Patent Description:
A wide variety of implantable medical devices (IMDs) that deliver a therapy to or monitor a physiologic or biological condition of a patient, or both, have been clinically implanted or proposed for clinical implantation in patients. An IMD may deliver therapy to and/or monitor a physiological or biological condition with respect to a variety of organs, nerves, muscles or tissues of the patients, such as the heart, brain, stomach, spinal cord, pelvic floor, or the like. The therapy provided by the IMD may include electrical stimulation therapy, drug delivery therapy, or therapy to reduce or eliminate a condition or symptoms of the condition of the patient.

The IMD may wirelessly communicate with another implanted device or an external device. An external device with which an IMD may communicate may be a programming device or a monitoring device (e.g., either attached to the patient or otherwise located near the patient). The information exchanged may be information related to a condition of the patient, such as physiological signals measured by one or more sensors, or information related to a therapy delivered to the patient. This information may be previously stored or real-time information. The IMD may also receive information from the external device, such as configuration information that may be used to configure a therapy to be provided to the patient. An IMD may communicate with another implanted device to control the operation of the other implanted device and/or to receive physiological data sensed by the other implanted device.

In some examples, an IMD wirelessly communicates with an implanted or external device via tissue conductance communication (TCC). During TCC, current is driven through the tissue between two or more electrodes of the transmitting IMD (or external device). The current spreads through the thorax, producing a potential field. The receiving IMD (or external device) may detect the TCC signal by measuring the potential difference between two or more of its electrodes.

<CIT> relates to an atrial synchronized ventricular pacing system using an intracardiac pacemaker and extracardiac atrial sensing. <CIT> relates to tissue conduction communication transmission. <CIT> relates to a communication dipole for implantable medical devices.

The claimed subject-matter is defined by independent claim <NUM>. In the following any of the methods are not claimed and the method directed to operating the implantable medical device is disclosed for illustrative purposes only.

In general, this disclosure is directed to techniques for improving the sensing of physiological electrical signals and the transmission and receipt of TCC signals via electrodes of an implantable medical device. For lower frequency physiological electrical signals, the electrodes, or active portion of the electrodes, may be spaced apart for a longer dipole and higher amplitudes. For higher frequency TCC signals, the surface area of the electrodes, or active portion of the electrodes, may be expanded to reduce the impedance of the electrodes, thereby increasing the current capability. Additionally, in some examples, the surface area of the electrodes, or active portion of the electrodes, may be smaller for delivery of lower frequency electrical stimulation therapy.

As one example, the disclosure is directed to an implantable medical device comprising a plurality of electrodes, sensing circuitry configured to sense a physiological electrical signal via the plurality of electrodes, and communication circuitry configured to transmit and/or receive a tissue conductance communication (TCC) signal via the plurality of electrodes, wherein at least one electrode of the plurality of electrodes comprises a lower-capacitance portion and a higher-capacitance portion.

In some examples, the disclosure is directed to a method for manufacturing an implantable medical device, the method comprising forming a first material of an electrode of the implantable medical device, depositing a mask on at least part of the first material, depositing a second material on the mask and the first material to form a higher-capacitance portion of the electrode, and removing the mask from the first material to expose a lower-capacitance portion of the electrode.

In some examples, the disclosure is directed to a method for manufacturing an implantable medical device, the method comprising: forming an electrode on the implantable medical device; depositing a mask on a first portion of the electrode; depositing a dielectric material on the mask and a second portion of the electrode; and removing the mask from the first portion of the electrode and leaving the dielectric material on the second portion of the electrode, wherein the first portion of the electrode has a higher capacitance than the second portion of the electrode.

In some examples, an implantable medical device comprises at least four electrodes, sensing circuitry configured to sense a physiological electrical signal via a first electrode and a second electrode of the at least four electrodes, communication circuitry configured to transmit a transconductance communication signal via at least a third electrode and a fourth electrode of the at least four electrodes; and switching circuitry configured to connect the first electrode and the second electrode to the sensing circuitry, and connect the at least four electrodes to the communication circuitry. The implantable medical device also comprises processing circuitry configured to control the switching circuitry to connect the first electrode and the second electrode to the sensing circuitry to sense the physiological electrical signal, and control the switching circuitry to connect at least the third electrode and the fourth electrode to the communication circuitry to transmit or receive tissue conductance communication (TCC) signals.

As another example, the disclosure is directed to method for operating an implantable medical device, the method comprising: controlling switching circuitry of the implantable medical device to connect a first electrode and a second electrode to sensing circuitry of the implantable medical device; sensing a physiological electrical signal via the first electrode and the second electrode; controlling the switching circuitry to connect at least a third electrode and a fourth electrode to communication circuitry of the implantable medical device; transmitting tissue conductance communication (TCC) signals via at least the third electrode and the fourth electrode; and receiving TCC signals via at least the third electrode and the fourth electrode.

This summary is intended to provide an overview of the subject matter described in this disclosure. It is not intended to provide an exclusive or exhaustive explanation of the apparatus and methods described in detail within the accompanying drawings and description below.

The details of one or more examples of this disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of this disclosure will be apparent from the description and drawings, and from the claims.

The drawings and the description provided herein illustrate and describe various examples of the inventive methods, devices, and systems of the present disclosure. However, the methods, devices, and systems of the present disclosure are not limited to the specific examples as illustrated and described herein, and other examples and variations of the methods, devices, and systems of the present disclosure, as would be understood by one of ordinary skill in the art. , are contemplated as being within the scope of the present application.

An implantable medical device (IMD) may transmit and receive signals at high and low frequencies for communication, sensing physiological signals, and delivering therapeutic signals. For example, the IMD may sense heart beat signals and transmit and/or receive tissue conductance communication (TCC) signals. For low-frequency signals, such as sensing physiological signals, the IMD may include relatively smaller surface area electrodes spaced at a relatively long dipole length, resulting in higher amplitude of the low-frequency signals. For relatively high-frequency signals, such as transmitting TCC signals, the IMD may include larger electrodes for a reduced load and source impedance to enable higher current capability during TCC signal transmission and higher received signal strength during TCC signal reception as well as reduced power consumption.

The IMD may include an electrode for receiving physiological electrical signals and transmitting and receiving TCC signals. The electrode may include a higher-capacitance portion configured to transmit and receive both relatively lower-frequency and relatively higher-frequency signals. The higher-capacitance portion of the electrode may provide a relatively lower impedance at both relatively lower frequencies and relatively higher frequencies. The electrode may also include a relatively lower-capacitance portion configured to transmit and receive relatively higher-frequency signals. The lower-capacitance portion of the electrode may provide a relatively higher impedance at relatively lower frequencies and a relatively lower impedance at relatively higher frequencies, such that the lower-capacitance portion of the electrode may not significantly affect, e.g., participate in or be active during, the receipt of lower-frequency signals, e.g., physiological electrical signals, by die higher-capacitance portion of the electrode. For purposes of this disclosure, low impedance or lower impedance may be defined as less than one thousand ohms.

The techniques of this disclosure may allow for a relatively long dipole length between the higher-capacitance portions of one or more electrodes, while increasing the effective surface area of the one or more electrodes for higher-frequency signals. The longer dipole length at relatively lower frequencies creates a higher transimpedance and higher amplitude for low-frequency signals. The relatively larger surface area of the electrode at high frequencies effectively lowers the impedance seen by higher-frequency signals, enabling higher current capability during TCC transmission and higher received signal strength during TCC reception. Lower impedance may also reduce the power consumption of the IMD when communicating via higher frequencies. For purposes of this disclosure, lower-frequency signals may include frequencies less than about one hundred Hertz (Hz), and higher-frequency signals may include frequencies greater than about fifty kiloHertz (kHz).

<FIG> is a conceptual drawing illustrating an example medical device system 8A in conjunction with a patient 2A. In the illustrated example, medical device system 8A includes an implantable medical device (IMD) <NUM>, also referred to as implantable monitoring device <NUM> or an implantable hub device, in communication with external device 14A. Medical device system 8A also includes implantable pressure sensing device <NUM>, also referred to as sensor device <NUM><NUM>. 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 techniques of this disclosure.

As shown in <FIG>, implantable sensor assembly <NUM>, including sensor device <NUM>, may be implanted within pulmonary artery 6A of heart 4A. In some examples, sensor assembly <NUM> is implanted within a left pulmonary artery, whereas in other examples, sensor assembly <NUM> is implanted within a right pulmonary artery. For the sake of clarity, a fixation assembly for sensor assembly <NUM> is not depicted in <FIG>.

In the illustrated example, IMD <NUM> is an insertable cardiac monitor (ICM) capable of sensing and recording cardiac electrogram (EGM) signals from a position outside of heart 4A via electrodes, and will be referred to as ICM <NUM> hereafter. In some examples, ICM <NUM> 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 and/or posture, blood flow, or respiration. ICM <NUM> may monitor a physiological parameter indicative of patient state, such as posture, heart rate, activity level, heart rate, and/or respiration rate. ICM <NUM> may be implanted outside of the thorax of patient 2A, e.g., subcutaneously or submuscularly, such as the pectoral location illustrated in <FIG>. In some examples, ICM <NUM> may take the form of a Reveal LINQ™ ICM, available from Medtronic pic, of Dublin, Ireland.

Sensor device <NUM> may be implanted, as one example, within a pulmonary artery of patient 2A and may include pressure sensing circuitry configured to measure the cardiovascular pressure of patient 2A. In some examples, sensor device <NUM> may be a part of sensor assembly <NUM>. If sensor device <NUM> determines that the current time is within a predetermined window that may be stored in memory of sensor device <NUM>, sensor device <NUM> may measure and transmit the cardiovascular pressure of patient 2A to ICM <NUM>. In some examples, sensor device <NUM> may autonomously store the pressure measurement data and transmit the stored data to ICM <NUM> at some time after the time of measurement. In some examples, sensor device <NUM>. may include wireless communication circuitry configured to receive a trigger signal from ICM <NUM> via TCC. The pressure sensing circuitry of sensor device <NUM> may be configured to measure the cardiovascular pressure of patient 2A in response to receiving the trigger signal. In either case, sensor device <NUM> may be configured to transmit the measured pressure values to ICM <NUM> via TCC.

ICM <NUM> may transmit measurements and data acquired by ICM <NUM> or sensor device <NUM> to external device 14A. ICM <NUM> may also receive signals from external device 14A. In some examples, ICM <NUM> may communicate with external device 14A and sensor device <NUM> via RF signals and/or TCC.

External device 14A may be a computing device, e.g., used in a home, ambulatory, clinic, or hospital setting, to wirelessly communicate with ICM <NUM>. External device 14A may be coupled to a remote patient monitoring system, such as Carelink®, available from Medtronic pic, of Dublin, Ireland. External device 14A may be, as examples, a programmer, external monitor, or consumer device, e.g., smart phone.

External device 14A may be used to program commands or operating parameters into ICM <NUM> for controlling its functioning, e.g., when configured as a programmer for ICM <NUM>. External device 14A may be used to interrogate ICM <NUM> to retrieve data, including device operational data as well as physiological data accumulated in IMD memory. The interrogation may be automatic, e.g., according to a schedule, or in response to a remote or local user command. Examples of communication techniques used by ICM <NUM> and external device 14A include TCC or RF telemetry, which may be an RF link established via Bluetooth, WiFi, or medical implant communication service (MICS).

<FIG> is a conceptual drawing illustrating another example medical device system 8B in conjunction with a patient 2B. In the illustrated example, medical device system 8B includes an IMD <NUM> coupled to a ventricular lead <NUM> and an atrial lead <NUM>. IMD <NUM> is an implantable cardioverter-defibrillator (ICD) capable of delivering pacing, cardioversion and defibrillation therapy to the heart 4B of a patient 2B, and will be referred to as ICD <NUM> hereafter.

Ventricular lead <NUM> and atrial lead <NUM> are electrically coupled to ICD <NUM> and extend into the patient's heart 4B. Ventricular lead <NUM> includes electrodes <NUM> and <NUM> shown positioned on the lead in the patient's right ventricle (RV) for sensing ventricular EGM signals and pacing in the RV. Atrial lead <NUM> includes electrodes <NUM> and <NUM> positioned on the lead in the patient's right atrium (RA) for sensing atrial EGM signals and pacing in the RA. Electrodes <NUM> and <NUM> may also be configured as coil electrodes and used to deliver cardioversion and defibrillation shocks. The term "anti-tachyarrhythmia shock" may be used herein to refer to both cardioversion shocks and defibrillation shocks.

ICD <NUM> may use both ventricular lead <NUM> and atrial lead <NUM> to acquire cardiac electrogram (EGM) signals from patient 2B and to deliver therapy in response to the acquired data. Medical device system 8B is shown as having a dual chamber ICD configuration, but other examples may include one or more additional leads, such as a coronary sinus lead extending into the right atrium, through the coronary sinus and into a cardiac vein to position electrodes along the left ventricle (LV) for sensing LV EGM signals and delivering pacing pulses to the LV. In other examples, a medical device system may be a single chamber system, or otherwise not include atrial lead <NUM>.

Processing circuitry, sensing circuitry, and other circuitry configured for performing the techniques described herein are housed within a sealed housing <NUM>. Housing <NUM> (or a portion thereof) may be conductive so as to serve as an electrode for pacing or sensing or as an active electrode during defibrillation. As such, housing <NUM> is also referred to herein as "housing electrode" <NUM>. Housing <NUM> may include one or more electrodes with a higher-capacitance portion and a lower-capacitance portion. The higher-capacitance portion and the lower-capacitance portion may be formed using two different materials.

ICD <NUM> may transmit EGM signal data and cardiac rhythm episode data acquired by ICD <NUM>, as well as data regarding delivery of therapy by ICD <NUM>, to an external device 14B. External device 14B may be a computing device, e.g., used in a home, ambulatory, clinic, or hospital setting, to communicate with ICD <NUM> via wireless telemetry. External device 14B may be coupled to a remote patient monitoring system, such as Carelink®, available from Medtronic plc, of Dublin, Ireland. External device 14B may be, as examples, a programmer, external monitor, or consumer device, e.g., smart phone.

External device 14B may be used to program commands or operating parameters into ICD <NUM> for controlling its functioning, e.g., when configured as a programmer for ICD <NUM>. External device 14B may be used to interrogate ICD <NUM> to retrieve data, including device operational data as well as physiological data accumulated in IMD memory. The interrogation may be automatic, e.g., according to a schedule, or in response to a remote or local user command. Examples of communication techniques used by ICD <NUM> and external device 14B include TCC and RF telemetry, which may be an RF link established via Bluetooth, WiFi, or medical implant communication service (MICS).

In some examples, as illustrated in <FIG>, medical device system 8B may also include an intracardiac pacing device (IPD) <NUM>. In the illustrated example, IPD <NUM> is implanted in the left-ventricle of patient 2B. In some examples, one or more IPDs <NUM> may additionally or alternatively be implanted within other chambers of heart 4B, or attached to the heart epicardially, or in other anatomical areas of interest such as the pulmonary artery.

IPD <NUM> is configured to sense electrical activity of heart 4B and deliver pacing therapy, e.g., bradycardia pacing therapy, cardiac resynchronization therapy (CRT), anti-tachycardia pacing (ATP) therapy, and/or post-shock pacing, to heart 4B. IPD <NUM> may be attached to an interior wall of heart 4B via one or more fixation elements that penetrate the tissue. These fixation elements may secure IPD <NUM> to the cardiac tissue and retain an electrode (e.g., a cathode or an anode) on the housing of IPD <NUM> in contact with the cardiac tissue. In addition to delivering pacing pulses, IPD <NUM> may be capable sensing electrical signals using the electrodes carried on the housing of IPD <NUM>. These electrical signals may be electrical signals generated by cardiac muscle and indicative of depolarizations and repolarizations of heart 4B at various times during the cardiac cycle.

In some examples, ICD <NUM> and IPD <NUM> may both be configured to deliver pacing therapy. In such examples, ICD <NUM> and IPD <NUM> may delivery pacing therapy to the right and left ventricles of heart 4B, respectively, to provide CRT pacing. Additionally, ICD <NUM> and IPD <NUM> may both be configured to detect tachyarrhythmias, and deliver anti-tachyarrhythmia therapy.

ICD <NUM> and IPD <NUM> may be configured to coordinate their cardiac rhythm detection and treatment activities. In some examples, ICD <NUM> and IPD <NUM> may engage in wireless communication to facilitate such coordinated activity. The wireless communication may be via TCC, and may be one-way communication in which one device is configured to transmit communication messages and the other device is configured to receive those messages, or two-way communication in which each device is configured to transmit and receive communication messages.

In some examples, medical device system 8B may include one or more devices depicted in <FIG>. Alternatively or additionally, medical device system 8B may include devices not depicted in <FIG>. For example, ICD <NUM> may be a subcutaneous ICD or a substernal ICD that has a subcutaneous lead or a lead with a distal end under the sternum.

In some examples, medical device system 8B may include a single device, such as a pressure sensing device, an ICM, an IPD, and/or any other suitable device. The single device may be configured to communication via TCC signals with an external device. In some examples, medical device system 8B may include two IPDs configured to coordinate the heart rate of patient 2B using dual chamber sensing and/or pacing.

<FIG> is a top-view conceptual diagram of an example configuration of ICM <NUM> of <FIG>. In the illustrated example, ICM <NUM> includes two electrodes 60A, 60B formed on an insulative cover <NUM> of the housing of ICM <NUM>, and an antenna <NUM>, which may be below insulative cover <NUM> and within the housing. In some examples, ICM <NUM> may not include insulative cover <NUM>, such as if ICM <NUM> includes metallic housing. ICM <NUM> may include other components, including internal components and/or external components, that are not depicted in <FIG>. Antenna <NUM> may be optional, and used for RF communication with external device 14A.

When IMD <NUM> is sensing relatively lower-frequency physiological electrical signals, such as an electrical signal of the heart including an R-wave, higher-capacitance portions 64A, 64B of electrodes 60A, 60B may receive the physiological electrical signals. Higher-capacitance portions 64A, 64B may present relatively lower impedance for relatively lower-frequency signals. Higher-capacitance portions 64A, 64B may be positioned at the distal ends of insulative cover <NUM> to increase the dipole length <NUM> for sensing physiological electrical signals, which may increase the amplitude of signals sensed by higher-capacitance portions 64A, 64B. Signal amplitude may be approximately proportional to the dipole length between the active portions of electrodes 60A, 60B, which in the case of lower-frequency signals is dipole length <NUM> between higher-capacitance portions 64A, 64B.

Higher-capacitance portions 64A, 64B may include a relatively higher-capacitance material such as titanium nitride. Titanium-nitride can be deposited in a manner to form three-dimensional structures, effectively increasing the surface area of an electrode. A capacitor (known as a double-layer capacitor) is formed when the titanium-nitride is placed in an ionic medium. In some examples, the increased surface area results in higher capacitance than for the bare metal. In some examples, each of higher-capacitance portions 64A, 64B may have a capacitance per unit area of greater than approximately one microfarad per square millimeter. In some examples, higher-capacitance portions 64A, 64B may include a cutoff frequency of less than fifty mHz to allow for sensing of relatively lower-frequency signals. Assuming a surface area of eleven square millimeters for higher-capacitance portions 64A, 64B, a fifty-mHz signal may be attenuated by <NUM> dB compared to a hypothetical electrode with infinite capacitance per unit area.

It may be desirable for higher-capacitance portions 64A, 64B to include a capacitance of less than three hundred nanofarads at five hundred mHz to allow an ECG amplifier to drive a one megaohm impedance at frequencies down to five hundred mHz. The ratio of capacitances between higher-capacitance portions 64A, 64B and lower-capacitance portions 62A, 62B may be larger than ten for a range of frequencies from five hundred mHz to one hundred Hz to ensure that higher-capacitance portions 64A, 64B include much lower impedance than lower-capacitance portions 62A, 62B at these frequencies. The capacitance of the lower-capacitance portions 62A, 62B may be higher than ten nanofarads at one hundred kHz so that the impedance of lower-capacitance portions 62A, 62B may be less than two hundred ohms at one hundred kHz.

When IMD <NUM> is transmitting or receiving relatively higher-frequency signals, such as TCC signals, higher-capacitance portions 64A, 64B and lower-capacitance portions 62A, 62B of electrodes 60A, 60B may transmit and receive the signals. Lower-capacitance portions 62A, 62B may include low impedance for relatively high-frequency signals, such as TCC signals, which may include a frequency between about fifty kHz and two hundred kHz, as examples. Thus, higher-capacitance portions 64A, 64B may be configured to sense physiological electrical signals and transmit or receive TCC signals, and lower-capacitance portions 62A, 62B may be configured to transmit or receive TCC signals but generally not sense physiological electrical signals. By using both higher-capacitance portions 64A, 64B and lower-capacitance portions 62A, 62B to transmit and receive high-frequency signals, the effective surface area for higher-frequency signals may be larger than the effective surface area for lower-frequency signals.

In some examples, each of lower-capacitance portions 62A, 62B may have a larger percentage of the surface area of electrodes 60A, 60B than each of higher-capacitance portions 64A, 64B. In some examples, the surface area of each of lower-capacitance portions 62A, 62B may be at least approximately <NUM>%, <NUM>%, or <NUM>% larger than the surface area of one of higher-capacitance portions 64A, 64B. Lower-capacitance portions 62A, 62B may include material such as titanium (i.e., bare titanium). A capacitor (known as a double-layer capacitor) is formed when the bare titanium is placed in an ionic medium. Alternatively, the titanium can be oxidized to form titanium dioxide, which acts as the dielectric of the capacitor. Another method to form the lower-capacitance portions 62A, 62B is to deposit a thin layer of dielectric material on top of the bare metal. In some examples, lower-capacitance portions 62A, 62B may include titanium nitride with a dielectric layer to reduce the capacitance. In some examples, the dielectric material may also be deposited on a portion of one or both of higher-capacitance portions 64A, 64B to form a lower-capacitance portion 62A, 62B of an electrode 60A, 60B.

In some examples, lower-capacitance portions 62A, 62B may include a first conductive material, and higher-capacitance portions 62A, 62B may include a second conductive material, where the first and second conductive materials are different elements or compounds. One portion may include a titanium, and the other portion may include a metallic material such as platinum, gold, copper, tin, and/or any other suitable conductive material. An electrode may include platinum and a dielectric material covering a portion of the platinum to create a higher-capacitance portion of the electrode. In some examples of this disclosure that employ higher voltages for certain uses of an electrode, a portion of an electrode may include tantalum pentoxide that insulates at lower voltages.

Impedance of electrodes 60A, 60B may be proportional to the surface area of electrodes 60A, 60B, as shown in equation (<NUM>): <MAT> <MAT> <MAT> <MAT>.

In equation (<NUM>), R represents the resistance, ρ represents die resistivity, L represents the length of a resistive element, and A represents the cross-sectional area of the resistive element. In equation (<NUM>), P represents the power dissipated in conducting a signal, and I represents the electrical current of the signal. In equation (<NUM>), ε is the dielectric constant of the dielectric layer, A is the area of the capacitor, and d is the thickness of the dielectric. By increasing the effective surface area of electrodes 60A, 60B, lower-capacitance portions 62A, 62B may reduce the resistance and impedance, which may enable higher current capability for TCC transmission and higher received signal strength for TCC reception. If the signal source impedance is relatively large, compared to the load impedance within IMD <NUM>, the received signal may be attenuated, causing a low signal to noise ratio. The load impedance within IMD <NUM> may be low due to electromagnetic interference suppression capacitors. Lower impedance may also reduce the power dissipation in electrodes 60A, 60B, as shown by equation (<NUM>). By increasing the thickness of the dielectric, the capacitance in equation (<NUM>) may be reduced. In equation (<NUM>), the impedance or capacitive reactance Xc is inversely proportional to the frequency f and the capacitance C. Thus, lower-capacitance portions 62A, 62B may include higher impedance than higher-capacitance portions 64A, 64B. It may be desirable for the capacitive reactance to be negligible compared to the resistance of electrodes 60A, 60B.

For sensing physiological electrical signals, the length of the dipole between electrodes 60A, 60B may be important to the signal quality sensed by electrodes 60A, 60B. Dipole length <NUM> may represent the effective length between the center of mass of higher-capacitance portion 64A and the center of mass of higher-capacitance portion 64B. For higher-frequency signals such as TCC signals, dipole length <NUM> may represent the effective length between the center of mass of electrode 60A and the center of mass of electrode 60B. As depicted in <FIG>, dipole length <NUM> may be longer than dipole length <NUM>. In some examples, IMD <NUM> may include dipole length <NUM> of approximately thirty-four millimeters, dipole length <NUM> of approximately forty millimeters, and width <NUM> of approximately eight millimeters. The <NUM>% reduction in dipole length for relatively higher-frequency signals may be compensated by the much larger increase in surface area for the higher-frequency signals.

Although dipole length <NUM> may be important for communicating via high-frequency signals, current capability and power consumption may be more important considerations in some examples. Current capability may be important with lower-voltage batteries because high impedance may reduce the electrical current through electrodes 60A, 60B. Power consumption may decrease as the surface area of electrodes 60A, 60B is increased. Lower-capacitance portions 64A, 64B being active during transmission of higher-frequency signals may reduce power consumption of ICM <NUM> by a much larger percentage than power consumption is increased by the resulting reduction in dipole length <NUM> and the corresponding reduction in transimpedance.

Moreover, the impedance of the tissue surrounding IMD <NUM> may be important. For example, bone and fat may have a relatively high resistance to electrical signals, while muscle and blood may have a relatively low resistance to electrical signals. At a frequency of eight kHz, the median impedance of tissue in patients may be about nine hundred and forty ohms. If IMD <NUM> includes a battery voltage of two volts, driving a TCC current of one or two milliamperes into the tissue may require relatively low impedance. A higher impedance may prevent a two-volt battery from driving an adequate TCC current. Thus, in some examples, a reduction in impedance for higher-frequency signals may be desirable.

In one example, each of lower-capacitance portions 62A, 62B may include a surface area of approximately fourteen square millimeters, and each of higher-capacitance portions 64A, 64B may include a surface area of approximately twenty-eight square millimeters. The dipole length and transimpedance may decrease by fifteen percent for transmission of higher-frequency signals, but the impedance may also decrease by sixty-six percent for the higher-frequency signals. Thus, the power efficiency in this example may improve by more than fifty percent: <MAT>.

In some examples, insulative cover <NUM> and/or can <NUM> (<FIG>) may act as an elongate housing that contains processing circuitry, sensing circuitry, and/or communication circuitry. Each of higher-capacitance portions 64A, 64B may be closer to the distal ends of insulative cap <NUM> than each of lower-capacitance portions 62A, 62B. Likewise, each of lower-capacitance portions 62A, 62B may be closer to antenna <NUM> than each of higher-capacitance portions 64A, 64B.

<FIG> is a conceptual, cross-sectional side-view diagram of the example configuration of ICM <NUM> of <FIG>, the cross-section taken along line A - A' of <FIG>. <FIG> depicts each of higher-capacitance portions 64A, 64B as being formed or placed above or on each of lower-capacitance portions 62A, 62B, each of which may be formed or placed above or on top of insulative cover <NUM>. Although shown as a layer above lower-capacitance portions 62A, 62B, higher-capacitance portions 64A, 64B may, in some examples, be formed by treating a portion of a surface of the lower-capacitance portions, e.g., treating a portion of a surface of a bare titanium electrode to form high-surface area titanium-nitride, producing higher-capacitance portion <NUM>.

ICM <NUM> as depicted in <FIG> may include a wafer-scale insulative cover <NUM> positioned over a can <NUM> to form the housing of the ICM. Circuitry of ICM <NUM>, e.g., processing circuitry, sensing circuitry, and/or communication circuitry, may be formed on insulative cover <NUM>, e.g., using flip-chip technology. For example, circuitry may be formed on a side of insulative cover <NUM>, and insulative cover <NUM> may be flipped onto can <NUM>. When flipped and placed onto can <NUM>, the circuitry may be positioned on the bottom side of insulative cover <NUM> in a gap <NUM> defined by can <NUM>. The circuitry on insulative cover <NUM> may be electrically connected to electrodes 60A, 60B through vias 82A, 82B formed through insulative cover <NUM>. Insulative cover <NUM> may include additional vias not shown in <FIG>. Insulative cover <NUM> may be formed of sapphire and/or any other suitable insulating material. Can <NUM> may be formed from bare titanium or any other suitable material.

In some examples, insulative cover <NUM> may be sapphire and have a thickness of approximately three hundred micrometers to six hundred micrometers. Can <NUM> may be titanium and have a thickness of approximately two hundred micrometers to five hundred micrometers. Lower-capacitance portion 62A may be titanium and have a thickness of approximately fifty micrometers to one hundred micrometers. Higher-capacitance portion 64A may be titanium nitride and have a thickness of approximately five hundred nanometers to ten micrometers. Higher-capacitance portion 64A may be more mechanically robust and have a higher capacitance for higher thicknesses. These materials and dimensions are examples only, and other materials and other thicknesses are possible for devices of this disclosure.

<FIG> is a conceptual side-view diagram of an IMD <NUM> including two electrodes 60A and <NUM>. Electrode 60A is described above with respect to <FIG> and <FIG>. IMD <NUM> may include dielectric material <NUM> deposited on can <NUM>. Higher-capacitance portion <NUM> of electrode <NUM> may be formed or placed on can <NUM>. Higher-capacitance portion <NUM> of electrode <NUM> may include similar properties to higher-capacitance portions 64A and 64B. These properties may include capacitance, frequency response (amplitude as a function of frequency), ratios of surface area for higher- and lower-capacitance portions, and any other properties. In one example, can <NUM> is formed of bare titanium, and higher-capacitance portion <NUM> is formed by treating a portion of the exposed portion of can <NUM> to form a titanium-nitride higher-capacitance portion <NUM>. In some examples, material <NUM> (i.e., "the tub" or "the can") may be connected to a terminal of sensing circuitry and communication circuitry of IMD <NUM>.

<FIG> is a conceptual side-view diagram of another example configuration of an IMD <NUM> including two electrodes 110A, 110B. IMD <NUM> may include an elongate housing including header <NUM> and can <NUM>. Header <NUM> and/or can <NUM> may house the processing circuitry, sensing circuitry, and/or communication circuitry of IMD <NUM>.

Header <NUM> may include insulating material such as plastic or any other suitable insulating material. Electrode 110A may include higher-capacitance portion 114A and lower-capacitance portion 112A, and may be attached to header <NUM>. Electrode 110A may be electrically connected to sensing circuitry and communication circuitry within IMD <NUM>. Higher-capacitance portions 114A and 114B may include similar properties to higher-capacitance portions 64A and 64B.

In some examples, can <NUM> may include conductive material such as bare titanium. Dielectric material <NUM> may be formed on can <NUM>. The area of can <NUM> may not be covered with dielectric material <NUM> may operate as lower-capacitance portion 112B. Higher-capacitance portion 114B may be formed or placed on or over lower-capacitance portion 112B. In some examples, can <NUM> may include a surface area of approximately seventy square millimeters, while electrode 110A may include a surface area of approximately nine square millimeters. Lower-capacitance portions 112A and 112B may include similar properties to lower-capacitance portions 62A and 62B.

<FIG> is a conceptual diagram illustrating an example configuration of an implantable medical lead <NUM> including electrodes 120A and 120B. Electrodes 120A and 120B may be used for sensing physiological electrical signals, transmitting and/or receiving TCC signals, and/or delivering therapeutic signals to a patient. Electrode 120A may include lower-capacitance portion 122A and higher-capacitance portion 124A, and electrode 120B may include lower-capacitance portion 122B and higher-capacitance portion 124B. Electrodes 120A and 120B may be connected to a lead body <NUM> which may connect to an IMD or external device. Electrode 120B at distal end <NUM> of lead body <NUM> may include at least a portion formed as a fixation structure, e.g., helix, to fix lead <NUM> to the tissue of the heart and hold the lead in place, or distal end of lead <NUM> may otherwise include a fixation structure.

Higher-capacitance portions 124A and 124B may be configured to operate as an electrode for communication, sensing, and delivering therapy pulses. Lead <NUM> may be configured to sense physiological signals and deliver therapeutic signals, e.g., pacing pulses, through higher-capacitance portions 124A and 124B. In some examples, a longer dipole may be desirable, such that the can (not shown in <FIG>) of a medical device may be used as an electrode. Lead body <NUM> may be connected to the can of the medical device. A medical device coupled to lead <NUM> may communicate using TCC through lower-capacitance portions 122A and 122B higher-capacitance portions 124A and 124B of electrodes 120A and 120B, or the lower capacitance and higher capacitance portions of one of the electrodes <NUM> and the can. Electrode 120B may be similar to electrode <NUM> in <FIG>, and electrode 120A may be a ring electrode, or a coil electrode similar to electrode <NUM> in <FIG>.

For defibrillation, electrode 120B (when configured as a coil electrode <NUM> (<FIG>)) and possibly the housing of IMD <NUM> in <FIG>, may transmit a lower-frequency and higher-voltage signal to the heart tissue. Defibrillation may include relatively higher voltages as compared to other pacing signals and therapy signals. The relatively higher voltages of defibrillation may produce relatively higher electrical currents. A lower defibrillation voltage may be desirable to reduce the need for high-voltage capacitors. A larger electrode surface area may also be desirable for defibrillation to reduce the voltage needed to generate an adequate current for defibrillation.

<FIG> are conceptual diagrams of an example technique for manufacturing an electrode <NUM> including a higher-capacitance portion <NUM> and a lower-capacitance portion <NUM>. The method may begin with insulative cover <NUM>. <FIG> depicts conductive material <NUM> formed on insulative cover <NUM>. Conductive material <NUM> may include, as examples, bare titanium or titanium dioxide.

<FIG> depicts mask <NUM> formed on part of conductive material <NUM>. <FIG> depicts another material <NUM> on mask <NUM> and conductive material <NUM>. Mask <NUM> may prevent material <NUM> from contacting some of conductive material <NUM>. The portion of material <NUM> that contacts conductive material <NUM> may form higher-capacitance portion <NUM>. Material <NUM> may include an element or compound, e.g., nitrogen, such that higher-capacitance portion <NUM> may include a mixture or composition of material <NUM> and conductive material <NUM>, such as titanium nitride. <FIG> depicts the removal of mask <NUM> and material <NUM> that was disposed on mask <NUM>, exposing conductive material <NUM>. The exposed portion of conductive material <NUM> may become lower-capacitance portion <NUM>. The process of <FIG> may be repeated, or performed on two areas of insulative cover <NUM> substantially simultaneously, to construct a second electrode <NUM>, and thereby construct two electrodes 60A, 60B, e.g., as illustrated in <FIG> and <FIG>.

<FIG> is a flowchart illustrating an example technique <NUM> of manufacturing an electrode including a higher-capacitance portion <NUM> and a lower-capacitance portion <NUM>. Technique <NUM> may be implemented in the construction of any one of the implantable medical devices (IMDs) discussed above because each one of the IMDs is configured to include an electrode including a higher-capacitance portion and a lower-capacitance portion. The technique of <FIG> may be described in the context of electrode <NUM> of <FIG>.

The technique of <FIG> may include forming insulative cover <NUM>, which may include sapphire in some examples (<NUM>). Insulative cover <NUM> may include circuitry attached to one side. The technique of <FIG> may also include forming first material <NUM> on insulative cover <NUM> (<NUM>). First material <NUM> may include bare titanium or another conductive material. The technique of <FIG> may also include depositing mask <NUM> on a portion of first material <NUM> (<NUM>). The technique of <FIG> may also include depositing second material <NUM> on first material <NUM> and mask <NUM> (<NUM>). Mask <NUM> may block material <NUM> from contacting all of first material <NUM>. The technique of <FIG> may also include removing mask <NUM> and material <NUM> that is deposited on mask <NUM>, leaving the second material deposited on first material <NUM> as higher-capacitance portion 64A on electrode 60A (<NUM>).

<FIG> are block diagrams of a method of manufacturing an electrode <NUM> using dielectric material <NUM>. The method may begin with insulative cover <NUM>. <FIG> depicts conductive material <NUM> formed on insulative cover <NUM>. Conductive material <NUM> may include, as examples, bare titanium or any other conductive material in some examples. <FIG> depicts another material <NUM> formed on conductive material <NUM>. Material <NUM> may be formed by treating conductive material <NUM> with an element or compound, e.g., nitrogen, on conductive material <NUM>, such that titanium nitride or another material forms with a higher capacitance.

<FIG> depicts mask <NUM> deposited on a portion of material <NUM>. <FIG> depicts dielectric material <NUM> deposited on material <NUM> and on mask <NUM>. Mask <NUM> may block dielectric material <NUM> from attaching to a portion of material <NUM>. Dielectric material <NUM> may include any suitable dielectric material, such as parylene. <FIG> depicts the removal of mask <NUM> and dielectric material <NUM> that was attached to mask <NUM>, exposing material <NUM> to form a higher-capacitance portion. The portion of material <NUM> that is covered by dielectric material <NUM> may operate as a lower-capacitance portion.

<FIG> is a flowchart illustrating an example technique <NUM> of manufacturing an electrode using dielectric material. Technique <NUM> may be implemented by any one of the implantable medical devices (IMDs) discussed above because each one of the IMDs is configured to include at least one electrode.

The technique of <FIG> includes forming conductive material <NUM> on insulative material <NUM> (<NUM>). Conductive material <NUM> may include a conductive material with low capacitance such as titanium or titanium oxide. The technique of <FIG> further includes treating conductive material <NUM> with a second material, such as nitrogen, to form material <NUM> (<NUM>). The technique of <FIG> further includes placing mask <NUM> on a portion of material <NUM> (<NUM>). The technique of <FIG> further includes forming dielectric material <NUM> on mask <NUM> and material <NUM> (<NUM>). The technique of <FIG> further includes removing mask <NUM> and the portion of dielectric material <NUM> on mask <NUM> (<NUM>). The remaining dielectric material <NUM> may lower the capacitance of the portion of material <NUM> that dielectric material <NUM> covers.

The capacitance per unit area of materials used for electrodes may be adjusted by changing the surface finish of the electrode. For example, the capacitance of a material such as anodized titanium may be modified by controlling the thickness of an oxide layer. The thickness of the oxide layer may be controlled by changing the voltage, current, time, and/or solution used in the anodizing process.

<FIG> is a block diagram of an example configuration of an IMD <NUM> including two electrodes 60A, 60B. IMD <NUM> may include processing circuitry <NUM> for controlling sensing circuitry <NUM>, TCC circuitry <NUM>, switching circuitry <NUM>, memory <NUM>, optional RF circuitry <NUM>, and optional therapy generation circuitry <NUM>. Switching circuitry <NUM> may include one or more switches, such as metal-oxide-semiconductor field-effect transistors (MOSFETs) or bipolar transistors. Processing circuitry <NUM> may control switching circuitry <NUM> to connect electrodes 60A, 60B to sensing circuitry <NUM> to sense a physiological electrical signal. IMD <NUM> may sense the physiological electrical signal, which may have a relatively lower frequency content through higher-capacitance portions 64A, 64B of electrodes 60A, 60B.

Moreover, processing circuitry <NUM> may control switching circuitry <NUM> to connect electrodes 60A, 60B to TCC circuitry <NUM> to transmit or receive TCC signals. Because TCC signals may have a relatively higher frequency content, e.g., than physiological signals or therapy signal, both higher-capacitance portions 64A, 64B and lower-capacitance portions 62A, 62B of electrodes 60A, 60B may be active during transmission and receipt of TCC signals. In some examples, processing circuitry <NUM> may control switching circuitry <NUM> to connect electrodes 60A, 60B to therapy generation circuitry <NUM> to deliver a therapy pulse, such as a pacing pulse to the heart. IMD <NUM> may deliver the therapy pulse, which may have a relatively lower frequency content, through higher-capacitance portions 64A, 64B. Lower-capacitance portions 62A and 62B may be configured to not deliver the therapy pulse because lower-capacitance portions 62A and 62B may have a relatively high impedance for lower-frequency therapy pulses. Lower-capacitance portions 62A and 62B may conduct lower-frequency therapy pulses, but the amplitude of the current through lower-capacitance portions 62A and 62B may be much lower than the amplitude of the current through higher-capacitance portions 64A, 64B. As a result, the electrical current through higher-capacitance portions 64A, 64B may be at least approximately ten, twenty, or one hundred times higher than the electrical current through lower-capacitance portions 62A and 62B. Therapy generation circuitry <NUM> and/or processing circuitry <NUM> may control the frequency, amplitude, and other characteristics of the therapy pulses. Therapy generation circuitry <NUM> may deliver the therapy pulses to electrodes 60A, 60B when switching circuitry <NUM> connects therapy generation circuitry <NUM> to electrodes 60A, 60B.

Processing circuitry <NUM> may control switching circuitry <NUM> by sending control signals to the control terminals of one or more switches of switching circuitry <NUM>. The control signals may control whether the switches of switching circuitry <NUM> conduct electricity between the load terminals of the switches. If switching circuitry <NUM> includes MOSFET switches, the control terminals may include gate terminals, and the load terminals may include drain terminals and source terminals.

<FIG> is a conceptual top-view diagram of an example configuration of an implantable medical device <NUM> including four electrodes 240A, 240B, 242A, 242B and an antenna <NUM>. IMD <NUM> may include switching circuitry configured to connect some or all of electrodes 240A, 240B, 242A, 242B to sensing circuitry and/or communication circuitry of IMD <NUM>. The switching circuitry is configured to connect the sensing circuitry to electrodes 242A, 242B, e.g., and not to TCC electrodes 240A, 240B, to sense a physiological electrical signal through electrodes 242A, 242B.

Dipole length <NUM> between the centers of electrodes 242A, 242B may be longer than dipole length <NUM> between the centers of the combination of electrodes 240A, 242A and the combination of electrodes 240B, 242B. Dipole length <NUM> may provide a larger transimpedance and signal amplitude than dipole length <NUM> for sensing physiological electrical signal. In some examples, all of electrodes 240A, 240B, 242A, 242B may include the same material, such as titanium nitride. To increase the dipole length, the processing circuitry of IMD <NUM> may control the switching circuitry to switch off electrodes 240A, 240B when sensing or transmitting low-frequency signals.

To increase the surface area for transmitting or receiving high-frequency signals, the processing circuitry may control the switching circuitry to switch on all of electrodes 240A, 240B, 242A, 242B, with electrodes 240A and 242A acting as one effective electrode, and electrodes 240B and 242B acting as another effective electrode. The communication circuitry may be configured to transmit or receive a TCC signal through electrodes 240A and 242A, and electrodes 240B and 242B.

In some examples, IMD <NUM> may include three electrodes for TCC and sensing, The battery of IMD <NUM> may be positioned near a single electrode in place of electrodes 240A, 242A. The single electrode may be used for both TCC and sensing signals. A three-electrode version of IMD <NUM> may include electrodes 240B and 242B on an opposite end from die single electrode to increase the dipole distance for sensing and lower the impedance for TCC.

<FIG> is a block diagram illustrating an example configuration of an implantable medical device <NUM> including four electrodes 240A, 240B, 242A, 242B. Switching circuitry <NUM>, as controlled by processing circuitry <NUM> may be configured to connect electrodes 242A, 242B to sensing circuitry <NUM> to sense physiological electrical signals. Switching circuitry <NUM> may include one or more switches, such as metal-oxide-semiconductor field-effect transistors (MOSFETs) or bipolar transistors. Switching circuitry <NUM> may be configured to connect electrodes 240A, 240B, 242A, 242B to transmit or receive TCC signals.

In some examples, processing circuitry <NUM> may control switching circuitry <NUM> to connect electrodes 242A, 242B to therapy generation circuitry <NUM> to deliver a therapy pulse, such as a pacing pulse to the heart. In some examples, electrodes 242A, 242B may have a higher capacitance than electrodes 240A, 240B. IMD <NUM> may deliver the therapy pulse, which may have a relatively lower frequency content, through electrodes 242A, 242B. Therapy generation circuitry <NUM> and/or processing circuitry <NUM> may control the frequency, amplitude, and other characteristics of the therapy pulses. Therapy generation circuitry <NUM> may deliver the therapy pulses to electrodes 242A, 242B when switching circuitry <NUM> connects therapy generation circuitry <NUM> to electrodes 242A, 242B.

<FIG> is a flowchart illustrating an example technique <NUM> of operating an implantable medical device <NUM> including two electrodes 60A, 60B. If IMD <NUM> is sensing physiological signals (<NUM>), IMD <NUM> may receive the physiological signals from higher-capacitance portions 64A, 64B of electrodes 60A, 60B (<NUM>;). If IMD <NUM> is communicating via TCC signals (<NUM>), both lower-capacitance portions 62A, 62B and higher-capacitance portions 64A, 64B of electrodes 60A, 60B may be active (<NUM>).

<FIG> is a flowchart illustrating an example technique <NUM> of operating an implantable medical device <NUM> including four electrodes 240A, 240B, 242A, 242B. If IMD <NUM> is sensing physiological signals (<NUM>), switching circuitry <NUM> may switch off electrodes 240A, 240B (<NUM>). Switching circuitry <NUM> may switch off electrodes 240A, 240B by disconnecting electrodes 240A, 240B from sensing circuitry <NUM> and connecting electrodes 242A, 242B to sensing circuitry <NUM>. Sensing circuitry <NUM> may receive a physiological signal from electrodes 242A, 242B (<NUM>).

If IMD <NUM> is communicating via TCC signals (<NUM>), switching circuitry <NUM> may switch on electrodes 240A, 240B, 242A, and 242B (<NUM>). IMD <NUM> may communicate via TCC signals by receiving signals from a control module. Switching circuitry <NUM> may switch on electrodes 240A, 240B, 242A, and 242B for communicating TCC signals by connecting electrodes 240A, 242A and electrodes 240B, 242B to respective nodes (e.g., input or output) of TCC communication circuitry <NUM>. TCC communication circuitry <NUM> may transmit and/or receive TCC signals via the connected electrodes 240A, 240B, 242A, 242B (<NUM>). In some examples, TCC communication circuitry <NUM> may transmit and/or receive TCC signals via at least electrodes 240A, 240B.

<FIG> is a diagram of an implantable medical device <NUM> including a tip electrode <NUM>. IPD <NUM> may be configured to be implanted in the left ventricle of the heart of a patient, as depicted in <FIG>. As shown in <FIG>, IPD <NUM> includes case <NUM>, cap <NUM>, electrode <NUM>, electrode <NUM>, fixation mechanisms <NUM>, flange <NUM>, and opening <NUM>. Together, case <NUM> and cap <NUM> may be considered the housing of IPD <NUM>. In this manner, case <NUM> may form a hermetical seal around the various electrical components, e.g., circuitry, within IPD <NUM>. Cap <NUM> may hold in place the tip electrode assembly and the tine assembly, as shown in <FIG>. Case <NUM> may enclose substantially all of the electrical components and create the hermetically sealed housing of IPD <NUM>. Although IPD <NUM> is generally described as including one or more electrodes, IPD <NUM> may typically include at least two electrodes (e.g., electrodes <NUM> and <NUM>) to deliver an electrical signal (e.g., therapy such as cardiac pacing) and/or provide at least one sensing vector.

Electrodes <NUM> and <NUM> are carried on case <NUM> and cap <NUM>, respectively. In this manner, electrodes <NUM> and <NUM> may be considered leadless electrodes. In the example of <FIG>, electrode <NUM> is disposed on the exterior surface of cap <NUM>. Electrode <NUM> may be a circular electrode positioned to contact cardiac tissue upon implantation. Electrode <NUM> may be a ring or cylindrical electrode disposed on the exterior surface of case <NUM>. Although not depicted in <FIG>, electrode <NUM> may include a higher-capacitance portion and a lower-capacitance portion. Both case <NUM> and cap <NUM> may be electrically insulating.

Electrode <NUM> may be used as a cathode and electrode <NUM> may be used as an anode, or vice versa, for delivering cardiac pacing such as bradycardia pacing, CRT, ATP, or post-shock pacing. However, electrodes <NUM> and <NUM> may be used in any stimulation configuration. In addition, electrodes <NUM> and <NUM> may be used to detect intrinsic electrical signals from cardiac muscle. Tip electrode <NUM> may be configured to contact cardiac tissue such as an interior wall of the left ventricle, the right ventricle, or the right atrium. Tip electrode <NUM> may be configured to contact cardiac tissue epicardially or intracardially.

Fixation mechanisms <NUM> may attach IPD <NUM> to cardiac tissue. Fixation mechanisms <NUM> may be active fixation tines, screws, clamps, adhesive members, or any other mechanisms for attaching a device to tissue. As shown in the example of <FIG>, fixation mechanisms <NUM>. may be constructed of a memory material, such as a shape memory alloy (e.g., nickel titanium), that retains a preformed shape. During implantation, fixation mechanisms <NUM> may be flexed forward to pierce tissue and allowed to flex back towards case <NUM>. In this manner, fixation mechanisms <NUM> may be embedded within the target tissue.

Flange <NUM> may be provided on one end of case <NUM> to enable tethering or extraction of IPD <NUM>. For example, a suture or other device may be inserted around flange <NUM> and/or through opening <NUM> and attached to tissue. In this manner, flange <NUM> may provide a secondary attachment structure to tether or retain IPD <NUM> within the heart if fixation mechanisms <NUM> fail. Flange <NUM> and/or opening <NUM> may also be used to extract IPD <NUM> once the IMD needs to be explanted (or removed) from the patient if such action is deemed necessary.

IPD <NUM> is one example of a pacing device configured to include one or more electrodes according to this disclosure. However, other implantable medical devices may be configured to include one or more electrodes similar to those described with respect to IPD <NUM>. <FIG> are conceptual diagrams of an implantable medical device <NUM> including a tip electrode <NUM>. <FIG> depicts an example that is similar to IPD <NUM> in <FIG>, while <FIG> depicts IPD <NUM> including conductive layer <NUM> adjacent to tip electrode and thin insulating overcap <NUM> deposited over conductive layer <NUM>. IPD <NUM> in <FIG> may be a depiction of cap <NUM> and tip electrode <NUM> before conductive layer <NUM> and overcap <NUM> are added. Tip electrode <NUM> may have a relatively higher capacitance than conductive layer <NUM> as covered by insulating overcap <NUM>. Consequently, tip electrode <NUM> may act as a higher-capacitance portion for delivering or sensing lower frequency signals, e.g., pacing signals or physiological signals, while the combination of tip electrode <NUM> and conductive layer <NUM> as covered by insulating overcap <NUM> may provide an increased surface area for transmission or receipt of high-frequency signals, e.g., TCC signals, but generally not conduct lower frequency signals. Moreover, material may be applied to electrode <NUM> to create a lower-capacitance portion and a higher-capacitance portion.

IPD <NUM> may engage in one-way or two-way communication with other devices, such as IMD <NUM> or another IPD <NUM>. IPD <NUM> may receive commands and data from IMD <NUM>, and IPD <NUM> may transmit data to IMD <NUM> or external device 14B. When IPD <NUM> is transmitting signals, a larger electrode with lower impedance may be desirable to enable higher current capabilities during TCC transmission and higher received signal strength during TCC reception. In some examples, lower impedance may also reduce power dissipation and consequently increase the battery life of IPD <NUM>. Larger surface area of an electrode may also reduce the likelihood that a TCC transmission from IPD <NUM> will inadvertently cause tissue stimulation.

In some examples, tip electrode <NUM> may include a surface area of approximately two square millimeters. In experiments using a similar tip electrode in several patients, the impedance had a mean value of six hundred and eighteen ohms. To reduce the effective impedance at high frequencies without significantly increasing the pacing current at low frequencies, conductive layer <NUM> may provide an impedance in parallel with tip electrode <NUM>. If the capacitance of conductive layer <NUM> is too high, the pacing current at low frequencies may increase significantly. If the capacitance of conductive layer <NUM> is too low, conductive layer <NUM> may not adequately reduce the impedance at high frequencies.

In some examples, the charge delivered to tip electrode <NUM> for a pacing current may be estimated at three hundred and ninety nanocoulumbs, based on a voltage of one volt, an average impedance of six hundred and twenty ohms, and a time period of two hundred and forty microseconds. Adding conductive layer <NUM> with overcap <NUM> may increase the pacing current. For an increase in the pacing current of ten percent, the additional charge would be thirty-nine nanocoulumbs. Thus, an estimate of the maximum capacitance is forty nanofarads for some pacing devices. An estimate of the minimum capacitance may be obtained using fifty kHz as a lovv-end frequency for TCC communication and one thousand ohms as a high-end impedance: <MAT> <MAT>.

<FIG> is a cross-section diagram of an example configuration of a distal portion of an implantable medical device having a tip electrode <NUM>. Tip electrode <NUM> may include a cylinder surrounding steroid eluting silicone plug <NUM>. Metal cylinder <NUM> may be positioned underneath tip electrode <NUM>. Metal cylinder <NUM> may include a flat base to hold silicone plug <NUM> in place, support tip electrode <NUM>, and provide a connection to feedthrough pin <NUM>. Insulating overcap <NUM> may include a silicone rubber piece that wraps around metal cylinder <NUM> to insulate the metallic pieces within IPD <NUM> from external fluids. Insulating overcap <NUM> may be adhered to other components of IPD <NUM> by a silicone medical adhesive.

Insulating overcap <NUM> may include a surface area of eight square millimeters. In some examples according to this disclosure, insulating overcap <NUM> may be replaced by an insulated metal piece with a similar surface area and a capacitance of approximately ten nanofarads, e.g., conductive layer <NUM> covered by insulating overcap <NUM> as depicted in <FIG>. In some examples, tip electrode <NUM> may include an impedance of one thousand ohms, and overcap <NUM> may include an impedance of three hundred ohms in series with ten nanofarads. The combined impedance of tip electrode <NUM> and overcap <NUM> may be three hundred and thirty ohms, two hundred and sixty ohms, and two hundred and forty ohms at fifty kHz, one hundred kHz, and two hundred kHz, respectively. Thus, overcap <NUM> may reduce impedance for high-frequency signals. However, the increase in pacing current may be relatively low, such as five nanoamperes or two-and-one-half percent.

To design overcap <NUM> and other components of IPD <NUM> as a lower-capacitance portion of tip electrode <NUM>, several materials may be used. Titanium nitride or bare titanium may provide a relatively high capacitance. A ten-micrometer layer of parylene for overcap <NUM> may result in a capacitance of approximately twenty-one picofarads, which may be too low for TCC signals. Anodized titanium may include a capacitance of five nanofarads per square millimeter at certain thicknesses, such as two hundred nanometers.

Various aspects of the techniques may be implemented within one or more processors, including one or more microprocessors, DSPs, ASICs, FPGAs, or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components, embodied in programmers, such as physician or patient programmers, electrical stimulators, or other devices.

In one or more examples, the functions described in this disclosure may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on, as one or more instructions or code, a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media forming a tangible, non-transitory medium. Instructions may be executed by one or more processors, such as one or more DSPs, A SICs, FPGAs, general purpose microprocessors, or other equivalent integrated or discrete logic circuitry. Accordingly, the term "processor," as used herein may refer to one or more of any of the foregoing structure or any other structure suitable for implementation of the techniques described herein.

Claim 1:
An implantable medical device comprising:
a plurality of electrodes;
sensing circuitry (<NUM>) configured to sense a physiological electrical signal via the plurality of electrodes (60A, 60B); and
communication circuitry configured to transmit and/or receive tissue conductance communication (TCC) signals via the plurality of electrodes,
wherein at least one electrode of the plurality of electrodes comprises a lower-capacitance portion (62A, 62B) and a higher-capacitance portion (64A, 64B), wherein the higher-capacitance portion is configured to sense the physiological electrical signal.