COMPOSITIONS COMPRISING POLYSACCHARIDE GUMS

The present invention relates to the use of polysaccharide gums in adhesives, especially in denture adhesives. It further relates to modified forms of naturally occurring polysaccharide gums, including ghatti, karaya and kerensis gums, which are especially useful in this respect, either alone or blended with unmodified gums. The present invention further relates to the methods of preparation of such modified gums, and specifically to the control of hydrogel content of the modified gums, which is key for their performance in the aforementioned adhesive compositions. There are several known naturally occurring polysaccharide gums, of which gum ghatti, gum karaya and gum kerensis are examples.

Gum ghatti, also known as Indian gum, is the dried exudate of Anogeissus latifolia, a large tree found abundantly in the dry deciduous forests of India. Gum ghatti was originally developed around 1900 as a substitute for gum arabic. However, several studies have demonstrated that the raw form of the gum is not currently suitable for certain applications due to the variation in solubility and viscosity of the raw material. Consequently, gum ghatti has never been established as major tree gum for industrial use.

The viscosity of natural gum ghatti has been shown to vary from 30 to 40OcP for 5% solutions. Collected nodules are not entirely soluble in cold water; however, breaking up and heating the gum at 90°C has been shown to improve the solubility. Although largely water soluble, individual samples may contain varying proportions of insoluble gel but are not chemically different to the soluble component.

The unpredictable physical and chemical properties of gum ghatti have prevented its commercial application. Natural gum exudates, including gum ghatti, have adhesive qualities which are dependent on their natural viscoelasticity. These adhesive qualities can be variable, due to the unpredictability of the molecular properties of the gum. This is common for natural exudates whose properties often dependent on the geography, climate and soil origin of the tree from which the gum is obtained. One of the problems associated with gum ghatti is the variation in viscosity from batch to batch. This variation is often due to the difference in insoluble hydrogel content.

As noted, this variation is to be expected for natural material and has been shown with, for example, gum arabic, whose properties and molecular parameters are dependent upon the age and location of the acacia trees from which it is derived. This variation also leads to unpredictable functional applicability. Gum karaya is obtained from Sterculia urens R and other Sterculia spp.

(Sterculiaceae, Malvales) and has been used as a denture adhesive/fixative agent in its neat (powder) form. Similarly to gum ghatti, gum karaya exhibits a natural variation of the proportion of soluble and insoluble fraction depending on the location of the tree or season when the gum is extracted. Gum kerensis is a gum exudate obtained from acacia tree (otherwise known as A.senegal war. karensis.) by tapping.

Hydrogels are three-dimensional cross-linked network structures in a polymeric system which are insoluble in water but which are able to absorb many times their weight of water. Hydrogels also tend to respond to the surrounding environment and to stimuli such as for example temperature, pH and the presence of electrolytes. The procedure of producing hydrogel (gel) often involves chemical reagents to cross-link the polymer chains to produce the three dimensional network structures. Hydrogels have been developed from many synthetic or natural polymers (hydrocolloids) as well as mixtures of the two. Due to their high water absorption capacity, hydrogels have been used in wound dressing, drug delivery, agriculture, sanitary pads as well as transdermal systems, denture materials, implants, injectable polymeric systems, ophthalmic applications and hybrid-type organs (encapsulated living cells) . Hyaluronan and alginate are among the most frequently utilised natural polysaccharides (hydrocolloids) hydrogels. Cellulose and its derivatives (carboxymethyl cellulose, hydroxypropyl methyl cellulose) also play an important role in the realisation of systems for the controlled release of drugs.

It is, therefore, important to be able to prepare hydrogels with controlled hydrodynamic properties, swelling, water absorption and hydration rate.

It is therefore the aim of the present invention to eliminate the natural variability that exists in samples of naturally occurring polysaccharide gums and produce products with reproducible viscoelasticity.

Surprisingly it has been found that thermal modification of naturally occurring gums, in the solid state, leads to an unexpected improvement in their hydrodynamic properties, their ability to form stable hydrogels and their consequent performance in adhesive materials, with much greater reliability and consistency than had previously been the case with unmodified materials.

The present invention concerns the use of a range of natural gums which contain proportions of soluble and insoluble fractions which can be used alone or blends thereof in a denture adhesive, wherein the gum may comprise alone or as part of a blend of one or more modified forms of gum that can be produced by heating natural gum in the dry solid state under specific conditions of temperature and atmospheric pressure to control the ratio of insoluble to soluble fraction. The modified gums have physical properties that are suitable for denture adhesive and which are not always solely present in the naturally occurring gum; in particular, the modified gum products have higher viscoelastic properties, increased swelling capacities and slow hydration compared with the naturally occurring gums. Such modification of the natural materials using this defined process allows the material to be adapted to fit the requirements of an effective denture adhesive, i.e. viscoelastic, an elastic dominance over a wide frequency range and a high dynamic viscosity.

According to a first aspect of the present invention, there is provided an denture adhesive comprising a polysaccharide gum, wherein the average insoluble hydrogel content of the gum is >10%. Preferably the polysaccharide gum is, or is derived from, one or more of natural karaya, ghatti or kerensis gums, most preferably ghatti gum.

According to a further aspect of the present invention there is provided a modified form of a polysaccharide gum for use in a denture adhesive according to the first aspect of the present invention, characterised in that it comprises a gel fraction of > 10 wt% (based on the total weight of the gum) and has a hydration value of < 90 wt% (based on the total weight of the gum).

Preferably the gel fraction of the modified gum is > 10 wt%, more preferably > 20 wt%, most preferably > 30 wt%, especially > 80 wt%. Preferably the hydration value is < 90 wt%, more preferably < 60 wt%, most preferably < 40 wt%, especially < 20 wt%.

An especially preferred modified gum has a gel fraction of between 10 and 80 wt%, and a hydration value of between 90 and 10 wt %.

Gel fraction and hydration value are each measured by the experimental technique set out in the Examples section below. Hydration value is defined in Table 1 as the solubility or the % of the soluble fraction (based on the total weight of the gum).

Preferably the gum modified according to the first aspect of the present invention is obtained after heating a sample of the gum in a dry solid state at 110 ⁰ C and atmospheric pressure for at least 12 hours, preferably at least 24 hours, more preferably at least 3 days, further preferably at least 4 days, most preferably at least 6 days, especially at least 7 days. Heating time can be greatly reduced if the material is heated in spray dried form or changing the ambient conditions to a controlled pressure and humidity.

The gel content and hydration value of the gum(s) will influence their ability to be dissolved in water or other solvent systems, and will determine the properties of such solutions or suspensions, in particular the viscosity of such solutions or suspensions. In a preferred embodiment of the first aspect of the present invention, there is provided a modified or naturally occurring polysaccharide gum which, when suspended or dissolved as a 10wt% dispersion in water, has a dynamic viscosity of at least 10 Pa. s at an oscillation frequency of 0.1 Hz. In a preferred embodiment, the adhesive according to the first aspect of the present invention may comprise a single gum or a blend of two or more gums. The two or more gums may be from the same natural source (e.g. both ghatti gums) or from different sources (e.g. ghatti and karaya gums) and may also be a mixture of naturally occurring gums of natural and modified gums. Preferably the adhesive comprises a blend of at least two gums wherein the total hydrogel content is in the range of 10 - 80% and the molecular weight of the soluble fraction is > 1.0 million.

Preferably the denture adhesive according to the first aspect of the invention is in the form of an ointment, cream or gel.

Preferably the denture adhesive according to the first aspect of the invention comprises one or more adjuvants selected from but not restricted to oils, waxes, gums, further adhesive substances, fragrances and dyes.

In a preferred embodiment, the denture adhesive composition according to this aspect of the invention comprises:

• 0.1 - 60 wt% of at least one modified polysaccharide gum according to the first aspect of the invention;

• 0.1 - 90 wt% of a base for the cream, ointment or gel; and,

• 0.1 - 50 wt% of at least one further adhesive substance based on the overall composition.

Preferably the combined amounts in wt% of the at least one modified polysaccharide gum and the at least one further adhesive substance is less than 60 wt%, based on the overall composition.

Preferably the oil/wax is selected from the group consisting of paraffin oil, sunflower oil, soy oil, petroleum jelly (Vaseline), glycerine, sorbitol, soya bean, vegetable wax (which may include blends of oils and waxes such as, but not limited to, castor seed oil, hydrogenated castor oil and carnuba wax) and hydrophobic oleogels; more preferably the oil/wax is selected from petrolatum, sunflower oil or paraffin oil or mixtures thereof. Preferably, the denture adhesive according to this aspect of the invention may further comprise one or more additional active adhesive substances, selected from the group consisting of sodium carboxymethylcellulose, sodium alginate, copolymer salts of methyl vinyl ether/maleic anhydride, polyvinyl acetate, polyvinyl pyrrolidone, hydroxyethyl cellulose, polyoxymethylene, polyacrylamides, polyethylene oxides and mixtures thereof.

Preferably the denture adhesive according to this aspect of the invention comprises an additional active adhesive substance which is a derivative of cellulose. Preferred is carboxymethylcellulose (CMC).

Further preferred as additional active adhesive substances are copolymer salts of methyl vinyl ether/maleic anhydride. In one particularly preferred embodiment of the

present invention, a mixture of sodium carboxymethylcellulose and copolymer salts of methyl vinyl ether/maleic anhydride are added as additional adhesive substances.

It is preferred that the denture adhesive according to this aspect of the present invention has a viscosity of between 5 and 50 Pa. s at 10s ^"1 , measured using a cone and plate Rheometer at 25°C and atmospheric pressure; and, a tube extrusion force of <16kg, measured using a texture analyser at 25°C and atmospheric pressure.

In a further aspect of the present invention, there is provided a process for the manufacture of a modified form of a gum for use in the denture adhesive of the first aspect of the present invention which comprises the steps of heating the natural gum, in the solid state, at atmospheric pressure at a temperature of between 100 to 130 ⁰ C for at least 12 hours, more preferably 1 day, further preferably 3 days, most preferably 7 days, especially 14 days. The preferred heating time will depend on the ambient conditions and on the method used to produce the initial unmodified solid gum.

According to a yet further aspect of the present invention there is also provided a method of manufacture of a denture adhesive according to a aspect of the invention, comprising the steps of:

• weigh the oil/gel phase components

• add the solid components

• mix until homogeneous. Preferably the weighing of the oil/gel phase components is carried out at a temperature above ambient temperature, more preferably at 70 ⁰ C or above, and the resultant mixture is allowed to cool before the solid phase components are added, preferably to below 50 ⁰ C, more preferably to below 40 °C.

Examples

The invention will now be further described by way of the following experimental data with reference to the accompanying drawings in which:

Figure 1 illustrates the dynamic viscosity values for control and modified (processed) gum ghatti samples according to the present invention as a function of oscillation

frequency. The recorded measurements were performed at 25°C on 10% w/w (based on loss on drying) in water containing 0.005% sodium azide (NaN ₃ )

Figure 2 illustrates the viscoelastic parameter, G' (open symbols) and G" (closed symbols), of 10% w/w control and modified ghatti dispersion as a function of oscillation frequency.

Figure 3 illustrates the % gel fraction and % solubility/hydration values as a function of processing time at 110 ⁰ C in days.

Figure 4 illustrates a series of ghatti gum dispersions (15% w/w) for control and selected processed samples from Table 1 for which images were taken 30 minutes after inverting a universal container into which the samples were places.

Figure 5 illustrates a graph of dynamic viscosity values (n') plotted as a function of oscillation frequency for karaya gum, a control sample of ghatti gum (lot 40138) and modified ghatti gum according to the present invention (heated for 168 hours at 110°C).

Experimental - gum modification (1 ) Initial modification studies

Commercial ghatti gum (Gums & Colloids India, regular gum ghatti Lot 40138) was used. Dust, bark and particulate matter associated with the gum nodules were removed manually from the samples and the nodules were kibbled (1-2mm). Controlled heating of the kibbled material was carried out at 110 ⁰ C, at atmospheric pressure, for various times using a Catterson Smith convection oven. Heating the gum in the dry state results in increasing the weight average molecular weight and viscoelasticity. Processing of the gum using the standard conditions outlined above was found to also change the hydration capacity and increase the gel content of the gum. The results are provided in Table 1 and each parameter described in turn together with the methodology used to determine the characteristics of the new materials.

Table 1. Values obtained for the % loss on drying, gel fraction as a % and the solubility of treated gum ghatti (Lot No.40138) as a function of time.

Experimental - gum properties

A) Percentage loss on drying

The percentage loss on drying of the samples of gum was determined as follows: around 1.0g of each sample was weighed in duplicate into pre-weighed evaporation dishes and placed in an oven SANYO convection oven MOV-212F at 105°C, for 16 hours. After heating, the samples were taken out of the oven and cooled in a desiccator.

The respective weights of each sample were then recorded. The dry matter was calculated from the loss of weight after heating.

It was observed that processing for just 1 day resulted in drastically reducing the percentage (%) loss on drying from 12 - 1 %. Further processing for up to eight weeks was found to not greatly change the % loss on drying.

B) Changes in viscoelastic parameters

The viscoelastic parameters namely the storage modulus (G'), viscous modulus (G") and dynamic viscosity of a control sample and selected samples from Table 1 were determined using known controlled stress rheometer conditions at 10% w/w dispersions (based on dry solid content).

The changes recorded for the dynamic viscosity, storage modulus (G') and viscous modulus (G") are given in Figures 1 and 2 respectively. The results provided in Figure 1 show an increase in viscosity with increasing processing time. The results illustrate that gum ghatti can be produced with increased viscosity values and an increase to 200 Pa. s can be achieved from the starting value of around 0.1 Pa. s of control ghatti with 2% insoluble (gel) materials.

The control sample showed typical diluted solution behaviour where G" is greater than G' over the entire range of the frequency studied (Figure 2). The modified samples of gum ghatti had increased G' and G" and showed concentrated solution behaviour where there is a cross over of G' and G" for samples after processing for 24 and 48 hours. Gel-like systems can also be produced where G' and G" are almost independent of oscillation frequency over the entire range (Figure 2).

The results given above demonstrate that the viscosity and viscoelasticity of gum ghatti can be controlled by careful selection of the processing time.

C) Variation in solubility and gel content

The ghatti samples, listed in Table 1 , were subjected to the following procedure: A 70 mm glass fibre paper (Fisher Scientific, 0.7 μm pore size) was dried in an oven (SANYO convection oven MOV-212F) at 105 ⁰ C for 1 hour, transferred to a desiccator containing silica gel, left to cool and weighed (W1 ). About 0.1 g of the test material (S) was weighed in a plastic universal container (30 ml capacity) and 20 ml of distilled water was added and left to hydrate overnight on a roller mixer. The solution was filtered through the glass fibre paper under vacuum. Water, 10-2OmI was then used to rinse the universal container and remove any residual insoluble material. The filter paper was dried at 105 ⁰ C for 1 hour (or longer if a result showed that there was still water present), and transferred to a desiccator containing silica gel, left to cool and then the paper

weighed (W2). The total insoluble (gel fraction) was calculated as follows wherein S is the weight of the test material:

Total insoluble (%) = (W2-W1 ) Formula (1 ) Sx100

It is to be noted that the materials retained on the filter appeared to be swollen particles of gum of dimension greater than 0.7μm and are referred here to as gel.

The results given in Table 1 are also shown in Figure 3, demonstrate that the % gel fraction and the degree of solubility can be controlled. The maximum gel % is achieved after processing for 21 days and further processing for up to 8 weeks does not greatly increase the maximum yield. However processing for up to 8 weeks can result in an 80% gel fraction.

A further demonstration of the increase in the viscoelasticity and adhesion of the modified gum ghatti samples is given in Figure 4. Figure 4 shows 15% w/w (based on dry weight) dispersions of selected samples listed in Table 1 contained in a universal container. Following overnight hydration, the containers were inverted and all image taken after 30 minutes. The images showed that processed gum ghatti samples (3days and 7 days) do not flow and adhere increasingly to the surface of the container. The sample control showed the best flowability, and followed by the processed samples subjected to increasing processing time. With increasing processing time flowability decreased and adhesion increased.

D) Changes in water binding (swelling) capacity Each sample of modified gum ghatti (0.1 gm (based on solid dry content) was weighed and 0.5ml of water added. The water was immediately absorbed by the modified gum sample (21 days) whereas the water associated with the control sample remained fluid. After 30 seconds, the water was completely retained by the processed gum sample and it was possible to invert the weighing boat without losing any material.

The same procedure was repeated but adding 1ml instead of 0.5ml. After 2 minutes, it was possible to invert the modified sample with very little flow of sample. After another 30 seconds the modified sample did not flow at all.

The water associated with the control sample during this time showed large water droplets and did not show any water binding capacity but rather dissolved the gum with time. The results are shown in Table 1 , and demonstrate that the solubility and water absorption can be controlled.

It is therefore demonstrated above that the water solubility and absorption of the water by the gum can be changed as desired.

E) Producing physical properties in gum qhatti which allow specific characteristics of other hvdrocolloids to be duplicated

Gum ghatti in the insoluble (gel) new material form according to a first aspect of the present invention can duplicate the functionality of other hydrocolloids, notably gum karaya, which has a wide range applicability in for example drug delivery and adhesion.

Gum karaya, due to its high viscoelasticity has applications as a denture adhesive. The same characteristics of gum karaya have therefore been targeted in the present invention which give this gum its excellent water absorption capability and enable its use in food products such as mayonnaise and salad dressing, and in medical and pharmaceutical applications such as an adhesive agent for colostomy bags.

For the matching controlled experiments we have used a Karaya gum sample supplied by the Gums and Colloids Group (India) as white cleaned nodules. A sample was prepared by grinding into a powder and the proportion of insoluble (gel) fraction was determined using the method already described above. The insoluble (gel) proportion was 81%, which can be duplicated by the ghatti product obtained after modification (see

Table 1 ).

The evidence of success of the present invention in duplicating the viscoelastic properties of gum karaya is given in Figure 5. This shows a plot of the dynamic viscosity of karaya gum at 30% compared with control ghatti at 30% concentration and the new ghatti gum produced by our invention at 10% w/w concentration. The results show that

the new gum ghatti can match these characteristics of karaya and can do this at a much lower concentration. This invention similarly allows the modified gum ghatti to match the viscoelasticity characteristics of many other hydrocolloids used in pharmaceutical and cosmetics applications. Modifying ghatti results in higher dynamic viscosity measurements at lower concentrations compared to standard ghatti measured at higher concentrations. These characteristics of the modified ghatti are also comparable to karaya at higher concentrations.

F) Adhesion to surface A series of experiments was undertaken to demonstrate the stronger adhesion characteristics of the modified gum ghatti produced by according to the present invention.

The gum produced after heating up to 72 hours and preferably up to 168 hours at 110 ⁰ C in the solid state and preferentially for periods up to 8 weeks at 110 ⁰ C was compared with a natural gum ghatti exudate which had not been subjected to the process described in this invention.

The modified sample of gum ghatti was found to be have stronger adhesion for up to 72 hours processing, and preferably up to 168 hours and more preferably up to 8 weeks processing when compared to natural gum ghatti exudate on the following surfaces: paper to paper adhesion; tackiness and sticking to skin and membrane of the human lip/mouth; adhesion to glass surfaces which stick the gum such that it will not fall when inverted and shaken; and, adhesion to plastic surfaces so that it adheres and will not fall off when inverted and shaken.

2) Extended studies

Further gum samples For more extended studies of the behaviour of the modified gums in compositions such as denture adhesives, 5 gums were selected, along with two blends, as detailed in Table

2 along with the physical properties of these gums.

Denture adhesive formulation(s)

Denture adhesive examples 1 to 7 were made using the gums detailed in Table 2, as shown in Table 3

Table 3

These denture adhesives were then tested for performance in comparison with two commercial products, Commercial product 1 and Commercial product 2, and the following properties were measured:

• Dynamic Vapour Sorption (DVS) - this is a measurement of the uptake/loss of mass of a sample depending on time and relative humidity at specific temperatures, and was measured here at 25 ⁰ C on a sample of mass 10 mg, with a measurement cycle comprising the steps of drying the sample at humidity 0%, increasing the relative humidity to 98% for 3 hours, measure the uptake of moisture and subsequent change in mass as a function of time (dm/dt), then dry the sample again at humidity 0%. The results are plotted as a graph, from which the area under the curve of dm/dt vs time is measured.

• 3 minute hold, assessed using a Texture analyser by measuring the separation force exerted by a gel between a circular PMMA top plate and a bottom plate after a 3 minute contact time, wherein the sample comprises 0.25g test material + 1.5 gm of deionised water or artificial saliva, and the plates are of diameter 6.5 cm.

• Long term hold measurement of the separation force of initially unhydrated denture fixative using a texture analyser with a circular PMMA plate of diameter 6.5 cm over a moistened semi-permeable membrane forming part of an artificial mouth or over soft rubber. Force separation measurements are taken at 15 second intervals over a 90 minute period as the fixative hydrates and the assessment is made qualitatively in comparison with standard commercial products .

• Dynamic viscosity at 10 s ^"1 (measured according to industry standard using a controlled stress rheometer).

• G ¹ and G" values (measured as detailed above using an oscillation frequency sweep within LVR region using a controlled stress rheometer).

From the results in Tables 4 and 5 below it can be seen that samples incorporating the modified gums singly or as blends showed at least comparable performance as denture adhesives to the commercial products tested.

Table 4

Without wishing to be bound by theory, it is believed that there is a direct link between the strength of the adhesive (as assessed by the 3 minutes hold and the long term hold), the DVS area, elastic modulus G ¹ , viscous modulus G", dynamic viscosity and viscosity. The modified gums present serve to increase the long term hold of the adhesive by imparting the correct properties of elasticity and viscosity on the mixture through slow hydration, and that this is only possible through being able to carefully control the properties of the gums through the presently claimed modification. The gums used here also surprisingly interact in a synergistic manner with other components, such as further adhesive substances (e.g. cellulose based products such as carboxymethyl-cellulose) and bases for creams, gels and ointments such as paraffin oil or petrolatum.

Table 5