POLY(ETHYLENE GLYCOL) METHACRYLATE MICROGELS, PREPARATION METHOD AND USES

Abstract

The invention relates to poly(oligo(ethylene glycol) methacrylate) microgels, to the process for preparing same and the uses thereof in various fields of application such as optics, electronics, pharmacy and cosmetics.

These microgels have the advantage of being monodisperse, pH-responsive and temperature-responsive. They can carry magnetic nanoparticles or biologically active molecules. These microgels may also form transparent films, which have novel optical and electromechanical properties.

Claims

1-13. (canceled)

14. Microgels obtainable via precipitation polymerization of at least three monomers in an aqueous phase, in the presence of a crosslinking agent, said monomers being: di(ethylene glycol) methyl ether methacrylate (M(EO).sub.2MA), an oligo(ethylene glycol) methyl ether methacrylate (M(EO).sub.nMA) n being an integer ranging from 3 to 12, a monomer of formula CR.sub.1R.sub.2═CR.sub.3R.sub.4 in which R.sub.1, R.sub.2, R.sub.3 and R.sub.4 represent a hydrogen, a halogen or a hydrocarbon group, on condition that at least one of the four groups comprises a —COOH or —COO.sup.−M.sup.+ group, M.sup.+ representing a cation.

15. The microgels according to claim 14, wherein M(EO).sub.2MA represents 50 mol % to 90 mol % of the total number of moles of the three monomers, M(EO).sub.nMA represents 10 mol % to 50 mol % of the total number of moles of the three monomers and the monomer of formula CR.sub.1R.sub.2═CR.sub.3R.sub.4 represents 0.1 mol % to 20 mol % of the total number of moles of the three monomers.

16. The microgels according to claim 14, wherein the monomer of formula CR.sub.1R.sub.2═CR.sub.3R.sub.4 is methacrylic acid.

17. The microgels according to claim 14, wherein the crosslinking agent is an oligo(ethylene glycol) diacrylate (OEGDA) comprising from 1 to 10 ethylene glycol units.

18. The microgels according to claim 14, wherein said microgels comprise metal or metal oxide nanoparticles.

19. The microgels according to claim 14, wherein said microgels comprise magnetic nanoparticles.

20. The microgels according to claim 14, wherein said microgels comprise a compound selected in the group consisting of pigments, dyes and sunscreens.

21. A process for preparing hybrid microgels, said process comprising the steps of: preparing a first aqueous colloidal dispersion of magnetic nanoparticles that are positively charged at their surface, preparing a second aqueous colloidal dispersion of microgels via a precipitation polymerization process comprising a step of bringing into contact three monomers in an aqueous phase, in the presence of a crosslinking agent, at a temperature of between 40° C. and 90° C., wherein the three monomers are: di(ethylene glycol) methyl ether methacrylate (M(EO).sub.2MA), an oligo(ethylene glycol) methyl ether methacrylate (M(EO).sub.nMA) n being an integer ranging from 3 to 12, and a monomer of formula CR.sub.1R.sub.2═CR.sub.3R.sub.4 in which R.sub.1, R.sub.2, R.sub.3 and R.sub.4 represent a hydrogen, a halogen or a hydrocarbon group, on condition that at least one of the four groups comprises a —COOH or —COO.sup.−M.sup.+ group, M.sup.+ representing a cation, mixing the first and the second aqueous colloidal dispersions, and adjusting pH of the obtained mixture at a pH value that is above isoelectric point of the magnetic nanoparticles, and recovering the hybrid microgels.

22. The process according to claim 21, wherein M(EO).sub.2MA represents 50 mol % to 90 mol % of the total number of moles of the three monomers, M(EO).sub.nMA represents 10 mol % to 50 mol % of the total number of moles of the three monomers and the monomer of formula CR.sub.1R.sub.2═CR.sub.3R.sub.4 represents 0.1 mol % to 20 mol % of the total number of moles of the three monomers.

23. The process according to claim 21, wherein the monomer of formula CR.sub.1R.sub.2═CR.sub.3R.sub.4 is methacrylic acid.

24. The process according to claim 21, wherein the crosslinking agent is an oligo(ethylene glycol) diacrylate (OEGDA) comprising from 1 to 10 ethylene glycol units.

25. A cosmetic or pharmaceutical product consisting of or containing the microgels as claimed in claim 14.

26. The cosmetic or pharmaceutical product according to claim 12, further comprising at least one compound selected in the group consisting of surfactants, oils, biologically active products, pigments and dyes.

27. A kit comprising a magnet and the cosmetic product according to claim 25, said magnet and said cosmetic product being packaged together.

28. A makeup or skin care method comprising a step of applying on skin a cosmetic product according to claim 25.

29. A film comprising at least one layer of microgels according to claim 14.

30. Monodisperse, temperature-responsive and magnetic hybrid microgels, wherein said microgels are based on poly(oligo(ethylene glycol) methacrylate) and contain magnetic nanoparticles.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0086] FIG. 1 presents the chemical structures of the monomers used for the synthesis of the biocompatible microgels of the invention.

[0087] FIG. 2 is a synthetic scheme of the pH-responsive and temperature-responsive biocompatible microgels based on poly(MEO.sub.2MA-co-OEGMA-co-MAA) of the invention.

[0088] FIG. 3 is a scheme for preparing γ-Fe.sub.2O.sub.3 particles.

[0089] FIGS. 4 and 5 are schematic representations respectively of the 1.sup.st and of the 2.sup.nd synthesis step of the process for preparing hybrid microgels according to the invention containing γ-Fe.sub.2O.sub.3 particles.

[0090] FIG. 6 represents a scheme of the process for forming films of microgels according to the invention.

[0091] FIG. 7 represents an assembly for characterizing the electromechanical effect of the films of microgels of the invention.

[0092] FIG. 8 is a schematic representation of the period of compression and relaxation of a film de microgels of the invention.

[0093] FIG. 9 represents the diagram of a compression and relaxation program of microgels of the invention.

[0094] FIG. 10 is an image of films of microgels of the invention in the dry state and swollen in solution.

[0095] FIG. 11 is an image of a dry film of hybrid microgels of the invention containing γ-Fe.sub.2O.sub.3 nanoparticles taken at various viewing angles.

[0096] FIG. 12 is an image of a dry film of hybrid microgels of the invention containing γ-FeO.sub.3 nanoparticles taken on a light surface and on a dark surface.

[0097] FIG. 13 is an image of a dry film of hybrid microgels of the invention containing γ-Fe.sub.2O.sub.3 nanoparticles with and without magnet.

[0098] The invention is also illustrated by the following examples.

Example 1: Synthesis of a Microgel Based on Poly(Oligo(Ethylene Glycol) Methacrylate) According to the Invention

[0099] The following monomers were used: di(ethylene glycol) methyl ether methacrylate (M(EO).sub.2MA, Mn 250 g.Math.mol.sup.−1), oligo(ethylene glycol) methyl ether methacrylate (M(EO).sub.4-5MA also denoted OEGMA hereinafter, Mn 475 g.Math.mol.sup.−1), and methacrylic acid (MAA). The crosslinking agent was oligo(ethylene glycol) diacrylate (OEGDA, Mn 250 g.Math.mol.sup.−1).

[0100] Experimental Protocol:

[0101] 0.966 g of MEO.sub.2MA (5.14×10.sup.−3 mol), 0.272 g of OEGMA (5.73×10.sup.−4 mol) and 0.029 g of OEGDA (1.17×10.sup.−4 mol) are introduced into a volume of 57.5 mL of water and left under magnetic stirring until the monomers have completely dissolved. The mixture is then filtered and introduced into a three-necked flask having a volume of 250 mL that is equipped with a mechanical stirrer before being degassed under nitrogen for 45 min with mechanical stirring (150 rpm:). An aqueous solution of MAA (0.026 g, 3.05×10.sup.−4 mol dissolved in 2 mL of water) is then introduced into the reaction medium. The mixture is left at 70° C. for 20 min before introducing an aqueous solution of potassium persulfate (KPS, 0.0143 g dissolved in 2.5 mL of water) previously degassed under nitrogen. The addition of KPS makes it possible to initiate polymerization and the reaction medium is left under mechanical stirring (50 rpm) at 70° C. for 6 h.

[0102] The polymerization is then stopped by addition of oxygen and left to cool to ambient temperature. The microgels are then separated from the reaction medium by centrifugation (10 000 rpm, 30 min) and the reaction medium is replaced by pure water (of milliQ grade); the step is repeated five times.

[0103] The final solution is then composed of a colloidal dispersion of P(MEO.sub.2MA-co-OEGMA-co-MAA) microgels in the aqueous phase, this dispersion is kept at ambient temperature.

[0104] Property of the Microgel

[0105] The synthesis of the microgels was characterized by kinetic monitoring of the monomers using proton nuclear magnetic resonance (.sup.1H NMR) spectroscopy. A complete conversion of the monomers and also a homogeneous composition of the microgels, with a homogeneous distribution of the crosslinking points and of the methacrylic acid units, are observed.

[0106] The final yield of crosslinked microgel was analyzed by solids content of the reaction medium and makes it possible to determine a yield of 70 wt % of crosslinked microgel.

[0107] The content of methacrylic acid incorporated was determined by acid-base titration of the purified microgels. The pH-responsive nature of the microgels in aqueous solution and also the incorporation of 70 mol % of the initial MAA monomer were able to be verified.

[0108] The microgels were observed by transmission electron microscopy (TEM) and their sizes were determined by dynamic light scattering. The microgels observed are monodisperse, with a size of 400 nm in the dried state and that may range up to 1000 nm in the wet state.

Example 2: Preparation of Solutions of a Microgel Based on Poly(Oligo(Ethylene Glycol) Methacrylate)

[0109] 1.2 mL of a solution of microgels prepared according to Example 1 (containing 15 g.Math.L.sup.−1 of microgels) are dispersed in 10 mL of a solution of pure water (milliQ grade). The pH of the dispersion is adjusted by addition of a 0.1 mol.Math.L.sup.−1 solution of hydrochloric acid or of potassium hydroxide. The mixture is left under magnetic stirring until the pH has stabilized. The size of the microgels in solution is measured by dynamic light scattering and the temperature of the solution is controlled during the analysis.

[0110] By studying the size of the microgels in solution by dynamic light scattering, it was possible to evaluate the impact of the pH and of the temperature of the medium on the capacity of the microgels to swell or to shrink in water.

[0111] The microgels are responsive to pH variations of the medium, changing from a size of 400 nm at pH<5.5 to a size of 1000 nm at pH>6.0.

[0112] The microgels are temperature-responsive and change from a swollen state at 20° C. to a shrunken state at high temperature. The shrinkage temperature depends on the pH (35° C. at pH<6.0 and 55° C. at pH>7.0). Finally, the volume of the swollen microgel at 20° C. decreases up to 3 times relative to its initial volume when the temperature goes beyond the shrinkage temperature.

Example 3: Preparation of a Microgel Based on Poly(Oligo(Ethylene Glycol) Methacrylate) Containing Magnetic Nanoparticles

[0113] Synthesis of the Maghemite γ-Fe.sub.2O.sub.3 Nanoparticles

[0114] The reactants used are: ferrous chloride tetrahydrate (FeCl.sub.2.4H.sub.2O), ferric chloride hexahydrate (FeCl.sub.3.6H.sub.2O), 28-30% w/w ammonium hydroxide (NH.sub.4OH), iron nitrate (Fe.sup.III(NO.sub.3).sub.3.9H.sub.2O), 36% v/v hydrochloric acid (HCl) and nitric acid (HNO.sub.3).

[0115] The maghemite nanoparticles used during this study were synthesized by coprecipitation of the metal salts (Fe.sub.II and Fe.sup.III). This method of synthesis consists in forming nanoparticles of magnetite (Fe.sub.3O.sub.4) by coprecipitation of ferrous chloride (FeCl.sub.2) and ferric chloride (FeCl.sub.3) in a basic medium by addition of ammonium hydroxide (NH.sub.4OH). The magnetite is then oxidized to form the maghemite (γ-Fe.sub.2O.sub.3) variety. The oxidation of the magnetite to maghemite makes it possible to establish pH-responsive hydroxyl functions at the surface of the nanoparticles, these functions having a point of zero charge at a neutral pH (pH≈7.2). Thus, at acidic or basic pH values, these nanoparticles have a colloidal state in the aqueous phase by electrostatic repulsion of anionic charges (at basic pH) or cationic charges (at acidic pH). This is a versatile method for synthesizing magnetic nanoparticles that are stabilized in the aqueous phase, commonly referred to as “cationic ferrofluids” or “anionic ferrofluids” depending on the pH of stabilization.

[0116] Experimental Protocol

[0117] Step 1: Formation of the Magnetite.

[0118] 12.2 g of ferric chloride hexahydrate FeCl.sub.3.6H.sub.2O (0.0451 mol) are introduced into a 3 L beaker containing 500 mL of pure water. 4.49 g of ferrous chloride tetrahydrate FeCl.sub.2.4H.sub.2O (0.0226 mol) are dissolved in 24.3 mL of a 1.5 mol.Math.L.sup.−1 solution of hydrochloric acid (HCl) is added to the 3 L beaker and everything is left mixing under gentle mechanical stirring (initial Fe.sup.II/Fe.sup.III ratio=0.5). A volume V=43 mL of 28/30% w/w ammonium hydroxide is then added to the beaker with vigorous mechanical stirring and at ambient temperature. The addition of ammonium hydroxide leads to the formation of flocculated magnetites (Fe.sub.3O.sub.4) in basic aqueous solution (pH>10), the magnetite flocs are then left to settle under the effect of a magnetic attraction generated by a permanent magnet, then the supernatant is removed and replaced by pure water (milliQ grade). The washing step is repeated twice in order to remove the excess ammonium hydroxide.

[0119] Step 2: Desorption of the Ammonium Counterions and Surface Oxidation.

[0120] After the successive steps of washing the magnetite, a volume V=28.6 mL of a 2 mol.Math.L.sup.−1 aqueous solution of nitric acid HNO.sub.3 is added to the magnetite flocs and is left under mechanical stirring for 30 min in order to treat the surface of the magnetite particles.

[0121] The addition of nitric acid makes it possible to acidify the medium and to induce a desorption of the excess ammonium NH.sub.4.sup.+ counterions at the surface of the nanoparticles by ion exchange with the nitrate NO.sub.3.sup.− ions. The oxidation of the particles at the surface also makes it possible to dissolve the ferrous ions that have not precipitated and that are present at the surface of the nanoparticles.

[0122] The surface-treated magnetite flocs are left to settle under a permanent magnet, then the supernatant is removed and replaced by pure water, this step is repeated twice.

[0123] Step 3: Oxidation of the Core of the Nanoparticles.

[0124] After the Successive Steps of Washing the Surface-Treated Magnetites, a volume V=85.7 mL of a freshly prepared 0.33 mol.Math.L.sup.−1 solution of ferric nitrate Fe.sup.III(NO.sub.3).sub.3.9H.sub.2O is added at boiling to the magnetite flocs and is left under reflux and under mechanical stirring for 45 min.

[0125] The introduction of the Fe.sup.3+ ions by the ferric nitrate makes it possible to oxidize the Fe.sup.II of the particles thus forming the maghemite γ-Fe.sub.2O.sub.3 variety. After complete oxidation of the particles, the maghemite floc is left to settle under permanent magnet and the supernatant is removed then replaced by pure water, the operation is repeated twice.

[0126] Step 4: “Peptization” of the Magnetite Nanoparticles.

[0127] A volume V=28.6 mL of a 2 mol.Math.L.sup.−1 solution of nitric acid HNO.sub.3 is added to the maghemite floc and left at ambient temperature and under mechanical stirring for 30 min. The addition of nitric acid makes it possible to introduce hydronium H ions at the surface of the maghemite. The cationic maghemite floc is left to settle then washed three times with acetone. A volume V=70 mL of water is then added to the nanoparticles enabling a “peptization” of the nanoparticles in the water, the dispersion of nanoparticles is then stabilized by electrostatic repulsion of positive charges at the surface of the nanoparticles. Lastly, the residual acetone is removed by evaporation under vacuum at 40° C.

[0128] Synthesis of P(MEO.sub.2MA-Co-OEGMA-Co-MAA)/γ-Fe.sub.2O.sub.3 Hybrid Microgels

[0129] The hybrid microgels are synthesized by simple mixing of an aqueous dispersion of P(MEO.sub.2MA-co-OEGMA-co-MMA) microgels with a dispersion of maghemite nanoparticles that is stabilized at pH 2 (nanoparticles with cationic charges). The encapsulation of the nanoparticles within the microgels is carried out in 2 steps: [0130] A first step consists in adding the cationic nanoparticles to a solution of microgels dispersed at pH 3 and at ambient temperature. These mixing conditions make it possible to retain the cationic charge at the surface of the γ-Fe.sub.2O.sub.3 nanoparticles. The nanoparticles will preferentially interact with the microgels owing to the carboxylic acid groups resulting from the methacrylic acid units contained within the microgels. Specifically, the carboxylic acid groups have the ability to be adsorbed at the surface of particles of metal oxide such as iron oxide and furthermore the positive charge at the surface of the nanoparticles enables a favored interaction. In this sense, upon addition of γ-Fe.sub.2O.sub.3 nanoparticles, the latter will preferentially be located within the microgels (step summarized in FIG. 4). [0131] A second step consists in increasing the pH of the medium (microgels+nanoparticles) starting from pH 3 up to pH 7.5. This rise in the pH induces: 1) A destabilization of the cationic γ-Fe.sub.2O.sub.3 nanoparticles within the mixture. Specifically, since the nanoparticles have a point of zero charge at neutral pH (isoelectric point=7.2), the latter flocculate at this pH due to lack of electrostatic repulsion. 2) The creation of negative charges within the microgels derived from the carboxylic acid (COOH) functions in the form of carboxylate (COO.sup.−) groups. The concomitance of these two phenomena makes it possible to anchor the magnetic nanoparticles within the microgel and to improve the stability of the hybrid microgels owing to the negative charges of the carboxylate functions (step summarized in FIG. 5).

[0132] Experimental Protocol

[0133] A volume of 40 mL of an aqueous dispersion of P(MEO.sub.2MA-co-OEGMA-co-MAA) microgels having a weight concentration of 1.45 g.Math.L.sup.−1 is introduced into a 100 mL round-bottomed flask and left under magnetic stirring, the pH of the dispersion is adjusted to 3.0 by addition of a 0.1 mol.Math.L.sup.−1 solution of nitric acid (HNO.sub.3). Next, a volume of 10 mL of a dispersion of cationic magnetite nanoparticles at pH 3 having a weight concentration of 1.34 g.Math.L.sup.−1 is added dropwise to the mixture at ambient temperature and under magnetic stirring, this corresponds to an amount of nanoparticles per hybrid microgels of ˜18.8%. The reaction mixture is left under stirring and at ambient temperature for 12 h. The pH of the reaction mixture is then increased by dropwise addition of a 0.5 mol.Math.L.sup.−1 solution of potassium hydroxide (KOH). Finally, the hybrid microgels are separated from the reaction medium by centrifugation (5000 rpm, 20 min) and the reaction medium is replaced by pure water (of milliQ grade). The final solution is then composed of a colloidal dispersion of P(MEO.sub.2MA-co-OEGMA-co-MAA) microgels in water, this dispersion is kept at ambient temperature. Various syntheses have been carried out by varying the theoretical weight fraction of nanoparticles per hybrid microgel between 0 and 33%.

[0134] Property of γ-Fe.sub.2O.sub.3/P(MEO.sub.2MA-Co-OEGMA-Co-MAA) Hybrid Microgels

[0135] The hybrid microgels were characterized in the dry state by transmission electron microscopy (TEM) and in the wet state by dynamic light scattering. The hybrid architecture of the microgels was demonstrated by TEM, the observation of the microgels in the dry state makes it possible to reveal the good encapsulation of the magnetic nanoparticles within the microgels which are not expelled during the drying treatment. The content of nanoparticle fillers encapsulated was determined by thermogravimetric analysis, the analysis confirms a quantitative and significant encapsulation of the nanoparticles (filler contents tested ranging from 0 to 33 wt % of nanoparticles per hybrid microgel). The temperature-responsive properties of the hybrid microgels in aqueous solution at neutral pH are also demonstrated with a shrinkage of the hybrid microgels which change from 1000 nm at 20° C. to 450 nm with a shrinkage temperature of 37° C. This shrinkage at neutral pH takes place regardless of the magnetic nanoparticle filler content.

Example 4: Films of Microgels

[0136] The compositions of microgels summarized in Table 1 are used to prepare the films.

[0137] The films are prepared by a drying process presented in FIG. 6, starting from a colloidal dispersion of monodisperse microgels (having a size that may vary between 500 and 1000 μm in solution) having a weight concentration of microgels that varies from 1.4 to 5 wt % in water (solution 1). A constant volume of solution is introduced into a plastic mold and left to dry until the water has completely evaporated (step 1 from FIG. 6). The film remaining at the bottom of the mold is then composed of several layers of monodisperse and completely dry microgels (having a size that may vary between 350 and 450 μm in the dry state). The film is carefully recovered and reintroduced into an aqueous solution (solution 2). Various parameters are varied: 1.) The weight concentration of dispersion from 1.4 to 5 wt % makes it possible to vary the swollen film thickness (end of step 3: thickness from 200 μm to 1000 μm). 2.) The pH of solution 2 is varied between 5.5 and 7.5.

[0138] Experimental Protocol: Formation of the Films of Temperature-Responsive Microgels Based on Poly(Oligo(Ethylene Glycol) Methacrylate.

[0139] A volume of 5 mL of a colloidal dispersion of microgels at a weight concentration of 1.4 to 5 wt % is introduced into a plastic mold and left to dry at a temperature of 32° C. (+/−2° C.). After complete evaporation of the solvent, the film is carefully recovered then introduced into an aqueous solution and left to swell at ambient temperature.

[0140] The recovered films were observed by atomic force microscopy in the dry state and characterized using a rheometer in the wet state (end of step 3 of FIG. 6). The microgels form an elastic film composed of several layers of microgels (step 2) and this being irrespective of the composition of the microgels (microgels 1 to 5 in table 1). Conversely, the microgels lose their mechanical properties when they are swollen in water but do not re-disperse in solution.

[0141] Microgels 1 and 2: the film thickness is varied from 200 to 1000 μm, the multiplication of the layers of microgels does not modify the “film formation” phenomenon: and the films keep their elasticity in the dry state.

[0142] Microgels 3, 4 and 5: A thickness of the order of 300 μm (swollen films in step 3) were studied in the case of the hybrid microgels. The addition of nanoparticles does not modify the film-forming properties of the microgels. On the contrary, the mechanical properties of the films are greatly improved in the wet state.

[0143] Evaluation of the Electromechanical Properties of the Films

[0144] 1. Characterization of the Electromechanical Properties of the Films

[0145] The electromechanical properties of the films of microgels were studied. This is a question of demonstrating the capacity of the microgels to generate an electric current when a pressure is exerted on these microgels. More particularly, the idea of the invention is to generate an electric current by pressing the material at the surface of a substrate. This electric potential may be generated from a material having ionic functions attached covalently (or polyelectrolyte material). Specifically, since the ionic groups are attached in the microgel, only the counterions of each carboxylate group have a mobility in the microgel. When a unidirectional pressure/deformation is exerted on these polyelectrolyte microgels, the mobility of the counterions is favored, thus creating a polarization between the positive charges of the mobile counterions and the negative charges of the attached carboxylate groups. This ionic gradient results in an electric potential at the interface. Thus, the presence of ionizable functions in the microgel would make it possible to create a polarization within the material and to generate an electric potential.

[0146] An assembly is used in order to demonstrate the electromechanical properties of the films. For this, an Anton Paar MCR301 rheometer is used in plate-plate geometry within which two flat and conductive electrodes based on indium tin oxide or ITO (entity 1. from FIG. 7) are attached on either side of the geometry. The lower electrode is fixed and the upper electrode is removable. A wet film of microgels (entity 3. from FIG. 7) is deposited on the surface of the lower electrode and the upper electrode is lowered in order to exert a compression of force F.sub.N on the film. By controlling the distance between the two electrodes, it is possible to control the crushing force (F.sub.N) exerted on the film.

[0147] A program of compression/relaxation is carried out in order to vary the force exerted on the film. Firstly, the initial thickness of wet film (denoted L.sub.0) was determined and the distance between the two electrodes was gradually reduced by a distance ΔL by lowering the upper electrode. The program is distinguished by a short period of crushing (τ=2 seconds) with a final distance L then a long period of relaxation (τ=20 seconds) with a return to the initial state L, everything making it possible to simulate a “touch-sensitive” type action on the film (FIG. 8).

[0148] In the crushing period, a normal force F.sub.N (in newtons) is recorded. This force F.sub.N is proportional to the crushing thickness (ΔL). The program is arranged as such: the film is compressed to a distance L=L.sub.0−ΔL with ΔL=(3×10%.Math.L.sub.0, then (3×) 20%.Math.L.sub.0, then (3×) 25%.Math.L.sub.0 etc. An example of a program used is given in FIG. 9.

[0149] Lastly, the upper and lower electrodes are connected to a converter/amplifier, in order to record the potential difference (denoted E) generated between the two electrodes throughout the program.

[0150] 2. Study of the Films of Microgels

[0151] Films of P(MEO.sub.2MA-Co-OEGMA-Co-MAA) Microgels

[0152] The films of P(MEO.sub.2MA-co-OEGMA-co-MAA) biocompatible microgels were characterized by varying 3 parameters:

[0153] 1.) The effect of the multiplication of the compressions: for each film, a compression of the same force is repeated successively (3 times as represented in FIG. 9) and the potential of each compression is analyzed.

[0154] 2.) Film thickness: two film thicknesses were tested in order to determine the impact of the thickness on the ability of the films to generate an electric potential at the interface (˜200 μm and ˜900-1000 μm).

[0155] 3.) Composition of carboxylic acid functions: the composition of MAA units was varied from 0 to 3.5 mol % of MAA (microgels 1 and 2) in order to evaluate the impact of the MAA units on the electric potential.

[0156] 4.) The pH of the solution in which the films are swollen: the pH makes it possible to vary the amount of ionic functions (COO.sup.−) within the microgel. Specifically, the carboxylic acid functions are present in: the form of two protonated (COOH) and deprotonated or ionized (COO.sup.−) species. The proportion of these two species depends on the pH of the solution with an increase in the ionized species COO.sup.− when the pH is increased (pH 5.5.fwdarw.% COO.sup.−=0; pH 6.5.fwdarw.% COO.sup.−=50%; pH 7.5.fwdarw.% COO.sup.−=75%).

TABLE-US-00001 TABLE 2 Summary of the samples studied. Thickness MAA γ-Fe.sub.2O.sub.3 of swollen Sample (mol %) (wt %) film (μm) pH E.sub.max (mV) F.sub.N (N) Effect of the film thickness Microgel 2 3.5 0 180 6.5  5.4-11.1 0.38-0.40 Microgel 2 3.5 0 850 6.5 2.0-5.6 0.30-0.35 Effect of the MAA composition Microgel 1 0 0 275 6.5 0.43-0.9  0.30-0.35 Microgel 2 3.5 0 180 6.5  5.4-11.1 0.38-0.40 Effect of the pH Microgel 2 3.5 0 310 5.5 2.0-9.9 0.7-0.9 Microgel 2 3.5 0 180 6.5  5.4-11.1 0.38-0.40 Microgel 2 3.5 0 350 7.5  5.5-11.2 0.18-0.23 Effect of γ-Fe.sub.2O.sub.3 Microgel 2 3.5 0 180 6.5  5.4-11.1 0.38-0.40 Microgel 2 3.5 4.7 310 6.5  8.7-12.2 0.65-1.13 Microgel 2 3.5 9.1 320 6.5 2.4-3.7 1.0-1.2 Microgel 2 3.5 16 180 6.5 3.7-3.8 3.0-3.1

[0157] Results:

[0158] By observing the change in the electric signal during the compression of the films of microgels, an electromechanical effect is demonstrated over all of the films characterized. This electromechanical effect is a function of the force F.sub.N exerted on the films with an electric potential that increases with the compression force. Furthermore, a trend seems to emerge as a function of the analysis parameters: [0159] Effect of the repetition of the compressions: the electric potential recorded is very high during the first compression. Whilst during the 2.sup.nd and 3.sup.rd compression, the potential generated is lower. This first observation may be due to a significant movement of the ions in the first compression of the film creating a high instantaneous electric potential (˜12 mV). After a relaxation time of 20 s, the following compressions of the same force do not appear sufficient to bring about this same movement of the ions with a generated potential that decreases. [0160] Effect of the film thickness: the electric potential generated by the compression of the films of different thicknesses shows a weak electromechanical effect when the film is too thick (E=2−5.6 mV for F.sub.N,max=0.35 N). Conversely, a greater electromechanical effect is seen when the film thickness is small ranging from 11 to 5 mV for forces F.sub.N of 0.38 to 0.4 N. Too large a thickness would not therefore make it possible to have a sufficient impact on the mobility of the ions. (cf. Table 2. Effect of the film thickness) [0161] Composition of carboxylic acid functions: the presence of methacrylic acid appears to improve the electromechanical properties of the films. Specifically, the films appear more sensitive to the compression effects with an electric potential measured for weak forces (11−5 mV at 0.4 N with MAA versus 1−0.5 mV at 0.4 N without MAA). Furthermore, the measurement made at pH 6.5 highlights the importance of the ionized carboxylate groups derived from the MAA units (50% of ionized MAA group) on the sensitivity of the films of microgels. (cf. Table 2. Effect of the M44 composition) [0162] The pH of the solution from 5.5 to 7.5 on the films of microgels of the same composition does not appear to modify the electric potential value of the films but when the pH of the solution is increased, the loss of the electric potential in the face of the compression repetitions appears to be reduced. Specifically, at pH 5.5, the repetition of the compression makes the electric potential drop to 2 mV whereas at a higher pH, the potential drops to 5.4 mV. This is probably due to the increase in the proportion of the ionized carboxylate functions (% COO.sub.pH 5.5=0%; % COO.sub.pH 6.5=50%; % COO.sub.pH 7.5=75%), increasing the polarization capacity of the microgels that form the film. The films are then more sensitive when the pH is increased. (cf. Table 2. Effect of the pH)

[0163] Films of P(MEO.sub.2MA-Co-OEGMA-Co-MAA)/γ-Fe.sub.2O.sub.3 Hybrid Microgels

[0164] The films of hybrid microgels were characterized at pH 7.5 and were compared to a film of microgels without magnetic nanoparticles (NPs). A film of microgels without nanoparticles has a maximum potential of 6 mV for a compression force F.sub.N=0.4 N. For the films of microgels with nanoparticles, the potential generated depends on the amount of nanoparticles incorporated: [0165] For ˜5 wt % of magnetic nanoparticles incorporated, the nanoparticles have no effect on the electric potential generated and the response of the film to the compressions is 6-7 mV. [0166] For 9 and 17 wt % of magnetic nanoparticles incorporated, a reduction of the electric potential, which reaches 2.5 mV irrespective of the compression force, is observed. This loss of electric potential may be attributed to the reduction of charges derived from the carboxylic acid units at pH 7.5 since they already interact with the NPs. Specifically, the incorporation of the nanoparticles takes place by adsorption of the latter at the ionic sites (COO.sup.−) contained in the microgels. This adsorption appears to reduce the fraction of ionic sites still available within the microgel and thus to reduce the polarization capacity of the microgels. An amount of 5 wt % of nanoparticles incorporated does not influence the polyelectrolyte behavior of the hybrid films (cf. Table 2. Effect of γ-Fe.sub.2O.sub.3).

[0167] Optical Properties of the Microgels

[0168] Besides the electromechanical properties, the films of microgels are distinguished by their optical properties, linked to the ability of these films to diffract light. A disparity is observed as a function of the composition of the films:

[0169] Films of P(MEO.sub.2MA-Co-OEGMA-Co-MAA) Microgels

[0170] During the drying of a colloidal dispersion of microgels without nanoparticles, the films formed are transparent in the dry state and iridescent in the wet state (FIG. 10). It appears that during the swelling of the microgels, the diameter and the distance between the particles favor a diffraction of light in the visible region, this diffraction is demonstrated by the observation of photonic crystals.

[0171] Films of P(MEO.sub.2MA-Co-OEGMA-Co-MAA)/γ-Fe.sub.2O.sub.3 Hybrid Microgels

[0172] During the drying of a colloidal dispersion of microgels, a film that is transparent and colored in the dry state is obtained which has iridescent properties in reflection at very small viewing angles. The material is then brown (color due to the magnetic nanoparticles) when viewed at 90° and iridescent when viewed at smaller angles (FIG. 11).

[0173] The photonic properties are visible in particular in: reflection (on dark background) and very little in transmission (on light background) as seen in FIG. 12.

[0174] Mechanical and Magnetic Properties of the Hybrid Films

[0175] Besides the film formation properties of the hybrid microgels and their optical properties, the addition of magnetic nanoparticles also makes it possible to orient the microgels during the drying. FIG. 13 clearly illustrates these properties since it is possible to concentrate the microgels during the drying at a precise point by application of a magnetic field (in our case the magnet was placed underneath the dispersion). The drying makes it possible, on the one hand, to set everything at a targeted point and, on the other hand, to modify the tint of the final film via a localized concentration of the hybrid microgels while retaining the iridescent properties in reflection (Solution dried with magnet, FIG. 13).