The ‘ultrathin, soft, radiative-cooling interface’ layer. Photo: City University of Hong Kong.
The ‘ultrathin, soft, radiative-cooling interface’ layer. Photo: City University of Hong Kong.

Overheating of wearable skin-like electronic devices increases the risk of skin burning and results in performance degradation. A team led by researchers at the City University of Hong Kong (CityU) has now invented a photonic material-based ‘soft, ultrathin, radiative-cooling interface’ that greatly enhances heat dissipation in such devices. Able to achieve temperature drops of more than 56°C, this material offers an alternative for effective thermal management in advanced wearable electronics.

“Skin-like electronics are an emerging development in wearable devices,” said Yu Xinge, associate professor in the Department of Biomedical Engineering (BME) at CityU, who co-led the research. “Effective thermal dissipation is crucial for maintaining sensing stability and a good user experience. Our ultrathin, soft, radiative-cooling interface, made of dedicatedly designed photonic material, provides a revolutionary solution to enable comfortable, long-term healthcare monitoring, and virtual and augmented reality (VR/AR) applications.”

In electronic devices, heat can be generated by both internal electronic components, when an electric current passes through a conductor, a process known as Joule heating, and external sources, such as sunlight and hot air. To cool down the devices, both radiative (i.e. thermal radiation – emitting heat energy from the device surface) and non-radiative (i.e. convection and conduction – losing heat to the layer of still air around the device and through direct contact with a cold object) heat-transfer processes can play a role.

However, current technologies rely mostly on non-radiative means to dissipate the accumulated Joule heat. Moreover, the materials in these devices are usually bulky and rigid and offer limited portability, hindering the flexibility of wireless wearable devices.

To overcome these shortcomings, the research team developed a multifunctional composite polymer coating with both radiative and non-radiative cooling capacity that demonstrates advances in wearability and stretchability. They report this coating in a paper in Science Advances.

The cooling interface coating is composed of hollow silicon dioxide (SiO2) microspheres, for improving infrared radiation, and titanium dioxide (TiO2) nanoparticles and fluorescent pigments, for enhancing solar reflection. It is less than 1mm thick, lightweight (about 1.27g/cm2), and has robust mechanical flexibility.

When heat is generated in an electronic device with this coating, the heat flows to the cooling interface layer and dissipates to the ambient environment through both thermal radiation and air convection. The open space above the interface layer provides a cooler heat sink and an additional thermal exchange channel. The interface also exhibits excellent anti-ambient-interference capability due to its lower thermal conductivity, making it less susceptible to environmental heat sources that would affect the cooling effect and the performance of the device.

To examine its cooling capacity, the researchers coated the cooling interface layer onto a metallic resistance wire – a typical electronic component that can cause a temperature rise. With a coating thickness of 75μm, the temperature of the wire dropped from 140.5°C to 101.3°C, compared with an uncoated wire, at an input current of 0.5A. A coating thickness of 600μm caused the temperature of the wire to fall to 84.2°C, a drop of more than 56°C.

“It is necessary to keep the device temperature below 44°C to avoid skin burns,” said Yu. “Our cooling interface can cool down the resistance wire from 64.1°C to 42.1°C with a 150μm-thick coating.”

The efficient passive radiative cooling capacity and the sophisticated nonradiative thermal design significantly improved the performance of several skin electronic devices developed by the team. This included improving the efficiency of wireless power transfer to light emitting diodes (LEDs) and the signal stability of a skin-interfaced wireless sensor under environmental obstructions (e.g. sunlight, hot wind and water).

“The intrinsically flexible nature of the cooling interface allows the electronic devices to undergo stable cooling even under extreme deformation, such as bending, twisting, folding and stretching many times,” said Lei Dangyuan, associate professor in the Department of Materials Science and Engineering (MSE) at CityU, another co-leader of the study.

Using this coating, the researchers developed a cooling-interface-integrated stretchable wireless-based epidermal lighting system that showed higher illumination intensity and maintained stable performance even upon repeated stretching (1000 times) from 5% to 50%.

Next, the research team will focus on practical applications of the cooling interfaces for advanced thermal management of wearable electronics in the healthcare monitoring, wireless communications and VR/AR fields.

This story is adapted from material from the City University of Hong Kong, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.