a biodegradable vehicle and filler , which can be mixed with one or more biologically active substances , or can be used as a biodegradable filler to fill in cavities or body tissues in animals , birds and humans . the consistency and rheology , hydrophilicity and hydrophobicity , and in vivo degradation rates of the biodegradable vehicle is controlled by modulating the molecular weight of polymers and copolymers , concentration of plasticizers , ratios of two or more plasticizer in the blends , types of polymers and copolymers , copolymer ratios , and ratios of blends of polymers with different molecular weights or different copolymers . the biodegradable vehicle is mixed with one or more bas just prior to use . mixing of the bas with the biodegradable vehicle can be accomplished by simply stirring the mixture with a stirring device , or by triturating the mixture or employing an ointment mill or a suitable device or apparatus or equipment that can be used for blending / mixing . alternatively , a device , which resembles two syringes , attached together with a removable partition or a valve assembly can also be used to uniformly mix the bas with the biodegradable vehicle . the mixing is performed in order to dissolve or uniformly suspend the bas particles in the biodegradable vehicle . modulating the polymer to plasticizer ratio , polymer molecular weight , copolymer ratio , and hydrophobicity and hydrophilicity of the plasticizer controls the release of the bas from the biodegradable vehicle .