Cotton Fabric-Reinforced Hydrogels with Excellent Mechanical and Broad-Spectrum Photothermal Antibacterial Properties

Antibacterial hydrogel wound dressings hold great potential in eliminating bacteria and accelerating the healing process. However, it remains a challenge to fabricate hydrogel wound dressings that simultaneously exhibit excellent mechanical and photothermal antibacterial properties. Here we report the development of polydopamine-functionalized graphene oxide (rGO@PDA)/calcium alginate (CA)/Polypyrrole (PPy) cotton fabric-reinforced hydrogels (abbreviated as rGO@PDA/CA/PPy FHs) for tackling bacterial infections. The mechanical properties of hydrogels were greatly enhanced by cotton fabric reinforcement and an interpenetrating structure, while excellent broad-spectrum photothermal antibacterial properties based on the photothermal effect were obtained by incorporating PPy and rGO@PDA. Results indicated that rGO@PDA/CA/PPy FHs exhibited superior tensile strength in both the warp (289 ± 62.1 N) and weft directions (142 ± 23.0 N), similarly to cotton fabric. By incorporating PPy and rGO@PDA, the swelling ratio was significantly decreased from 673.5% to 236.6%, while photothermal conversion performance was significantly enhanced with a temperature elevated to 45.0 °C. Due to the synergistic photothermal properties of rGO@PDA and PPy, rGO@PDA/CA/PPy FHs exhibited excellent bacteria-eliminating efficiency for S. aureus (0.57%) and E. coli (3.58%) after exposure to NIR for 20 min. We believe that the design of fabric-reinforced hydrogels could serve as a guideline for developing hydrogel wound dressings with improved mechanical properties and broad-spectrum photothermal antibacterial properties for infected-wound treatment.


Introduction
Bacterial infection can reduce the healing process of damaged wounds, thereby hindering the restoration of skin tissue, impacting the quality of life, and even threatening lives.For more severe cases, bacterial-infected wounds may lead to sepsis and multiorgan failure, which has a staggering 20-40% mortality rate [1][2][3].Notably, over USD 31 billion has been spent annually on chronic wound treatment and management, bringing about a substantial burden to the national healthcare system [4].Hydrogel wound dressings hold great potential in eliminating bacteria and accelerating the healing process [5][6][7].To tackle bacterial infections at the wound site and create a better pro-healing microenvironment, various antibacterial agents (i.e., curcumin [8], polypeptide nisin [9]) and antibiotics (i.e., penicillin, vancomycin, and gentamicin) were integrated into hydrogel wound dressings to achieve efficient antibacterial effects, which would inevitably lead to unpredictable side effects.In addition, drug activity cannot be ensured during the entire complex woundhealing process [10,11].Furthermore, more than 70% of wound-infected bacteria clinically appear resistant to at least one antibiotic.
By incorporating photothermal therapy agents, hydrogel wound dressings can convert optical energy into local heating and cause physical damage to pathogens [12][13][14][15].Completely different from conventional antimicrobial therapy, photothermal therapy can prevent bacteria from developing drug resistance and has gained mounting attention in the treatment of wound infections due to its broad-spectrum antibacterial properties, noninvasiveness, and remote controllability [16].Among various photothermal materials, polydopamine-functionalized graphene oxide (rGO@PDA) exhibits excellent thermal stability, high photothermal conversion capability, and superior biocompatibility, which is suitable for photothermal antibacterial wound therapy [17][18][19][20].For example, photothermally responsive antimicrobial gelatin methacryloyl hydrogels have been fabricated for the treatment of wound infections based on rGO@PDA [21].
One major challenge for hydrogel wound dressings is their poor mechanical properties [22,23], which cannot provide enough physical support and are even deformed or broken when applied to the wound site [24,25].To overcome this practical restriction, researchers have focused on constructing interpenetrating hydrogel networks [26][27][28].For example, Wenzheng Li and coworkers [29] synthesized the double-network polyvinyl alcohol-polypyrrole hydrogels by the freeze-thaw and in situ polymerization method, leading to more stable structures.Results showed that the double-network structure significantly improved hydrogel elongation (156.4%),much higher than that of pristine polyvinyl alcohol hydrogels (103.0%).In addition, fabrics have been used to reinforce hydrogels to enable excellent mechanical properties for various applications including oil-water separation, soft sensors, tissue scaffolds, and wound dressing [30][31][32][33].For example, Ran Zhang and coworkers reported that fabric-reinforced poly (vinyl alcohol) composite hydrogels exhibited significant improvement in mechanical properties compared with cotton fabric and neat PVA gel [31].Compared to pure hydrogels, reinforcement with cotton fabric is straightforward, and cost-effective, which can provide robust mechanical properties in the long term.It should be noted that alginate hydrogels have been widely used in wound management because of their remarkable liquid adsorption, hemostatic properties, and biosafety, which are beneficial for wound healing [24,25,34].Nevertheless, their poor mechanical properties often lead to structural collapse when applied at wound sites, especially for long-term use, which makes it difficult to achieve the desired treatment efficacy.To this end, it remains a challenge to fabricate hydrogel wound dressings that simultaneously exhibit excellent mechanical and photothermal antibacterial properties.
In this work, we present polydopamine-functionalized graphene oxide (rGO@PDA)/ calcium alginate (CA)/Polypyrrole (PPy) fabric-reinforced hydrogels (abbreviated as rGO@PDA/CA/PPy FHs) for tackling bacterial infections.Notably, alginate hydrogels were selected as the main materials of wound dressings to directly interact with wound skin due to their superior biocompatibility and the capability to retain wound exudate for promoting healing process.In addition, alginate hydrogels also played an important role in integrating other functional components into composite wound dressings.Moreover, cotton fabric was employed to improve the mechanical properties of wound dressings by reinforcement.In addition, the mechanical properties of alginate hydrogels were further enhanced by generating an interpenetrating hydrogel network with PPy.Furthermore, PPy and rGO@PDA were employed to enable hydrogel wound dressings with superior broad-spectrum photothermal antibacterial properties based on the photothermal effect.Results indicated that rGO@PDA/CA/PPy FHs exhibited excellent tensile strength in the warp (289 ± 62.1 N) and weft directions (142 ± 23.0 N), similarly to cotton fabric.By incorporating PPy and rGO@PDA, the swelling ratio was significantly decreased from 673.5% to 236.6%, while photothermal conversion performance was significantly enhanced with a temperature elevated to 45.0 • C. Due to the synergistic photothermal conversion properties of rGO@PDA and PPy, rGO@PDA/CA/PPy FHs exhibited excellent bacteriaeliminating efficiency for S. aureus (0.57%) and E. coli (3.58%) after exposure to NIR for 20 min.Our work may facilitate the development of cotton fabric-reinforced hydrogels for infected-wound management.

Synthesis of rGO@PDA
The synthesis of rGO@PDA was carried out according to a previous report [20] with minor changes.Specifically, 100 mg GO and 50 mg DA were dissolved in 200 mL of 10 mM Tris-HCl (pH = 8.5).Under ice conditions, the obtained solution was dispersed in an ultrasonic machine for 10 min.Next, it was heated at 60 • C under magnetic stirring for 24 h.Finally, the mixture was filtered and washed with deionized water.The product (rGO@PDA) was collected after drying for further use.

Preparation of Cotton Fabric-Reinforced Hydrogels
CaCO 3 was dissolved in sodium alginate solution (0.02 g/mL, 12 g) under ultrasonic mixing with CaCO 3 -sodium alginate mass ratio of 0.287.Then, rGO@PDA was added into the mixture solution at different rGO@PDA-sodium alginate mass ratios (i.e., 0, 0.02, 0.04, 0.08, 0.16).Next, cotton fabric (5 cm × 5 cm) was immersed in the solution and then transferred to 25 g glucono-δ-lactone solution with a mass concentration of 0.9620% for 24 h.Obtained hydrogels were then immersed in FeCl 3 •6H 2 O solution (0.1024 M, 100 mL) for 24 h.After being washed with deionized water several times, the treated samples were put into PPy solution (0.0512 M, 100 mL) in the ice bath.HCl (0.1 M) was used to adjust the pH of the solution to 6.After 6 h of reaction and deionized water washing, rGO@PDA/CA/PPy FHs were finally obtained.For CA FHs and rGO@PDA/CA FHs, the PPy polymerization was skipped.In addition, there was no rGO@PDA for CA FHs during the fabrication process.

General Characterizations
A scanning electron microscope (TM-3000 SEM, Hitachi Co., Ltd., Shanghai, China) was used to observe the morphology.Fourier transform infrared spectroscopy was examined by an FTIR spectrometer (Nicolet 6700, Thermo Fisher Scientific, Waltham, MA, USA), with a wavenumber range of 4000-400 cm −1 .A thermogravimeter (CLARUS SQ8-STA8000, Shanghai PerkinElmer Co., Ltd., Shanghai, China) equipped with a highly sensitive analytical balance was used for thermogravimetric analysis (TGA).Each sample placed in a corundum crucible was heated up to 800 • C (heating rate 10 • C•min −1 ) in a dynamic atmosphere of nitrogen with a flow rate of 30 mL•min −1 .

Evaluation of Mechanical Properties
Tensile tests were conducted on an electronic fabric strength tester (YG(B)026G-500, Darong Textile Instrument Co., Ltd., Wenzhou, China) to characterize the mechanical prop-erties.Experimental conditions were as follows: temperature 20 ± 3 • C, relative humidity 65% ± 5%, sample size 10 mm × 50 mm, tensile speed 10 mm/min, and clamping distance 40 mm.An average value was taken from three repeated experiments for each group.

Evaluation of the Swelling Behavior
The swelling behavior was quantified after rehydration of the samples in deionized water by measuring the weight changes as a function of immersion time.Samples with different rGO@PDA mass ratios were compared.
After measuring dry weights, wet weights were determined at different time points (i.e., 1 h, 2 h, 4 h, 8 h, 12 h, 24 h, 48 h) by gentle blotting with filter papers to remove exceeding surface liquid.The swelling ratio was calculated according to Equation (1) [21]: where W d and W s are the weights of the samples in the dry and the swollen states, respectively.For each sample group, the results were taken as the mean values of three replicate samples under each condition.

Evaluation of Photothermal Properties
The obtained samples (sample size 10 × 10 mm) were placed into plastic centrifuge tubes that contained 1 mL of deionized water and irradiated with an 808 nm NIR light at 0.75 W/cm 2 with a distance of 10 cm using a NIR irradiation device (HW808AD1000-34F, Shenzhen Infrared Laser Technology Co., Ltd., Shenzhen, China) for 20 min.Pure deionized water was used as the control group.Temperature changes and infrared images were recorded using a photothermal imager (TG165, FLIR, Portland, ME, USA) at different time points (i.e., 1 min, 2 min, 4 min, 8 min, 12 min, 16 min, 20 min), and the curve was drawn according to the temperature change.

Evaluation of In Vitro Antibacterial Efficiency
Antibacterial activity against Staphylococcus aureus (S. aureus, ATCC 25923) and Escherichia coli (E.coli, ATCC 25922) was measured by the spread plate assay.Briefly, 10 µL of PBS buffers (pH = 7.4) containing 10 9 CFU•mL −1 bacteria were inoculated on a series of the experimental groups, including cotton fabric, CA FHs, CA/PPy FHs, and rGO@PDA/CA/PPy FHs.Then samples were exposed to NIR irritation for 20 min with the same parameters as mentioned in Section 2.7.Afterward, bacteria eluants from experimental samples were diluted 10 4 times, and then the bacterial suspension (10 µL) from each group was spread onto agar plates.The plates were incubated for 18 h at 37 • C. The number of colonies on the agar plates was recorded, and the bacterial survival rate was calculated.Results were taken from the mean values of three replicate samples.

Design and Fabrication of rGO@PDA/CA/PPy FHs
We developed rGO@PDA/CA/PPy FHs based on several considerations.Firstly, cotton fabric was used to reinforce the CA hydrogels and thus enhance mechanical properties.Secondly, rGO@PDA [35] was incorporated into the wound dressing to enable broadspectrum antibacterial properties based on the photothermal effect (Figure 1a).Thirdly, in situ polymerization of pyrrole was employed to induce an interpenetrating hydrogel network for further improving the mechanical properties (Figure 1b,c).To fabricate cotton fabric-reinforced CA hydrogels, cotton fabric was soaked in sodium alginate, CaCO 3 , and rGO@PDA solution.To accelerate the crosslinking process, cotton fabric was transferred to a glucono-δ-lactone reaction system, which would react with CaCO 3 and produce Ca 2+ , thereby forming an eggshell structure with sodium alginate for more efficient crosslinking progress (Figure 1d).In addition, the obtained fabric-reinforced hydrogels were treated by an oxidant agent of iron (III) chloride hexahydrate (Figure 1b), enabling the polymerization of PPy after being transferred to the PPy solution.
network for further improving the mechanical properties (Figure 1b,c).To fabricate cotton fabric-reinforced CA hydrogels, cotton fabric was soaked in sodium alginate, CaCO3, and rGO@PDA solution.To accelerate the crosslinking process, cotton fabric was transferred to a glucono-δ-lactone reaction system, which would react with CaCO3 and produce Ca 2+ , thereby forming an eggshell structure with sodium alginate for more efficient crosslinking progress (Figure 1d).In addition, the obtained fabric-reinforced hydrogels were treated by an oxidant agent of iron (III) chloride hexahydrate (Figure 1b), enabling the polymerization of PPy after being transferred to the PPy solution.

Structure and Mechanical Properties of Cotton Fabric-Reinforced CA Hydrogels
To confirm the feasibility of fabricating cotton fabric-reinforced CA hydrogels, SEM images were obtained (Figure 2a,b).Typical woven structures showing yarns and fibers were observed for the pristine cotton fabric (Figure 2a).After reinforcing CA hydrogels, both yarns and the gaps between yarns were covered by CA hydrogels, although the yarn shape still existed, indicating the successful fabrication of cotton fabric-reinforced CA hydrogels (Figure 2b).Notably, CA hydrogels were evenly attached to the fabric surface, which can ensure the structural stability of cotton fabric-reinforced hydrogels.

Structure and Mechanical Properties of Cotton Fabric-Reinforced CA Hydrogels
To confirm the feasibility of fabricating cotton fabric-reinforced CA hydrogels, SEM images were obtained (Figure 2a,b).Typical woven structures showing yarns and fibers were observed for the pristine cotton fabric (Figure 2a).After reinforcing CA hydrogels, both yarns and the gaps between yarns were covered by CA hydrogels, although the yarn shape still existed, indicating the successful fabrication of cotton fabric-reinforced CA hydrogels (Figure 2b).Notably, CA hydrogels were evenly attached to the fabric surface, which can ensure the structural stability of cotton fabric-reinforced hydrogels.
To validate the successful incorporation of desired functional components, FTIR spectra were obtained to characterize the functional groups (Figure 2c).Specifically, the FTIR spectrum of GO and rGO@PDA emerged at 1637 cm −1 , which should be ascribed to the vibrations of C=C.In addition, the peak intensity at 1725 cm −1 (C=O) of rGO@PDA was lower than that of GO, indicating that the number of oxygen-containing groups of rGO@PDA decreased.Meanwhile, rGO@PDA exhibited a low intensity at 1456 cm −1 due to the bending vibration of N-H, demonstrating that GO has been successfully reduced by PDA.As for all FHs including calcium alginate FHs, calcium alginate/PPy FHs, and rGO@PDA/CA/PPy FHs, the -OH peak shifted to the lower frequency of 3419 cm −1 compared to that of cotton fabric (3443 cm −1 ).Compared to that of the pure cotton fabric, the peaks at 1629 cm −1 and 1400 cm −1 of all these FHs samples should be attributed to the asymmetrical -COO − stretching vibration and symmetric -COO − stretching vibrations of calcium alginate [24].The characteristic peaks of PPy at 1030 cm −1 and 1317 cm −1 were present in calcium alginate/PPy FHs and rGO@PDA/CA/PPy FHs, verifying the successful deposition of PPy on FHs [25,27].Taken together, the FTIR results verified that rGO@PDA/CA/PPy FHs were successfully fabricated.To validate the successful incorporation of desired functional components spectra were obtained to characterize the functional groups (Figure 2c).Specifical FTIR spectrum of GO and rGO@PDA emerged at 1637 cm −1 , which should be ascri the vibrations of C=C.In addition, the peak intensity at 1725 cm −1 (C=O) of rGO@PD lower than that of GO, indicating that the number of oxygen-containing grou rGO@PDA decreased.Meanwhile, rGO@PDA exhibited a low intensity at 1456 cm to the bending vibration of N-H, demonstrating that GO has been successfully re by PDA.As for all FHs including calcium alginate FHs, calcium alginate/PPy FH rGO@PDA/CA/PPy FHs, the -OH peak shifted to the lower frequency of 3419 cm − pared to that of cotton fabric (3443 cm −1 ).Compared to that of the pure cotton fabr peaks at 1629 cm −1 and 1400 cm −1 of all these FHs samples should be attributed asymmetrical -COO − stretching vibration and symmetric -COO − stretching vibrati calcium alginate [24].The characteristic peaks of PPy at 1030 cm −1 and 1317 cm − present in calcium alginate/PPy FHs and rGO@PDA/CA/PPy FHs, verifying the succ deposition of PPy on FHs [25,27].Taken together, the FTIR results verified rGO@PDA/CA/PPy FHs were successfully fabricated.
To investigate whether the introduction of cotton fabric and the employment interpenetrating hydrogel network will improve mechanical performances, tensil  c) FTIR spectra of GO, rGO@PDA, cotton fabric, calcium alginate hydrogels, CA FHs, CA/PPy FHs, and rGO@PDA/CA/PPy FHs.(d) Warp breaking strength and breaking elongation of cotton fabric, CA/PPy FHs, and rGO@PDA/CA/PPy FHs with different rGO@PDA:SA mass ratios.(e) Weft breaking strength and breaking elongation of cotton fabric, CA/PPy FHs, and rGO@PDA/CA/PPy FHs with different rGO@PDA:SA mass ratios.(f) The swelling performance of CA FHs, CA/PPy FHs, and rGO@PDA/CA/PPy FHs with different rGO@PDA:SA mass ratios.
To investigate whether the introduction of cotton fabric and the employment of the interpenetrating hydrogel network will improve mechanical performances, tensile tests were then conducted for cotton fabric, calcium alginate FHs, calcium alginate/PPy FHs and rGO@PDA/CA/PPy FHs with different rGO@PDA:SA mass ratios (Figure 2d,e).It should be noted that pure calcium alginate hydrogel broke very easily in the process of tensile experiments because of poor mechanical properties, so tensile experimental data could not be measured.In contrast, the warp and weft breaking strength of CA FHs were 213 ± 21.5 N, 117 ± 29.5 N, respectively, suggesting that reinforcement with cotton fabric can significantly improve the mechanical properties of CA hydrogels.Notably, breaking strength was further elevated in both warp (289 ± 62.1 N) and weft directions (142 ± 23.0 N) after polymerization of PPy, higher than that of cotton fabric (warp and weft breaking strength were 272 ± 15.3 N, 132 ± 16.8 N, respectively), which should be attributed to the existence of an interpenetrating structure.Specifically, the additional polymer networks of PPy in the alginate hydrogels provided the wound dressing with a more stable and compact structure against external force.Additionally, carboxyl groups of calcium alginate hydrogel could form a coordination structure with Fe 3+ [28], which can further improve the overall toughness.It should be noted that no significant changes were detected after incorporating rGO@PDA.Nevertheless, our results indicated that the mechanical properties of hydrogels could be greatly improved by reinforcing with cotton fabric and employing the interpenetrating hydrogel network.
Besides tensile tests, swelling behaviors were also investigated to characterize the hydrogel stability (Figure 2f).It was obvious that all samples exhibited a faster swelling process at the early stage and then reached swelling equilibrium gradually.Inspiringly, the swelling ratio of calcium alginate/PPy FHs was 281.9% after 48 h, significantly lower than that of calcium alginate FHs, which reached 673.5% after 48 h, indicating that the introduction of PPy into the CA FHs can significantly reduce the swelling rate.The decreased swelling rate should be attributed to the existence of hydrophobic PPy in the CA FHs.In addition, the tight interpenetrating network caused by the crosslinking of the PPy molecular chain could also reduce the hydrogel water-binding capacity [26].After adding rGO@PDA into calcium alginate/PPy FHs, the swelling ratio was further lowered in all rGO@PDA/CA/PPy FHs samples.In particular, 0.16rGO@PDA/CA/PPy FHs showed the lowest swelling ratio of 236.6% after 48 h, which should be ascribed to the fact that the presence of the layer structure of rGO@PDA could hinder the spatial extension of polymeric segments inside the hydrogel network.Furthermore, the hydrophobicity of the aromatic C=C of the graphene plane also affected the absorption capacity of the material [18,20].To this end, the introduction of PPy and rGO@PDA can significantly improve the hydrogel stability by decreasing the swelling ratio.Remarkably, all rGO@PDA/CA/PPy FHs samples still retained a certain swelling capacity to ensure they absorb wound exudate in a timely fashion when serving as a wound dressing.

Thermal Properties of Cotton Fabric-Reinforced CA Hydrogels
To verify the stability of cotton fabric-reinforced CA hydrogels for use in photothermal therapy, thermogravimetric curves were obtained for CA/PPy FHs and rGO@PDA/CA/PPy FHs (Figure 3a,b).To indicate the accumulation of weight loss, the differential thermogravimetry (DTG) curves, the first-order derivative of the corresponding TGA curves, were also obtained.For CA/PPy FHs, the weight loss occurred at about 294.8 • C, which should be assigned to the thermal decomposition of the organic groups and oxygen-containing functional groups of materials.After adding rGO@PDA, the initial decomposition temperature of samples was upshifted from 294.8 • C to 313.84 • C. According to the DTG curves, CA/PPy FHs showed a sharp peak at about 354.8 • C and a derivative weight of 19.6%/min, while rGO@PDA/CA/PPy FHs exhibited a strong peak at around 373.0 • C and a derivative weight of 18.1%/min, indicating that the addition of rGO@PDA shifted the decomposition temperature and increased the thermal stability.The above shifting should result from the fact that the proper dispersion of rGO@PDA in the composites enhanced interfacial interactions with the molecular chain, limiting the movement of the macromolecular chains.Meanwhile, the layered structure of rGO@PDA could also limit the small molecules generated during the pyrolysis process from leaving the system, leading to reduced mass loss [17,19].Importantly, the weight loss below 100 • C belonged to the loss of water in the hydrogel, which was negligible, indicating that both CA/PPy FHs and rGO@PDA/CA/PPy FHs can meet the temperature requirement for the subsequent photothermal experiment.
To quantify photothermal conversion performance, the temperature was measured for CC/PPy FHs and rGO@PDA/CA/PPy FHs with different rGO@PDA:SA mass ratios exposed to 808 nm near-infrared light (NIR) laser for 20 min (Figure 3c,d).Notably, photothermal conversion performance was highly dependent on the existence of PPy and rGO@PDA.Specifically, the temperature of CA FHs showed negligible changes, while the temperature was enhanced from 26.1 • C to 40.7 • C after incorporating PPy.By increasing the rGO@PDA:SA ratio to 0.16, the temperature was further elevated to 45.0 • C.These changes demonstrate that rGO@PDA/CA/PPy FHs presented a synergistic photothermal effect from rGO@PDA and PPy.Specifically, rGO@PDA and PPy could exhibit strong optical absorption and then convert optical energy to thermal energy based on the energy-level transition principle, making it a promising photothermal wound dressing for antibacterial treatment.To quantify photothermal conversion performance, the temperature was measure for CC/PPy FHs and rGO@PDA/CA/PPy FHs with different rGO@PDA:SA mass rati exposed to 808 nm near-infrared light (NIR) laser for 20 min (Figure 3c,d).Notably, ph tothermal conversion performance was highly dependent on the existence of PPy an rGO@PDA.Specifically, the temperature of CA FHs showed negligible changes, while th temperature was enhanced from 26.1 °C to 40.7 °C after incorporating PPy.By increasin the rGO@PDA:SA ratio to 0.16, the temperature was further elevated to 45.0 °C.The changes demonstrate that rGO@PDA/CA/PPy FHs presented a synergistic phototherm effect from rGO@PDA and PPy.Specifically, rGO@PDA and PPy could exhibit strong o tical absorption and then convert optical energy to thermal energy based on the energ level transition principle, making it a promising photothermal wound dressing for an bacterial treatment.

Photothermal Antibacterial Properties of Cotton Fabric-Reinforced CA Hydrogels
Next, we further evaluated the in vitro photothermal antibacterial properties of cotto fabric-reinforced CA hydrogels against two bacterial strains (i.e., Gram-positive S. aure and Gram-negative E. coli).After 18 h incubation, the colony formation units (CFU) of th viable bacteria were then counted (Figure 4).As expected, the survival rate of bacteria o cotton fabric and CA FHs showed no significant difference whether treated with NIR radiation or not.Specifically, their S. aureus (Figure 4a,c) and E. coli (Figure 4b,d) bact rial survival rates were above 90%.Similarly, samples containing rGO@PDA or PPy d not show bacteria-killing capability when NIR irradiation was not involved.In contra the bacterial survival rate of CA/PPy FHs was 35.2% and 36.6% for S. aureus and E. co respectively, which should be attributed to the excellent photothermal conversion perfo mance of PPy.Notably, rGO@PDA/CA FHs and rGO@PDA/CA/PPy FHs exhibited sup

Photothermal Antibacterial Properties of Cotton Fabric-Reinforced CA Hydrogels
Next, we further evaluated the in vitro photothermal antibacterial properties of cotton fabric-reinforced CA hydrogels against two bacterial strains (i.e., Gram-positive S. aureus and Gram-negative E. coli).After 18 h incubation, the colony formation units (CFU) of the viable bacteria were then counted (Figure 4).As expected, the survival rate of bacteria on cotton fabric and CA FHs showed no significant difference whether treated with NIR irradiation or not.Specifically, their S. aureus (Figure 4a,c) and E. coli (Figure 4b,d) bacterial survival rates were above 90%.Similarly, samples containing rGO@PDA or PPy did not show bacteria-killing capability when NIR irradiation was not involved.In contrast, the bacterial survival rate of CA/PPy FHs was 35.2% and 36.6% for S. aureus and E. coli, respectively, which should be attributed to the excellent photothermal conversion performance of PPy.Notably, rGO@PDA/CA FHs and rGO@PDA/CA/PPy FHs exhibited superior antibacterial effects regardless of bacterial type, which should be attributed to their effective heating for bacteria [14,15].Notably, the rGO@PDA:SA mass ratio also showed a significant influence on the antibacterial properties.In particular, 0.16 rGO@PDA/CA/PPy FHs possessed the lowest bacterial survival rates of 0.57% and 3.58% for S. aureus and E. coli, respectively.These excellent photothermal antibacterial results were consistent with photothermal conversion results, suggesting that the mechanism of killing bacteria was attributed to direct thermal bacteria ablation.Specifically, rGO@PDA and PPy can convert optical energy into thermal energy upon NIR irradiation, resulting in high temperatures and then destroying the structural integrity of the bacterial cell wall and cell membrane.Meanwhile, the high temperature could also lead to protein enzyme denaturation, and even DNA damage, causing irreversible damage to bacteria [12][13][14].
Polymers 2024, 16, 1346 9 of 12 attributed to direct thermal bacteria ablation.Specifically, rGO@PDA and PPy can conv optical energy into thermal energy upon NIR irradiation, resulting in high temperatur and then destroying the structural integrity of the bacterial cell wall and cell membran Meanwhile, the high temperature could also lead to protein enzyme denaturation, a even DNA damage, causing irreversible damage to bacteria [12][13][14].To this end, we fabricated rGO@PDA/CA/PPy FHs that simultaneously exhibit exc lent mechanical and broad-spectrum properties enabled by NIR irradiatio All these properties are essential and required for materials when used as wound dre ings.Our work has proven that rGO@PDA/CA/PPy FHs could achieve the required pro erties for practical wound management and serve as a robust physical barrier to avo bacterial infection during the healing process.For standard clinical treatment with anti otics (i.e., penicillin, vancomycin, and gentamicin), more than 70% of wound-infected ba teria clinically appear resistant to at least one antibiotic.In contrast, rGO@PDA/CA/P FHs with excellent broad-spectrum antibacterial properties can effectively kill both gra positive and gram-negative bacteria with minimal side effects, providing an effective s lution to addressing the clinical challenge of antibiotic resistance.We believe that the d sign of rGO@PDA/CA/PPy FHs could serve as a guideline for infected-wound clini management.Notably, all materials including alginate [34], cotton fabric [9], rGO [3 PDA [37], and PPy [38] have been proven to exhibit superior biocompatibility in the lit ature.In addition, rGO nanomaterials were incorporated into the composite wound dre ings, which can eliminate general safety concerns.Nevertheless, the current work focus on the antibacterial properties of rGO@PDA/CA/PPy FHs enabled by NIR irritation.Eli inating bacterial infection is just one major goal of wound dressings; functional comp nents that promote wound healing are also needed to achieve desired wound manag ment.Therefore, we believe that rGO@PDA/CA/PPy FHs hold great promise for medi use after further refining the wound-healing properties by combining other function components.To this end, we fabricated rGO@PDA/CA/PPy FHs that simultaneously exhibit excellent mechanical and broad-spectrum antibacterial properties enabled by NIR irradiation.All these properties are essential and required for materials when used as wound dressings.Our work has proven that rGO@PDA/CA/PPy FHs could achieve the required properties for practical wound management and serve as a robust physical barrier to avoid bacterial infection during the healing process.For standard clinical treatment with antibiotics (i.e., penicillin, vancomycin, and gentamicin), more than 70% of wound-infected bacteria clinically appear resistant to at least one antibiotic.In contrast, rGO@PDA/CA/PPy FHs with excellent broad-spectrum antibacterial properties can effectively kill both gram-positive and gram-negative bacteria with minimal side effects, providing an effective solution to addressing the clinical challenge of antibiotic resistance.We believe that the design of rGO@PDA/CA/PPy FHs could serve as a guideline for infected-wound clinical management.Notably, all materials including alginate [34], cotton fabric [9], rGO [36], PDA [37], and PPy [38] have been proven to exhibit superior biocompatibility in the literature.In addition, rGO nanomaterials were incorporated into the composite wound dressings, which can eliminate general safety concerns.Nevertheless, the current work focused on the antibacterial properties of rGO@PDA/CA/PPy FHs enabled by NIR irritation.Eliminating bacterial infection is just one major goal of wound dressings; functional components that promote wound healing are also needed to achieve desired wound management.Therefore, we believe that rGO@PDA/CA/PPy FHs hold great promise for medical use after further refining the wound-healing properties by combining other functional components.

Conclusions
In summary, we developed rGO@PDA/CA/PPy FHs with improved mechanical properties and photothermal antibacterial properties for wound management.Due to the cotton fabric reinforcement and the interpenetrating structure, rGO@PDA/CA/PPy FHs overcame the weakness of fragile calcium alginate hydrogels and exhibited excellent tensile strength in the warp (289 ± 62.1 N) and weft directions (142 ± 23.0 N), similarly to cotton fabric.By incorporating PPy and rGO@PDA, the swelling ratio was also significantly decreased from 673.5% to 236.6%, while photothermal conversion performance was significantly enhanced with a temperature elevated to 45.0 • C. Furthermore, the antibacterial experiments showed that rGO@PDA/CA/PPy FHs had excellent bacteria-eliminating efficiency for S. aureus (0.57%) and E. coli (3.58%) after exposure to NIR for 20 min due to the synergistic photothermal antibacterial properties of rGO@PDA and PPy.We believe that rGO@PDA/CA/PPy FHs hold great promise for medical use after further refining the wound-healing properties by combining other functional components.In addition, the design of fabric-reinforced hydrogels could serve as a guideline for developing hydrogel wound dressings with improved mechanical properties and broad-spectrum photothermal antibacterial properties for infected-wound treatment.

Figure 2 .
Figure 2. Structure and mechanical properties of cotton fabric-reinforced CA hydrogels.(a,bimages of (a) cotton fabric and (b) rGO@PDA/CA/PPy FHs.(c) FTIR spectra of GO, rGO@PD ton fabric, calcium alginate hydrogels, CA FHs, CA/PPy FHs, and rGO@PDA/CA/PPy FHs.(d breaking strength and breaking elongation of cotton fabric, CA/PPy FHs, and rGO@PDA/C FHs with different rGO@PDA:SA mass ratios.(e) Weft breaking strength and breaking elon of cotton fabric, CA/PPy FHs, and rGO@PDA/CA/PPy FHs with different rGO@PDA:SA mass (f) The swelling performance of CA FHs, CA/PPy FHs, and rGO@PDA/CA/PPy FHs with d rGO@PDA:SA mass ratios.

Figure 3 .
Figure 3. Thermal properties of cotton fabric-reinforced CA hydrogels.(a,b) Thermogravimet curves and differential thermogravimetry curves of (a) CA/PPy FHs and (b) rGO@PDA/CA/PP FHs.(c) The photothermal performance of CA FHs, CA/Ppy FHs, and rGO@PDA/CA/PPy FHs e posed to 808 nm NIR light laser for 20 min at 0.7 W cm −2 : (c) photothermal heating curves and ( the thermal images.

Figure 3 .
Figure 3. Thermal properties of cotton fabric-reinforced CA hydrogels.(a,b) Thermogravimetric curves and differential thermogravimetry curves of (a) CA/PPy FHs and (b) rGO@PDA/CA/PPy FHs.(c) The photothermal performance of CA FHs, CA/Ppy FHs, and rGO@PDA/CA/PPy FHs exposed to 808 nm NIR light laser for 20 min at 0.7 W cm −2 : (c) photothermal heating curves and (d) the thermal images.

Figure 4 .
Figure 4. (a,b) Photographs of bacterial colonies formed by (a) S. aureus and (b) E.coli treated w various samples with or without 808 nm laser irradiation (0.7 W cm −2 ).(c,d) The bacterial viabilit of (a) S. aureus and (b) E.coli were measured using the plate count method.

Figure 4 .
Figure 4. (a,b) Photographs of bacterial colonies formed by (a) S. aureus and (b) E.coli treated with various samples with or without 808 nm laser irradiation (0.7 W cm −2 ).(c,d) The bacterial viabilities of (a) S. aureus and (b) E.coli were measured using the plate count method.