The Cellular Substrate Mechanics And Stretchability Of Flexible Electronics

As a kind of high compatible device,flexible electronics can not only bend but also stretch,compress,twist,and fold while maintaining a high level of performance and reliability to adapt to complex application surfaces.Compared with traditional electronics,flexible electronice have a much wider application in the field of biomedical,nation security and information technology,and can better meet the needs of accuracy,portability and comfort in human-computer interaction.The most common approach achieve stretchability is to have all functional components to reside on rigid device “islands” that are electrically and mechanically linked by deformable interconnects,which are called “islands-bridge structure”.The structure is always integrated on substrate through transfer printing process.Then the deformable interconnects are exploited to isolate functional components from strains associated with overall system deformation.Most of the existing research mainly focused on the design of interconnects to improve the stretchability of the flexible electronics,but not the optimization of the substrate.A cellular substrate,which can achieve much lower modulus and higher permeability than the solid substrate,is proposed in the flexible electronics design.Focus on the mechanics of the cellular substrate and the stretchability of the electronics,the analytic model of the cellular substrate is established,the interfacial shear stress between the electronics and skin is analyzed and the stretchability of the electronic devices is optimized in this paper.The accomplished studies are as follows:1.The cellular substrate has been proposed into design of the flexible electronics.Parameters have been defined and the advantages of such cellular substrates have been verified as its low equivalent modulus,which could be much lower than that of the virgin material,therefore reduces the interfacial stresses between the flexible electronics and skin and makes the electronics soft,stretchable and comfortable.2.Anisotropic mechanics of regular hexagonal cellular substrate has been completed.An analytic model for the anisotropic regular hexagonal cellular substrate under finite stretching along any direction has been established.The equivalent stress-strain curve of the substrate agrees well with the FEA results without any parameter fitting in defferent porosities,even for the engineering strain as large as 80%.The stress-strain curve shows that equivalent modulus of the regular hexagonal cellular substrate decreases with the increase of the porosity and the lower equivalent modulus can be achieved by stretching along the cell wall direction.For a regular hexagonal cellular substrate and an equivalent solid substrate bonded to a skin,FEA has been used to determine the interfacial shear stress between the substrate and skin.The results show that the interfacial shear stress for stretching along cell wall direction is smaller than that for other directions,and the interfacial shear stress for the equivalent solid substrate is on the same trend as the regular hexagonal cellular substrate.3.Mechanics analysis and structural optimization of the irregular hexagonal cellular substrate have been accomplished.The irregular hexagonal cellular substrate has been proposed and the parameter angle(?)has been defined in this paper,which can achieve much lower modulus than the regular hexagonal cellular substrate in a particular direction.An analytic model for the irregular cellular substrate under finite stretching along a particular direction has been established.The equivalent stress-strain curve of the substrate agrees well with the FEA results without any parameter fitting in defferent porosities.FEA has been used to determine the interfacial shear stress between the substrate and skin,the results show that the interfacial shear stress increases as angle(?)increases for the fixed porosity and decreases as porosity increases for the fixed angle(?).For the contradiction that an irregular hexagonal cellular substrate with small angle(?),the lower equivalent modulus can be achieved easier but with a higher risk of self-collapse,a structural optimization has been proposed to find a balance point for these two properties.4.The stretchability of the flexible electronics based on the 2D serpentine interconnects and the cellular substrate has been analyzed and optimized.The result shows that the cell size of the cellular substrate at fixed porosity has little effect on the stretchability of the flexible electronics.An equivalent solid substrate,which has the stress-strain model for the cellular substrate with fixed porosity,yields an effective way to estimate the lower bound of elastic stretchability for the 2D serpentine interconnects bonded to a cellular substrate.The optimized designs of the size and modulus of the 2D serpentine interconnects are accomplished based on the equivalent solid substrate.Then a generic two-stage encapsulation strategy has been reported in this paper considering a possible poor environment.The results suggest substantial enhancements of the stretchability can be achieved through the use of the reported generic two-stage encapsulation strategy.5.The stretchability of the flexible electronics based on the 3D helical interconnects and the cellular substrate has been analyzed and optimized.The result shows that the cell size of the cellular substrate at fixed porosity also has little effect on the stretchability.An equivalent solid substrate,which has the stress-strain model for the cellular substrate with fixed porosity,yields an effective way to estimate the lower bound of elastic stretchability for the 3D helical interconnects bonded to a cellular substrate.The optimized designs of the size and modulus of the 3D helical interconnects,which are suitable for different pre-strain,have been accomplished based on the equivalent solid substrate.A generic two-stage encapsulation strategy has been reported and the advantages are analyzed through a systematic comparison.In summary,this paper presents a systematic study of the flexible electronics based on the cellular substrate,through combination of the analytical modeling and FEA numerical simulation.Innovative results have been achieved in analytic model for different porosities,structural design and the stretchability.This study is useful to the design and stretchability optimization of flexible electronics bonded to the cellular substrate.