THIN FILM FABRICATION TECHNIQUE FOR IMPLANTABLE ELECTRODES

Abstract: An elongated implantable electrode assembly (50) includes a set of electrode pads (12, 14) arranged in a pre-selected pattern, and a plurality of longitudinal wires (34, 36), each wire being connected to at least one pad. The electrode assembly is formed by first depositing the pads (12, 14) on a sacrificial layer (10), adding the wires (34, 36) to the pads (12, 14), embedding the pads and wires in a carrier (46) and then removing the sacrificial layer (10). These steps can be performed using photolithographic techniques.

Description: THIN FILM FABRICATION TECHNIQUE FOR IMPLANTABLE ELECTRODES TECHNICAL FIELD This invention relates to a method of making elec¬ trodes particularly suited for implantation into an organism, using thin film technology normally used for the manufacture of integrated circuits. BACKGROUND ART A microelectrode system for neural stimulation or recording consists of a series of conducting pads which are placed close to the target neurones. The pads are connected to recording or stimulating electronics via conductors which are insulated from each other and from the surrounding medium. One such electrode arrangement which is in common use today is made by the Cochlear Limited of Lane Cove, New South Wales, Australia, under the name Cochlear Implant CI22M. This device consists of a number of platinum rings or balls positioned along a flexible insulating carrier, generally made from silicon rubber. Attempts to use thin film processing techniques to fabricate these electrodes have failed to produce a reliable electrode suitable for use in humans. For example, electrodes have been constructed with silicon substrates. These micromachined devices may be suitable for cochlear implant electrodes. See Frazier AB, Ann CH, Allen MG, "Development of micro machined devices using polyamide-based processes" Sensors and Actuators 1994 ; A45; 47-55. However, they are extremely thin (5-15 um) in order that they can be flexible enough to be inserted into the cochlea. Because of this extremely small dimension, these electrodes are very brittle and diffi¬ cult to handle. In general, the reason why thin film techniques have not been applied to commercial cochlear electrode produc¬ tion or the production of other neural electrodes lies in the difficulty of selecting materials which are com¬ patible with both the processing techniques and with the target tissue in which the device has to be used. The choice of conductor is limited to noble metals such as platinum or iridium. There are much wider choices in the insulating material, but the following properties must be main¬ tained: 1. Biocompatibility 2. Stability to the processes that are used to fabricate the metalization patterns. 3. Flexibility and mechanical compatibility with the target organ. So far, materials which adequately fulfil the require¬ ments of point 1 and 2 above do not have the required mechanical properties of point 3. Thus clearly there is a need for an improved technique for commercial production of electrodes, such as cochlear implants, and the like, which can be used relatively inexpensively and efficiently, taking advantage of photolithographic techniques. 5 DISCLOSURE OF THE INVENTION In view of the above, it is an object of the present invention to provide a method of making implantable electrode systems or arrays using thin film technology. According to one aspect of the invention there is 0 provided a method of making an elongated implantable electrode assembly comprising the steps of : providing a sacrificial layer; forming a plurality of pads on said sacrificial layer, said pads being made of an electrically conduct- 5 ing, biocompatible material; forming a plurality of wires, each wire being connected to at least one of said pads; embedding said pads and said wires in an elec¬ trically nonconductive material to form an assembly o body; and removing said sacrificial layer from said body. According to another aspect of the invention there is provided a method of forming an electrode assembly having a plurality of electrodes and wires extending 5 longitudinally through the assembly and connected through said electrodes, said method comprising the steps of forming said wires; forming a carrier about said wires and pads, said carrier being made of a flexible non-conducting material; and removing said sacrificial layer. The application of thin film techniques has obvious advantages over the more laborious, time consuming hand assembly techniques. Thin film photolithographic tech¬ niques can be used to fabricate small structures rapidly with higher density of electrodes than is possible with hand assembly. This invention allows the conductors to be formed by photolithographic processes. Elastomeric insulation is then added to free standing conductor patterns thus formed. This technique has the added advantage that the metal-polymer adhesion can be more easily optimized as the polymerization is a low energy process. Achieving a good adhesion between metal films and polymer supports ensures the mechanical integrity of the device. Most metal deposition processes intrinsically require high energy levels. For instance, in thermal evaporation or sputtering metal atoms are used which have a high energy level when they impact the polymer surface. Chemical re¬ arrangements which occur as a result of this bombardment lower the metal polymer adhesion. A solution proposed herein is to add the polymer to the metal rather than the other way around. The following is a description of a process based on standard thin film techniques which allows this to be done. The technique is illustrated with a cochlear implant electrode but other neural stimulating or recording electrodes could be made in a similar way. Briefly, in accordance with an embodiment of this invention, an electrode array suitable for implantation into a patient's body consists of a plurality of electrode pads disposed in a preselected pattern and imbedded into an elongated carrier made of a relatively stiff, non-conductive material. The carrier must be sufficiently rigid to allow the electrode assembly to be inserted smoothly into the body of a patient through an incision made for this purpose, or through a natural cavity or opening. Preferably the carrier has a minimal cross-section to ensure that during implantation, it requires only a small incision or opening, and that it does not interfere with the patient's tissues. The carrier includes a distal end adjacent to which at least some of the pads are disposed, and a proximal end. A plurality of connecting wires extend through the carrier body from the pads to the proximal end. At the proximal end, these wires are connected by coupling means to electronic circuitry for sending to and/or receiving electrical signals from the pads. A photolithographic process is used to produce the electrode assembly using a sacrificial layer as the initial base. More particularly, first, pads are formed on the base in the preselected pattern. Next, pad connectors are added to the pads. The third step con¬ sists of forming wires extending from the pad connectors to the proximal end. The wires, connectors and pads are then embedded or encapsulated in a plastic sheath which forms the carrier, and the base is eliminated. Optionally, the wires are formed with strain relief zones constructed and arranged to allow the wires to flex during manufacture and implantation without breaking. These zones advantageously also provide a strong bond with the encapsulating sheath. BRIEF DESCRIPTION OF THE DRAWINGS Figures IA- II show in perspective the steps of making an electrode assembly in accordance with this invention; Figures 2A-F shows a side sectional view of the electrode assembly during some of the steps of Figure 1; Figure 3 shows a flow chart for the subject method; Figure 4A-C show the masks used to make an elec¬ trode assembly in accordance with a first embodiment of the invention; Figures 5A-I show the masks used to make an elec- trode assembly in accordance with a second embodiment; Figure 5J shows a cross sectional view of an elec¬ trode assembly constructed by using the masks of Figures 5A-I; and Figure 6 shows a cross-sectional view of an alter- native embodiment of the invention. MODES FOR CARRYING OUT THE INVENTION In one embodiment of the invention, electrode production is carried out on a sacrificial layer made of a material selected to withstand all of the conditions of the processing steps but which can be removed by dissolution, by peeling or by heating. The electrode pads, electrode and wires are formed by a photolithographic technique, mainly by plating platinum or another suitable metal through a polyimide mask. The process relies on a combination of three basic techniques: 1. Thermal or electron-beam evaporation of platinum; 2. Masking and etching; and 3. Film thickening by electrolytic deposition. Alteratively, platinum can be deposited directly on a copper substrate through the desired mask by electroplating. Each wire is attached to an electrode pad through connectors. The connectors are arranged so that when the polyimide mask is removed the wires will be self-support¬ ing in mid-air. Preferably the wires are formed with steps or flexible zones etched into the polyamide to im¬ prove electrode longitudinal flexibility and facilitate structure encapsulation in the corner. The polyimide in the structures can be removed by either plasma etching or by chemical etch. A polymer backing, parylene or silicone rubber, is then added to the free standing electrode structure in two steps. Firstly, the back of the electrode is formed, then the sacrificial layer (e.g., copper substrate) is removed. If the sacrificial layer is a material which can be etched then it is possible that the polymer may be added in just a single step. This is achieved by forming a channel in the substrate material with a simple under¬ cut etch. Referring now to the Figures, in order to make an electrode assembly, in accordance with this embodiment of the invention, first, a sacrificial layer 10 is provided, made for example of polished copper (step 100) (Figs. IA, 2A, and 3) . This layer must have a thickness selected so that it is strong enough to withstand the chemical action the other processing steps require to manufacture the electrode assembly. Next, a plurality of electrode pads 12, 14 are formed on layer 10 (step 102) . These pads are preferably made of a highly conductive biocompatible material such as platinum, iridium or other similar material. As previously mentioned, preferably the pads are formed on layer 10 using thin film techniques well known in the art of manufacturing integrated circuits. More specifically, as shown in Figures IB-ID, step 102 consists of a number of substeps. First, (Figure IB) the layer 10 is covered, for example by spinning, with a thin layer 16 of photopolymerisable polymeric (PR) material. Next, a photolithographic technique is used to make apertures 18, 20 in layer 16, having the dimensions of the pads 12, 14. The photolithographic technique involves applying a mask (not shown) to the layer 16, ex¬ posing the mask and layer 16 to light having a specific wavelength, removing the mask, and developing the layer 16 to dissolve the portions exposed to light to generate the apertures 18, 20. These steps are well known in the art and need not be described in more detail herein. Next, platinum is applied to layer 16, for example by sputtering, or thermal electron-beam evaporation to form the pads 12, 14 in holes 18, 20. Finally, the layer 16 is dissolved leaving the pads 12, 14, as shown in Figures ID and 2B. Alternatively, pads 12, 14 could be formed by apply- ing a mask onto layer 10, electroplating platinum through apertures in the mask, and then removing the mask. In the following step 104 the pad contacts are added. This step 104 is performed essentially by repeat¬ ing the substeps used to form pads 12, 14. As shown in Figure IE, first a layer 22 of PR is applied to both layer 10 and pads 12, 14. Next, two round apertures 24, 26 are formed in layer 22 on top of pads 12, 14. A third aperture 28 is also formed on top layer 10 in a location between pads 12, 14, as shown in Figure IE. Next, plat- inum is deposited (Figure IF) , for example by vacuum or by electroplating in apertures 24, 26 for making contacts 30, 32. The resulting structure is shown in Figure 2C. Next, as indicated by step 106, the wires 34, 36 for the pads 12, 14, as well as a flexure zone 38 are formed. For this purpose, channels 40 and 42 are etched by using another lithographic mask (not shown) . Channel 40 leads from contact 30 to a proximal end of the assembly (not shown) and channel 42 leads from contact 32 also to the proximal end. Next, platinum is introduced into the channels 40 and 42, for example, by electroplating. Finally, layer 22 is removed, leaving the structures shown in Figures IH and 2D. Importantly, the wire 34 includes a step which dips down toward layer 10 and goes back up again, as best seen in Figure 2D, thereby forming a reinforcing or flexible zone 38. This step strengthens the wire 34 so that it is substantially self-supporting as it extends through the air above pad 14. In the steps described above the PR may be removed by either plasma etching or by chemical etch. Next, an insulating material 46, made for example of parylene or silicone rubber is applied on the pads 12, 14, contacts 30, 32 and wires 34, 36 to embed the same and form a cohesive elongated body 48 (step 108, Figure 2E) . Finally (step 110) , the sacrificial layer 10 is removed, leaving the completed electrode assembly 50 (Figures II, 2F) . The material 46 may be further molded to or laminated to a preselected shape. Figures 1 and 2 show the steps for making the electrode assembly at a microscopic level. Figures 4A-C and Figs 5A-J show two different schemes for making the electrode assembly at a macroscopic level. More particularly, Figures 4A-C show some of the masks used to make the apertures of Figure 1 and 2 and their respective relationship. Figure 4A shows mask 160 with square zones 161. This mask is applied to layer 16 (Figure IB) to generate the holes 18, 20. Figure 4B shows a mask 162 consisting of a plurality of bar shaped zones 164. At the distal end, the mask 162 is formed with round 166. This mask is used to generate the apertures 24, 26 and 28 in Figure IE which result 5 respectively in contacts 30, 32 and steps 38. Each zone 164 forms a step 38. Figure 4C shows a mask 168 used to generate the longitudinal apertures 40, 42, (Figure IG) which generate the wires 34, 36 (Figure IH) As seen in Figure 4C, the ιo mask includes several substantially parallel dark lines 170. Each line 170 includes a proximal end 172 for con¬ nection to an outside terminal (not shown) and a distal end 174 electrically coupled to a connector such as 30. Thus, each pad is connected to a corresponding wire ex- 15 tending through the length of the electrode system 50. The process described above may be used to manufac¬ ture an electrode assembly having a large number of elec¬ trodes. For example, in Figures 4A-4C the masks can be used to make an assembly with 22 electrodes, wherein the 20 wires are all arranged essentially in a common layer. However, for an assembly with a larger number of electrodes and/or if the electrode assembly must be limited in cross-sectional area, a multilayered approach is necessary. Figures 5A-5J show the masks for making a 25 22-electrode assembly in three layers. Figures 5A-5C and 5D-5F show the layers for eight electrodes each, while Figures 5G-5I show the masks for six additional elec¬ trodes. Preferably the bases of masks in Figures 5B, 5E and 5H are aligned so that the steps formed in the wires are superimposed as shown in the cross-sectional view of Figure 5J of an electrode assembly made using the masks of Figures 5A-I. In this embodiment, the electrodes are made in three different sets, marked in the figures as 212A, 212B and 212C, each being provided with their own connector pads and wires by using the techniques described for the embodiment of Figures 4A-4C. After the three sets are completed, they are disposed in a juxtaposed position, and sandwiched as shown in Figure 5J. Thereafter, sheath 246 is applied to form the electrode carrier. In an alternative embodiment of the invention shown in Figure 6, sacrificial layer 310 is made of a material which can be etched so that two grooves 302, 304 are formed therein prior to the formation of the electrode assembly. The electrode assembly includes electrode 312, connector 330 and wire 334, all disposed in a sheath or carriers 346. The carrier 346 is formed around the elec¬ trodes, connectors and wires, such that it flows into the grooves 302, 304. Similar grooves are formed and filled with insulating material underneath flexible zones 38 (Figures IH and 2D) of the wires. After the carrier is set, the whole electrode assembly 350 can be peeled off sheet 310 in a single step. The current electrode production techniques rely on tedious and costly hand assembly techniques. The photo¬ lithographic techniques disclosed herein provide several distinct advantages: 1. Low cost and increased throughput of devices; 2. Much higher density electrodes possible; 3. Costly hard tooling (e.g., molds) can be eliminated; and 4. Flexible and rapid design cycles. Although the invention has been described with ref¬ erence to several particular embodiments, it is to be un¬ derstood that these embodiments are merely illustrative of the application of the principles of the invention. Accordingly, the embodiments described in particular should be considered exemplary, not limiting, with re¬ spect to the following claims. INDUSTRIAL APPLICABILITY An implantable electrode assembly made in accordance with the method of the invention may be used in cochlear implants.

Claims: Claims 1. A method of making an elongated implantable electrode assembly comprising the steps of : providing a sacrificial layer; forming a plurality of pads on said sacrificial layer, said pads being made of an electrically conduct¬ ing, biocompatible material; forming a plurality of wires, each wire being connected to at least one of said pads; embedding said pads and said wires in an elec¬ trically nonconductive material to form an assembly body; and removing said sacrificial layer from said body. 2. The method of claim 1 wherein said pads and wire are formed by lithographic techniques. 3. The method of claim 1 wherein said pads and wires are formed of platinum. 4. The method of claim 1 further comprising the step of forming pad conductors between said wires and corresponding pads. 5. The method of claim 1 further comprising forming in said wires reinforcing zones. 6. The method of claim 5 wherein said zones are longitudinally offset. 7. A method of forming an electrode assembly having a plurality of electrodes and wires extending longitudinally through the assembly and connected through said electrodes, said method comprising the steps of forming said wires; forming a carrier about said wires and pads, said carrier being made of a flexible non-conducting material; and removing said sacrificial layer. 8. The method of claim 7 wherein said pads are formed on said sacrificial layer in a pre-selected pattern. 9. The method of claim 7 wherein said pads are formed substantially simultaneously. 10. The method of claim 7 wherein said pads are partitioned into a number of sets, said method comprising forming the sets of pads separately, each set having its own wires, and then assembling said sets. 11. The method of claim 7 wherein said wires are formed with reinforcing steps.

Inventors: PARKER JOHN, TREABA CLAUDIU, PARKER, JOHN, TREABA, CLAUDIU
Applicants: COCHLEAR LTD, COCHLEAR LIMITED
Application year: 1996
Main classification: H04R25
IPC codes: A61N1, H01L21, H04R25, H05K3