A low-cost, adjustable newborn thermal model for development of newborn thermal management devices

Introduction:: Newborn hypothermia is a significant mortality risk in in low resource settings (LRS) and the use of auto-regulating (servo-mode) newborn thermal management devices can improve patient outcomes[1]. A challenge in developing low-cost servo-mode devices in LRS is having an adequate benchtop model simulating the baby to fine-tune and validate the control loop. A benchtop model that adequately represents thermal properties of a newborn and accounts for the common clinical practice of swaddling can allow for device iteration prior to clinical testing on patients. We have developed a low-cost newborn thermal model (Simubaby) that can be used to support development of newborn thermal management devices.


Materials and Methods:: We created a benchtop thermal simulator (Simubaby) consisting of a flexible polyethylene canteen with lid (076340, Nalgene), water pump (PP333LV, Jebao), heater (DERNORD-147, Dernord), and thermistor (GA10K3A1IA, Digi-Key). The heater was connected to a DC variable power supply (STP3005DH,Toppower) to vary the total heat input to the system. Simubaby was characterized in two configurations: unwrapped, and wrapped to simulate swaddling. All testing occurred at an ambient room air temperature of 25 ± 5 ˚C. Simubaby was filled with 1.5L of ambient temperature water, placed on insulative material (Foamular 150, Owens Corning) and the water pump and heater were turned on. The ambient room air temperature (25 ± 5 ˚C), volume of water in Simubaby (1.5L), and the heat input (1.1-8.3W) were controlled. The Simubaby water temperature was measured at steady state, which varied between 2-22 hours based on fluctuating ambient room temperature. The unwrapped Simubaby was characterized in 31 trials at 6 discrete heat inputs varying from 2.2W to 8.3W. The wrapped Simubaby was characterized in 31 trials at 9 discrete heat inputs varying from 1.1W to 4.1W.


Results, Conclusions, and Discussions:: Thermal conductance (W/°C) was empirically calculated by dividing total heat input over the difference between ambient room air temperature and the steady state Simubaby temperature. The thermal conductance of an unwrapped and wrapped Simubaby was 0.72 ± 0.05 W/°C and 0.38 ± 0.04 W/°C , respectively. The thermal conductance (hA) was used in a generalized equation, Q_input=hA*(Tb-T_a), to determine the heat input (Q_input) required to maintain a desired Simubaby temperature (Tb) at a given ambient temperature (Ta). The increase in Simubaby temperature above the ambient room air temperature as a function of the heat input is plotted in Figure 1, where the inverse of the slope is the thermal conductance.

Simubaby thermally models a very low birthweight (VLBW) newborn (1.5 kg) because small, preterm newborns are at increased risk for hypothermia [2]. Further efforts are currently underway to characterize an extremely low birthweight (ELBW) newborn (1.0 kg) and as well as a high birthweight (HBW) newborn (4.0 kg) [3]. Simubaby has been used in pre-clinical benchtop testing for control loop tuning and disturbance variable testing not feasible to access during a clinical study.

Simubaby parameters (Q_input and Ta) can be adjusted through the generalized equation, Q_input=hA*(Tb-Ta), to thermally model newborns at a variety of physiological temperatures. Simubaby weight (water volume), and clinical practice (swaddling) can be adjusted through additional characterization. High quality benchtop models are a key step in the development of medical devices. This low-cost, adjustable newborn thermal model can aid medical device developers to effectively test and develop newborn thermal management devices prior to verification on newborn patients.


Acknowledgements (Optional): : This work was supported by generous commitments from the John D. & Catherine T. MacArthur Foundation, the Bill & Melinda Gates Foundation, the Children’s Investment Fund Foundation, The ELMA Philanthropies, the Lemelson Foundation, the Sall Family Foundation, the Ting Tsung and Wei Fond Chao Foundation, and individual donors to NEST360

References (Optional): : 1. Sinclair, J. C. (2002). Servo-control for maintaining abdominal skin temperature at 36C in low birth weight infants. Cochrane Database of Systemic Reviews. https://doi.org/10.1002/14651858.cd001074

2. Laptook, A. R., Salhab, W., & Bhaskar, B. (2007). Admission temperature of low birth weight infants: Predictors and associated morbidities. Pediatrics, 119(3). https://doi.org/10.1542/peds.2006-0943

3. Gomella, T. L. (2013). Neonatology: Management, Procedures, On-Call Problems, Diseases, and Drugs (7th ed.). McGraw-Hill. ISBM 978-0-07-176801-6