WO2021089110A1

WO2021089110A1 - Photocatalytically active and superhydrophobic elastic films, method for preparing the same and applications thereof, in particular as a wound dressing

Info

Publication number: WO2021089110A1
Application number: PCT/EP2019/080105
Authority: WO
Inventors: Jie Liu; Werner Steffen; Hans-Jürgen Butt
Original assignee: Max-Planck-Gesellschaft Zurförderung Der Wissenschaften E. V.
Priority date: 2019-11-04
Filing date: 2019-11-04
Publication date: 2021-05-14

Abstract

The present invention relates to photocatalytically active and superhydrophobic elastic composite films comprising or consisting of a cross-linked assembly of modified photocatalytically active metal oxide nanoparticles, in particular titanium dioxide nanoparticles, wherein the metal oxide nanoparticles are modified by a polysiloxane copolymer grafted on the surfaces thereof and wherein the modified nanoparticles are cross-linked by a coupling reaction between Si-H-groups of the polysiloxane copolymer and functional vinyl groups of a vinyl-terminated polysiloxane crosslinker. A further aspect of the invention relates to an article comprising a layer of such an elastic composite film provided on a substrate surface, preferably a surface of an elastic substrate. The photocatalytically active and superhydrophobic elastic composite films of the invention and the articles comprising the same have antibacterial properties and are capable to provide hemostasis and reducing the adherence of dry blood. Consequently, these elastic composite films and articles have medical applications, in particular for blood-repellent wound dressings. Still further aspects of the invention relate to methods for preparing said photocatalytically active and superhydrophobic elastic composite films as well as articles comprising the same.

Description

PHOTOCATALYTICALLY ACTIVE AND SUPERHYDROPHOBIC ELASTIC FILMS, METHOD FOR PREPARING THE SAME AND APPLICATIONS THEREOF, IN PARTICULAR AS A WOUND DRESSING Field of the Invention The present invention relates to photocatalytically active and superhydrophobic elastic films as well as to a method for preparing the same. Further and more specific aspects of the invention relate to various applications of such films and composite materials comprising the same, in particular as a blood-repellent wound dressing. Background of the invention Durable and biocompatible superhydrophobic surfaces are of significant potential for various applications and in particular for use in physiological fluid repellent applications in medicine. A well-known blood characteristic is its high propensity to form thrombogenesis or blood clots when it comes into contact with foreign surfaces. This is due to the blood’s intrinsic hemostatic mechanisms, induction of coagulation, and platelet activation. This coagulation effect inevitably results in the blood’s strong adhesion to substrates, making it stick to surfaces and causing an immune response. Blood coagulation can pose a serious side-effect when it occurs on medical implants used in vivo, leading to deterioration in the condition of the patient. Blood adhesion to surfaces such as clothes and wound dressings can cause contamination, waste and even infections. In medical textiles, exposure and transfer of blood between patients and medical personnel during first-response care, or generally within hospitals, is a cause for high concern, potentially causing bacterial or viral infections. Therefore, developing efficient blood repellent surfaces is not only highly desirable, but basically essential. Superhydrophobic surfaces have already been extensively investigated and applied in various fields e.g. to improve heat transfer, in microfluidics, or to prevent icing, etc. In recent studies, superhydrophobic surfaces have been reported to have great potential in repelling blood (see a) S. Moradi, N. Hadjesfandiari, S. F. Toosi, J. N. Kizhakkedathu, S. G. Hatzikiriakos, Acs Appl. Mater. Inter.2016, 8, 17631; b) M. Paven, P. Papadopoulos, S. Schottler, X. Deng, V. Mailander, D. Vollmer, H. J. Butt, Nat. Commun.2013, 4, 2512; c) S. Movafaghi, V. Leszczak, W. Wang, J. A. Sorkin, L. P. Dasi, K. C. Popat, A. K. Kota, Adv. Healthc. Mater.2017, 6, 1600717; d) V. Jokinen, E. Kankuri, S. Hoshian, S. Franssila, R. H. A. Ras, Adv. Mater.2018, 30, 1705104; e) T. Zhu, J. Wu, N. Zhao, C. Cai, Z. Qian, F. Si, H. Luo, J. Guo, X. Lai, L. Shao, J. Xu, Adv. Healthc. Mater.2018, 7, 1701086; f) Q. Wei, C. Schlaich, S. Prevost, A. Schulz, C. Bottcher, M. Gradzielski, Z. H. Qi, R. Haag, C. A. Schalley, Adv. Mater.2014, 26, 7358.). The hierarchical structures with a trapped air plastron can effectively reduce the contact area between blood and surface, and reduce platelet adhesion through hydrodynamic effects (V. Jokinen, E. Kankuri, S. Hoshian, S. Franssila, R. H. A. Ras, Adv. Mater.2018, 30, 1705104). Paven et al. (Nat. Commun.2013, 4, 2512) reported that a superamphiphobic membrane consisting of a fractal-like network of fluorinated silicon oxide nanospheres can resist blood adhesion. However, the practical use of superhydrophobic surfaces described in the literature for blood-repellency in medical applications still poses some problems which are related to limited chemical and mechanical stability, flexibility and biocompatibility of the known materials. In particular also in view of ongoing regulations and concerns about using highly fluorinated substances for medical and other applications, the research and development of effective substitutes for use in medical and other life science applications continues. In view of these drawbacks of the prior art, the main object of the present invention was to provide novel superhydrophobic materials with improved chemical and mechanical properties in particular for use in medical applications such as wound dressings. This objective has been achieved generally by providing the elastic film according to present claim 1 and the method for preparing the same according to claim 12. Additional aspects and more specific and/or preferred embodiments of the invention, such as a blood-repellent elastic wound dressing, are the subjects of further claims. Description of the invention The present invention provides an elastic, photocatalytically active and superhydrophobic composite film, comprising or consisting of a cross-linked assembly of modified photocatalytically active metal oxide nanoparticles, wherein the metal oxide nanoparticles are modified by a polysiloxane copolymer grafted on the surfaces thereof and wherein the modified nanoparticles are cross-linked by a coupling reaction between Si-H-groups of the polysiloxane copolymer and functional vinyl groups of a vinyl-terminated polysiloxane crosslinker. The term “polysiloxane” as used herein refers to any oligomer or polymer comprising the characteristic siloxane repeating element -(Si-O-Si)-. The term “nanoparticles” as used herein generally refers to particles having a mean diameter in the range from 1 nm to 999 nm, preferably from 10 nm to 100 nm. Typically, in the elastic composite film of the invention the polysiloxane copolymer is coupled to the nanoparticle surfaces by direct covalent Si-O-Metal bonds between the polysiloxane copolymer and the metal oxide and/or the polysiloxane copolymer is selected from the group of polysiloxane copolymers comprising -[Si(CH3)2H]n-groups, in which 0 < n < 1000. More specifically, the polysiloxane copolymer is selected from the group of copolymers comprising a compound having a repeating element of the general formula –[R1R2Si-O- R3R4Si]x-, wherein x is an integer from 5 to 5,000,000, preferably from 10 to 50,000, and R1, R2, R3, R4 independent from each other represent H, halogen, an organic residue, in particular alkyl, or -OSiR5R6R7, with R5, R6, R7 being substituents as defined for R1 to R4. Preferably, each alkyl substituent is a C₁-C₁₀ alkyl substituent, such as methyl, ethyl, propyl, butyl etc. In one specific embodiment, each substituent R1 to R4 (and/or optionally R5 to R7) is the same, preferably an alkyl group as defined above. For example, the polysiloxane may be PDMS, preferably having a molecular weight in the range from 0.5 to 10,000 kDa. Preferably, the corresponding surfaces of the polysiloxane copolymer-grafted modified metal oxide nanoparticles are completely fluorine-free. Principally, the metal oxide nanoparticles used herein may be any nanoparticles of a photocatalytically active metal oxide. The photocatalytically active metal oxide as used herein may also comprise or represent a plurality of metals or metal oxides and/or the metal of said photocatalytically active metal oxide may also comprise or represent a metal alloy. The term “metal oxide” as used herein also includes a heterogeneous system of two or more metal oxide mixtures and/or a mixture of a metal oxide with other inorganic material. More specifically, the metal oxide is selected from the group comprising TiO2, ZnO2, SnO2, CeO₂, Fe₂O₃, Ag₂O, WO₃, Al₂O₃, Nb₂O₅, ZnS, CuO, MoO₃, ZrO₂, MnO₂, MgO, and V₂O₅ or a mixture thereof. TiO2 is especially preferred. In one preferred embodiment of the composite film according to the present invention, the metal oxide nanoparticles are TiO2 nanoparticles, the polysiloxane copolymer is a methylhydrosiloxane-dimethylsiloxane copolymer, in particular a copolymer of methylhydrosilane and dimethylsiloxane with the methylhydrosilane component being present in a portion of 25-30 Mol% of the total copolymer and the dimethylsiloxane component copolymer being present in a portion of 70-75 Mol% of the total copolymer, abbreviated in the following text as a “(25-30% methylhydrosiloxane)-dimethylsiloxane copolymer”, and the vinyl-terminated polysiloxane crosslinker has a general structural formula CH2=CH–[(CH3)2Si-O]x-Si(CH3)2-CH=CH, wherein x is an integer from 0 to 10000, and a Mw in the range from 0.1 kDa to 1000 kDa, preferably from 10 kDa to 200 kDa, in particular a Mw in the range from 50 kDa to 100 kDa. Specifically, the methylhydrosiloxane-dimethylsiloxane copolymer may have the structural formula Si(CH3)₃-O-[(CH₃)(H)Si-O]_m-[(CH3)₂Si-O]_n-Si(CH3)₃ with m being an integer in the range from 1 to 10000 and n being an integer in the range from 0 to 10000. Typically, in said composite film the mass ratio of copolymer-modified metal oxide nanoparticles to the cross-linker is 0.025 < m_TiO2/m_vinyl-PDMS < 1, m_TiO2 and m_vinyl-PDMS respectively represent the weight of TiO₂ nanoparticles and vinyl-terminated PDMS. As already mentioned above, the composite film of the invention advantageously has both elastic and superhydrophobic properties. Here, surfaces are defined to be “superhydrophobic” when exhibiting a static contact angle of more than 150^○ with respect to water as determined with 5 µl sized drops of water. Typically, said composite film has a modulus of elasticity of at least 5 MPa, preferably at least 20 MPa, 40 MPa, 50 MPa or 90 MPa, and/or an advancing or static contact angle of larger than 150^○, preferably larger than 165^○, for water, blood, and aqueous solutions. The present invention also relates to an article comprising a layer of the composite film according to the present invention on a substrate surface. The substrate may be an elastic or rigid substrate, but preferable the substrate is an elastic substrate, e.g. a substrate having a modulus of elasticity of at least 5 MPa, preferably at least 20 MPa. The substrate may be selected from a wide variety of substrates, in particular from the group comprising woven or non-woven textiles or fabrics, meshes, papers, rubber, cardboards, glass, plastics, and membranes. Typically, the substrate has an essentially planar or 2-dimensionally extended shape. In a preferred embodiment of the invention, said article represents or comprises a wound dressing, in particular a blood-repellent wound dressing. In this embodiment, the substrate may be any material conventionally used for a wound dressing, in particular any flexible or elastic material suitable or adapted for this purpose. The fact that the composite film of the invention is also elastic allows a very favorable combination with any conventional flexible or elastic wound dressing materials and, thus, for the first time to impart both superhydrophobic and photocatalytic properties to such elastic wound dressings as well. Advantageously, an article comprising a layer of the composite film of the invention on a substrate surface is capable to repel blood, provide hemostasis and reduce the adherence of dry blood. Besides, the structure is stable enough to resist breakage after deformation of the substrates. The superhydrophobic property is not impaired after 1000 times deformation and the blood-repellent property is maintained even after several tens of deformation cycles. The photocatalytic activity endows the modified surfaces to be self-cleaning even after contamination of chemicals such as surfactants. This property also promotes the anti- bacterial property of the surfaces. Thus, a further closely related aspect of the invention is the use of the composite film of the invention or of an article comprising the same for medical applications, i.a. for providing hemostasis and reducing the adherence of dry blood. A still further aspect of the present invention relates to a method for preparing the elastic composite film as described above. Typically, said method comprises at least the following steps: a) providing a mixture of photocatalytically active metal oxide nanoparticles, preferably pretreated with oxygen plasma, and a polysiloxane copolymer; b) irradiating the mixture with light in a wavelength range from 180 nm to 550 nm for a sufficient time to generate reactive moieties in said polysiloxanes and to form covalent Si-O- Me (Me = metal) bonds between said reactive moieties of the polysiloxanes and the metal oxide surface; c) optionally separating unreacted polysiloxane molecules from the modified polysiloxane- grafted metal oxide nanoparticles; d) providing a mixture of the modified metal oxide nanoparticles with a vinyl-terminated polysiloxane crosslinker in a suitable organic solvent in the presence of a catalyst, in particular a Pt-catalyst; e) reacting the mixture of step d) for a predetermined time period, typically in the range from 10 min to 12 h, preferably from 0.5 h to 6 h, at an elevated temperature, preferably in the range from 60 °C to 80 °C, to form an elastic film of cross-linked modified metal oxide nanoparticles; f) optionally removing unreacted reactants and solvent from the elastic film of cross-linked modified metal oxide nanoparticles. Steps a) - c) of the above method may be performed according to known techniques, e.g. the polysiloxane-grafting method disclosed in WO 2017/137154 A1. Typically, the pretreatment of metal oxide nanoparticles with commercial oxygen plasma cleaner is effected for a time period in the range from 0 s to 600 s with a power setting in the range from 10% to 100%. More specifically, the metal oxide nanoparticles used in the method of the invention are selected from the group comprising TiO2, ZnO2, SnO2, CeO2, Fe2O3, Ag2O, WO3, Al2O3, Nb2O5, ZnS, CuO, MoO3, ZrO2, MnO2, MgO, and V2O5 nanoparticles, and the polysiloxane copolymer is selected from the group of copolymers comprising -[Si(CH3)2H]n-groups, in which 0 < n < 1000, preferably with n in the range from 10 to 100. In one preferred embodiment, the metal oxide nanoparticles are TiO2 nanoparticles, the polysiloxane copolymer is a methylhydrosiloxane-dimethylsiloxane copolymer, in particular a (25-30% methylhydrosiloxane)-dimethylsiloxane copolymer, and the vinyl-terminated polysiloxane crosslinker has a structural formula CH2=CH–[(CH3)2Si-O]x-Si(CH3)2-CH=CH, wherein x is an integer from 0 to 10000 and a M_w in the range from 0.1 kDa to 1000 kDa, preferably from 10 kDa to 200 kDa, in particular a M_w in the range from 50 kDa to 100 kDa. In another specific embodiment of said method, the mass ratio of modified metal nanoparticles and vinyl-terminated polysiloxane cross-linker in the reaction mixture in step d) is in the range from 0.025 to 1, preferably from 0.05 to 0.6, especially preferred from 0.1 to 0.2, and/or the volume ratio of cross-linker to solvent in the reaction mixture is in the range from 0.1 to 0.5. In still another specific embodiment of said method, the concentration of the modified metal nanoparticles in the reaction mixture in step d) is in the range from 0.2 wt% to 33.3 wt%, preferably from 0.4 wt% to 20 wt%, especially preferred from 0.9 wt% to 6.7 wt%, and the concentration of the vinyl-terminated polysiloxane cross-linker is in the range from 10 wt% to 50 wt%, preferably from 20 wt% to 33 wt%, especially preferred from 20 wt% to 25 wt%. The catalyst used in the method of the present invention, may be principally any catalyst used in the art for this kind of crosslinking reaction. Preferably, the catalyst is selected from the group comprising noble metal complexes, preferably platinum complexes, such as platinum-divinylterminated-siloxane complexes, in particular platinum(0)-1,3-divinyl-1,1,3,3- tetramethyldisiloxane complex solution. Typically, said catalyst is present in an amount in the range of from 0.004 wt% to 0.01 wt% of the total mixture of step d). The reaction time may be controlled by the concentration of the catalyst. A further specific aspect of the invention relates to a method for preparing an elastic composite film-coated article as described above, wherein the generation of the elastic film and the bonding of said film to the substrate surface is effected essentially simultaneously. In this embodiment, the elastic composite film is prepared as outlined above and the substrates to be coated with the layer of the composite film are immersed in a reaction mixture comprising modified metal oxide nanoparticles with a vinyl-terminated polysiloxane crosslinker in a suitable organic solvent in the presence of a catalyst, in particular a Pt- catalyst (such as the reaction mixture of step d) of the method for preparing the composite film outlined above) for the duration of the crosslinking-reaction for forming the elastic film (such as the reaction of step e) of the method outlined above). However, it is also possible to provide the elastic film on the respective substrate surface in a subsequent step. In this case, the strength of the attachment/bonding of the film to the substrate surface is affected by the number of hydroxyl groups on the surface, which increase the adhesion between film and substrate. Preferably, in both cases, the substrate surface has been also pretreated to provide a superhydrophilic surface prior to the bonding of the film. The treatment may be effected, e.g. with oxygen plasma for 5 minutes and a power setting of 100 %, similar to the pretreatment of the metal oxide nanoparticles described above. Summarizing, the method of the invention enables to prepare blood-repellent superhydrophobic elastic films and composite materials by using hierarchical structures, in particular composed of biocompatible titanium dioxide (TiO2) nanoparticles and polydimethylsiloxane (PDMS). The PDMS provides surface superhydrophobicity directly without fluorinated modification, and additionally stability under UV illumination. The assembled nanoparticles connected by cross-linked PDMS offer flexibility and thus more stability under external forces. In addition, TiO2 nanoparticles provide the surface function of photo catalysis, which helps the surface to be anti-bacterial under UV-A illumination. These flexible, photocatalytic, superhydrophobic surfaces can be used very advantageously in medical applications, in particular for use as a blood-repellent wound dressing. Brief Description of the Figures Fig.1 Superhydrophobic and photocatalytic active films on surfaces. a) Fabrication of cross- linked PDMS/TiO₂ film from PDMS-copolymer modified TiO₂ nanoparticles and vinyl-PDMS. b) Morphology of polyester fabrics covered with cross-linked PDMS/TiO2 film at different magnifications c) Dependent formation of the superhydrophobic surface on the amount of modified TiO₂ particles, vinyl-PDMS and solvent used for the preparation. d) Sequence shows the periodical sliding of a water droplet (5 µL) on a bent polyurethane tape. Fig.2 Mechanical stability of the crosslinked-PDMS/TiO₂ film. a) The advancing contact angle (Θ_ACA, ■), receding contact angles (Θ_RCA, □), and contact angle hysteresis (Θ_CAH, ▲) of the crosslinked-PDMS/TiO₂ film on PU tape as a function of stretching elongation. b) The advancing contact angle (Θ_ACA, ■), receding contact angles (Θ_RCA, □), and contact angle hysteresis (Θ_CAH, ▲) of the crosslinked-PDMS/TiO₂ film on PU tape after repeated bending cycles. c) Force recorded as a function of insertion depth when the indenter presses and withdraws on the crosslinked-PDMS/TiO₂ structure. d) Force as a function of insertion depth when the indenter presses and withdraws on the inorganic surface structure. e) Forces recorded as functions of insertion depth during the repeated indentation at one certain position. f) The Young’s modulus and hardness of a crosslinked-PDMS/TiO₂ structure measured during repeated indentation cycles. Fig.3 Photocatalytic activities of a crosslinked-PDMS/TiO2 film. a) The advancing contact angle (ΘACA, ■), receding contact angles (ΘRCA, □), and contact angle hysteresis (ΘCAH, ▲) of water on the crosslinked-PDMS/TiO2 film as functions of the UV-A illumination time. b) Images of the receding contact angle of water on the crosslinked-PDMS/TiO2 film before (left), and after oleic acid contamination (middle), as well as after UV-A illumination for 2 h (right). c) Degradation ratio for Nile red over time under illumination with UV-A light in the presence of crosslinked-PDMS/TiO2 film. d) UV–vis spectra of Rhodamine B aqueous solution after being degraded by crosslinked-PDMS/TiO2 film for different times. e, f) Confocal microscopy images of E. coli on the bare glass, (e) and crosslinked-PDMS/TiO2 film, (f) after incubation for 210 min under UV-A illumination. Fig.4 Hemostasis ability of wound dressing modified with the crosslinked-PDMS/TiO2 film. a) Hybrid structure of the textiles covered with crosslinked-PDMS/TiO2 film hindering blood permeation. b) Repellency of blood flowing on modified wound dressing at different elongations. c, d) Shielding ability of the modified wound dressing thus avoiding blood leakage into water (c) or air (d). e) Air permeability of the modified wound dressing under water. f, g) Simulated blocking of bleeding experiment. The following examples illustrate the present invention in more detail, however, without limiting the same to the specific parameters and conditions thereof. General materials and methods Measurement of the mechanical property of a composite film: A MFP Nanoindenter (Asylum Research, Santa Barbara, CA) equipped with a diamond Berkovich indenter was used. Young’s modulus and hardness of the structures were measured by the nanoindentation. A setpoint of the loaded force on the tip was 100 μN. Bacterial cell viability assay: E.coli K12 MG1655 were transformed with a fluorescent protein expression plasmid (GFP-pTrc99A, ampicillin resistant, Isopropyl β-D-Thiogalactoside (IPTG) inducible).10 mL of Lysogeny broth (LB) medium containing 50 μg/mL ampicillin and 0.5 mM IPTG were inoculated with a single colony and incubated overnight at 37 °C at 250 rpm. The bacteria were diluted with phosphate buffer saline (PBS), or LB medium, to the desired density (OD600 = 0.1). The glass slides (control) and crosslinked-PDMS/TiO2 film-covered glass slides were fixed on the bottom of sterile borosilicate 2-well plates (ibidi μ-Slide, glass bottom).2 mL of bacterial solution was added and incubated for up to 210 min in the dark and under UV-light irradiation (5 ± 0.5 mW cm⁻²), respectively. To determine the bacterial viability after incubation with the sample surfaces, the bacteria were mixed with propidium iodide (PI, 1 μL mL⁻¹ bacterial culture), which selectively enters cells with damaged membranes, and incubated for 15 min in order to stain the dead cells. Stained bacteria were observed using a Leica SP8 laser scanning confocal microscope with excitation wavelengths of 488 and 610 nm. All bacteria display green fluorescence due to the expression of the GFP protein. Dead bacteria display red fluorescence of PI. Hemostasis experiment: The PDMS/TiO2 layer was laid on the commercial stretchable wound dressing. A polyvinyl chloride (PVC) tube was used to simulate a blood vessel. An incision was cut into the tube to simulate a wound. The PVC tube was fixed on the forefinger with the incision site located at the finger joint, simulating the state of a wound near a joint. Contrasting experiments were conducted with the incision being respectively covered by layers of modified (3 layers) and unmodified (3 layers) wound dressing. The hemostasis effect of the wound dressing was observed after adding human blood into the tube during which the forefinger was bent. Human blood was obtained from the Department of Transfusion Medicine Mainz from ten healthy donors after physical examination and after obtaining their informed consent in accordance with the Declaration of Helsinki. The use of blood was approved by the local ethics committee “Landesärztekammer Rheinland-Pfalz” (837.439.12 (8540-F)). EXAMPLE 1 Preparation and morphology of crosslinked-PDMS/TiO₂ films Modification of titanium dioxide nanoparticles: 0.5 g titanium dioxide nanoparticles (TiO2, diameter: 21 ± 5 nm, P25, Sigma) were dispersed in tetrahydrofuran (THF, 10 mL) by sonication. 30 mL (25-35% methylhydrosiloxane)-dimethylsiloxane copolymer (PDMS-copolymer, Mw: 1.9-2.0 kDa, Gelest Inc.) was added and mixed uniformly with TiO2 nanoparticles. The molecule structures are shown in Figure 1a. THF was evaporated for 24 h, and the particle dispersion was illuminated with UV-A light (intensity: 10 mW cm⁻²) and stirred for 10 hours. After the reaction, the modified TiO2 nanoparticles were purified by centrifugation at 10000 rpm for 10 min and redispersed in THF. This process was repeated five times. The modified nanoparticles become easily disperse in organic solvents such as toluene, THF, and hexane. The mass of modified TiO2 nanoparticles dispersed in THF was measured by weighing the deposition of 10 μL dispersion with a balance (Sartorius Genius ME, Mettler Toledo) after evaporation of THF. The concentration of particles was finally controlled to be 7.0 wt%. Preparation of anti-breaking superhydrophobic and photocatalytic active films: The modified TiO₂ nanoparticles (dispersed in THF, 7.0 wt%) were mixed with the vinyl-terminated PDMS (vinyl-PDMS, Mw: 62.0 kDa, Gelest) as well as Pt-catalyst (0.005 wt% relative to vinyl-PDMS, Platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex solution in xylene, Gelest) at a certain ratio in solvent toluene. Various substrates (glass, polyester fabrics, polyurethane, commercial wound dressings) were modified to be superhydrophilic by pre-treatment with oxygen plasma (5 min, power: 100%). Then the substrates were immersed in the mixture and allowed to react for 6 h at 60 ^oC in a closed chamber. The crosslinking reaction between PDMS molecules occurred both in bulk and on the substrates’ surfaces and formed a weak gel. After washing the samples with toluene to remove PDMS residues, hierarchical nanostructures formed on the surface. Fig.1: Superhydrophobic and photocatalytic active films on surfaces. a) Schemes illustrate fabrication of cross-linked PDMS/TiO₂ film from PDMS-copolymer modified TiO₂ nanoparticles and vinyl-PDMS. b) Morphology of polyester fabrics covered with cross-linked PDMS/TiO₂ film at different magnifications. From left to right, the scale bars are 10 μm (inset: 200 μm) and 2 μm. m_TiO2/m_vinyl-PDMS =0.1, m_TiO2/m_vinyl-PDMS= 3, in which, m_TiO2/m_vinyl- _PDMS: mass ratio of modified TiO₂ nanoparticles to vinyl-PDMS; V_solvent/V_vinyl-PDMS: volume ratio of solvent to vinyl-PDMS. c) Dependent formation of the superhydrophobic surface on the amount of modified TiO₂ particles, vinyl-PDMS and solvent used for the preparation. ○: formation of superhydrophobic surface (light green background); ×: failed formation of superhydrophobic surface (pink and gray background). Inset shows the morphology of a 5 μL water droplet on the crosslinked-PDMS/TiO₂ films. d) Sequence shows the periodical sliding of a water droplet (5 µL) on the bent polyurethane tape. Fig.1b shows the morphology of crosslinked-PDMS/TiO2 film on polyester fabrics. After treated with oxygen plasma and rinsed in the precursor mixture, the crosslinked-PDMS/TiO2 film uniformly covers the textile surface, even the surfaces inside the fabrics. The water contact angle on this surface is Θ = 165^○ ± 1^○. The modified TiO2 nanoparticles determine the formation of micro/nano structures. A surface prepared from unmodified TiO2 nanoparticles has poor water-repellent properties with low advancing contact angle smaller than 140°. The unmodified particles tend to aggregate in PDMS and as a result, the roughness of the films is not sufficient to allow for the formation of a superhydrophobic surface. The amount of modified TiO2 particles, vinyl-PDMS and solvent used for the preparation all determine whether superhydrophobic surfaces are obtained or not (Figure 1c). Here, surfaces are defined to be superhydrophobic when exhibiting a static contact angle > 150°. When using too many TiO2 nanoparticles (pink part in Figure 1c, mTiO2/mvinyl-PDMS = 0.5), the static contact angle of water on the crosslinked-PDMS/TiO₂ film becomes smaller than 150° which is arround 142°. In contrast, when the particles and vinyl-PDMS are too diluted (gray part in Figure 1c, m_TiO2/m_vinyl-PDMS < 0.025), no connective film forms. This is because of the low crosslink density between vinyl-PDMS and modified TiO₂ nanoparticles that fails to connect particles together. Therefore, in order to prepare superhydrophobic films, the contents of the components during fabrication should be in the green part (0.025 < m_TiO2/m_vinyl-PDMS < 1, and 1 < m_TiO2/m_vinyl-PDMS < 5) as indicated in Figure 1c. Other surfaces could also be coated, such as polyurethane (PU) tape. After coating with a layer of crosslinked-PDMS/TiO₂ film, the PU plate surface becomes superhydrophobic demonstrating ultra-low adhesion to water droplets. In Figure 1d, a water drop easily slides periodically on the bending PU surface covered with crosslinked-PDMS/TiO₂ film, indicating low energy dissipation of the droplet during sliding. EXAMPLE 2 Mechanical stability of crosslinked-PDMS/TiO₂ films Fig.2: Mechanical stability of the crosslinked-PDMS/TiO2 film of Example 1. a) The advancing contact angle (Θ_ACA, ■), receding contact angles (Θ_RCA, □), and contact angle hysteresis (Θ_CAH, ▲) of the crosslinked-PDMS/TiO2 film on PU tape as a function of stretching elongation. b) The advancing contact angle (ΘACA, ■), receding contact angles (ΘRCA, □), and contact angle hysteresis (ΘCAH, ▲) of the crosslinked-PDMS/TiO2 film on PU tape after repeated bending cycles. The bending of the crosslinked-PDMS/TiO2 film was carried out with bending angles ranging from 90^○ to 180^○, and the film maintained stable super-repellency to flowing water after bending 1000 times. c) Force recorded as a function of insertion depth when the indenter presses and withdraws on the crosslinked-PDMS/TiO2 structure. The film was fabricated with a component ratio of mTiO2/mvinyl-PDMS = 0.1, Vsolvent/Vvinyl-PDMS = 1. d) Force as a function of insertion depth when the indenter presses and withdraws on the inorganic surface structure. e) Forces recorded as functions of insertion depth during the repeated indentation at one certain position. f) The Young’s modulus and hardness of a crosslinked- PDMS/TiO2 structure measured during repeated indentation cycles. A flat PU tape was used as a substrate to investigate the stability of the crosslinked- PDMS/TiO2 film under stretching and bending conditions (Fig.2a, b). This is reflected by the variation of the advancing (Θ_ACA), receding (Θ_RCA) contact angles and contact angle hysteresis (ΘACA - ΘRCA) of the crosslinked-PDMS/TiO2 film. In Fig.2a, when the elongation of the film is smaller than 180%, the film demonstrates outstanding superhydrophobicity, Θ_ACA = 166^○ ± 1^○ and ΘRCA = 165^○ ± 1^○ with an extremely low contact angle hysteresis，ΘCAH ≈ 1^○. For example, a 5 μL water droplet easily slides off the crosslinked-PDMS/TiO2 PU surface stretched to 140% with a tilting angle of 1^○. Though the receding contact angle of the crosslinked-PDMS/TiO2 film decreases to 150^○ ± 1^○ at the elongation of 200%, it is still superhydrophobic. In addition, Figure 2b shows that the crosslinked-PDMS/TiO2 film has good resistance to bending, reflected by the stable advancing and receding contact angles and low contact angle hysteresis after bending the surface between 90^○ and 180^○ for 1000 times. The crosslinked-PDMS/TiO2 film also demonstrates stable repellency to water flow in the bending state. The mechanical properties of the surface were measured using a MFP Nanoindenter with a loaded force of 100 μN. By pressing and withdrawing a diamond Berkovich indenter into the film and detecting the applied force-vs-indentation (Figure 2c-f), the elasticity, modulus and hardness could be measured. Figure 2c shows the force curves measured on the crosslinked- PDMS/TiO₂ film. This was compared with the indentation recorded on an inorganic superhydrophobic surface (Figure 2d). The inorganic superhydrophobic surface was prepared by heating the crosslinked-PDMS/TiO₂ film at 500 °C for 30 min. Therefore, it has a similar structure. According to the smooth force line in Figure 2c, during pressing no break occurred in the structure. While, the obvious lagging of the retracting force illuminates the deformed structure can just recover partially, demonstrating the plasticity of the structures. In contrast, the jumps of the force line recorded during pressing inorganic superhydrophobic structures (Figure 2d) indicate the damaged of structures. Though the inorganic structure shows a higher modulus and hardness (data not shown), its capability to resist deformation is much lower than the crosslinked-PDMS/TiO₂ film. In addition, multiple test cycles of pressing and withdrawing indicate the good mechanical durability of the crosslinked- PDMS/TiO₂ film. Figure 2e shows that no obvious breaking occurs in the structure concluded from the smooth force lines even after 100 cycles of indentation. The surface Young’s modulus of the crosslinked-PDMS/TiO2 film (mTiO2/mvinyl-PDMS = 0.1, Vsolvent/Vvinyl-PDMS = 1) increases from 92.7 MPa to 135.1 MPa (the hardness increases from 8.5 MPa to 13.9 MPa) before 30 cycles of test and changed little after that (Figure 2f). EXAMPLE 3 Superhydrophobic and photoctalytic properties of crosslinked-PDMS/TiO₂ films Fig.3: Photocatalytic activities of a crosslinked-PDMS/TiO₂ film (m_TiO2/m_vinyl-PDMS =0.1, V_solvent/V_vinyl-PDMS = 3). a) The advancing contact angle (Θ_ACA, ■), receding contact angles (Θ_RCA, □), and contact angle hysteresis (Θ_CAH, ▲) of water on the crosslinked-PDMS/TiO₂ film as functions of the UV-A illumination (5 ± 0.5 mW cm⁻²) time. b) Images of the receding contact angle of water on the crosslinked-PDMS/TiO₂ film before (left), and after oleic acid contamination (middle), as well as after UV-A illumination for 2 h (right). The diagram shows the variation of the Θ_RCA of a water drop on the crosslinked-PDMS/TiO₂ film via oleic acid contamination and UV-A (5 ± 0.5 mW cm^-2) illumination for 6 cycles. c) Degradation ratio for Nile red over time under illumination with UV-A light (10 ± 1 mW cm^-2) in the presence of crosslinked-PDMS/TiO₂ film.10 μg mL^-1 of Nile red was dissolved in silicone oil (10 cSt). d) UV–vis spectra of Rhodamine B (1 μg mL^-1, 3 mL) aqueous solution after being degraded by crosslinked-PDMS/TiO2 film for different times (10 ± 1 mW cm^-2). e, f) Confocal microscopy images of E. coli on the bare glass, (e) and crosslinked-PDMS/TiO₂ film, (f) after incubation for 210 min under UV-A illumination (5 ± 0.5 mW cm^-2). Green and red fluorescence indicates whole and dead E. coli, respectively. As mentioned above, the crosslinked-PDMS/TiO2 film possesses superhydrophobic and photocatalytic properties. Even after UV-A illumination (5 ± 0.5 mW cm^-2) for 33 hours, the advancing contact angle of the film is around 166^○ ± 1^○ and the contact angle hysteresis is around 1^○ (Fig.3a). This is attributed to the stable resistance of PDMS molecules to UV degradation. After contamination by oleic acid, the receding contact angle of the crosslinked-PDMS/TiO2 film decreased from 165^○ ± 1^○ to 0^○, but recovers after UV-A illumination (5 ± 0.5 mW cm^-2) for 2h. After six cycles of contamination using oleic acid and successive photodegradation, the crosslinked-PDMS/TiO2 film remained superhydrophobic. Thus, crosslinked-PDMS/TiO2 films possess self-cleaning properties that can deal with both physical (sand contamination, data not shown) and chemical contamination. The crosslinked-PDMS/TiO2 films can also purify solvents based on photocatalytic activities. As an example (Figure 3c), Nile red (10 μg mL^-1, 3 mL) in silicone oil (viscosity: 10 cSt) is degraded by crosslinked-PDMS/TiO₂ film in 5 hours under the illumination of UV-A light (5 ± 0.5 mW cm^-2). The fluorescent intensity became weaker after longer UV-A illumination (data not shown). With low surface tension, the Nile red solution is in the Wenzel state on the film, where the dye molecules can have maximal contact with the TiO₂ nanoparticles. In contrast, an aqueous solution is in the Cassie state on the film ˗ an air layer exists between solution and the film. As shown in Figure 3d, the UV–vis absorption spectrum shows that the Rhodamine B (1 μg mL^-1, 3 mL) in aqueous solution can be completely degraded in 4 hours under UV-A illumination (5 ± 0.5 mW cm^-2), presenting the lighter color of the dye solution with longer UV-A illumination time (data not shown). The crosslinked-PDMS/TiO2 films showed anti-bacterial properties (Figure 3e,f) under UV-A illumination. To demonstrate this, glass slides covered with crosslinked-PDMS/TiO₂ were placed at the bottom of an E. coli dispersion in fresh LB-ampicillin media (OD600 = 0.1) in sterile borosilicate 2-well plates. The bare glass slide was used as a control. Giving the opacity of glass to UV light, a quartz slide (thickness: 170 µm) was used as a cap to inhibit liquid evaporation. The states (dead or live) of the E. coli on the glass and crosslinked- PDMS/TiO2 film after incubation for 210 min under UV-A illumination (5 ± 0.5 mW cm^-2) and in the dark were detected using confocal microscopy. Green and red fluorescence respectively highlighted the whole and dead E. coli. Bacteria incubated in the dark remain alive after 210 min. In comparison, some of the E coli bacteria are killed under UV illumination on both a bare glass surface and on crosslinked-PDMS/TiO2 film. Compared to the bare glass (Figure 3e), more bacteria are killed on the crosslinked-PDMS/TiO2 film (Figure 3f), indicating a superior and more obvious anti-bacterial effect of the film. The TiO2 nanoparticles on the film generate radicals when illuminated by UV light. These radicals enhance the destruction of bacteria present on the crosslinked-PDMS film. EXAMPLE 4 Blood repellency of crosslinked-PDMS/TiO2 films Fig.4: Hemostasis ability of wound dressing modified with the crosslinked-PDMS/TiO2 film. a) Scheme illustrates the hybrid structure of the textiles covered with crosslinked-PDMS/TiO₂ film (m_TiO2/m_vinyl-PDMS =0.1, V_solvent/V_vinyl-PDMS = 3) hindering blood permeation. The gray circle: textile of the wound dressing; red part: blood; green part: crosslinked-PDMS/TiO2 structure. b) Repellency of blood flowing on modified wound dressing at different elongations. c, d) Shielding ability of the modified wound dressing thus avoiding blood leakage into water (c) or air (d). e) Air permeability of the modified wound dressing under water. f, g) Simulated blocking of bleeding experiment. A polyvinyl chloride (PVC) tube is applied to simulate the human blood vessel and a small incision is made simulating the wound. The arrows represent the direction of the blood flow in the tube. PDMS and TiO₂ are both biocompatible. Therefore, the crosslinked-PDMS/TiO₂ film can well be used to cover bleeding wounds. The common method for stopping bleeding is bandaging using a medical dressing which is always hydrophilic. During bandaging, blood is lost by adsorption into the hydrophilic wound dressings. The crosslinked-PDMS/TiO₂ superhydrophobic films prevent blood loss while stopping bleeding. As shown in Fig.4a, the hybrid micro/nano superhydrophobic structures supported by textile and the crosslinked- PDMS/TiO2 film provide a stable air layer when exposed to blood, thus efficiently hindering bleeding. Figure 4b shows that blood repellency of the crosslinked-PDMS/TiO2 film modified wound dressing is still maintained when the film is stretched. There was no blood adhesion when flowing blood impacted onto the stretched and modified wound dressing, elongated by 200%. The stability of the modification was further confirmed by a deformation test (data not shown). After twisting the dressing tens of times, it still maintained its blood repellency. For medical applications, the crosslinked-PDMS/TiO2 film modified wound dressing demonstrated several more advantages including the ability to prevent blood from leakage when exposed to water or air, and its waterproof and air permeability. Blood leakage was completely prevented in the test bottle by using a crosslinked-PDMS/TiO2 modified wound dressing (Figure 4c, b); in contrast, the blood easily penetrated the unmodified wound dressing and thus from entering the water (Figure 4c). A crosslinked-PDMS/TiO2 modified wound dressing can also prevent blood exposure to the air (Figure 4d). The blocking effect of blood into water and air can effectively reduce the loss of blood from a wound and protect the wound from infection. In addition, the crosslinked-PDMS/TiO2 film modified wound dressing offers good air permeability in a water environment. Figure 4e shows a strong reflection at the contact area of the film and the water indicates a distinctive interface on account of the superhydrophobicity of the film. At the same time, the air bubble, which was injected with a needle, was easily adsorbed by the film. Efficient air permeability provides sufficient oxygen to the wound and promotes healing. Figures 4f and g show an experimental simulation of a bleeding process. A small incision ws cut into a PVC tube to simulate a wound in a blood vessel. After pumping blood through the tube ‘bleeding’ took place. By fixing the tube on a forefinger and locating the incision at the finger joint, a wound was simulated. In contrast with a continuously bleeding wound bandaged up with an unmodified wound dressing, the modified wound dressing efficiently stopped bleeding. Figure 4g shows that when the incision was bandaged by the crosslinked- PDMS/TiO₂ modified wound dressing, no bleeding was observed even if the finger joint was bent. This is attributed to the high durability of the crosslinked-PDMS/TiO₂ structure (Figure 4b). In contrast, bleeding continued when the incision was bandaged with an unmodified wound dressing (Figure 4f), and blood penetrated the wound dressing when the joint was bent.

Claims

22750/PCT CLAIMS 1. An elastic, photocatalytically active and superhydrophobic composite film, comprising or consisting of a cross-linked assembly of modified photocatalytically active metal oxide nanoparticles, wherein the metal oxide nanoparticles are modified by a polysiloxane copolymer grafted on the surfaces thereof and wherein the modified nanoparticles are cross-linked by a coupling reaction between Si-H-groups of the polysiloxane copolymer and functional vinyl groups of a vinyl-terminated polysiloxane crosslinker.

2. The composite film according to claim 1, wherein the polysiloxane copolymer is coupled to the nanoparticle surfaces by direct covalent Si-O Me bonds between the polysiloxane copolymer and the metal oxide and/or wherein the polysiloxane copolymer is selected from the group of polysiloxane copolymers comprising -[Si(CH3)2H]n-groups, in which 0 < n < 1000.

3. The composite film according to any one of claims 1 or 2, wherein the polysiloxane copolymer is selected from the group of copolymers comprising a compound having a repeating element of the general formula –[R1R2Si-O-R3R4Si]x-, wherein x is an integer from 5 to 5,000,000, preferably from 10 to 50,000, and R1, R2, R3, R4 independent from each other represent H, halogen, an organic residue, in particular alkyl, or -OSiR₅R₆R₇, with R₅, R₆, R₇ being substituents as defined for R₁ to R₄.

4. The composite film according to any one of claims 1-3, wherein the metal oxide nanoparticles are selected from the group comprising TiO₂, ZnO₂, SnO₂, CeO₂, Fe₂O₃, Ag₂O, WO₃, Al₂O₃, Nb₂O₅, ZnS, CuO, MoO₃, ZrO₂, MnO₂, MgO, and V₂O₅ nanoparticles.

5. The composite film according to any one of claims 1-4, wherein the metal oxide nanoparticles are TiO₂ nanoparticles, the polysiloxane copolymer is a methylhydrosiloxane-dimethylsiloxane copolymer, in particular a (25-30% methylhydrosiloxane)-dimethylsiloxane copolymer, and the vinyl-terminated polysiloxane crosslinker has a structural formula CH₂=CH–[(CH₃)₂Si-O]_x-Si(CH₃)₂- CH=CH, wherein x is an integer from 0 to 10000, and a Mw in the range from 0.1 kDa to 1000 kDa, preferably from 10 kDa to 200 kDa, in particular a Mw in the range from 50 kDa to 100 kDa.

6. The composite film according to any one of claims 1-5, wherein the mass ratio of copolymer-modified metal oxide nanoparticles to the cross-linker is 0.025 < mTiO2/mvinyl-PDMS < 1.

7. The composite film according to any one of claims 1-6, which has a modulus of elasticity of at least 5 MPa, 20 MPa, 40 MPa, 50 MPa or 90 MPa, and/or a static contact angle larger than 150^○, preferably larger than 165^○, for water, blood, and aqueous solutions.

8. An article comprising a layer of the composite film according to any one of claims 1-7 provided on a substrate surface, preferably a surface of an elastic substrate, in particular a surface of a substrate selected from the group comprising woven or non-woven fabrics, meshes, rubber, papers, cardboards, plastics, glass, membranes.

9. The article according to claim 8, which represents or comprises a wound dressing, in particular a blood-repellent wound dressing.

10. The article according to claim 8 or 9, which is capable to provide hemostasis and reduce the adherence of dry blood.

11. Use of the composite film according to any one of claims 1-7 or of the article according to claims 8 to 10 for medical applications, in particular for providing hemostasis and reducing the adherence of dry blood.

12. A method for preparing the composite film according to any one of claims 1-7, comprising at least the following steps: a) providing a mixture of photocatalytically active metal oxide nanoparticles, preferably pretreated with oxygen plasma, and a polysiloxane copolymer; b) irradiating the mixture with light in a wavelength range from 180 nm to 550 nm for a sufficient time to generate reactive moieties in said polysiloxanes and to form covalent Si-O-Me (Me = metal) bonds between said reactive moieties of the polysiloxanes and the metal oxide surface; c) optionally separating unreacted polysiloxane molecules from the modified polysiloxane-grafted metal oxide nanoparticles; d) providing a mixture of the modified metal oxide nanoparticles with a vinyl- terminated polysiloxane crosslinker in a suitable organic solvent in the presence of a catalyst, in particular a Pt-catalyst; e) reacting the mixture of step d) for a predetermined time period in the range from 10 min to 12 h, preferably from 0.5 h to 6 h at an elevated temperature, preferably in the range from 60 °C to 80 °C, to form an elastic film of cross-linked modified metal oxide nanoparticles; f) optionally removing unreacted reactants and solvent from the elastic film of cross- linked modified metal oxide nanoparticles.

13. The method according to claim 12, wherein the metal oxide nanoparticles are selected from the group comprising TiO₂, ZnO₂, SnO₂, CeO₂, Fe₂O₃, Ag₂O, WO₃, Al2O3, Nb2O5, ZnS, CuO, MoO3, ZrO2, MnO2, MgO, and V2O5 nanoparticles, and the polysiloxane copolymer is selected from the group of copolymers comprising -[Si(CH3)2H]n-groups, with 0 < n < 1000.

14. The method according to claim 12 or 13, wherein the metal oxide nanoparticles are TiO2 nanoparticles, the polysiloxane copolymer is a methylhydrosiloxane- dimethylsiloxane copolymer, in particular a (25-30% methylhydrosiloxane)- dimethylsiloxane copolymer, and the vinyl-terminated polysiloxane crosslinker has a structural formula CH2=CH–[(CH3)2Si-O]x-Si(CH3)2-CH=CH, wherein x is an integer from 0 to 10000, and a Mw in the range from 0.1 kDa to 1000 kDa, preferably from 10 kDa to 200 kDa, in particular a Mw in the range from 50 kDa to 100 kDa.

15. The method according to any one of claims 12-14, wherein the mass ratio of modified metal nanoparticles and vinyl-terminated polysiloxane cross-linker in the reaction mixture in step d) is in the range from 0.025 to 1, preferably from 0.05 to 0.6, especially preferred from 0.1 to 0.2, and/or wherein the volume ratio of cross-linker to solvent in the reaction mixture in step d) is in the range from 0.1 to 0.5.

16. The method according to any one of claims 12-15, wherein the concentration of the modified metal nanoparticles in the reaction mixture in step d) is in the range from 0.2 wt% to 33.3 wt%, preferably from 0.4 wt% to 20 wt%, especially preferred from 0.9 wt% to 6.7 wt%, and the concentration of the vinyl-terminated polysiloxane cross-linker is in the range from 10 wt% to 50 wt%, preferably from 20 wt% to 33 wt%, especially preferred from 20 wt% to 25 wt%.

17. The method according to any one of claims 12-16, wherein the catalyst is selected from the group comprising platinum-divinylterminated-siloxane complexes, in particular platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex solution and which is preferably present in an amount in the range of from 0.004 wt% to 0.01 wt% of the total mixture of step d).

18. A method for preparing an article according to any one of claims 8-10, wherein the elastic composite film is prepared by the method according to any one of claims 12- 17 and the substrates to be coated with the layer of the composite film are immersed in the reaction mixture of step d) of the method according to claim 12 for the duration of the crosslinking-reaction of step e) of the method according to claim 12 so that the generation of the elastic film and the bonding of said film to the substrate surface is effected simultaneously.