WO2009129579A1

WO2009129579A1 - A method of coating an implantable medical device

Info

Publication number: WO2009129579A1
Application number: PCT/AU2009/000510
Authority: WO
Inventors: Suel Savci; Mark Berelowitz
Original assignee: Cochlear Limited
Priority date: 2008-04-24
Filing date: 2009-04-23
Publication date: 2009-10-29

Abstract

A method of coating an implantable medical device (10) with a fluid dispensed from a jet nozzle (45) is disclosed. The method includes the steps of: determining a desired surface area of coating for the device; defining a set of discreet points within the desired surface area; dispensing a charge of liquid from the jet nozzle (45) towards a first of the points; driving the jig (30) or the jet nozzle (45), or both the jig (30) and the jet nozzle (45) into another relative position and orientation adjacent to another of the points; dispensing a charge of liquid towards said another of the points; and repeating the above two steps until a desired coating is achieved. The method may also include the feedback steps of: capturing images of portions of the desired surface area of coating on which a charge or charges of fluid have been dispensed; analysing the images; and modifying at least one of the above steps of driving or dispensing based on the analysis of the images, thereby improving the coating.

Description

A METHOD OF COATING AN IMPLANTABLE MEDICAL DEVICE

FIELD OF THE INVENTION

The invention relates to implantable medical devices. In particular, the invention relates to coating medical devices.

INCORPORATION BY REFERENCE

The following documents are hereby incorporated by reference:

- Australian Provisional Patent Application No. 2008902047 titled "A METHOD OF SEALBSfG AN IMPLANTABLE MEDICAL DEVICE"

- Australian Provisional Patent Application No. 2008904592 titled "METHOD AND APPARATUS FOR REDUCING ENERGY DRAIN IN A MEDICAL DEVICE"

BACKGROUND OF THE INVENTION Medical implants are used in many areas of medicine to enhance the length and/or quality of the life of the implant recipient. Such implants include pacemakers, controlled drug delivery implants and cochlear implants.

A cochlear implant allows for electrical stimulating signals to be applied directly to the auditory nerve fibres of the user, allowing the brain to perceive a hearing sensation approximating the natural hearing sensation. These stimulating signals are applied by an array of electrodes implanted into the user's cochlea. The electrode array is connected to a stimulator unit which generates the electrical signals for delivery to the electrode array. The stimulator unit in turn is operationally connected to a signal processing unit which also contains a microphone for receiving audio signals from the environment, and for processing these signals to generate control signals for the stimulator.

The signal processing unit is in practice, located externally to the user and the stimulator is implanted within the user, usually near the mastoid on the user's skull and underneath the surrounding tissue. The processor and stimulator may communicate by various wireless means including by a radio frequency link.

Medical devices, including but not limited to cochlear implants and their stimulators, that are implanted within a user's body require sealing against the uncontrolled ingress of liquids in many cases. For example, medical devices having metallic or ceramic surfaces may require over-moulding at certain locations to prevent the uncontrolled ingress of fluids. Over-moulds may be made from silicone for instance. Known methods of sealing implantable medical devices to prevent the uncontrolled ingress of fluids include manual methods of applying silicone adhesive or primer onto predefined regions on the surface of an implantable medical device. After application of the silicone adhesive or primer, an over-mould part is bonded to the device to provide a finished assembly that prevents fluid ingress.

A disadvantage of the above described manual process is that it is difficult to achieve consistent coating thickness. Furthermore, such manual processes are time consuming and suffer quality control problems. Such problems can lead to delamination and to fluid ingress.

In addition to, or instead of, requiring coats of adhesive or primers, many implantable medical devices (IMDs) require, or benefit from, other coatings of various types. For instance, coatings may be provided to prevent biofilm growth or to promote resistance against infection. Coatings may also be provided to prevent or enhance tissue growth and protein attachment, adhesion, bonding or adsorption. Manual application of such coatings to DVIDs can result in inconsistencies and defects.

It is an object of the invention to provide an improved method of coating an implantable medical device that overcomes or assists with overcoming at least some of the above described problems or at least offers the user some choice.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a method of coating an implantable medical device with a fluid dispensed from a jet nozzle, the method including the steps of:

(a) determining a desired surface area of coating for the device;

(b) defining a set of discreet points within the desired surface area; (c) loading the device onto a jig;

(d) driving the jig or the jet nozzle, or both the jig and the jet nozzle, into a first relative position and orientation adjacent to a first of the points;

(e) dispensing a charge of liquid from the jet nozzle towards the first of the points;

(f) driving the jig or the jet nozzle, or both the jig and the jet nozzle into another relative position and orientation adjacent to another of the points;

(g) dispensing a charge of liquid towards said another of the points;

(h) repeating the above two steps until each point is covered in liquid dispensed from the jet nozzle; and

(i) allowing the liquid to flow such that at least most of the charges of liquid join to an adjacent charge of liquid. According to a second aspect of method of sealing an implantable medical device against uncontrolled fluid ingress, the method including the steps of:

(a) determining a desired surface area of coating for the device;

(b) defining a set of discreet points within the desired surface area; (c) loading the device onto a jig, the jig having a platform rotatable about two non-parallel axes with respect to a jig frame, the jig frame positionable adjacent to a jet assembly, the jet assembly including a jet nozzle movable with respect to the jig frame in x, y and z directions, the jet nozzle arranged to dispense liquid in the z direction;

(d) rotating the jig platform and hence the device about the two axes into an initial orientation with respect to the z direction;

(e) driving the jet to an x y position adjacenta first of the points and driving the jet in a z direction to a predetermined height h above the first of the points;

(f) dispensing a charge of liquid from the jet nozzle towards the first of the points;

(g) driving the jet to an x y position adjacentanother of the points and driving the jet in a z direction to a predetermined height h above said another of the points;

(h) dispensing a charge of liquid towards said another of the points;

(i) repeating the above two steps until each point is covered in liquid dispensed from the jet nozzle;

(j) allowing the liquid to flow such that most of the charges of liquid join to an adjacent charge of liquid; (k) placing an over-mould component onto the device so as to substantially cover the liquid, whereby the liquid bonds the over-mould component to the device.

In one form the method includes the feedback steps of: capturing images of portions of the desired surface area of coating on which a charge or charges of fluid have been dispensed; analysing the images; and modifying at least one of the above steps of driving or dispensing based on the analysis of the images, thereby improving the coating.

According to a third aspect of method of coating an implantable medical device with a fluid dispensed from a jet nozzle, the method including the steps of:

(a) determining a desired surface area of coating for the device;

(b) defining a set of discreet points within the desired surface area; (c) dispensing a charge of liquid from the jet nozzle towards a first of the points;

(d) driving the jig or the jet nozzle, or both the jig and the jet nozzle into another relative position and orientation adjacent to another of the points; (e) dispensing a charge of liquid towards said another of the points; and

(f) repeating the above two steps until a desired coating is achieved.

In one form the method of the third aspect of the invention includes the feedback steps of: capturing images of portions of the desired surface area of coating on which a charge or charges of fluid have been dispensed; analysing the images; and modifying at least one of the above steps of driving or dispensing based on the analysis of the images, thereby improving the coating.

A specific embodiment of the invention will now be described in some further detail with reference to and as illustrated in the accompanying figures. This embodiment is illustrative, and is not meant to be restrictive of the scope of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION

A preferred embodiment of the invention is illustrated in the accompanying representations in which:

Figure 1 is a diagrammatic isometric view of an apparatus for performing a method according to the invention;

Figure 2 is an isometric view of an implantable medical device having external surfaces to which the method of the invention can be applied;

Figure 3 is a similar view to Figure 2a but shows the same medical device from an opposite side;

Figures 4 and 5 show a further medical device having internal surfaces that can be sealed by the method of the invention;

Figure 6 is a diagrammatic cross-sectional view of the medical device of Figures 2a, 2b and 3;

Figure 7 is an isometric view of the medical device of Figures 2a, 2b and 3 with an over-mould in place;

Figure 8a is an isometric view of an implantable medical device having external surfaces to which the method of the invention can be applied; Figure 8b is a close up view of the portion of the implantable medical device shown in Figure 8a as indicate by circle A;

Figure 9 is an isometric view of an implantable medical device having extra cochlear electrode to which the method of the invention can be applied.

Figure 10 is a similar view to that of figure 1, but shows a camera provided to enable feedback. Referring first to Figure 1, an automated gimbal jig 30 is shown. Above the gimbal jig 30 is a jet assembly 40 having a jet nozzle 45 which is moveable with respect to the jig frame 35 in x,y and z directions. The jet nozzle 45 is arranged to dispense liquid in the z direction.

The jig 30 has a jig platform 50 that is rotatable about two perpendicular axes 32 and 34 with respect to the jig frame 35. This allows a medical device such as the medical device 10 as illustrated in Figures 2a, 2b and 3 to be presented in any desired orientation with respect to the jet nozzle 45.

An example of how the method of the invention may be performed is as follows.

Sealing of an implantable medical device 10, such as that shown in Figure 2a, to prevent uncontrolled fluid ingress, includes the steps of determining a desired surface area 20 of coating for the device and then defining a set of discreet points within the desired surface area 20 as illustrated in Figure 2b. The device 10 is loaded into the jig 30. This is shown in Figure 1. Once loaded, the device 10 can be rotated about axes 32 and 34 as indicated by rotation arrows θ and β. The jig platform 50 and hence the device 10 is rotated about the two orthogonal axes 32 and 34 into an initial orientation with respect to the z direction indicated in Figure 1. The jet is then driven to an x,y position over a first of the points (such as the points illustrated in Figure 2b) and is also driven in a z direction to a predetermined height h above the first of the points. A charge of liquid is then dispensed from the jet nozzle 45 onto the first of the points. The jet is then driven through an x,y position over another of the points and is driven in a z direction to a predetermined h above said another of the points. Again, a charge of liquid is dispensed from the jet nozzle to another of the points. The above two steps are repeated until each point is covered in liquid dispensed from the jet nozzle. The fluid is allowed to flow such that most of the charges of liquid joint to an adjacent charge of liquid. It has been found that using this method a desired surface area 20 can be accurately coated with silicone adhesive or primer in such a way as to minimize thickness variability. With a uniform coating of silicone adhesive or primer, an over-mould can be placed onto the device so as to substantially cover the silicone. The silicone adhesive bonds the over-mould component to the device.

Referring again to Figure 1 it can be seen that the jet assembly 40 is movable in three mutually perpendicular directions x, y and z. In the embodiment of the invention described, the x and y directions he in a horizontal plane and the z direction is a vertical direction. In other embodiments of the invention, the axes may be orientated differently. The jig 30 is spaced from the jet assembly 40 in the z direction as shown in Figure 1. The jig 30 allows rotation of the platform 50 and hence the device 10 to be coated about two mutually perpendicular axes 32 and 34. Thus, there are a total of five controllable position variables: x, y, z, θ and β. Generally the silicone adhesive or priming material will be enclosed in a specified pressure. Computerized control systems of various types can be used to define a set of discreet points within a desired surface area as illustrated in Figure 2b. Manual methods or automated methods using algorithms may be used to determine the optimum spacing and density of the points taking into account the flow properties of the liquid being dispensed (typically silicone) so as to provide an optimum coating 21 over the desired surface area 20. The viscosity of the silicone adhesive or primer can be diluted to the desired viscosity either using a suitable solvent or heat in order to enable the desired thickness of coating to be applied (eg. 4OcP will provide a 50 micron cured coating thickness).

The amount of liquid dispensed at each point can be tightly controlled. Hence, the precision with which the silicone adhesive is dispensed translates to a very high level of accuracy with respect of its thickness and location. This enables design requirements to be fulfilled without the need for marking regions that don't need to be coated. Consequently, rework/waste and variability in the thickness of the coating are minimized using the method described above.

The above-described method can be used to coat various liquids including silicone adhesive or primer. For example silicone MED2-4213 diluted with two parts n-heptane resulting in a viscosity between 40 and 5OcP can be used. This has been found to bond well with silicone MED-4860. The aforementioned method can be used to adhere to various surfaces or substrates including platinum, titanium and ceramic.

The above-described method has been found to prevent failures previously caused by fluid reaching electrical conductors within the medical device.

The function of liquid silicone adhesive or primer is to provide a coating 21 that provides adhesion between a substrate surface and the over-moulded silicone 80 illustrated in Figure 7 resulting in a physical barrier for preventing uncontrolled fluid ingress. Fluid ingress can occur as a result of breaking of the covalent bond at the interface rather than a diffusion mechanism. Generally speaking most silicones are at equilibrium with water at 1%. The problem from fluid ingress stems from damage at the interface between the substrate and silicone adhesive or primer such as nicks/cuts from surgical tools, delamination during handling, due to an initial poor adhesion or a reduction in adhesion from aging effects during the product life cycle of the implanted devices (eg. oxidative, enzymatic or hydro lytic damage) thereby resulting in a pathway for fluid to enter the electrical conductors resulting in failures in electric conduction. The effectiveness of preventing fluid ingress is believed to be a function of the material composition of the silicone adhesive or primer and the adhesion of the substrate to the over- moulded silicone. It is also dependent on ensuring complete coverage of surfaces to be protected from fluid ingress. Investigations have revealed that the peel strength correlates with the thickness of the silicone adhesive or primer hence is an important parameter to control. The implantable medical device 10 shown in Figures 2a, 2b and 3 have regions 20 where primer is desired. These can be uniformly coated using methods according to the invention as described above.

The adhesion of the coating onto the substrate depends on the chemistry of the coating being applied (i.e. number of reactive sites), the surface finish of the substrate and the surface contamination on the substrate. The affect of the contamination can vary from inhibiting cure to preventing adhesion depending on the type of contamination. For instance, sulphur and tin will inhibit the cure of MED2- 4213 while silicone oils, graphite, greases, surfactants and titanium soot will prevent its adhesion onto the substrate. The surface finish can be controlled using roughness measurements and the surface contamination can be measured using chemical analysis techniques such as x-ray photoelectron spectroscopy (XPS) by quantifying the measure of elemental composition by irradiating a material with a beam of X-rays. Alternatively a correlation between adhesion (i.e. via peel strength) and surface tension or surface contact angle can be established in order to determine cohesive adhesion failure where failure occurs at the bulk of the silicone material rather than at the interface with the substrate. Cohesive adhesion is important for preventing uncontrolled fluid ingress otherwise delamination at the interface can easily occur particularly under fatigue loading thereby providing a pathway for fluid ingress.

Referring to Figures 2a, 2b and 3, an example of the medical device 10 that is implantable into a user's body is shown. With this device, a discreet set of points within the desired surface area of the coating are illustrated in the enlarged view of Figure 2b. After the liquid, in this case silicone adhesive, has been dispensed from the jet nozzle onto each of the points illustrated in Figure 2b, it flows so that the liquid droplets join to form a continuous surface. By appropriate selection of the position of the points, the thickness of the silicone adhesive can be controlled accurately. This is the case even for internal surfaces 20 such as illustrated in Figures 4 and 5.

The rotation of the device 10 about the axes 34 and 32 illustrated in Figure 1 can be achieved in a number of conventional ways. For instance, stepper motors can be located on the jig to fully automate the process. Furthermore, the jig assembly can be a removable jig assembly. With a removable jig assembly, once the silicone adhesive has been dispensed onto the device 10, the jig assembly together with the device 10 can be transferred to a curing station. Furthermore, devices that do not require rotation with respect to the z axis of the jet nozzle 45 can be located on a separate non-rotatable jig. A hot air blower can be employed to assist with curing before or after over-moulding.

It has been found that the peel strength of the over-mould away from the device correlates with the thickness of the silicone adhesive or primer and hence is an important parameter to control. Further, it has been found that using the method described above, a superior peel strength can be obtained. The method of coating an implantable medical device may be used in conjunction with other technologies such as the application of anti-microbial coatings (for instance containing active ingredients such as silver, chitosan, selenium, PEG, peptides) or with drug delivery coatings (for example DEX) or for application of self assembling monolayers (SAMs) for prevention of protein adhesion. The coatings in these instances typically on top of the over-moulded silicone to metal surfaces (that is outer surfaces) in order to prevent biofilm growth or resistance against infection effectively. For instance, referring to Figure 8a an isometric view of an implantable medical device having external surfaces to which the method of the invention can be applied is shown. The method can be used to apply an anti-microbial coating to the region indicated by arrow 23 (or any other desired area).

Figure 8b is a close up view of the portion of the implantable medical device shown in figure 8a as indicate by circle A. Again the method of the invention can be used to precisely deliver coating material as desired.

Use of the coating with the described above in the application of SAMs to EvIDs will now be described. IMDs such as cochlea implants have both electrically non-conductive regions and electrically conductive regions. Electrically conductive regions can include elctrodes. The electrodes deliver electrical signals to neural sensors of the cochlear. An Extra Cochlear Electrode (ECE) 22 is shown in Figure 9. The generation and delivery of these electrical stimulating signals requires energy, which is provided by a power source such as a battery. Battery life is a critical feature of IMD design. Energy drain of a power source for a medical implant can severely affect the effectiveness and performance of the implant. Furthermore, a power source that is drained more quickly necessitates more frequent recharging or replacement of the power source. In some instances, this may require surgery, which is highly undesirable for the patient.

The applicant's Provisional Patent Application No. 2008904592 titled "Method and apparatus for reducing energy drain in a medical implant" (the entire contents of which is hereby incorporated by reference) discloses a medical implant that has a self assembling monolayer (SAM) on the surface of its electrode contacts. These SAMs help prevent or reduce build up of impedance-inducing materials such as fibrous tissue encapsulations or protein adhesions or other attachments to the surfaces of the implant.

Different types of SAMs can be used on other portions of IMDs to enhance tissue growth and protein attachment, adhesion, bonding, or adsorption.

Known methods of coating MDs with SAMs include placing the IMD (or part of the IMD) into an appropriate solution. Such methods typically require selective masking of areas on the surface of the The method of coating part of the surface of an MD with a silicone adhesive or primer to bond an overmould described above can also be used to facilitate coating of portions of IMDs with SAMs. In particular, the jetting processes provides for precise and accurate delivery of liquid to particular locations on the surface of the IMD. This enables masking to be dispensed with and minimises any rework or waste that may otherwise arise with manual coating methods.

An Extra Cochlear Electrode (ECE) 22 on an MD is shown in figure 9. As described above, application of a self assembling monolayer (SAM) on the surface of electrode contacts, such as ECE 22, can prevent or reduce build up of impedance-inducing materials such as fibrous tissue encapsulations or protein adhesions or other attachments to the surfaces of the implant. Again the method of the invention can be used to produce a SAM over the ECE 22. The method can also be used to apply an anti-microbial coating to the magnet 25, or surfaces adjacent to the magnet 25 of the MD.

Referring to Figure 10, the method of the invention can include the use of imaging to improve the coating applied. For instance a camera 90 having a lens 92 can be used to capture images of portions of the desired surface area of coating on which a charge or charges of fluid have been dispensed. These images can then be analysed. The steps of driving or dispensing can then be modified based on the analysis of the images to thereby improving the coating.

The jetting processes described above are not limited to a particular jetting process. Depending on the material being deposited, the jetting technology may be varied. With the method described above, a jetting system employing a solenoid controlled needle valve has been found to be effective. The solenoid pulses the needle allowing the silicone adhesive to be precisely dispensed from a pneumatically pressurised supply chamber. In other embodiments of the invention, piezoelectric ink jets or thermal inkjet designs may be suitable alternatives for the jetting process.

Throughout the specification and the claims that follow, unless the context requires otherwise, the words "comprise" and "include" and variations such as "comprising" and "including" will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement of any form of suggestion that such prior art forms part of the common general knowledge.

While the present invention has been described in terms of preferred embodiments in order to facilitate without departing from the principles of the invention. Therefore, the invention should be understood to include all such modifications within its scope.

Claims

THE CLAIMS:

1. A method of coating an implantable medical device with a fluid dispensed from a jet nozzle, the method including the steps of:

Q) determining a desired surface area of coating for the device; (k) defining a set of discreet points within the desired surface area;

(1) loading the device onto a jig; (m) driving the jig or the jet nozzle, or both the jig and the jet nozzle, into a first relative position and orientation adjacent to a first of the points;

(n) dispensing a charge of liquid from the jet nozzle towards the first of the points; (o) driving the jig or the jet nozzle, or both the jig and the jet nozzle into another relative position and orientation adjacent to another of the points; (p) dispensing a charge of liquid towards said another of the points; (q) repeating the above two steps until each point is covered in liquid dispensed from the jet nozzle; and (r) allowing the liquid to flow such that at least most of the charges of liquid join to an adjacent charge of liquid.

2. A method of sealing an implantable medical device against uncontrolled fluid ingress, the method including the steps of: (1) determining a desired surface area of coating for the device;

(m) defining a set of discreet points within the desired surface area;

(n) loading the device onto a jig, the jig having a platform rotatable about two non-parallel axes with respect to a jig frame, the jig frame positionable adjacent to a jet assembly, the jet assembly including a jet nozzle movable with respect to the jig frame in x, y and z directions, the jet nozzle arranged to dispense liquid in the z direction;

(o) rotating the jig platform and hence the device about the two axes into an initial orientation with respect to the z direction; (p) driving the jet to an x y position adjacenta first of the points and driving the jet in a z direction to a predetermined height h above the first of the points; (q) dispensing a charge of liquid from the jet nozzle towards the first of the points;

(r) driving the jet to an x y position adjacentanother of the points and driving the jet in a z direction to a predetermined height h above said another of the points; (s) dispensing a charge of liquid towards said another of the points;

(t) repeating the above two steps until each point is covered in liquid dispensed from the jet nozzle; (u) allowing the liquid to flow such that most of the charges of liquid join to an adjacent charge of liquid; whereby the liquid bonds the over-mould component to the device.

3. A method as claimed in either one of claims 1 or 2 including the feedback steps of: capturing images of portions of the desired surface area of coating on which a charge or charges of fluid have been dispensed; analysing the images; and modifying at least one of the above steps of driving or dispensing based on the analysis of the images, thereby improving the coating.

4. A method of coating an implantable medical device with a fluid dispensed from a jet nozzle, the method including the steps of:

(g) determining a desired surface area of coating for the device;

(h) defining a set of discreet points within the desired surface area; (i) dispensing a charge of liquid from the jet nozzle towards a first of the points;

(j) driving the jig or the jet nozzle, or both the jig and the jet nozzle into another relative position and orientation adjacent to another of the points;

(k) dispensing a charge of liquid towards said another of the points; and

(1) repeating the above two steps until a desired coating is achieved.

5. A method as claimed in claim 4 including the feedback steps of: capturing images of portions of the desired surface area of coating on which a charge or charges of fluid have been dispensed; analysing the images; and modifying at least one of the above steps of driving or dispensing based on the analysis of the images, thereby improving the coating.