What is our approach to make stretchable electronics?

We have integrated several approaches to attain the maximum stretching while maintaining mechanical integrity. As our first approach, we reported the design and fabrication of an all silicon based network of hexagonal islands connected through spiral springs to form an ultra-stretchable arrangement for complete compliance to highly asymmetric shapes. Several design parameters are considered and their validation is carried out through finite element analysis. The fabrication process is based on conventional microfabrication techniques and the measured stretchability is more than 1000% for single spirals and area expansions as high as 30 folds in arrays. The reported method can provide ultra-stretchable and adaptable electronic systems for distributed network of high-performance macro-electronics especially useful for wearable electronics and bio-integrated devices. 

J. P. Rojas, A. Carreno, I. G. Foulds, M. M. Hussain*, “Design and Characterization of Ultra-Stretchable Monolithic Silicon Fabric”, Appl. Phys. Lett. 105, 154101 (2014).

What is the concept of unit cell to make stretchable electronics?

We have introduced the concept of a unit-cell, which mimics the mechanical behavior of full complex arrays. The elongation of each spiral changes linearly with prescribed displacement and depends on spiral's position in the respective unit-cell. Our results show that stress/strains linearly change with prescribed displacement. The elongation and stress of each of the spirals reduce when more number of spirals are added and stretched under the same displacement. The square array with the cross-linked spiral has advantages of improved connectivity, areal density, and mechanical robustness by reducing the stress from ∼1570 MPa to ∼1100 MPa. We have verified our FEM results by using the 3D printed PLA specimens. These PLA specimens were stretched with the prescribed displacement of 10 cm. Comparison of our FEM and experimental results was found to be in good agreement with one another. The results of this study provide better understanding of spiral based stretchable interconnects which can lead to efficient designs for wearable electronics and devices.

Mechanical response of spiral interconnect arrays for highly stretchable electronics

N. Qaiser, S. M. Khan, M. Nour,  M. U. Rehman, J. P. Rojas, M. M. Hussain

Appl. Phys. Lett. 111, 214102 (2017); https://doi.org/10.1063/1.5007111

Can we make stretchable silicon platform, which can be folded too?

Yes, a CMOS-compatible integral strategy for stretching and folding thick silicon is described here. We decouple the effective bending radius of the substrate from the device layer thickness. We can then tune the effective bending radius for most wearable applications. We used the finite element analysis to validate the spring design and then we demonstrated experimentally that the spring expands to 490%, which translates to a 5.6-fold gain in area for this particular flexible device assembly. More importantly, the spring design allows the islands to fold elastically on top of one another, which makes our approach a promising replacement for silicon vias. Our monocrystalline siliconplatform, unlike other flexing strategies, is sufficiently robust to be compatible with flip-chip bonding. Given the mechanical stability of the islands and interconnectors, this strategy has the advantage that the springs can be patterned before or after device fabrication. Mechanical cleavage adds no trace impurities or additional layers to the fabricated devices. Consequently, no further design considerations are required. Our approach with islands and springs has the potential to be used in applications in compliant electronics.

Stretchable and foldable silicon-based electronics

A. C. Cavazos Sepulveda, M. S. Diaz Cordero, A. A. A. Carreño, J. M. Nassar, M. M. Hussain

Appl. Phys. Lett. 110, 134103 (2017); https://doi.org/10.1063/1.4979545