Examples for useful swellable materials are xanthan gum and derivatives thereof; guar gum and derivatives thereof, e.g. hydroxypropyl guar or carboxymethylhydroxypropyl guar (e.g. Jaguar??? gums); alginates, polysaccharides, e.g. dextran; pectins; cellulose derivatives, such as sodium or calcium carboxymethylcellulose, hydroxypropylcellulose or hydroxypropylmethylcellulose (e.g. Methocel E4M Prem.