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Article

Multi-Modal Mechanical Response of Self-Healing Double-Network Hydrogel Coatings Based on Schiff Base Bond

by
Yanan Li
1,†,
Wenbin Hu
1,†,
Qike Gao
2,†,
Jincan Yan
1,
Guan Wang
1,
Sheng Han
1,*,
Chenchen Wang
1,* and
Xiaozheng Hou
2,*
1
School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai 201418, China
2
Department of Joint Surgery, Yangquan Coal Industry (Group) General Hospital, Yangquan 045008, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Coatings 2025, 15(5), 552; https://doi.org/10.3390/coatings15050552
Submission received: 19 March 2025 / Revised: 17 April 2025 / Accepted: 29 April 2025 / Published: 5 May 2025
(This article belongs to the Special Issue Advanced Surface Engineering of Biocompatible Coatings: Innovation and Application)
Browse Figures
Versions Notes

Abstract

:
Ti6Al4V alloy is one of the most widely used orthopedic implants due to its low density, high strength and good biocompatibility, but surface tribology limits its service life and performance. In this paper, a layer of dynamic double-network hydrogel based on a Schiff base bond and a hydrogen bond was grafted on the surface of Ti6Al4V alloy by the mussel chemical self-assembly method. The -NH2 of acrylamide (AM) and -CHO of vanillin (VA) formed Schiff base bonds to form the first layer of a cross-linked network, a large number of hydrogen bonds were formed between the -OH of vanillin and the -OH of sodium alginate (SA) to provide the second layer of the cross-linked network and the network was properly regulated by introducing core–shell polymer nanoparticles (PDCS). Dynamic self-healing bonds, Schiff base bonds and hydrogen bonds endow qPDCS/SA/VA/AM hydrogels with self-healing ability, and the network structure destroyed under high strain (250%) can be rebuilt under low strain (1%). In the second cycle, G’ and G can recover almost the same value. PDCS/SA/VA/AM hydrogel coating can achieve dynamic repair through reversible Schiff base bond dissociation–recombination during friction, while 1000ppmPDCS/SA/VA/AM hydrogel coating can achieve stable friction reduction and low wear under multiple loads. Under 0.5 N load, the average friction coefficient of 1000ppmPDCS/SA/VA/AM hydrogel coating is as low as 0.157, which is 67.74% lower than the uncoated Ti6Al4V surface under the same load. Under 2 N load, 1000ppmPDCS/SA/VA/AM hydrogel coating remains stable and low-friction, and the average coefficient of friction (ACOF) can reach 0.130, which is 59.27% lower than the uncoated Ti6Al4V surface under the same load. The design idea of the hydrogel network regulated by core–shell polymer nanoparticles (PDCS) to achieve low friction and low wear provides a new strategy for biolubricating materials.

1. Introduction

In daily life, articular cartilage can be damaged due to mechanical injuries (such as sports injuries) or acute trauma, and joint injuries are more likely to cause osteoarthritis (OA). Arthritis is a chronic degenerative disease characterized mainly by the degeneration of articular cartilage. It often causes persistent pain and joint dysfunction, and in severe cases, it can lead to limited mobility and even disability. Due to the weak ability of cartilage to repair itself, OA rarely heals itself and eventually requires artificial joint replacement surgery to treat it [1,2,3]. This undoubtedly increases the application of titanium alloy as an artificial joint replacement material, which has high strength and good ductility and can be used as a raw material for implant devices such as scaffolds, spines, indirect bone joints, cochlear implants and nerve stimulators. Among them, titanium alloy products represented by Ti6Al4V have excellent comprehensive properties, but their implants still have problems such as poor friction resistance, unsatisfactory biocompatibility, stress barrier and physiological toxicity, which may eventually lead to implantation failure [4,5]. For example, Liu et al. [6] developed a unique medical titanium alloy material, Ti10Mo6Zr4Sn3Nb titanium alloy (referred to as Ti-B12). Compared with Ti6Al4V alloy, Ti-B12 improved the poor biocompatibility of Ti6Al4V, and compared with Ti6Al4V, Ti-B12 titanium alloy material had more advantages in promoting osteoblast adhesion and ALP secretion (p < 0.05). The expression of OCN and Runx2 genes in the TiB12 group was higher than that in the Ti6Al4V group and the blank control group and had better bone-binding performance than traditional titanium, Ti6Al4V. The quick advancement of surface modification technology in recent years has opened new possibilities for enhancing the chemical and physical characteristics and capabilities of medical metal materials. How to use metal surface modification technology to improve the functional properties and service life of titanium and titanium alloy implants has become the focus of many researchers. Hydrogels have become an ideal biomimetic joint cartilage material due to their high hydrophilic and slippery properties and similar mechanical properties to natural cartilage and have shown important application potential in the field of biolubricating materials [7,8,9]. In recent years, researchers at home and abroad have also carried out a lot of research on the tribological properties of hydrogels, hoping to prepare hydrogels that can be used as replacement materials for human articular cartilage. As a result, creating a hydrogel coating on the surface of titanium alloy has emerged as a successful technique to address the metal’s wear resistance and biocompatibility issues. Designing a self-healing hydrogel with high biocompatibility and wear resistance is therefore essential. Vanillin, a renewable lignin derivative with good biocompatibility [10,11], which has attracted our interest.
Vanillin is a renewable, lignin-derived compound that has gained widespread application in food processing owing to its non-toxic nature and excellent biocompatibility. And the molecule features two reactive functional groups: hydroxyl and aldehyde groups. [12,13]. A dynamic covalent Schiff base bond can be formed between the aldehyde group of vanillin and the amino group of acrylamides, which makes the hydrogel have excellent self-healing ability, and it is expected to be used as a repair material for artificial articular cartilage after wear [2,14]. For example, Ni et al. [15] studied and synthesized a new vanillin-modified polyacrylate (VPA), combined the synthesized VPA with mesoporous silica-modified MXene (AMS-MXene) and at the same time performed green dynamic covalent cross-linking with chitosan (CS) to form a flexible composite conductive film specially designed for dual-mode sensors. This composite film exhibits excellent self-healing ability with AMS-MXene through reversible dynamic covalent and multiple hydrogen bond interactions. Even at a low temperature of 30 °C, this powerful self-healing function is still effective.
Vanillin can form Schiff base bonds with acrylamide, making hydrogels with excellent self-healing ability [10,16]. However, in general, this type of hydrogel has poor mechanical properties. For example, Xu et al. [17] prepared a chitosan/vanillin hydrogel based on Schiff base bonds, which can completely heal after 5 h. No cracks appeared at the healing line, and the two circular samples did not separate after stretching. Despite its good healing properties, the tensile strength of hydrogel is only 27.5 Kpa, and its elastic modulus (G’) is only about 8700 Pa. Therefore, it is necessary to add a nanoparticle with high strength to enhance the mechanical properties, friction coefficient and wear rate of the hydrogel. Core–shell polymer nanoparticles (PDCS) are an excellent toughening modifier for elastomers. Core–shell polymer nanoparticles (PDCS) have excellent compatibility with hydrogel and can form a high-strength interface when added into the hydrogel precursor solution and dispersed uniformly. When the hydrogel coating is subjected to external impact, it causes the shear yield of the material to create a small gap, absorb a lot of energy and improve toughness and impact resistance. Qiu et al. [18] prepared core–shell polymer nanoparticles (CSP)/epoxy cement (CSEC) composites by emulsion polymerization. The frictional properties of CSEC composite and epoxy cement were studied. The results show that the wear rate is reduced by 14.4 at a load of 20 N compared to the material with only epoxy cement (EC), which indicates that CSP has excellent friction resistance. Wang et al. [19] prepared PVA/PAA/GO/PDA hydrogel coatings with different graphene oxide (GO) contents (0 wt%, 0.1 wt%, 0.5 wt% and 1 wt%). The results showed that the hydrogel coating significantly reduced the friction coefficient (82%) and wear rate (76.4%) between the samples and the bone tissue. In this study, we used biofriendly vanillin and monomer acrylamide to form a Schiff base bond as the first layer of a cross-linking network and added natural biological macromolecule sodium alginate (SA) to form the second layer of the cross-linking network to enhance the mechanical strength of the hydrogel. Core-shell polymer nanoparticles (PDCS) were added to enhance the friction resistance of the hydrogel. The tribological properties of PDCS/SA/VA/AM hydrogel coating under different loads were investigated. The results showed that the prepared PDCS/SA/VA/AM hydrogel coating could effectively improve the poor tribological properties of Ti6Al4V alloy, and the PDCS/SA/VA/AM hydrogel coating was expected to become a replacement material for human articular cartilage.

2. Experimental Section

2.1. Materials

Putting the Ti6Al4V alloy into the acetone solution and cleaning the Ti6Al4V surface using ultrasonic waves about one hour, followed by three rinses with distilled water. Silicon nitride (Si3N4) balls were obtained from Ningbo Baiyue Hardware Accessories Co, Ningbo, China. The following chemicals were sourced from various suppliers: sodium alginate (SA) from China National Pharmaceutical Chemical Reagent Co., Ltd, Shanghai, China., ammonium persulfate (APS, 98% purity), acrylamide (AM, 99% purity), vanillin (VA, 99% purity), N,N’-methylenebisacrylamide (MBA, 99% purity), 3-aminopropyltriethoxysilane (APTES), methyl methacrylate (MMA, 99%) and butyl acrylate (BA, stable with MEHQ, 99%) were purified by activated Al2O3 column chromatography and used as monomers. Ethylene glycol dimethacrylate (EGDMA, 98%) was purified by activated Al2O3 column chromatography and used as a cross-linked monomer. Sodium dodecyl sulfate (SDS, 92.5%) was used as an emulsifier. Polyether amine (D400) was purchased from Shandong Jinan Xinglong Chemical Co., Ltd, Jinan, China. Dopamine (DA), potassium hydroxide (KOH, 85% purity), sodium hydroxide (NaOH, 98% purity), tetramethyl ethylenediamine (99.99%) and tris(hydroxymethyl)aminomethane were obtained from Shanghai Titan Technology Co., Ltd, Shanghai, China.

2.2. Mussel Chemical Self-Assembly Prepared on a Titanium Alloy Surface

The oil on the surface of Ti6Al4V was cleaned by ultrasonic cleaning in acetone solution for 1 h, washed with distilled water three times and then cleaned by ultrasonic cleaning in distilled water for 1 h. Then, the titanium alloy plate was immersed in 5 mol/L KOH solution and stirred for 24 h so that the surface of the titanium alloy was completely grafted with hydroxyl. Then the hydrogenated titanium alloy plate was soaked in 0.3 wt.% APTES solution for 1 h, and the pH was adjusted to 8.5 with hydrochloric acid. Subsequently, a dopamine (DA) solution with a concentration of 0.2 wt.% was configured, the pH was adjusted again to 8.5 using tri-methylate aminomethane and the plates were placed in the dopamine solution and stirred for approximately 24 h. The amino group on the dopamine grafted on the substrate formed a number of hydrogen bonds with the core–shell polymer nanoparticles to enhance the bonding strength with the hydrogel coating.

2.3. Preparation of Core–Shell Nanoparticles (PDCS)

The synthesis of core–shell polymer nanoparticles (PDCS) referred to previous work of Qiu et al. [18]. Initially, 100 mL of n-butyl acrylate, 2 mL of cross-linker EGDMA and 60 mL of surfactant solution (1.0 g sodium dodecyl sulfate (SDS) in deionized water) underwent ultrasonic shear emulsification for 10 min, yielding approximately 162 mL of core monomer pre-emulsion.
Subsequently, 54 mL of the freshly prepared core pre-emulsion was combined with 60 mL of deionized water containing 0.125 g potassium persulfate (KPS). The mixture was degassed via three nitrogen purge cycles followed by thermal initiation at 82°C under constant stirring. Upon visual observation of bluing (indicating nucleation), an additional 5 mL aqueous solution containing 0.125 g KPS was introduced. The remaining two-thirds (108 mL) of core pre-emulsion was then fed via controlled dropwise addition over 1.5 h. The system was maintained at reaction temperature for 1 h before final maturation at 90°C for 1 h to complete core particle formation.
For shell formation, 50 g of core latex was blended with 60 mL KPS solution (0.08 g in deionized water) and degassed similarly. A pre-emulsified shell component containing 15 mL methyl methacrylate, 0.1 g SDS and 10 mL deionized water was subsequently fed over 1 h at 82°C. After post-reaction maturation for 1 h, the resultant core–shell latex was subjected to spray-drying (following appropriate dilution) to obtain dry particulate products.

2.4. Synthesis of the SA/VA/AM and PDCS/SA/VA/AM Hydrogels

An amount of 0.7 mg core–shell nanoparticles with butyl acrylate as core and methyl acrylate as shell was added into 6.5 mL deionized water, stirred evenly until fully dissolved and dispersed, and we used ultrasonic cleaning to eliminate bubbles. Then 0.1 g sodium alginate, 0.4 g vanillin, 5 g acrylamide, 0.015 g N, N-methylene bisacrylamide, 0.03 g ammonium persulfate, 0.5 mL NaOH (0.1 mol/L) and 20 μL catalyst tetramethyl ethylenediamine were added. And finally, the ultrasonic wave was used to eliminate the gas bubble, then 3 mL of the prepared mixed solution was poured into a polystyrene square box with a length of 4 cm, a width of 4 cm and a height of 2 cm, and the PDCS/SA/VA/AM hydrogel was obtained by vacuum heat polymerization in a vacuum-drying oven at 60 °C for 1 h. The Schiff base bond-based dynamic hydrogel added to PDCS is referred to as PDCS/SA/VA/AM. PDCS were not added to the blank control group, which was named SA/VA/AM hydrogel. The hydrogels with 3.5 mg, 7 mg and 8.75 mg core–shell nanoparticles were, respectively, named 500ppmPDCS/SA/VA/AM, 1000ppmPDCS/SA/VA/AM and 1250ppmPDCS/SA/VA/AM.

2.5. Analysis Methods

To examine the structural characteristics of PDCS, SA/VA/AM and 1000ppm/PDCS/SA/VA/AM hydrogels, Fourier-transformed infrared spectroscopy (FTIR, Nicolet In10, Waltham, MA, USA) was used. To investigate the surface morphology of PDCS, a scanning electron microscope (SEM, Nova NanoSEM450, FEI, Waltham, MA, USA) was utilized. SEM combined with energy-dispersive X-ray spectroscopy (EDX) was used to examine the surface appearance of Ti6Al4V, SA/VA/AM and 1000ppm/PDCS/SA/VA/AM. The three-dimensional profiles of wear scars and their rate of wear were assessed using a white-light interferometer (WLI, Bruker Contour GT-K0, Billerica, MA, USA).

2.6. Adhesion Strength of Coatings and Film Thickness Test

The adhesive ability of the hydrogel covering was assessed by an impact test. The titanium alloy plate grafted with dopamine was put into a polystyrene square box with a length of 4 cm, a width of 4 cm and a height of 2 cm, and PDCS/SA/VA/AM precursor solution was added (for the preparation of precursor solution, please refer to Section 2.4 in this article). Then, it was put into a vacuum-drying oven at 60 °C for vacuum-heat polymerization for 1 h and then used to test the adhesion of hydrogel and titanium alloy. The experiments were performed using a UMT TriboLab (Bruker) under specified conditions: an acceleration rate of 80 N/min, a scratching velocity of 0.135 mm/s and an absolute maximum pressure of 50 N. When there is the first significant fluctuation in the acoustic signal, it shows that the adhesion between the hydrogel coating and titanium alloy is broken, and the corresponding friction force and friction coefficient will also show a significant change at this time. The critical load value obtained is the bonding strength of the hydrogel coating. To guarantee data precision, each hydrogel coating underwent five tests. The thickness of the hydrogel coating was measured by a WLI three-dimensional white-light interferometer (ContourGT-K0, Bruker, USA). The preparation method of the hydrogel used to test the thickness of the coating was basically the same as that of the hydrogel used to test the adhesion test. The difference is that half of the surface of the titanium alloy was shielded by an anti-static film. When the film thickness test was carried out with a three-dimensional white-light interferometer, the shielded anti-static film was removed and then film thickness was measured.

2.7. Swelling Test

The newly prepared equilarge hydrogel sample (3.5 mm diameter, 3 mm thick disc, evenly divided into two pieces) was taken, and the original mass was measured and fully immersed in the same amount of deionized water (200 mL). We measured the sample weight every three hours. We blotted the surface water with absorbent paper before each weighing and changed the liquid at each time point. We repeated this three times for each group of samples until the swelling was balanced.

2.8. Dynamic Oscillatory Rheometric and Mechanics Performance Testing

The rheological and self-healing properties of the hydrogels were evaluated using a HAAKE MARS 40 rheometer (Thermo Scientific, Waltham, MA, USA). For the rheological tests, hydrogel discs with a diameter of 35 mm and a thickness of 1 mm were prepared and placed between parallel plate fixtures with a diameter of 35 mm and a gap of 1 mm. The following experiments were performed: (1) The linear viscoelastic region of hydrogels containing different concentrations of SA/VA/AM, and 1000ppm/PDCS/SA/VA/AM hydrogels was evaluated using a dynamic strain sweep test. The test was performed at a frequency of 1 Hz and a deformation range of 0.1% to 1000%. (2) Dynamic frequency sweep tests to verify internal friction energy dissipation, at a temperature of 25 °C, a frequency of 1 Hz and a dynamic frequency sweep scope of 0.01–100 rad/s. (3) Dynamic temperature measurements to verify thermal stability. The temperature varies from 25 °C to 45 °C with a thermal speed of 3 °C per minute and a frequency of 1 Hz. (4) Alternating strain sweep at a constant horizontal frequency of 10 rad/s at an average temperature of 25 °C, with a recovery strain of 1% and a structural failure strain of 200%, with each strain test lasting 70 s.

2.9. Tribological Properties

In the experimental setup, the upper friction component was a Si3N4 ball, while the lower friction component consisted of titanium alloy samples coated with different hydrogels: Ti6Al4V, SA/VA/AM and 1000ppmPDCS/SA/VA/AM. The Si3N4 balls had a diameter of 6 mm, a G4 grade, a surface arithmetic average roughness (Ra) of 0.02 μm and a Rockwell hardness of 78. Reciprocating tests were carried out at room temperature in simulated body fluid (SBF) solution under the following conditions: upper friction pair loading of 0.5 N, 1 N, 1.5 N and 2 N, reciprocating length of 4 mm, frequency of 1 Hz and time of 1800 s. Each tribological experiment was repeated at least three times to ensure reliable results. The composition of the SBF solution is detailed in Table S1. This experimental design allowed for a thorough evaluation of the frictional behavior between the Si3N4 ball and the various titanium alloy samples under consistent and controlled conditions.
The wear rate (k) of the Si3N4 balls on the various titanium alloy samples was determined using the following Equation (1) (mm3·(N m)−1):
k = V X · F N
where V is the wear volume, X is the total sliding distance and FN is the vertical load.

3. Results and Discussion

3.1. Design Rationale and Characterization of PDCS/SA/VA/AM Hydrogel

To design a kind of hydrogel coating with dynamic damage self-healing, excellent corrosion resistance and antibacterial properties, PDCS/SA/VA/AM hydrogel was prepared by the hot polymerization method. First, natural biological macromolecule sodium alginate (SA) was added, and the pH was adjusted to an alkaline environment by sodium hydroxide. Next, PDCS, acrylamide (AM), a cross-linking agent (N, N dimethyl bisacrylamide)a and an initiator (amine persulfate) were added and finally poured into a mold. PDCS/SA/VA/AM hydrogels were successfully prepared by vacuum-drying at 60 °C for about 2 h. We used four main components, PDCS, VA, AM and SA, and chose PDCS to provide good mechanical stability for the hydrogel. The AM monomer provided adjustable mechanical properties for the hydrogels. Secondly, the -NH2 of AM formed a dynamic self-healing Schiff base bond with the -CHO of VA. The introduction of sodium alginate (SA) and acrylamide (AM) formed a second layer of the cross-linked network, which enhanced the mechanical properties of the hydrogels. The mechanical strength of PDCS/SA/VA/AM hydrogel coating was derived from hydrogen bonds, Schiff base bonds and the addition of PDCS, which had excellent self-healing and mechanical properties.
Due to the unique structure of core–shell nanoparticles (the core is hard, while the shell is relatively light), PDCS can withstand certain forces to narrow the friction gap between the upper friction pair silicon nitride pellets (Si3N4) and the hydrogel, avoid direct contact between the friction pairs and reduce frictional wear (Figure 1). This explains the excellent wear resistance and low friction (ACOF = 0.130) of the PDCS/SA/VA/AM hydrogel coating at a test load of 2 N. Dynamic Schiff base bonds and hydrogen bonds in PDCS/SA/VA/AM hydrogels provide dynamic damage self-healing capabilities and allow for rapid self-healing during wear and friction. This can reduce the average friction coefficient by 59.27%.
Figure 2a shows the Fourier transform infrared spectrum of vanillin. The wide peak at 3552 cm−1 is generated by stretching vibration of the -OH in vanillin [15], the characteristic peak at 1695 cm−1 is the -C=O group of vanillin and at 3000–3500 cm−1 is the characteristic peak of the N–H stretching vibration confirmed the presence of the primary amide group of acrylamides. The characteristic peak of the Schiff base bond (C=N) formed after the reaction between the amino group (-NH2) of acrylamide and the aldehyde group of vanillin (-CHO) is 1668 cm−1 and 1661 cm−1, which can be seen from the Fourier transform infrared spectrum of the SA/VA/AM and 1000ppmPDCS/SA/VA/AM hydrogels. In addition, compared with the SA/VA/AM hydrogel, after adding PDCS, the number of hydrogen bonds inside the hydrogel increased, the -OH peak shifted towards a lower wavenumber and a double peak appeared. The 1000ppmPDCS/SA/VA/AM hydrogel showed characteristic peaks like those of PDCS, and the characteristic peak of 2833 cm−1 is the methylene (-CH₂-) of n-butyl acrylate, the solvent required for the synthesis of PDCS. The characteristic peak of methylene (-CH₂-) can also be observed at 2826 cm−1 of the PDCS/SA/VA/AM hydrogel. Figure 2b shows the Raman spectrum, which can also confirm that at 1667 cm−1 is the C=N stretching vibration peak, while at 1578 cm−1 is the N-H bending vibration peak [20]. When the Schiff base reaction is complete, the N-H bending vibration peak of 1000ppmPDCS/SA/VA/AM hydrogel completely disappears and the ester bond of PDCS can also be proved. At 532 cm−1 is the characteristic peak of the in-planebending vibration of the C=O bond, while at 1208 cm−1 is the characteristic peak of the C-O single bond asymmetric stretching in an ester bond [21].
Generally, hydrogels can absorb a large amount of free water because of their hydrophilic network, so the swelling rate is also an important indicator that supports the degree of cross-linking of the network. As shown in Figure 2c, the swelling rate of hydrogel materials with different amounts of nanoparticles was basically the same and reached equilibrium after 9 h. The swelling rate of the SA/VA/AM hydrogel network without PDCS nanoparticles was the highest, reaching about 400%, and it decreased slightly with the swelling rate, while the swelling rate of the hydrogel system with 1250 ppm PDSC/SA/VA/AM nanoparticles was about 330%. A lower swelling rate means better network cross-linking, which may be due to the addition of nanoparticles to increase the cross-linking density of the hydrogel network, and the better entanglement of polymer chains under the control of nanoparticles [22].
The scanning electron microscopy (SEM) morphology of PDCS is shown in Figure 2c. It can be seen from the Figure that the spherical PDCS were of uniform size at about 5 μm in diameter. The microstructure of the SA/VA/AM and 1000ppmPDCS/SA/VA/AM hydrogels was observed and characterized by SEM. As shown in Figure 2d,e, the hydrogels are highly porous. Compared with the SA/VA/AM hydrogel, the network structure of the 1000ppmPDCS/SA/VA/AM hydrogel is denser, and the cross-linking density is enhanced, which is because after adding PDCS nanoparticles, hydrogels are more porous. Because PDCS can form more hydrogen bonds with groups on the hydrogel chain. The cross-linking density of 1000ppmPDCS/SA/VA/AM hydrogel was enhanced.

3.2. Bonding Strength and Film Thickness Test of Hydrogels

To accurately evaluate the adhesion strength between SA/VA/AM and 1000ppmPDCS/SA/VA/AM hydrogel coatings and Ti6Al4V alloy, adhesion tests were performed using a friction and wear testing machine (UMT) based on the normal force (critical load), acoustic emission signals and friction fluctuations at the time of coating separation. The adhesion test results are shown in Figure 3a,b. The critical load sequence of the sample is as follows: SA/VA/AM (15 N) < 1000ppmPDCS/SA/VA/AM (22.5 N) data analysis showed that due to the addition of PDCS nanoparticles, a large number of hydrogen bonds were formed with the amino groups on the grafted dopamine on the substrate [2,23]. Therefore, the interface bonding strength between the 1000ppmPDCS/SA/VA/AM hydrogel coating and the substrate is improved. Figure 3c,d shows the thickness of SA/VA/AM and 1000ppmPDCS/SA/VA/AM hydrogel coating. The thickness of SA/VA/AM hydrogel coating and 1000ppmPDCS/SA/VA/AM hydrogel coating is very similar, about 15 μm. This shows that the hydrogel coating can be uniformly prepared on the titanium alloy.

3.3. Mechanics Performance Testing

Rheological and tensile properties are often used to verify the mechanical response of hydrogel cross-linked networks [16,24,25]. Hydrogels with different PDCS ratios were named A (SA/VA/AM hydrogel), B (500ppmPDCS/SA/VA/AM hydrogel), C (1000ppmPDCS/SA/VA/AM hydrogel) added) and D (1250ppmPDCS/SA/VA/AM hydrogel), respectively. In terms of rheological testing, it was found in dynamic oscillation amplitude scanning that hydrogel samples with different PDCS additions reflect different viscoelastic properties, as shown in Figure 4a. The shear strain corresponding to the end point of the linear viscoelastic region of the hydrogel sample without core–shell polymer PDCS is the largest, which may be due to the carbonylated nanoparticles breaking the cross-linking state of the hydrogel network to some extent. The shear strain corresponding to the end point of the linear viscoelastic region of the sample with the addition of the amount of PDCS of 1000 ppm is the smallest, and the corresponding energy storage modulus in the linear viscoelastic region is the largest, which indicates that with the cooperation of PDCS, 1000 ppm is the better addition amount in terms of the mechanical properties obtained from the rheological test. This view is also confirmed in the dynamic oscillation frequency scanning and dynamic temperature scanning. In the process of dynamic oscillation frequency scanning from high frequency to low frequency, the energy storage modulus of each added amount of hydrogel sample remained stable. Among them, the order of the energy storage modulus (G”) of the hydrogel from large to small is as follows: G“ (SA/VA/AM) < G“ (500ppmPDCS/SA/VA/AM) < G“ (1250ppmPDCS/SA/VA/AM) < G” (1000ppmPDCS/SA/VA/AM). It can be found that with the increase in the addition amount of PDCS, the energy storage modulus also gradually increases, but when it exceeds 1000 ppm, the energy storage modulus decreases. This might be due to the stacking of PDCS, which forms local rigid regions, hindering the uniform entanglement of polymer chains, resulting in a decrease in the effective cross-linking density and subsequently a reduction in the energy storage modulus, as shown in Figure 4b [26,27]. Figure S1 shows the variation in tan δ with the scanning frequency at 1% strain. The tan δ of the 1000ppmPDCS/SA/VA/AM hydrogel is the smallest, indicating that its energy loss is small and the internal friction is small, further enabling the hydrogel network to respond quickly to external forces. This phenomenon indicates that the reasonably designed dual-network hydrogel has impact resistance. Figure 4c shows the variation curves of G’ and G’’ of the hydrogel coating with temperature (25–45 °C). the curve shows that the energy storage modulus and loss modulus of the hydrogels remain basically unchanged with the increase in temperature, which may be because the cross-linked network of the hydrogels is very close; we also verified the temperature stability of the hydrogel material [28,29,30], and the energy storage modulus is much higher than the loss modulus. Therefore, the SA/VA/AM and PDCS/SA/VA/AM hydrogels are still mainly showing elastic deformation, showing good dynamic mechanical properties.
In oscillatory rheological tests, solid-prone hydrogel structures are destroyed at high shear strain. The topological network structure of hydrogel samples with a dynamic self-healing network can be reconstructed when high shear strain is removed. Therefore, the oscillatory rheological test of alternating strain was used to verify the self-healing function of hydrogel materials. The PDCS/SA/VA/AM hydrogel network is destroyed under high shear stress (strain = 250%), the network structure is reconstructed when the critical stress is withdrawn (strain = 1%) and G’ and G” can recover almost the same value during the second cycle, as shown in Figure 4d. This may be related to dynamic bonds, Schiff base bonds (VA and AM) and hydrogen bonds (VA and SA) [31,32,33]. Overall, the high energy storage modulus, impact resistance, variable temperature stability and self-repair properties of dual-network hydrogels with the addition of PDCS make these hydrogels have biomechanical material potential.
In addition, tensile and compression tests can also be used to verify the mechanical properties of hydrogel networks [34,35,36]. In the constant rate tensile test, a larger slope implies a higher elastic modulus, and the sample without PDCS exhibits the largest tensile elastic modulus, which is consistent with the results of the end strain in the highest linear viscoelastic region of the sample without PDCS in the dynamic amplitude scan, as shown in Figure 4e. Similarly, a hydrogel sample with an addition of 1000 ppm PDCS achieves a high–tough network with a tensile fracture strain of 750 % due to the optimal coordination of nanoparticles in its cross-linked network. In the compression test of hydrogel samples, the compression test curves of samples with different PDCS addition levels reflect that the samples without a core–shell polymer nanoparticle (PDCS) addition exhibit the highest compressive strength, and the stress–strain curve slope of the compression test is the largest, as shown in Figure 4f. The deformation of the hydrogel sample with the addition of 1000 ppm PDCS occurred at nearly 85%, which also means that the reasonable addition of nanoparticles brings excellent toughness to the hydrogel sample [37].

3.4. Biotribological Response Under Different Loads

According to the Hertz contact theory, the vertical static pressure between a ball and plate can cause a contact pressure of about 2–4 MPa, which is a good simulation of the mechanical scene when artificial joint material is in service [19,38,39]. In this work, a multi-step load (0.5 N–2 N) with high-frequency reciprocating friction under simulated body fluid (SBF) is designed for the study of bio-tribology. Under the same load, compared with the samples of Ti6Al4V alloy with SA/VA/AM hydrogel coating with and without PDCS, the surface of Ti6Al4V alloy without hydrogel coating has more serious friction and wear, as shown in Figure 5. For the same sample, the friction coefficient under small load is significantly greater than that under large load, as shown in Figure 5b,b1–b3). At the same time, the wear rate can be calculated according to the wear volume measured by the white-light interferometer. The volume wear caused by high contact pressure is obviously greater [40,41].
From the friction coefficient curve, compared with the high friction coefficient and huge fluctuation of the value of the surface of Ti6Al4V alloy without grafted self-assembly coating, the friction coefficient curve of the coated sample is more stable. Under different loads, the hydrogel-coated sample with the 1000 ppm core–shell polymer nanoparticle (PDCS) addition (1000ppm/PDCS/SA/VA/AM) shows a lower friction coefficient than the sample without PDCS addition (SA/VA/AM). Therefore, the dynamic hydrogel based on Schiff base bonds as a coating on a Ti6Al4V alloy sheet is conducive to enhancing its anti-friction performance, and the 1000ppm/PDCS/SA/VA/AM hydrogel network as a coating has a better anti-friction effect.
Three-dimensional white-light interferometry can measure the three-dimensional morphology of a worn surface and analyze the influence of friction and wear on the material. After the friction and wear test, an obvious furrow was formed on the surface of the uncoated Ti6Al4V alloy, and the wear marks of the sample after the reciprocating wear test with greater load were wider and deeper, as shown in Figure S2. The width of the wear marks after the reciprocating test with a load of 0.5 N was 0.22 mm, and the depth was 3.05 μm; under the 2 N load, the width and depth of the wear marks reached 0.55 and 13.5 μm. However, due to the soft elasticity of the hydrogel coating, this phenomenon is not obvious in the Ti6Al4V alloy samples with SA/VA/AM hydrogel coating and those without PDCS, which may be the result of elastic recovery of the dual-network hydrogel coating after load removal and network reconstruction after wear [39,42,43], as shown in Figure 6a1,a2,b1,b2 and Figure S2a1,a2,b1,b2. Micron particles can be observed in the furrow gaps on the surface of uncoated Ti6Al4V alloy, which may be caused by the wear of Ti6Al4V alloy and the upper friction by silicon nitride debris. Although such debris will fill the wear crevices to a certain extent to form a lubricating film, more debris will continue to aggravate the abrasive wear [44,45]. Compared with the Ti6Al4V alloy sample without SA/VA/AM hydrogel coating, the sample with 1000 ppm PDCS created a lower roughness surface, which may be because of the addition of nanoparticles on the cross-linking of the hydrogel network. The sample with lower elastic modulus has better wear repair ability.
Because the upper friction pair is Si3N4 pellets, the material’s ultra-low surface roughness (G4 class) and ultra-high elastic modulus (300 GPa) tend to show lower wear in friction experiments. As shown in Figure 7a,b and Figure S3a,b, a platform of about 2 μm can be found at the top of the cross-section profile of the silicon nitride pellets, while the corresponding Si3N4 pellets with hydrogel coating can hardly be observed to lose volume. In addition, micron particles are clearly visible in the three-dimensional topography, as shown in Figure 7a,b and Figure S3a,b, which may be from the adhesion of Ti6Al4V alloy chips. However, this phenomenon is not obvious on the samples with the hydrogel coating (Figure 6a1,a2,b1,b2 and Figure S2a1,a2,b1,b2), which is because the Ti6Al4V alloy does not have direct contact with the hydrogel coating. Smaller particles can be observed on the surface of the Si3N4 ball, as can be observed in the Figure 7a2,b1 and Figure S3b,b1,b2, which may come from the adhesive wear of the hydrogel coating. The hydrogel fragments dispersed into the lubricant during the frictional procedure stick to the Si3N4 ball, and this process of shedding and adhesion undoubtedly means greater wear of the coating [46,47]. In Figure 7b2 and Figure S3b1,b2, it is more obvious in high load experiments, which is in line with the findings from the wear rate and friction coefficient.

3.5. Wear Mechanisms

The tribological properties of a polymer surface are related to its network composition and structure, and a weak network can be designed as a sacrifice bond to enhance the toughness of the whole structure [48]. The precursor liquid system with nanoparticle dispersion will affect the entanglement of the polymer chain to achieve the regulatory network structure. Weak interactions such as the hydrogen bond and Schiff base bond can be re-established after being destroyed, and this destruction–repair sequence can achieve a certain degree of dynamic damage repair [49,50]. Under these guiding theories, hydrogels with low friction, low wear and dynamic damage repair function can be designed. During the ball-and-plate friction experiment, the balls with the upper friction pair fixed passed through the surface of the material in the form of point contact one after another. The hydrogel coating constructed with polyacrylamide and sodium alginate as the main structure was more conducive to energy dissipation due to its low elastic modulus (compared with Si3N4) [51,52], as shown in Figure 8. More energy is effectively dissipated during friction operation, which means less direct mechanical damage, which is macroscopic in the form of lower interface wear. The instantaneous friction in the relative frictional motion is related to the adsorption and relative slip of the material surface chains. The aldehyde group and the amino group of polyacrylamides in vanillin molecules can form Schiff base bonds, and this dynamic covalent interaction of moderate strength can be regarded as the fixation of the polyacrylamide chain. In addition, the sodium alginate polysaccharide chain and the polyacrylamide chain will also be entangled due to hydrogen bonds. Therefore, the forced movement of the polyacrylamide chain due to friction is dynamically limited, and the macroperformance is of low-interface friction. There are also spheroidal core–shell polymer nanoparticles (PDCS) that play a ball-bearing role in hydrogel material networks, where the sliding friction between some chains is converted into chain–ball–chain rolling friction, and this conversion of friction types also means a substantial reduction in the coefficient of friction [53].
From another point of view, in the ball-and-plate friction model, the point contact between the ball and the plate is sequential, which provides conditions for material network damage repair [54,55]. When the internal stress of the material reaches the critical value of the hydrogel network, the weak network will be destroyed first, such as the hydrogen bond between the sodium alginate polysaccharide chain and the polyacrylamide chain and the Schiff base bond between the VA and the PAM chain. However, this internal stress distribution will shift with the continuous movement of the upper friction pair, and the dynamic mechanical bond will have a chance to rebuild when the internal stress is reduced. The addition of a reasonable proportion of nanoparticles can avoid excessive entanglement of polymer chains. In addition, the relative position of nanoparticles will be forced to float up in space due to reciprocating friction, which may generate more coordination opportunities in the newly established network structure of the contact surface. The establishment of this interaction between nanomaterials and polymer chains may supplement the mechanical strength of materials. This efficient reconstruction mechanism have established a more optimal response paradigm for internal stress distribution, which effectively explain the low friction and wear of the material compared with the non-optimal network structure.
In addition, the benzene ring and polar groups (-OH, -OCH3, -CHO) of vanillin can be adsorbed on the material surface to form a monolayer, reducing friction. The sodium alginate polymer chain is rich in sodium carboxylate (-COONa), which can highly hydrate in water to form a hydration layer and reduce the direct contact of the friction interface [56]. The -CHO group in vanillin and the -NH2 group in acrylamide can form a dynamic Schiff base bond. This design is expected to endow the hydrogel with self-healing ability and achieve dynamic repair through reversible host–guest dissociation–recombination during the friction process [57]. In addition, the core–shell structure of PDCS nanoparticles, which is hard on the outside and soft on the inside, enables them to withstand certain loads and achieve a rolling effect during friction [18], thereby reducing friction. The multiple lubrication mechanisms allow the 1000ppmPDCS/SA/VA/AM hydrogel to remain stable on the surface of Ti6Al4V under a load of 2 N. The average coefficient of friction (ACOF = 0.130) is 67.74% lower than that of the Ti6Al4V surface under the same load.

4. Conclusions

Ti6Al4V alloy is one of the most widely used orthopedic implants due to its low density, high strength and good biocompatibility, but surface tribology limits its service life and performance. In this paper, a layer of a dynamic self-healing double-network hydrogel based on dynamic bonds, Schiff base bonds (VA and AM) and hydrogen bonds (VA and SA) was constructed on the surface of Ti6Al4V alloy. By introducing core–shell nanoparticles (PDCS), the structure of the network can be adjusted appropriately, which realizes the dynamic repair of PDCS/SA/VA/AM hydrogels under the cooperative regulation of core–shell polymer nanoparticles (PDCS). The PDCS/SA/VA/AM hydrogel network is destroyed under high shear stress (strain = 250%), the network structure is reconstructed when the critical stress is withdrawn (strain = 1%) and G’ and G can recover almost the same value in the second cycle, which indicates that the PDCS/SA/VA/AM hydrogel has excellent self-healing ability. This well-designed PDCS/SA/VA/AM hydrogel coating also has excellent lubrication and wear resistance, while the 1000ppmPDCS/SA/VA/AM hydrogel coating can achieve stable friction reduction and low wear under multiple loads. Under a 0.5 N load, the average friction coefficient of the Ti6Al4V surface is as low as 0.157, 77.47% lower than that of a Ti6Al4V surface under the same load. Under a 2 N load, the Ti6Al4V surface remains stable, and the average friction coefficient (ACOF = 0.130) is 67.74% lower than that of the Ti6Al4V surface under the same load. Excellent low-friction and low-wear properties are inseparable from the dynamic damage repair of Schiff base bonds (VA and AM) and hydrogen bonds (VA and SA). The design concept of this hydrogel network is to achieve low friction and low wear by regulating the core-shell polymer nanoparticles (PDCS), which provides a new strategy for bio-lubricant materials.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/coatings15050552/s1, Figure S1. The tan δ (G″/G′) under the strains of 1% and frequencies of 0.1−10 Hz. Figure S2. 3D micrographs of wear scars of different hydrogels; (a–b) Ti6Al4V; (a1–b1) SA/VA/AM; (a2–b2) 1000ppmPDCS/SA/VA/AM, Different load conditions of (a-a2): 0.5 N; (b-b2): 1.5 N. Figure S3. 3D micrographs of of wear scars on Si3N4 samples against titanium alloys. (a–b) Ti6Al4V; (a1–b1) SA/VA/AM; (a2–b2) 1000ppmPDCS/SA/VA/AM; Different load conditions (a–a2):0.5 N; (b–b2):1.5 N.

Author Contributions

Conceptualization, Y.L., W.H., X.H. and C.W.; methodology, Q.G.; software, G.W.; validation, C.W., G.W. and J.Y.; writing—original draft preparation, Y.L., W.H. and G.W.; writing—review and editing, C.W., Q.G., J.Y., S.H. and X.H.; visualization, Y.L. and W.H.; supervision, S.H.; funding acquisition, C.W. and S.H. All authors have read and agreed to the published version of the manuscript.

Funding

The authors are grateful for the support of the “ChenGuang” project (22CGA75) supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation and Shanghai “Science and Technology Innovation Action Plan” Morning Star Cultivation (Sailing Program 22YF1447500) and the Fundamental Research Funds for the Central Universities (project number 24X010301321) and sponsored by Collaborative Innovation Center of Fragrance, Flavour and Cosmetics.

Institutional Review Board Statement

Not Applicable.

Informed Consent Statement

This research does not involve human beings.

Data Availability Statement

Data is contained within the article or Supplementary Material.

Conflicts of Interest

The authors declare no conflicts of interest.

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Coatings 15 00552 g001
Figure 1. Synthesis of 1000ppmPDCS/SA/VA/AM hydrogel: 7 mg PDCS was added to 6.5 mL deionized water for ultrasonic dispersion for 0.5 h, then 0.1 g sodium alginate (SA), 5 g acrylamide (AM), 0.015 g N, N‘-methylene bis-acrylamide, 0.03 g ammonium persulfate and 20 uL tetramethyl ethylenediamine were added in sequence to fully dissolve them. Then, the titanium alloy plate grafted with polyamine was immersed in 3 mL of the precursor solution and followed by thermal polymerization for 1 h.
Figure 1. Synthesis of 1000ppmPDCS/SA/VA/AM hydrogel: 7 mg PDCS was added to 6.5 mL deionized water for ultrasonic dispersion for 0.5 h, then 0.1 g sodium alginate (SA), 5 g acrylamide (AM), 0.015 g N, N‘-methylene bis-acrylamide, 0.03 g ammonium persulfate and 20 uL tetramethyl ethylenediamine were added in sequence to fully dissolve them. Then, the titanium alloy plate grafted with polyamine was immersed in 3 mL of the precursor solution and followed by thermal polymerization for 1 h.
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Figure 2. (a) FTIR spectra of VA, AM, core–shell polymer nanoparticles (PDCS), 1000ppmPDCS/SA/VA/AM hydrogel; (b) Raman spectra of VA, core–shell polymer nanoparticles (PDCS) and 1000ppmPDCS/SA/VA/AM hydrogel; (c) swelling of hydrogels; (d,d1) SEM images of core–shell polymer nanoparticles (PDCS); (e,e1) SEM images of 1000ppmPDCS/SA/VA/AM hydrogel; (f,f1) SEM images of 1000ppmPDCS/SA/VA/AM hydrogel.
Figure 2. (a) FTIR spectra of VA, AM, core–shell polymer nanoparticles (PDCS), 1000ppmPDCS/SA/VA/AM hydrogel; (b) Raman spectra of VA, core–shell polymer nanoparticles (PDCS) and 1000ppmPDCS/SA/VA/AM hydrogel; (c) swelling of hydrogels; (d,d1) SEM images of core–shell polymer nanoparticles (PDCS); (e,e1) SEM images of 1000ppmPDCS/SA/VA/AM hydrogel; (f,f1) SEM images of 1000ppmPDCS/SA/VA/AM hydrogel.
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Figure 3. (a,b) Adhesion of alloy SA/VA/AM and 1000ppmPDCS/SA/VA/AM hydrogel coating on titanium; (c,d) film thickness test of SA/VA/AM and 1000ppmPDCS/SA/VA/AM hydrogel coatings.
Figure 3. (a,b) Adhesion of alloy SA/VA/AM and 1000ppmPDCS/SA/VA/AM hydrogel coating on titanium; (c,d) film thickness test of SA/VA/AM and 1000ppmPDCS/SA/VA/AM hydrogel coatings.
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Figure 4. Rheological and mechanical properties of hydrogels: (a) strain amplitude scan test at a fixed angular frequency (10 rad/s) at 25 °C (γ = 0.1 ~ 1000%) and the rheological characteristics of every hydrogel: (A): SA/VA/AM; (B): 500ppmPDCS/SA/VA/AM; (C): 1000ppmPDCS/SA/VA/AM; (D): 1250ppmPDCS/SA/VA/AM; (b) Dynamic frequency scanning curve of hydrogel at 10% strain; (c) curve of G’ and G’’ of hydrogel with temperature (25–45 °C); (d) alternate step strain scanning tests from low strain (1%) to subsequent high strain (250%); (e) tensile stress–strain curve of hydrogel; (f) compressive stress–strain curve of hydrogel.
Figure 4. Rheological and mechanical properties of hydrogels: (a) strain amplitude scan test at a fixed angular frequency (10 rad/s) at 25 °C (γ = 0.1 ~ 1000%) and the rheological characteristics of every hydrogel: (A): SA/VA/AM; (B): 500ppmPDCS/SA/VA/AM; (C): 1000ppmPDCS/SA/VA/AM; (D): 1250ppmPDCS/SA/VA/AM; (b) Dynamic frequency scanning curve of hydrogel at 10% strain; (c) curve of G’ and G’’ of hydrogel with temperature (25–45 °C); (d) alternate step strain scanning tests from low strain (1%) to subsequent high strain (250%); (e) tensile stress–strain curve of hydrogel; (f) compressive stress–strain curve of hydrogel.
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Figure 5. (a–a3) Friction coefficient curves of Ti6Al4V, SA/VA/AM and 1000ppm/PDCS/SA/VA/AM samples under different loads (0.5 N, 1 N, 1,5 N, 2 N); (b–b3) the average friction coefficient (ACOF) of Ti6Al4V, SA/VA/AM and 1000ppm/PDCS/SA/VA/AM hydrogels; (c–c3) friction wear rate of Ti6Al4V, SA/VA/AM and 1000ppm/PDCS/SA/VA/AM hydrogel coatings.
Figure 5. (a–a3) Friction coefficient curves of Ti6Al4V, SA/VA/AM and 1000ppm/PDCS/SA/VA/AM samples under different loads (0.5 N, 1 N, 1,5 N, 2 N); (b–b3) the average friction coefficient (ACOF) of Ti6Al4V, SA/VA/AM and 1000ppm/PDCS/SA/VA/AM hydrogels; (c–c3) friction wear rate of Ti6Al4V, SA/VA/AM and 1000ppm/PDCS/SA/VA/AM hydrogel coatings.
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Figure 6. Three-dimensional micrographs of wear scars of different hydrogels; (a,b) Ti6Al4V; (a1,b1) SA/VA/AM; (a2,b2) 1000ppmPDCS/SA/VA/AM, different load conditions of (a–a2) 1 N; (b–b2) 2 N.
Figure 6. Three-dimensional micrographs of wear scars of different hydrogels; (a,b) Ti6Al4V; (a1,b1) SA/VA/AM; (a2,b2) 1000ppmPDCS/SA/VA/AM, different load conditions of (a–a2) 1 N; (b–b2) 2 N.
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Figure 7. (a,b) Three-dimensional micrographs of wear scars on Si3N4 samples against titanium alloys. (a,b) Ti6Al4V; (a1,b1) SA/VA/AM; (a2,b2) 1000ppmPDCS/SA/VA/AM; different load conditions (a–a2): 1N; (b–b2): 2 N.
Figure 7. (a,b) Three-dimensional micrographs of wear scars on Si3N4 samples against titanium alloys. (a,b) Ti6Al4V; (a1,b1) SA/VA/AM; (a2,b2) 1000ppmPDCS/SA/VA/AM; different load conditions (a–a2): 1N; (b–b2): 2 N.
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Figure 8. Schematic diagram for biotribological mechanisms of 1000ppmPDCS/SA/VA/AM hydrogel: the PDCS/SA/VA/AM hydrogel may be worn due to long-term friction, but the dynamic Schiff base bond can make the PDCS/SA/VA/AM hydrogel dynamically repair the worn area.
Figure 8. Schematic diagram for biotribological mechanisms of 1000ppmPDCS/SA/VA/AM hydrogel: the PDCS/SA/VA/AM hydrogel may be worn due to long-term friction, but the dynamic Schiff base bond can make the PDCS/SA/VA/AM hydrogel dynamically repair the worn area.
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MDPI and ACS Style

Li, Y.; Hu, W.; Gao, Q.; Yan, J.; Wang, G.; Han, S.; Wang, C.; Hou, X. Multi-Modal Mechanical Response of Self-Healing Double-Network Hydrogel Coatings Based on Schiff Base Bond. Coatings 2025, 15, 552. https://doi.org/10.3390/coatings15050552

AMA Style

Li Y, Hu W, Gao Q, Yan J, Wang G, Han S, Wang C, Hou X. Multi-Modal Mechanical Response of Self-Healing Double-Network Hydrogel Coatings Based on Schiff Base Bond. Coatings. 2025; 15(5):552. https://doi.org/10.3390/coatings15050552

Chicago/Turabian Style

Li, Yanan, Wenbin Hu, Qike Gao, Jincan Yan, Guan Wang, Sheng Han, Chenchen Wang, and Xiaozheng Hou. 2025. "Multi-Modal Mechanical Response of Self-Healing Double-Network Hydrogel Coatings Based on Schiff Base Bond" Coatings 15, no. 5: 552. https://doi.org/10.3390/coatings15050552

APA Style

Li, Y., Hu, W., Gao, Q., Yan, J., Wang, G., Han, S., Wang, C., & Hou, X. (2025). Multi-Modal Mechanical Response of Self-Healing Double-Network Hydrogel Coatings Based on Schiff Base Bond. Coatings, 15(5), 552. https://doi.org/10.3390/coatings15050552

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