Thermal management of micro-scale inorganic light-emittng diodes on an orthotropic substrate for biointegrated applications

The orthotropic material with the in-plane thermal conductivity much larger than the off-plane one can control the heat flow direction. This feature provides unique benefits in thermal management of micro-scale inorganic light-emitting diodes (μ-ILEDs) device for biointegrated applications by helping the heat dissipation from μ-ILEDs along the in-plane directions to lower the μ-ILED temperature and prevent the heat dissipation to the tissue along the off-plane direction to ensure a low tissue temperature. Three-dimensional analytical models, accounting for the coupling between the Fourier heat conduction in the μ-ILED device and the Pennes bioheat transfer in the human skin, are established to investigate the thermal behaviors of μ-ILEDs on an orthotropic substrate integrated with the human skin. Both the operations of μ-ILEDs in a constant mode and pulsed mode are studied. The maximum temperature increases of μ-ILED and in the tissue are derived and their dependences on various parameters such as the thermal conductivities of the orthotropic substrate, substrate thickness, and loading parameters (e.g., duty cycle, pulse period) are investigated. These results pave the theoretical foundation for the thermal management of μ-ILED devices for biointegrated applications.