Yufeng Xing

One-Dimensional Thermal Analysis of the Flexible Electronic Devices Integrated with Human Skin

A one-dimensional analytic thermal model for the flexible electronic devices integrated with human skin under a constant and pulsed power is developed. The Fourier heat conduction equation is adopted for the flexible electronics devices while the Pennes bio-heat transfer equation is adopted for the skin tissue. Finite element analysis is performed to validate the analytic model through the comparison of temperature distributions in the system. The influences of geometric and loading parameters on the temperature increase under a pulsed power are investigated. It is shown that a small duty cycle can reduce the temperature increase of the system effectively. A thin substrate can reduce the device temperature but increase the skin surface temperature. The results presented may be helpful to optimize the design of flexible electronic devices to reduce the adverse thermal influences in bio-integrated applications.

3D thermal analysis of rectangular microscale inorganic light-emitting diodes in a pulsed operation

Microscale inorganic light-emitting diodes (µ-ILEDs) have attracted much attention due to their excellent performance in biointegrated applications such as optogenetics. The thermal behaviors of µ-ILEDs are critically important since a certain temperature increase may degrade the LED performance and cause tissue lesion. The µ-ILEDs in a pulsed operation offer an advantage in thermal management. In this paper, a 3D analytic model, as validated by finite element analysis, is developed to study the thermal response of rectangular µ-ILEDs in a pulsed operation. A scaling law for the maximum normalized temperature increase of rectangular µ-ILEDs in terms of non-dimensional parameters is established. The influences of geometric (i.e. shape factor) and loading parameters (e.g. duty cycle and period) on the temperature increase are systematically investigated. These results are very helpful in designing µ-ILEDs by providing guidelines to avoid adverse thermal effects.

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.

A simple analytical thermo-mechanical model for liquid crystal elastomer bilayer structures

The bilayer structure consisting of thermal-responsive liquid crystal elastomers (LCEs) and other polymer materials with stretchable heaters has attracted much attention in applications of soft actuators and soft robots due to its ability to generate large deformations when subjected to heat stimuli. A simple analytical thermo-mechanical model, accounting for the non-uniform feature of the temperature/strain distribution along the thickness direction, is established for this type of bilayer structure. The analytical predictions of the temperature and bending curvature radius agree well with finite element analysis and experiments. The influences of the LCE thickness and the heat generation power on the bending deformation of the bilayer structure are fully investigated. It is shown that a thinner LCE layer and a higher heat generation power could yield more bending deformation. These results may help the design of soft actuators and soft robots involving thermal responsive LCEs.

Thermal analysis of epidermal electronic devices integrated with human skin considering the effects of interfacial thermal resistance

Epidermal electronic devices (EEDs) have similar mechanical properties as those of human skin such that they can be integrated with human skin for potential applications in monitoring of human vital signs for diagnostic, therapeutic or surgical functions. Thermal management is critical for EEDs in these applications since excessive heating may cause discomfort. Comprehensive analytical studies, finite element analysis and experiments are carried out to study the effects of interfacial thermal resistance between EEDs and human skin on thermal properties of the EED/skin system in this paper. The coupling between the Fourier heat transfer in EEDs and the bio-heat transfer in human skin is accounted in the analytical model based on the transfer matrix method to give accurate predictions on temperatures, which agree well with finite element analysis and experimental measurements. It is shown that the maximum temperature increase of the EED for the case of imperfect bonding between EED and skin is much higher t...

Bandgap Structures of SH-Wave in a One-Dimensional Phononic Crystal with Viscoelastic Interfaces

Phononic crystal is an artificial periodic structure with the ability to regulate and control the wave propagation of particular frequencies and has been widely used in many applications. The adhesive layer bonding different constituents in the periodic structure of phononic crystals is usually a viscoelastic material, which has frequency-dependent material properties. In this paper, an analytical model based on the transfer matrix method is developed to study the bandgap structures of SH-wave (a shear wave with the propagation direction normal to the motion plane) in a one-dimensional phononic crystal consisting of two different elastic constituents bonded by the viscoelastic adhesive layer. The results show that the viscosity of the adhesive layer has a significant influence on the bandgap structure at the region of high frequency. The effects of various material parameters of the viscoelastic adhesive layer such as the relaxation time, the final-state modulus and the initial-state modulus are systemati...