In this article, AZoRobotics explores the role of thin film heat exchangers in the thermal management of wearable robots.
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What is a Wearable Robot?
A wearable robot is a type of wearable device used to improve the physical abilities and motion of a person. These robots represent the integration of wireless technologies, big data, and sophisticated new hardware. Several wearable robot models are used for rehabilitation or post-surgery purposes.
Wearable robot interfaces can be programmed in different ways. Sensors in these robots receive behavioral or verbal input to facilitate specific types of movement. Thus, these wearable devices have received significant attention for several medical applications. Specifically, these robots are greatly beneficial for disabled or paralyzed individuals.
Why is Thermal Management Important for Wearable Robots?
Technological advancements in the past two decades have led to the development of several types of wearable robots for industrial and medical settings. However, thermal discomfort due to wearable robots has led to their disuse and hindered long-term adoption, emphasizing the importance of efficient thermal management of these wearable devices.
Effective thermal management in wearable robots is also necessary for thermal safety due to the extreme proximity of these devices to human skin. Additionally, the electronic components in wearable robots must remain within specific temperature ranges to ensure functionality, performance, and reliability.
In wearable robots, using bulky heat sinks or other heat transfer augmentation elements for thermal management is not feasible, necessitating the identification of new materials and structures, such as thin films.
Thermal Management of Wearable Robots Using Thin Films
In several wearable robots, the heat generated during the typical operation of the electronic components must be spread effectively and then dissipated to the ambient to prevent device failures, discomfort, or injury.
Thus, materials with high thermal conductivities, mechanically compatible with the skin, and good breathability are required for thermal management in wearable robots. For instance, thin polyethylene films with a high thermal conductivity of 62 Wm−1 K−1 can be used in wearable devices.
These films consist of aligned polyethylene nanofibers with amorphous and crystalline regions, with the amorphous region possessing an exceptionally high thermal conductivity of 16 Wm−1 K−1.
Similarly, a continuous array of boron nitride nanosheets can be synthesized on an elastomer substrate with pre-formed tetrahedral structures to create three-dimensional (3D) thermal paths and realize high stretchability and flexibility.
Thermoelectric materials and devices can be used for flexible cooling applications, including wearable personal thermoregulation devices. Flexible thermoelectric generators can be operated in reverse to achieve cooling through local heat dissipation.
Intrinsically flexible organic thermoelectric materials can be used for cooling in wearable devices as they are lightweight and have low thermal conductivity. Additionally, these materials are cheap, possess low toxicity, and can be manufactured easily owing to their solution processability.
Several inorganic flexible thermoelectric materials, such as materials based on copper iodide thin film, have also displayed good ZT values and bendability at room temperature.
Flexible electrocaloric cooling materials can also be used for active thermal control in wearable devices. Temperature changes of more than 10 K can be realized using thin films with high dielectric breakdown fields.
For instance, a giant electrocaloric effect of 0.48 K/V has been realized in 350-nanometer PbZr0.95Ti0.05O3 films near the ferroelectric Curie temperature of 222 °C. Thin electrocaloric films have also been synthesized using a prototype electrocaloric cooler.
Several studies have demonstrated the synthesis of flexible electrocaloric coolers using inorganic electrocaloric nanowire arrays and flexible thin films. In a recent study, researchers demonstrated a flexible electrocaloric cooler based on polymeric electrocaloric films and elastomeric substrates.
Electrostatic actuation was employed to realize alternating thermal contacts between the electrocaloric films and the hot/cold side of the cooler to complete a thermodynamic cycle. However, the cooling power and temperature differential were insufficient for practical applications of flexible electrocaloric coolers.
Thus, more research is required to improve the reliability of thin electrocaloric films under repeated removal and application of electric fields and to enhance the thermal diffusivity of electrocaloric films to increase cycle frequency and cooling power.
Solid-liquid phase-change materials, such as inorganic salt hydrates, polyols, fatty acids, and paraffin waxes, are commonly used for thermal control and management applications. Several studies have successfully displayed the synthesis of intrinsically flexible materials that can undergo solid-solid phase changes.
For instance, intrinsically flexible thin phase change material films synthesized by chemically grafting toluene-2,4-diisocyanate and melamine with polyethylene glycol (PEG) demonstrated stable phase transition under 1000 cooling-heating cycles, high latent heat of more than 100 J/g, and tunable phase-change temperatures.
To summarize, thin films will be used more extensively for the thermal management and control of wearable robots in the future. However, focused research and development efforts are necessary to increase the commercial deployment of thin films for thermal management.
References and Further Reading
Rykaczewski. K. (2022) Thermophysiological aspects of wearable robotics: Challenges and opportunities. Temperature. doi.org/10.1080/23328940.2022.2113725
Ju, Y. S. (2022). Thermal management and control of wearable devices. IScience, 25(7), p. 104587. https://www.sciencedirect.com/science/article/pii/S2589004222008598
Mischenko, A. S., Zhang, Q., Scott, J. F., Whatmore, R.W., Mathur, N. D. (2006). Giant Electrocaloric Effect in Thin-Film PbZr0.95Ti0.05O3. Science. doi.org/10.1126/science.1123811
Yang, C., Souchay, D., Kneiß, Max., Bogner, M., Wei, H., Lorenz, M., Oeckler, O., Benstetter, G., Fu, Y. Q., Grundmann, M. (2017). Transparent flexible thermoelectric material based on non-toxic earth-abundant p-type copper iodide thin film. Nature Communications, 8, p. 16076. https://www.nature.com/articles/ncomms16076
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