WO2000001373A1

WO2000001373A1 - Materials and methods relating to encapsulation

Info

Publication number: WO2000001373A1
Application number: PCT/GB1999/002159
Authority: WO
Inventors: Artur Bartkowiak; David Hunkeler
Original assignee: Ecole Polytechnique Federale De Lausanne; Kiddle, Simon
Priority date: 1998-07-06
Filing date: 1999-07-06
Publication date: 2000-01-13
Also published as: EP1094792A1; GB9814619D0; AU4636699A

Abstract

A method for encapsulating a core material within a capsule having a permeable or semipermeable membrane is disclosed using a complex formation reaction between oppositely charged polymers, wherein one of the polymers is an oligosaccharide such as chitosan and the reaction is carried out at a pH between about 6.6. and 7.5. The method overcomes the poor solubility of such high molecular weight polysaccharides at neutral and alkaline pH, providing capsules having useful strength and permeability.

Description

Materials and Methods Relating to Encapsulation

Field of the Invention

The present invention relates to materials and methods relating to encapsulation, and in particular to encapsulating biological materials in capsules by complex formation between oppositely charged polymers.

Background of the Invention The encapsulation of materials in polymeric microcapsules having permeable or semipermeable membranes surrounding a liquid core is well known in the art. A wide variety of different approaches, based on different polymer chemistry, different processes for membrane formation and different encapsulation technologies have been tried.

An important application of these techniques is in the encapsulation of biologically active species. In general, these techniques have the potential for the treatment of diseases requiring enzyme or endocrine replacement as well as in nutrient delivery via the encapsulation of enzymes and bacteria. Encapsulation is currently employed in the food, agriculture and biotechnology and biomedical industries. Examples of these and potential applications are the encapsulation of islets for the treatment of diabetes mellitus, the use of encapsulated bioartificial organs targeted at treating neurodegenerative disorders such as Parkinson's disease, Alzheimer's disease and Huntington' s chorea, and in the control of chronic pain and the administration of human growth factors .

The immunoisolation of biologically active materials has been the subject of recent reviews, see illaert & Baron (1996) and Jen et al (1996) . The technology of encapsulation (Colton, 1996) , the inflammatory response of transplanted tissues (Morris, 1996) , the modification of polymer surfaces to enhance biocompatibility (Hubbell, 1994) , the use of polyacrylates for encapsulation (Sefton & Stevenson, 1993; Stevenson & Sefton, 1993), the use of water-soluble polymers for immunoisolation (Hunkeler, 1998) , a review of polymers for bioartificial organs (Hunkeler, 1997) and an overview of tissue engineering (Baldwin & Saltzman, 1996) have also been examined by researchers in this field.

One approach for forming capsules is based on complex formation between oppositely charged, high molecular weight polymers. In this method, a solution of a first polymer including a material to be encapsulated is preformed in droplets between about 0.2 -5.0mm in diameter and contacted with the second oppositely charged polymer. Typically, droplets of the first polymer and the material to be encapsulated are generated using a capillary or spraying device and contacted with the second polymer, e.g. by falling into a precipitation bath. The reaction between the polymers at the surface of the droplet forms a membrane around a liquid core including the encapsulated material . These polyelectrolyte complexes (PECs) have been studies due to their potential application as microcapsules for medical implants. A variety of approaches, based on various polymer chemistries, processes for membrane formation and encapsulation technologies have been evaluated. However, over the past two decades the overwhelming majority of scientists have restricted their studies on the alginate/poly-L-lysine polyelectrolyte complex system, where solid alginate/calcium beads are coated with a solution of oppositely charged poly-L-lysine (PLL) and subsequently converted into a permeable capsule by liquefying the ionotropically gelled anionic polysaccharide .

GB 2 135 954 A (Dautzenberg et al) describes a method of forming capsules from a pair of oppositely charged polyelectrolytes or a single polyelectrolyte and low molecular weight organic counter ions. In this method, preformed particles of a polyelectrolyte are contacted with a second polyelectrolyte or organic counter ions so that a membrane forms around the droplet thereby producing a capsule. The examples in this reference employ high molecular weight polyelectrolytes such as cellulose sulphate, carboxymethylcellulose or alginate. The lowest molecular weight polyelectrolyte disclosed is polydiallylmethylammonium chloride (40kD) , from which capsules were formed with sodium cellulose sulphate at pH 7.0.

However, the reaction described in this application requires high levels of the organic counter ionic material to provide the necessary the mass transfer driving force to form a dense membrane skin. Further, the use of the organic counter ions suffers from the disadvantage that these materials can be toxic, the method does not provide a satisfactory trade-off between the permeability and the mechanical strength of the capsules, and the reaction times are exceedingly long (tens of minutes) .

EP 0 152 898 A (MIT) describes the use of oppositely charged polymers solutions to encapsulate biological materials, exemplifying the use of chitosan and alginate. This method uses high molecular weight polymers and consequently is carried out below pH 6.6 to ensure that these materials are soluble. Further, it relies on polyvalent metal cations (e.g. Ca²⁺ ions) present in the cationic polymer solution to gel or harden the anionic polymer during capsule formation.

US Patent No:4,808,707 (University of Delaware) discloses a method of forming capsules using chitosan and alginate, employing a pH of 6.1 or less and esterifying a proportion of the alginate in an attempt control the permeability of the membrane around the capsules. The reaction was also carried out in the presence of polyvalent metal ions (Ca²⁺) , as with EP 0 152 898 A to promote gelling of beads during capsule formation.

US Patent No: 5, 462, 866 (Vanderbilt University) describes a method for producing uniform capsules by forming capsules by individually enveloping droplets of polyanion solution with a collapsing annular sheet of polycation solution while the sheet is travelling downward at the same velocity as the droplets. This approach was employed as high molecular weight polyelectrolytes form viscous solutions leading to polyanion droplets distorting on impact with the surface of the polycation solution, and form irregular and highly distorted capsules. By way of example, the patent discloses that the problem arises as a 0.2% solution of high molecular weight chitosan (>100kD) is about 10 times as viscous as a calcium chloride solution. Example 4 in this patent mentions "a 0.2% citosan [sic] solution with an approximate molecular weight of 20,000 daltons" . However, in view of the whole thrust of the patent towards dealing with high viscosity polymer solutions, and as a 0.2% solution of 20kD chitosan would have a viscosity similar to water, the chitosan used in this example is a high molecular weight polymer, probably having a molecular weight of 200kD.

GB 2 145 992 A (Damon Biotech Inc) discloses a method for encapsulating cells by cross-linking droplets of chitosan by exposing the droplets to anionic materials such as polyaspartic acid and polyglutamic acid. In common with other prior art references, the chitosan has a high molecular weight and requires acidic pH to form solutions in water.

US Patent No: 5, 089,272 (Shioya et al) relates to a method of producing capsules using ionic strength to adjust the permeability of the capsule. In the examples, capsules are formed from a polyanionic polysaccharide and chitosan obtained from natural source materials having a molecular weight of 2800kD and 1600kD.

It remains a problem in the art in finding methods for controlling the balance between the permeability of the capsules and their mechanical strength. There is also a problem in the art in finding reactants and reactions conditions which are compatible with biological materials, in particular as many of the biological materials that might be usefully encapsulated, for example mammalian cells, require neutral or slightly alkaline pH to remain biologically viable. Summary of the Invention

Broadly, the present invention relates to methods of producing capsules from oppositely charged ionic polymer solutions, wherein one of the polymers is oligomeric. Surprisingly, the use of an oligomeric polymer provides microcapsules having good mechanical properties and allows capsules to be formed at neutral or alkaline pH. Thus, the use of oligomers in conjunction with higher molecular weight polyelectrolytes can help to ameliorate some of the problem associated with the prior art methods discussed above.

Accordingly, in one aspect, the present invention provides a method for encapsulating a core material within a capsule having a permeable or semipermeable membrane using a complex formation reaction between oppositely charged polymers, the method comprising: forming a droplet from a solution of an inner polymer comprising at least one positively or negatively charged polymer and the core material; and, contacting the droplet with a solution of an outer polymer comprising at least one polymer having an opposite charge to the polymer or polymers forming the droplet so that the polymers react to form a membrane around the droplet, wherein one of the polymers is an oligosaccharide with a molecular weight (MW) of less than 10. OkD and the reaction is carried out at a pH between about 6.6 and 7.5.

While the prior art has employed polysaccharides such chitosan as encapsulating polymers, these materials are only soluble at acidic pH, typically below pH 6.5. The poor solubility of such polysaccharides at neutral and alkaline pH means that the prior art does not provide any teaching to overcome the problem of producing polymer solutions which are sufficiently concentrated to allow capsule formation under physiological conditions (e.g. pH 7.0/0.9% NaCl) .

The present inventors surprisingly found that there is a significant correlation between the solubility of polysaccharides such as chitosan and their molecular weight, and that the use of an oligomeric polymer can both improve the solubility of the polymer at physiologically relevant pH and provide capsules having good mechanical properties. Thus, in a preferred embodiment, the present invention provides capsules having useful strength and permeability at physiologically appropriate pH by employing chitosan having a molecular weight of less than 20. OkD and more preferably, less than 10. OkD.

Conveniently, the oligosaccharide is produced from a commercially available natural source materials such as chitosan (modified chitin) . While these materials are readily available, in general they have relatively high molecular weights and may require pretreatment to reduce their molecular weight so that they can then be used in the above method. Conveniently, the pretreatment is carried out using radical degradation to break down the polysaccharide so that it has a molecular weight below 10. OkD and is soluble at the pH of the encapsulation reaction, e.g. using methods such as radical degradation with peroxide, hydrolytic-enzymatic degradation, γ-ray irradiation or acidic hydrolysis. Pretreating the polysaccharide in a controlled reaction has the further advantage that it is degraded in a controlled way, providing a consistent material and a degree of quality control required in many biomedical applications. In preferred embodiments, the oligosaccharide has a degree of deacetylation >90%, and more preferably >95% and/or a polydispersity in MM less than 2.0, more preferably less than 1.8, and still more preferably less than about 1.5.

The possibility of producing capsules using under neutral and slightly basic conditions is advantageous as prior art approaches, e.g. those employing the chitosan/alginate polymer system, use high molecular weight polymers and so typically need to employ pHs below 6.5 in order to ensure that the polymers remain soluble and do not precipitate. This feature of the method is particularly useful as the encapsulation of some biologically active materials, including mammalian cells, typically employ physiological conditions with a pH between 6.8 to 7.4, e.g. pH 7.0/0.9% NaCl . Thus, the method has the advantage that it can be carried out under physiologically compatible conditions that promote maintenance of biological activity.

In encapsulation reactions employing carrageenans , monovalent metal cations induce gelation through the creation of double helix aggregation structures. These sulfonated natural occurring polysaccharides form two types of gels: "strong" gels in the presence of K⁺ and Rb⁺, and "weak" gels in the presence of Li⁺ and Na⁺, with the gel structure and stability being strongly dependent on the temperature. Therefore, the properties of the gel can be modulated by type of the metal cation and temperature .

However, generally speaking, the method does not require the use of high temperatures or polyvalent metal ions to promote gelling of the polymer solution and/or the use of organic counter ions in a precipitation bath. Commonly in the literature, multivalent ions such as calcium, barium or iron are used to gel the polyelectrolyte. These beads are then coated with a solution of an oppositely charged polyelectrolyte. The solid bead is then converted into a permeable capsule by liquefying the gel, generally via the addition of a chelating agent such as EDTA or sodium citrate. As noted above, these materials can be toxic and have a detrimental effect when present in methods for encapsulating biological materials. Further, the method is preferably a single step reaction, in contrast to many of the prior art multi-step processes.

Preferably, the polymers are naturally occurring polymers to promote biocompatibility with encapsulated biological materials. The use of oligosaccharides or polysaccharides is especially preferred. The polymers can be negatively charged polymers such as alginate, i - carrageenan or κ-carrageenan or carboxymethylcellulose. Chitosan is a preferred positively charged polymer.

Preferably, the oligosaccharide has a molecular weight less than 20. OkD, more preferably less than 10. OkD, and more preferably between about 0.5 and about 9. OkD, more preferably between about 0.6 and about 6. OkD and most preferably between about 1.0 and about 3. OkD. Typically, the other polymer has a molecular weight in the range 100-lOOOkD, more preferably in the range 100-500kD. In preferred embodiments, the oligomeric polymer is positively charged, e.g. using the combination of oligochitosan and alginate.

In some embodiments, the polymer solutions may include low molecular weight monovalent metal salts (e.g. Na⁺ or K⁺) , or agents to promote isotonicity (e.g. mannitol) or buffer solutions. In the encapsulation of biological materials (cells, bacteria etc) , these and other additives can be included to help to maintain the osmotic pressure between the cells and polymer solutions in equilibrium.

By way of example, suitable anionic polymers for capsule formation include alginate, carboxymethylcellulose, xanthan, hyaluronic acid, gellan gum, cellulose sulphate, carrageenans (kappa- or iota-) and polyacrylic acid. Preferred cationic polymers include chitosan, glycol chitosan, chitosan derivatives, polyallylamines, quaternised polyamines, polydiallyldimethyl ammonium chloride (polyDADMAC) , polyDADMAC-acrylamide, polytrimethylammoethylacrylate-co-acrylamide, polymethylene-co-guanidine, polyvinylamine .

Conveniently, the capsules can be produced by dropping one of the polymer solutions into a bath of the other. Preferably, the concentration of the polymers is in the range 0.1 to 5.0%, more preferably in the range 0.5 to 2.0%. The droplets can be formed using passing one of the polymers through a capillary, by extrusion with gas or a liquid, using a spinning disk or by electrostatic generation. Preferably, the droplets are formed from the higher molecular weight polymer, with the oligomeric polymer being the outer polymer.

Preferably, the capsules produced using the method have a diameter between 50μm and 5.0mm. The diameter of the capsules can be controlled by varying the flow rate of the inner polymer solution or by controlling physical parameters such as the applied voltage, if electrostatic droplet generation is used, or the fluid stripping velocity, if the capsules are made by air/fluid stripping.

The membrane around the capsules is typically 5 to 400/xm thick, and more preferably 50 to 200μm thick. Preferably, the membrane is permeable or semi-permeable . The encapsulation method can also allow the molecular weight cut off of the membrane to be controlled between about 1 to about 300kD by selecting the polyelectrolyte, oligomer or monovalent salt concentration, pH, temperature or reaction time.

Conveniently, the core material can be encapsulated by mixing it with the polymer solution which is formed into droplets. The method is particularly useful for encapsulating biological or biologically active materials, including cells, bacteria, enzymes, antibodies, drugs, cytokines or hormones, as it can be carried out biologically compatible pH, preferably at a pH greater than about 6.5, more preferably at a pH between about 6.6 and 7.5, more preferably at a pH between about 6.8 and 7.4 and more preferably at a pH between about 7.0 and 7.4, and more preferably at a pH between about 7.2 and 7.4. In a further aspect, the present invention provides a method which additionally comprises the steps of: selecting a physical property required of a capsule; selecting the molecular weight of an oligosaccharide between 0.5 and 20. OkD that will provide capsules having the physical property when reacted with a core material and an oppositely charged polymer; reacting the polymers and core material as described above to produce the capsules; wherein the physical property is deformability and/or mechanical strength.

In this embodiment of the invention, preferably the capsules have one or more of : (a) a high deformability of >70%, more preferably

>75%, and most preferably >80%, as determined in the experimental section below;

(b) good mechanical strength, as represented by a bursting force >400mN, more preferably >500mN, and most preferably >1500mN, as determined in the experimental section below.

In a further aspect, the present invention provides a method which comprises following the encapsulation of a core material as described above, the further step of formulating the capsules in a composition or other product .

In a further aspect, the present invention provides a polymeric capsule comprising a semipermeable membrane encapsulating a core material produced by the above method. In a further aspect, the present invention provides a polymeric capsule comprising a semipermeable membrane encapsulating a core material produced by the above method for use in a method of medical treatment .

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings .

Brief Description of the Figures

Figure 1: Schematic representation of capsule formation via polyelectrolyte complexation. A: droplet generation apparatus, B: contacting of oppositely charged polymers, C: capsule with membrane.

Figure 2 : Variation in the mechanical properties of the chitosan-alginate capsules with changes in the molecular weight of chitosan.

Figure 3 : Degree of protonation of chitosans of various MMs as a function of pH.

Figure 4: Permeability of capsules synthesized in various solutions (for conditions see Table 1) . Solute diffusion of dextran of different molar masses.

Figure 5: Relative cross linking density of capsule membranes in function of membrane/capsule volume ratio for conditions see Table 1. The arrows indicate the direction of the pH increase (from 6.0 to 7.0) .

Figure 6 : Conversion of chitosan during capsule membrane formation in water. Reaction between a 1% alginate (Keltone HV) solution and a 1% chitosan solution (M_n=2.6kD) .

Detailed Description Capsules with a permeable, or semipermeable wall, and a liquid core, can be formed by introducing the liquid droplets from aqueous solution of an anionic polyelectrolyte into an aqueous solution of a countercharged cationic polyelectrolyte (Figure 1A) . The capsule wall consists of polymer complex which is a product of the reaction of oppositely charged polyelectrolytes (Figure IB) . Although the anionic polymer is generally interior to the cationic polymer, the reverse can also be applied. Figure 1C shows a schematic of a membrane generated by the interaction of the two polymers .

The following sections summarize exemplary preparation conditions and characteristics of the microcapsules of the invention. Section A and examples 1 to 23 describe microcapsules production with different polymers and reaction conditions. Following on from this, a series of thermodynamic and kinetic experiments using the preferred chitosan/alginate system are described in section B. Finally, in vivo implantation experiments are described in section C.

Materials and Methods

Alginate: Keltone HV - Sodium Alginate (lot. 54650A) was obtained from Kelco/NutraSweet (San Diego, CA, USA) . An intrinsic viscosity [η] of 780mL/g was measured in 0. IM NaCl at 20°C in a capillary viscosimeter (Viscologic TI 1, SEMA Tech, France) . This reflects to MM of 390kD. Chitosan: Samples with varying molar masses (l-10kD) were obtained in controlled radical degradation via continuous addition of hydrogen peroxide (0.8-6.4mMol/g of polysaccharide) to 2.5% chitosan solution of pH 3.5 to 4.0 at 80°C. Chitosan with a molar mass of 50kD and a degree of deacetylation >97% was used as the starting material (Hutchinson/McNeil Int., Philadelphia, USA, product E-055) . All samples after degradation as a chloride salts had similar polydispersities in MM (1.5- 1.8) and high degrees of deacetylation (>95%) .

Molar masses of chitosan samples were estimated by GPC at a flow-rate of 0.5 mL/min. (LaChrom L-7110 isocratic pump, Merck, Darmstadt, Germany) equipped with a refractometric detection (LaChrom Rl detector L-7490, Merck) . A Shodex OHpak SB-803 HQ column (Showa-Denko Company, Tokyo, Japan) was employed as the stationary phase, using 0.5M acetic acid/0.5M sodium acetate as an eluent, as recommended. Polyethylene glycol standards (PSS, Mainz, Germany) were used for column calibration and as a relative reference for MM calculation. All other reagents were of analytical grade.

The degree of chitosan amino group ionization (α) at different pHs were determined from potentiometric titration curves, where 20 ml of 0.1% polymer solutions (pH 2.5) were titrated with 0.02M NaOH. The degree of protonation is defined as α=α' + [H₃0⁺] /c, where α' is the degree of neutralization, [H₃0⁺] is the proton concentration (deducted from pH) and c is equivalent concentration of chitosan.

Measurements of pH were made at ambient temperature with a pH meter 744 (Metrohm, Herisau, Switzerland) , calibrated at pH 4.0 and 7.0.

Microcapsule Preparation: Capsules were produced from a pair of oppositely charged polysaccharides. A 0.5-2% aqueous sodium alginate was prepared in deionized water or 0.9% NaCl. Approximately 4mL of this solution was introduced into a 5mL disposable syringe with a 0.4mm flat-cut needle (Becton Dickinson AG, Basel, Switzerland) . The droplets were sheared off for 60 sec at a flow rate lmL/min (kdScientific syringe pump - Bioblock Scientific, Frenkendorf, Switzerland) into 20mL of solution of 1% chitosan (M_n varied between 2-3kD) at pH 6, 6.5 and 7.0 previously adjusted with IM NaOH respectively. The resulting microcapsules (2.5-3mm in diameter) were allowed to harden for 20 minutes under gentle stirring (200rpm) with small magnetic bar, filtered and rinsed with the solvent used for preparation of the polysaccharide solutions. Collected microcapsules (approx. 2mL in volume) were stored at 4°C in 0.9%

NaCl/0.01% sodium azide. The entire capsule formation procedure described herein was performed at ambient temperature .

The effect of chitosan molar mass on the relative mechanical strength of the prepared chitosan/alginate capsules was determined as follows. In brief, the oligochitosan generally had a "lower critical oligomer chain length" (M_n >l-2kD) for stable capsule formation, with a maximum in mechanical capsule strength observed at a MM of 2-3kD. All oligochitosan samples used in the mechanism and kinetics studies were prepared within this range (2.2-2.8kD) . Chitosan Conversion Determination: Samples of 400mL of chitosan solutions were withdrawn from the reaction bath every 5 minutes during capsule formation and characterized by using the aforementioned chromatographic method. By assuming that the total concentration of the solute is proportional to the GPC elution curve area (Beers' Law) , the conversion and MM of chitosan were calculated from the respective chromatogram derivatives.

Permeability Measurements: Two mL of a 0.1% polymer standard solution (dextran 40, 70 and 11OkD in 0.9% NaCl) , were added under agitation to ImL of microcapsules placed in a lOmL vial. Aliquots were withdrawn after 3 hours and injected into a liquid chromatograph equipped with Shodex SG-G and SB-803 HQ columns. The eluent, 0.9% NaCl/0.01% sodium azide, was applied at a flow rate of 0.5mL/min. The dextran concentrations were proportional to the maximum heights of the detected chromatographic peaks (Beers' Law) and calculated with respect to the initial polymer standard concentration, which is the concentration of the dextran standard with the defined MM at time 0 (immediately following the addition of 2ml of standard solution into 1ml of capsules) . The membrane permeability, calculated from decrease in dextran concentration, was maximal at 33.3% of polymer diffusion. The cut-off of the microcapsules was defined as the lowest MM of dextran for which diffusion was smaller than 2% after 3 hours.

Mechanical Characterization: The mechanical resistance of microcapsules was determined on a Texture Analyzer (TA- 2xi, Stable Micro Systems, Godalming, UK) . The mechanical deformation tests were performed at 0.1 mm/sec mobile probe speed until bursting was observed. The force (itiN) exerted by the probe on the capsule was recorded as a function of the compression distance leading to a force vs. strain relation. Twenty capsules per batch were analyzed in order to obtain statistically relevant data.

Microscopic Observation: Capsules size and membrane thickness were visually examined under a fluorescence confocal microscope and a standard inverted light microscope (LSM 410 Invert Laser Scan Microscope and Axiovert 100, respectively; Carl Zeiss Jena GmbH, Jena, Germany) . The capsules were stained in 0.01% fluorescence dye solution (Rhodamine or Eosine, both from Fluka, Buchs, Switzerland) for 30 minutes and than washed with 0.9% NaCl. The Eosine dye is more suitable due to it anionic character and ability of specific binding to amino groups of chitosan. Images were acquired by normal light microscopy followed by a confocal scan. The thickness of the optical section was on the order of several micrometers. Changing the plane of focus vertically up or down by 10mm had no effect on the apparent membrane thickness. All fluorescence pictures were analyzed using the Scion Image software (Scion Corporation, Maryland, USA) .

A: Capsule Production

In examples 1 to 23, capsules were produced according to the following protocol. (a) An alginate solution (Table I) was dropped into a low molecular weight (0.5-10.0 kD) oligochitosan solution.

(b) Solvents used included water, as well as aqueous solution of low molecular weight monovalent metal salts, and isotonic or buffer solutions.

(c) The reaction was carried out at a temperature between 0 and 50°C.

(d) The reaction time varied between 1 minute and three hours. The capsules are stable after a few seconds, but 1 minute is a preferred minimum reaction time .

(e) Alginate can be replaced by either iota-or kappa-carrageenan or carboxymethylcellulose (CMC) (Table I) •

(f) Alginate, carrageenan, CMC and chitosan were used at a concentration of 0.5-5.0%.

Table I : Properties of Polymers

Capsules were produced having characteristics within the following ranges:

(a) Capsule size: 50μm to 5.0mm in diameter, exemplifying the ranges 3 -4mm, 0.4 -0.6mm and 0.8 -1.0mm.

(b) Wall thickness: 0.5 to 400μm.

(c) Permeability: molecular weight cutoff varied from 10 to 30OkD.

Optimization:

The following parameters have been optimized by experimental study:

(a) Figure 2 shows a schematic graph correlating the mechanical strength of the capsules with the molecular weight (MW) of the outer polymer (chitosan) . This shows that below 600 daltons, no capsules were precipitated under the conditions used, while above 10. OkD, the membrane formed had low mechanical strength. Between these extremes, stable capsule were generated with a maximum at a molecular weight of 2,000 daltons. (b) The viscosity of the inner polymer solution

(controlled by changing the molecular weight or concentration) . By increasing the viscosity, the capsules are formed with a lower permeability and higher mechanical strength. (c) The charge density of the inner and outer polymer chains. This corresponds to the degree of deacetylation for chitosan.

(d) The concentration of monovalent metal salts in solution. By increasing the monovalent metal ion salt concentration, the capsules are formed with higher permeability and lower mechanical strength.

(e) The solution pH. By increasing the solution pH, the capsules prepared in water are formed with higher permeability and higher mechanical strength. (f) Temperature. By increasing the temperature of the polycation solution, when carrageenan is used as polyanion, the obtained capsules have thicker and more permeable membranes .

The following examples are provided by way of illustration to demonstrate how the capsules can be produced under a range of different conditions. Example 1

An alginate solution of 0.75-1.5% w/v sodium alginate in water was added dropwise to precipitation bath of 0.5-2% w/v oligochitosan solution in water. The chitosan had molecular weight of 1-4 KD and a solution pH of 7.0.

Immediately after entry into the precipitation bath, the droplets become covered with a membrane of the alginate/chitosan complex. After 20 minutes, the capsules were separated by decanting and washed with water. The spherical transparent microcapsules with good mechanical properties have a diameter of 3 -4mm.

Example 2

Following the procedure of Example 1, 0.75-1% w/v of sodium alginate solution in 0.9% NaCl was added dropwise to precipitation bath of 1-2% w/v oligochitosan solution also in 0.9% NaCl. The chitosan had molecular weight of 3-6 KD and a solution pH of 6.7. Immediately after entry into the precipitation bath, the droplets become covered with membrane of the alginate/chitosan complex. After 10 minutes, the capsules were separated by decanting and washed with 0.9% NaCl. The spherical semi-transparent microcapsules with good mechanical properties have a diameter of 3 -4mm.

Example 3

Following the procedure of Example 1, 0.75-1% w/v iota- carrageenan solution in water was added dropwise to precipitation bath of 0.5-2 %w/v oligochitosan solution in water. The chitosan had molecular weight 1-4 KD and its solution pH 6.9. Immediately after entry into the precipitation bath, the droplets became covered with a membrane of the iota-carrageenan/chitosan complex. After 10 minutes, the capsules were separated by decanting and washed with 0.9% NaCl. The spherical semi-transparent microcapsules with good mechanical properties have a diameter of 3 -4mm.

Example 4

Following the procedure of Example 1, 1-1.5% w/v carboxymethylcellulose with a degree of substitution higher than 1.2 in water was added dropwise to a precipitation bath of oligochitosan solution also in water. The chitosan had molecular weight of 3-10 KD and a solution pH of 7.2. Immediately after entry into the precipitation bath, the droplets became covered with a membrane of the carboxymethylcellulose/chitosan complex. After 10 minutes, the capsules were separated by decanting and washed with 0.9% NaCl. The spherical semi- transparent microcapsules with good mechanical properties have a diameter of 3 -4mm.

Example 5

This example was carried out under the conditions used in Example 1 with a pH of 6.6.

Example 6 This example was carried out under the conditions used in Example 1 with a reaction time of 40 minutes.

Example 7

This example was carried out under the conditions used in Example 1 with a diameter of 0.8 -lmm.

Example 8

This example was carried out under the conditions used in Example 1 with a diameter of 0.4 -0.6mm.

Example 9

This example was carried out under the conditions used in Example 2 with a pH of 7.2 and the chitosan with a molecular weight of l-3kD.

Example 10

This example was carried out under the conditions used in Example 2 with a reaction time of 30 minutes.

Example 11

This example was carried out under the conditions used in Example 2 with a diameter of 0.8 -lmm

Example 12

This example was carried out under the conditions used in

Example 2 with a diameter of 0.4 -0.6mm.

Example 13

This example was carried out under the conditions used in Example 3 with a pH of 7.2

Example 14 This example was carried out under the conditions used in Example 3 with a reaction time of 30 minutes.

Example 15

This example was carried out under the conditions used in Example 3 with a diameter of 0.8-lmm.

Example 16

This example was carried out under the conditions used in Example 3 with a diameter of 0.4-0.6mm.

Example 17

This example was carried out under the conditions used in Example 4 with a pH of 6.7

Example 18

This example was carried out under the conditions used in Example 4 with a reaction time of 30 minutes.

Example 19

This example was carried out under the conditions used in

Example 4 with a diameter of 0.8 -lmm.

Example 20

This example was carried out under the conditions used in Example 4 with a diameter of 0.4 -0.6mm.

Example 21 This example was carried out under conditions used in Examples 1,6,7 and 8 where alginate and chitosan solutions were prepared in MOPS or HEPES buffer solutions .

Example 22

This example was carried out under conditions used in Examples 3, 13, 14, 15 and 16 where iota-carrageenan solution was prepared in MOPS or HEPES buffer solutions and chitosan solution was prepared in 0.9% NaCl.

Example 23

This example was carried out under conditions used in Examples 3, 13, 14, 15 and 16 where iota-carrageenan solution was at 37°C and temperature of chitosan solution was varied between 4°C and 40°C.

B: Detailed Studies 1. Membrane Size and Shape:

Examination of the microcapsules under the optical microscope revealed an approximately spherical shape with a diameter in the range 2 -2.5mm. In most cases capsules possessed transparent membranes. Using both confocal fluorescence and light microscopy, it was possible to distinguish the capsule wall thickness. Moreover, from fluorescence intensity one can observe that capsules have asymmetric membrane structures with higher concentrations of chitosan close to the surface, gradually decreasing towards capsule centre. This agrees with a former description of asymmetric capsule membrane formation between two oppositely charged polyelectrolytes as a two- step process. According to this model, membrane formation begins with the spontaneous creation of a semi- solid complex "skin" at the droplet surface as a result of the phase separation process. After this initial step, a macroporous "trabecular" membrane is build up, a process which is controlled by diffusion of the cationic polymer.

2. Thermodynamic Aspects Related to Capsule and

Membrane Formation Capsule permeability is controlled by the crosslinking density of the polymer network. As the membrane formation is an electrostatic process, pH and ionic strength influence complexation, with the protonation degree of chitosan being the key variable. Effect of Chitosan Protonation:

Chitosan is weak base and has therefore a limited solubility in the higher pH region, i.e. precipitation occurs when the pH exceeds 6.0-6.5. We found that in some cases for chitosans with MMs lower than 8kD, precipitation occurs at higher pH values. The solution properties of this natural polycation are control by the degrees of acetylation and amino group protonation. Figure 3 shows the ionization degrees of 50kD chitosan and of two oligochitsan samples as a function of pH. At a pH 4.0 all samples are in their fully protonated form. However, with increasing pH protonation becomes highly dependent on the MM. For high MM chitosan (5OkD) , ionization of 50% of the cationic groups is observed at pH 6.1, whereas half of the oligochitosan (2.8kD) chains become ionized at pH 6.5. A possible explanation for the enhanced solubility of lower MM chitosans in neutral, or even slightly basic solutions is a shift of their pK_a's towards higher values .

The capsules obtained at different pHs (6, 6.5 and 7) in water or saline varied in their surface roughness, membrane thickness and morphology. Capsules prepared in water had smooth surfaces and their membrane thickness increased as a function of pH. These differences can be explained by changes in the protonation degree of chitosan (35%, 50% and 65% respectively; see Figure 3) which influences the crosslinking density of the polyelectrolyte complexes produced. Interestingly, by introducing a monovalent metal salt during capsule formation (in 0.9% NaCl) this trend changed and the membrane thickness decreased with increasing pH. Furthermore, the capsules were less homogeneous and rougher in their structure in comparison to those prepared in pure water. This may indicate, that the salt shielded the polyanion, decreasing the extent of intermolecular interaction.

Control of Crosslinking Density:

Taking into account the difference in charge density (potentiometric titration) , the conversion of chitosan (from GPC measurements) and the membrane thickness (microscopic observation) of capsules prepared with 20 min reaction time, one can calculate the relative crosslinking density (CD) given by the -NH3⁺/-COO^" ratio (see Table 1 below) . Generally, in pure water the CD increased with the lowering of pH, resulting in denser and less permeable membranes. From dextran permeability measurements one can observe a significant difference in membrane porosity. At pH 6.0 the MM cut-off was below 40 KD, at pH 6.5 it was between 70 and HOkD, and at pH 7.0 it exceeded 11OkD (Figure 4) .

The presence of low MM salt (0.9% NaCl) during the binary capsule formation accelerated the diffusion of the oligocations and led to thicker capsule walls with lower CD (Table 1) . As a consequence, these capsules had similar porosities with a higher cut-off (>110kD) . This implies that the differences in diffusion kinetics are associated with a variation in membrane thickness.

Mechanism: The structure of the chitosan/alginate microcapsule membrane may be represented as alternative sequences of ionic interchain bonds and loop-like regions incorporating the uncoupled units of both chains. More compact membranes, with smaller "loops" and lower cutoffs, are formed at lower pHs, though only in the simple aqueous system (capsule prepared in water) . The introduction of low MM ions changes the polyelectrolyte solution conformation. In particular, longer chains, as in alginate, transform from elongated into more compact coil structures. As a result, the short oligochitosan chains can more easily penetrate the alginate chain network and form thicker and less dense membranes. This apparent difference in the mechanism of membrane formation is well visualized in a plot of CD in function of membrane/capsule volume ratio (Figure 5) . While for the pure water system we observed a strong decrease (high slope) of the CD with increasing pH, for the saline system the changes went into the opposite direction with a significantly lower variation in CD values (lower slope) . This implies that, for all but non-neutral conditions, the charge suppression on the macroions due to increased salt reduces the extent of complexation. However, both curves cross over at approximately neutral pH, where capsules have comparable membrane thickness and porosity (Table 1) .

3. Kinetics of Alginate-Oligochitosan Complexation Effect of Molar Mass Distribution

Table 2 summarizes the chitosan MM bound within the capsule membrane after 5 and 20 min of reaction. A very selective binding with respect to the MM of chitosan was observed during capsule formation in solutions which differ in pH and ionic strength. This effect is most prominent between capsules prepared in solutions with varying ionic strengths, such as water and 0.9% NaCl. Generally, the lower MM portion of chitosan is involved in membrane formation in salt free system. This specific complexation is caused by the so called polyelectrolyte effect. It has been shown for a number of interpolymer reactions between mixtures of oligomers that preferential binding to the longest chains takes place. However, this conclusion was primarily based on hydrogen bond complexation systems. In the case of polyelectrolytes in salt-free solutions, inter- and intramolecular electrostatic interactions occur which strongly influence the solution properties as well as the mechanism of complex formation. Recently, this observation was experimentally confirmed for polyelectrolyte complexes between poly (diallyldimethylamonium chloride) PDADMAC and poly(styrene sulfonate) PSS of different molecular weights. In the salt free solution and at extremely low salt concentrations, preferential binding of PSS (8kD) was found. At higher ionic strength, a pronounced complexation of higher MM PSS (356kD) was observed due to the shielding effect of sodium cations during polyelectrolyte complex formation.

Influence of the Kinetics on Capsule Properties :

During capsule formation in pure water the lower MM part of chitosan is selectively incorporated into the membrane. With increasing polycation protonation degree, through lowering the pH, a clear shift toward shorter oligomeric chains is observed. This leads to more dense and thinner membranes and consequently to mechanically less stable capsules (Table 3) . When the cut-off of the primary membrane is too low, polycation molecules may not be able to diffuse, and membrane growth is slowed down or even stopped. This was the case when chitosan of MM higher than 3 OkD was used or when capsules were prepared at lower pH in salt free solutions. Therefore, at neutral pH in water, the conversion of chitosan increased proportionally with the reaction time and the reaction rate was significantly reduced at lower pHs (Figure 6) .

By introducing sodium chloride into the reaction media the mechanism of the membrane formation was changed. In this case, the MM of the complexed oligocation was higher than the starting material and with increasing of pH shifted into the direction of larger MM values. One also observes that the conversion rate is independent of the pH and is proportional to the reaction time, similarly to the conversion in water at neutral pH (Figure 6) . As the result of this complex phenomenon, a formation of capsules with similar mechanical and structural properties was observed, which was independent of the protonation degree of the chitosan.

For capsules prepared in water at pH 7.0, Tables 2 and 3 show that capsules with the best mechanical strength (1,850 mN) and flexibility (85% bursting strain) were obtained when chitosan reacted without any preferential reaction early in the process and hence uniformly with time (Table 2) . Therefore, uniformity in kinetics leads to improved mechanical properties. Moreover, although capsules prepared at pH 7.0 in water or saline had similar relative crosslinking densities, membrane thicknesses and porosities, they showed about 10 times higher mechanical resistance and very high (approx. 85%) deformability in water alone. This is evidence that by simply varying the ionic strength of the solvent at neutral pH we can significantly influence the mechanism of capsule formation and decouple the mechanical properties from permeability. The uniformities of the capsules prepared in water and saline are different due to the shift of MM of chitosan built into the membrane towards higher values with introducing the salt during the membrane formation.

4. Capsule Preparation under Physiological Conditions

Bioencapsulation in general, and applications in the biomedical field in particular require physiological conditions for immobilization and immunoisolation of living cells. Therefore, an additional set of capsules was prepared at pH 7.0 in 0.9% NaCl in order to demonstrate the versatility of the new capsule chemistry.

The Effect of Reaction Time:

The properties of the capsules vary remarkably as a function of the reaction conditions with the reaction time having the most profound influence on the process of binary capsule formation. Generally, the mechanical strength of the capsules increases with reaction time due to the increase of the capsule wall thickness (Table 4) . However, no significant differences in capsule permeability and cut-off were measured. This indicates that the process of "skin" formation during the first minutes of reaction controls the cut-off of the membrane. However the subsequent diffusion controlled building-up of the inner membrane is responsible for the capsule's mechanical resistance.

The Effect of Chitosan MM and Concentration:

Table 5 illustrates that lower MM chitosans (2.6kD) form slightly denser membrane skins which limit oligocation diffusion and membrane build-up and, as a consequence, lead to mechanically less resistant capsules. In addition, decreasing the oligochitosan concentration changes the kinetics of membrane formation, engendering a significant reduction in mechanical stability. However, in both cases no significant difference in membrane permeability was observed.

The Effect of Alginate Concentration:

The capsule membrane grows more slowly at higher alginate concentrations (Table 6) . Oligocation diffusion seems to be retarded by the higher concentration of the polyanion chain network, which leads to thinner but due to the higher charge density, denser membrane with lower cut-off and higher mechanical resistance.

6. Conclusions

The ionic strength and the pH of the solution utilized during the capsule formation process influence the structure of the alginate/oligochitosan membrane. The presence of a low MM salt (0.9% NaCl) accelerates the diffusion of the oligocations and leads to thicker capsule walls, although with lower relative crosslinking density. We have shown that sodium chloride diminishes the effect of chitosan charge density on the permeability and the mechanical properties of capsules and shifts the cut-off of prepared membranes towards higher MM values. The permeability of the new alginate-oligochitosan microcapsules, therefore, depends mainly of two factors: (1) the density of the membrane which forms the outer shell influences the cut-off, which is particularly important for capsules prepared in water, and (2) the final membrane thickness controls the kinetics of solute diffusion and is more important for capsules synthesized in 0 . 9% NaCl .

The mechanical properties of capsules prepared at physiological conditions are primarily influenced by membrane thickness and can be controlled by parameters such as reaction time, oligochitosan MM and concentration. On the other hand, the alginate concentration significantly effects both mechanical and porosity characteristic of the capsule membrane.

C: In Vivo Implantation Experiments

1.2 % Keltone HV (Kelco Chemical Company, UK), 500mL in 0.9% NaCl; was filtered over 5mm and 0.22mm filters (high pressure sterile filtration; 200mL) . A receiving bath was prepared containing 1% oligochitosan (2.8kD, PD=1.7), lOOmL in 0.9% NaCl, pH adjusted to 7.0 with 3% NaOH, which had been filtered over 5mm and 0.45 mm (non- sterile) and sterile filtered using a cellulose acetate membrane system ZAPCAPS 0.22mm (Schleicher & Schuell) . The polyanion was extruded at 5mL/h using air stripping apparatus (22 G needle-Becton Dickinson, Ireland) into the chitosan receiving bath (30mL of 1% oligochitosan solution) and allowed to react in the bath for 5 minutes. After a further 5 minutes, the reaction solution was discarded over sterile plastic tube with nylon net. The capsules were then washed several times with PBS solution in a plastic tube (transferred 2x into the fresh PBS solution) .

The capsules were transplanted into 3 mice (1000 capsules of 6mm diameter tubes in each batch) and explanted after 3 weeks. After this, non-fibrotic capsule overgrowth was observed indicating high level of biocompatibility . D: Applications

The method described herein can be applied to the encapsulation of a wide variety of materials. The method is particularly relevant to the encapsulation of biological or biologically active materials. The encapsulation can be used to physically protect the encapsulated material, e.g. from mechanical or environmental, or where the capsules are for implantation into a human or animal, to help to prevent or reduce an immune rejection to the material. Particularly preferred applications include:

(a) Encapsulation of living animals and plant cells, for example cells for implantation in a patient, e.g. to replace cells lost through disease, cells which are transformed to express a therapeutically useful polypeptide, artificial cells, and bioartificial organs.

(b) Encapsulation of bacteria/nutrients, particularly for use in the food industry, as all components utilized in the method can be of food grade. (c) Encapsulation of vitamins, drugs, polypeptides and enzymes, particularly for use in the cosmetics and pharmaceutical industries.

Table 1. Properties of Capsules Prepared at Different pH and Ionic Strength"

Degree of Membrane V^v mem//V^v cap Chitosan Membrane Crosslinking

Solvent pH Chitosan Thickness Conversion CHIT/ALG^C Density

Protonation -NH₃V-COO^"

Of (-) (mm) (%) (%) (g/g) (mol/mol) e.o 0.65 13 3.1 3.08 19.9 14.8

Water

6.5 0.50 72 16.3 5.19 6.37 3.94

7.0 0.35 105 23.1 6.59 5.70 2.43

0.9% 6.0 0.65 300 56.1 7.25 2.59 1.93

6.5 0.50 L90 39.0 7.08 3.63 2.28

7.0 0.35 75 16.9 6.74 7.98 3.40

^aCapsules (2.5 mm in diameter) were obtained through a reaction between a 1% alginate (Keltone HV) solution and a 1% chitosan M_n=2.6 kD (20 min. reaction time, ambient temperature). ^bMembrane/capsule volume ratio. ^cChitosan/alginate ratio in the capsule membrane.

Table 2. Molar Masses of Chitosan Bonded during Capsule Formation*

Reaction Conditions M ^{1 l}n M M_w/M_n kD KD

Chitosan - starting 2.60 4.70 1.8 solution pH 6.0 5 min. 1.80 4.20 2.3

Water

20 min. 1.80 4.20 2.3 pH 6.5 5 min. 1.95 4.20 2.2

20 min. 1.85 3.00 1.6 pH 7.0 5 mm. 2.80 4.50 1.6 20 min. 2.85 4.55 1.6 pH 6.0 5 mm. 2.60 5.50 2.1

0.9% NaCl

20 min. 2.80 5.20 1.9 pH 6.5 5 mm. 2.80 4.40 1.6

20 min. 4.10 8.60 2.1 ^aThe preparation is identical to that described in Table 1.

Table 3. Mechanical Properties of Capsules Prepared at Different pH and Ionic Strength*

Membrane CD Bursting Strain

Solvent pH Thickness -NH₃V-COO^" Force at Bursting

(mm) (mol/mol) (mN) (%)

6.0 13 14.8 30+15 64

Water

6.5 72 3.94 180+60 77

7.0 105 2.43 1,850±500 85

6.0 300 1.93 100±30 72

0.9%

NaCl

6.5 190 2.28 220±90 72

7.0 75 3.40 190+30 72

^aThe preparation is identical to that described in Table 1.

Table 4. Properties of Capsules as Function of Reaction Time*

Reaction Membrane Permeabil:ity (after Bursting

Time Thi .ckness 3h) (%) Force

(min) (mm) (mN)

40 kD 110 kD

5 40 33 24 50±30

10 60 33 23 110±60

15 80 32 23 200±70

20 95 33 22 370+150 ^aCapsules (2.5 mm in diameter) were obtained through a reaction between a 1% alginate (Keltone HV) solution and a 1% chitosan M_n=2.8 kD (pH 7.0, 0.9% NaCl, ambient temperature).

Table 5. Properties of Capsules obtained with Chitosan of Different MM and Concentration*

Chitosan M_n Of Membrane Per eability Bursting

Cone. Chitosan Thickness (after 3h) Force

(%) (kD) (mm) (%) (mN)

40 kD 110 kD

1 2.8 95 33 22 370±100

1 2.6 75 33 21 190±30

0.5 2.6 125 33 25 50+20

"Capsules (2.5 mm in diameter) were obtained through a reaction between a 1% alginate (Keltone HV) solution and a chitosan solutions (pH 7.0, 0.9% NaCl, 20 min. reaction time, ambient temperature) .

Table 6. Properties of Capsules Prepared with Different Concentration of Alginate*

Alginate Membrane Permeabil: Lty (after Bursting

Cone . Thickness 3h) (%) Force

(%) (mm) (mN)

40 kD 110 kD

1.0 95 33 22 370+100

1.2 80 33 18 450+110

1.5 70 24 10 570+150

"Capsules (2.5 mm in diameter) were obtained through a reaction between alginate (Keltone HV) solutions and 1 chitosan M_n=2.8 kD (pH 7.0, 0.9% NaCl, 20 min. reaction time, ambient temperature) .

References :

The following references are expressly incorporated by reference .

Willaert & Baron, Rev. Chem. Eng.,12:5, 1996. Jen et al, Biotechnol . Bioeng., 50:357, 1996.

Colton, Trends Biotechnol ., 14 : 158 , 1996.

Morris, Trends Biotechnol., 14:163, 1996.

Hubbell, Trends Poly. Sci . , 2:20, 1994.

Sefton & Stevenson, Adv. Polym. Sci., 107:143, 1993. Baldwin & Saltzman, in Fundamentals of Animal Cell

Encapsulation and Immobilization, Goosen ed, CRC Press, pp 144-181, 1993.

Hunkeler et al , Advances in Polymer Science, 136:1,

1998. Hunkeler, Trends in Polymer Science, 5:286, 1997.

GB 2 135 954 A (Dautzenberg) .

EP 0 152 898 (MIT) .

US Patent No:4,808,707 (University of Delaware).

US Patent No:5,462,866 (Vanderbilt University). GB 2 145 992 A (Damon Biotech) .

US Patent No: 5, 089, 272 (Shioya et al) .

Claims

Claims :

1. A method for encapsulating a core material within a capsule having a permeable or semipermeable membrane using a complex formation reaction between oppositely charged polymers, the method comprising: forming a droplet from a solution of an inner polymer comprising at least one positively or negatively charged polymer and the core material; and, contacting the droplet with a solution of an outer polymer comprising at least one polymer having an opposite charge to the polymer or polymers forming the droplet so that the polymers react to form a membrane around the droplet, wherein one of the polymers is an oligosaccharide with a molecular weight of less than 10. OkD and the reaction is carried out at a pH between about 6.6 and 7.5.

2. The method of claim 1, wherein the oligosaccharide is produced by subjecting a precursor polysaccharide to a radical degradation reaction to reduce its molecular weight to below lOkD.

3. The method of claim 1 or claim 2, wherein the radical degradation reaction comprises treating the precursor polysaccharide with peroxide or exposing the precursor polysaccharide to radiation.

4. The method of claim 2 or claim 3 , wherein the pretreatment step increases the solubility of the oligosaccharide at the pH of the encapsulation reaction.

5. The method of any one of the preceding claims, wherein the oligosaccharide has a molecular weight between about 0.5 and 9. OkD .

6. The method of any one of the preceding claims, wherein the oligosaccharide has a molecular weight between about 0.6 and 6. OkD.

7. The method of any one of the preceding claims wherein the oligosaccharide has a molecular weight between about 1.0 and 3. OkD.

8. The method of any one of the preceding claims, wherein the oligosaccharide is chitosan.

9. The method of any one of the preceding claims, wherein the encapsulation reaction is carried out in the absence of polyvalent metal cations or organic counter ions .

10. The method of any one of the preceding claims, wherein the reaction between the polymers is carried out at a pH between about 6.8 and 7.4.

11. The method of any one of the preceding claims, wherein the reaction between the polymers is carried out at a pH between about 7.0 and 7.4.

12. The method of any one of the preceding claims, wherein the negatively charged polymer is alginate, carrageenan or carboxymethylcellulose.

13. The method of claim 12, wherein the alginate or carrageenan has a molecular weight between about 100 and lOOOkD.

14. The method of any one of the preceding claims, wherein droplets of the negatively charged polymer are dropped into a reaction bath of the positively charged oligosaccharide .

15. The method of any one of the preceding claims, wherein the capsules are between 50╬╝m and 5.0mm in diameter.

16. The method of any one of the preceding claims wherein the capsules have a membrane thickness of 0.5 to 200╬╝m.

17. The method of any one of the preceding claims wherein the membrane has a permeability cut off between 1.0 and 300kD.

18. The method of any one of the preceding claims, wherein the core material is a biological material.

19. The method of claim 18, wherein the biological material are selected from cells, bacteria, enzymes, hormones, antibodies, drugs or cytokines .

20. The method of any one of the preceding claims, which additionally comprises the initial steps of: selecting a physical property required of a capsule; selecting the molecular weight of an oligosaccharide between 0.5 and 10. OkD that will provide capsules having the physical property when reacted with a core material and an oppositely charged polymer; reacting the polymers and core materials as described above to produce the capsules; wherein the physical property is deformability and/or mechanical strength.

21. A method which comprises following the encapsulation of a core material using the method of any one of claims 1 to 20, the further step of formulating the capsules produced in a composition or other product.

22. A polymeric capsule comprising a semipermeable membrane encapsulating a core material produced by the method of any one of claims 1 to 20.

23. A polymeric capsule comprising a semipermeable membrane encapsulating a core material produced by the method of any one of claims 1 to 20 for use in a method of medical treatment .