Patent Application
20070250142
The disclosure relates generally to implantable medical devices (“IMDs”) coated using atomic layer deposition (“ALD”) techniques to improve, among other things, hermeticity, biocompatibility, biostability, surface characteristics, and electrical properties.
The implantation of medical devices into the human body for extended periods of time has become a common practice with constantly evolving and expanding applications. Devices designed for long-term use within the body should be designed to minimize the device's undesirable impact on the body environment while at the same time limiting the environment's undesirable impact on the device. Examples of undesirable impact of the device on the body include thrombogenic response, cellular breakdown, and aggravated immune responses. Conversely, the device may be impacted by corrosion and other body fluids, moisture and ionic contaminants that decrease the useful life or performance of the device.
The importance of making implantable medical devices more compatible with the human body becomes significant as device technology enables miniaturization. This is in part because smaller devices require minute components that are susceptible to failure if exposed to otherwise minor bio-contaminants in the body. Further, increased use of media-exposed devices presents new challenges in controlling and possibly eliminating undesireable interactions between an implanted device and the body environment.
Embodiments of the invention are generally related to coatings applied to a wide variety of medical devices using atomic layer deposition techniques and methods for applying such coatings. In one embodiment, a feedthrough conductor used to convey electrical signals through the walls of a canister of an implantable medical device, without exposing the interior of the canister to the exterior environment, may be coated using the atomic layer deposition process. In another embodiment media-exposed transducers, possibly installed on an electrical lead, may be coated using the atomic layer deposition process. Embodiments of the present invention also include methods for coating implantable medical devices such as, but not limited to, electrical feedthroughs and media exposed devices with a coating using the atomic layer deposition process.
The following detailed description should be read with reference to the drawings, in which like elements in different drawings are numbered identically. The drawings depict selected embodiments and are not intended to limit the scope of the invention. It will be understood that embodiments shown in the drawings and described below are merely for illustrative purposes, and are not intended to limit the scope of the invention as defined in the claims.
Many IMDs include integrated circuits for processing information obtained from sensors (i.e., pressure sensors, electrical activity sensors, etc.) and for administering therapies (i.e., pacing pulses, defibrillation shocks, etc.). In some cases these integrated circuits are housed in hermetically sealed containers or “cans” to minimize undesirable interactions between the integrated circuit and the body. Feedthroughs are used to provide electrical connection between therapy delivery, sensing and detection components (outside the can), while maintaining a hermetic seal at the connection point.
At the proximal end, the feedthrough pin 40 includes connection pins (not shown) to provide electrical connection with circuits or other controls in IMD can 50. The other end of the feedthrough pin 40 may be connected to a lead (not shown) outside of the IMD can 50. The lead may be connected to a sensor, a therapy delivery device, or other device requiring electrical connection with the contents of the IMD can 50. The feedthrough body 20 may be in electrical communication with the IMD can 50, but the feedthrough pin 40 is isolated from the can by the insulating element 30.
The feedthrough pin 40 may be secured to the insulating element 30 via a brazing process. The braze 60 may be made of a different material of construction than the feedthrough body 20 and/or the feedthrough pin 40. In one exemplary embodiment the feedthrough body 20 is titanium, the feedthrough pin 40 is an alloy of platinum and iridium, and the braze 60 is gold. The braze 60 may wick down the pin 40 and fill most of the annular gap 80. However in some instances bodily fluids may migrate along the feedthrough pin 40 through the annular space 80 around the pin 40. These bodily fluids may corrode the braze joint 60. This problem is more likely to manifest itself in feedthroughs when the electrical current sent through the feedthrough pin 40 is direct current (DC) (i.e., sensors) rather than AC or AC-like as when electrical pulses are intermittently sent through the feedthrough pin 40.
The exterior well 90 of the feedthrough 10 may optionally be filled with a polymer 70. Examples of polymers usable in this application include, but are not limited to, epoxies, polyimides, silicones, and polyurethanes. This polymer fill acts as a hermetic seal against the migration of bodily fluid into the feedthrough 10 and also absorbs any mechanical stress that may act on the feedthrough pin 40 to protect the braze 60 and the insulating element 30. However, even with this polymer barrier, it is possible that additional protection for the feedthrough will be desirable.
Atomic layer deposition (“ALD”) is a process that allows thin conformal coatings of specific chemistries to be applied to a device or surface in a very controlled fashion. A flow chart of an exemplary ALD process is shown in
It is also possible to create laminated coatings by using different precursors in sequence. For example a layer of aluminum oxide may be followed by a layer of titanium oxide, tantalum pentoxide, or any combination of suitable materials that may be used and will occur to those of ordinary skill in the art. Once an initial layer is created by the execution, perhaps multiple times, of steps 302-305, a second volatile precursor may be introduced into the reaction chamber 306 to react with the previously formed metal oxide. The chamber may be optionally purged 307. An oxygen precursor is now added to the reaction chamber to form a self-limited layer of a second metal oxide 308. Excess precursors and byproducts are then purged from the reaction chamber 309. These steps may be repeated in various sequences to created layered coatings of two or more metal oxides in a very well controlled fashion.
In certain embodiments, an ALD coating of aluminum oxide (Al2O3) is applied to a gold brazed feedthrough as described herein. In certain embodiments, a tantalum pentoxide (Ta2O5) and a laminate of alternating layers of Al2O3 and Ta2O5 were also applied. In one case a feedthrough was coated with a 200-nanometer coating of Al2O3 and Ta2O5. Testing was performed in a saline electrolyte with an electrical differential applied between the feedthrough pin 40 and the feedthrough body 20. The coated feedthrough lasted several months without significant corrosion while the uncoated feedthrough failed in three days. The feedthrough was coated without a sensor or lead attached, but the coating process is amenable to coating the feedthrough itself or the entire device since the process is vapor phase and can coat surfaces that are not “line-of-sight” coatable. That is, because vapor phase coating processes, such as the ALD process, simply allow the coating material to envelope the item to be coated and then the coating is formed on the surface through surface reactions, these processes can coat, for example, the inside surface of an enclosed object through a relatively small opening that can allow access of the vapors to the inside of such an enclosed object. Vapor phase coatings can coat multiple surfaces of an object without the need to rotate the object or move the source of the coating material relative to the object. Other coating processes require a “line of sight,” or a direct line between the source of the coating material and the area of the object to be coated.
The ALD coatings, as just mentioned, are not limited by line of sight. In fact, ALD coatings may have an aspect ratio of 80:1 or more, meaning that a coating may be applied through an opening and coat an area eighty (80) times greater in depth than the effective diameter of the opening itself. This property of ALD coatings enables coating of the interior elements of the IMD 2 through a small opening that may be sealed after the coating process. For example, the interior of an IMD can 50 (and interior components) can be coated effectively through a relatively small opening in the can such as an opening configured to accept a feedthrough. The feedthrough can then be installed in the opening completing the construction of the device.
ALD coating techniques, consistent with the present invention, can be used on other implantable devices such as, for example, media-exposed sensors, integrated circuits, micro-electro-mechanical systems, and other media-exposed devices for use with or as an IMD.
Can 210 includes transducer 270. The transducer 270 includes an integrated circuit 220 and sensor 230. The transducer 270 of this embodiment has a sensor, such as a pressure sensor, an optical sensor, a biochemical sensor, a protein sensor, a motion sensor (e.g., an accelerometer or a gyroscope), a temperature sensor, a chemical sensor (e.g., a pH sensor), a genetic sensor, and the like. In some embodiments, the transducer 270 is a sensing transducer, an actuating transducer, an IC-only transducer, or combinations thereof, or other suitable transducers. Examples of sensing transducers include a sensor and an integrated circuit integrated on a chip (e.g., a silicon substrate or micro-electro-mechanical system (MEMS) device or nano-electro-mechanical system (NEMS) device), a sensor element without an integrated circuit built into a substrate (e.g., glass, ceramic, silicon, or other suitable material), and a sensor element built into a substrate with an integrated circuit hermetically encapsulated or packaged into a substrate. Examples of actuating transducers include an actuator and an integrated circuit integrated on a chip (e.g., a silicon substrate or MEMS device or NEMS device), a actuator element without an integrated circuit built into a substrate (e.g., glass, ceramic, silicon, or other suitable material), and an actuator element built into a substrate with an integrated circuit hermetically encapsulated or packaged into a substrate. Such actuating transducers may include a piezoelectric element actuator for vibration. Examples of IC-only transducers include an integrated circuit on a silicon substrate (e.g., an IC-logic multiplexer on a lead or a memory chip for sensor calibration coefficients) and an integrated circuit hermetically encapsulated or packaged into a substrate (e.g., glass, ceramic, silicon, or other suitable material).
In the embodiment shown in
A micro-mechanical pressure sensor that is integrated with the silicon of an integrated circuit 220 may be fabricated by bonding two wafers of silicon together to form a 4-8 μm thick diaphragm or by etching such a diaphragm out of the silicon. Other methods of creating a micro-mechanical pressure sensor are known in the art.
The transducer module 200 of the embodiment shown in
Conductors 300 connect the feedthrough pins 260 to the integrated circuit 220. The conductors 300 electrically connect the feedthrough pins to pads 240 on the integrated circuit 220. The pads 240 may be made of tantalum, niobium, titanium, or other similar electrically conductive material.
In the embodiment shown in
The inventors have learned that such media-exposed transducers have certain performance characteristics that may be preferable in certain circumstances over transducers that are hermetically sealed. It is also believed that some non-hermetic IMDs are easier and less expensive to construct and maintain and may be more reliable in some applications than hermetically sealed devices. Coating these devices with ALD coatings as described above makes them even more usable as the devices can be protected from the environment and the body can be protected from impacts from the device.
Once the embodiment of the transducer module 200 of
The ALD coating in this embodiment may coat the integrated circuit 220, sensor 230, and other components of the transducer module 200 with a thin and conformal coating. This coating can protect the silicon from etching by blood, for example.
Some integrated circuits contain a passivation layer. A passivation layer on the integrated circuit reduces the reactivity of the substrate of the integrated circuit substrate once the circuit has been formed. Such a layer may, for example, be phosphosilicate glass, silicon nitride, polyamide, and combinations thereof. The ALD coating may protect these layers from bodily fluids that may lead to breakdown of the passivation layer. For example, ALD coatings may eliminate or reduce the leaching of bonding agents that are used to connect the glass to the integrated circuit substrate.
Integrated circuits may contain dopants or metals to allow for certain functionality of the integrated circuit. For example, runners or pad out metallization may allow for electrical connectivity within the integrated circuit or between the integrated circuit and other elements or devices. Such runners or metallization may, for example, include metals such as aluminum, aluminum-copper alloy, aluminum-copper-silicon alloy, copper, titanium-tungsten alloy, titanium nitride and others known to those of skill in the art, and this invention contemplates these materials and materials yet to be applied to integrated circuits. As mentioned above, the pads 240 that allow connection of the conductors 300 and feedthrough pins 260 to the transducer 270 may be made of tantalum, niobium, titanium, or other similar electrically conductive material. The ALD coatings may protect these metals from corrosive or other attacks from bodily fluids and prevent these metals from leaching into the body where they may have deleterious effects.
Integrated circuits may also include discrete elements such as capacitors that are soldered onto the integrated circuit. A “lid” may be placed on these integrated circuits for physical protection of the circuit and to provide insulation to prevent short-circuiting among the elements. Often this lid is glass that includes components such as, for example, boron, to help bond the glass to the substrate. ALD coatings on these media-exposed devices may help to prevent the leaching of boron or other dopants from the glass or solder from the discrete elements into the bodily fluids. As the dopant leaches from the glass the glass may lose its dielectric properties. Further, the solder bond may weaken and perhaps fail if enough solder is compromised.
Thus, embodiments atomic layer deposition coatings for implantable medical devices are disclosed. One skilled in the art will appreciate that atomic layer deposition coatings for implantable medical devices can be practiced with embodiments other than those disclosed. Moreover, it should be understood that the features of the disclosed embodiments may be used in a variety of combinations. The combinations of features of the disclosed embodiments are compiled only for purposes of illustration and not limitation. The present invention is limited only by the claims that follow.
