Multifunctional composite film based on biodegradable grape skin and polyvinyl alcohol

In winemaking, large amounts of grape skin (GS) are generated as by-products, which contain not only abundant degradable cellulose, hemicellulose, and pectin but various functional polyphenols. In contrast to most studies focusing on the utilization of extractives, the current study investigates the use of ultrasonicated grape skin (UGS) containing all components to develop a multifunctional composite film. Owing to dissociation during ultrasonication, all GS components were well dispersed in water to obtain the UGS suspension. Transmission electron microscopy (TEM) indicated that the celluloses were successfully transformed into cellulose nanofibers, which can improve the uniformity of the composite film. Subsequently, biodegradable and multifunctional composite films were fabricated by combining the UGS and polyvinyl alcohol (PVA). The UGS and PVA formed a good interface, which was attributable to strong hydrogen bonds, and the resulting films exhibited excellent thermal stability and moisture-sensitive mechanical properties. The polyphenols in the UGS suspension endowed the composite film with multiple functions, including pH-responsive color change, excellent antioxidant activity, ultraviolet shielding, and antimicrobial properties. The use of PVA enhanced the flexibility, strength, and elongation of the UGS film. Therefore, the easily prepared, tailored, multifunctional, and biodegradable UGS/PVA composite film exhibits superior potential for application in agriculture, cosmetics, and healthcare.


Introduction
Grape wine is a widely known drink, and the grape industry has a large market share. More than 60 million tons of grapes are harvested annually, * 75 % of the harvest allocated to winemaking (Fig. 1a), resulting in * 14.5 million tons of grape pomace byproduct (Beres et al. 2017;Ferrari et al. 2019;Fan et al. 2020). Grape skin (GS) is the major component of grape pomace, roughly comprising half of the material weight (Mendes et al. 2013). GS is currently not considered as a highly profitable waste and is discarded, causing environmental problems. Only a small proportion of GS is used for animal feed. The lack of sufficient applications for GS, as well as the increasing environmental concern, has drawn widespread interest. GS, which is similar to a porous hydrogel, is presented in Fig. 1b. The porous skeleton of GS-from the outside to the inside-consists of the cuticle, intermediate epidermis, and hypodermis ( Fig. S1) (Ortega-Regules et al. 2008). Dried GS consists of cellulose, hemicelluloses, polyphenols, pectin, proteins, lipids, and soluble sugar (Mendes et al. 2013). The hypodermis contains most of the polyphenols in GS (Lecas and Brillouet 1994). Although cellulose represents the largest proportion of GS, most studies have focused on the extraction and use of various extractives, such as anthocyanins, hydroxycinnamic acids, flavanols, and flavonol glycosides (de Moura et al. 2002;Esquivel-Alvarado et al. 2020;Lavelli et al. 2016;Quijada-Morín et al. 2015;Xu et al. 2018a, b;Zhu et al. 2015). These extractives represent only a small proportion of the GS, and a significant portion is wasted. Considerable progress effort is required to achieve the comprehensive utilization of the whole GS.
The increasing environmental concern has resulted in the strong inclination to use ''green materials'' to replace petroleum-based non-degradable plastic films. Owing to their environmentally friendly characteristics, biodegradable plant-based materials (cellulose, starch, and polylactic acid) are desirable alternatives to petroleum-based products Wang et al. 2020;Yu et al. 2020). However, as market demand rises, the overuse of plant-based films may create a new ecological and food crisis (Han et al. 2018;Papadopoulou et al. 2019;Sun et al. 2019;Tu et al. 2020;Wang et al. 2018). Large amounts of plant cellulose (fruit skins, stems, crop husk, leaves, and corncobs) are wasted as by-products in the food industry. Owing to their high biodegradability, most by-products end up in landfills. Cellulose resources are significantly wasted, and the polyphenols in the byproducts may cause groundwater pollution (Jin et al. 2020). Therefore, the efficient utilization of cellulose in food by-products not only minimizes the imbalance between the supply and demand of biodegradable materials but also reduces the wastage of resources and environmental pollution. As one of by-product of winemaking, GS shows significant potential because of its high cellulose content. Naturally dried GS is brittle because of its high pectin content and deficiency in the mechanical properties required for film formation (López de Lerma et al. 2014;Š ešlija et al. 2018). Thus, the combination of GS with a suitable material provides a good solution. Poly(vinyl alcohol) (PVA) is a water-soluble and biocompatible polymer that has been studied extensively because of its excellent filmforming properties and complete biodegradability (Liu et al. 2018). Their excellent interfacial compatibility and the simplicity of the preparation have prompted the research and development of many cellulose/PVA composite films. These films exhibit satisfactory mechanical strength and flexibility (Kim et al. 2019;Yang et al. 2020;Xu et al. 2018a, b). Moreover, cellulose and PVA can be combined in an aqueous solution, which is beneficial for maintaining the activities of functional polyphenols in GS.
In the present study, a biodegradable functional composite film was successfully fabricated by mixing an ultrasonicated GS (UGS) suspension with PVA (Fig. 1c). The cellulose nanofibers (CNFs) formed a three-dimensional (3D) porous skeleton to support the composite film. The hemicellulose and pectin, which acted as the mechanical support and adhesive substance in the plant cell wall (Kozioł et al. 2017), promoted the formation of the film and improved the interface bonding between the UGS and PVA. The self-standing UGS-PVA composite film was formed after the mixed solution was dried at 35 8C for 12 h. Moreover, the polyphenols endowed the film with desirable multiple functions, including pH-responsive color change, antioxidant activity, ultraviolet (UV) shielding, and antimicrobial properties. The multifunctional biodegradable composite film has considerable potential in cosmetics, health care, and agriculture.

Preparation of the GS
The fresh GS was washed with distilled water until no stains were visible on the outside surface of the skin, and no grape pulp adhered to the inside surface of the skin. The washed GS was then freeze-dried (Scientz-30ND, Ningbo Scientz Biotechnology Co., Ltd., Ningbo, China) to obtain oven-dried GS. To prevent the growth of yeast and mildew, 75 % C 2 H 5 OH was sprayed onto the dry GS. The sterilized GS was again dried in a vacuum drying oven (DZ-1 AII, Tianjin City Taisite Instrument Co., Ltd., Tianjin, China) at 35 8C for 12 h and then stored in a desiccator.

Preparation of the UGS suspension
The GS was ground into powder by using a pulverizer (BO-2500Y, BoouHardware Products Co., Ltd., Zhejiang, China) and sieved through a 600 mesh screen to prepare the UGS suspension. The GS powder was mixed with distilled water (1 % wt) and stirred for 15 min at 800 rpm. The obtained GS powder suspension was treated with an ultrasonic cell disruptor (JY 99-IID, Ningbo ScientzBiotechnology Co., Ltd., Ningbo, China) at 800 W for 4 h to dissociate the cell walls and obtain an initial UGS suspension. The initial UGS suspension was then centrifuged (H1850, Hunan XiangYi Laboratory Instrument Development Co., Ltd., Changsha, China) at 150 rpm for 10 min to remove the large segments. The obtained UGS suspension was adjusted to 0.1 % wt. for further use.
Preparation of the UGS-PVA film PVA and distilled water were added to a beaker at a ratio of 1:9 (wt). The sealed beaker was placed in a water bath and then heated (95 8C) and stirred (200 rpm) until the PVA was completely dissolved.

Characterization
The chemical structure of the UGS was characterized using a Fourier transform infrared (FTIR) spectrometer (ZN-04, KINGSLH, China) and a solid-state 13 C NMR spectrometer (AVANCE III 400 MHz WB, Bruker, Switzerland). The surface morphology of the films was analyzed using an atomic force microscope (AFM, Dimension Icon, Bruker, Germany). Transmission electron microscopy (TEM) was performed on a Tecnai G20 electron microscope (JEM 2100) with an acceleration voltage of 200 kV. Scanning electron microscopy (SEM, JSM-7500 F, JEOL, Japan) was conducted to observe the morphology of the films. The crystalline structure of the films was characterized using an optical polarizing microscope (CX40P, Ningbo, China). The degree of crystallinity of the films was determined using an X-ray diffractometer (XRD, D8 Advance, Bruker, Germany). The thermal stability of the film was analyzed with a thermogravimetric analyzer (TGA/DSC 1/1600 HF Mettler-Toledo, Switzerland) at a heating rate of 10°C min -1 in a nitrogen environment (from room temperature to 700°C). The crystallization mechanism was characterized using a differential scanning calorimeter (DSC) (DSC Q20 V24.10 Build 122, USA) at a 10°C min -1 heating and cooling rate. The optical properties of the films were analyzed using a UVvisible (UV-vis) spectrophotometer (TU-1950, Beijing Purkinje General Instrument Co., Ltd., Beijing, China). A testing machine (AI-7000 S TC160701511, Gotech, Taiwan) was used to evaluate the mechanical properties of the films. The UV stability was evaluated by using a QUV accelerated weathering tester (Accelerated Weathering Tester, QV/Spray), which provided constant UV light of 340 nm, at 45 8C for 96 h. The average irradiance was set to 0.77 W/m 2 .
The XRD deconvolution analysis was performed using Peak Anylyzer of OriginPro 8.5 software in order to deconvolute crystalline and amorphous regions. The crystalline and amorphous peaks were deconvoluted based on the assumption of Gaussian function. The crystallinity CrI was calculated using the Eq. 1 (Popescu 2017): where A cr is the area of all the crystalline peaks and A t is the total peaks area of the diffractogram.

Moisture adsorption test
Two sealed glass containers with internal relative humidity (RH) levels of 56 and 93 % were prepared using the saturated salt solution method with (Mg(NO 3 ) 2 and KNO 3 at a constant temperature of 20 8C. Dried PVA and UGS-PVA films measuring 1 mm 9 1 mm were placed in the containers until the film weight increased by \ 0.005 g within 24 h at a given RH. The moisture adsorption of the film was based on moisture content (MC), which was determined using Eq. 2: where m 0 is the dry weight of the films, m 1 is the equilibrium weight at a given RH, and MC is equal to the equilibrium MC. Eight replicates of each film type were used for each RH.

Antioxidant activity test
With the UV-vis spectrophotometer, the antioxidant activity of the UGS-PVA films was measured based on the disappearance of the absorption band at 517 nm of the DPPH free radical. A 0.1 mol/L test solution was prepared by dissolving DPPH in CH 3 OH, and the solution was covered with foil to shield it from light. Film-soaking solutions were prepared by soaking 0.15 g of the film in 3 mL of CH 3 OH for 30 min. Subsequently, 2 mL of the DPPH solution and 2 mL of the film soaking solution were added and mixed in glass vials wrapped in foil for 30 min. The mixed solution was ultimately analyzed using the UV-vis spectrophotometer. The radical scavenging activity (RSA) of the films was calculated using Eq. 3: where A ctr and A sample are the absorbance values of the control and the films at a wavelength of 517 nm, respectively. The antioxidant property of the film was further demonstrated by attaching it to a fresh apple. The apple was first washed, and part of the skin was peeled off (3 peeled regions). The PVA and UGS-PVA film were then attached to two of the peeled regions, while the other one served as the control. Finally, the apple was placed in atmospheric environment for 3 h, and the films were subsequently removed. Each step was photographed.
pH responsiveness test The level of pH responsiveness was measured using a portable colorimeter (CM-2300D, MINOLTA) equipped with a CIELAB system. Aqueous solutions with different pH values (1, 3, 5, 7, 9, 11, and 13) were prepared using distilled water, 0.1 mol/L HCl, and 0.1 mol/L NaOH. The solutions were dropped onto the UGS 15 -PVA film measuring 1 cm 9 1 cm, respectively. After sealing and resting for 30 min, the color of the films was determined.

Antimicrobial simulation test
Two plastic Petri dishes 6 cm in diameter were sterilized by UV light, and 20 mL of milk was placed into both Petri dishes. The PVA film and UGS 15 -PVA film measuring 2 cm 9 2 cm were placed into the two Petri dishes. Both Petri dishes were sealed and stored for 3 days to ensure the migration of polyphenols in the film into the milk. The seal was subsequently removed, and the dishes were placed in the laboratory for about 3 weeks. The final state of the milk was determined using photographs.

Results and discussion
Preparation of the UGS and fabrication of the UGS-PVA films The dried GS with a porous skeleton was first pulverized into powder measuring 10-20 lm (Fig. 2a  and c), which further underwent ultrasonication treatment and for sufficient dissociation and approximate 90 % of GS was successfully made into good dispersion of the UGS suspension. The ultrasonic impact disintegrated the grape cellulose into the CNF with a high specific surface area and a diameter of 4-10 nm, resulting in the sufficient explosion of large amounts of the hydroxyl group (Fig. 2d). Thus, the CNF and aqueous solution formed a uniform and stable 3D network attributable to hydrogen bonds (Chen et al. 2011). In addition, the TEM image showed the presence of aggregates, which formed as core-shell structures measuring * 100 nm (Fig. 2d and e). Consisting of proteins or lipids, the core was encapsulated by polyphenols as shells due to hydrogen bonds (Liu et al. 2019a;Zhang et al. 2020).
In addition to efficient dissociation of cellulose into CNFs, the polyphenols, pectin, and proteins with multiple functions were successfully maintained after ultrasonication, as determined by FTIR and NMR spectroscopy (Fig. 2f, g). As shown in Fig. 2f, the UGS filtrate has adsorption peaks at 2918 and 2849 cm -1 (i) which are ascribed to the asymmetric and symmetric stretching vibration of CH 2 in the lipids. The peaks at 1732 and 1716 cm -1 (ii) are attributed to the C=O stretching vibration of the aliphatic polyesters. Proteins showed typical peaks at 1689 cm -1 (iii) owing to the C=O stretching vibration; however, the peak was weak because the protein content of the GS was less than 5 % (Pinelo et al. 2006). The peak at 1605 cm -1 (iv) is ascribed to the C=O asymmetric stretching vibration of the pectin and C = C stretching vibration of the polyphenols. The pectin had adsorption peaks at vi, vii, and viii attributed to the CH 2 , OH, C-O, and C-C stretching vibrations, respectively. The peaks at ix are ascribed to C-H deformation in the polyphenols. The FTIR peaks are described in detail in Table S1. The polyphenols were analyzed by solid-state 13 C NMR. As shown in Fig. 2 g, the filtered UGS primarily consists of pectins (170-182; 105.2; 53.6 ppm), holocellulose (62-106 ppm), residual polyphenols (115-155 ppm), and lipids (30 ppm) (Castillo-Muñoz et al. 2009;Farooque et al. 2018;Matharu et al. 2016;Mendes et al. 2013;Zhu et al. 2014). The dominant polyphenols included tannins, anthocyanins, proanthocyanidins, and flavonols, which exhibited typical peaks at 55-155 ppm (Mateus et al. 2002;Prozil et al. 2012). The UGS filtrate also contained pectins, proteins, and lipids, which combined with the polyphenols and formed functional aggregates (Fig. 2e). Details of the NMR results are listed in Table S2.
Transparent and self-standing brown UGS-PVA films were fabricated by solvent casting of the mixtures of UGS and PVA solution. After the water evaporated, transparent films with homogenously distributed UGS were obtained (Fig. 2a). The surface roughness of the obtained film increased slightly relative to that of the pure PVA film ( Fig. 3a and b, and  S2). The homogeneous distribution of UGS in the composite films is ascribed to sufficient dissociation by ultrasonication and the presence of numerous hydroxyl groups, which can strongly interact with PVA chains via hydrogen bond formation. This occurrence led to superior compatibility between PVA and UGS at the interface and prevented the debonding of the UGS substrate from the matrix, as observed in the SEM images (Fig. 3b 2 ). By contrast, GS without ultrasonication, which contained microscaled particles, could not feasibly be used to fabricate such homogenous films but instead, only films with considerably aggregated GS particles (Fig. S3). Thus, the nano-dimension of UGS by ultrasonication is critical for the formation of homogenous transparent and self-standing composite films. Moreover, solely drying UGS failed to generate stable self-standing films and only resulted in small cracks (Fig. S4).
The addition of UGS effectively influenced the crystal nucleation of PVA. As shown in Fig. 3a 1 and b 1 , pure PVA exhibits a relatively smooth morphological surface. This smoothness can be attributed to the low crystallization of pure PVA chains, rendering most PVA molecular chains amorphous during water evaporation. However, the surface of the UGS-PVA composite film was uneven and rough, with numerous grains and gullies (Fig. 3b 1 ). These qualities could be caused by the improvement of crystallization after UGS was added; some PVA chains crystallized into ordered domains, whereas the others were amorphous. The crystal nucleation property of the films was further characterized by DSC. As shown in Fig. S5, the crystallization temperature and enthalpy of the film increase with an increasing in UGS content. The addition of UGS promoted the composite film to form more stable and abundant crystal structure. The crystallinity index (C r I) was significantly increased as UGS was added (Fig. 3c). Pure PVA had a C r I of 28.86 %, and UGS 15 -PVA had a C r I of 66.27 % (Fig. S6). On the basis of the polarized light microscopy measurement, well-distributed bright domains were visible in the composite film, with a higher density than that of the pure PVA film. The difference indicated significant improvement in the crystallization of the UGS-PVA composite film (Fig. S7). Moreover, the presence of typical peaks ascribed to UGS within the FTIR spectra confirmed the successful maintenance of all UGS components in the composite film, particularly polyphenols, which are critical for the introduction of new functions in the composite films (Fig. 3d). The UGS-PVA composite films exhibited thermal stability superior to that of the pure PVA film owing to the higher thermal stability of the cellulose and pectin than that of the PVA (Einhorn-Stoll et al. 2007;Kaboorani et al. 2012) (Fig. 3e). Specifically, with an increase in UGS content from 0 to 15 %, the first thermal decomposition rate peak of the film shifted from 249 8C to 289 8C, and the rate decreased. This improvement in thermal stability would contribute to the practical application of the UGS-PVA composite films.

Moisture-sensitive mechanical properties of the UGS-PVA films
As shown in Fig. 4a, the dried UGS 5 -PVA film with excellent flexibility and toughness can be easily folded into an airplane; no damage occurs after the film is unfolded. Similar to the pure PVA film, the composite films exhibited moisture-switchable mechanical properties. In its wet state, the UGS 5 -PVA film could attach to the finger and be easily deformed without damage when the finger was bent, exhibiting excellent softness and adhesion (Fig. 4b). The wet UGS 5 -PVA film could also be stretched to about twice its original length and then restored to its original shape (Fig. 4c), revealing an elastic mechanical behavior after stretching. To evaluate the influence of moisture on mechanical properties, moisture-dependent analyses were further performed. As shown in Fig. 4d, the MCs of all films increase as RH rises from 56 to 93 %. The pure PVA film with a semi-crystalline structure could be easily destroyed by moisture, resulting in efficient moisture adsorption at increased RH (Konidari et al. 2011). Unlike that in the PVA and UGS-PVA films with similar MC levels at a low RH level (56 %), the addition of UGS impeded moisture adsorption at a high RH level (93 %) because of the enhancement of the crystalline structure (Fig. 3c). This effect reduced the MC of the UGS-PVA film relative to that of the PVA film (Fig. 4d). In Fig. 4e, the crystalline domains in the UGS 15 -PVA film almost disappears after moisture equilibrium treatment at high RH and vice versa after drying. This occurrence endowed the composite film with moisture-sensitive mechanical properties.
The tensile curves of all films at different RH levels are presented in Fig. 4f and g. In its dry state, the pure PVA film exhibits the highest strength of 95.3 MPa, with a breaking elongation of 5.6 %. As the UGS content increased from 5 to 15 %, the strength of the UGS-PVA films decreased from 76.6 MPa to  Fig. 2 f. e Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves with temperatures ranging from room temperature to 700 8C. The inserts in a 1 and b 1 were the photographs of the films 13 MPa, and the breaking elongation was reduced from 3.2 to 1.2 % which was ascribed to the increase in fragility. After the films were equilibrated at RH levels of 56 and 93 %, their strengths decreased, and their breaking elongations increased significantly (Fig. 4 g-h and Fig. S8). The UGS 15 -PVA film treated at RH of 93 % exhibited its lowest strength of 12.1 MPa and breaking elongation of 412.3 %, which still satisfied the standard requirement for polyethylene film in packaging applications ((strength C 10.5 MPa and breaking elongation C 100 %).(ASTM D3981-09a 2016) The significant increase in breaking elongation at a high MC was attributed to the dissociation of the crystalline structure, which would facilitate the application of UGS-PVA films in packaging, wound dressing, and cosmetics, among others.

Multiple functions of UGS-PVA films
The polyphenols in the UGS endowed the UGS-PVA films with multiple functions. As shown in Fig. 5a, the UGS 15 -PVA film exhibits pH-induced color switch ability. The color could change from soft pink to blue/ brown with an increase in pH from 1 to 13. This change was ascribed to the change in the chemical structure of anthocyanins in the GS (Fig. S9). The anthocyanin gradually became deprotonated as the pH changed from acidic to basic (Fig. S10). Red flavylium cations (AH ? ) formed and became predominant at pH 1-3. When the pH increased to 4-6, a colorless hemiketal formed, and yellow chalcone developed via ringopening. The AH ? was ultimately transformed into purple-blue quinonoid base isomers (pH [ 6) (Liu et al. 2019b;Sigurdson et al. 2017). Moreover, Fig. 4 Images of the flexible UGS 5 -PVA film under different conditions: a the dry film is folded and unfolded without damage; b the film wetted in deionized water for 5 minis attached to the finger joint and is deformed when the finger is bent; c stretching and retraction of the wet film. d Moisture content of the films at relative humidity (RH) levels of 56 and 93 %. e Polarizing microscope images of the UGS 15 -PVA film in dry and wet states. f-g Stress-strain curves of the films after equilibrating f at an RH of 0 %, and g at RH levels of 56 and 93 %. h Tensile strength of the films after equilibrating at different RH levels the anthocyanin also exhibits considerably high antioxidant activity (Serra et al. 2008;Št'avíková et al. 2011). The combination of anthocyanin with other GS antioxidants, such as proanthocyanidins and resveratrol, endowed the UGS-PVA films with high antioxidant activity (Fig. 5b and c). DPPH is a stable free radical with a characteristic absorption wavelength of 517nm. The pure PVA showed no antioxidant activity, as indicated by the similar peak strength in the control group. The UGS-PVA composite films exhibited excellent antioxidant activity, as indicated by an evident reduction in peak strength. RSA increased from 20 to 85 % as the UGS content increased. The color of the DPPH solution changed from purple to colorless, which was attributed to the reduction in DPPH by the antioxidants in UGS (Fig. S11). Usually, when fruits and vegetables suffered physical injuries, the injured areas would brown due to the polyphenol oxidation (Le Bourvellec et al. 2004). Thus, the antioxidant activity of the UGS-PVA film was further demonstrated by attaching the film to a fresh apple. As shown in Fig. 5e, the peeled area to which the UGS 15 -PVA film was attached showed the slightest discoloration after the apple was placed in the atmospheric environment for 3 h. This observation was attributable to the excellent antioxidant activity and the superior adhesion of the composite film (Fig. 4b) which could effectively isolate oxygen. After they were placed in atmospheric environment for 3 h, the films were removed, and the blue area with the UGS 15 -PVA film exhibited the slightest discoloration. f Fungal growth in milk in the presence of PVA or UGS 15 -PVA films after 21 days in the laboratory. The blue dashed area in the magnified image appears transparent owing to the dissolution and release of the colored polyphenols Therefore, both aforementioned functions endowed the composite film with considerable application potential in food safety and packing.
The polyphenols with high UV absorption capacity (Fig. S12) endowed the UGS-PVA films with excellent UV-shielding. Compared with the pure PVA films, the UGS-PVA films showed superior UVshielding performance with low UV transmittance (Fig. 5d).The positive synergistic effect of excellent antioxidant activity and UV-shielding performance provides such composite films great application potential for the repair of skin damage caused by UV illumination Wang et al. 2019). Moreover, the UGS-PVA film possessed antimicrobial properties (Fig. 5f). After immersion, the films in milk and storage at room temperature for 21 days, as well as the milk containing the PVA film, was entirely covered by fungi. By contrast, the UGS 15 -PVA film inhibited fungal growth, and the milk contained only a small number of fungi. These observations are assumed to be related to the antimicrobial activity of grape polyphenols, which migrated from the composite film into the milk (Hassan et al. 2019;Katalinić et al. 2010). In addition, polyphenols had other functions, such as enhancing visual acuity and providing anti-inflammatory and anti-cancer properties (Ju and Howard 2003;Udenigwe et al. 2008). The polyphenols endow the composite film multiple functions, in turn, the PVA protected the activity of those matters. For instance, the UGS-PVA had long-term stability that the UGS 15 -PVA maintained the excellent pH responsiveness after placed for 8 months (Fig. S13). Subsequently, the tensile property of films was measured after pH responsiveness, although the strength of films was decreased owing to the breaking or weakening of hydrogen-bonds by strong acid and alkali, it still satisfied the service demand. For more, the UGS-PVA films exhibited superior UV stability. As showed in Fig. S14, the UGS-PVA films retained UV-shielding and tensile strength after accelerated aging for 96 h. Thus, the outstanding stability of UGS-PVA films expanded the potential applications.

Conclusions
In summary, a facile process via ultrasonication dissociation of GS and further compositing with PVA was developed to prepare a multifunctional UGS-PVA composite film. The efficient dissociation by ultrasonication resulted in a transparent and homogeneously distributed UGS-PVA composite film. Similar to the pure PVA film, the UGS-PVA film retained its moisture-sensitive mechanical properties, particularly the significant improvement in breaking elongation at high MC. Notably, the retention of polyphenols in the UGS resulted in the multiple functions of the UGS-PVA films, such as pH-sensitive color change, excellent antioxidant properties with an RSA of 85 %, excellent UV-shielding performance, and antimicrobial properties. These findings provide a new route for the resourceful utilization of waste GS. The UGS-PVA films exhibit significant potential for application in packaging, cosmetics, and healing materials.