Implantable medical device

Abstract

An implantable medical device (1) comprising a header (2) and can (4). The can (4) is formed by a two-part outer casing of first and second shells (6, 7) that are welded together and enclose an electronics unit (8) and a sensor unit (9). The electronics and sensor units (8, 9) communicate with each other using near-field communication. The electronics unit comprises an NFC reader device and the sensor unit comprises a passive or active NFC target device. Near-field communication is thus enabled over-the-air through a wireless communication path inside the outer casing. The absence of a wired connection between the sensor unit (9) and the electronics unit (8) provides for ease of assembly, since precise arrangement of the casing shells (6, 7) and the electronics and sensor units (8, 9) is not necessary.

Description

[0001] IMPLANTABLE MEDICAL DEVICE

[0002] The invention relates to an implantable medical device (IMD).

[0003] Example IMDs are a cardiac pacemaker or an implantable pulse generator (IPG) for a spinal cord stimulator (SCS).

[0004] In a conventionally constructed IPG there are two principal units: a header and a can.

[0005] The header is the interface area for conductive leads, such as the leads for SCS which are embedded in a nerve trunk of the patient. These carry therapeutic stimulation pulses and also may be used as sensing leads for sensing evoked compound action potential (ECAP) signals. The header may also include other components such as telemetry antennas and recharging antennas or coils. The header is hermetically isolated from the housing.

[0006] The can is a module containing the electronics, telemetry (sensors and sensor data) and wireless communications hardware, as required for external communication as well as a battery (primary cell or rechargeable) to power the electronics and provide power for the header, e.g., as needed for applying stimulation pulses to the electrode of an SCS. The can has a metal outer casing, made of a biocompatible metal, inside which the electronics, battery and other components are hermetically sealed. The outer casing is typically formed in two halves, referred to as shells, that are welded together to form a hermetically sealed unit.

[0007] From time to time, it is necessary to recharge the chargeable battery that powers the electronics unit. During wireless charging, the metal outer casing heats up as a result of magnetic fields generated by the external wireless charger. For safety, it is important to limit the heating of the metal outer casing, since it is in direct contact with the patient. The industry standard ISO 14708-3 :2017 specifies temperature limits. The definition is based on a thermal dose limit expressed in cumulative equivalent minutes at 43°C (referred to as CEM43) where the accumulation is defined by the following integral.

[0008] Jt CLTR43~T^ o

[0009] ( 0.5 if T > 43°C R ~ (0.25 if T < 43°C

[0010] As can be seen from the above formula, the CEM43 dose increases exponentially when the temperature exceeds 43°C. The general aim when wireless charging is to maximize the charging rate of the battery while keeping the thermal dose within the dose limit.

[0011] Two known approaches to tracking the temperature of the outer casing are:

[0012] - Temperature estimation based on an estimated a priori thermal impedance; and

[0013] - Temperature sensor integrated with the electronics which is connected to the outer casing with a thermally conductive bridge.

[0014] If temperature estimation is used, then uncertainty in the estimation must be taken account of. Temperature estimation relies on taking a value for thermal impedance, but this parameter is difficult to measure and test. Moreover, it is also not straightforward to calculate temperature accurately from a thermal impedance value. Therefore, if temperature estimation is used, a conservative approach to the thermal dose needs to be adopted resulting in reduced charging rate.

[0015] On the other hand, if the approach is taken to integrate a temperature sensor on the electronics printed circuit board and thermally connect it to the shell via a bridge piece, this causes complication during assembly, specifically when welding together the two shells, since precise fitting between the two shells and the electronics frame is needed.

[0016] According to first aspect of the disclosure there is provided an implantable medical device comprising: an outer casing forming a hermetically sealed enclosure; an electronics unit accommodated in the outer casing, the electronics unit including control electronic components for controlling the implantable medical device, communication electronic components for external wireless communication and a battery for providing electrical power to the control and communication electronic components; and a sensor unit accommodated in the outer casing, the sensor unit including a sensor and a near-field communication, NFC, target device, wherein the electronics unit further includes an NFC reader device arranged and configured to communicate with the NFC target device over a wireless communication path inside the outer casing.

[0017] The NFC communication between the sensor unit and electronics unit can be performed with radio-frequency identification (RFID) technology, as used widely for applications such as contactless payment methods, automotive access, door entry access, and Internet of Things (loT) sensors. NFC is a short-range wireless communication method with many available communication protocols defined by respective standards. NFC components are also widely available from many vendors. NFC communication is governed by industry group “NFC Forum” and there are multiple standards which govern different operating modes (ISO / IEC 14443, ISO 15693). NFC operates at 13.56 MHz ISM band with low data rates on the order of 10 to 100 kbit / sec. In NFC communication, a reader device generates a magnetic field which couples to a target device, e.g., in the form of a radio frequency identification (RFID) tag. The NFC target can be an active component or a passive component, in the sense of having or not having a local electrical power source. In a passive implementation, the NFC target device derives its electrical power from the magnetic field of an interrogating reader device. On interrogation by a reader device, the target device powers up and communicates with the reader device, which may be to receive data from the reader device, such as new configuration data, or to transmit data to the reader device, such as sensor data, or both. According to an embodiment, the sensor of the sensor unit derives its electrical power from the magnetic field of an interrogating reader device.

[0018] According to an embodiment, the NFC target device and the NFC reader device are configured to transmit and receive NFC signals. According to an embodiment of the present invention, the sensor unit and the electronics unit are galvanically isolated. The sensor unit and the electronics unit are communicating via NFC signals, using the NFC target device and the NFC reader device. This reduces electrical wiring and simplifies the design of the electronic assembly for an implantable medical device. For instance, the sensor unit can be placed further away from the electronics unit without having constraints of electrical wiring design.

[0019] In certain embodiments, the outer casing is made of a biocompatible metal. Suitable biocompatible metals include titanium, titanium alloy, cobalt-chromium alloy, and stainless steel.

[0020] In some embodiments, the sensor is a temperature sensor. In particular, it is useful to arrange a temperature sensor to measure temperature of the outer casing. For example, the temperature sensor can be mounted in direct contact with an inner surface of the outer casing.

[0021] According to an embodiment, the temperature sensor is one from the type of a NTC Thermistor, Resistance Temperature Detector (RTD), thermocouple, optical sensor (infrared), or a semiconductor based sensor.

[0022] In other embodiments, the sensor is an impedance sensor, for example one used to measure impedance between the casing and patient body tissue. In still further embodiments, the sensor is a gas sensor. The gas sensor may be used to measure the presence of a particular atomic or molecular species by a light absorption to detect a characteristic ro-vibrational transition energy. This could be to measure unwanted outgassing of the electronics or to measure outside the casing with optical access being provided through a window formed in the casing.

[0023] The outer casing may comprise first and second casing shells that are fitted together in hermetically sealed contact with each other. The first and second casing shells may form two halves of the outer casing and be welded together to form a hermetically sealed unit.

[0024] In certain embodiments, the sensor unit is secured to the second casing shell, in particular to an inner surface of the first casing shell. This facilitates assembly, since then the second casing shell, with the sensor unit attached to it can be fitted together with the first casing shell over the electronics unit, which may be held in a suitable frame, such as a plastics frame. The first and second casing shells can then be bonded together with a hermetic seal, e.g., by a welding process. As a further aid to assembly, the electronics unit may be secured to the first casing shell, so that the second casing shell, with the sensor unit attached to it, can be fitted together with the first casing shell, with the electronics unit attached to it; followed by the bonding process. In the assembled device, an over-the-air wireless communication path for near-field communication is provided between the NFC reader device of the electronics unit and the NFC target device which is connected with the sensor of the sensor unit. A wired connection between the sensor unit and electronics unit can thus be avoided, which facilitates assembly and also avoids wiring that could disturb operation of a temperature sensor by thermally conducting heat from components of the electronics unit, which will generate heat during operation, to the temperature sensor. In some embodiments, the sensor unit has no physical connection to the electronics unit other than via the outer casing.

[0025] According to an embodiment, the sensor unit and the electronics unit can be attached to the same casing shell. According to another embodiment, the sensor unit and the electronics unit are attached to different locations of the outer casing of the implantable medical device.

[0026] The sensor unit can be based on active or passive NFC. In a passive NFC implementation, the sensor unit does not require a battery but rather is electrically powered by radio frequency coupling to an electromagnetic field generated by the NFC reader device of the electronics unit (so-called passive RFID sensor). In an active NFC implementation for the sensor unit, the sensor unit has its own battery connected to provide electrical power to the sensor and the NFC target device, e.g., a long-life battery with a specified lifetime of 5 years or more (so-called active RFID sensor).

[0027] The above-described can may be combined with a suitable header to form the implantable medical device. According to another aspect of the disclosure, the above-described implantable medical device is integrated in a medical device communication system, e.g., as described by way of example in the detailed description below.

[0028] According to a further aspect of the disclosure there is provided a method of assembling an implantable medical device, the method comprising: providing an electronics unit, the electronics unit including control electronic components for controlling the implantable medical device, communication electronic components for external wireless communication, a near-field communication, NFC, reader device, and a battery for providing electrical power to the control and communication electronic components; providing a sensor unit, the sensor unit including a sensor and a near-field communication, NFC, target device configured to communicate with the NFC reader device; providing first and second casing shells shaped such that when fitted together they form an outer casing with an interior enclosure shaped and sized to accommodate the electronics unit and sensor unit; securing the sensor unit to the second casing shell; fitting the first and second casing shells together over the electronics unit to provide a wireless communication path inside the outer casing for near-field communication between the NFC reader and target devices; and forming a hermetic seal between the first and second casing shells.

[0029] Certain embodiments of the invention may have one or more of the following further advantages:

[0030] - The absence of any physical connection between the sensor unit and the electronics unit provides for ease of assembly, since precise arrangement of the casing shells and the electronics and sensor units is not necessary, especially if the casing shells are laser- welded.

[0031] - In the sensor unit, the sensor and its integrated NFC transceiver can be supported by standard integrated circuit components such as microcontrollers or system-on-a-chip (SoC) devices.

[0032] - The sensor unit is implementable in a compact way, so it does not occupy a significant volume within the can. - Sensor data, in particular accurate casing temperature data, can be collected by the sensor, wirelessly transmitted from the sensor to the electronics unit and transmitted onwards by the electronics unit from the implantable medical device to an external device, which can then use the temperature data as a control parameter or for logging purposes. For example, if the external device is an intelligent charger that is charging the battery inside the can, then the intelligent charger can select an optimum compromise between charging speed and heating of the outer casing.

[0033] This invention will now be further described, by way of example only, with reference to the accompanying drawings.

[0034] Fig. 1 is a schematic diagram of a two-part IMD with header and can, the IMD being integrated in a medical device communication system (MDCS).

[0035] Fig. 2A is a schematic diagram of the can’s first casing shell.

[0036] Fig. 2B is a schematic diagram of the can’s electronics unit.

[0037] Fig. 2C is a schematic diagram of the can’s second casing shell and sensor unit.

[0038] Fig. 3 is a block schematic diagram showing details of the components relevant for near-field communication (NFC) between the electronics unit and the sensor unit.

[0039] Fig. 4 is an exploded perspective view of an example IMD comprising a header and a can according to an embodiment of the invention with the inset showing the assembled IMD.

[0040] Fig. 5A is a perspective view of the can without the first casing shell.

[0041] Fig. 5B is a cross-section view of the can without the first casing shell. Fig. 6 is a perspective view of an active sensor unit (target), which is a self-contained unit with its own battery.

[0042] Fig. 7 is an exploded perspective view of the active sensor unit of Figure 6 installed in the can’s second casing shell and the electronics unit.

[0043] Fig. 8 is a perspective view of a passive sensor unit (target) fitted in the can’s second casing shell.

[0044] In the following detailed description, for purposes of explanation and not limitation, specific details are set forth in order to provide a better understanding of the present disclosure. It will be apparent to one skilled in the art that the present disclosure may be practiced in other embodiments that depart from these specific details.

[0045] In particular, the embodiments described in the following are described by way of example only in relation to an SCS system.

[0046] An SCS device is a type of IMD that is used for chronic pain therapy of a patient. An SCS device uses electrical current sent through electrodes on one or more leads implanted in the epidural space dorsal to the spinal cord in an SCS device. Control parameters include at least: pulse width, frequency, amplitude, and electrode selection. Additionally, duty cycling and pulse density modulated stimulation can be introduced as further control parameters to modulate therapy and better manage battery consumption through the introduction of alternating periods of carrier and envelope stimulation.

[0047] An IMD typically forms part of a medical device communication system (MDCS) that is configured to upload clinical data from the IMD on a continual basis via a MDCS to a remotely located data repository, which may be a data center. In an SCS device, clinical data includes both patient data relating to physiological monitoring of a patient's health, for example as collected by sensors of the IMD, and device data related to status and operation of the IMD, for example data logging each time an SCS device is used and at what stimulation level in terms of electrical pulse width and pulse frequency as well as control signals issued to the SCS device to optimize delivery of pain relief. These clinical data can then be accessed by health care staff accessing the data center using suitably designed software applications. Treatment plans and, if necessary, interventions can then be decided upon based on analysis of these clinical data. The analysis may be expert analysis by a health care professional or computer-automated analysis with a software program, e.g., running on computing resource at a neuro-service data center, or a combination of both.

[0048] Figure 1 shows a standard architecture of a MDCS 100 for communication between an IMD 1 implanted in a patient and a remotely located data center 60, which is accessible to health care staff using suitable application software. The IMD 1 comprises a header 2 and a can 4 which have respective outer casings 3 and 5. The can 4 includes therapeutic components for patient treatment, such as an SCS driver circuit 16 (or another IMD stimulator) and optionally an SCS sensing circuit for sensing ECAP signals (not shown).

[0049] The can 4 contains an electronics unit 8 including a system-on-a-chip (SoC) 10 with control electronic components for controlling the IMD 1 and communication electronic components for external wireless communication. A rechargeable battery 17 provides electrical power to the electronics unit 8. The SoC 10 delivers control signals to the electrode driver circuit 16 according to a treatment plan.

[0050] The header 2 is attached to a lead arrangement comprising multiple leads 80i to 80N with each lead incorporating multiple electrodes 90. a to 90. h. In the illustrated example, there are multiple leads but in other examples there may be only one lead. The electrode driver circuit 16 provides a suitable drive current to each of the electrodes 90. a to 90. h according to a set of control parameters that follow a treatment plan as controlled by the SoC 10.

[0051] When the IMD 1 is implanted, the rechargeable battery 17 can be charged in a contactless manner by an external charger 41, for example a resonant inductive charger, which is placed on the patient's skin adjacent the implanted can 4 to charge its rechargeable battery 17. The charger 41 is itself provided with an energy source 42, here a rechargeable battery 42, as well as an external power connection 43, for example an external power jack, to power the charger 41 so that its rechargeable battery 42 can be recharged. Alternatively, the charger's battery 42 can be recharged wirelessly by placing the charger 41 on a mains-powered charging pad. The patient is provided with a patient remote controller 33 (hereinafter also called “patient remote 33”), for example a smartphone with wireless transceivers 34, 35, 36 respectively for WPAN (e.g. BLE), LPWAN, cellular (e.g., 4G / LTE / 5G) and WLAN communication. If a smartphone is used, this is a smartphone that is possessed, e.g., owned, by the patient in which the IMD 1 is implanted and on which a software application ('app') is installed. The app is then a so-called Software as a Medical Device (SaMD) which is defined by the United States Food and Drug Administration (FDA) as software intended to be used for one or more medical purposes that perform these purposes without being part of a hardware medical device.

[0052] The cellular transceiver 35 and WLAN transceiver 36 provide two different data communication paths for the patient remote 33 to upload clinical data from the IMD 1 via an internet connection 50 to a remotely located data center 60 (hereinafter also called “backend 60”) acting as repository for storage of clinical data and / or as a host for the services, for example a neuro-service data center. The remote's WLAN transceiver 36 can upload data to the backend 60 via a router 38 and a telephone line 40 (or telephone network 40) using a wired internet connection 50. The internet connection 50 may provide access to one or more distributed networks and / or cloud services. The remote's cellular transceiver 35 can upload data to the backend 60 via one or more cellular network base stations 45 (cellular towers), for example LPWAN-capable cellular network base station. In some cases, instead of a public communication network, a dedicated point-to-point transmission, e.g., via a dedicated telephone line, may be provided for uploading clinical data. In all these scenarios, uploading of clinical data from the IMD 1 to the backend 60 takes place via the intermediary of the patient remote 33, the latter thereby acting as a relay device.

[0053] Health care staff, such as health care professionals (HCPs), clinical specialists and representatives and remote care team members have access to the backend 60 via suitable portals 70 with the aid of a software application running on the backend 60 and / or the portal 70 to provide the necessary user interfacing, diagnostics and so forth. At least one portal 70 is provided by at least one workstation for health care staff to access a data center, e.g., the backend 60. Analysis and diagnostic software may also be run at the backend 60 to analyze clinical data from individual patients or groups of patients. Figure 2A is a schematic diagram of the can’s first casing shell 6. Figure 2B is a schematic diagram of the can’s electronics unit 8. Figure 2C is a schematic diagram of the can’s second casing shell 7 and a sensor unit 9. The first and second casing shells 6 and 7 are welded together to form the can outer casing 5, with the first and second casing shells 6, 7 forming a hermetically seal with each other. The electronics unit 8 is held in a frame 31, which may be made of a plastics material.

[0054] The SoC 10 includes a microcontroller 13 which in turn includes a memory 14 and processor 15. The memory 14 stores a control program for the IMD functions that can be loaded into and run on the processor 15. It will be understood that the IMD functions may be implanted wholly in hardware or in any suitable combination of hardware, firmware (e.g. programmable logic array(s)) and / or software elements. The SoC 10 delivers control signals to the electrode driver circuit 16 according to a treatment plan devised by the computer program. The can 4 may also include one or more sensors for patient monitoring (not shown).

[0055] The hardware and software components of the electronics unit 8 operate collectively to provide an intelligent pulse generator to deliver electrical pulses to the leads 80 conforming to a particular set of parameters, including amplitude, pulse width, frequency and / or duty cycle to provide stimulation therapy. For SCS, the electrodes are implanted at or near a patient’s spinal cord to direct electrical signals into the patient’s tissue for spinal cord stimulation. The SCS device is implanted subcutaneously.

[0056] The electronics unit 8 further includes a wireless personal area network (WPAN) transceiver 11 to handle the external communications to the patient remote 33. The WPAN transceiver 11 uses a suitable WPAN protocol such as Bluetooth Low Energy (BLE), Medical Implant Communication System (MICS) or Medical Device Radiocommunications Service (MedRadio), the latter two being almost identical protocols.

[0057] The electronics unit 8 further includes a near-field communication (NFC) reader device 12 for communicating wirelessly with a corresponding NFC target device 18 of the sensor unit 9. The sensor unit 9 further comprises one or more sensors, for example temperature sensors and in some but not all embodiments also its own battery 20. For can assembly, the sensor unit 9 is secured to the second shell 7. The electronics unit 8 is held in the frame 31. The first shell 6 and the second shell 7 (carrying the sensor unit 9) are then fitted together enclosing the electronics unit frame 31 and welded together. Optionally, the electronics unit frame 31 may be secured to the first shell 6 before fitting the two shell halves together. Alternatively, the electronics unit frame 31 may be secured to the second shell 7, after securing the sensor unit 9, before fitting the two shell halves together.

[0058] Figure 3 is a block schematic diagram showing details of the components relevant for the near-field communications between the electronics unit 8 and the sensor unit 9. The electronics unit 8 comprises NFC reader functionality and the sensor unit 9 comprises NFC target functionality. The NFC target may either be active, i.e., with an on-board battery, or passive, i.e., with no on-board battery and instead being powered by an interrogating electromagnetic field generated by the NFC reader. The dashed arrowed lines schematically show the power flow in a passive embodiment. The double-headed solid lines show bidirectional communication paths between components. The NFC reader device 12 in the electronics unit 8 is under control of the microcontroller 13 and includes an antenna 21 and its own controller 22, for example an application specific integrated circuit (ASIC). The NFC target device 25 in the sensor unit 9 comprises an antenna 26, controller 27 and ASIC 28. The ASIC 28 is connected to receiver sensor signals from the sensor(s) 19.

[0059] Figure 4 is an exploded perspective view of an example IMD 1 comprising a header 2 and can 4 with the inset showing the assembled IMD 1. The components 19, 26, 27, 28 of the sensor unit 9 are mounted on a printed circuit board (PCB) 30, e.g., made of polyimide. The PCB 30 may be secured to the inside surface of the second shell 7 by any suitable bonding or fastening means. The components of the electronics unit 8 are mounted on a PCB 29, e.g., made of polyimide, including the antenna 21 and controller 22. The two antenna 21, 26 are preferably arranged coaxially with respect to their planar spiral coils as illustrated to maximize their coupling.

[0060] Figures 5A and 5B are perspective and cross-sectional views of the can without the first casing shell. The antennae 21, 26 have their spiral coils coaxially aligned as illustrated by the dashed vertical line. Figure 6 is a perspective view of an active sensor unit (target) 9, which is a self-contained unit in its own housing 32 which is illustrated with a cylindrical shape. The sensor(s) 19, controller 27 and ASIC 28 are arranged on a PCB 30 secured to the base of the housing. The antenna 26 is arranged under the roof of the housing 32 with a watch-form-factor battery 20 arranged intermediate the roof and base.

[0061] Figure 7 is an exploded perspective view of the active sensor unit 9 of Figure 6 installed in the can’s second casing shell 7. The sensor unit housing 32 may have its base secured to the PCB 30. The sensor housing base may be mounted on the first surface of the PCB 30.

[0062] Alternatively, a hole may be provided in the PCB 30 so that the base of the sensor unit housing 32 can be arranged in direct physical contact with the inner surface of the second casing shell 7 so there is thermal conductive path from the casing shell 7 to the sensor unit housing 32.

[0063] Figure 8 is a perspective view of a passive sensor unit (target) fitted in the second casing shell 7. Here a passive antenna coil 26 is embedded in the PCB 30.

[0064] Reference numerals

[0065] 1 implantable medical device (IMD)

[0066] 2 header

[0067] 3 header outer casing

[0068] 4 can

[0069] 5 can outer casing

[0070] 6 outer casing first shell

[0071] 7 outer casing second shell

[0072] 8 electronics unit

[0073] 9 sensor unit

[0074] 10 system-on-a-chip (SoC)

[0075] 11 wireless personal area network (WPAN) transceiver

[0076] 12 near-field communication (NFC) reader device

[0077] 13 microcontroller

[0078] 14 microcontroller memory

[0079] 15 microcontroller processor

[0080] 16 driver circuit, e.g., SCS device with attached leads

[0081] 17 electronics unit rechargeable battery

[0082] 18 NFC target device

[0083] 19 sensor, e.g., temperature sensor

[0084] 20 sensor unit battery

[0085] 21 NFC reader antenna

[0086] 22 NFC reader controller

[0087] 25 NFC target device

[0088] 26 NFC target antenna

[0089] 27 NFC target controller

[0090] 28 NFC target application specific integrated circuit (ASIC)

[0091] 29 electronics unit printed circuit board (PCB)

[0092] 30 sensor unit PCB

[0093] 31 electronics unit frame

[0094] 32 sensor unit housing

[0095] 33 patient remote controller (patient remote) 34 smartphone wireless personal area network (WPAN) transceiver

[0096] 35 smartphone cellular transceiver

[0097] 36 smartphone wireless local area network (WLAN) transceiver

[0098] 38 router 40 telephone line / telephone network

[0099] 41 wireless charger

[0100] 42 wireless charger battery

[0101] 43 wireless charger external power connection

[0102] 45 cellular network base station (cellular tower) 50 internet connection

[0103] 60 remotely located data center (backend)

[0104] 70 portal

[0105] 80i to 80N leads

[0106] 90. a to 90. h electrodes

[0107] 100 medical device communication system (MDCS)

Claims

Claims1. An implantable medical device (1), comprising: an outer casing (3) forming a hermetically sealed enclosure; an electronics unit (8) accommodated in the outer casing, the electronics unit including control electronic components for controlling the implantable medical device, communication electronic components for external wireless communication and a battery (17) for providing electrical power to the control and communication electronic components; and a sensor unit (9) accommodated in the outer casing, the sensor unit including a sensor (19) and a near-field communication, NFC, target device (18), wherein the electronics unit (8) further includes an NFC reader device (12) arranged and configured to communicate with the NFC target device over a wireless communication path inside the outer casing.

2. The implantable medical device of claim 1, wherein the NFC reader device (12) and the NFC target device (18) are configured to transmit and receive NFC signals.

3. The implantable medical device of claim 1 or 2, wherein the sensor unit (9) and the electronics unit (8) are galvanically isolated.

4. The implantable medical device of claim 1, 2 or 3, wherein the sensor (19) is at least one of a temperature sensor, an impedance sensor, or a gas sensor.

5. The implantable medical device of claim 4, wherein the sensor is a temperature sensor arranged to measure temperature of the outer casing.

6. The implantable medical device of any one of the preceding claims, wherein the outer casing comprises first and second casing shells (6, 7) that are in hermetically sealed contact with each other.

7. The implantable medical device of claim 6, wherein the sensor unit is secured to the second casing shell.

8. The implantable medical device of claim 6 or 7, wherein the electronics unit is secured to the first casing shell.

9. The implantable medical device of any one of claims 1 to 8, wherein the sensor unit has its own battery (20) connected to provide electrical power to the sensor and the NFC target device (18).

10. The implantable medical device of any one of claims 1 to 8, wherein the sensor unit does not have its own battery and is electrically powered by radio frequency coupling to an electromagnetic field generated by the NFC reader device (12).

11. The implantable medical device of any one of the preceding claims, wherein the sensor unit has no physical connection to the electronics unit other than via the outer casing.

12. The implantable medical device of any one of the preceding claims, further comprising a header (2) secured to the can (4).

13. A medical device communication system comprising an implantable medical device according to any one of the preceding claims.

14. A method of assembling an implantable medical device (1), the method comprising: providing an electronics unit (8), the electronics unit including control electronic components for controlling the implantable medical device, communication electronic components for external wireless communication, a near-field communication, NFC, reader device (12), and a battery (17) for providing electrical power to the control and communication electronic components; providing a sensor unit (9), the sensor unit including a sensor (19) and a near-field communication, NFC, target device (18) configured to communicate with the NFC reader device;providing first and second casing shells (6, 7) shaped such that when fitted together they form an outer casing with an enclosure shaped and sized to accommodate the electronics unit and sensor unit; securing the sensor unit to the second casing shell (7); fitting the first and second casing shells together over the electronics unit to provide a wireless communication path inside the outer casing for near-field communication between the NFC reader and target device; and forming a hermetic seal between the first and second casing shells. The assembly method of claim 14, wherein the electronics unit is secured to the second casing shell prior to fitting the first and second casing shells together.

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