one embodiment of the invention will now be described , however , it is noted that the present invention is of wide scope and that many possible embodiments fall within the general inventive concept . referring to fig1 the electrode array 12 is initially preferably formed in a generally straight cylindrical or semi - cylindrical shape . the materials are selected such that this shape will generally be maintained outside of the human body at normal conditions of temperature and humidity . in this form , it is relatively simple to control the insertion of the electrode into the cochlea 30 . it will be appreciated that the illustration of the cochlea is purely schematic and is not intended to be anatomically accurate . immediately after insertion , the electrode generally rests against the radially outer wall of the scala tympani 31 , as can be seen in fig4 and 5 , at position 1 . the electrode is preferably constructed in essentially two layers . referring to fig2 and 3 , the first layer ( 10 , 20 ) is formed from a suitable bio - compatible material , such as a silicone polymer , and contains the electrode contacts and connecting leads in the form of a long strip structure . it is preferably constructed from a material which is flexible for bending in at least one direction , and stable in its dimensions both outside and inside the living body . in this fashion , the correct spacings between electrodes can be maintained , and the proper electrical functioning predicted accurately in use . the second layer ( 11 , 21 ) is formed from a bio - compatible material which is adhered , co - extruded or moulded to the first layer . this material has the important property of a controlled rate of expansion when exposed to the water contained in body fluids . any suitable water - expanding material which fulfils the above criteria may be utilised in the invention . a suitable material may be a polymer which swells in response to contact with a fluid such as water . a first preferred material is silastic a ( trade mark ) silicone polymer mixed with a certain amount of finely ground nacl . the exact proportion of nacl depends on the geometry chosen and can be readily determined by those skilled in the art . a second preferred material is silastic a mixed with polyacrylic acid to form a hygroscopic layer in the electrode structure . both these materials will slowly expand in contact with water , thereby deforming the electrode structure from its original straight shape to a generally spiral shape with a curvature equal to or smaller than the scala tympani 31 . it will be appreciated that in its broadest form the invention encompasses alteration of shape in situ by any means , including polymers or other materials which alter their volume in response to other stimuli , such as heat or electrical stimuli , or materials which merely alter their shape so as to deform the array . it is the post insertion curving which is at the core of the present invention . thus , after a suitable period of time , typically hours or days , the electrode array 12 gradually moves into substantially position 2 shown in fig4 and 5 . the electrodes are always positioned on their correct side because of the expansion geometry involved , so that they are in close proximity to the modiolus . it will be appreciated that the insertion is a surgical procedure and so will not always result in theoretical results , owing to differences in human anatomy , the skill and experience of the surgeon , and other factors . fig7 illustrates one structure incorporating the features of the present invention . a first layer 10 is constructed from a mechanically stable and bio - compatible polymer strip , with metal electrodes 13 deposited on one side and leads 14 on the other side . any suitable material may be used for the first layer 10 , although polyamide foil is preferred . the electrodes 13 and leads 14 may be formed from any suitable material , for instance sputtered platinum . the other side of the electrode is formed from a generally semi - cylindrical portion 11 of water absorbing polymer . preferably , the strip 10 is adhered to the material 11 during the moulding process . fig8 illustrates an alternative embodiment of the inventive structure . the first layer 20 is formed from a number of wires 23 placed side by side to form a flat ribbon connected to the platinum disk electrodes 22 . this complete structure 20 is then moulded into the water absorbing polymer 21 . fig9 shows the connection of the discs 22 by lead wires 23 . the electrode is preferably formed with mechanical characteristics as shown in fig6 . the electrode ( in its straight form ) should be flexible in a direction which is perpendicular to the plane of the electrodes , or in other words in the radial direction in the inserted structure , shown in fig6 as direction d . it should , however , be stable along the plane of the electrodes , and relatively stiff across the plane of the electrodes , or in other words in directions perpendicular to the radial and coming out of the page in the illustration of fig6 shown as directions b and c . it will be appreciated that suitable polymer selection and the exact volumes of the various portions of the array will enable a suitable degree of curvature to be provided . other differential expansion or contraction mechanisms using the same or other material properties , or other arrangements of water expanding polymer , for instance in shaped sections , are encompassed within the present invention . the array need not be precisely straight when manufactured -- it may incorporate a small curve at the end to aid insertion . it will also be appreciated that embodiments are possible which do not have uniform characteristics at all points in the array . it will be appreciated that the present invention relates to a principle of general application and the particular embodiments disclosed herein are not to be considered as limitative . variations and additions will be apparent to those skilled in the art and are encompassed within the general scope of the invention .
an electrode array for cochlear implant is formed by two layers , wherein the first layer is formed from a bio - compatible material and contains electrode contacts and connecting leads and the second layer is formed from a bio - compatible material having a property of a controlled rate of expansion when exposed to the water contained in body fluids . this material expands in use and curves the implanted array and enables more effective stimulation .