ANTIMICROBIAL MEMBRANES BASED ON POLYDIMETHYLSILOXANE AND TiO 2

Abstract

The present invention describes a method for the realization of an antimicrobial membrane based on electrospun polydimethylsiloxane fibers (ROMS) functionalized with Titanium dioxide (TiO2) particles. The invention also describes the crystallization of these TiO2 particles on the surface of ROMS fibers by hydrothermal reaction, the properties of the membrane thus obtained and the numerous hygienic-sanitary applications of the same.

Description

[0001] Antimicrobial membranes based on polydimethylsiloxane and TI02

[0002] * * * * *

[0003] Technical field

[0004] The present invention relates to the preparation of antimicrobial membranes based on polydimethylsiloxane (PDMS) and Ti02 deposited by hydrothermal synthesis and to the membranes thus obtained.

[0005] Known art

[0006] Electrospinning is a well-known technique that allows the formation of nonwoven fabrics with a micro- or nano-fibrous appearance through the application of a strong potential difference (in the order of kV) between a polymeric solution of adequate viscosity and a collector where the fabric is collected.

[0007] Membranes obtained by electrospinning are used for diversified applications but one of all, i.e. the use as a support for the deposition of antimicrobial agents, has acquired great scientific interest in the research for new technical solutions.

[0008] In fact, the current scientific literature reports the pressing need to develop new strategies, especially against those microbial species that are drug-resistant. In fact, some microorganisms have developed pan-resistance, and this means that they have become resistant to many of the pharmacological principles used in the clinic.

[0009] Of all the chemical species capable of performing antibacterial and antimicrobial functions, titanium dioxide is one of the main interpreters.

[0010] TiO2 is a metal oxide with semiconductive properties, which acts by activating itself whenever it is irradiated by a light source. In titanium dioxide, the interaction with photons from the light source causes a rearrangement of the electronic structure which in turn promotes the formation of oxygen-based radicals, which are very reactive and have high oxidizing power.

[0011] These oxygen radicals are in turn capable of inducing the oxidation of the structural components of microorganisms, triggering a microbicidal mechanism. This mechanism goes beyond the development of any resistance by the pathogen as what is affected is the structure of biological macromolecules, which is oxidized and therefore destroyed. It is also known that nanometric TiO2 powders, although with bacteriostatic or antibacterial characteristics, are scarcely exploitable if not fixed on a specific support. For instance, for the use of titanium dioxide in air purification devices and small dehumidifiers, TiO2 must necessarily be deposited in a stable way, both to withstand the high temperatures that these devices can reach and not to be dragged away by the air that is forced to pass over the surfaces that contain it.

[0012] However, the flip side of the coin is that, by fixing the TiO2 particles on supports, their surface area is usually decreased. Therefore, regardless of the substrate used (e.g., glass, ceramic materials, textile substrates, etc.), the fixing operation reduces the biocidal activity of Titanium dioxide.

[0013] As previously mentioned, the electrospinning process is widely known in the literature, as well as the production of polydimethylsiloxane fibers (PDMS) for the production of non-woven fabrics (NWF) suitable for different applications.

[0014] For instance, the scientific article "Electrospinning of Poly(Dimethyl Siloxane) by Sol-Gel Method (2009), Journal of Applied Polymer Science", describes the preparation of PDMS fibers starting from a solution of single-molecular-weight hydroxyl terminated PDMS in THF, in the presence of TEOS and HNO3 as a crosslinking agent and as an acidic catalyst, respectively. The solution is then flushed from the narrow nozzle of a needle to a collector (for collecting the fibers) between which a potential difference is applied to induce electrospinning. Subsequently, for further heat treatment of the PDMS fibers obtained, the cross-linking reaction is completed.

[0015] The same authors of the previous article also described in "Fabrication and Characterization of TiO2 / Poly(dimethylsiloxane) Composite Fibers with Thermal and Mechanical Stability (2009), Journal of Applied Polymer Science" the synthesis of a composite fibrous material of PDMS / TiC using the hybrid electrospinning technique, i.e. the parallel realization of PDMS and TiO2 fibers according to the procedure described above. However, the properties of the composite material are conditioned by the fragility and low mechanical resistance offered by TiO2 ceramic fibers. Furthermore, as reported in the article, the use of the PDMS / TiC composite material as a photocatalyst in an aqueous environment has a degree of abatement of only 78% after 8 hours. In the scientific work "Poly(dimethylsiloxane) / TiO2 Photocatalytic Membranes Obtained by Different Electrospinning Systems (2016), Journal of Nanoscience and Nanotechnology" the authors describe the realization of a composite material based on electrospun PDMS and TiO2 (commercial) wherein, in the various configurations in which the material has been made, Titanium dioxide is incorporated into the PDMS and totally absent on the surface. In the rare forms wherein Titanium dioxide was predicted externally, however, the SEM-EDS analysis did not reveal its presence on the surface. However, TiCte is either mixed into the PDMS polymer solution or spun in parallel from a powder suspension in THF using two parallel needles or a coaxial needle. However, the materials obtained with this procedure are not suitable for photocatalytic activity and the synthetic procedure developed is not suitable to guarantee the surface adhesion of commercial Titanium dioxide on the PDMS fiber.

[0016] "Antimicrobial activity from polymeric composites based polydimethylsiloxane / TiCte / GO: evaluation of filler synthesis and surface morphology (2017), Polymer Bulletin" describes the synthesis and characterization of the antibacterial activity of a PDMS - TiC -based composite (commercial). Since powdered Titanium dioxide is used, the latter cannot be crystallized on the surface of the PDMS fibers but will be entirely contained within the fibers themselves. Precisely for this reason, like the previous scientific article, the material produced will have a very low photocatalytic capacity. Not only that, since the material obtained is made in the form of a film instead of a membrane, the exposed surface is really small. The lack of surface Titanium dioxide combined with a rather limited exposed surface negatively affects antimicrobial activity.

[0017] "Construction and performance of composite nanoporous PVDF membrane loaded with TiC -based catalysts for organic pollutants' removal (2023), Journal of Material Science" describes a research about the design of nanoporous membranes based on PVDF (polyvinylidene fluoride) loaded with TiC -based catalysts and hypercrosslinked polystyrene nanoparticles for the removal of organic pollutants. Here the polymer matrix is synthesized by electrospinning, but the TiCte nanoparticles are dispersed in solvent by ultrasound and this solution is in turn dispersed on the membrane already formed by electrospray. The membrane loaded with the photocatalyst is dried in vacuum at 40°C for 12h. However, already starting from the second but even more significantly from the fifth recycling test, there is a marked loss of efficiency probably caused by the lack of stability over time of TiO2 on the PVDF membrane.

[0018] Further, the article "Decoration of Cauliflower-Like TiO2 on Nanofibrous PVDF Membranes: A Strategy for Wastewater Treatment (2023), ACS Applied Polymer Materials" describes the electrospinning of hydrophobic PVDF membranes, with wide contact angles, thermally resistant, and supported with TiO2 particles obtained by hydrothermal synthesis.

[0019] Finally, the review " TiO2 Supported in Polymethyl methacrylate (PMMA) Properties, Preparation, and Photocatalytic Activity for the Degradation of Synthetic Dyes (2023), Letters in Applied NanoBioScience" describes various techniques for the deposition of titanium dioxide on polymer substrates, in particular polymethyl methacrylate (PMMA), including hydrothermal synthesis and sol-gel technique.

[0020] In the PhD thesis entitled "Preparation of PVDF / TiC core / shell nanofibrous membranes and investigation of piezo-potential effect on the photocatalytic performance (2023)" the formation of PVDF / TiO2-based nanofibrous membranes is discussed. Among the different possibilities of synthesis and processing, electrospinning and hydrothermal treatment for the growth of TiO2 on electrospun PVDF are described.

[0021] As can be seen from the well-known art, even though the technical landscape is rather crowded with inventions, the problems and application limits that still remain are numerous and evident.

[0022] In particular, the electrospun polymer fibers described in the literature are functionalized with pre-formed TiO2 nanoparticles that are dispersed in solvent and then adhered to the membrane or directly dispersed in the polymer matrix before electrospinning. These are synthetic approaches that limit and hinder the catalytic and antimicrobial activity of the photocatalyst as they negatively affect the reactivity and surface availability of metal atoms. The latter are in fact mostly incorporated into the polymeric support and therefore catalytically inefficient, while the residual particles of metal oxide that remain exposed superficially are weakly adhered to the fibrous matrix of the polymer and therefore subject to erosion and easy detachment with a consequent collapse of antibacterial activity. It therefore follows that there are no devices that exploit heterogeneous photocatalysis, that we have a high surface area covered with an active photocatalyst and that are recyclable without loss of effectiveness.

[0023] Even the choice of making membranes using electrospun fibers directly in TiO2 is particularly inefficient not only because of the mechanical fragility and therefore application of these fibers but also because the catalytically active surface appears extremely reduced.

[0024] In addition, the use of polymeric supports such as PVDF for the production of membranes with antimicrobial activity is inadequate due to the low thermal stability offered by this polyfluorinated polymer.

[0025] This feature reduces the range of possible applications of the PVDF membrane, as well as prevents the use of high-temperature functionalization treatments.

[0026] Finally, the antimicrobial electrospun polymeric membranes described in the literature prevent the transmission of electromagnetic radiation through the substrate. The result of this is to have only one of the two faces of the membrane catalytically active since the other will be adhered to the surface to which the membrane is applied, drastically limiting the activity of the photocatalytic metal as a whole exposed on the surface.

[0027] The paper ALBERTI STEFANO ET AL: "Porous polydimethylsiloxane membranes loaded with low-temperature crystallized T1O2NPS for detachable antibacterial films", JOURNAL OF MATERIAL SCIENCE, KLUWER ACADEMIC PUBLISHERS, DORDRECHT, vol. 54, no. 2, 21 September 2018 (2018-09-21 ), pages 1665-1676, XP036625151 , ISSN: 0022-2461 , DOI: 10.1007 / S10853-018-2881 -4 describes the preparation of a TiO2sol-gel. This is used to form anatase colloidal suspension crystals with the help of some TiO2nanoparticles to initiate nucleation. Anatase is deposited on PDMS so that titanium dioxide is dispersed between the PDMS fibers.

[0028] The formation of polydimethylsiloxane membranes loaded with TiO2 is carried out with the electrospinning equipment illustrated in Fig. 1 wherein is shown that the spinning and spraying of PDMS and TiO2 gels take place simultaneously using two parallel needles.

[0029] However, the processes described in the 2018 article do not allow a consistency of reproducibility and therefore cannot be industrialized. In addition, the membranes electrospun with the equipment in Fig. 1 are not homogeneously coated as the TiO2gel is applied by electrospray on the membrane and is unable to completely wet all the fibers, consequently not ensuring an adequate distribution of the coverage of crystalline TiO2 nanoparticles on all the fibers of the membrane.

[0030] It was therefore felt necessary to provide an improved product that would allow membranes with a more homogeneous distribution of TiO2 particles to be obtained.

[0031] Unless specifically excluded in the detailed description below, this chapter is to be considered as an integral part of the detailed description of the invention.

[0032] Summary of the invention

[0033] A purpose of the present invention is to describe a method to obtain antimicrobial membranes based on electrospun polydimethylsiloxane fibers and TiO2 titanium dioxide nanoparticles.

[0034] Another purpose of the present invention is to describe the crystallization and thus deposition of TiO2 nanoparticles on the surface of polydimethylsiloxane fibers.

[0035] A further purpose is to describe the characteristics of antimicrobial membranes obtained according to the method of the present invention.

[0036] Additional purpose is to report the range of possible applications that antimicrobial membranes according to the present invention can have.

[0037] Further purposes and aspects of the invention will be evident from the detailed description that follows.

[0038] The claims describe preferred embodiments of the invention, forming an integral part of the present description.

[0039] Brief description of the Figures

[0040] Figure 1 : A) Cut sample of membrane adhered to a vertical wall; B) Cut sample of membrane held with tweezers; C-D) SEM electron microscope images of PDMS fibers on which TiO2 is accreted; E-F) Various membrane samples spread and rolled over a Petri dish; G) Bacterial cultures of Escherichia coir, H) Salmonella spp after contact with the device. Definitions

[0041] • In this document, the term "olation" refers to a process of chemical condensation that leads to the formation of hydroxide bridges between two metal species, both if they are the same or different.

[0042] • In this document, the term "oxolation" refers to a process of chemical condensation that leads to the formation of oxygen bridges between two metal species, both if they are the same or different.

[0043] • In this document, the term "about" means a variation of + / - 10% for the indicated values.

[0044] • In this document, the term "aging" refers to the time it takes for the prepolymer solution to develop olation and oxolation reactions (at a temperature different from room temperature) that lead to an increase in the viscosity value of the solution itself, thus making it suitable for processing by electrospinning.

[0045] Detailed Description

[0046] The membranes made through the process of the present invention are electrospun membranes, e.g. synthesized and processed by electrospinning, obtained by a causal, disordered and random interweaving of the electrospun polymer fibers, and appear as a sheet of microfibrous non-woven fabric (NWF), macroscopically white, soft to the touch and adhesive by electrostatic.

[0047] According to the method of the present invention, these membranes are made using a polydimethylsiloxane polymer (PDMS).

[0048] Polydimethylsiloxane fibers are transparent to visible radiation and when assembled according to random orientations for the realization of the electrospun membrane they give the fabric said white color.

[0049] By the process of the present invention, a photocatalytic material is grown on these membranes, wherein said photocatalytic material is preferably of an inorganic nature.

[0050] More preferably, this photocatalytic material is an oxide and can be chosen as a non-limiting example in the group comprising zinc oxide (ZnO), tungsten trioxide (WO3), titanium dioxide (TiC ).

[0051] According to a preferred embodiment, the photocatalytic material suitable for covering / functionalizing the electrospun membrane is titanium dioxide (TiC ).

[0052] The fabric thus obtained can be adhered to different types of surfaces (especially metal) by activating environmental sanitation thanks to photocatalysis triggered by artificial or natural light and operated by TiO2.

[0053] The membranes are produced using already known chemical synthesis and processing techniques that are however combined with a method for the growth of TiO2 nanoparticles directly on the membrane.

[0054] These TiO2 nanoparticles through the method according to the present invention are therefore firmly bound / fixed / anchored to the electrospun membrane and uniformly distributed on it.

[0055] These TiO2 nanoparticles grown and uniformly dispersed on the membrane according to the method of the present invention are such as to result in a minimal reduction of the area exposed by the membrane itself, which in itself, being electrospun, has a high porosity and a large surface area.

[0056] Therefore, compared to the synthesis of TiO2 fibers described in known art, the growth of titanium dioxide nanoparticles (i.e. particles with a high surface / volume ratio) by the method of the present invention allows to significantly increase the availability of titanium atoms superficially active and available to trigger photocatalysis processes when hit by light radiation.

[0057] Again, with respect to what is described in known art, the method according to the present invention allows to fix these metal oxide nanoparticles, preferably TiO2, on the electrospun membrane in a firm and immovable way.

[0058] This fixing operation therefore allows these nanometric particles of TiO2 dispersed on the membrane to resist any mechanical stress deriving from the various applications that these electrospun and functionalized membranes may have.

[0059] Furthermore, the transparency to visible radiation of the electrospun PDMS fibers constituting the non-woven membrane combined with the morphology and distribution of the TiO2 nanoparticles allow the incident light to be able to easily penetrate the thin layer of fibrous material by photoactivating even the titanium nanoparticles located on the face of the membrane opposite the light source and adhering to the surface (preferably metallic) on which this membrane has been adhered to.

[0060] The operation of nucleation and growth of metal photocatalyst nanoparticles, preferably titanium dioxide, on electrospun polymeric fibers assembled in a disorderly manner to form the membrane is carried out by hydrothermal synthesis.

[0061] This hydrothermal synthesis is operated on the electrospun membrane starting from a gel-phase precursor of the metal photocatalyst obtained in turn by sol-gel technique.

[0062] Electrospun membrane synthesis

[0063] A solution of a polymer suitable for the electrospinning process, preferably polydimethylsiloxane (PDMS), more preferably hydroxy-terminated polydimethylsiloxane dissolved in a solvent, preferably organic, more preferably tetrahydrofuran (THF), is initially subjected to an "aging" process, i.e. to a process of mixing the precursors following a certain order, according to a predetermined temperature and according to appropriate timing, to obtain the conditions suitable for electrospinning.

[0064] This "aging" process, operated by the addition of a cross-linking agent, preferably TEOS, in solution, allows the chains of PDMS in solution to bind together to form a large three-dimensional lattice that drastically increases the viscosity of the solution.

[0065] Subsequently, the solution is conveyed in a constant flow to a collector while it is subjected to the action of an electric potential.

[0066] The electric charge that hits and precisely charges the flow of solution coming out of the nozzle of the flushing channel contrasts with the cohesion force (induced by the high viscosity and surface tension) until it overwhelms it.

[0067] This imbalance of forces thus induces the formation of the Taylor cone, at the apex of which the decomposition of the cone itself into a jet of fibers takes place, which can in turn take on different shapes depending on the type of potential applied and the order of magnitude of the potential difference. The fibers, whose size is regulated precisely by the experimental conditions of electrospinning, are then pushed and collected on the collector, accumulating in the thickness of the membrane itself. In fact, it is precisely the macroscopic accumulation with random orientation of all the fibers that are extracted from the polymeric solution subjected to electrospinning that leads to the formation of the membrane, i.e. of the non-woven fabric. The membrane becomes "MEMBRANE" when a minimum thickness (100 pm) is reached, sufficient to be able to handle it and subject it to the hydrothermal support process with TiO2.

[0068] Polymer fibres, preferably PDMS, obtained by the method of the present invention have a diameter of 1 -60 pm, preferably between 3-55 pm, preferably between 5-50 pm, preferably between 5-45 pm, preferably between 5-40 pm, preferably between 5-35 pm, preferably between 5-30 pm, preferably between 5-25 pm, preferably between 5-20 pm.

[0069] According to a particularly preferred embodiment, the polymer fibers obtained, preferably in PDMS, have a diameter between 5-30 pm.

[0070] Membranes obtained from the random, casual and disordered assembly of electrospun polymer fibers, preferably in PDMS, have a thickness between 10-2000 pm, preferably between 50-1500 pm, preferably between 80-1200 pm, preferably between 100-1000 pm. This thickness is a function of the amount of polymer solution that is electrospun.

[0071] According to a particularly preferred embodiment, membranes obtained by disordered, chaotic and random assembly of electrospun polymer fibers, preferably in PDMS, have a thickness between 100-1000 pm.

[0072] Pre-polymer preparation

[0073] Two pre-polymers, i.e. high molecular weight hydroxy-terminated polydimethylsiloxane (PDMS-OH) (PDMS-OH HW) having a viscosity of about 50000 cSt under standard conditions (Aldrich, CAS: 70131 -67-8, density: 0.98 g / mL at 25°C) and low molecular weight hydroxy-terminated polydimethylsiloxane, PDMS-OH LW, having a viscosity between 18000 and 22000 cSt under standard conditions (Aldrich CAS: 70131-67-8), are weighed and dispersed in an organic solvent, preferably aprotic, which is added immediately after the weighing of the two pre-polymers.

[0074] In a preferred embodiment, the organic solvent employed is tetrahydrofuran (THF).

[0075] The pre-polymers are left under stirring (e.g., mechanical, magnetic) until completely dissolved.

[0076] Preferably, this dissolution can be achieved at a temperature between 60-100°C, preferably between 65-95°C, preferably between 70-90°C, preferably between 75- 85°C, more preferably about 80°C to promote mobility, reduce viscosity and therefore facilitate dispersion. These values are to be considered with a tolerance of 2%.

[0077] A cross-linking agent is then added which, by means of consecutive hydrolysis and condensation reactions (oxolation and olation reactions), induces a three- dimensional cross-linking of the hydroxy-terminated PDMS chains through the formation of oxygen bridges (Si-O-Si) and hydroxide bridges (Si-OH-Si) which then leads to a three-dimensional network.

[0078] Preferably, the cross-linking agent used is TEOS (tetraethyl orthosilicate).

[0079] Preferably, TEOS (Aldrich, CAS: 78-10-4, density 0.933 g / mL, Reagent grade, 98%) is added with a percentage ratio by weight to PMDS (of both viscosities) lower than or equal to 5%.

[0080] The polymer solution is left to "age" (an operation that allows a thermal balancing of the reactants and the solvent) at a temperature between 75-85°C, more preferably between 78-82°C. The aging time, in combination with the various possible temperatures, can be about 20 minutes + / - 10%.

[0081] Preferably, this aging operation is carried out by solvent reflux.

[0082] The "aging" operation, i.e. thermal balancing, allows to have reproducibility in obtaining the final product.

[0083] Once the "aging" process is complete, the cross-linked polymer solution in the organic solvent is brought to a temperature between 15-30°C, preferably between 18-28°C, preferably between 20-26°C, more preferably at a temperature of 25°C.

[0084] Once the solution has cooled, an acidic catalyst is added. Preferably, the acidic catalyst can be chosen from the group including, but not limited to, hydrochloric acid (HCI), sulphuric acid (H2SO4), nitric acid (HNO3), phosphoric acid (H3PO4).

[0085] More preferably, the acidic catalyst is nitric acid (HNO3).

[0086] Preferably, the acidic catalyst is added with a percentage ratio by weight to the PDMS (of both viscosities) lower than or equal to 0.2%.

[0087] Once the addition has been made, the cross-linked polymer solution, preferably PDMS, in the presence of an acidic catalyst, preferably nitric acid, is "aged" by bringing it under stirring (e.g. mechanical, magnetic) to a temperature between 40- 60°C, preferably between 45-55°C, more preferably about 50°C for 1 day, preferably more than one day, preferably 2 days, preferably more than 2 days, more preferably 3 days, preferably more than 3 days.

[0088] The completion of the aging operation is established by rheological tests aimed at evaluating the viscoelastic behaviour of the polymer solution; the solution is in fact considered suitable for electrospinning when the elastic modulus exceeds the viscous modulus, after crossing it.

[0089] This evaluation of the correct aging time was possible thanks to a dynamicmechanical rheological analysis (TS test), with which it was possible to evaluate the viscous modulus (also called Loss Modulus) and the elastic modulus (Storage Modulus) of the polymer solution, which defines the change in behaviour from viscous to elastic when, with the measurement, it is possible to appreciate an inversion of the two modulus (the absolute value of the Loss Modulus is greater than the Storage Modulus, module inversion occurs when the absolute value of the Storage Modulus equals and exceeds the Loss Modulus).

[0090] The range of values of the modules can be between 10 and 1000 Pa and the inversion of the two modules must take place within a time of between 10 and 100 minutes.

[0091] Once this "aging" operation has been completed, the solution is to be considered ready for electrospinning.

[0092] Electrospinning The electrospinning process is performed on said viscosity-controlled pre-polymer solution by exploiting the application of a potential difference between the solution ejection site into an outlet from a flushing channel and a collection manifold. The operating mode and the technical details for performing electrospinning are well known to an expert in the field.

[0093] Preferably, electrospinning carried out according to the method of the present invention uses a potential between 5 and 15 kV, a flux between 2 and 500 pL per minute, a temperature between 15 and 25°C and a relative humidity not exceeding 45%, preferably 30 RH%.

[0094] The thickness of the non-woven membranes obtained by electrospinning is a function of the amount of electrospun pre-polymer solution.

[0095] Preferably, the thickness is higher than 500 pm.

[0096] T / O2 preparation

[0097] Titanium dioxide, TiO2, is synthesized by sol-gel technique from titanium tetraisopropoxide (TTIP, 97 %, Sigma Aldrich) into iso-propanol (Sigma Aldrich) and deionized water.

[0098] Preferably, the synthesis reaction of TiO2 is carried out at room temperature (about 25°C).

[0099] Preferably, the synthesis reaction of TiO2 is performed at room pressure (about 1 atm).

[0100] The oxolation and olation reactions that occur in solution, releasing isopropanol molecules (iPrOH), allow the formation of oxygen bridges (Ti-O-Ti) and hydroxide bridges (Ti-OH-Ti) between titanium atoms, thus allowing the formation of a three- dimensional network of TiO2 in gel form.

[0101] The reaction is almost immediate when water is added to the iPrOH and TTIP solution.

[0102] The reaction is evaluated macroscopically: the starting solution based on iPrOH and TTIP appears transparent with yellowish reflections while once H2O has been added, the formation of a white gelatinous substance is highlighted which, under stirring, is evenly distributed throughout the solution, giving the medium the characteristics of a milky, white and "thick" gel.

[0103] It is defined as a sol-gel reaction precisely because of the transition of behaviour from solution (sol) to gel.

[0104] The stirring is preferably maintained for an hour (it can also be maintained for much longer) with the aim of evaporating part of the solvent (H2O and iPrOH).

[0105] Once the reaction has taken place, titanium dioxide in gel form is chemically and physically suitable for hydrothermal treatment.

[0106] Hydrothermal process

[0107] The process of deposition and engraftment of the antimicrobial metal photocatalyst, preferably TiO2, to the membrane cannot be carried out by mixing a nanometric powder of TiO2 to the pre-polymer solution (potentially feasible at any stage of synthesis and aging) as the active substance can perform its antimicrobial function only if placed in direct contact with the micro-organism, i.e. the antimicrobial agent must be evenly distributed over the surface of the polymer support.

[0108] In fact, since it is a heterogeneous photocatalysis, i.e. a catalysis operated between two different phases (solid-liquid or solid-gas; where the solid is made up of TiO2 particles), TiO2 must necessarily be exposed to the outside, so that it comes into contact with the other phase. For this reason, a hydrothermal process is used in a suitable hydrothermal reactor wherein the precursor of the substance in question (the TiO2 gel prepared according to the method described in the previous paragraph) is placed in contact with the previously electrospun polymer membrane, preferably PDMS.

[0109] A commercial TiO2 powder, being made up of preformed particles of Titanium dioxide, would not be suitable for hydrothermal synthesis, which is why the preparation of a TiO2 gel is used.

[0110] The hydrothermal reaction is performed in a temperature range 50-150°C.

[0111] The hydrothermal reaction is performed in a time interval between 3-6 hours (calculated from the moment the reaction medium has reached the desired temperature).

[0112] Preferably, the pressure is not actively controlled but develops naturally according to the temperature at which the treatment is conducted, the filling degree of the reactor and the type of solvent present in the gel.

[0113] Preferably, the hydrothermal reactor consists of a Teflon vessel inserted into a steel container.

[0114] The hydrothermal reactor can be advantageously equipped with a pressure gauge for the evaluation of the pressure reached.

[0115] One (or more) piece of membrane to be subjected to the hydrothermal process is placed inside the hydrothermal reactor, the size of which depends on the size (and therefore the volume) of said hydrothermal reactor, preferably having dimensions 5 cm x 2 cm and with a thickness between 700 pm and 1 mm, and the TiO2 gel.

[0116] The amount of TiO2 gel must be sufficient to impregnate (soak) the membrane entirely. The excess of TiO2 gel must therefore be evaluated according to the size and thickness of the membrane to be processed.

[0117] Preferably, the filling percentage of the hydrothermal reactor with the TiO2 gel and the membrane piece is 75% (the remaining 25% can be air).

[0118] Once the reaction is complete, the pieces of membrane are washed with solvent, preferably iPrOH, and left to dry (in the air and / or in a stove) for at least one night.

[0119] Hydrothermal synthesis allows to obtain a double effect: the crystallization of titanium dioxide from the amorphous phase (the precursor gel) to its photocatalytically active crystalline form (anatase) and the growth of TiO2 nanoparticles directly on the surface of the silicone membrane, ensuring stability and external exposure.

[0120] Titanium dioxide nanoparticles obtained hydrothermally from an amorphous phase of TiO2 gel have a surface area of between 150-200 m2 / g compared to that of commercial TiO2 particles which is around 50 m2 / g. It is evident, therefore, that this synthesis allows an increase in the surface quantity of active catalyst available for photocatalysis.

[0121] The electrospinning technique makes it possible to obtain a microfibrous support of non-woven fabric suitable for effective impregnation by the TiO2 gel, i.e. the precursor that provides the crystalline TiO2 that grows on the fibers of the membrane during the hydrothermal treatment, and which performs the antibacterial properties.

[0122] The fibers, both superficially and in the deep layers, following the hydrothermal process retain and expose TiO2 nanoparticles grown with high surface area (figures 1 C and 1 D); this anchoring to the surface of the electrospun fibers makes it possible to avoid the loss of photocatalytic activity.

[0123] Hydrothermal synthesis as described here allows nucleation and subsequent growth of the size of metal photocatalyst (TiC ) nanoparticles on electrospun polymer fibers, preferably PDMS, assembled to form the membrane.

[0124] This increase is made possible by a combination of the temperature applied and the pressure exerted, which in turn derives from the type of solvent used and the filling degree of the reactor. In particular, in working conditions, the pressure is such as to allow operation at reduced temperatures in order to obtain a specific crystalline phase, anatase. Anatase is usually obtained with heat treatments of the order of 300-350°C, while with the solvo-thermal or hydrothermal synthesis of the present invention it is possible to obtain it at about 150°C thanks to the development of this internal pressure.

[0125] The methodology described in the present invention, according to which the membrane previously electrospun and impregnated with TiO2 gel is subjected to hydrothermal synthesis, makes it possible to overcome a series of limitations due to the different method described in the 2018 article cited above, which instead involves the simultaneous electrospinning of PDMS and TiO2, the latter previously subjected to hydrothermal treatment.

[0126] A further difference can be found in the "Materials and Methods" paragraph of the 2018 article where the reagents used for the synthesis of TiO2 and PDMS are described, as reported in the present patent application. However, in the paragraph "Polydimethylsiloxane solution" there is a difference with respect to the method reported in the present invention, namely, the aging of the solution, which is described as being carried out in the oven for 3 days at 50°C, has been modified and in the present invention is carried out at the same temperature (50°C) but by reflux and magnetic stirring for 3 days, because the method as reported in the article did not guarantee reproducibility in the aging treatment. However, as mentioned above, the main difference between the 2018 article and the present invention lies in the spinning of the PDMS membrane which is carried out in parallel with a spray of the TiO2 solution. To be more specific, the article mentions hydrothermal treatment as a method of crystallization of TiO2 gel starting from sol-gel synthesis, with the essential difference that, as shown in Fig. 1 of the article, two syringes were used, one containing the polymeric solution of PDMS and the other containing the TiO2 gel (previously thermally treated in hydrothermal process). Therefore, what is obtained is an electro-spinning of PDMS in parallel with an electro-spray of TiO2 (also due to the application of the electric potential between needle and collector); only that, in the absence of a viscous solution, it is not possible to form fibers but instead spray droplets on the forming membrane.

[0127] This method of applying the TiO2 gel is not effective as the spray was not stable in the horizontal conformation of the electrospinning, consequently not suitable for long treatment times (the TiO2 gel placed in the syringe sediments after a while, separating from the propanol solvent); moreover, as can be seen from Fig. 1 of the article, TiO2 is not homogeneously distributed on the fibers but rather localized only in some massive points.

[0128] The non-homogeneity of the TiO2 coating represents a clear disadvantage for photocatalytic activity.

[0129] Therefore, to overcome this specific problem, it was decided to carry out the deposition of TiO2 directly on the fibers of the PDMS membrane using the hydrothermal treatment applied to the portions of the membrane, previously electrospun and soaked in TiO2 gel.

[0130] This made it possible to obtain the crystallization of the TiO2 gel during the treatment precisely allowing it to grow directly above the fibers.

[0131] The result of the change in the method has first of all allowed the process to be:

[0132] • reproducible, i.e. providing the same type of result after each synthesis, without the variability that was observed using the method reported in the article (aging in oven);

[0133] • scalable towards larger quantities of pre-polymer solutions, essential characteristics for industrial production. In conclusion, the method of crystallization of the TiO2 gel on top of the fibers, usually not used because, as a type of treatment, the hydrothermal one is aggressive, has allowed the membrane to be used directly as a support for the crystallization of TiO2 germs, which has made it possible to obtain an artifact (the membrane) that has been homogeneously covered as the gel can completely wet all the fibers; consequently, ensuring an optimal distribution of the coverage of crystalline TiO2 nanoparticles on all membrane fibers.

[0134] This new methodology has also made it possible to overcome the problems of instability of the TiO2 gel during electrospraying, a technique mentioned in the article but abandoned as a method of TiO2 deposition as it is absolutely ineffective for obtaining products homogeneously covered with TiO2 and therefore provided with improved antibacterial properties.

[0135] The electrospun membrane obtained or obtainable with the method according to the invention is transparent to light radiation and allows the photoactivation of titanium nanoparticles (TiC ) both on the face exposed to the light source and on the opposite face.

[0136] As previously mentioned, TiO2 particles synthesized according to the method of the present invention appear to have a much larger surface area than commercial particles, which is why they can exert an increased (photocatalytic) antibacterial and antimicrobial activity.

[0137] In fact, since it is a question of "heterogeneous photocatalysis", i.e. catalysis that develops at the interface between two different phases, it is consequential to expect that the greater the exposed area, the greater the activity.

[0138] The electrospun PDMS membrane functionalized with crystallized TiO2 nanoparticles is characterized by chemical and thermal resistance, a high surface area, antimicrobial properties and adhesion stability.

[0139] The antimicrobial efficacy of PDMS / TiO2 membranes was tested in the laboratory using a prototype lamp capable of simulating solar radiation (Osram ULTRA - VITALUX of 300W) on different bacterial colonies exposed to artificial light irradiation at a constant temperature (about 30°C both in contact with the PDMS / TiO2 membrane and in its absence). The results obtained demonstrate a reduction in the viability of bacterial cells, determined by the ability to form colonies when cultured on solid growth medium. The high bactericidal action is confirmed by the small number of bacterial colonies present in the culture substrate, when compared to those that are simply exposed to light but not to the membrane.

[0140] In fact, as shown in figure 1 H, bacterial proliferation (specifically the species Salmonella spp) after exposure to the PDMS / TiC membrane according to the present invention is particularly reduced compared to that of the same bacterial species that remained free from contact with this membrane. (Figure 1 G)

[0141] The membranes of the invention (figures 1A and 1 B) look like a sheet of non-woven, soft, white and electrostatic fabric, which therefore has excellent adhesion characteristics to surfaces of different kinds without worsening their appearance (even if it is opaque). The presence of TiO2 nanoparticles, which is crystallized and grown directly on the silicone structure of the sheet already formed through hydrothermal synthesis, therefore ensures the possibility of generating oxygenbased radical species every time the same nanoparticles are irradiated by a light source, even artificial. For this reason, the surfaces on which the membrane is adhered are sterilized when lighted.

[0142] The silicone nature of the support material (the membrane) guarantees stability against the oxidative activity of TiO2, thermal stability up to about 300°C and mechanical stability, as it is elastic and not brittle. Furthermore, due to its characteristics, the photocatalytic action of the device has no time limit.

[0143] The membranes of the invention are designed for the coating of flat and complex surfaces, fixed or mobile in environments in the clinical and medical sector (operating rooms, ambulances and rescue vehicles) that are at risk of microbial contamination.

[0144] Membranes can ensure constant disinfection of healthcare environments where healthcare-related infections include infections that result from the exposure of patients - but also of operators or other people who attend the facility - to infectious agents deriving from direct contact with other living beings (e.g. contaminated hands of operators) or indirect contact with reusable medical devices (e.g. surgical instruments) or with the contaminated inanimate environment (e.g. surfaces, dust). In addition, the membranes can be applied to all surfaces that are at risk of bacterial contamination (in work, domestic, healthcare, transport environments) and on which it is necessary to reduce the microbial load. These membranes therefore represent a valid alternative to the use of chemical or physical devices for the disinfection of environments as they do not require additional energy sources to those already used for the lighting of the rooms wherein the surfaces are located.

[0145] With the membranes of the invention, it is certainly possible to save on the use of disinfecting agents or other processes in general that are currently used for the disinfection of surfaces and objects at risk of microbial contamination.

[0146] The following examples are given for the sole purpose of illustrating the invention and are in no way to be considered as limiting its scope.

[0147] EXAMPLES

[0148] Example 1

[0149] Preparation of the polymer solution to be electrospun.

[0150] Two prepolymers are weighed: 0.88 g of high molecular weight hydroxy-terminated polydimethylsiloxane (PDMS-OH) with a viscosity of 50000 cSt (Sigma Aldrich), referred to as PDMS-OH HW, and 3.52 g of low molecular weight PDMS-OH with a viscosity between 18000 and 22000 cSt (Sigma Aldrich), referred to as PDMS-OH LW. The two pre-polymers are dispersed in 8.34 mL (7.43 g) of tetrahydrofuran (THF, Sigma Aldrich, 99.9%) which is added immediately after weighing the prepolymers. Dissolution is then favored by magnetic stirring at 80°C. Once the solution is completed, TEOS (0.54 mL, which corresponds to 0.51 g) is added in solution to trigger the three-dimensional cross-linking of the PDMS chains. The polymer solution is then left at 80°C for 20 minutes under solvent reflux conditions. After that, the solution is air-cooled to room temperature. Nitric acid (0.02 g. or 16 pL) is added, and the temperature is brought to 50°C under stirring. It is thus left to age for 3 days to obtain the solution suitable for the electrospinning process.

[0151] With the synthesis route described above, it is possible to recover about 13.4 mL (which corresponds to 12.35 g) of solution that can be processed by electrospinning.

[0152] The same synthesis route has proved to be compatible for a scale up since working with a scale factor of 1 :10 it was possible to prepare about 134 mL of polymer solution to be electrospun.

[0153] Example 2

[0154] Electrospinning on a polymeric PDMS solution.

[0155] Take 50 mL of polymer solution with which to cover a rotating roller with a surface area of 1200 cm2and set the following experimental parameters (set up):

[0156] Operating in these conditions, it is therefore possible to obtain the electrospun membrane base with a dimensional range between 10 and 30 pm. As a technical expedient, the collector is coated with a very thin metal film (e.g. aluminium) to facilitate the recovery of the electrospun polymer. In order to complete the crosslinking process of the properly aged pre-polymer solution, the surface of the metal foil is wetted with a 5% solution of DBDLS (Dibutyltin dilaurate, Sigma Aldrich 95%) in THF. By covering the entire surface of the aluminium foil, as soon as the fibres that form at the apex of the Taylor Cone are attracted to the collector, they can be blocked in their final morphology and thus allow them to accumulate for the duration of the electrospinning process.

[0157] The accumulation of all the fibers on the collector results in the formation of the membrane, which is itself the non-woven fabric (NWF). So basically, the membrane assembles itself thanks to the simple accumulation of fibers on top of each other that takes place throughout the time of the electrospinning itself.

[0158] Example 3 Antimicrobial membrane activity in PDMS functionalized with T1O2.

[0159] The antimicrobial efficacy of PDMS / TiC membranes was tested in the laboratory, using a prototype of a 300W Osram ULTRA - VITALUX lamp capable of simulating solar radiation. Specifically, Gram- (Salmonella spp.) and Gram+ (Staphylococcus aureus) bacteria were exposed to artificial light irradiation for 30 min at a 30 cm distance from the light source and at a constant temperature of 30°C, both in contact with the PDMS / TiO2 membrane and in its absence.

[0160] The results obtained demonstrate a reduction in the viability of bacterial cells, determined by the ability to form colonies when cultured on solid growth medium, The high bactericidal action is confirmed by the small number of bacterial colonies present in the culture substrate, when compared to those that are simply exposed to light but not to the membrane.

Claims

CLAIMS1. A method for the realization of antimicrobial membranes based on electrospun fibers of polydimethylsiloxane (PDMS) and titanium dioxide (TiC ) deposited by hydrothermal synthesis comprising the steps of: i. preparing a pre-polymeric solution of polydimethylsiloxane in an organic solvent starting from hydroxy-terminated polydimethylsiloxane; ii. electrospinning said pre-polymeric solution to obtain electrospun fibers; iii. assembling the electrospun fibers to obtain an electrospun membrane; iv. preparing a titanium dioxide gel using the sol-gel technique and applying it by impregnation on the previously electrospun membrane; v. subjecting the membrane impregnated with titanium dioxide (TiC ) gel from the previous step to a hydrothermal reaction to crystallize the TiO2 nanoparticles on it.

2. The method according to the above claim wherein the step of preparing the pre-polymer solution involves the steps of:• dissolving a hydroxy-terminated polydimethylsiloxane with a viscosity of 50000 cSt under standard conditions and a hydroxyterminated polydimethylsiloxane with a viscosity between 18000 and 22000 cSt under standard conditions in an organic solvent at a temperature between 60 and 100°C;• adding tetraethyl orthosilicate (TEOS) and leaving to stir at a temperature between 60-100°C and for more than 10 minutes;• adding an acidic catalyst, preferably HNO3; leaving the solution under stirring at a temperature between 40- 60°C for more than 1 day, preferably 3 days.

3. The method according to anyone of the preceding claims wherein the organic solvent and temperature in the phase of dissolving said polydimethylsiloxane polymers are respectively tetrahydrofuran (THF) and 80°C.

4. The method according to anyone of the preceding claims wherein said electrospun fibers have a diameter between 5-30 pm and said electrospun membrane has a thickness between 100-1000 pm.

5. The method according to anyone of the preceding claims wherein said titanium dioxide (TiC ) gel is obtained from titanium tetraisopropoxide (TTIP) in isopropanol and deionized water at 25°C and 1 atmosphere.

6. The method according to anyone of the preceding claims wherein said hydrothermal reaction is conducted on the membrane previously electrospun and impregnated with titanium dioxide gel in a hydrothermal reactor operating at a temperature of 50-150°C and for a time of 3-6 hours.

7. The method according to anyone of the preceding claims wherein titanium dioxide TiO2 nanoparticles crystallize on the surface of polydimethylsiloxane polymer fibers by hydrothermal reaction.

8. The method according to anyone of the preceding claims wherein titanium dioxide (TiC ) crystallizes on the surface of the electrospun fibers of said electrospun membrane in the photocatalytically active crystalline form of anatase.

9. An electrospun membrane obtained by the method according to anyone of the preceding claims wherein the TiO2 particles have a surface area between 150-200 m2 / g.

10. The electrospun membrane obtained or obtainable by the method according to anyone of the preceding claims that is transparent to light radiation.

11. Artifacts made with the electrospun membrane of the previous claim.

12. Use of the electrospun membrane in accordance with any of claims 9-10 to coat flat and complex fixed or mobile surfaces in clinical and medical environments such as operating theatres, ambulances and rescue vehicles that are at risk of microbial contamination, on all surfaces that are at risk ofbacterial contamination such as in workplaces, domestic, health, transport.

Images