Sustainable polymer coating for stainproof fabrics

The excessive use of synthetic detergents in laundry operations is an important source of environmental pollution. As a result, sustainability-driven innovations are receiving increasing attention to enable eco-friendly textiles characterized by properties that allow for minimized consumption of detergents. Here we propose a coating-at-will (CAW) strategy to create an extra layer on top of a textile fabric to introduce stain resistance. The coated layer is based on conjugated polymers from lysozyme (Lyz) and zwitterionic poly(sulfobetaine methacrylate) (pSBMA), which, once exposed to the fabric, form a robust nanofilm on the surface. Remarkably, this hydrophilic layer exhibits excellent underwater superoleophobicity, and the coated fabrics can be cleaned simply with water without detergents. Optically transparent and biocompatible, this polymer nanofilm does not compromise the clothing comfort of the fabric and reduces the carbon footprint by more than 50% compared with detergents, according to a life cycle analysis. Moreover, our CAW strategy can be applied to the surfaces of various materials, including metals, glasses, plastics and ceramics, suggesting a versatile solution to the environmental risks posed by cleaning products. Laundry detergents usually contain chemicals that are problematic to the environment. The authors introduce a polymer nanofilm that renders fabrics and many more materials stain resistant and detergent free.

The excessive use of synthetic detergents in laundry operations is an important source of environmental pollution. As a result, sustainability-driven innovations are receiving increasing attention to enable eco-friendly textiles characterized by properties that allow for minimized consumption of detergents. Here we propose a coating-at-will (CAW) strategy to create an extra layer on top of a textile fabric to introduce stain resistance. The coated layer is based on conjugated polymers from lysozyme (Lyz) and zwitterionic poly(sulfobetaine methacrylate) (pSBMA), which, once exposed to the fabric, form a robust nanofilm on the surface. Remarkably, this hydrophilic layer exhibits excellent underwater superoleophobicity, and the coated fabrics can be cleaned simply with water without detergents. Optically transparent and biocompatible, this polymer nanofilm does not compromise the clothing comfort of the fabric and reduces the carbon footprint by more than 50% compared with detergents, according to a life cycle analysis. Moreover, our CAW strategy can be applied to the surfaces of various materials, including metals, glasses, plastics and ceramics, suggesting a versatile solution to the environmental risks posed by cleaning products.
As a major global challenge, sustainable ecological development is continuously challenged by the aggravated use of chemical surfactants in daily life and industrial production 1 . Among the numerous forms of surfactants, homecare detergents (for example, laundry and kitchenware detergents) account for more than 40% of the global surfactant market [2][3][4] . In the past two decades, the output of synthetic detergents in China has grown at an annual rate of 0.36 million tons, increasing from 3.48 million tons in 2001 to 10.77 million tons in 2021 (see https://data.stats.gov.cn/english/easyquery.htm?cn=C01). As a result, the ever-increasing use of detergents has made the treatment of surfactant-containing wastewater a growing burden. For instance, wastewater treatment in 2017 consumed approximately 4% of all the electrical power produced in the United States 5 . It is further estimated that the electricity required for wastewater treatment will increase by 20% over the next 15 years in developed countries, substantially increasing CO 2 emissions and energy consumption 6 . Moreover, in economically underdeveloped regions, household detergents are directly discharged into the environment without pretreatment 7 , resulting in the long-term accumulation of surfactants in the ecosystem. This detergent invasion has incredibly adverse effects on aquatic life, plants, animals and humans 8 . Regarding the abovementioned surfactant issue, although a few commercially available natural detergents have On the basis of the classical antifouling property of the pSBMA block, the good features of the PTL-pSBMA layer were further highlighted by its excellent biofouling resistance (Fig. 2a); this aspect is another highly desirable property for functional textiles. As examined by quartz crystal microbalance with dissipation (QCM-D), in contrast to bare Au, the PTL-pSBMA nanofilm showed superior resistance to the non-specific adsorption of a series of biofluid mixtures, adhesive proteins (Fetal bovine serum (FBS), β-lactoglobulin (β-Lg), fibrinogen (Fibri), human serum albumin (HSA), concanavalin A (Con A), horseradish peroxidase (HRP) and Bovine Serum Albumin (BSA)) and extracellular matrix components (Fig. 2b) (refs. 26-28). For example, the adsorption capacities of the PTL-pSBMA nanofilm to milk emerged 9-13 , their low production yields, high production costs, poor cleaning efficiency, irritability to sensitive skin and damage to wool and silk fabrics have limited their scale 14 .
In recent years, various superhydrophilic and underwater superoleophobic surfaces have been developed due to the extraordinary hydration capabilities of hydrophilic polymers. In particular, zwitterionic polymers contain a pair of oppositely charged groups in their repeating units. When these oppositely charged groups are uniformly distributed at the molecular level, the resultant molecule exhibits an overall neutral charge. Through ionic solvation, these zwitterionic polymers produce strong hydration, with profound implications for interface research on zwitterionic materials [15][16][17][18] . To construct these materials, surface-initiated atom transfer radical polymerization (SI-ATRP) technology 19 has been widely used due to its controllable degree of polymerization. However, the complex chemical procedures, dependence on substrate chemical structure, energy consumption and waste liquid/gas emissions limit the large-scale application of this technology. Moreover, SI-ATRP is often accompanied by chemical residues (for example, heavy metals) that irritate the human body. In addition to SI-ATRP, classical surface priming methods involve polydopamine (PDA) and polyphenol/Fe complex chemistry 20,21 . These methods typically impart surfaces with undesirable dark colours and rough topographies, with poor stability in alkaline pH environments (Supplementary Table 1).
Here we propose a coating-at-will (CAW) strategy to impart any surface with underwater superoleophobicity that can be maintained throughout the life cycle. This strategy is that any target surface can be engineered to impart underwater anti-oil staining properties via a rapid one-step aqueous coating with an antifouling hydrophilic biopolymer. Compared with normal detergent-based washing protocols, the present coating methods avoid the use of detergent in the laundry process and increase the cleaning efficiency for fabrics. We found that water and energy use can be decreased by at least 40~50%, with reduction in carbon emissions of more than 50% and at least 200 cycles of coating regeneration. According to its high cost-effectiveness (US$1,620 ton −1 of clothes) and low recommended dosage (0.9 g kg −1 of clothes), this method holds great commercial potential to transform the present mainstream laundry detergent-based cleaning protocol for fabrics washing. This strategy may pave the way for developing a series of CAW-derived coating designs that can provide a low-carbon-footprint sustainable solution to the environmental risks posed by cleaning products.

Design of PTL-pSBMA nanofilms
Protein-polymer conjugate was prepared via a 'grafting-to' strategy starting with the synthesis of poly(sulfobetaine methacrylate) (pSBMA), followed by coupling of lysozyme and pSBMA (Lyz-pSBMA) (Supplementary Figs. 1-6). Typically, after reducing the intramolecular disulfide bond of Lyz-pSBMA with tris(2-carboxyethyl)phosphine (TCEP) 22,23 , the corresponding amyloid-like aggregation of Lyz-pSBMA rapidly formed a stable two-dimensional phase-transitioned Lyz-pSBMA nanofilm (PTL-pSBMA) on various substrates (Fig. 1a). During this aggregation process, the conformational transition of the proteins from an α-helix to a β-sheet was reflected by a signal enhancement from the β-sheet structure in the circular dichroism (CD) (216 nm) and Fourier transform infrared (FTIR) (1,625 cm −1 ) spectra of PTL-pSBMA (  and human serum albumin were as low as 10 ng cm −2 , while the adsorption capacities of bare Au to those substances were >1,400 ng cm −2 (Supplementary Fig. 29). Resistance to bacterial adhesion by the nanofilm was directly visualized by incubating the micropatterned PTL-pSBMA nanofilm ( Supplementary Fig. 30) in bacterial liquid for a certain culture period. In this manner, micropatterned bacterial adsorption was observed on the blank area but not on the PTL-pSBMA-coated region. The clear contrast between the blank and PTL-pSBMA-coated surfaces indicated the effective inhibition of non-specific bacterial adhesion on the PTL-pSBMA nanofilm (Extended Data Fig. 2). In addition to bacteria, the PTL-pSBMA nanofilm showed good resistance to the adhesion of typical fungi, microbes, human platelets, whole blood and a fibroblast cell line (L929) (Supplementary Figs. 31 and 32).

Characterization of PTL-pSBMA-modified fabrics
The robustness and universal adhesion capability of PTL-pSBMA highlight its great value for the development of functionalized fabrics and smart textiles. So far, few universal methods have functionalized fabric surfaces without changing the original properties of the fabric, such as its optical appearance, biocompatibility and breathability. In this context, the present work offered a universal strategy for creating stealthy coatings on various mainstream fabrics (Fig. 3a). By using FITC-labelled Lyz-pSBMA in the phase transition, the resultant PTL-pSBMA coatings on the textile and substrate surfaces were clearly visualized under a fluorescence microscope (Fig. 3b,c and Supplementary Figs. 33 and 34). In contrast to conventional universal coatings, the PTL-pSBMA coating was optically transparent and had a thickness of 14 nm. This thin proteinaceous coating did not affect the wearing comfort of clothing based on air and moisture permeability (Fig. 3d,e). With the introduction of the hydrophilic zwitterionic pSBMA polymer, the subsequent WCA test indicated a large increase in hydrophilicity on the modified fabrics ( Supplementary Fig. 35). As a result, water-based ink droplets spread completely on the PTL-pSBMA-coated silk surface while maintaining poor wetting on the pristine silk surface (Fig. 3f,g). The high hydrophilicity of PTL-pSBMA offered a surface hydration layer on the modified fabric surface, supporting good underwater oil repellence by preventing oil droplets from approaching the fabric surface 29 . In addition, organic solvents and edible oils showed underwater oil

Sustainable coating for stainproof fabrics
The above results shed important light on easy oil removal from textile surfaces, since oil stains on fabric (that is, clothes) are a major issue that consumes large amounts of water, detergent and energy during washing. In this context, the improvements in the hydrophilicity and resultant superoleophobicity after attaching the PTL-pSBMA nanofilm coating to the fabric surface would be highly desirable and holds great promise for developing a detergent-free oil removal protocol by pure water washing; this protocol would dramatically reduce the consumption of precious water and energy resources and decrease the ecological impact from detergents. For this purpose, the ability of PTL-pSBMA-modified fabrics to resist common stains, including chili oil, ketchup, grass and coffee ( Supplementary Fig. 45), was examined. In this test, the same amounts of these four kinds of stains were applied to the surfaces of pristine fabrics and PTL-pSBMA@fabrics (polyester, vinylon, silk and cotton). The washing steps were divided into four groups: pristine fabrics, PTL-pSBMA@fabrics, dishwashing liquid (DWL) and laundry powder (LP). By photographing and determining the detergency and whiteness retention of the fabrics for water-soluble stains (for example, ketchup, coffee and whole blood), we noted that the detergency and whiteness retention characteristics in the PTL-pSBMA (Supplementary Figs. 46 and 47), DWL and LP groups were basically the same, and all exceeded those in the blank group. This result indicated that with the use of the PTL-pSBMA coating, simple machine washing with water without the use of detergent was sufficient to achieve conventional detergent-based cleaning performance for water-soluble stains ( Supplementary Fig. 48). Furthermore, by quantifying the detergency and whiteness retention of the four groups of fabrics polluted with oil-soluble stains, including chili oil (Fig. 4a) and grass (Extended Data Fig. 4), we found that the PTL-pSBMA-modified fabric group presented the same or better cleaning performance than the DWL and LP groups, and the values in these three groups were ~4-fold enhanced from those in the blank group (Fig. 4b,c and Supplementary Video 2). For another control, the anti-oil-stain performance of the fabrics after dip-coating in phase-transitioned lysozyme (PTL) or pSBMA was basically the same as that in the blank group; both performance levels were lower than that of PTL-pSBMA ( Supplementary Fig. 49). This result further proved that the phase transition of Lyz-pSBMA was key for supporting excellent oil resistance for detergent-free water cleaning. Finally, the following two aspects were critical to the cleaning ability of the PTL-pSBMA coating: the adhesion of oil droplets at the interface and the repulsion of the surface hydration layer 30,31 . As observed by in situ AFM imaging, the pSBMA component could attenuate the adhesion of hydrophobic alkyl chains on the surface, providing a molecular basis for superoleophobicity ( Supplementary Fig. 50). This attenuation might result from the strong internal hydration effects of zwitterionic polymer chains of pSBMA. The underwater superoleophobicity was further ascribed to the dynamic morphology transformation on the PTL-pSBMA nanofilm surface in the water phase. We speculated that the surface structures of PTL-pSBMA nanofilms underwent a morphological transition from an aggregate layer to a polymer layer during the transition from the air to the water phase.   PTL-pSBMA dispersion, and the refreshed PTL-pSBMA@fabrics showed excellent oil resistance after periodic washing and regeneration cycles (Fig. 5a). By this coating regeneration process (CAW strategy), the excellent detergency performance of PTL-pSBMA-modified silk, vinylon and polyester was well maintained for at least 200 cycles of washing and regeneration (Fig. 5b-d). Second, to broaden the application scope, the PTL-pSBMA-supported detergent-free textile cleaning method was further extended to clean kitchenware, such as plates, bowls and dishes (Extended Data Fig. 5). In the chili oil resistance test, the cleaning efficiencies of the PTL-pSBMA-coated dishes were all above 99.6%, which were enormously higher than those of the DWL (43.8%) and blank (32.8%) groups (Supplementary Videos 4 and 5). Moreover, due to the ease of oil stain cleaning, the cleaning time (correlating to energy) and water consumption characteristics in the PTL-pSBMA group were decreased by at least 50% compared with those in the DWL group ( Supplementary Fig. 65). These results indicated that PTL-pSBMA could reduce the large dosage of detergent needed for kitchenware cleaning and sharply decrease the consumption of water and energy (more than 50%) during tableware oil cleaning.

Biocompatibility and life cycle assessment (LCA) of PTL-pSBMA
Furthermore, due to the proteinaceous nature and biocompatibility of betaine derivatives, we conducted many experiments on the biological safety and toxicity of PTL-pSBMA, such as HT22 (Supplementary Fig. 66) and L929 cytotoxicity in vitro, haemolysis assay on PTL-pSBMA coated on bandage, toxicity on various viscera after in vivo injection in mice (Supplementary Fig. 67 and Table 2), mice skin irritation 32 , zebrafish and hydroponic lettuce toxicity (Extended Data Fig. 6). The above experimental results show that PTL-pSBMA has no effect on the toxicity of mouse skin, nerve cells, and growth of fish and plants, which provides a solid foundation to support the good biosafety of PTL-pSBMA. For excellent biosafety, a series of requirements for sustainable scale-up applications were further explored. From the perspectives of material cost and environmental impact, the cost to wash 1 kg of clothes by PTL-pSBMA was approximately US$1.62, which was similar to those of plant enzymes and natural plant extract detergent (Supplementary  Table 3). Furthermore, a complete LCA (Fig. 6a), including environmental considerations, was conducted to evaluate the overall sustainability and safety of the material. According to the LCA, for the   (Fig. 6b). Therefore, the environmental impact of PTL-pSBMA in the washing process was hugely lower than that of DWL and LP. Furthermore, the carbon footprint analysis to wash 1 kg of clothes with PTL-pSBMA indicated that the carbon dioxide produced by electricity and tap water accounted for more than 80% of the total (Fig. 6c), showing that after industrializing PTL-pSBMA, the emission of carbon dioxide was further reduced in quantity (Supplementary Data 1 and  Table 3). Moreover, to evaluate whether the PTL-pSBMA dispersion could be degraded, we cocultured it with microorganisms and then determined the biochemical oxygen demand (BOD) to measure its degradation ratio. The experimental results showed that PTL-pSBMA could exceed 80% degradation after 30 d, showing reasonable biodegradability 34 (Fig. 6d) PTL-pSBMA utilization could reduce water consumption by 50% and electricity consumption by 40% (Fig. 6e).

Discussion
To summarize, we have developed a CAW strategy for the design of stain-resistant fabrics that no longer require laundry detergents and thus minimize detergent pollution. This strategy features the manipulation of an amyloid-like aggregation of lysozyme-pSBMA, a protein-zwitterionic polymer conjugate. The resultant PTL-pSBMA imparts the surface of a coated substance with high hydrophilicity and thus excellent underwater superoleophobicity. The application on fabrics could autonomously remove oily stains by the hydrophilic polymer coating, leading to facile water-induced removal of the oily stains without adding detergent. This method achieves a cleaning efficiency comparable to or even better than that of traditional detergent-based cleaning. The CAW strategy can even serve to regenerate the PTL-pSBMA-primed surface at any time by simply recoating with PTL-pSBMA. In this manner, the coating-cleaning mode supported by the PTL-pSBMA coating on fabrics could be maintained for an almost unlimited period. Moreover, with the PTL-pSBMA system, the water and energy consumption could be reduced by 40-50%,  and the carbon footprint by more than 50%. Further, in addition to its cost-effectiveness (US$1,620 ton −1 of clothes), a low washing dosage (0.005 wt%), good optical transparency (~100%), good biocompatibility and excellent anti-biofouling properties could be achieved. The CAW strategy allows for household and industrial detergent-free cleaning with outstanding cleaning efficiency and ecological safety, contributing to a sustainable future.    Article https://doi.org/10.1038/s41893-023-01121-9

Preparation of PTL
the flask was purged with nitrogen gas for 30 min to remove the oxygen in the system and the flask was sealed. The solution was heated to 70 °C in an oil bath to initiate polymerization and the reaction lasted for 12 h. Then, the solution was cooled to room temperature. Finally, pSBMA was obtained by precipitating the solution in methanol and dimethyl sulfoxide, and vacuum drying at 40 °C for 12 h (refs. 36,37). Lyz-pSBMA was synthesized through the reaction of an amine with an N-succinimidyl activated ester under ambient conditions 38 . Briefly, lysozyme (5 mg, 0.35 μmol), pSBMA (45 mg, 0.485 mmol), ethyl (dimethylamino propyl) carbodiimide (EDC) (23.5 mg, 0.122 mmol) and N-hydroxysuccinimide (NHS) (5.4 mg, 25 μmol) were dissolved in a sodium phosphate buffer solution (0.1 M, pH 7.4) and reacted for 2 h while stirring at room temperature. Then, the solution was dialysed in water using a dialysis tube with a molecular weight (MW) of 7,000 Da for 48 h. Finally, the Lyz-pSBMA powder was obtained through lyophilisation and stored at 5 °C. In addition, Lyz-pSBMA samples with initial lysozyme and pSBMA molar ratios of 1:20, 1:30 and 1:50 were synthesized. The molecular weight of the synthesized Lyz-pSBMA was characterized using matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) spectra.
Equal volumes of Lyz-pSBMA solution (2 mg ml −1 ) and TCEP solution (50 mM, pH 7.0) were mixed and incubated for 2 h to obtain the PTL-pSBMA dispersion. To prepare the protein-polyzwitterionic polymer conjugate, zwitterionic pSBMA was first synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization (Supplementary Fig. 1). The resultant polymer product had a molecular weight of 4,640 (degree of polymerization =17) and a polydispersity index of 1.17, as analysed by hydrogen-1 nuclear magnetic resonance ( Supplementary Fig. 2) and gel permeation chromatography (Supplementary Fig. 3). Then, a typical protein-polymer conjugate was obtained via an EDC/NHS-mediated coupling reaction between the amine groups of native lysozyme and the carboxyl groups of pSBMA ( Supplementary Fig. 1a). By this process, one lysozyme molecule could attach to 1, 2 or 3 pSBMA graft chains, producing the protein-polymer conjugates Lyz-1pSBMA, Lyz-2pSBMA and Lyz-3pSBMA, respectively ( Supplementary Fig. 4). These products were properly characterized using MALDI-TOF, FTIR and corresponding XPS spectra, demonstrating an exact shift in the molecular mass of the protein after polymer grafting; additionally, these spectra displayed the characteristic signals for the sulfobetaine side chains in the FTIR (Supplementary Fig. 1b) and XPS ( Supplementary Fig. 5) spectra of the Lyz-pSBMA conjugate 39,40 . The native bioactivity of lysozyme was highly preserved after polymer conjugation, further indicating a negligible influence of the conjugation process and polymer chain on protein conformation and bioactivity ( Supplementary Fig. 6). For simplicity, we mainly used Lyz-2pSBMA (herein referred to as Lyz-pSBMA for simplicity, unless otherwise noted) to complete the experiments.

Coating of PTL-pSBMA nanofilms on various substrates
Fabrics (silk, linen, cotton, modal, polyester, flannel and vinylon), mica, silicon and polycarbonate (PC) plates were soaked in the PTL-pSBMA dispersion for 2 h, removed from the solution and then washed and dried.

PTL-pSBMA@fabrics regeneration
To verify the regeneration of PTL-pSBMA in fabrics, we used a household washing machine to wash the PTL-pSBMA@fabrics for 0, 3, 5, 10, 15, 20, 30 and 200 times. After washing for the indicated number of times, the washed fabric was re-coated with PTL-pSBMA.

Oil resistance of PTL-pSBMA@fabrics
Four types of textile fabric (polyester, silk, cotton and vinylon) were cut into 10 × 10 cm 2 pieces and soiled with the same amount of chili oil, grass stains, ketchup and coffee to simulate dirty stains on clothes. The fabrics were dried in open air for at least 24 h. The following washing steps were divided into four groups: (1) the blank group consisting of pristine fabrics with four stains and machine washed with water; (2) the PTL-pSBMA@fabrics group consisting of the PTL-pSBMA-modified cloths with four stains and machine washed with water; (3) the DWL group consisting of pristine cloths with four stains and machine washed with DWL and (4) the LP group consisting of pristine cloths with four stains and machine washed with LP. The soiled fabrics were immersed in a solution of DWL or LP at a concentration of 2 mg ml −1 . OMO washing powder was used as LP, the main ingredient of which is sodium dodecyl benzenesulfonate. OMO detergent was used as DWL, the main components of which are sodium alkyl sulfonate and sodium fatty alcohol ether sulfate. According to GB/T 13171.1-2009 and QB/T 1224-2012, the contents of DWL and LP in the washing solution were both 0.2 wt%. Deionized water without any detergent was used for the blank control and the PTL-pSBMA@fabrics. The contaminated fabrics were washed in a washing machine (AUX XQB42-A1508 4.2 kg) for 32 min at 25 °C.

Quantitative characterization of detergency of modified fabrics
In this experiment, a WSD-3C automatic whiteness colorimeter (Beijing Instrument KangGuang Optical Instrument) was used to test the L*, a*, b* and CIE whiteness values (W) of the contaminated fabrics before and after washing. During the test, the wavelength of the light source was 360-700 nm, and the sample was measured at least 5 times to obtain an average value. The corresponding detergency (D) was calculated according to equation (1): (1) where ΔE represents color differences, L* represents brightness, a* represents the colour component in the red and green directions, and b* represents the colour component in the yellow and blue directions. The subscript W stands for the stained area after washing, and S and B stand for the stained area and the blank area of the fabric before washing, respectively 41 .

Whiteness retention test
Whiteness retention, which reflects anti-redeposition properties (washed dirty stains that cannot re-stain washed fabrics), was determined according to equation (2): where the parameters are the whiteness values (%) of the pristine (R 0 ) and the soiled cloths before (R 1 ) and after (R 2 ) washing, measured using a WSD-3C automatic whiteness colorimeter 42 .

Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request. Correspondence and requests for materials should be addressed to P.Y. Source data are provided with this paper.