JAJU678 January   2019

 

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2.2 Design Considerations

Carefully consider the placement of the patch to ensure that the temperature reading is aligned with the expected results. The average temperature reading of an oral thermometer in a healthy adult is 98.6ºF, but measurements taken from other areas of the body will differ in temperature. For example, a temporal thermometer will read a temperature that can be up to one degree (1ºF) lower than that of an oral thermometer.

The crucial distinction here is between core temperature and skin temperature. The primary goal of this design is to demonstrate effective techniques for the measurement of skin temperature. The temperature at the surface of a patient's skin will not normally be identical to their core temperature. The most accurate methods of obtaining core temperature are internal, such as with oral or rectal thermometers. In certain applications however, such as long-term patient monitoring in the incubator of a NICU, skin temperature monitoring is often the only practical method.

When using skin temperature to try and obtain a measurement close to core temperature, the preferred sites are traditionally the underarm (axillary) or the forehead (temporal). The form factor of this patch demonstrates a technique which can be employed for underarm measurements. Extending the TMP117 away from the primary portion of the board that contains the RF antenna not only thermally isolates the device, but also allows the RF antenna to be exposed on one side while the sensor is enclosed underneath the user's arm. In production systems that use axillary measurements, users can move this long flexible arm to place the antenna in an area that can ensure RF performance.

The effective Bluetooth range of the design will depend on many factors, such as walls and objects between the patch and the smart device. When worn, however, the primary source of signal loss will likely be the patch wearer. To improve the range, the CC2640R2F can be programed to increase its Bluetooth output power, but this will decrease the battery life of the patch. The patch is powered by a thin-film cell, therefore low power Bluetooth modes are recommended for this design to extend operating time. As characterized, this design uses the maximum output power of the CC2640R2F to attain a longer range. Alternatively, antenna and pi-network matching may be performed while the device is worn to increase the signal range without increasing the output power level of the CC2640R2F.