"Monitoring the patient's temperature is necessary for medical service providers and patients, but it can cause interference. Wireless thermometer can measure body temperature regularly in a non-invasive way in clinical environment and home. This function will be welcomed by medical service providers and patients. However, for developers, the corresponding solutions often can not meet the needs of high-precision and low-power wireless operation for a long time, so it is difficult to ensure a satisfactory user experience.
This paper will introduce the key requirements of clinical thermometer, and explain how developers can combine the high-precision digital temperature sensor of Texas Instruments with wireless microcontroller to meet these seemingly contradictory requirements.
Clinical thermometer requirements
In the field of medical care, body temperature, heart rate, blood pressure and respiratory rate are listed as the four main vital signs. Body temperature is not only used to distinguish cold or influenza, but also an important clinical index. Small changes in body temperature can be the earliest indication to judge the adverse reactions of patients to treatment such as medication or blood transfusion. Therefore, accurate thermometry is essential to ensure continuous care and signal the need for intervention in the event of complications.
The significance of small changes in body temperature is very important, so clinical thermometer must meet the accuracy and calibration requirements specified in ASTM e1112 and iso-80601-2-56 standards. ASTM e1112 standard was developed by ASTM International (formerly the American Society for testing and materials). It is required that thermometers used in clinical applications must meet the maximum error rate requirements of the following temperature ranges:
At 37.0 ˚ C to 39.0 ° C (usually indicating slight to moderate heating), with a maximum error of ± 0.1 ° C
At 35.8 ˚ C to 36.9 ° C (may indicate hypothermia in some individuals), with a maximum error of ± 0.2 ° C
At 39.1 ˚ C to 41.0 ° C (indicating more serious physical conditions, including high fever or hyperthermia), with a maximum error of ± 0.2 ° C
When the body temperature is lower than 35.8 ° C or higher than 41.0 ° C, the maximum error is ± 0.3 ° C
Although clinical temperature monitoring is very important in clinic, it used to rely on expensive bedside monitors to provide the required level of accuracy. In order to continuously monitor body temperature, medical service providers have to connect cables to patients, which is at least inconvenient or even impossible in some environments (such as neonatal ward). Wireless temperature monitoring can be an effective alternative, but it has been difficult for developers to create a wireless design that can meet many requirements at the same time. In addition to meeting the basic requirements of clinical accuracy and low-power battery work, the design of such wireless monitors must also ensure the comfort of patients, never interfere with patients during work lasting for several hours or even days, and ensure that the battery life can ensure long-term reliable work. For designs that meet these requirements, the tmp117maidrvt temperature sensor of Texas Instruments can be used as a key support component.
Clinical grade temperature sensor
Tmp117maidrvt (hereinafter referred to as tmp117) combines the analog temperature sensing subsystem with I2C serial interface, EEPROM and control logic. It also has programmable alarm function and can send signals when the temperature exceeds the specified range. In the temperature sensing subsystem, the sensor regulating circuit supplies the output of the bipolar junction transistor (BJT) silicon bandgap temperature sensor to the 16 bit on-chip analog-to-digital converter (ADC) (Fig. 1).
Tmp117 is specially designed to support clinical application and can fully meet the requirements of ASTM e1112 and iso-80601-2-56 standards for clinical electronic thermometer. The device can not only meet the maximum error requirement of ± 0.1 ° C in the range of 37.0 ° C to 39.0 ° C, but also achieve this accuracy level in the range of - 20 ° C to 50 ° C without any calibration. Tmp117 has excellent accuracy performance over the recommended working range of - 55 ° C to 150 ° C, so it can even be used as a substitute for Class AA resistance temperature detector (RTD) (Figure 2).
The tmp117 is packaged in a 2 mm x 2 mm 6-pin package with an operating voltage of 1.8 to 5.5 volts. The average current consumption is only 3.5 microamps (µ a) (the conversion rate is 1 Hz), and only 150 Na (NA) in off mode. In addition, developers can use the single conversion function of the device to maximize the time when tmp117 is in ultra-low power off mode.
The single pass mode enables the device to enter the off mode immediately after the active conversion phase. In contrast, the device's default continuous conversion mode allows it to operate at 1.25 in a programmable time μ A remains active in standby mode. In single mode, each temperature measurement involves an activity conversion phase, which lasts about 15.5 milliseconds (MS) and consumes about 135 seconds in total μ A current.
These two modes enable developers to sacrifice power consumption for higher conversion rate, while the average mode allows them to sacrifice power consumption for higher noise immunity. In average mode, the device will automatically perform 8 consecutive conversions to generate average results. Using this mode, the device can achieve repeatability with the least significant bit (LSB) of ± 1 in the converted digital results, while the LSB is ± 3 in the non average mode.
Design challenges
With integrated functions such as single mode and average mode, tmp117 can provide a complete digital temperature measurement sensor in 2 mm x 2 mm WSON (small ultra-thin lead-free) package, and only uses 6 pins: V + power supply, grounding, serial data, serial clock, serial bus address selection and alarm function. Therefore, the workload of hardware interface design will not exceed that of any typical I2C serial device. However, in practice, the design challenge of this or any other high-precision temperature sensor lies more in the physical layout design optimized for thermal management than in the design of hardware interface.
On board heat management function: an interesting problem of digital thermometer
For the body temperature sensor, its design must minimize the heat conduction path from other heat sources and maximize the heat conductivity to the patient. In order to minimize the impact from other heat sources, developers can install the sensor at the end of the narrow arm of the PC board away from the main board. This can effectively insulate the sensor from the heat source in the main design. However, even if there is an ideal heat insulation mechanism, any electronic device will be affected by self heating effect, which may damage the accuracy of temperature sensor. The low power consumption of tmp117 helps to minimize the self heating effect in this case. Over time, the autothermal response of the device increases in proportion to its supply voltage, but the change is only millidegree Celsius (MC) (Fig. 3). By using single pass mode, developers can shorten the active working time of the device and keep the self heating effect at the single digit MC level.
A more difficult design challenge is how to optimize the heat conduction path between the device and the patient's skin. In order to help improve the thermal conductivity to the bottom layer plate or component, the device package includes a large bare thermal conductive pad, which is not grounded, but is purely used to improve the heat transfer performance to the bjst silicon bandgap sensor through the package. Texas Instruments recommends filling solid copper-clad under the heat conduction pad of the device to optimize the heat conduction path between the device and the PC board.
However, for final contacts in contact with the skin, Ti recommends the use of vias and final coatings using biocompatible materials such as thermally conductive polymers, rather than the continued use of copper coating. This is because copper may cause skin corrosion or other reactions. Finally, the recommended components are simple double-layer laminations designed to reduce manufacturing costs while providing the necessary thermal conductivity between the device and the skin (Fig. 4).
Reference design of low power wireless digital thermometer
Texas Instruments demonstrates how to use tmp117 with appropriate thermal management methods in the comprehensive reference design of wireless clinical thermometer. In this design, Texas Instruments combines tmp117 with low-power Texas Instruments cc2640r2f Bluetooth microcontroller. In addition to the arm used as the main processor ® Cortex ®- In addition to the M3 32-bit core, the cc2640r2f also integrates a dedicated radio frequency (RF) core subsystem and its own dedicated arm Cortex-M0 core and RF transceiver (Fig. 5).
With the integrated function of the MCU itself, this design can provide a complete battery power supply solution by using only one 3V thin-film battery (such as 0132990001 of Molex) and several additional passive components. The constructed design can be adhered to the human body through medical tape to provide continuous monitoring for several days, although the capacity of thin-film flexible battery is relatively limited. This reference design provides a complete solution to insulate the 2 mm x 2 mm tmp117 IC using a flexible PC board with the above extended arm (Figure 6).
Ti also provides a related sample application to demonstrate how to use the Bluetooth advertising protocol to transmit the temperature reading of the skin patch to the mobile device. Bluetooth advertising protocol aims to provide short messages to nearby Bluetooth devices, allowing developers to add a few bytes of data to the standard Bluetooth advertising package.
The sample software is built in ti-rtos working environment, including a module TIDA_ 01624. C, this module demonstrates how to use TI's low-power Bluetooth (ble) stack to transmit tmp117 temperature reading in Bluetooth advertising package. Although the use of ble stack may be complex, the Ti software architecture can abstract the data flow through the stack. For the specific application instance simpleperipheral, the application is in the task function simpleperipheral_ Executed within the main loop in taskfxn(). After initializing the application, the event management service of the software framework will bring the control flow to a piece of code, which will read the tmp117 sensor (sensorread()), load the generated temperature measurement value into the payload of the advertisement package, and start the Bluetooth advertisement using the generated data package (Listing 1).
In addition to the basic initialization and configuration, the software interaction with tmp117 is also simple and direct. For example, the sensorread () function used in the main loop of the above application simply executes the I2C transaction used to transfer the measurement results (Listing 2).
In addition to demonstrating the use of Bluetooth stack and ti-rtos, the sample software also provides ready-made applications for transmitting temperature readings to mobile devices running Ti simplelink SDK Explorer mobile applications (with IOS and Android versions). In addition to the pre built applications, Ti also provides simplelink SDK Explorer application distribution package with complete source code for each mobile platform, as well as Ti SDK Explorer Bluetooth plug-in for cc2640r2 MCU.
summary
It is difficult to design a user-friendly and effective clinical wireless thermometer because it can not meet the needs of high measurement accuracy and long battery life at the same time. The tmp117 temperature sensor of Texas Instruments has low power consumption and clinical accuracy, so it provides an effective solution. As demonstrated in the comprehensive reference design, developers can combine tmp117 with the cc2640r2 wireless Bluetooth microcontroller of Texas Instruments to build a complete wireless thermometer design suitable for medical applications.
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