GB2204553A - Microencapsulation - Google Patents

Abstract

Microcapsules of a polymeric material comprising gelatin and the polysaccharide XM6 obtainable from the bacterium NCIB 11870 can be prepared by complex coacervation of these polymers over a wide range of pH and concentration.

Description

MI CROENCAPSULATION Background of the invention 1. Field of the invention This invention relates to microencapsulation, that is to say the formation of capsules consisting of a polymeric envelope enclosing a solid or liquid "core".

2. Description of the prior art Gelatin is commonly used in microencapsulation. It is obtainable in an acid-extracted or an alkali-extracted form. In a common type of microencapsulation process, the acid-extracted gelatin is contacted with a second polymer such as gum arabic or pectin or gellan gum and the material to be encapsulated, under conditions in which the gelatin acquires an overall positive charge and the second polymer an overall negative charge, whereby "complex coacervation" takes place.That is, a dispersion is formed from an aqueous solution containing the polymers and an oily or solid phase comprising the core material to be encapsulated, the polymeric mixture solidifies around the core in a thin layer or skin at the interface of the phases and the aqueous phase can then be removed to allow the polymer to harden, an operation aided by first cross-linking the gelatin with glutaraldehyde. The process of solidification of the polymeric mixture is assisted by forming the dispersion at above the gelling temperature of gelatin and also, if it is a gelling agent, of the negatively charged polymeric component, adjusting the pH and then allowing the dispersion to cool to below the gelling temperature.

The process of complex coacervation is a delicate one. In order to make it reasonably easy to carry out repeatedly, it is best to avoid use of plant polysaccharides such as pectin or gum arabic. Variations in their structure or properties between batches can cause the conditions proposed for their use in microencapsulation not to be reliable. Further, the process of complex coacervation requires control of polymer concentration, pH and temperature within critical limits, to prevent precipitation of the polymer as an extensive mass or-flocculate of polymer solids. It has been a problem to extend the range of one or more of these parameters so as to make the process more reliable to carry out.

According to a brochure "CD-31" issued by the Kelco division of Merck Inc., it is believed in February 1983, gellan gum, a bacterial polysaccharide comprising glucuronic acid, rhamnose and glucose units, forms microcapsules by coacervation with gelatin over a wide pH range but no details are given of the range.

Gel lay gum, being a bacterial polysaccharide, is not subject to the above-mentioned objection to gum arabic and pectin. However, it has a pronounced tendency to self-gelation at the higher concentrations of gellan gum in aqueous solution.

Summary of the invention It has now been found that acid-extracted gelatin and a bacterial polysaccharide known as "XM6" form microcapsules by complex coacervation over a wide pH range, which is generally from 3.0 to 4.5 and a wide range of concentrations, including low concentrations of XM6. The XM6 polysaccharide, described in U.S.

Patent 4,638,059 or the corresponding European Patent 134649 (NRDC), is obtainable from the already publicly accessible deposit NCIB 11870 (NCIB, Aberdeen, Scotland).

XM6 is itself a gelling agent, showing particularly good thermoreversible gelling properties when in association with calcium or other cations of the same group of the Periodic Table. For the present purposes, self-gelation of XM6 is positively undesired and it is therefore used in association with an alkali metal or quaternary ammonium cation, which reduces this tendency. Because XM6 undergoes self-gelation it is rather a surprising choice as the partner for gelatin in a complex coacervation process and it was particularly unexpected to find that it can be used with such ease.

Accordingly, an important aspect of the present invention provides microcapsules having a core enclosed within a polymeric capsule wherein the polymer comprises gelatin and the polysaccharide XM6 obtainable from the bacterium NCIB 61870.

The invention also includes . a process of preparing microcapsules or a slurry or suspension thereof, which comprises forming aqueous solutions of acid-extracted gelatin and the polysaccharide XM6 obtainable from the bacterium NCIB 11870, when in the form in which it is associated with alkali metal or quaternary ammonium cations, together with a core material to be encapsulated, at a temperature above the gelation temperature of the gelatin and at a pH of from 95X upwards of the pI (isolonic point) of the acid-extracted gelatin, typically above 7, reducing the pH to within a range which allows complex coacervation, typically 3.0 to 4.5, allowing the dispersion to cool to a suspension of microcapsules at a temperature below the gelation temperature of the gelatin, and, if desired, recovering the microcapsules or a slurry thereof from the suspension.

Also within. the scope of the invention are the products of the above process, including suspensions and slurries of microcapsules, and a kit for making microcapsules comprising the said gelatin, XM6 and core components, each in separate containers.

Brief description of the drawing The drawing is a graph showing concentrations of gelatin (y-axis) and XM6 (x-axis) which will provide the coacervation required for microencapsulation.

Description of the preferred embodiments The polymeric mixture of gelatin and XM6 can be regarded as a coacervate of these two polymers, the gelatin being initially In acid-extracted form and the XM6 initially associated with alkali metal or quaternary ammonium cations.

The gelatin can be any which is acid-extracted, preferably having an isolonic point between 7 and 10. Examples of such gelatins can be found in the literature, for example in "The Science and Technology of Gelatin" edited by A.G.Ward and A. Courts, Academic Press, 1977, page 87, Acid-extracted gelatin products have different "bloom strengths": bloom strength is relatable to molecular weight.

The XM6 can be obtained from NCIB 11870 itself or from an XM6-producing mutant or variant thereof. It is ordinarily produced in a mixed salt form containing cations comprising calcium, magnesium, sodium and potassium cations, whence it can be exchanged into the hydrogen-ion form and then the hydrogen ions re-exchanged with cations of an alkali metal, preferably sodium or potassium, or quaternary ammonium cations NR1R2R3R4 + where each of R1 to R4 is hydrogen or an organic group preferably alkyl, aryl or aralkyl. It is easily possible to prepare the XM6 in, say, sodium form without any problem of self-gelation. This is demonstrated by tests reported in Example 1 hereinafter.

The core material to be used can be liquid or solid, but it must be dispersible in the aqueous solutions of gelatin and XM6 used. Thus when the core material is liquid it should be oily or hydrophobic. Indeed, the basic process of microencapsulation is inapplicable to liquids which dissolve in water. It can be a single liquid, mixture of liquids or a solution of a solid in a liquid, for example. Typically (but not exclusively), the core material will be a substance to be ingested orally, for example a drug or vitamin supplement, or a material used in food manufacture. The invention is however applicable in any field in which microencapsulation is useful.

The immediate objective in combining the two polymers and the core is to obtain a dispersion of the core in the aqueous phase.

The term "dispersion" is used herein to include an emulsion of liquid phase core and a suspension of solid phase core. Such a dispersion can be achieved by mixing the three components approximately simultaneously or by mixing the core with the gelatin to form a two-component dispersion and later mixing this with the XM6. Naturally, to form the dispersion, the gelatin and XM6 must be above their gelation temperatures, at which they set to form a gel. The gelation temperature of XM6 depends on the nature and concentration of associated cations and is usually in the range 30 to 45 C. The greater the concentration of XM6 or of the associated cations, up to a -limit, the higher the gelation temperature. The gelation temperature of gelatin varies according to its molecular weight and the precise conditions of gel formation, but is usually in the range 18-35 C.In normal practice, the dispersion of gelatin and XM6 solutions and core material should be heated to a temperature well above the melting temperature of gelatin, in order to ensure solubility of the gelatin in the aqueous phase. Since the XM6 in the specified salt form has less tendency to gel, the same criterion does not normally arise in relation to it. Usually a temperature of 50 to 65 C, most preferably about 60 C will be suitable, but a higher temperature can be used, subject only to the requirement that it must not cause degradation of the gelatin, XM6 or core material.

The initial pH of the gelatin and XM6 solutions and therefore also of the said dispersion can be any which is of the order of or exceeds the pI of the gelatin, usually above 7 and generally in the range about 7.5 to 11, the upper figure being the maximum pH which the XM6 will tolerate without being degraded.

Preferably the pH is from 8 to 10 and most preferably 8.5 to 9.5.

The ratio of gelatin to XM6 and their concentrations in aqueous solution can be any which provide complex coacervation without flocculation of the polymer or solidification of the entire aqueous phase. Guidance can be obtained from the accompanying drawing in which weight percentage concentrations of gelatin and the sodium salt of XM6 in the dispersion at 60 C are shown on the y- and x- axis respectively on a logarithmic scale.

They are based on the solutions when mixed. Thus 4% gelatin in the initial solution thereof, before it is mixed with an equal volume of XM6 solution is represented as 2X gelatin in the drawing. The gelatin and XM6 concentrations normally usable are indicated by the area enclosed by a chain dotted line (1)- while the smaller, hatched area enclosed by a broken line (2) ------------, , is that which normally gives superior results, and the most preferred concentrations are those in the circular region (3) of 15 gelatin, 0.1% XM6 shown by the solid line.The area bounded by (1) encompasses a range of gelatin concentrations of from 0.01 to 5X. At low concentrations of gelatin, say less than 0.5r, the concentration of XM6 has to be above about 0.4X.

Above about 1.0% of XM6 no coacervation occurs, the medium being completely liquid for gelatin concentrations below 0.01%, and flocculated for gelatin concentration above 0.01Z. At high concentrations of gelatin (0.5it or higher) the concentration of XM6 required to avoid formation of flocs, is relatively wide, viz from about 0.01 up to a value in the 0.8 to 1 percent region.

While the graph relates to sodium salt XM6, in the absence of significant added electrolyte, it is not expected that with other monovalent ions or other ionic concentrations thereof, the graph will vary greatly. The skilled man will in any case easily be able to determine the precise limits applicable to his own chosen conditions.

When the dispersion has been formed, the pH is reduced by adding any convenient and compatible acid, for example hydrochloric acid. The pH can be reduced to any level at which the gelatin acquires an overall positive charge and the XM6 an overall negative charge, whereby, at appropriate concentrations of these polymers as above-described, complex coacervation takes place. Generally stated, this pH is in the 3.0 to 4.5 range.

Preferably, the pH is reduced to about 4. At this stage the dispersion becomes turbid (goes pearly) and undergoes a decrease in viscosity, signalling the onset of coacervation. The dispersion is then cooled. It is ordinarily convenient to cool the dispersion or, as it can now be called, suspension of capsules in the aqueous liquid, to room temperature - say below 30"C and preferably below 25"C, but bearing in mind that it should be cooled to below the gelation temperature of the gelatin used.

Although the suspension of microcapsules is a useful product in its own right at this stage and is included as such in the invention, it is preferable to cross-link or fix the polymeric coat of the capsules for ease of handling the suspension. Any of the known cross-linking agents interactive with amino groups of proteins can be used, of which glutaraldehyde is preferred.

The suspension of capsules can be stored for a ònsiderable tlme (a time of about a month having been specifically tested They could be sold in this form, but it is more sensible from. a handling viewpoint to dehydrate the suspension, either to produce a more concentrated suspension (a slurry) or dries microcapsules. In either case it is best to remove water fros the polymeric coatings by use of an organic solvent which replaces the aqueous liquid and the solvent decanted and/or evaporated off. The capsules can finally be dried by gentile heating of the suspension to yield a free-flowing powder.

The following Examples illustrate the invention.

EXAMPLE 1 The polysaccharide XM6 was prepared as described by M.t O8Neill et al., Carbohydrate Research 148, 63-69 (1986). A logllitre solution of freeze-dried XM6 was pumped down a columns containing 50 cm.3 of "Dowex (H) 50W-X8", a hydrogen cation-exchange resin, in order to convert the XM6 from the natural salt form in which it was prepared to the hydrogen form!.

The eluate was then neutralised with a small amount of 0.lem sodium hydroxide to pH7, thus putting the polymer into a sodium salt form, and the solution was freeze dried.

The natural salt form of XM6 obtained as described in U.S.

Patent 4,638,059 contains bivalent cations which enhance its.

ability to gel. Thus an 0.2X solution of natural salt XM6 gels upon adding 509 mM sodium chloride solution. Preparation of the sodium salt inhibits gelation. Thus, when 3M aqueous sodium chloride solution was added to a 1X solution of the above-prepared sodium salt of XM6 at room temperature, the XM6 dld not gel. When O.5M aqueous sodium chloride solution was added to a 2X solution of sodium salt XM6 at room temperature, a weak gel was formed, indicating that the threshold of gelation had only just been reached.

A solution of acid-extracted pig-skin gelatin of Bloom strength 175, obtained from Sigma Chemical Co. as- type 2625 (29/100 ml) and a solution of XM6 prepared as described above (0.2g/100 ml) were prepared. Each solution was heated separately to 60"C and its pH adjusted to 9.0 with O.lM sodium hydroxide.

Equal volumes of the gelatin and XM6 solutions and of sunflower oil to which a red dye had been added, as core material, were mixed at 60"C with a magnetic stirrer, producing an emulsion.

This temperature is above the gelation temperature of both gelatin and XM6. The pH of the emulsion, still at 60"C, was adjusted with O.lM hydrochloric acid to 4.0. As the pH was reduced the emulsion became pearly in appearance and its viscosity decreased at the same time. The emulsion was then allowed to cool. On cooling to room temperature a suspension of microencapsulated droplets of sunflower oil of size in the range 1 to 400m was obtained. The microcapsules were viewed under the microscope, the red dye in the sunflower oil making them readily visible. They were seen to be coated with polymer.

The microcapsules can be isolated by washing the suspension with water, adding from 2.5 to 6% w/v glutaraldehyde as a fixing agent, again washing, dehydrating the suspension with 70% isopropanol and concentrating the suspension to a slurry, by decantation, and if desired drying the slurry.

EXAMPLE 2 Example 1 was repeated using colourless paraffin as the core material, with similar formation of microcapsules.

EXAMPLE 3 Example 1 was repeated using aluminium powder in the proportion of lg powder/ml of gelatin or XM6 solution, with similar formation of microcapsules. The suspension, cross-linked and then dehydrated with isopropanol, can be dried until a free-flowing powder is obtained.

Claims (14)

1. Microcapsules having a core enclosed within a polymeric capsule wherein the polymer comprises gelatin and the polysaccharide XM6, obtainable from the bacterium NCIB 11870.

2. Microcapsules according to claim 1 wherein the polymer is a coacervate of acid-extracted gelatin and an alkali metal or quaternary ammonium salt of the polysaccharide XM6, obtainable from the bacterium NCIB 11870.

3. A process of preparing microcapsules or a slurry or suspension thereof, which comprises forming a dispersion of a mixture of aqueous solutions of acid-extracted gelatin and the polysaccharide XM6 obtainable from the bacterium NCIB 11870, when in the form in which it is associated with alkali metal or quaternary ammonium cations, together with a core material to be encapsulated, at a temperature above the gelation temperature of the gelatin and at a pH above 955 of the isoionic point of the gelatin, reducing the pH to within a range which allows complex coacervation, allowing the dispersion to cool to a suspension of microcapsules at a temperature below the gelation temperature of the gelatin and XM6, and, if desired recovering the microcapsules or a slurry thereof from the suspension.

4. A process according to claim 3 wherein the microcapsules or a slurry thereof are recovered by adding a fixing agent for the gelatin to the suspension, washing it and dehydrating it.

5. A process according to claim 3 or 4 wherein the XM6 is initially in the form of a sodium salt.

6. A process according to claim 3, 4 or 5 wherein the pH is reduced to within the range 3.0 to 4*5.

7. A process according to any one of claims 3 to 6 wherein the ratio of gelatin to XM6 is within a range enclosed by chain dotted line (1) in the accompanying drawing.

8. A process according to claim 7 wherein the said ratio is within the area enclosed by the broken line (2).

9. A process according to claim 8 wherein the said ratio is within the area enclosed by the solid line (3).

10. A process according to any one of claims 3 to 9 wherein the dispersion is formed by mixing the gelatin and XM6 solutions and; the core material substantially simultaneously.

11. A process according to any one of claims 3 to 9 wherein a first dispersion is formed by mixing the gelatin solution with the core and a second dispersion by mixing the first dispersion with the XM6 solution, all at a temperature above the gelling temperature of gelatin.

12. A process according to any one of claims 3 to 11 wherein the pH at which the solutions and core material are mixed is from 7.5 to 9.

13. A process of preparing microcapsules or a slurry or suspension thereof, from gelatin and the polysaccharide XME substantially as hereinbefore described with reference to the accompanying drawing and/or Example 1, 2 or 3.

14. Microcapsuies or a slurry or suspension thereof when prepared by a process claimed in any one of claims 3 to 13.