Abstract
Temperature is the most commonly collected vital sign in all of clinical medicine; it plays a critical role in care decisions related to topics ranging from infection to inflammation, sleep, and fertility. Most assessments of body temperature occur at isolated anatomical locations (e.g. axilla, rectum, temporal artery, or oral cavity). Even this relatively primitive mode for monitoring can be challenging with vulnerable patient populations due to physical encumbrances and artifacts associated with the sizes, weights, shapes and mechanical properties of the sensors and, for continuous monitoring, their hard-wired interfaces to data collection units. Here, we introduce a simple, miniaturized, lightweight sensor as a wireless alternative, designed to address demanding applications such as those related to the care of neonates in high ambient humidity environments with radiant heating found in incubators in intensive care units. Such devices can be deployed onto specific anatomical locations of premature infants for homeostatic assessments. The estimated core body temperature aligns, to within 0.05 °C, with clinical grade, wired sensors, consistent with regulatory medical device requirements. Time-synchronized, multi-device operation across multiple body locations supports continuous, full-body measurements of spatio-temporal variations in temperature and additional modes of determining tissue health status in the context of sepsis detection and various environmental exposures. In addition to thermal sensing, these same devices support measurements of a range of other essential vital signs derived from thermo-mechanical coupling to the skin, for applications ranging from neonatal and infant care to sleep medicine and even pulmonary medicine.
| Original language | English (US) |
|---|---|
| Article number | 115545 |
| Journal | Biosensors and Bioelectronics |
| Volume | 237 |
| DOIs | |
| State | Published - Oct 1 2023 |
Funding
Fig. S1 shows the circuit-level aspects of device operation. In the main sensor unit, two LDOs (TPS7A02, Texas Instruments) convert the transferred power to a constant voltage of 1.8 V. One of the two LDOs remains active to enable Bluetooth communication; the other activates only when collecting data, to minimize standby power consumption (Fig. S2). The BLE SoC (ISP 1807, Insight SIP) controls these LDOs and provides power to the sensors through Analog-to-Digital Converting (ADC) pins with 16-bit resolution and digital General-Purpose Input/Output (GPIO) pins. Voltage divider circuits that include the NTC thermistors (10 kΩ, 3380 K, Murata Electronics) and reference resistors (10 kΩ, 0.1%, Vishay Dale) support robust capabilities in temperature sensing (Fig. S3). The IMU (BMI160, Bosch) can effectively capture not only changes in body orientation but also high frequency signals originating from cardiac cycles, respirations, and vocalizations such as crying. The resonance frequency of the Rx coil was set to the NFC standard (13.56 MHz) by adjusting capacitors connected in parallel. The rectifier (BAS4002, Infineon Tech.) converts the transferred AC voltage into a DC voltage, with a voltage regulator (LP2985, Texas Instruments) that maintains the output at 3.2 V. A pair of supercapacitors act as a charge reservoir to ensure stable operation during periods of low power transfer. A comparator (MIC842, Microchip Tech.) activates the LDO when the supercapacitors have sufficient charge to operate the main sensor unit.The output of the PMIC supports skin- and ambient-facing temperature measurements and mechano-acoustic data collection using two NTC thermistors and a 3-axis Inertial Measurement Unit (IMU), respectively. These data pass to an embedded flash memory on the BLE SoC and then wirelessly transmit to a user interface on a BLE-enabled device such as a smartphone. An LED indicator serves as a visual alarm that can be activated based on the measured data and various, user-defined threshold settings. These data can also be transferred to external life support systems for automated control. An example involves active regulation of the temperature of a neonatal incubator according to the body temperature. Fig. 1F and G shows a closed-loop system that includes a button device, a customized user interface, and a wireless module for heating the local environment of the incubator. Regulation of the temperature follows from comparisons of the temperature of the baby to a desired body temperature (Tset = 36 °C, here). Multiple devices can be located at different regions of the body, for time-synchronized operation. The data can also be uploaded to a secure Cloud platform for remote monitoring and analysis.Fig. 2A shows a simplified schematic diagram of the battery-powered embodiment. In this sensor, a voltage divider circuit that includes an NTC thermistor (thermal sensor) and a reference resistor supports robust and stable temperature sensing capabilities. The sensing performance depends on the change in the temperature of the NTC itself, as influenced by three main factors: i) the external ambient environment, ii) the thermal mass of the NTC thermistor and the surrounding materials, and iii) the thermal contact between the NTC and the target. Six layers of a thermal insulating film (RTAZS, 0.2 mm, 0.038 W/mK, Blueshift) that cover the NTC on the side opposite to the skin minimize the influence of the external environmental on the measurements. This insulation increases, however, the thermal mass of the sensor and therefore increases the thermal response time. An air cavity (3 × 3 × 1.2 mm3) structure compensates for this effect by thermally isolating the NTC thermistor from the other body parts of the sensor. From a heat transfer perspective, reductions in the thickness of the encapsulating material between the NTC thermistor and the target improve the efficiency of heat transfer. Thus, the bottom encapsulating layer is selected to be thin (<120 μm). A silicone elastomer (Ecoflex 00–30, 0.15 W/mK, Smooth-On) fills the other areas of the device. A molded elastomer with a different chemical formulation forms an encapsulating structure for the entire system (Silbione RTV-4420, 0.15 W/mK).The authors acknowledge funding from the Querrey-Simpson Institute for Bioelectronics and from grants from the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2021R1C1C2010180) and the DHA SBIR Phase II award (W81XWH22C0106). The authors acknowledge funding from the Querrey-Simpson Institute for Bioelectronics and from grants from the National Research Foundation of Korea ( NRF ) grant funded by the Korea government ( MSIT ) (2021R1C1C2010180) and the DHA SBIR Phase II award (W81XWH22C0106).
Keywords
- Bioelectronics
- Health monitoring
- Thermoregulatory responses
- Wireless sensors
ASJC Scopus subject areas
- Biophysics
- Biotechnology
- Biomedical Engineering
- Electrochemistry