The present invention relates to a biodegradable ceramic-polymer composite and to a method for preparing it. A variety of inorganic materials are nowadays used as insulation material and flame retardant material in industry and especially for construction purposes. Conventional inorganic materials such as rockwool and glass fibres are however associated with several problems. Firstly, these materials are artificial and not degradable, which makes their application from environmental perspective questionable. Secondly, their production is energy intensive. In addition, these materials need to be handled with proper care since they give raise to skin irritation. In view of the above, there is clearly room to develop more attractive products for use as insulation or flame retardant materials. Object of the present invention is to provide a product that can suitably be used as an insulation material or a flame retardant material, and which deals with the above problems. Surprisingly, it has now been found that this object can be established when use is made of a product comprising nano -particles of an inorganic material which particles are contained in a particular network of a natural polymer. Accordingly, the present invention relates to a ceramic-polymer composite comprising inorganic nano-particles that are contained within a network of a natural polymer which network is formed by means of ionogenic interactions. The composites of the present invention display excellent isolation and flame retardant properties, and do not produce toxic fumes during thermal degradation. In addition, they are easy to make; their production is less energy intensive when compared to conventional materials; they are biodegradable; they are highly flexible and maintain their strength; and they are user-friendly in the sense that they can be handled without special care, like the use of gloves, since they do not cause skin irritation. Hence, it will be clear that the composites in accordance with the present invention constitute a significant improvement over the known materials. Suitably, the ionogenic interactions are multivalent interactions between ionogenic groups of the natural polymer, which multivalent interactions are facilitated by means of multivalent ions that originate from a cross-linking agent. Suitably, in the composite according to the present invention the inorganic nano -particles are present in an amount of at least 25% by weight, based on total composite. Preferably, the inorganic nano-particles are present in an amount in the range of from 50 - 90% by weight, based on total composite. More preferably, the inorganic nano-particles are present in an amount in the range of from 60-75% by weight, based on total composite. One of the surprising aspects of the present invention is that the composites according to the present invention can suitably contain large amounts of inorganic material, whilst displaying a high flexibility and maintaining strength over a long period of time. The natural polymer to be used in accordance with the present invention can suitably be chosen from the group consisting of (hetero)polysaccharides, polypeptides and proteins. The composite of the present invention can suitably comprise one or more types of natural polymer. Preferably, however, the composite comprises one type of natural polymer. Suitable natural polymers include starches, celluloses, chitosans, alginates, inulins, pectins, caseins and derivatives thereof. Derivatives that may be used are for example esters, such as acetylated starch, or carboxy methylated cellulose, and ethers, such as hydroxypropylated starch. Specific examples include acetylated starch and hydroxypropylated cellulose. Preferred polysaccharides include chitosans, aliginates, pectins, modified starches, whey proteins, caseins, gelatines, and mixtures of two or more of these compounds. Preferably, the natural polymer comprises one or more aliginates. More preferably, the natural polymer comprises one type of aliginate. An advantage of the use of such natural polymers is that they are biodegradable and not inflammable. In the context of the invention, a biodegradable material is a material, which, in a biological environment, degrades to compounds, which are preferably water soluble and non-toxic. The degradation may proceed under influence of air, water, soil and/or micro¬ organisms. The composite according to the present invention comprises inorganic nano-particles. In the context of the present invention, a nano- particle is a particle having in at least one direction a length in the range of nanometers. A wide variety of inorganic nano-particles can be used in the composition of the present invention. The inorganic nano-particles may have been derived from one or more types of inorganic material. Preferably, however, the inorganic nano-particles are derived from one type of inorganic material. Suitable inorganic nano-particles include those derived from a smectite-like clay mineral, a kaolinite, a hydroxy apatite or other biocompatible calciumphosphates, a hydrotalcite or a metal oxide. Preferably, the inorganic nano-particles are derived from a montmorrillonite. The nano-particles can suitably be derived from a clay having a layered structure and a cation exchange capacity of from 30 to 250 milliequivalents per 100 gram. When this capacity exceeds the above upper limit, it proves difficult to finely disperse the clay on a molecular level because of the strong mutual interaction of the clay layers. When the cation exchange capacity is lower than the above lower limit, it turns out that the clay is hard to modify, owing to the fact that the interaction with the natural polymer is small. Preferably, a clay having a cation exchange capacity of from 50 to 200 milliequivalents per 100 gram is used. If a clay having a layered structure and a cation exchange capacity of from 30 to 250 milliequivalents per 100 gram is used, and the natural polymer happens to contain hydrophobic groups, such as acetyl, alkylcarboxyl or arylcarboxyl ester groups, the clay may first be ion exchanged with a modifying agent, such as an onium ion. Suitable examples of onium ions include ammonium, phosphonium and sulfonium ions. This modification, which is known for incorporation of clay into other materials, has the goal to compatabilize the layers of the clay with the natural polymer. It utilizes suitable surfactants in an ion exchange reaction. The surfactant should have an onium functionality in addition to a functionality compatible with the natural polymer, such as one or more OH groups, COOH groups and the like. Preferably, the surfactant has alkyl chains with a length ranging from a number of 6 to 22 carbon atoms. The surfactant will typically be employed in an amount of from 5 to 70 wt.%, preferably from 20 to 40 wt.%, with respect to the clay. Suitable clay types based on layered silicates that can be used in accordance with the present invention, such as layered phyllosilicate are composed of magnesium and/or aluminum silicate layers, with a thickness between 0.7 and 1.2 nm. Especially preferred are smectite-like clay minerals, such as montmorillonite, saponite, hectorite, fluorohectorite, beidellite, nontronite, vermiculite, halloysite and stevensite. These materials impart very favourable mechanical properties and a great heat resistance to a biodegradable thermoplastic material. The composite according to the present invention is suitable for the production of various articles of manufacture. Examples of suitable applications of the material include fibres, coatings, films, packaging materials, carrier materials, construction materials and the like. The present composites are especially useful in isolation and/or flame retardant materials to be used in the building industry. The invention accordingly further relates to articles manufactured from the composite according to the present invention. Preferably, such articles are made entirely of inflammable components, including the composite according to the present invention. The present invention also relates to medical articles manufactured from the composite according to the present invention, wherein the inorganic nano-particles are derived from a hydroxy apatite or another biocompatible calcium phosphate. A suitable medical article is for instance suture. The present invention further relates to a method for preparing the composite according to the present invention, wherein a mixture is obtained by mixing a first solution that comprises inorganic nano-particles, a natural polymer and a solvent with a second solution that comprises a cross-linking agent that facilitates the formation of the network of the natural polymer, after which the composite in accordance with the invention is recovered from the mixture. Such recovery can suitably be established by evaporating the liquid components of the mixture. The skilled person will understand that the extent to which the formation of the network will take place may vary considerably. The extent to which the formation of the network occurs will depend on the number of ionogenic groups present in the natural polymer as well as the amount of the cross-linking agent to be used in the mixture. Suitably, the cross-linking agent is present in an amount of at least 0.5% by weight, based on total mixture. Preferably, the cross-linking agent is present in an amount in the range of from 0.5 to 5.0% by weight, based on total mixture. The mixture comprising the inorganic nano-particles, the natural polymer and the solvent can be prepared by mixing these ingredients, preferably in a kneader or in an extruder. Preferably, temperatures are used ranging from 10 to 2000C, more preferably from 18 to 180°C. The cross-linking agent facilitates the formation of the network of the natural polymer. Preferably, the cross-linking agent facilitates multivalent interactions between ionogenic groups of the natural polymer. Hence, the cross-linking agent preferably comprises multivalent ions. More preferably, the multivalent ions are cations. Suitably, the cross-linking agent is an inorganic agent. Suitable agents include calcium salts, barium salts, aluminium salts, magnesium salts but also di-and polyamines and imines but also phosphates, sulphates, carbonates and also di-and polycarboxylic acids, di and polysulfonium compounds and mixtures of these. Also possible are betaines and other substances compromising different ionic groups as well as di-and polypeptides and proteins. Preferably, the agent comprises calcium salts, carbonates, phosphates or polypeptides. Preferably, the method according to the present invention is a solution spinning process, which type of process is as such well known to the skilled person. From the fibres obtained with such a solution spinning process various types of articles can subsequently be made, such as building elements for isolation and/or flame retardancy purposes. The composite according to the present invention may further comprise any conventional additives, such as pigments, stabilizers, processing aids, aroma substances, anti-fouling agents and the like. Under specific circumstances, it may be desired that the material comprises a compatibilizing agent that assists in providing an optimal combination of the inorganic nano -particles and the natural polymer. The invention will now be further elucidated by the following, non- restrictive examples. Examples

Preparation a 5wt% sodium-alginate solution:

75g sodium-alginate (from brown algae, Fluka) is added slowly to 1425 g water at 60°C under stirring (mechanical stirrer) in a reactor. The solution is kept stirring at 600C for 4 hours or longer till all sodium-alginate is dissolved.

Preparation of a 5wt% clay solution:

50g Sύdchemie EXM 757 clay is added to 950 g water, the mixture is stirred with a magnetic bar for minimal one night, preferably over 3 nights.

Preparation of the sodium-alginate / clay solution:

a) 50/50 wt/wt% sodium-alginate / clay solution

50g of the 5wt% sodium-alginate solution is added to 5Og of the 5wt% clay solution and mixed to a homogeneous solution.

b) 30/70 sodium-alginate / clay solution

3Og of the 5wt% sodium-alginate solution is added to 7Og of the 5wt% clay solution and mixed to a homogeneous solution

Preparation of 1, 2, 3, 4 wt% calcium chloride solutions:

10g/20g/30g/40g CaCl2 is added to 990g/980g/970g/960g water, the mixture is stirred with a magnetic bar till all CaCl2 is dissolved. Preparation of sodium-alginate / clay fibres:

20ml of the 50/50 wt/wt% sodium-alginate/clay solution and 20ml of the a 70/30 wt/wt% sodium-alginate / clay solution were respectively put into a 20 ml syringe with a 2 mm hole and spun into 100ml of a 1, 2, 3 or 4wt% CaCb solution. The prepared fibres stay in the used CaCk solution for one night after which they are dried in air at room temperature. Subsequently, the flexibility of the fibres that were obtained from the 50/50 wt/wt% sodium-alginate/clay solution was determined using a tensile testing machine. From the results shown in Figure 1, it will be clear that the fibres obtained in accordance with the present invention displayed an excellent flexibility.