Patent Publication Number: US-2022212017-A1

Title: Temperature sensors in medical implants

Description:
This application claims the benefit of U.S. Provisional Patent Application No. 63/134,436, filed Jan. 6, 2021, the entire contents of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The disclosure relates to medical devices and, more particularly, temperature sensors for sensing temperature of implantable medical devices. 
     BACKGROUND 
     Implantable medical devices (IMDs) may be used to monitor a patient condition and/or deliver therapy to the patient. In long term or chronic uses, IMDs may include a rechargeable power source (e.g., comprising one or more capacitors or batteries) that extends the operational life of the medical device to weeks, months, or even years over a non-rechargeable device. 
     When the energy stored in the rechargeable power source has been depleted, the patient may use an external charging device to recharge the power source. Since the rechargeable power source is implanted in the patient and the charging device is external to the patient, this charging process may be referred to as transcutaneous charging. In some examples, transcutaneous charging may be performed via inductive coupling between a primary coil in the charging device and a secondary coil in the IMD. 
     When a current is applied to the primary coil and the primary coil is aligned with the secondary coil, electrical current is induced in the secondary coil within the patient. Circuitry associated with the IMD uses the current to charge a rechargeable power source, such as a battery, within the IMD. Therefore, the external charging device does not need to physically connect with the rechargeable power source for charging to occur. 
     SUMMARY 
     In general, the disclosure is directed to devices, systems, and techniques for monitoring the temperature of a medical device, e.g., monitoring the temperature as feedback for controlling charging of a rechargeable power source, general operation of the device, or indication of malfunctioning components. An implantable medical device (IMD) may include a rechargeable power source that can be transcutaneously charged. The IMD, an external charging device, or other medical device associated with charging the rechargeable power source may include a temperature sensor that monitors the temperature of the respective device (e.g., the IMD, external charging device, or other medical device) during a charging session. Since recharging may be limited by the temperature of the housing of the IMD to reduce a risk of unwanted heating of tissue, one or more components of the system may monitor the temperature of the device to control charging of the rechargeable power source and avoid exposing patient tissue to undesirable temperatures. 
     The temperature sensor may be configured to sense the temperature of a portion of the device being monitored while being thermally-coupled to this portion of the device being monitored for temperature changes. In other words, the temperature sensor may utilize indirect temperature measurement techniques to sense the temperature of a particular surface or material within a device. 
     In one or more examples, the disclosure is directed to an implantable device that includes a medical device comprising a housing. A temperature sensor may be disposed within the housing and configured to sense a temperature of a portion of the medical device, wherein the temperature sensor is configured to be thermally coupled to the portion. At least one processor may be configured to control charging of a rechargeable power source based on the sensed temperature. In one example, the disclosure describes an implantable medical device may include a housing with at least one support disposed within the housing, and a temperature sensor thermally coupled to the interior surface of the housing, wherein the temperature sensor is disposed within the housing and configured to sense a temperature of a portion of the housing. At least one physically compliant material is disposed between the at least one support and the temperature sensor, where the physically compliant material is configured to provide a physical bias against the temperature sensor and towards the interior surface of the housing. 
     In one example, a method includes disposing a temperature sensor within a housing, the housing comprising an exterior surface and an interior surface, the housing having at least one support coupled to the housing, disposing at least one physically compliant material between the at least one support and the temperature sensor, where the at least one physically compliant material is configured to physically bias the temperature sensor towards the interior surface of the housing with the physically compliant material, and thermally coupling the temperature sensor to the housing, the temperature sensor configured to sense a temperature of a portion of the housing, the temperature sensor including a sensor surface. 
     In one example, the disclosure describes an implantable medical device may include a housing with at least one support disposed within the housing, and a temperature sensor thermally coupled to the interior surface of the housing, wherein the temperature sensor is disposed within the housing and configured to sense a temperature of a portion of the housing. At least one physically compliant material is disposed between the at least one support and the temperature sensor, where the physically compliant material is configured to provide a physical bias against the temperature sensor and towards the interior surface of the housing. A hybrid board may be disposed within the housing, where a flexible circuit may electrically couple the temperature sensor to the hybrid board, and processing circuitry may be configured to receive a temperature signal from the temperature sensor and control at least one function of the implantable medical device based on the temperature signal. 
     The details of one or more example are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a conceptual diagram illustrating an example system that includes an implantable medical device (IMD) and an external charging device that charges a rechargeable power source of the IMD. 
         FIG. 2  is a block diagram of the example IMD of  FIG. 1 . 
         FIG. 3  is a block diagram of the example external charging device of  FIG. 1 . 
         FIG. 4A  is a cross-sectional view of an example IMD that includes a temperature sensor disposed within the IMD. 
         FIG. 4B  is a cross-sectional view of an example IMD that includes a temperature sensors disposed within the IMD. 
         FIG. 4C  is a cross-sectional view of an example IMD that includes a temperature sensor disposed within the IMD. 
         FIG. 4D  is a cross-sectional view of an example IMD that includes a temperature sensor disposed within the IMD. 
         FIG. 4E  is a cross-sectional view of an example IMD that includes a temperature sensor disposed within the IMD. 
         FIG. 5  is a conceptual diagram illustrating an example IMD that includes temperature sensor(s) disposed within the IMD. 
         FIG. 6  is a conceptual diagram illustrating an example IMD that includes temperature sensor(s) disposed within the IMD. 
         FIG. 7  is a conceptual diagram illustrating an example IMD that includes temperature sensor(s) disposed within the IMD. 
         FIG. 8  is a conceptual diagram illustrating an example support configured to be disposed within an IMD. 
         FIG. 9  is a cross-sectional view of an example IMD with temperature sensor(s) disposed within an IMD. 
         FIG. 10  is a flow diagram that illustrates an example technique for manufacturing an IMD. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure is generally directed to devices, systems, and techniques for sensing a temperature of a portion of a medical device, such as the housing of the medical device. Since medical devices come into contact with human tissue, it may be beneficial to monitor the temperature of the medical device over time and/or during certain events. Some medical devices can determine temperature or heat based on indirect measurements of power used within the medical device and/or power applied to the medical during wireless recharge, for example. However, these indirect measurements may not be an accurate representation of the temperature of the medical device or the temperate at a certain location on or within the medical device. In other medical devices, a circuit board may include a temperature sensor that senses the temperature of the medical device at that position on the circuit board. However, the temperature sensed at the circuit board may not be indicative of the temperatures of the housing of the medical device or other external surfaces of the medical device that contact patient tissue. Placement of a temperature sensor in contact with a desired portion of a housing provides packaging and manufacturing challenges for a medical device, but more accurate measurements of certain portions of the medical device may provide operational and safety benefits. 
     Implantable medical devices (IMDs) may be implanted within a patient and used to monitor a parameter of the patient and/or deliver a therapy to the patient. To extend the operational life of the IMD, the IMD may include a rechargeable power source (e.g., one or more capacitors or batteries). When the rechargeable power source is being recharged, the power transmitted to the IMD generates heat via increased current to the battery and eddy currents on the housing, for example, that increases the temperature of the IMD. In addition, an external charging device (e.g., another medical device) placed against the skin of the patient may increase in temperature when power is transmitted during the recharging session. Higher charging power may reduce charging times while also increasing heating of the IMD. This may result in heating of tissue proximate the IMD and/or proximate the external charging device. In order to prevent undesirable temperatures, the system may monitor sensed temperatures in the IMD and/or external charging device. Without accurate temperature sensing, the system may be conservative with high power recharge to reduce the risk of undesirable high temperature. Therefore, accurate temperature sensing of one or more portions of the IMD or other device associated with charging may reduce recharge times and increase patient comfort. 
     As described herein, a medical device may include one or more temperature sensors thermally coupled to target portions of the medical device. Using the sensed temperature, a system can monitor the temperature the specific portions of the device, such as the temperatures occurring during recharge of a rechargeable power source within the medical device. An IMD may include a housing and a temperature sensor, in one or more examples, physically attached and/or thermally coupled to the surface of the housing within the IMD. For example, the one or more sensors may be positioned between a mounting structure and an inner surface of the housing. One or more compliant materials may be configured to provide a bias to the temperature sensor in order to maintain the thermal coupling between the temperature sensor and the housing. 
     The IMD or other device associated with charging may use the output from one or more temperature sensors as feedback for controlling the charging of the implanted rechargeable power source. The IMD and/or external charging device may thus monitor one or more temperatures to control charging and effectively limit temperatures of patient tissue adjacent the IMD and/or external charging device. For example, one or more processors (e.g., processing circuitry) may reduce the power used during the charging session, cycle the power to control heat imparted to tissue (e.g., cycle it on and off), or terminate the charging session in response to a temperature exceeding a threshold. In other examples, the IMD or other device may employ the temperature sensed by a temperature sensor to perform other or additional functions. For example, a processor may compare the sensed temperature to a fault condition threshold and disconnect the rechargeable power source from at least one electrical circuit when the sensed temperature exceeds the fault condition threshold. By including temperature sensors thermally coupled to the target structure of the medical device, such as the inner surface of the housing, the IMD or other device may receive accurate temperature information to precisely control charging power during a recharge session. 
       FIG. 1  is a conceptual diagram illustrating an example system  10  that includes implantable medical device (IMD)  14  and external charging device  22  that charges rechargeable power source  18  of IMD  14 . Although the techniques described in this disclosure are generally applicable to a variety of medical devices including medical devices such as patient monitors, electrical stimulators, or drug delivery devices, application of such techniques to implantable neurostimulators will be described for purposes of illustration. More particularly, the disclosure will refer to an implantable neurostimulation system for use in spinal cord stimulation therapy, but without limitation as to other types of medical devices. In other examples, temperature sensors may be included in other non-medical devices such as personal electronics, mechanical devices, or any other type of structure. 
     As shown in  FIG. 1 , system  10  includes an IMD  14  and external charging device  22  shown in conjunction with a patient  12 , who is ordinarily a human patient. In the example of  FIG. 1 , IMD  14  is an implantable electrical stimulator that delivers neurostimulation therapy to patient  12 , e.g., for relief of chronic pain or other symptoms. Generally, IMD  14  may be a chronic electrical stimulator that remains implanted within patient  12  for weeks, months, or even years. In the example of  FIG. 1 , IMD  14  and lead  16  may be directed to delivering spinal cord stimulation therapy. In other examples, IMD  14  may be a temporary, or trial, stimulator used to screen or evaluate the efficacy of electrical stimulation for chronic therapy. IMD  14  may be implanted in a subcutaneous tissue pocket, within one or more layers of muscle, or other internal location. IMD  14  includes rechargeable power source  18 , such as a rechargeable battery, and IMD  14  is coupled to lead  16 . 
     Electrical stimulation energy, which may be constant current or constant voltage based pulses, for example, is delivered from IMD  14  to one or more targeted locations within patient  12  via one or more electrodes (not shown) of lead  16 . The parameters for a program that controls delivery of stimulation energy by IMD  14  may include information identifying which electrodes have been selected for delivery of stimulation according to a stimulation program, the polarities of the selected electrodes, i.e., the electrode configuration for the program, and voltage or current amplitude, pulse rate, pulse shape, and pulse width of stimulation delivered by the electrodes. Electrical stimulation may be delivered in the form of stimulation pulses or continuous waveforms, for example. 
     In the example of  FIG. 1 , lead  16  is disposed within patient  12 , e.g., implanted within patient  12 . Lead  16  tunnels through tissue of patient  12  from along spinal cord  20  to a subcutaneous tissue pocket or other internal location where IMD  14  is disposed. Although lead  16  may be a single lead, lead  16  may include a lead extension or other segments that may aid in implantation or positioning of lead  16 . In addition, a proximal end of lead  16  may include a connector (not shown) that electrically couples to a header of IMD  14 . Although only one lead  16  is shown in  FIG. 1 , system  10  may include two or more leads, each coupled to IMD  14  and directed to similar or different target tissue sites. For example, multiple leads may be disposed along spinal cord  20  or leads may be directed to spinal cord  20  and/or other locations within patient  12 . 
     Lead  16  may carry one or more electrodes that are placed adjacent to the target tissue, e.g., spinal cord  20  for spinal cord stimulation (SCS) therapy. One or more electrodes may be disposed at a distal tip of lead  16  and/or at other positions at intermediate points along lead  16 , for example. Electrodes of lead  16  transfer electrical stimulation generated by an electrical stimulation generator in IMD  14  to tissue of patient  12 . The electrodes may be electrode pads on a paddle lead, circular (e.g., ring) electrodes surrounding the body of the lead, conformable electrodes, cuff electrodes, segmented electrodes, or any other type of electrodes capable of forming unipolar, bipolar or multipolar electrode configurations for therapy. In general, ring electrodes arranged at different axial positions at the distal ends of lead  16  will be described for purposes of illustration. 
     In alternative examples, lead  16  may be configured to deliver stimulation energy generated by IMD  14  to stimulate one or more sacral nerves of patient  12 , e.g., sacral nerve stimulation (SNS). SNS may be used to treat patients suffering from any number of pelvic floor disorders such as pain, urinary incontinence, fecal incontinence, sexual dysfunction, or other disorders treatable by targeting one or more sacral nerves. Lead  16  and IMD  14  may also be configured to provide other types of electrical stimulation or drug therapy (e.g., with lead  16  configured as a catheter). For example, lead  16  may be configured to provide deep brain stimulation (DBS), peripheral nerve stimulation (PNS), gastric stimulation to treat obesity or gastroparesis, tibial nerve stimulation, or other deep tissue or more superficial types of electrical stimulation. In other examples, lead  16  may provide one or more sensors configured to allow IMD  14  to monitor one or more parameters of patient  12 . The one or more sensors may be provided in addition to, or in place of, therapy delivery by lead  16 . 
     IMD  14  delivers electrical stimulation therapy to patient  12  via selected combinations of electrodes carried by lead  16 . The target tissue for the electrical stimulation therapy may be any tissue affected by electrical stimulation energy, which may be in the form of electrical stimulation pulses or waveforms. In some examples, the target tissue includes nerves, smooth muscle, and skeletal muscle. In the example illustrated by  FIG. 1 , the target tissue for electrical stimulation delivered via lead  16  is tissue proximate spinal cord  20  (e.g., one or more target locations of the dorsal columns or one or more dorsal roots that branch from spinal cord  20 . Lead  16  may be introduced into spinal cord  20  via any suitable region, such as the thoracic, cervical or lumbar regions. Stimulation of dorsal columns, dorsal roots, and/or peripheral nerves may, for example, prevent pain signals from traveling through spinal cord  20  and to the brain of the patient. Patient  12  may perceive the interruption of pain signals as a reduction in pain and, therefore, efficacious therapy results. For treatment of other disorders, lead  16  may be introduced at any exterior location of patient  12 . 
     Although lead  16  is described as generally delivering or transmitting electrical stimulation signals, lead  16  may additionally or alternatively transmit electrical signals from patient  12  to IMD  14  for monitoring. For example, IMD  14  may utilize detected nerve impulses to diagnose the condition of patient  12  or adjust the delivered stimulation therapy. Lead  16  may thus transmit electrical signals to and from patient  12 . 
     A user, such as a clinician or patient  12 , may interact with a user interface of an external programmer (not shown) to program IMD  14 . Programming of IMD  14  may refer generally to the generation and transfer of commands, programs, or other information to control the operation of IMD  14 . For example, the external programmer may transmit programs, parameter adjustments, program selections, group selections, or other information to control the operation of IMD  14 , e.g., by wireless telemetry or wired connection. 
     In some cases, an external programmer may be characterized as a physician or clinician programmer if it is primarily intended for use by a physician or clinician. In other cases, the external programmer may be characterized as a patient programmer if it is primarily intended for use by a patient. A patient programmer is generally accessible to patient  12  and, in many cases, may be a portable device that may accompany the patient throughout the patient&#39;s daily routine. In general, a physician or clinician programmer may support selection and generation of programs by a clinician for use by stimulator  14 , whereas a patient programmer may support adjustment and selection of such programs by a patient during ordinary use. In other examples, external charging device  22  may be included with, or form part of, an external programmer. In this manner, a user such as a clinician, other caregiver, or patient may program and charge IMD  14  using one device, or multiple devices. 
     IMD  14  may be constructed of any polymer, metal, or composite material sufficient to house the components of IMD  14  (e.g., components illustrated in  FIG. 2 ) within patient  12 . In this example, IMD  14  may be constructed with a biocompatible housing, such as titanium or stainless steel, or a polymeric material such as silicone or polyurethane, and surgically implanted at a site in patient  12  near the pelvis, abdomen, or buttocks. The housing of IMD  14  may be configured to provide a hermetic seal for components, such as rechargeable power source  18 . In addition, the housing of IMD  14  may be selected of a material that facilitates receiving energy to charge rechargeable power source  18 . 
     As described herein, rechargeable power source  18  may be included within IMD  14 . However, in other examples, rechargeable power source  18  could be located external to a housing of IMD  14 , separately protected from fluids of patient  12 , and electrically coupled to electrical components of IMD  14 . This type of configuration of IMD  14  and rechargeable power source  18  may provide implant location flexibility when anatomical space for implantable devices is minimal. In any case, rechargeable power source  18  may provide operational electrical power to one or more components of IMD  14 . 
     Rechargeable power source  18  may include one or more capacitors, batteries, or components (e.g. chemical or electrical energy storage devices). Example batteries may include lithium-based batteries, nickel metal-hydride batteries, or other materials. Rechargeable power source  18  is also rechargeable. In other words, rechargeable power source  18  may be replenished, refilled, or otherwise capable of increasing the amount of energy stored after energy has been depleted. Rechargeable power source  18  may be subjected to numerous discharge and recharge cycles (e.g., hundreds or even thousands of cycles) over the life of rechargeable power source  18  in IMD  14 . Rechargeable power source  18  may be recharged when fully depleted or partially depleted. 
     Charging device  22  may be used to recharge rechargeable power source  18  and IMD  14  when implanted in patient  12 . Charging device  22  may be a hand-held device, a portable device, or a stationary charging system. In any case, charging device  22  may include components necessary to charge rechargeable power source  18  through tissue of patient  12 . For example, charging device  22  may include housing  24 , charging cable  28 , and charging head  26 . Housing  24  may enclose or house at least some of the operational components of charging device  22 . For example, housing  24  may include a user interface, processor, memory, power source, and other components. Charging cable  28  may electrically couple charging head  26  to the power source within housing  24 , such that charging cable  28  is configured to transmit power and/or information to charging head  26 . Charging head  26  may include a coil (e.g., a component of charging head  26 ) for inductive coupling or components used to transmit power from charging head  26  to rechargeable power source  18 . In other examples, charging cable  28  and/or charging head  26  may also be contained within or disposed on housing  24 , or various ones of the components associated with charging device  22  may be carried by cable  28  and/or charging head  26 . Although a user may control the recharging process with a user interface of charging device  22 , charging device may alternatively be controlled by another device (e.g., an external programmer). 
     In some examples, charging device  22  may only perform charging of rechargeable power source  18 . In other examples, charging device  22  may be an external programmer or other device configured to perform additional functions. For example, when embodied as an external programmer, charging device  22  may transmit programming commands to IMD  14  in addition to charge rechargeable power source  18 . In another example, charging device  22  may communicate with IMD  14  to transmit and/or receive information related to the charging of rechargeable power source  18 . For example, IMD  14  may transmit information regarding temperature of IMD  14  and/or rechargeable power source  18 , received power during charging, the charge level of rechargeable power source  18 , charge depletion rates during use, or any other information related to power consumption and recharging of IMD  14  and rechargeable power source  18 . 
     Charging device  22  and IMD  14  may utilize any wireless power transfer techniques that are capable of recharging rechargeable power source  18  of IMD  14  when IMD  14  is implanted within patient  14 . In one example, system  10  may utilize inductive coupling between a coil of charging device  22  (e.g., a coil within charging head  26 ) and a coil of IMD  14  coupled to rechargeable power source  18 . In inductive coupling, charging device  22  is placed near implanted IMD  14  such that a primary coil of charging device  22  is aligned with, i.e., placed over, a secondary coil of IMD  14 . Charging device  22  may then generate an electrical current in the primary coil based on a selected power level for charging rechargeable power source  18 . As described further below, the power level may be selected to control the temperature of IMD  14  and/or the charge rate of rechargeable power source  18 . When the primary and secondary coils are aligned, the electrical current in the primary coil may magnetically induce an electrical current in the secondary coil within IMD  14 . Since the secondary coil is associated with and electrically coupled to rechargeable power source  18 , the induced electrical current may be used to increase the voltage, or charge level, of rechargeable power source  18 . Although inductive coupling is generally described herein, any type of wireless energy transfer may be used to charge rechargeable power source  18 . 
     During the energy transfer process that charges rechargeable power source  18 , some of the energy involved in the charging process may be converted into heat at rechargeable power source  18 , other components of IMD  14 , and/or in charging head  26 , for example. When increased energy levels are used to charge rechargeable power source  18  at a higher rate, the temperature of IMD  14  and/or charging device  22  may also increase. Although the temperature of the IMD  14  housing may not achieve a temperature sufficient to burn or necrose tissue adjacent to the housing of IMD  14 , elevated temperatures may be undesirable and could cause discomfort in some cases. Therefore, one or more devices may monitor temperatures of any device or component that may come into contact with or otherwise affect tissue of patient  12 . The sensed temperature may be used as feedback in a closed-loop or partially closed-loop temperature control system. For example, charging device  22  may control the power level, power cycle times, and/or charging time used to charge rechargeable power source  18  to reduce or minimize any undesirable temperatures of IMD  14  that could be caused by charging rechargeable power source  18 . In addition, monitoring the temperature of IMD  14  and/or the temperature of tissue adjacent to the housing of IMD  14  may minimize patient discomfort during the charging process and reduce the amount of time to recharge the IMD. 
     As described herein, system  10  may utilize one or more temperature sensors to sense, measure, or otherwise detect the temperature of a portion of a device. In one example, a temperature sensor of system  10  may sense the temperature of a portion of a medical device (e.g., charging head  26  or IMD  14 ). A processor of system  10  (e.g., a processor housed by either charging device  22  or IMD  14 ) may be configured to control charging of rechargeable power source  18  based on the sensed temperature. In this manner, the temperature sensor may provide feedback for controlling the charging of rechargeable power source  18 . For example, charging device  22  may control current applied to a primary coil within charging head  26  based on the sensed temperature. Charging device  22  may control current, for example, by controlling a current amplitude, duty cycle, or other characteristic of the charging current. In some examples, the temperature sensor may be disposed within a housing of the medical device (e.g., a housing of charging head  26 , housing  24 , or a housing of IMD  14 ). In this manner, the temperature sensor may be disposed in a medical device that is either external to patient  12  or implanted within patient  12 . In one or more examples, processing circuitry may be configured to receive a temperature signal from the temperature sensor and may control at least one function of the implantable medical device based on the temperature signal. 
     Temperature sensors described herein may take different forms and utilize different temperature sensing techniques. In one example, the temperature sensor is a top-sensing temperature sensor that is thermally coupled to the inside surface of the IMD housing. 
     System  10  may utilize one or more temperature sensors in one or more medical device. For example, each of charging head  26  and IMD  14  may include a single temperature sensor. In another example, each of charging head  26  (e.g., external of patient  12 ) and/or IMD  14  (e.g., implanted within patient  12 ) may include two or more temperature sensors. Multiple temperature sensors within the same device may be provided for different reasons. For example, each of the multiple temperature sensors may be oriented to sense the temperature of the same portion of the device for redundant, backup, composite, or cross-correlated temperature measurement. If multiple temperature sensors are used, the multiple sensors may be similar or may instead be sensors of different types of temperatures sensors described herein. 
     Alternatively, two temperature sensors may be oriented to sense temperature of different surfaces and/or components within the same device. A first temperature sensor may be configured to sense a first portion of the device and a second temperature sensor may be configured to sense a second portion of the device. The two portions may be of different components or different areas of the same component. In one example, the first portion may be a one housing surface within the device, and the second portion may be another housing surface within the device. Since temperatures within a device may be non-uniform due to component location, thermal transfer within the device, or other external factors, the multiple temperature sensors may be used to identify temperature variations or “hot spots” of the device. In some cases, a one or multi-dimension array of temperature sensors may be provided to sense one or more portions of the IMD  14  or external device (e.g., recharger). 
     In some examples, two surfaces being sensed for temperature may be located adjacent to one another (e.g., different locations of a generally planar surface). In this example, two temperature sensors may be mounted to the same interior surface of the IMD housing. In other examples, the two surfaces may be generally opposed to one another (e.g., two different interior surfaces of the IMD housing and separated by the hybrid board). In this example, each temperature sensor may be mounted on opposing interior surfaces of the IMD housing such that one sensor senses temperature on one side of IMD housing and the other sensor senses temperature on the opposite side IMD housing. 
     Each temperature sensor may sense temperatures simultaneously such that system  10  may process multiple temperatures at the same time. Alternatively, one or more temperature sensors may be selectively enabled by one or more processors. This selective temperature sensing may reduce power consumption from unnecessary temperature sensors. In addition, selective temperature sensing may reduce power consumption and/or processing speed needed to process signals from unneeded temperature sensors. 
     System  10  may control the charging of rechargeable power source  18  using one or more techniques. Using the sensed temperature, a processor may compare the sensed temperature to a threshold temperature. The sensed temperature may be from a temperature sensor located within IMD  14  and/or charging device  22 . The threshold temperature may be a value stored by a memory. The threshold temperature may be selected based on tissue models, patient history, or any other information that may be used to determine when a charging session should be modified. The processor may then determine when the sensed temperature exceeds the threshold temperature. When the sensed temperature exceeds the threshold temperature, the processor may control charging of rechargeable power source  18  by adjusting a power level used to charge rechargeable power source  18 . In other words, the processor may reduce the power level when the temperature threshold is exceeded, turn the power off for a predetermined period of time before the power is again provided (e.g., cycle the power on and off) or even terminate the charging session. Reducing the power level may reduce the energy used to charge rechargeable power source  18  and/or the rate at which rechargeable power source  18  is recharged. 
     When sensing a temperature of a component of IMD  14 , a processor of IMD may merely transmit the calculated temperature or data representative of the temperature to charging device  22 . A processor of charging device  22  may then determine how to control the charging session. Alternatively, the processor of IMD  14  may determine how to control the charging session and transmit a respective command to charging device  22 . 
     Charging device  22  may thus charge rechargeable power source  18  using one or more power levels or cycle times in some examples. In one example, charging device  22  may select a high power level when first starting a charging session. Charging device  22  may then select a low power level, relative to the high power level, in response to one or more temperature sensors exceeding a threshold. In this manner, the high power level may charge rechargeable power source  18  at a high rate to reduce charging time while increasing the temperature of IMD  14 . Charging device  22  may select the low power level to charge rechargeable power source  18  at a slower rate to reduce the temperature of IMD  14 . The low power level may be sufficiently minimal so that any increase in temperature of IMD  14  may have minimal or no effect on surrounding tissue. 
     A high power level and a low power level may be subjective and relative to the charging power that charging device  22  is capable of generating and transmitting to IMD  14 . In some cases, the high power level may be the maximum power that charging device  22  can generate. This high power level may be referred to as a “boost” or “accelerated” charging level because of the high rate of charge induced in rechargeable power source  18 . This high rate of charge may minimize the amount of time patient  12  needs to recharge rechargeable power source  18 . By monitoring the temperature of one or more portions of charging head  26  and/or IMD  14 , charging device  22  may charge rechargeable power source  18  with the high power level for a longer period of time without damaging tissue surrounding IMD  14 . 
     In one example, the high power level may be approximately 2.5 Watts and the low power level may be approximately 1.0 Watt (W). Of course other power levels and ranges may be selected for use, with such levels falling either within the above-described range or outside of this range. For instance, a low power level may be much lower than 1.0 Watt in an example wherein there is good coupling between primary and second coils and wherein recharge is to be conducted relatively slowly. An example charge current level may be approximately 100 milliamps (mA) for the high power level and approximately 60 mA for the low power level. An example primary coil voltage and current for a high power may be approximately 450 V and approximately 800 mA, respectively, and an example primary coil voltage and current for a low power level may be approximately 250 V and approximately 500 mA. These values are merely examples, and other examples may include higher or lower values in accordance with the techniques described herein. In additional more than two levels may be defined (e.g., low, one or more intermediate levels, and a high level) to control charging. 
     In some cases, charging device  22  may cycle the driving of the primary coil. For instance, charging device  22  may drive the coil during a first period of time, and may discontinue driving the coil for a second period of time following the first period of time. This may be repeated multiple times, with the first and second time periods being selected to control an overall transmission of power (and hence heat dissipation.). 
     In some examples, IMD  14  may directly adjust the power level for charging (e.g., limit the charge current) instead of relying on a change in power level at charging device  22 . For example, as IMD  14  receives an alternating charging current, IMD  14  may employ a circuit that may change from full-wave rectification to half-wave rectification to reduce the charge rate and temperature of IMD  14  during charging. In other words, IMD  14  may utilize half-wave rectification as a means to reduce the electrical current delivered to rechargeable power supply  18  instead of reducing the overall power received by IMD  14 . Alternatively, IMD  14  may employ other mechanisms such as current and/or voltage limiters that may limit the charging rate of rechargeable power supply  18 . 
     In other examples, a processor of charging device  22  and/or IMD  14  may perform actions other than changing a power level for charging in response to temperature changes. For example, charging device  22  may instruct a user to replace a phase change material cartridge attached to charging head  26  of charging device  22 . The phase change material cartridge may act as a heat sink and increase the amount of time charging device  22  can charge rechargeable power source  18  at a high power level. In one example, a processor of charging device  22  may calculate a temperature change rate from the multiple sensed temperatures when rechargeable power source  18  is charging. The temperature change rate may be representative of how fast the temperature of charging head  26  is changing. As described above, charging head  26  may include a primary coil that transfers power wirelessly to a secondary coil within IMD  14 . The processor may then determine when the temperature change rate increases subsequent to the temperature change rate decreasing during the charging. In response to determining that the temperature change rate has increased, the processor may control a user interface to present a notification that instructs a user to replace a phase change material cartridge thermally coupled to the device. 
     As described herein, a temperature sensor may be used to sense a temperature of a portion of IMD  14  (including rechargeable power source  18 ), charging head  26 , and/or housing  24 . A processor that controls an aspect of the charging session may be housed by IMD  14 , charging head  26 , or housing  24 . In this manner, a processor configured to perform some or all of the functions described herein may be housed together with a temperature sensor or separate from the temperature sensor. 
     Although an implantable rechargeable power source  18  is generally described herein, techniques of this disclosure may also be applicable to a rechargeable power source  18  that is not implanted. For example, rechargeable power source  18  may be external to the skin of patient  12  and in physical contact with the skin. Therefore, charging device  22  may control the charging of rechargeable power source  18  with temperature sensed within charging head  26  or IMD  14  even when the power source is external to patient  12 . 
       FIG. 2  is a block diagram illustrating example components of IMD  14 . In the example of  FIG. 2 , IMD  14  includes temperature sensor  42 , coil  40 , processing circuitry  30 , therapy circuitry  34 , recharge circuitry  38 , memory  32 , telemetry circuitry  36 , and rechargeable power source  18 . In other examples, IMD  14  may include a greater or a fewer number of components. For example, in some examples, IMD  14  may not include temperature sensor  42 . 
     In general, IMD  14  may comprise any suitable arrangement of hardware, alone or in combination with software and/or firmware, to perform the various techniques described herein attributed to IMD  14  and processing circuitry  30 . In various examples, IMD  14  may include one or more processors  30 , such as one or more microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components. IMD  14  also, in various examples, may include a memory  32 , such as random access memory (RAM), read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electronically erasable programmable read only memory (EEPROM), flash memory, comprising executable instructions for causing the one or more processors to perform the actions attributed to them. Moreover, although processing circuitry  30 , therapy circuitry  34 , recharge circuitry  38 , and telemetry circuitry  36  are described as separate circuitries, in some examples, processing circuitry  30 , therapy circuitry  34 , recharge circuitry  38 , and telemetry circuitry  36  are functionally integrated. In some examples, processing circuitry  30 , therapy circuitry  34 , recharge circuitry  38 , and telemetry circuitry  36  correspond to individual hardware units, such as ASICs, DSPs, FPGAs, or other hardware units. 
     Memory  32  may store therapy programs or other instructions that specify therapy parameter values for the therapy provided by therapy circuitry  34  and IMD  14 . In some examples, memory  32  may also store temperature data from temperature sensor  42 , instructions for recharging rechargeable power source  18 , thresholds, instructions for communication between IMD  14  and charging device  22 , or any other instructions required to perform tasks attributed to IMD  14 . Memory  32  may be configured to store instructions for communication with and/or controlling one or more temperature sensors  42 . As described herein, the temperature sensor  42  may be an IR sensor, a phosphor temperature sensor, or any other non-contact sensor or sensor (whether or not contact) that senses temperature by means other than thermal coupling. 
     Generally, therapy circuitry  34  may generate and deliver electrical stimulation under the control of processing circuitry  30 . In some examples, processing circuitry  30  controls therapy circuitry  34  by accessing memory  32  to selectively access and load at least one of the stimulation programs to therapy circuitry  34 . For example, in operation, processing circuitry  30  may access memory  32  to load one of the stimulation programs to therapy circuitry  34 . In such examples, relevant stimulation parameters may include a voltage amplitude, a current amplitude, a pulse rate, a pulse width, a duty cycle, or the combination of electrodes  17 A,  17 B,  17 C, and  17 D that therapy circuitry  34  uses to deliver the electrical stimulation signal. Therapy circuitry  34  may be configured to generate and deliver electrical stimulation therapy via one or more of electrodes  17 A,  17 B,  17 C, and  17 D of lead  16 . Alternatively, or additionally, therapy circuitry  34  may be configured to provide different therapy to patient  12 . For example, therapy circuitry  34  may be configured to deliver drug delivery therapy via a catheter. These and other therapies may be provided by IMD  14 . 
     IMD also includes components to receive power from charging device  22  to recharge rechargeable power source  18  when rechargeable power source  18  has been at least partially depleted. As shown in  FIG. 2 , IMD  14  includes secondary coil  40  and recharge circuitry  38  coupled to rechargeable power source  18 . Recharge circuitry  38  may be configured to charge rechargeable power source  18  with the selected power level determined by either processing circuitry  30  or charging device  22 . Recharge circuitry  38  may include any of a variety of charging and/or control circuitry configured to process or convert current induced in coil  40  into charging current to charge power source  18 . Although processing circuitry  30  may provide some commands to recharge circuitry  38 , in some examples, processing circuitry  30  may not need to control any aspect of recharging. 
     Secondary coil  40  may include a coil of wire or other device capable of inductive coupling with a primary coil disposed external to patient  12 . Although secondary coil  40  is illustrated as a simple loop of in  FIG. 2 , secondary coil  40  may include multiple turns of conductive wire. Secondary coil  40  may include a winding of wire configured such that an electrical current can be induced within secondary coil  40  from a magnetic field. The induced electrical current may then be used to recharge rechargeable power source  18 . In this manner, the electrical current may be induced in secondary coil  40  associated with rechargeable power source  18 . The induction may be caused by electrical current generated in the primary coil of charging device  22 , where the level of the current may be based on the selected power level. The coupling between secondary coil  40  and the primary coil of charging device  22  may be dependent upon the alignment of the two coils. Generally, the coupling efficiency increases when the two coils share a common axis and are in close proximity to each other. Charging device  22  and/or IMD  14  may provide one or more audible tones or visual indications of the alignment. 
     Although inductive coupling is generally described as the method for recharging rechargeable power source  18 , other wireless energy transfer techniques may alternatively be used. Any of these techniques may generate heat in IMD  14  such that the charging process can be controlled by matching the sensed temperature to one or more thresholds, modeling tissue temperatures based on the sensed temperature, or using a calculated cumulative thermal dose as feedback. 
     Recharge circuitry  38  may include one or more circuits that process, filter, convert and/or transform the electrical signal induced in secondary coil to an electrical signal capable of recharging rechargeable power source  18 . For example, in alternating current induction, recharge circuitry  38  may include a half-wave rectifier circuit and/or a full-wave rectifier circuit configured to convert alternating current from the induction to a direct current for rechargeable power source  18 . The full-wave rectifier circuit may be more efficient at converting the induced energy for rechargeable power source  18 . However, a half-wave rectifier circuit may be used to store energy in rechargeable power source  18  at a slower rate. In some examples, recharge circuitry  38  may include both a full-wave rectifier circuit and a half-wave rectifier circuit such that recharge circuitry  38  may switch between each circuit to control the charging rate of rechargeable power source  18  and temperature of IMD  14 . 
     Rechargeable power source  18  may include one or more capacitors, batteries, and/or other energy storage devices. Rechargeable power source  18  may deliver operating power to the components of IMD  14 . In some examples, rechargeable power source  18  may include a power generation circuit to produce the operating power. Rechargeable power source  18  may be configured to operate through hundreds or thousands of discharge and recharge cycles. Rechargeable power source  18  may also be configured to provide operational power to IMD  14  during the recharge process. In some examples, rechargeable power source  18  may be constructed with materials to reduce the amount of heat generated during charging. In other examples, IMD  14  may be constructed of materials that may help dissipate generated heat at rechargeable power source  18 , recharge circuitry  38 , and/or secondary coil  40  over a larger surface area of the housing of IMD  14 . 
     Although rechargeable power source  18 , recharge circuitry  38 , and secondary coil  40  are shown as contained within the housing of IMD  14 , in alternative implementations, at least one of these components may be disposed outside of the housing. For example, in some implementations, secondary coil  40  may be disposed outside of the housing of IMD  14  to facilitate better coupling between secondary coil  40  and the primary coil of charging device  22 . These different configurations of IMD  14  components may allow IMD  14  to be implanted in different anatomical spaces or facilitate better inductive coupling alignment between the primary and secondary coils. 
     IMD  14  may also include temperature sensor  42 . Temperature sensor  42  may include one or more temperature sensors configured to measure the temperature of respective portions of IMD  14 . As described herein, a temperature sensor is thermally coupled to, and/or may be directly attached to, the portion of the device from which temperature is to be measured. In one instance, the temperature sensor is directly coupled with a portion of the interior of the IMD housing, such as an interior surface of the housing. 
     Temperature sensor  42  may be oriented to measure the temperature of a component, surface or structure (e.g., the housing) of IMD  14 . Temperature sensor  42  may be disposed internal of the housing of IMD  14 . Processing circuitry  30 , or charging device  22 , may use this temperature measurement as the tissue temperature feedback to control the power levels or charge times (e.g., cycle times) used during the charging session. Although a single temperature sensor may be adequate, multiple temperature sensors may provide more specific temperature readings of separate components or different areas of the housing. Although processing circuitry  30  may continually measure temperature using temperature sensor  42 , processing circuitry  30  may conserve energy by only measuring temperature during recharge sessions. Further, temperature may be sampled at a rate necessary to effectively control the charging session, but the sampling rate may be reduced to conserve power as appropriate. 
     Processing circuitry  30  may also control the exchange of information with charging device  22  and/or an external programmer using telemetry circuitry  36 . Telemetry circuitry  36  may be configured for wireless communication using radio frequency protocols or inductive communication protocols. Telemetry circuitry  36  may include one or more antennas configured to communicate with charging device  22 , for example. Processing circuitry  30  may transmit operational information and receive therapy programs or therapy parameter adjustments via telemetry circuitry  36 . Also, in some examples, IMD  14  may communicate with other implanted devices, such as stimulators, control devices, or sensors, via telemetry circuitry  36 . In addition, telemetry circuitry  36  may be configured to transmit the measured tissue temperatures from temperature sensor  42 , for example. 
     In other examples, processing circuitry  30  may transmit additional information to charging device  22  related to the operation of rechargeable power source  18 . For example, processing circuitry  30  may use telemetry circuitry  36  to transmit indications that rechargeable power source  18  is completely charged, rechargeable power source  18  is fully discharged, or any other charge status of rechargeable power source  18 . Processing circuitry  30  may also transmit information to charging device  22  that indicates any problems or errors with rechargeable power source  18  that may prevent rechargeable power source  18  from providing operational power to the components of IMD  14 . 
       FIG. 3  is a block diagram of the example external charging device  22 . While charging device  22  may generally be described as a hand-held device, charging device  22  may be a larger portable device or a more stationary device. In addition, in other examples, charging device  22  may be included as part of an external programmer or include functionality of an external programmer. In addition, charging device  22  may be configured to communicate with an external programmer. As shown in  FIG. 3 , charging device  22  includes two separate components. Housing  24  encloses components such as a processor  50 , memory  52 , user interface  54 , telemetry circuitry  56 , and power source  60 . Charging head  26  may include power circuitry  58 , temperature sensor  62 , and coil  48 . A different partitioning of components is also possible, such as including one or more of the foregoing components within a circuitry carried by the cord of charging device  22 . 
     A separate charging head  26  may facilitate optimal positioning of coil  48  over coil  40  of IMD  14 . However, charging circuitry  58  and/or coil  48  may be integrated within housing  24  in other examples. Memory  52  may store instructions that, when executed by processor  50 , cause processor  50  and external charging device  22  to provide the functionality ascribed to external charging device  22  throughout this disclosure. 
     External charging device  22  may also include one or more temperature sensors  62 , similar to temperature sensor  42  of  FIG. 2 . Temperature sensor  62  may be disposed within charging head  26  and/or housing  24 . For example, charging head  26  may include one or more temperature sensors positioned and configured to sense the temperature of coil  48  and/or a surface of the housing of charging head  26 . In some examples, charging device  22  may not include temperature sensor  62 . 
     In general, charging device  22  comprises any suitable arrangement of hardware, alone or in combination with software and/or firmware, to perform the techniques attributed to charging device  22 , and processor  50 , user interface  54 , telemetry circuitry  56 , and charging circuitry  58  of charging device  22 . In various examples, charging device  22  may include one or more processors, such as one or more microprocessors, DSPs, ASICs, FPGAs, or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components. Charging device  22  also, in various examples, may include a memory  52 , such as RAM, ROM, PROM, EPROM, EEPROM, flash memory, a hard disk, a CD-ROM, comprising executable instructions for causing the one or more processors to perform the actions attributed to them. Moreover, although processor  50  and telemetry circuitry  56  are described as separate circuitries, in some examples, processor  50  and telemetry circuitry  56  are functionally integrated. In some examples, processor  50  and telemetry circuitry  56  and charging circuitry  58  correspond to individual hardware units, such as ASICs, DSPs, FPGAs, or other hardware units. 
     Memory  52  may store instructions that, when executed by processor  50 , cause processor  50  and charging device  22  to provide the functionality ascribed to charging device  22  throughout this disclosure. For example, memory  52  may include instructions that cause processor  50  to control the power level used to charge IMD  14  in response to the sensed temperatures, communicate with IMD  14 , or instructions for any other functionality. In addition, memory  52  may include a record of selected power levels, sensed temperatures, or any other data related to charging rechargeable power source  18 . Processor  50  may, when requested, transmit any of this stored data in memory  52  to another computing device for review or further processing. 
     User interface  54  may include a button or keypad, lights, a speaker for voice commands, a display, such as a liquid crystal (LCD), light-emitting diode (LED), or cathode ray tube (CRT). In some examples the display may be a touch screen. As discussed in this disclosure, processor  50  may present and receive information relating to the charging of rechargeable power source  18  via user interface  54 . For example, user interface  54  may indicate when charging is occurring, quality of the alignment between coils  40  and  48 , the selected power level, current charge level of rechargeable power source  18 , duration of the current recharge session, anticipated remaining time of the charging session, sensed temperatures, instructions for changing a phase change material cartridge of charging head  26 , or any other information. Processor  50  may receive some of the information displayed on user interface  54  from IMD  14  in some examples. 
     User interface  54  may also receive user input via user interface  54 . The input may be, for example, in the form of pressing a button on a keypad or selecting an icon from a touch screen. The input may request starting or stopping a recharge session, a desired level of charging, or one or more statistics related to charging rechargeable power source  18  (e.g., the cumulative thermal dose). In this manner, user interface  54  may allow the user to view information related to the charging of rechargeable power source  18  and/or receive charging commands. 
     Charging device  22  also includes components to transmit power to recharge rechargeable power source  18  associated with IMD  14 . As shown in  FIG. 3 , charging device  22  includes primary coil  48  and charging circuitry  58  coupled to power source  60 . Charging circuitry  58  may be configured to generate an electrical current in primary coil  48  from voltage stored in power source  60 . Although primary coil  48  is illustrated as a simple loop in  FIG. 3 , primary coil  48  may include multiple turns of wire. Charging circuitry  58  may generate the electrical current according to a power level selected by processor  50  based on the sensed temperature or temperatures received from IMD  14  or a temperature sensor within charging device  22 . As described herein, processor  50  may select a high power level, low power level, or a variety of different power levels to control the rate of recharge in rechargeable power source  18  and the temperature of IMD  14 . In some examples, processor  50  may control charging circuitry  58  based on a power level selected by processing circuitry  30  of IMD  14 . The sensed temperature used as feedback for control of the recharge power level may be from a temperature sensed by a temperature sensor within IMD  14  and/or charging device  22 . Although processor  50  may control the power level used for charging rechargeable power source  18 , charging circuitry  58  may include one or more processors configured to partially or fully control the power level based on the sensed temperatures. 
     Primary coil  48  may include a coil of wire, e.g., having multiple turns, or other device capable of inductive coupling with a secondary coil  40  disposed within patient  12 . Primary coil  48  may include a winding of wire configured such that an electrical current generated within primary coil  48  can produce a magnetic field configured to induce an electrical current within secondary coil  40 . The induced electrical current may then be used to recharge rechargeable power source  18 . In this manner, the electrical current may be induced in secondary coil  40  associated with rechargeable power source  18 . The coupling efficiency between secondary coil  40  and primary coil  48  of charging device  22  may be dependent upon the alignment of the two coils. Generally, the coupling efficiency increases when the two coils share a common axis and are in close proximity to each other. User interface  54  of charging device  22  may provide one or more audible tones or visual indications of the alignment. 
     Charging circuitry  58  may include one or more circuits that generate an electrical signal, and an electrical current, within primary coil  48 . Charging circuitry  58  may generate an alternating current of specified amplitude and frequency in some examples. In other examples, charging circuitry  58  may generate a direct current. In any case, charging circuitry  58  may be capable of generating electrical signals, and subsequent magnetic fields, to transmit various levels of power to IMD  14 . In this manner charging circuitry  58  may be configured to charge rechargeable power source  18  of IMD  14  with the selected power level. 
     The power level that charging circuitry  58  selects for charging may be used to vary one or more parameters of the electrical signal generated for coil  48 . For example, the selected power level may specify wattage, electrical current of primary coil  48  or secondary coil  40 , current amplitude, voltage amplitude, pulse rate, pulse width, a cycling rate that determines when the primary coil is driven, or any other parameter that may be used to modulate the power transmitted from coil  48 . In this manner, each power level may include a specific parameter set that specifies the signal for each power level. Changing from one power level to another power level (e.g., a high power level to a lower power level) may include adjusting one or more parameters. For instance, at a high power level, the primary coil may be substantially continuously driven, whereas at a lower power level, the primary coil may be intermittently driven such that periodically the coil is not driven for a predetermined time to control heat dissipation. The parameters of each power level may be selected based on hardware characteristics of charging device  22  and/or IMD  14 . 
     Power source  60  may deliver operating power to the components of charging device  22 . Power source  60  may also deliver the operating power to drive primary coil  48  during the charging process. Power source  60  may include a battery and a power generation circuit to produce the operating power. In some examples, a battery of power source  60  may be rechargeable to allow extended portable operation. In other examples, power source  60  may draw power from a wired voltage source such as a consumer or commercial power outlet. 
     Charging device  22  may include one or more temperature sensor  62  for sensing the temperature of a portion of the device. For example, temperature sensor  62  may be disposed within charging head  26  and oriented to sense the temperature of the housing of charging head  26 . In another example, temperature sensor  62  may be disposed within charging head  26  and oriented to sense the temperature of charging circuitry  58  and/or coil  48 . In other examples, charging device  22  may include multiple temperature sensors  62  each oriented to any of these portions of device to manage the temperature of the device during charging sessions. 
     Telemetry circuitry  56  supports wireless communication between IMD  14  and charging device  22  under the control of processor  50 . Telemetry circuitry  56  may also be configured to communicate with another computing device via wireless communication techniques, or direct communication through a wired connection. In some examples, telemetry circuitry  56  may be substantially similar to telemetry circuitry  36  of IMD  14  described herein, providing wireless communication via an RF or proximal inductive medium. In some examples, telemetry circuitry  56  may include an antenna, which may take on a variety of forms, such as an internal or external antenna. Although telemetry circuitries  56  and  36  may each include dedicated antennae, telemetry circuitries  56  and  36  may instead, or additionally, be configured to utilize inductive coupling from coils  40  and  48  to transfer data. 
     Examples of local wireless communication techniques that may be employed to facilitate communication between charging device  22  and IMD  14  include radio frequency and/or inductive communication according to any of a variety of standard or proprietary telemetry protocols, or according to other telemetry protocols such as the IEEE 802.11x or Bluetooth specification sets. In this manner, other external devices may be capable of communicating with charging device  22  without needing to establish a secure wireless connection. As described herein, telemetry circuitry  56  may be configured to receive a signal or data representative of a measured tissue temperature from IMD  14 . The tissue temperature may be indirectly measured by measuring the temperature of the internal surface of the IMD housing adjacent to rechargeable power source  18 . In some examples, multiple temperature readings by IMD  14  may be averaged or otherwise used to produce a single temperature value that is transmitted to charging device  22 . The sensed temperature may be sampled and/or transmitted by IMD  14  (and received by charging device  22 ) at different rates, e.g., on the order of microseconds, milliseconds, seconds, minutes, or even hours. Processor  50  may then use the received temperature to control charging of rechargeable power source  18  (e.g., control the charging level used to recharge power source  18 ). 
       FIGS. 4A-4E  are conceptual cross-sectional views of example temperature sensors  150  disposed within respective example IMDs  100 . Any of IMDs  100 A- 100 E are examples of IMD  14  of  FIGS. 1 and 2 . The IMDs described herein are generally shown with rectangular cross-sections. However, temperature sensors may be disposed within IMDs or any other devices of any shapes, dimensions, or sizes. 
     As shown in  FIG. 4A , IMD  100 A includes housing  110  that encloses hybrid board  140 , electronics, temperature sensor  150 , compliant material  160  and support  180 . Electronics may include various components such as a processor and memory and associated circuitry. A secondary coil and rechargeable power source may also be disposed within housing  110 . The hybrid board  140  may be electrically coupled with the temperature sensor  150  with a flexible circuit  142 , and may provide a flexible and electrical connection between the hybrid board  140  and the temperature sensor  150 . In one or more examples, a connector  144  may be disposed on and/or electrically coupled with the hybrid board  140  and may be coupled with the flexible circuit  142 , and may facilitate the electrical connection between the flexible circuit  142  and the hybrid board  140 . IMD  100  may include additional components in other examples, or in other areas of IMD  100  not shown in the particular cross-section of  FIG. 4A . 
     The housing  110  includes an exterior surface  112  and an interior surface  114 , and/or in some examples constructed of a titanium alloy. In some examples, the housing  110  is a titanium shield configured to protect components within the housing  110  such as a temperature sensor. In one or more examples, the housing  110  may include a cup shaped housing  126  and lid  128 , as shown in the IMD  100 D of  FIG. 4D . The components discussed above may be assembled and set within the cup shaped housing  126 . In some examples, a method of assembly may include welding the lid  128  to the cup shaped housing  126 , for example, at weld  118  after the components are set therein. In one or more examples, as shown in  FIG. 4E , IMD  100 E includes housing  110  which may include a sleeve  120  that connects with header  124 , where the sleeve  120  may be slid over the components therein and may be welded to the header  124  at weld  118 . 
       FIGS. 5-8  illustrate a support  180  in greater detail, where  FIGS. 5-6  show an IMD  500  with a translucent housing,  FIG. 7  shows a top view of an IMD, and  FIG. 8  shows a bottom view of the IMD with the support  180 . In one or more examples, at least one support  180  may be disposed within the housing  110 , and/or in one or more examples may be mechanically coupled with the housing and/or other components within the housing  110 , for example to the interior surface  114  of the housing  110 . In one or more examples, the at least one support  180  may extend from or is in contact with one or more locations of the interior surface  114  of the housing  110 . In one or more examples, the at least one support  180  may be in contact with two or more interior surfaces  114  of the housing  110 . In one or more examples, the at least one support  180  may be in contact with two or more interior surfaces  114  of the housing  110 , such as opposing side surfaces and/or top/bottom surfaces. In one or more examples, the at least one support  180  may have a clearance fit with the housing  110 . In one or more examples, the at least one support  180  may have a friction fit with the housing  110 . In some examples, at least one support  180  may include structure such as tabs, projections, and/or recesses to couple components therewith. For example, support  180  may include tabs in which the hybrid board  140  may be snapped. In some examples, support  180  may be formed together with housing  110  or attached using material coupling techniques such as welding or soldering. 
     In one or more examples, the at least one support  180 , or a portion thereof, may include a planar ledge configured to accommodate devices, including electronics or sensors, to be disposed thereon or therein. In one or more examples, the at least one support  180 , or a portion thereof, may include a curved cup-like structure configured to accommodate devices, including electronics or sensors, to be disposed thereon or therein. 
     In some examples, the at least one support may be a rigid support, for example as rigid as housing  110 . In one or more examples, at least one support  180  may be a rigid support, for example, lacking flexibility or unable to bed or be forced out of shape. In one or more examples, at least one support may be formed of machined plastic and/or metal. 
     In one or more examples, the housing  110  includes one or more curved portions  116 . For example, an upper edge of the housing may have curved edges as shown in  FIG. 9 . In one or more examples, an upper and lower edge of the housing  110  may have curved edges. In one or more examples, the support  180  may include a curved profile  182  as shown in  FIG. 8 , where the support curved profile  182  may be substantially similar to a curved side wall  108  ( FIG. 6 ) of the housing  110  such that the support  180  is adjacent to a curved side interior portion of the housing  110 , as shown in  FIG. 5 . 
     The IMD  100  includes one or more temperature sensor(s)  150 . In some examples, the temperature sensor  150  may be a high-precision digital temperature sensor and/or having low power consumption. In one or more examples, the temperature sensor  150  may include multiple sensors, as shown in  FIGS. 4B, 5-7, and 9 . For example, the temperature sensor  150  may be a first sensor and the IMD  100  includes one or more additional temperature sensors disposed at multiple locations within the housing  110  and thermally coupled to respective portions of the interior surface  114  of the housing  110 . In one or more examples, the temperature sensors  150 A,  150 B may be positioned to sense temperature on opposing sides of the housing  110 , as shown in  FIG. 4B . In some examples, at least four sensors may be disposed in the IMD  500 , as shown in  FIG. 9  In one or more examples, two sensors  150 A,  150 C may be disposed near a top portion of a housing, and/or two sensors  150 B,  150 D may be disposed near a bottom portion of the housing, for example as shown in  FIGS. 5, 6, and 9 . In some examples, sensing the temperature on opposing sides of IMD 500  may be beneficial if IMD  500  becomes flipped within the tissue pocket containing IMD 500  with the patient. 
     The temperature sensor  150  may be disposed within the housing  110 , and the temperature sensor  150  may be thermally coupled to the interior surface  114  of the housing  110 . In one or more examples, the temperature sensor  150  may be physically coupled to the housing  110  and/or directly coupled to the housing  110 . In one or more examples, the temperature sensor  150  includes a sensor surface  152  that may be thermally coupled to the interior surface  114  of the housing  110 . The temperature sensor may be configured to sense a temperature of a portion of the housing, for example the sensor surface  152  may be operable to sense a temperature. In one or more examples, the sensor surface  152  may be in direct contact with the interior surface  114  of the housing  110 . 
     Referring to  FIG. 4C , an exemplary IMD  100 C is shown with a temperature sensor  150  disposed therein. In addition to temperature sensor  150 , thermally conductive material  130  may be disposed within housing  110  and between housing  110  and temperature sensor  150  to transfer energy from the housing  110  to temperature sensor  150 . In some examples, conductive material  130  (or another energy transfer structure) may transfer the energy from the desired housing surface to be sensed to temperature sensor  150 . In the example of  FIG. 4C , thermally conductive material  130  may be configured to be thermally coupled to the interior surface  114  of housing  110 . 
     Although only one thermally conductive material  130  may be provided, IMD  100 C may include two or more components of thermally conductive material to transfer energy from multiple portions within IMD, as shown in  FIG. 4D . Examples of thermally conductive material may include a thermal pad such as a thermally conductive pad. The thermally conductive material  130  may be constructed of a solid structure, hollow structure, gel, paste, or any other configuration in which the thermally conductive material  130  conducts heat energy from the target surface (e.g., interior surface  114 ) to temperature sensor  150 . 
     Thermally conductive material  130  may be configured within housing  110  to physically contact the interior surface  114  of housing  110 . In some examples, thermally conductive material  130  may be constructed such that a free end of thermally conductive material  130  may be biased against curved portion  116  of housing  110  when housing  110  may be hermetically sealed around the interior components of IMD  100 . In other words, closing housing  110  may cause curved portion to contact thermally conductive material  130  such that the structural stiffness of thermally conductive material  130  retains physical contact between thermally conductive material  130  and curved portion  134  and a surface of the sensor  150 . In one or more examples, thermally conductive material may be configured to conform to the curved inner portion of the housing  110 . 
     In one or more examples as shown in  FIGS. 4A-4E , at least one physically compliant material  160  may be disposed between the at least one support  180  and the temperature sensor  150 . In one or more examples, the at least one physically compliant material  160  may be configured to provide a physical bias against the temperature sensor and towards the interior surface of the housing. In one or more examples, the physically compliant material  160  may be configured to provide the physical bias to the temperature sensor  150  to maintain contact between the interior surface  114  of the housing  110  The at least one physically compliant material  160  includes an elastomer in one or more examples. Physically compliant material  160  may be solid, viscoelastic solid, gel, paste, or any other material configured to provide the physical bias. The physical bias may be generated by a shape of compliant material  160  and/or material properties of compliant material  160 . The compliant material  160  make take up tolerance of the various components within the housing. In some examples, the compliant material  160  may be electrically insulative material. In some examples, compliant material  160  may be formed of one or more of elastic pad, compression pad, foam, rubber, or a diaphragm of plastic and/or metal configured to flex. In one or more examples, a stiffener  146  may be disposed between the compliant material  160  and the sensor  150 . The stiffener  146  may be provided under components that are electrically coupled, for example soldered, to the flexible circuit  142 . The stiffener  146  may prevent mechanical fatigue on solder joints and circuit components. 
       FIG. 10  is a flow diagram that illustrates an example technique for manufacturing an IMD, such as any of IMDs  100 A- 100 E. This technique may include disposing a temperature sensor within a housing, the housing comprising an exterior surface and an interior surface, the housing having at least one support in contact with the housing. In one or more examples, the at least one support may be coupled with an interior surface of the housing. In one or more examples, the method includes disposing multiple temperature sensors at multiple locations within the housing, and wherein each of the multiple temperature sensors are physically biased towards the interior surface of the housing with the physically compliant material. The method may further include disposing at least one physically compliant material between the at least one support and the temperature sensor, where the at least one physically compliant material may be configured to physically bias the temperature sensor towards the interior surface of the housing with the physically compliant material. In one or more examples, the method includes thermally coupling the temperature sensor to the housing, the temperature sensor configured to sense a temperature of a portion of the housing, the temperature sensor including a sensor surface. 
     In the example of  FIG. 10 , the manufacturing technique includes installing a hybrid board into the support, and may further include attaching a compliant pad to the support ( 302 ). The sensor may be attached to the support, and may be attached to the compliant pad ( 304 ). In some examples, the method includes thermally coupling the temperature sensor with a housing ( 306 ), for example the method includes disposing a sensing surface of the at least one temperature sensor directly adjacent to the interior surface of the housing wherein the sensing surface may be in direct contact with the interior surface of the housing. In one or more examples, the method may further include disposing a printed circuit board within the housing, and electrically coupling the temperature sensor with the hybrid board with a flexible circuit. 
     The method also includes thermally coupling the temperature sensor to the housing ( 304 ), where the temperature sensor may be configured to sense a temperature of a portion of the housing, and the temperature sensor may include a sensor surface. In one or more examples, the method may further include disposing thermally conductive material disposed between the sensor and the interior surface of the housing, and further optionally thermally connecting the thermally conductive material with the sensor surface and the interior surface of the housing. 
     Example 1: An implantable medical device includes a housing comprising an exterior surface and an interior surface; at least one support disposed within the housing; a temperature sensor comprising a sensor surface thermally coupled to the interior surface of the housing, wherein the temperature sensor is disposed within the housing and configured to sense a temperature of a portion of the housing; and at least one physically compliant material disposed between the at least one support and the temperature sensor, wherein the physically compliant material is configured to provide a physical bias against the temperature sensor and towards the interior surface of the housing. 
     Example 2: The implantable medical device of example 1, wherein the at least one physically compliant material is configured to provide the physical bias to the temperature sensor to maintain contact between the interior surface of the housing and the sensor surface of the temperature sensor. 
     Example 3: The implantable medical device of any of examples 1 and 2, wherein the at least one support contacts at least two opposing interior surfaces of the housing. 
     Example 4: The implantable medical device of any of examples 1 through 3, wherein the sensor surface is an operable sensing surface and the sensing surface is disposed directly in contact with the interior surface of the housing. 
     Example 5: The implantable medical device of any of examples 1 through 4, further includes a hybrid board disposed within the housing; and a flexible circuit electrically coupling the temperature sensor to the hybrid board. 
     Example 6: The implantable medical device of any of examples 1 through 5, wherein the at least one support is in contact with the interior surface of the housing. 
     Example 7: The implantable medical device of any of examples 1 through 6, wherein the at least one support is a rigid support. 
     Example 8: The implantable medical device of any of examples 1 through 7, wherein the physically compliant material is an elastic pad. 
     Example 9: The implantable medical device of any of examples 1 through 8, further comprising a thermally conductive material disposed between the sensor surface and the interior surface of the housing. 
     Example 10: The implantable medical device of example 9, wherein the housing comprises a curved inner portion, and wherein the thermally conductive material is configured to conform to the curved inner portion to couple the sensor surface and the interior surface of the housing. 
     Example 11: The implantable medical device of any of examples 1 through 10, wherein the temperature sensor is a first temperature sensor, and wherein the implantable medical device comprises one or more additional temperature sensors disposed at multiple locations within the housing and thermally coupled to respective portions of the interior surface of the housing. 
     Example 12: The implantable medical device of any of examples 1 through 11, wherein the housing is a titanium shield configured to protect the temperature sensor. 
     Example 13: The implantable medical device of any of examples 1 through 12, further comprising processing circuitry configured to receive a temperature signal from the temperature sensor and control at least one function of the implantable medical device based on the temperature signal. 
     Example 14: A method includes at least one physically compliant material at least one support where the at least one physically compliant material is configured to physically bias the temperature sensor towards the interior surface of the housing with the physically compliant material; and thermally coupling the temperature sensor to the housing, the temperature sensor configured to sense a temperature of a portion of the housing, the temperature sensor including a sensor surface. 
     Example 15: The method of example 14, further comprising disposing a sensing surface of the temperature sensor directly adjacent to the interior surface of the housing wherein the sensing surface is in direct contact with the interior surface of the housing. 
     Example 16: The method of any of examples 14 and 15, further comprising disposing a hybrid board within the housing, and electrically coupling the temperature sensor with the hybrid board with a flexible circuit. 
     Example 17: The method of any of examples 14 through 16, further comprising disposing thermally conductive material disposed between the temperature sensor and the interior surface of the housing. 
     Example 18: The method of example 17, further comprising thermally connecting the thermally conductive material with the sensor surface and the interior surface of the housing. 
     Example 19: The method of any of examples 14 through 18, further comprising disposing multiple temperature sensors at multiple locations within the housing, and wherein each of the multiple temperature sensors are physically biased towards the interior surface of the housing with the physically compliant material. 
     Example 20: An implantable medical device includes a housing comprising an exterior surface and an interior surface; at least one support disposed within the housing; a temperature sensor comprising a sensor surface thermally coupled to the interior surface of the housing, wherein the temperature sensor is disposed within the housing and configured to sense a temperature of a portion of the housing; at least one physically compliant material disposed between the at least one support and the temperature sensor, wherein the physically compliant material is configured to provide a physical bias against the temperature sensor and towards the interior surface of the housing. 
     According to the techniques and devices described herein, an IMD or external charging device may include one or more temperature sensors configured to sense the temperature of a portion of the device directly coupled or thermally coupled to the temperature sensor. These temperature sensors may be mounted on an interior surface of the IMD housing. In this manner, temperature sensors may obtain temperature information about one or more portions of the device while being thermally coupled to the interior surface of the IMD housing. The IMD and/or external charging device may then control charging of an implantable rechargeable power source using the sensed temperatures. 
     This disclosure is primarily directed to wireless transfer of energy between two coils (e.g., inductive coupling). However, one or more aspects of this disclosure may also be applicable to energy transfer involving a physical connection between a charging device and a rechargeable power supply. For example, aspects of this disclosure may be applicable to charging the power supply of an IMD by inserting a needle coupled to an external charging device through the skin and into a port of the IMD. Although physical connections for energy transfer may not introduce heat losses due to energy transfer between wireless coils, heat may still be generated and lost to the patient from components within the IMD (e.g., the battery being charged and circuits involved in the recharging of the power supply). In addition, the temperature sensors described herein may be used to monitor temperature for any application related to the operation, status, or condition of the device or device surroundings. 
     The disclosure also contemplates computer-readable storage media comprising instructions to cause a processor to perform any of the functions and techniques described herein. The computer-readable storage media may take the form of any volatile, non-volatile, magnetic, optical, or electrical media, such as a RAM, ROM, NVRAM, EEPROM, flash memory, or any other digital media. The computer-readable storage media may be non-transitory in that the storage media is not an electromagnetic carrier wave. However, this does not mean that the storage media is not transportable or that it non-volatile. A programmer, such as patient programmer or clinician programmer, may also contain a more portable removable memory type to enable easy data transfer or offline data analysis. 
     The techniques described in this disclosure, including those attributed to IMD  14 , charging device  22 , or various constituent components, may be implemented, at least in part, in hardware, software, firmware or any combination thereof. For example, 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, discrete, or analog logic circuitry, as well as any combinations of such components, embodied in programmers, such as physician or patient programmers, stimulators, image processing devices or other devices. The term “processor” or “processing circuitry” may generally refer to any of the foregoing logic circuitry, alone or in combination with other logic circuitry, or any other equivalent circuitry. 
     Such hardware, software, firmware may be implemented within the same device or within separate devices to support the various operations and functions described in this disclosure. While the techniques described herein are primarily described as being performed by processing circuitry  30  of IMD  14 , processor  50  of charging device  22 , or any one or more parts of the techniques described herein may be implemented by a processor of one of IMD  14 , charging device  22 , or another computing device, alone or in combination with each other. 
     In addition, any of the described units, circuitries or components may be implemented together or separately as discrete but interoperable logic devices. Depiction of different features as circuitries or units is intended to highlight different functional aspects and does not necessarily imply that such circuitries or units must be realized by separate hardware or software components. Rather, functionality associated with one or more circuitries or units may be performed by separate hardware or software components, or integrated within common or separate hardware or software components. 
     Various examples have been described. These and other examples are within the scope of the following claims.