A Flexible, Biocompatible, Passive Cooling Film for Electronic Devices

Wearable electronic devices have a great potential for applications in health monitoring, safety monitoring, prosthetics, robotics, and consumer electronics. One major challenge in the road is the incompatibility between the soft and curvilinear human body, and the rigid and planar electronics. The soft, flexible, and biocompatible material at the human machine interface is highly desired. On the other hand, the wearable electronic devices contain an increasing number of electronic and electrical components, including processors, batteries, displays, and data transmission systems, which all generate heat and are all constantly being housed in smaller spaces. Thermal management must be taken into consideration to improve device reliability and user experience. Passive cooling technology such as radiative cooling could be a solution since no additional energy input needed. Hence, a multifunctional material at the human machine interface is critical for the development of wearable devices.
Radiative cooling is a passive cooling technology that does not need any energy input to induce cooling. Radiative cooling is a spontaneous physical process that an object cools down by radiating thermal energy in the form of infrared radiation. Optical properties of radiative material must be precisely controlled over a wide wavelength range from UV to mid-infrared. Ideally, the radiative colling material should have zero absorptance in the solar spectrum (0.3-3 μm) to avoid solar heating. In the atmospheric transparency window (8–13 μm), the emittance should be 1 to effectively dissipate heat through radiation to outer space.
Here we proposed a material and process solution to realize a flexible, biocompatible, and passive cooling film for wearable electronic devices. A biocompatible fluorinated polymer polyvinylidene fluoride was selected owing to its intrinsically high emissivity in the atmospheric transparency window (8–13 μm) originated from the unique property of carbon-fluorine covalent bond. Electrospinning was chosen to process the polymer from a solution into a multifunctional film. Both the polymer solution parameters (molecular weight of polymer, solvent, and concentration of the solution) and electrospinning processing parameters (voltage, tip to collector distance, feed rate) were optimized to achieve the desired optical and physical properties.
The structural, optical, and physical properties of electro span free-standing PVDF film were systematically characterized. Electrospinning process has greatly changed the packing motif of the polymer chains. FTIR indicating that beta phase is the dominating crystalline phase in the crystalline PVDF, with a fraction over 94%. Whereas in the polymer pallet before electrospinning, the alpha phase is the dominating component at a fraction of 80%. Optical characterization of the multifunctional film in the UV-vis and infrared range were conducted with spectrometers and integration spheres. A high emissivity in the atmosphere transparent window (8-13 μm) of 94%, and a low absorption in the solar spectrum (0.3-3 μm) of 10% was recorded. The optical property ensures the film could effectively dissipate heat in the mid-IR range and minimize solar heating.
In summary, we demonstrated a material and process solution to prepare a flexible, biocompatible, and passive cooling film to address the thermal management challenge of wearable electronic devices. Our material strategy rooted in the fundamental of the structure-property relationship at molecular level, identify the fluorinated polymer candidate with intrinsic radiative cooling property. The electrospinning processing method not only further optimize the optical and physical properties of the free-standing film, but also bring in great potential in scaleup possibility. Our result indicating the potential application of fluorinated polymer as external coating of case for wearable electronic devices.