| Rapid demand of wearable electronic devices and integrated systems accelerates the development of flexible and stretchable electronics.Wearable integrated systems provide multiple functionalities through interconnecting various electronic devices.Therefore,reliable interconnecting strategy to realize stable electrical and mechanical interconnection between independent devices is crucial.Here,we designed stretchable interconnections with high electrical performance under strain.Firstly,materials and structures for the solderless stretchable interconnections（SLSIs）were studied to prepare bio-inspired conductive micro-bridges（CMBs）conductors.With regional distribution of adhesive polymer（AP）on CMBs conductors,interlayer conductivity with tight adhesion of the SLSIs was realized.Then,electrical performance of SLSIs under strain was tested and optimized.Based on the morphology observation of Au cracks,phenomenological model and finite element analysis（FEA）of stretchable conductors and interconnections,electrical conductivity mechanism and strain manipulation due to the presence of CMBs were explained.Lastly,all-stretchable electronic systems with SLSIs were constructed and studied.To design SLSIs,strain distribution of SLSIs under strain with different Young’s modulus of interconnecting layer was studied via FEA,and elastic material（polydimethylsiloxane,PDMS）and interlocking CMBs structures were chosen to realize stretchable interlocking interconnections.To avoid strain localization induced by mismatch of Young’s modulus at the soft/rigid interface of solder assisted interconnections,solderless interconnecting strategy was designed to navigate strain distribution.Then,we designed and synthesized poly-dopamine acrylic acid（poly-DA）as AP,it was highly adhesive on Au thin-film（with a shear adhesion of 420 k Pa,much higher than the shear adhesion of Au thin-film on PDMS）.Additional oxygen plasma treatment to the CMBs conductor resulted in super wettability（contact angel～0°）.Through wetting and de-wetting process,AP formed micro-ring around each CMB pattern.This further ensures the electrical and mechanical connecting of the SLSIs,where areas covered with AP generate tight interlayer adhesion and areas exposed enable interlayer conductivity between pairing CMBs（with a maximum strain of 52%and stress of 360 k Pa）.To study the electrical performance of stretchable CMBs conductors and SLSIs,we have tested and optimized the resistance change of CMBs conductors under strain（best performance reached a stretchability of 50%with R/R0≈11.27）.It was proved that CMBs can reduce the negative effect of AP on the electrical performance of SLSIs.And SLSIs with Au thickness of 60 nm and increasing CMBs’heights（from 30μm,70μm and 120μm）can offer reliable interconnecting performance.After optimization,the initial resistance of SLSI was as low as 19.9Ω and the stretchability of SLSIs was very promising（strain 34.0%,R/R0≈4.5）.Especially,our SLSIs offers highly stable performance even after 500 stretching cycles.Mechanism of the electrical conductivity under strain of SLSIs based on a phenomenological model was studied,it reflects how Au micro-cracks influence resistance change of the conductor.FEA modeling on SLSIs with different CMBs parameters revealed how CMBs contribute to the navigation of strain distribution.With further observation on morphology of detached CMBs conductor,the failure mode of SLSIs was predicted:a throughout crack of Au thin-film generated at the margin of connection due to concentrated strain may be the cause of the electrical failure of the SLSIs at higher strain.This explains the loss of electrical conductivity while the joints were still mechanically intact.To design and construct the all-stretchable electronic system,we further adopted SLSIs as stretchable interconnections to connect a stretchable strain sensor and a stretchable supercapacitor onto the stretchable circuit which resulted in an all-stretchable data acquisition platform.With the assistance of the all-stretchable DAQ platform,output current signals can first be recorded simultaneously with information of pain severity provided by patients.Then,through the calculation ofΔR/Δεof the applied strain sensor,the correlation between wrist bending range and pain severity can be established.When stretched at 15%strain with different moving rates,the platform can still provide reliable current data acquisition functionality. |
