 3D nanofiber composite hydrogel scaffold for heart valve tissue. This 3D composite scaffold inhibits the pathological differentiation of unhealthy human aortic valve interstitial cells into myofibroblasts and osteoblasts (Figure 12), providing appropriate support and microenvironment for natural ECM deposition and cell proliferation.
图12 3D复合支架生物学实验[97]

Fig. 12 Biological experiments on 3D composite scaffolds[97]

4.3 Conductive Sensor

Conductive hydrogels with good biocompatibility and tunable mechanical properties are considered suitable candidates for the synthesis of flexible sensors[49,98]. The main strategy for preparing conductive hydrogels is to incorporate conductive materials, such as graphene (graphene, GN), carbon nanotubes (carbon nanotube, CNT), and conductive polymers, etc[99,100]. However, GN and CNT tend to self-aggregate and distribute unevenly due to their inherent drawbacks[101]. Therefore, using nanofillers to overcome the aforementioned issues and achieve highly conductive hydrogels[102].
Li et al.[103] prepared chitin nanofiber (ChNF) composite PAM-based hydrogel (ChNF/PAM) through a one-pot method. The addition of ChNF endowed the ChNF/PAM with excellent mechanical properties (tensile strength of 261.2 kPa, elongation at break of 550.3%), high conductivity (conductivity 1.20 S·m-1) and strain sensitivity (gauge factor up to 4.29) (Figure 13). Jiao et al.[104] incorporated oxidized cellulose nanofibrils (TOCNF) into PANI/PAA hydrogel to prepare a new type of conductive self-healing composite hydrogel with high conductivity (conductivity 3.95 S·m−1), high sensitivity (gauge factor, GF=8.0), excellent repeatability, and stability. Tie et al.[49] introduced a composite hydrogel that integrates optical transparency, high conductivity, and good mechanical properties. The PPy nanofibers formed by radical polymerization were uniformly distributed in the CNF and well integrated with the PAM matrix, thus constructing highly interconnected pathways for excellent conductivity.
图13 ChNF/PAM的机械性能和导电性[103]

Fig. 13 Mechanical properties and electrical conductivity of ChNF/PAM[103]

4.4 Adsorption Materials

Hydrogels, as a three-dimensionally cross-linked polymer network, have shown tremendous application potential in wastewater treatment, but the low mechanical strength of traditional hydrogels is still a significant defect when used as adsorption materials[105]. Nanofiber composite hydrogels possess a stable 3D network structure and excellent mechanical properties, which can greatly overcome the limitations of traditional adsorption materials and may open up unprecedented possibilities for wastewater treatment[106,107].
Zhang et al.[108]developed a cellulose nanofiber composite carbon dot grafted polyacrylamide composite hydrogel (CZCH) for the adsorption of tetracycline. CZCH has a significant adsorption capacity for TC, reaching up to 810.36 mg/g, and after five consecutive adsorption/desorption cycles, CZCH still maintains 85.6% of its initial adsorption capacity. Huang et al.[109]prepared TiO2nanofiber composite alginate-polyacrylic acid hydrogel (NC), as an efficient adsorbent for Cd2+.The porous and loose three-dimensional cross-linked structure of NC allows the adsorption sites to be fully exposed to Cd2+,with a maximum adsorption capacity of 76.92 mg/g. Bora et al.[110]introduced modified cellulose nanofiber hybrid composite starch/itaconic acid/acrylic acid-based hydrogel (HNC). The incorporation of nanofiber hybrids enhances the adsorption capacity of HNC for Cu2+and Fe2+to 122 and 70 mg·g-1,respectively. It can be reused for 3 purification cycles without significantly reducing efficiency.

5 Conclusions and Prospects

Hydrogels and electrospun materials have attracted significant interest from researchers, but their inherent drawbacks to some extent limit their individual applications. Nanofiber composite hydrogels combine the advantages of both materials, possessing not only the biocompatibility and versatility of the hydrogel matrix, but also the excellent mechanical strength, high aspect ratio, and large specific surface area of nanofibers.
This review classifies nanofiber composite hydrogels, specifically, natural organic nanofiber composite hydrogels, synthetic nanofiber composite hydrogels, inorganic nanofiber composite hydrogels, and organic-inorganic hybrid nanofiber composite hydrogels. CNF, due to its excellent strength and stiffness, high dispersibility in aqueous media, and ease of surface chemical modification, is often used to prepare composite hydrogels with high mechanical properties. Synthetic nanofiber composite hydrogels, such as PLA nanofiber composite hydrogels and PCL nanofiber composite hydrogels, are more commonly used to prepare bio-composite hydrogels that mimic the structure of the ECM. Inorganic nanofiber composite hydrogels, on the other hand, are used to imitate natural bone tissue, supporting the attachment and proliferation of bone cells. Additionally, we summarize various methods for preparing nanofiber composite hydrogels, including blending, layer-by-layer stacking lamination, hydrogel encapsulation of nanofibers, coating, dual electrospinning/electrospraying, and electrospinning of hydrogel precursors; examples corresponding to each method are listed, and the advantages and disadvantages of various methods are analyzed: blending is simple and widely adopted but may result in uneven distribution of nanofibers, layer-by-layer stacking is typically used to prepare nanofiber composite hydrogels with 3D hierarchical structures, the encapsulation method can be used to prepare nanofiber composite hydrogels of various shapes and complex structures, and the formation of nanofiber composite hydrogels at the microscale is achieved through dual electrospinning/electrospraying. Finally, the applications of nanofiber composite hydrogels in wound dressings, tissue engineering, conductive sensors, and adsorbent materials are discussed in detail.
However, relatively speaking, the application of this new type of composite material has not been deeply and systematically developed and is still in its early stages. Therefore, more researchers need to focus on this composite material, continue to expand the application scope of nanofiber composite hydrogels, and explore large-scale pipeline synthesis processes suitable for industrialization.

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