Patent Publication Number: US-2021186422-A1

Title: Implantable medical device with metal and polymer housing

Description:
This application claims the benefit of U.S. Provisional Patent Application No. 62/951,617, filed Dec. 20, 2019, the entire content of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The disclosure generally relates to medical devices, and more particularly, to implantable medical devices. 
     BACKGROUND 
     Various implantable medical devices (IMDs) have been clinically implanted or proposed for therapeutically treating or monitoring one or more physiological conditions of a patient. Such devices may be adapted to monitor or treat conditions or functions relating to heart, muscle, nerve, brain, stomach, endocrine organs or other organs and their related functions. Advances in design and manufacture of miniaturized electronic and sensing devices have enabled development of implantable medical devices capable of therapeutic as well as diagnostic functions such as pacemakers, cardioverters, defibrillators, biochemical sensors, and pressure sensors, among others. Such devices may be associated with leads to position electrodes or sensors at a desired location, or may be leadless, with the ability to wirelessly transmit data either to another device implanted in the patient or to another device located externally of the patient, or both. 
     In some examples, implantable miniature sensors have been proposed and used in blood vessels to measure directly the diastolic, systolic, and mean blood pressures, as well as body temperature and cardiac output. As one example, patients with chronic cardiovascular conditions, particularly patients suffering from chronic heart failure, may benefit from the use of implantable sensors adapted to monitor blood pressures. As another example, subcutaneously implantable monitors have been proposed and used to monitor heart rate and rhythm, as well as other physiological parameters, such as patient posture and activity level. Such direct in vivo measurement of physiological parameters may provide significant information to clinicians to facilitate diagnostic and therapeutic decisions. If linked electronically to another implanted therapeutic device (e.g., a pacemaker), the data may be used to facilitate control of that device. Such sensors also, or alternatively, may be wirelessly linked to an external receiver. 
     SUMMARY 
     In some aspects, this disclosure describes implantable medical devices (IMDs) and example techniques for manufacturing such devices. The IMD may have a housing including a metal housing component joined to a polymer housing component along an interface between the two components. The housing of the IMD may contain electronic circuitry as well as other components such as a power source, e.g., that operates the medical device to sense one or more patient parameters or other operating functions of the 1 MB. 
     In some examples, the metal housing component and polymer housing component may be joined by positioning the respective components adjacent to each other along an interface, and then delivering energy to the metal housing component. Heat may be transferred to the polymer housing component from the metal housing component, e.g., via conductive heat transfer along the interface. The heating of the metal housing component may cause a portion of the polymer housing component to melt. The melted portion may wet on the surface of the metal housing component and then solidify by cooling to form the seal between the metal housing component and the polymer housing component. In some examples, the seal formed between the two components may be a hermetic seal. 
     In some examples, the disclosure is directed to a method for manufacturing the 1 MB. The method may include positioning a metal housing component adjacent to a polymer housing component so that there is an interface between the metal housing component and the polymer housing component. The method may further include forming a seal at the interface between the metal housing component and the polymer housing component to join the metal housing component and the polymer housing component, wherein the joined metal housing component and the polymer housing component form at least a portion of housing for the implantable medical device, wherein the housing of the implantable medical device contains electronic circuitry. 
     In some examples, the disclosure is directed to an 1 MB having electronic circuitry; and a housing, wherein the processing circuitry is contained within the housing, wherein the housing includes a metal housing component and a polymer housing component sealed to each other along an interface. 
     The details of one or more aspects of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a conceptual diagram illustrating an example medical device system, in accordance with some examples described in this disclosure. 
         FIG. 1B  is a conceptual diagram illustrating another example medical device system, in accordance with some examples described in this disclosure. 
         FIG. 2  is a conceptual diagram illustrating an example IMD including metal and polymer housing components. 
         FIGS. 3A-3D  are schematic diagrams illustrating an example IMD including metal and polymer housing components. 
         FIG. 4  is a flow diagram illustrating an example technique, in accordance with some examples described in this disclosure. 
         FIG. 5  is a schematic diagram illustrating a portion of an example IMD with an energy beam applied, in accordance with some examples described in this disclosure. 
         FIGS. 6-8  are micrographs showing cross-sectional views of various samples for experiments performed to evaluate aspects of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     As described above, the disclosure relates to implantable medical devices (IMDs) and example techniques for making IMDs. Various IMDs have been implanted or proposed for therapeutically treating or monitoring one or more physiological conditions of a patient. These IMDs may include a metal outer housing that contains electronic components capable of monitoring patient data, transmitting patient data, processing patient data, and/or delivering electrical stimulation into the body of the patient. The IMD may also include one or more electrodes located on the metal housing, e.g., to conduct electrical signals to and from the electronics within the metal housing. In some examples, the metal housing of the IMD may be formed of multiple metal housing components (e.g., top and bottom housing components) that are combined to form an outer housing of the IMD that forms a hermetically sealed enclosure for containing the electronics and other components of the IMD. 
     During the manufacturing process, the metal housing components may be joined to each other by a metal welding process. It may be important that metal housing components are adequately sealed to each other to protect the electronic components from liquid or vapor leaking into the device. Furthermore, having electrodes located on the outer metal housing may require complicated feedthroughs and/or other measures to electrically isolate the electrodes from the outer metal housing, e.g., to prevent shunting of electric fields sensed by the electrodes. However, the welding process to join two metal housing components as well as the design requirements to electrically isolate electrodes located on the metal housing may be relatively expensive. 
     In accordance with the disclosure, an IMD including a metal housing component and polymer housing component, and methods for manufacturing such IMD are described. The metal and polymer housing components may combine to form an enclosure within the outer housing of the IMD that contains electronic circuitry and/or other internal components of the 1 MB. In some examples, the IMD may include one or more electrodes on the polymer housing component. The polymer housing component may be formed of an electrically insulative polymer, e.g., to electrically isolate the electrodes from the metal housing component as well as electrically isolating respective electrodes from each other. 
     Examples of the disclosure may include techniques for joining a metal housing component to a polymer housing component to form an outer housing of the 1 MB. An example technique may include positioning a metal housing component adjacent to a polymer housing component, such that there is an interface between the two components. A seal may be formed at the interface between the metal housing component and polymer housing component by applying energy to the metal component to heat to the metal housing component. The heating of the metal housing component may in turn heat the polymer housing component to an elevated temperature at the interface, e.g., where the elevated temperature is equal to or greater than a melting temperature (and, e.g., lower than the decomposition onset temperature) of the polymer housing component. The elevated temperature may melt the polymer housing component to reflow in the area of the interface and wet the surface of the metal housing component at the interface. Upon cooling of the polymer, a seal may be formed between the metal housing component and the polymer housing component. 
     In some examples, the method of heating the metal housing component may include a laser welding process or other process in which a laser or other energy source is directed at the surface of the metal housing component. In one example, a laser welding process may be used in which a pulsed laser beam or continuous wave laser beam may be directed to a surface of the metal housing component to heat the metal housing component and, as a result, melt or otherwise cause a portion of the polymer housing component to reflow at an interface between the polymer housing component and metal housing component. In one example, the laser beam or other energy source may move relative to the metal housing component while forming the hermetic seal. As will be described below, other suitable energy sources and/or heating techniques may be employed in such a process and may include, e.g., inductive energy sources or process which furnace heating is employed to heat the metal part with subsequent insertion/assembly with the polymer housing component, resistance heating by current applied to the metal housing component, friction against the surface of the metal housing component, RF heating, heat from another focused light, hot air, conductive heat transfer or other heat transfer from contact, ultrasonic energy, and/or the like. 
     In some examples, the IMD having the combined metal and polymer housing components may include a miniaturized implantable medical device configured to sense various physiological parameters of a patient, such as one or more physiological pressures, electrical signals, and the like. Such devices may include a hermetic housing that contains a power source and electronic circuitry to operate the 1 MB. In some examples, the IMD may include one or more electrodes each defined by an electrically conductive surface on an outer surface of the polymer housing component. In some examples, the IMDs may include processing circuitry configured to at least sense electrical signals via the electrode(s). In some examples, the IMD may include processing circuitry configured to at least control delivery of electrical stimulation via the electrode(s). 
     In some examples, an IMD including metal and polymer housing components joined by a seal to define the outer housing of the IMD may provide one or more advantages. As noted above, the polymer housing component may function to electrically isolate the one or more electrodes on the housing from the metal housing component and/or other electrodes of the 1 MB. Additionally, or alternatively, a controlled process to join the polymer and metal housing components using an energy source to heat the metal component to melt the polymer housing at an interface between the components may allow for improved manufacturability and improved manufacturing efficiency. Joining a metal housing component and polymer housing component to form a hermetically sealed IMD housing may be performed with more precision and replicability than other sealing methods that involve an additional material such as a polymer adhesive. An IMD containing a hermetically sealed metal housing component and polymer housing component may prevent any disruptions in IMD performance for the entirety of the operating life of the device. An IMD containing a hermetically sealed metal housing component and polymer housing component may reduce the number of foreign materials introduced into a patient&#39;s body, and ensure the proper function of the IMD. 
     For ease of description, examples of the present disclosure are primarily described with regard to miniaturized sensing medical devices configured to be implanted within the heart of a patient, e.g., to monitor a pressure within the heart of the patient. However, examples are not limited to such devices and configurations. Other medical devices including multi-component outer housings, e.g., that house internal components within a hermetically sealed enclosure are contemplated. In some examples, the multi-component housings may be employed with IMDs such as implantable cardiac devices that deliver pacing, defibrillation and/or cardioversion therapy, or implantable medical devices that delivery neurostimulation therapy to patient. In some examples, the IMD may be configured to deliver electrical stimulation therapy to a patient, e.g., via one or more electrically coupled leads, and/or may sense bioelectrical signals of the patient. In other examples, the IMD may sense one or more physiological parameters of a patient but not delivery electrical therapy to the patient, e.g., to monitor one or more cardiac parameters of a patient without delivering electrical therapy to the patient. Other functions of an IMD of this disclosure beyond electrical sensing/stimulation may include capturing chemical parameters (e.g., glucose or oxygen sensor), capturing images, providing navigation assistance when delivering the device, and/or delivering fluids/other bioactive items. 
     In some examples, the medical device may be an IMD that is targeted for a relatively short-term implant in a patient and/or IMD that are relatively easy to explant from a patient. In some examples, the IMD may function the same or substantially similar to Reveal LINQ™ ICM, available from Medtronic plc, of Dublin, Ireland. In some examples, medical devices of this disclosure may be configured to be implanted in a patient subcutaneously, subpectorally, and/or in any other suitable implant location. 
       FIG. 1A  is a conceptual diagram illustrating an example of a medical device system  8 A. Medical device system  8 A includes IMD  15 A, which is implanted within patient  2 A, and external device  14 A. IMD  15 A may comprise an implantable or insertable cardiac monitor or an implantable hub device, in communication with external device  14 A. Medical device system  8 A also includes implantable sensor assembly  10 A, which comprises pressure sensing device  12 A. As shown in  FIG. 1A , implantable sensor assembly  10 A may be implanted within pulmonary artery  6 A of heart  4 A of patient  2 A. In some examples, pulmonary artery  6 A of heart  4 A may comprise a left pulmonary artery, whereas in other examples, pulmonary artery  6 A may comprise a right pulmonary artery. For the sake of clarity, a fixation assembly for sensor assembly  10 A is not depicted in  FIG. 1A . IMD of this disclosure are not limited to those configured to be implanted in a heart of a patient. 
     IMD  15 A comprises an insertable cardiac monitor (ICM) configured to sense and record cardiac electrogram (EGM) signals from a position outside of heart  4 A, and will be referred to as ICM  15 A hereafter. In some examples, ICM  15 A includes or is coupled to one or more additional sensors, such as accelerometers, that generate one or more signals that vary based on patient motion, posture, blood flow, or respiration. ICM  15 A may monitor a physiological parameter such as posture, heart rate, activity level, or respiration rate, and may do so at times when the one or more additional sensors, such as pressure sensing device  12 A, is configured with circuitry to measure the cardiovascular pressure of patient  2 A. ICM  15 A may be implanted outside of the thoracic cavity of patient  2 A, e.g., subcutaneously or submuscularly, such as at the pectoral location illustrated in  FIG. 1A . In some examples, ICM  15 A may take the form of a Reveal LINQTAIICM, available from Medtronic plc, of Dublin, Ireland. 
     ICM  15 A may transmit posture data, and other physiological parameter data acquired by ICM  15 A, to external device  14 A. ICM  15 A also may transmit cardiovascular pressure measurements received from pressure sensing device  12 A to external device  14 A. External device  14 A may be a computing device configured for use in settings such as a home, clinic, or hospital, and may further be configured to communicate with ICM  15 A via wireless telemetry. For example, external device  14 A may be coupled to a remote patient monitoring system, such as Carelink®, available from Medtronic plc, of Dublin, Ireland. External device  14 A may, in some examples, comprise a programmer, an external monitor, or a consumer device such as a smart phone. 
     External device  14 A may be used to program commands or operating parameters into ICM  15 A for controlling its functioning, e.g., when configured as a programmer for ICM  15 A. External device  14 A may be used to interrogate ICM  15 A to retrieve data, including device operational data as well as physiological data accumulated in the memory of ICM  15 A. The accumulated physiological data may include cardiovascular pressure generally, such as one or more of a systolic pressure, a diastolic pressure, and a mean pulmonary artery pressure, or medians of such pressures, although other forms of physiological data may be accumulated. In some examples, the interrogation may be automatic, e.g., according to a schedule. In other examples, the interrogation may occur in response to a remote or local user command. Programmers, external monitors, and consumer devices are examples of external devices  14 A that may be used to interrogate ICM  15 A. 
     Examples of wireless communication techniques used by ICM  15 A and external device  14 A include radiofrequency (RF) telemetry, which may be an RF link established via an antenna according to Bluetooth®, Wi-Fi™, or medical implant communication service (MICS), or transconductance communication (TCC), which may occur via electrodes of ICM  15 A. Examples of wireless communication techniques used by ICM  15 A and pressure sensing device  12 A may also include RF telemetry or TCC. In one example, ICM  15 A and pressure sensing device  12 A communicate via TCC, and ICM  15 A and external device  14 A communicate via RF telemetry. 
     Medical device system  8 A is an example of a medical device system configured to monitor a cardiovascular pressure of patient  2 A. Although not illustrated in the example of  FIG. 1A , a medical device system may include one or more implanted or external medical devices in addition to or instead of ICM  15 A and pressure sensing device  12 A. For example, a medical device system may include a vascular implantable cardiac defibrillator (ICD) or pacemaker (e.g., IMD  15 B illustrated in  FIG. 1B ). One or more such devices may generate physiological signals, and may include processing circuitry configured to monitor cardiovascular pressure. In some examples, the implanted devices may communicate with each other or with external device  14 A. 
       FIG. 1B  is a conceptual diagram illustrating another example medical device system  8 B. Medical device system  8 B includes sensor assembly  10 B implanted, for example, in left pulmonary artery  6 B of patient  2 B, through which blood flows from heart  4 B to the lungs, and another device, such as a pacemaker, defibrillator, or the like, referred to as IMD  15 B. For purposes of this description, knowledge of cardiovascular anatomy is presumed, and details are omitted except to the extent necessary or desirable to explain the context of the disclosure. 
     In some examples, IMD  15 B may include one or more leads  18 A- 18 C, that carry electrodes that are placed in electrical contact with selected portions of the cardiac anatomy in order to perform the functions of IMD  15 B, as is well known to those skilled in the art. For example, IMD  15 B may be configured to sense and record cardiac EGM signals via the electrodes on leads. IMD  15 B may also be configured to deliver therapeutic signals, such as pacing pulses, cardioversion shocks, or defibrillation shocks, to heart  4 B via the electrodes. In the illustrated example, IMD  15 B may be a pacemaker, cardioverter, or defibrillator. 
     In some examples, this disclosure may refer to IMD  15 B, particularly with respect to its functionality as part of a medical device system that monitors cardiovascular pressure and other physiological parameters of a patient  2 B, as an implantable monitoring device or implantable hub device. In some examples, IMD  15 B includes or is coupled to one or more additional sensors, such as accelerometers, that generate one or more signals that vary based on patient motion or posture, blood flow, or respiration. IMD  15 B may monitor posture of patient  2 B at or near the times when implantable pressure sensing device  12 B is measuring cardiovascular pressure. 
     IMD  15 B also may have wireless capability to receive and transmit signals relating to the operation of the device. IMD  15 B may communicate wirelessly to an external device, such as external device  14 B, or to another implanted device such as pressure sensing device  12 B of the sensor assembly  10 B, e.g., as described above with respect to IMD  15 A, external device  14 A, and pressure sensing device  12 A of  FIG. 1A . In some examples, the pressure sensing device may communicate wirelessly and directly with external device  14 B, rather than communicating with external device  14 B through the IMD  15 B. 
     Medical device system  8 B is an example of a medical device system configured to monitor the cardiovascular pressure of patient  2 B. One or more of IMD  15 B, implantable pressure sensing device  12 B, and external device  14 B, individually, or collectively, may include processing circuitry that allows medical device system  8 B to function as described herein. Such function may include measuring a cardiovascular pressure of patient  2 B, such as pulmonary artery pressure. In some examples, an implantable pressure sensing device  12  measures the cardiovascular pressure at a plurality of predetermined times during a day or a portion of a day, e.g., at night. 
     Sensor assembly  10 A and sensory assembly  10 B may include outer housings (not labelled in  FIGS. 1A and 1B ) that contain one or more components such as electronic circuitry and a power source. In some examples, the electronic circuitry may include, e.g., processing circuitry, telemetry circuitry, and/or the like, which allow assemblies  10 A and  10 B to operate as described herein. As will be described further below, the outer housings may be formed of at least one metal housing component and at least one polymer housing component. A seal may be formed between a metal housing component and a polymer housing component along an interface between the two components. In some examples, the seal may be a hermetical seal between the two components, e.g., to allow for the internal components to be contained within a hermetically sealed housing enclosure. In some examples, the seal between the components may be formed by positioning the components adjacent to each other along an interface, and then delivering energy to the metal housing component. Heat may be transferred to the polymer housing component from the metal housing component, e.g., via conductive heat transfer along the interface. The heating of the polymer housing component may cause a portion of the polymer housing component to melt. The melted portion may wet on the surface of the metal housing component and then solidify by cooling to form the seal between the metal housing component and the polymer housing component. 
       FIG. 2  is a conceptual diagram illustrating example IMD  20  including outer housing  23 . Housing  23  is defined by a combination of metal housing component  22  and polymer housing component  21 , in accordance with some examples described in this disclosure. Sensor assemblies  10 A and  10 B are examples of IMD  20 . Housing  23  defines an enclosure that houses internal components such as electronic circuitry  26  and power source  27 . In the example of  FIG. 3 , electronic circuitry  26  and power source  27  are shown being within metal housing component  22 . However, electronic circuitry  26  and power source  27  (as well as other internal components) may be located within either metal housing component  22 , polymer housing component  21  or a combination of the components (e.g., as metal housing component  22  and polymer housing component  21  may combine to define an overall hermetically sealed enclosure that contains the internal components). 
     Electrode  25  may define an electrically conductive surface that forms a portion of the outer surface of polymer housing component  21 . In cases in which polymer housing component  21  is formed of an electrically insulating material, polymer housing component  21  may function to electrically isolate electrode  25  from metal housing component  22 . While a single electrode is shown in  FIG. 2 , IMD  20  may include multiple electrodes electrically isolated from each other and from metal housing component  22 . In some examples, electrode  25  may be formed of a biocompatible conductive material. For example, electrode  25  may be formed from any of stainless steel, titanium, platinum, iridium, or alloys thereof. In addition, electrode  25  may be coated with a material such as titanium nitride or fractal titanium nitride, although other suitable materials and coatings for electrodes may be used. 
     Electrode  25  may be electrically coupled to electronic circuitry  26  and/or power source  27  within housing  23 . Using electrode  25 , electronic circuitry  26  and/or power source  27 , IMD  20  may sense electrical signals of a patient in which IMD  20  is implanted and/or deliver electrical signals to the patient. 
     Although not shown, IMD  20  may include telemetry circuitry to allow IMD  20  to communicate with other devices located internal or external to the patient. Under the control of the electronic circuitry  26  of IMD  20 , the telemetry circuitry may receive downlink telemetry from and send uplink telemetry to external devices with the aid of an antenna, which may be internal and/or external. 
     In some examples, IMD  20  may also include one or more other sensors configured to sense one or more physiological parameters of the patient. 
     Electronic circuitry  26  may include or may be one or more processors or processing circuitry, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable logic arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor” and “processing circuitry” as used herein may refer to any of the foregoing structure or any other structure suitable for implementation of the techniques described herein. 
     Depending on the function of IMD  20 , electronic circuitry  26  may include signal sensing circuitry and/or electrical signal generating circuitry. 
     Although not shown, IMD  20  may include a memory within housing  23 . The memory may include any volatile or non-volatile media, such as a random-access memory (RAM), read only memory (ROM), non-volatile RAM (NVRAM), electrically erasable programmable ROM (EEPROM), flash memory, and the like. The memory may be a storage device or other non-transitory medium. In general, memory of IMD  20  may include computer-readable instructions that, when executed by a processing circuitry of IMD  20 , cause it to perform various functions attributed to the device herein. For example, processing circuitry of IMD  30  may control the signal generator and sensing circuitry according to instructions and/or data stored on memory to deliver therapy to a patient and perform other functions related to treating condition(s) of the patient with IMD  20 . 
     The various components of IMD  20  may be coupled to a power source  27 . Power source  27  may be any suitable power source that provides operational power to IMD  20 . In some examples, power source  27  may be a primary or secondary battery, such as a lithium battery. Power source  27  may be capable of holding a charge for several years. 
     While a single polymer housing component  21  and a single metal housing component  22  are shown for IMD  20 , examples are contemplated in which housing  23  includes more than one metal housing component  22  and/or more than one polymer housing component  21  joined to each other. Furthermore, while a single electrode  25  is shown in  FIG. 2 , in some examples, IMD  20  may include multiple electrodes  25  on the same or different polymer housing components  21  of IMD  20 . 
     In some examples, polymer housing component  21  and metal housing component  22  may be joined together by one or more of the example methods disclosed herein, e.g., such that a hermetic seal is formed between the two components, thus protecting the internal components from fluids such as body fluids and/or gases. In the example depicted in  FIG. 2 , electrical feedthroughs (not shown) may provide electrical connection of electrode  25  to circuitry within housing  23 . Polymer housing component  21  and metal housing component  22  may be joined together at interface  24 . The seal formed at interface  24  between the respective housing components may define a hermetic seal that hermetically seals component within housing  23  from the environment external to housing  23 . 
     Polymer housing component  21  may be formed of a polymeric material. Any suitable polymer or combination of polymers may be used for polymer housing component  21 . The polymeric material may be a biocompatible polymer suitable for implantation in a patient. The polymeric material may be a material that forms a hermetic boundary between the environment external to housing  23  and the internal components. In some examples, the polymer material may have a relatively low permeability (e.g., to form a hermetic barrier). As described herein, polymer housing component  21  may be formed of a polymeric material that melts when heated, e.g., by heat transferred to polymer housing component  21  from metal housing component  22  along interface  24 . In some examples, the polymer housing component  21  may be formed of a polymer that is able to reflow and solidify without significant degradation. In some examples, the polymer may be a thermoplastic. 
     In some examples, polymer housing component  21  includes a single polymer material. In other examples, polymer housing component  21  includes a combination of polymers. Suitable polymers may include polyether ether ketone, polysulfone, polyetherimide, polyphenylsulfone, ultra-high molecular weight (UHMW) polyethylene (PE), and/or polyethersulfone (PES). Other suitable polymers may include those which are liquid crystalline polymers (LCPs), which may be highly adaptable to IMD applications. In some examples, polymeric housing component includes at least one polymeric polymer. In some examples, polyolefins and/or silicones may be employed for polymer housing component  21 . 
     In some examples, polymer housing component  21  may be formed of bulk or main polymer portion (e.g., PEEK or LCP) with a layer of a second polymer material (e.g., a suitable thermoplastic) that has a lower melting temperature in the area of contact with metal housing component  22  (e.g., at interface  24 ). In some examples, the second polymer material may be referred to a “tie layer,” and when melted and cooled, may have better adhesive properties than the bulk material to ensure a better bond with metal housing component  22 . 
     In some examples, a suitable polymer material may be selected based on how the material expands when heated. For example, a polymer with a chemical foaming agent blended in just above the melt temperature, but then begins to foam at a higher temperature may be selected. This foaming action may both increase the internal pressure inside the device (e.g., helping to force the polymer out), as well as helping ensure a better bond to the inside. 
     Metal housing component  22  may be formed of any suitable metal or alloy or combination of metals or alloys. Like that of polymer housing component  21 , metal housing component  22  may be formed of one or more metals and/or alloys that is biocompatible for implantation into a patient. The metal or alloy material may be a material that forms a hermetic boundary between the environment external to housing  23  and the internal components. In some example, the metal or alloy material may have surface morphology that has a low reflection for the outer surface of housing component  22 . For the surface of metal housing component  22  it may be desirable for a low surface roughness that wets well, e.g., to increase contact with reflow material from polymer housing component  21 . Suitable metal or alloy materials may include at least one of stainless steel, titanium (e.g., grades 1, 5, 9, 23, and the like), tantalum, niobium, platinum, or iridium. In some examples, a metal or alloy may be selected that has desirable thermal behavior (e.g., in terms of conduction/absorption from lasers in a laser heating process). 
     In some examples, metal housing component  22  may have surface modifications or other properties, e.g., surface roughness, cleanliness, oxides, in the area of interface  24  with polymer housing  21  that promote better bonding with the polymer. In some examples, larger scale features, like dovetail grooves, ridges, partial or even through holes, may serve to help seal/lock the polymer to the metal housing after being joined as described herein. Locking and/or sealing between the respective components may also be improved by mechanisms that tend to force the polymer housing component  21  into close contact with the metal housing component  22  once the polymer melts. Foaming agents in the polymer as described above may be one example mechanism, but other mechanisms may be employed. For example, a preloaded compression spring may be placed inside polymer housing component  21  in the area of interface  24 , e.g., at the rectangular hole in the center of polymer housing  31 A shown in  FIG. 3C . Once the polymer housing  31 A is softened by an external laser beam heating of metal housing  32 , the spring may deform the softened polymer into closer contact with the metal housing. Other approaches include making the polymer and metal housings components a relatively tight fit, and assembling the respective components inside a chamber with higher than atmospheric pressure so that pressure is captured inside. With the appropriate design, reheating the polymer outside the pressure chamber might cause the polymer to melt and be forced into good contact with the metal housing as a technique to improve the bond in the area of interface  24 . 
       FIGS. 3A-3D  are conceptual schematic diagrams illustrating various view of example IMD  30  including polymeric housing components  31 A and  31 B, and metal housing component  32 . IMD  30  may be an example of IMD  20  of  FIG. 2 . In some examples, IMD  30  may be configured to function as a monitoring device, such as ICM  15 A, pressure sensing device  12 A, or pressure sensing device  12 B, or as a device that monitors and/or delivers electrical therapy to a patient, such as IMD  15 B described above. In  FIGS. 3A and 3B , polymer housing components  31 A and  31 B are shown as being semitransparent for illustrative purposes, e.g., to show the one or more internal components of IMD  30 .  FIG. 3C  metal housing  32  is shown as being semitransparent and without internal components for illustrative purposes.  FIG. 3D  is a cross-sectional view of portion of IMD  30  along the longitude axis of IMD  30 .  FIG. 3D  does not show the internal components of IMD  30  but instead only show polymer housing component  31 A and metal housing component  32 . 
     IMD  30  includes outer housing  33  which may be the same or substantially similar to that described above for housing  23  of IMD  20  in  FIG. 2 . For examples, housing  33  includes first and second polymeric housing components  31 A and  31 B, which may be the same or substantially similar to that described for polymeric housing component  21  in  FIG. 2 . First polymer housing component  31 A and second polymer housing component  31 B may have substantially the same composition (e.g., formed of the same polymer composition) or may have different compositions (e.g., formed from different polymer compositions). 
     Housing  33  also includes metal housing component  32 , which may be the same or substantially similar to that described for metal housing component  22  in  FIG. 2 . Metal housing component  32  may have a tubular shape that define internal cavity  59  to house all or a portion of one or more of the internal components of IMD  30 . First polymer housing component  31 A is joined at one open end of metal housing component  32  and closes off that open end of metal housing component  32 . For example, as shown in  FIG. 3D , first polymer housing component  31 A is joined to metal housing component  32  along interface  56 . During assembly of IMD  30 , a seal such as a substantially hermetic seal may be formed between first polymer housing component  31 A and metal housing component  32  at interface  56  when first polymer housing component  31 A and metal housing component  32  are joined to each other. 
     Likewise, second polymer housing component  31 B is joined at the other open end of metal housing component  32  and closes off that open end of metal housing component  32 . Thus, in combination, housing components  32 ,  31 A and  31 B may form an outer housing  33  for IMD  30  that defines a sealed enclosure, e.g., a hermetically sealed enclosure, having inner cavity  59  that houses one or more components of IMD  30 . For example, like that described above for IMD  20 , housing  33  of IMD  20  may contain electronics and other internal components necessary or desirable for executing the functions associated with the device. In one example, housing  33  of IMD  30  includes one or more of processing circuitry, memory, a signal generation circuitry, sensing circuitry, telemetry circuitry, and a power source. In some examples, housing  33  encloses electronic circuitry  26  and protects the circuitry contained therein from fluids such as body fluids. 
     IMD  30  also includes two electrodes (first electrode  35 A and second electrode  35 B), which may the same or substantially similar to that described for electrodes  25  of IMD  20 . First and second electrodes  35 A and  35 B may be used by IMD  30  to sense electrical signals within a patient and/or delivery electrical signals generated by IMD  30  to one or more target sites within a patient. For example, first and second electrodes  35 A and  35 B may be used to sense cardiac EGM signals, e.g., ECG signals, when IMD  30  is implanted in the patient either sub-muscularly or subcutaneously. The signals may be sensed by IMD  30  using a unipolar or multipolar configuration. In some examples, the EGM signals may be stored in a memory of the IMD  30 , and data derived from the cardiac EGM signals may be transmitted via an integrated antenna to another medical device, which may be another implantable device or an external device, such as external device  14 A. In some examples, IMD  30  may function the same or substantially similar to that of Reveal LINQ® Insertable Cardiac Monitor (available from Medtronic plc., Dublin, IE). 
     As shown, first electrode  35 A is positioned on first polymer housing component  31 A and second electrode  35 B is positioned on second polymer housing component  31 B. As described above, first polymer housing component  31 A and second polymer housing component  31 A may each be formed of an electrically insulating material. In this manner, first polymer housing component  31 A may electrically isolate first electrode  35 A from metal housing component  32  and second electrode  35 B. Similarly, second polymer housing component  31 A may electrically isolate second electrode  35 B from metal housing component  32  and first electrode  35 A. While the examples of first and second electrodes  35 A and  35 B are shown as being located on the same major surface of housing  33  at distal and proximal ends of IMD  30 , respectively, and as defining flattened, outward facing conductive surfaces, other examples are contemplated. For example, one or both of first and seconds electrodes  35 A and  35 B may extend from first major surface, around rounded edges or an end surface, and onto the second major surface. Thus, the electrode may have a three-dimensional curved configuration. In some examples, all or a portion of first electrode  35 A may be located on first major surface of housing  33  and all or a portion of second electrode  35 B may be located on a second major surface of housing  33 . 
     During the manufacturing process for IMD  30 , first electrode  35 A and first polymer housing component  31 A may be formed separately from metal housing component  32 . The composite assembly of first electrode  35 A and first polymer housing component  31 A may then be joined to metal housing component  32 . For example, the electrically conductive structure of first electrode  35 A (and associated feedthroughs and other structure) may be fabricated and then the polymer material of first polymer component  31 A may be backfilled and/over-molded around the prefabricated structure. The composite component of first electrode  35 A and first polymer housing component  31 A may then be joined to metal housing component  32  along interface  56 . The composite structure of second electrode  35 B and second polymer housing component  31 B may be similarly manufactured, and subsequently joined to metal housing component  32  at the opposite end of housing component  32 . First polymer housing component  31 A and second polymer housing component  31 B may each be joined to metal housing component  32  to form outer housing  33  of IMD  30 , e.g., using one or more of the example techniques described herein. 
       FIG. 4  is a flow diagram illustrating an example technique for assembling an IMD, in accordance with some examples described in this disclosure. The example technique shown in  FIG. 4  may be used to form an IMD having an outer housing made from one or more polymeric housing components and one or more metal housing components that are sealed to each. The example technique shown in  FIG. 4  may be used to assemble the respective housing components of IMD  20  or IMD  30  described above. For ease of description, the example technique of  FIG. 4  is described with regard to the joining of first polymer housing component  31 A to metal housing component  32  for IMD  30 . However, it is recognized that such a process may be used to join second polymer housing component  31 B to metal housing component  32  at the opposite end of metal housing  32  and/or may be used to assemble any housing that includes a polymer housing component and a metal housing component joined to each other, e.g., to form a substantially hermetic seal. 
     As shown in  FIG. 4 , the example technique includes positioning metal housing component  32  adjacent to first polymer housing component  31 A, e.g., so that the respective components are directly adjacent to each other along interface  56  ( 42 ). Such an arrangement is shown, e.g., in  FIGS. 3C and 3D . In some examples, positioning metal housing component  32  adjacent to first polymer housing component  31 A ( 42 ) may include contacting surfaces of the polymeric and metal housing components  31 A and  32  with each other at interface  56 . 
     Any suitable technique may be employed to position metal housing component  32  and first polymer housing component  31 A as described. For example, the respective components may be manually positioned adjacent to each other or automated robotic equipment may be employed to position the respective components as described. In some examples, metal housing component  32  and first polymer housing component  31 A may be sized, shaped, and/or otherwise configured such that there is press fit (also referred to as an interference fit) formed at interface  56  to secure (e.g., temporarily hold) metal housing component  32  and first polymer housing component  31 A to each other (e.g., so a seal may be formed between the two components as describe below). 
     Once metal housing component  32  has been positioned adjacent to first polymer housing component  31 A ( 42 ), a seal may be formed between metal housing component  32  and first polymer housing component  31 A at interface  56  ( 44 ). For example, energy (represent by arrows  57  in  FIG. 3D ) may be applied to metal housing component  32 , e.g., in an area at or near interface  56 , such that the temperature of metal housing component  52  increases. As a result, energy, e.g., in the form of heat, may then be transferred from metal housing component  32  to first polymer housing component  31 A in the area of interface  56  (e.g., via conductive and/or convective heat transfer). The transferred energy may increase the temperature of first polymeric housing component  31 A at or near interface  56  to a threshold temperature at which the polymeric material of first polymer housing component  31 A softens and/or melts. Once softened and/or melted, the polymer material may reflow along interface  56  so that a seal if formed by between first polymer housing component  31 A and metal housing component  32  when the polymer material cools (e.g., by terminating the application of the energy source applied to metal housing  32 ). In some examples, the temperature of first polymeric housing  31 A in the area of interface  56  may be increase to or above the glass transition temperature of the polymer material and/or increase to or above the melting temperature and/or softening temperature of the polymer material. In some examples, the reflow of the polymeric material increases the contact between the adjacent surfaces of first polymer housing component  31 A (e.g., as compared to the contact between the surfaces prior to application of the energy to metal housing component  32 ). In some examples, contact between the surfaces increases as a result of this “wetting” of the surface of metal housing component  32  in contact with softened and/or melted polymer material along interface  56 . 
     In some examples, the seal formed along interface  56  by the process of  FIG. 4  may be a substantially hermetic seal. In some examples, a helium leak test may be employed to evaluate the hermeticity. A substantially hermetic seal may be defined by such a test with helium transfer below detectable levels at time=0. However, hermeticity of a seal may be measured using other metrics, e.g., pressure decay/pressure increase tests by creating a pressure differential between inside and outside of the housing. 
     Any suitable energy source may be employed to apply energy  57  to metal housing  32  ( 44 ). For example, a laser beam source may be employed that applies laser beam energy to the metal housing component  32 , in accordance with this disclosure. In some examples, the process may be a laser beam welding process. A laser energy source may offer desirable control and targeting for applied energy  57 . Other forms of heat sources and/or heating techniques may be employed and may include electron beam, electrical arc, plasma, resistance heating (e.g., by current applied to the metal housing component), electrical heating tools, inductive heating, pre-heating (e.g., in a furnace), friction against the surface of the metal housing component, RF heating, heat from another focused light, hot air, conductive heat transfer or other heat transfer from contact, ultrasonic energy, and/or the like. 
       FIG. 5  is a conceptual schematic diagram illustrating that application of laser beam energy  58  (or other type of suitable energy from an external source) to metal housing component  32  in the area of interface  56  (shown in  FIG. 3D ). To apply beam energy  58  to metal housing component  32  ( 44 ), beam energy  58  may be moved relative to metal housing along direction D in a substantially continuous or periodical fashion over the entire outer perimeter of metal housing  32  in the area of interface  56 . For example, energy  58  may be stationery and metal housing component  32  and first polymer housing component  31 A may be moved, or vice versa. Additionally, or alternatively, energy  58  may be moved and metal housing component  32  and first polymer housing component  31 A may be stationary. By moving the energy  58  along direction D all portions of metal housing component  32  may be heated and the heating may be controlled as desired. 
     During the process, energy  58  may be applied on a substantially continuous or periodic basis. In some examples, energy  58  may be applied according to an on/off duty cycle. The timing of the application of energy  58  and/or the relative movement of energy  58  relative metal housing component  32  (as well as other parameters such as beam energy source diameter, power, and the like) may be selected such that enough energy is delivered to metal housing component  31  to heat the adjacent polymer material of first polymer housing component  31 A to a temperature sufficient to form a seal (e.g., a substantially hermetic seal) along interface  56  upon cooling of the polymer material. 
     In some examples, sufficient heat is transferred to first polymeric housing component  31 A from metal housing component  32  to increase the temperature of the polymeric housing component to or above the glass transition temperature (Tg) of the polymer material. In some examples, sufficient heat is transferred to first polymeric housing component  31 A from metal housing component  32  to increase the temperature of the polymeric housing component to or above the melting temperature of the polymer material. In some examples, sufficient heat is transferred to first polymeric housing component  31 A from metal housing component  32  to increase the temperature of the polymeric housing component to or above the softening temperature of the polymer material. When the polymer that forms polymeric housing component  31 A reaches a temperature at or above its Tg, melting, and/or softening temperature, the polymer reflows and forms a seal with metal housing component  32  along interface  56  upon cooling. 
     Energy  58  may be applied using any suitable parameters for performing the process described for  FIG. 4 . The parameters for energy  58  may be dependent on a number of factors including, e.g., the composition (and other heat transfer properties such as thickness  58 ) of metal housing component  32  and the composition of polymer housing component  31 A. In some examples, when energy  58  is in the form of a continuous wave laser beam energy, energy  58  may have a power of about 10 Watts (W) to about 1000 W, such as about 100 W to about 300 W; a beam diameter of about 0.001 inches to about 0.030 inches, such as about 0.008 inches to about 0.026 inches. In some examples, energy source  58  may move relative to metal housing component  32  as a rate of about 1.0 inch per minute (ipm) to about 500 ipm, such as about 50 ipm to about 150 ipm. Other values are contemplated. Energy  58  can also be in the form of pulsed laser beam energy. 
     The application of energy  58  may be controlled to increase the temperature of the material of polymer housing component  31 A above the softening and/or melting point of the material but below a threshold maximum temperature for metal housing component  32  and/or polymer housing component  31 A. The threshold maximum temperature may be a temperature at which metal housing component  32  and/or polymer housing component  31 A that cause undesirable side effects to the housing components. For example, first electrode  35 A may include a thermally sensitive component. In such cases, it may be desirable to design the polymeric housing component using a polymer having a Tg, melting point, and/or softening point that is lower than a temperature which would harm first electrode  35 A (or other thermally sensitive component of IMD  30 ). For example, it may be known that temperatures above 150° C., need to be avoided when using a given thermally sensitive component. In such cases, the polymeric housing component may include a polymer having a Tg, melting point, and/or softening point of less than about 150° C. 
     In some examples, the temperature of the polymer housing component  31 A may be kept below the onset of polymer degradation or decomposition. The bonding strength may decrease when the polymer decomposes and may ultimately lead to loss of hermeticity. 
     In some examples, the temperature during the process may be controlled to keep the temperature below the degradation temperature of other polymer components inside the housing (e.g., of other internal polymer seals between battery cathode/anode), below a distortion temperature of polymer housing component  31 A, and/or degradation temperature of circuit devices of IMD  31 A. 
     In some examples, employing a laser as the energy source may be beneficial as it may provide control over the intensity of the energy, the duration that it is applied over, and/or the ability to focus the application of the energy to very specific areas so other areas remain unheated/undamaged. This may allow some parts of the polymer to hit very high temperatures quickly, and then cool down without heating adjacent areas to temperatures that can damage them. 
     While the example IMD  30  shown in  FIGS. 3A-3D  is configured such that there is a lap joint or half lap joint between polymer housing component  31 A and metal housing component  32 , other joints types may be employed. In some examples, a butt joint, tee joint, edge joint, or the like may be used. 
     EXAMPLES 
     Various experiments were carried out to evaluate one or more aspects of the disclosure. In each of the experiments, a titanium housing component, having an outer diameter of 0.748 inches and a wall thickness of 0.010 inches, was positioned adjacent to a polyethyl ethyl ketone polymer housing component having a diameter of 0.728 inches, a length of 0.125 inches, and an outer diameter of 0.748 inches. A laser welding process was used to heat the metal housing component which in turn heated the polymer housing component. The laser was operated in a continuous mode with a power of 200 watts and a beam diameter of 287 micrometers (11.3 mils). Weld speeds were varied for three separate samples and are included in the Table below. 
     Results are summarized in the Table below.  FIGS. 6-8  are micrographs showing cross-sectional views of the samples:  FIG. 6  for Sample A,  FIG. 7  for Sample B, and  FIG. 8  for Sample C. Region  69  in  FIG. 6  shows a region where material has reflowed. In each of  FIGS. 6-8 , the polymer component is on the “top” side and the metal component is on the “bottom” side. The region of the applied laser energy is shown in  FIGS. 6-8 . As illustrated in  FIGS. 6-8 , the two components were intimate contact to facilitate bonding during the laser welding process. 
     
       
         
           
               
               
               
               
               
             
               
                   
               
               
                   
                 Rotary Weld 
                 Weld Speed 
                   
                 Hermeticity 
               
               
                 Sample 
                 Speed (rpm) 
                 (ipm) 
                 Observation 
                 Check 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 A 
                 40 
                 94.0 
                 THC melted on small 
                 Hermetic 
               
               
                   
                   
                   
                 section of edge 
                   
               
               
                 B 
                 30 
                 70.5 
                 THC melted on edge 
                 Hermetic 
               
               
                 C 
                 50 
                 117.5 
                 No visible melt on 
                 Leaked 
               
               
                   
                   
                   
                 THC 
                   
               
               
                   
               
            
           
         
       
     
     One skilled in the art will appreciate that the present disclosure may be practiced with examples other than those disclosed herein. The disclosed examples are presented for purposes of illustration and not limitation, and the present disclosure is intended to be limited only by the claims, including insubstantial changes therefrom. 
     Various examples have been described in the disclosure. These and other examples are within the scope of the following clauses and claims. 
     Clause 1. A method for manufacturing an implantable medical device, the method comprising: positioning a metal housing component adjacent to a polymer housing component so that there is an interface between the metal housing component and the polymer housing component; and forming a seal at the interface between the metal housing component and the polymer housing component to join the metal housing component and the polymer housing component, wherein the joined metal housing component and the polymer housing component form at least a portion of housing for the implantable medical device, wherein the housing of the implantable medical device contains electronic circuitry. 
     Clause 2. The method of clauses 1 or 2, wherein positioning the metal housing adjacent to the polymer housing comprises contacting a surface of the metal housing component with a surface of the polymer housing component at the interface. 
     Clause 3. The method of any one of clause 1-3, wherein forming the seal at the interface between the metal housing component and the polymer housing component comprises: delivering energy to the metal housing component such that the metal housing component causes a portion of the polymer housing component to melt, wherein the melting of the portion of the polymer housing component increases contact between the metal housing component and the polymer housing component at the interface, and wherein the seal is formed between the metal housing component and the polymer housing component at the interface upon cooling of the melted portion of the polymer housing component. 
     Clause 4. The method of clause 3, wherein delivery in the energy to the metal housing component comprises delivering laser beam energy to the metal housing component. 
     Clause 5. The method of clause 4, wherein delivering the laser beam energy comprises delivering pulsed laser beam energy. 
     Clause 6. The method of clause 4, wherein delivering the laser beam energy comprises delivering continuous wave laser beam energy. 
     Clause 7. The method of clause 4, wherein the polymer housing component and the metal housing component are stationary during the delivery of the laser beam energy. 
     Clause 8. The method of any one of clauses 1-7, wherein the seal is a hermetic seal. 
     Clause 9. The method of any one of clauses 1-8, wherein positioning the metal housing component adjacent to the polymer housing component comprises forming a press fit of between the metal housing component and polymer housing component. 
     Clause 10. The method of any one of clauses 1-9, wherein the metal housing component comprises at least one of stainless steel, titanium, platinum, or iridium. 
     Clause 11. The method of any one of clauses 1-10, wherein the polymer housing component comprises polyether ether ketone. 
     Clause 12. The method of any one of clauses 1-11, wherein the polymer housing component comprises a liquid crystalline polymer. 
     Clause 13. The method of any one of clauses 1-12, wherein the polymer housing component comprises a polymer having a glass transition temperature (Tg) of less than about 150 degrees Celsius. 
     Clause 14. The method of any one of clauses 1-13, wherein the electronic circuitry is contained within the housing upon positioning the polymer housing component adjacent to the metal housing component. 
     Clause 15. The method of any one of clauses 1-14, wherein the polymer housing component includes an electrode on an outer surface of the housing. 
     Clause 16. The method of any one of clauses 1-15, wherein the implantable medical device comprises a cardiac monitor configured to sense and record cardiac electrogram signals. 
     Clause 17. An implantable medical device comprising: electronic circuitry; and a housing, wherein the processing circuitry is contained within the housing, wherein the housing includes a metal housing component and a polymer housing component sealed to each other along an interface. 
     Clause 18. The implantable medical device of clause 17, wherein the metal housing component comprises at least one of stainless steel, titanium, platinum, or iridium. 
     Clause 19. The implantable medical device of clauses 17 or 18, wherein the polymer housing component comprises polyether ether ketone. 
     Clause 20. The implantable medical device any one of clauses 17-19, wherein the polymer housing component comprises a liquid crystalline polymer. 
     Clause 21. The implantable medical device of any one of clauses 17-20, wherein the housing further comprises an electrode forming an outer surface of the implantable medical device. 
     Clause 22. The implantable medical device of clause 21, wherein the electrode is located on the polymer housing component, wherein the polymer housing component electrically isolates the electrode from the metal housing component. 
     Clause 23. The implantable medical device of any one of clauses 17-22, wherein the implantable medical device comprises a cardiac monitor configured to sense and record cardiac electrogram signals. 
     Clause 24. The implantable medical device of any one of clauses 17-23, wherein the seal comprises a hermetic seal.