WO1997002580A1

WO1997002580A1 - Magnetic field sensitive polymer gels and uses thereof

Info

Publication number: WO1997002580A1
Application number: PCT/HU1996/000036
Authority: WO
Inventors: Miklós ZRINYI; János GÁCS; Csabáné SIMON
Original assignee: Zrinyi Miklos; Gacs Janos; Simon Csabane
Priority date: 1995-07-06
Filing date: 1996-07-05
Publication date: 1997-01-23
Also published as: HU9502059D0; AU6366896A; HU213215B

Abstract

The present invention relates to novel magnetic field sensitive polymer gels and uses thereof. These gels comprise 1-90 % of a cross-linked macromolecule, 0.1-50 % of magnetic particles of colloidal size enclosed by the cross-linked network and a swelling agent in an amount corresponding to the balance up to 100 %. The gels of the invention can be used as parts of actuator controlled delivery systems, sensors, separators, seals, pistons, valves, artificial muscles and power transmission systems.

Description

MAGNETIC FIELD SENSITIVE POLYMER GELS AND USES THEREOF

Background of the Invention The present invention relates to novel magnetic field sensitive polymer gels called "ferrogels" and uses thereof. A ferrogel is a cross-linked polymer network swollen by a ferrofluid. When a magnetic field sensitive gel according to the present invention is placed into a spatially non-uniform magnetic field, the shape of the gel is controlled by the balance of magnetic and elastic interactions. A ferrogel can be bent, elongated and contracted repeatedly many times by magnetic moduiation. The response time of this new type of stimuli-responsive gels is less than a second, independently of the size.

Polymer gels are at least two-component systems having high elasticity. The amount of liquid immobilized into a polymer network strongly depends on the structure of the polymer which is cross-linked by physical or chemical interactions. Due to its cross-linked structure, the equilibrium volume of the gel can be altered by changing such environmental factors as pH, ionic strength, temperature, the presence of complex-forming compounds and composition of the surrounding liquid mixture. A small alteration in the above mentioned factors may result in significant change in the volume of the gel - due to the nature of the equilibrium - in reversible manner that can be of extensive technological applicability. The magnetic field sensitive gels, firstly prepared by the present inventors, represent a novel type of environmentally sensitive gels. In the presence of non-uniform magnetic field, these gels can significantly change their shape. Said change can be elongation, contraction, rotation, bending or a combination of these basic distortions. Polymer gels represent one class of intelligent materials in the state of the art. Their capability of being in thermodynamic equilibrium with their environment and the fact that their equilibrium state is in direct correlation with their volume makes it possible to change the volume of the gel by altering certain external (environmental) parameters. This possibility of changing the volume by applying an environmental effect gives the technical basis of the practical use of these materials.

Gels are known in the art the volume of which can be changed by altering temperature, solvent composition, pH, ionic strength, electric field, illumination, mechanical tension or by applying a specific chemical reaction (D. DeRossi, K. Kajiwara, Y. Osada, A. Yamauchi: "Polimer Gels Fundamentals and Biomedical Applications", Plenum Press, New York and London, 1991 ; K. Dusek (Ed): "Responsive Gels: Volume Transition I", Advances in Polymer Science Vol. 109 Springer-Verlag, Berlin, Heidelberg, 1993). There is no gel known in the art the volume or mechanical state of which could be changed by applying a magnetic field. It should be noted here that some gel systems containing colloidal size magnetic particles are known in the art like magnetic ceramics (Y. N. Jun, D. M. Dabbs, I. A. Aksay, S. Erramilli: "Processing of Monolithic Magnetic Gels for Magnetophoresis", Langmuir, 1994) and monodisperse magnetic latexes (J. Ugelstand, A. Berge, T. Ellingsen, J. Bjorgum, R. Schmid. P. Stenstad, O. Aune, T. N. Nilsen, S. Funderud, K. Nustad: "Future Directions in Polymer Colloids" NATO ASI Series, Series E. Applied Sciences, Martiuus Nijhoff Publ, 1987), nevertheless magnetic field cannot exert its effect via altering the shape or mechanical state of the material with respect to the magnetic gel systems known so far.

While responsive gels known in the art can only display a certain predetermined change of their shape as a consequence of a change in their volume, magnetic field sensitive gels developed by the present inventors can be elongated, contracted and bent, depending on the inhomogeneity of the magnetic field applied. There is a very significant difference between the response time of environmentally sensitive gels developed so far and that of the novel magnetic field sensitive gels of the present invention. The response time of traditional adaptive gels is substantially determined by the speed of mass transport processes via the swelling and shrinking kinetics, whereas the response time of magnetic field sensitive gels is determined by the speed of the fast magnetic polarization. This means that for traditional responsive gels the response time is proportional to the square of the characteristic linear size of the gel (that in turn determines the mechanical resistance) which is a major drawback regarding the practical applicability of the environmentally sensitive gels (so called "intelligent" gels) known in the art. In case of the present magnetic field sensitive gels, however, the response time is independent of the dimensions of the gel.

Brief Description of the Present Invention The magnetic field sensitive gel of the invention is an elastic material enclosing a considerable amount of liquid, and exhibiting strong super¬ paramagnetic (or ferromagnetic) properties. The amount of the enclosed swelling agent, as well as the elastic properties of the gel can be varied in a wide range by varying the degree of cross-linking. The magnetic properties are due to colloidal particles built in the gel that can not be removed from the gels via diffusion, sedimentation or other physical mechanisms.

The magnetic properties of the gels according to the present invention are mainly controlled by the initial susceptibility and saturation magnetization, as well as the size and concentration of the particles built in the gel. The shape and/or mechanical state of the gels can be changed by applying an extemal and also an intemal static or dynamic magnetic field. When applying a dynamically modulated magnetic field, the shape of the magnetic field sensitive gels of the invention keep changing that makes continuous transformation of the shape of these gels possible by applying an appropriate magnetic field. Description ofthe drawings The attached Fig. 1 and 2 demonstrates changes caused by extemal magnetic fields in the shape of magnetic field sensitive gels of the invention Fig. 1 shows the bending of a magnetic field sensitive gel by applying a magnetic field created by a permanent magnet. The gel seen on Fig IA is about 15 cm far from the magnet. When bringing the magnet closer, the axis of the gel becomes parallel to the edge of the circular magnet (Fig. IB). Fig. 2 shows the elongation and contraction of magnetic field sensitive gels caused by inhomogeneous magnetic fields. Fig. 2 A shows the gel without applying an extemal magnetic field. Fig. 2B, C and D show the changes in the shape of the gel caused by static magnetic fields created by an electromagnet. A significant elongation can be seen on Fig. 2B, while Fig. 2C and D show contractions. The present invention thus relates to magnetic field sensitive gels comprising 1 -90 % of a cross-linked macromolecule, 0J-50 % of magnetic particles of colloidal size enclosed by the cross-linked network and a swelling agent in an amount corresponding to the balance up to 100 %.

According to a preferred embodiment, the gels of the invention comprise, as the macromolecular ingredient, a polymer of synthetic or natural origin (a biopolymer) or a mixture thereof cross-linked by physical and/or chemical interactions.

According to another preferred embodiment, the gels of the invention comprise, as the magnetic particles, ferro-, ferri-, para- and superparamagnetic particles having an average diameter of 5 nm - 100 μm or the mixture of said particles.

According to another preferred embodiment, the gels of the invention comprise, as the swelling agent, water, aqueous solutions, organic solvents or mixtures thereof. According to a further preferred embodiment of the invention gels are provided having a) if prepared by chemical or radiochemical cross-linking, a cross-linking density varying between IO^"4 and 0.5, said ratio being defined as the ratio of moles of monomer units per moles of cross-linking agent applied when preparing the gel, and b) if prepared by physical gelation, an elastic modulus varying between IO^"3 and IO³ kPa.

The present invention further relates to the use of magnetic field sensitive gels according to the invention as parts of actuator controlled delivery systems, sensors, separators, seals, pistons, valves, artificial muscles and power transmission systems.

Detailed description of the invention Preparation of magnetic field sensitive gels:

For preparing magnetic field sensitive gels one has to solve the following tasks simultaneously:

- preparation of a cross-linked polymer network,

- preparation of colloidal particles having the required magnetic properties,

- stabilization of the sol (or suspension) of the magnetic particles,

- incoφoration of the colloidal magnetic particles into the polymer network. Preparation of the magnetic field sensitive gels of the invention does not require a special type of polymer or special magnetic particles. As a polymer network, one may use any organic and inorganic material, from which a spatial network structure can be prepared by physical and/or chemical cross-linking. Thus cross-linking polymerization of monomers or cross-linking of polymer chains or macromolecules by chemical bonds or by specific interactions [like H- bridge, electrostatic interactions and (micro)crystal formation] can be used for the preparation of the magnetic field sensitive gels of the invention. The elasticity of the gels depends on the flexibility of the chains, the degree of cross-linking and the concentration of the magnetic particles enclosed. The swelling agent applied can be of aqueous or organic origin, depending on the chemical stmcture of the polymer network.

There are several ways of preparing super-, para- or ferromagnetic particles and enclosing them in the gel. The particles can be prepared before, during and after the gelation process takes place. One can e.g. prepare and stabilize the magnetic sol first, and the stabilized sol can then be added to the solution to be gelled. When preparing a magnetic field sensitive gel according to the present invention, an important task to be solved is the stabilization of the sol of the magnetic particles. This can be achieved by the adsoφtion of certain ions on the surface of the particles, by applying surfactants, neutral or charged homo- and copolymers, poiyelectrolytes or by the polymer network itself As magnetic colloidal particles can be prepared and stabilized in both aqueous and organic media, it is possible to prepare magnetic field sensitive hydrogels and organogels as well. By altering the parameters of the cross-linking reaction (e.g. the concentration of the network forming material and the measure of cross-linking), the physical, physico-chemical, colloidal and magnetic properties of the gels can be widely varied. The magnetic properties of the gels prepared are highly dependent on the quality, concentration and size of the enclosed colloidal magnetic particles.

The magnetic properties of the gel are greatly influenced by the - saturation magnetization

- size and

- concentration of the colloidal particles, as well as the strength of the extemal magnetic field. The initial susceptibility at low field strength:

, ₌ JL^₀ _ _ (1)

18 kβT

where φ represents the volume quotient of the particles, μ = 4πl0^" Hm^"1 is the permeability of vacuum, M_d represents the magnetization of the solid particle, d represents the diameter of the colloidal particle, k_B represents the Boltzman constant, T represents the thermodynamic temperature. The saturation magnetization:

where H represents the strength of the magnetic field.

It can be seen from the above equations that by the appropriate selection of the supeφaramagnetic material (M_d) and by changing the size (d) and concentration (φ) of the particles both initial and saturation magnetization can be varied within a wide range.

The elastic behavior of the gel is determined mostly by the elastic properties of the cross-linked polymer network, the measure of swelling of the gel and the concentration of the enclosed colloidal size material. Elasticity modulus characterizing the mechanical properties of the gel can be given by the following equation:

where Grj represents the modulus of the cross-linked polymer without swelling agent and enclosed material which depends on the measure of cross-linking, and q represents the measure of swelling ofthe gel.

The following examples serve to further illustrate the invention, nevertheless the scope of the invention is no way limited to the specific embodiments actually presented in the examples.

Example 1 Preparation of magnetic field sensitive polyvinyl alcohol (PVA) hydrogel (I)

Starting materials:

Polyvinyl alcohol 7200 (Merck, Darmstadt, Germany),

25 % (w/v) aqueous solution of glutaric aldehyd (Merck, No. 820603), FeCl₂x4H₂0, pro anal. (Reanal, Budapest, Hungary), FeCl₃x6H₂0, pro anal. (Reanal, Budapest, Hungary), KOH, pro anal. (Reanal, Budapest, Hungary),

70 % (w/w) aqueous solution of HC10₄, pro anal. (Janpharm Laborchemie, Apolda, Germany),

37 % (w/w) aqueous solution of HCl, pro anal. (Reanal, Budapest, Hungary).

The preparation method is a two step process. In the first step the super¬ paramagnetic Fe₃0₄ sol is to be prepared then, in the following step, gelation is carried out. 100 g FeCl₂x4H₂0 is dissolved in 200 ml H₂0, 200 g FeCl₃x6H₂O is dissolved in 300 ml H₂0 and 360 g KOH is dissolved in 400 ml H₂0.

The FeCl₂ and FeCl₃ solutions are then mixed, whereafter the mixture is filtered. From the filtrate 200 μl is dropwise added to 200 ml H₂O with continuous mixing and the KOH solution is then added to the reaction mixture. The remaining FeCl₂-FeCl₃ solution is then added to the reaction mixture with continuous mixing. The suspension is then decanted and washed with distilled water several times. The pH of the system is then adjusted to 2 by applying HCl and the mixture is poured into a vessel of suitable geometry (e.g. tube or capillary). After the completion of gelation (ca. 20 hours), gels are removed from the reaction vessels. The prepared gels have significant magnetic character that can directly be sensed by applying a permanent or electromagnet.

Example 2 Preparation of magnetic field sensitive polyvinyl alcohol hydrogel (II)

The gel is prepared according to the recipe set forth in Example 1 , with the exception that the Fe₃0₄ sol is not prepared in advance but in the 8 % (w/w) PVA solution. Gelation is performed as in Example 1.

Example 3 Preparation of polyvinyl acetate gel swelled in acetone

In the first step, a PVA gel is prepared as described in Example 1 or 2. The gel is then placed into a mixture of acetic anhydride and pyridine (v/v = 1/1) so as to replace the aqueous medium. After repeating the replacement of the medium the gel is incubated at 90 °C for 8 hours. The medium is replaced with a fresh mixture of acetic anhydride and pyridine (v/v = 1/1) in every other hour. By the end of the 8 hour incubation the chemical reaction transforms PVA to polyvinyl acetate (PVAc) without damaging the cross-linked polymer network or the magnetic particles. Then the system is cooled to room temperature and the medium within the gel is entirely replaced with acetone in several steps. The medium of the so prepared magnetic field sensitive gel can be replaced with any solvent miscible with acetone and polyvinyl acetate. Example 4

Preparation of magnetic field sensitive polymethacrylic acid gel

50 ml of 20 % (w/w) glyceric solution of magnetic sol according to Example 1 or other supeφaramagnetic sol is prepared and mixed with 50 ml of 30 % (w/w) glyceric solution of methacrylic acid. To the mixture, 0J 5 g of α- cC-azo-bis-isobutyronitrile initiator is added. The mixture is homogenized and poured into a vessel of suitable geometry. The vessel containing the gelation mixture is then placed into a water bath of 80 °C. After the gelation took place (1-2 hours) the gel is removed from the vessel. A magnetic field sensitive polymethacrylic acid gel comprising glyceric medium is prepared this way, the medium of which can be replaced with other medium miscible with glycerol and polymethacrylic acid.

Claims

Claims:

1. Magnetic field sensitive gel comprising 1-90 % of a cross-linked macromolecule, OJ-50 % of magnetic particles of colloidal size enclosed by the cross-linked network and a swelling agent in an amount corresponding to the balance up to 100 %.

2. Gel according to claim 1 comprising, as the macromolecular ingredient, a polymer of synthetic or natural origin (a biopolymer) or a mixture thereof cross¬ linked by physical and/or chemical interactions.

3. Gel according to claims 1 or 2 comprising, as the magnetic particles, ferro-, ferri-, para- and supeφaramagnetic particles having an average diameter of 5 nm - 100 μm or the mixture of said particles.

4. Gel according to any of claims 1 to 3 comprising, as the swelling agent, water, aqueous solutions, organic solvents or mixtures thereof. 5. Gel according to any of claims 1 to 4 having a) if prepared by chemical or radiochemical cross-linking, a cross-linking density varying between 10^" and 0.

5 said density being defined as the ratio of moles of monomer units per moles of cross-linking agent applied when preparing the gel, and b) if prepared by physical gelation, an elastic modulus varying between IO^"3 and l0³ kPa.

6. Use of magnetic field sensitive gels according to any of claims 1 to 5 as parts of actuator controlled delivery systems, sensors, separators, seals, pistons, valves, artificial muscles and power transmission systems.