Stretchable Electronics: Devices, Materials, and Mechanics

Stretchable electronics combines the electronic performance of conventional wafer-based semiconductor devices and mechanical properties of a rubber band, and thus can have very broad applications that are impossible for hard, planar integrated circuits that exist today. Examples range from surgical and diagnostic implements that integrate with the human body to provide advanced therapeutic capabilities, to structural health monitors and inspection systems for civil engineering. In this research thrust, we develop materials and devices for stretchable electronics, we also study fundamental mechanics to further our understanding of the underlying physics and to guide the design and opimization. 

Bio-inspired Bug Eye Cameras (Artificial Compound Eye)

In arthropods, evolution has created a remarkably sophisticated class of imaging systems, with a wide-angle field of view, low aberrations, high acuity to motion and an infinite depth of field. A challenge in building digital cameras with the hemispherical, compound apposition layouts of arthropod eyes is that essential design requirements cannot be met with existing planar sensor technologies or conventional optics. In this study, we combined elastomeric compound optical elements with deformable arrays of thin silicon photodetectors into integrated sheets that can be elastically transformed from the planar geometries in which they are fabricated to hemispherical shapes for integration into apposition cameras.

References:

• Y. M. Song+, Y. Xie+, V. Malyarchuk+, J. Xiao+, I. Jung, K.-J. Choi, Z. Liu, H. Park, C. Lu, R.-H. Kim, R. Li, K. B. Crozier, Y. Huang, and J. A. Rogers, Digital Cameras With Designs Inspired By the Arthropod Eye, Nature 497, 95–99 (2013)  

Highlighted on the cover of Nature Magazine. 

Tunable Eyeball Cameras With Zoom

In our previously developed bio-inspired eyeball cameras, photodetectors are distributed on curvilinear surfaces to match the strongly nonplanar image surfaces (i.e., Petzval surfaces) that form with simple lenses. Although these systems provide advantages compared to those with conventional, planar designs, their fixed detector curvature renders them incompatible with changes in the Petzval surface that accompany variable zoom achieved with simple lenses.Here, we put stretchable photodetector arrays on thin elastomeric membranes, capable of reversible deformation into hemispherical shapes with radii of curvature that can be adjusted dynamically, via hydraulics. Combining this type of detector with a similarly tunable, fluidic planoconvex lens yields a hemispherical camera with variable zoom and excellent imaging characteristics.

References:

• I. Jung, J. Xiao, V. Malyarchuk, C. Lu, M. Li, Z. Liu, J. Yoon, Y. Huang, and J. A. Rogers, Dynamically tunable hemispherical electronic eye camera system with adjustable zoom capability, Proc. Natl. Acad. Sci. USA 108, 1788-1793 (2011) (featured on the cover of PNS magazine)

• C. Lü, M. Li, J. Xiao*, I. Jung, J. Wu, Y. Huang, K.-C. Hwang, and J.A. Rogers, Mechanics of tunable hemispherical electronic eye camera systems that combine rigid device elements with soft elastomers, Journal of Applied Mechanics-Transactions of the ASME (accepted)

• S. Wang, J. Xiao, J. Song, H. C. Ko, K.-C. Hwang, Y. Huang, and J. A. Rogers, Mechanics of curvilinear electronics, Soft Matter 6, 5757–5763 (2010)
• D.-H. Kim, J. Xiao, J. Song, Y. Huang and J. A. Rogers, Stretchable, Curvilinear Electronics Based On Inorganic Materials, Advanced Materials 22, 2108–2124 (2010)
• S. Wang, J. Xiao, I. Jung, J. Song, H. C. Ko, M. P. Stoykovich, Y. Huang, K.-C. Hwang and J. A. Rogers, Mechanics of Hemispherical Electronics, Appl. Phys. Lett. 95, 181912 (2009).
• J. Song, Y. Huang, J. Xiao, S. Wang, K.C. Hwang, H.C. Ko, D.H. Kim, M.P. Stoykovich, and J.A. Rogers, Mechanics of noncoplanar mesh design for stretchable electronic circuits, Journal of Applied Physics 105, 123516 (2009).
• H. C. Ko, M. P. Stoykovich, J. Song, V. Malyarchuk, W. M. Choi, C.-J. Yu, J. B. Geddes, J. Xiao, S. Wang, Y. Huang, and J. A. Rogers, A Hemispherical Electronic Eye Camera Based on Compressible Silicon Optoelectronics. Nature 454, 748-753 (2008). (featured on the cover of nature magazine)

Flexible, Ultrathin Sensors for Biomedical Applications

In biomedical practices, many electronic devices are used to perform diagnosis, treament and other functions. However, electronics are usually very hard, and cannot comply with the complex geometries and extreme deformabilities of biological tissues. This incompatibility greatly limits the application of electronics in biomedical areas (one extreme example is brain computer interface). We developed electronics that are capable of intimate, non-invasive integration with the soft, curvilinear surfaces of biological tissues, which offer important opportunities for diagnosing and treating disease and for improving interfaces between electronics and biological tissues. As shown on the left are neural sensors on cat's brains (top left and right) and cardiac sensor on pig's heart, for electrophysiology measurement.

References:

• Viventi et al., Flexible, Foldable, Actively Multiplexed, High-Density Electrode Array for Mapping Brain Activity in vivo, Nature Neuroscience 14, 1599–1605 (2011) 

• Kim et al., Dissolvable Films of Silk Fibroin for Ultrathin Conformal Bio-Integrated Electronics, Nature Materials 9, 511-517 (2010) (featured on the cover of Neural Sensors)

• Viventi et al., A Conformal, Bio-interfaced Class of Silicon Electronics for Mapping Cardiac Electrophysiology, Science Translational Medicine 2, 24ra22 (2010). (featured on the cover of Science Translational Medicine)

Stretchable Inorganic LEDs

Inorganic light-emitting diodes and photodetectors represent important, established technologies for solid-state lighting, digital imaging and many other applications. Eliminating mechanical and geometrical design constraints imposed by the supporting semiconductor wafers can enable alternative uses in areas such as biomedicine and robotics. We developed systems that consist of arrays of interconnected, ultrathin inorganic light-emitting diodes and photodetectors configured in mechanically optimized layouts on unusual substrates. As shown on the left are LEDs poked by a pencil tip (top left), twisted to different angles (top right), LEDs on a thread sutured underneath the skin (bottom left) and LEDs and photodetectors (PDs) integrated on a glove for detecting distance (bottom right). 

References:

• R.-H. Kim, D.-H. Kim, J. Xiao, B. H. Kim, S.-I. Park, B. Panilaitis, R. Ghaffari, J. Yao, M. Li, Z. Liu, V. Malyarchuk, D. G. Kim, A.-P. Le, R. G. Nuzzo, D. L. Kaplan, F. G. Omenetto, Y. Huang, Z. Kang, and J. A. Rogers, Waterproof AlInGaP optoelectronics on stretchable substrates with applications in biomedicine and robotics, Nature Materials 9, 929-937 (2010)

Flexible Solar Cells

The high natural abundance of silicon, together with its excellent reliability and good efficiency in solar cells, suggest its continued use in production of solar energy, on massive scales, for the foreseeable future. Although organics, nanocrystals, nanowires and other new materials hold significant promise, many opportunities continue to exist for research into unconventional means of exploiting silicon in advanced photovoltaic systems.Here,we developed modules that use large-scale arrays of silicon solar microcells created from bulk wafers and integrated in diverse spatial layouts on foreign substrates by transfer printing. The resulting devices can offer useful features, including high degrees of mechanical flexibility, user-definable transparency and ultrathin-form-factor microconcentrator designs.

References:

• Baca et al., Compact monocrystalline silicon solar modules with high voltage outputs and mechanically flexible designs, Energy & Environmental Science 3, 208-211 (2010) (featured on the cover of Solar EES)
• Yoon et al., Ultrathin silicon solar microcells for semitransparent, mechanically flexible and microconcentrator  module designs. Nat. Mater. 7, 907-915 (2008). (featured on the cover of Nature Materials)

Stretchable and Compressible Conductors

 

We developed stretchable Au thin films on elastomeric substrates of polydimethylsiloxane that are designed with sinusoidal, “wavy” features of surface relief. This approach eliminates the compressive strains introduced into the thin films through commonly adopted buckling approach, and therefore can provide both high stretchability and compressibility. Such systems can be useful as stretchable conductors in electronic or sensory devices.

References:

• Xiao et al., Stretchable and Compressible Thin Films of Stiff Materials on Compliant Wavy Substrates. Appl. Phys. Lett. 93, 013109 (2008)

• Xiao et al., Analytical and Experimental Studies of the Mechanics of Deformation in a Solid with a Wavy Surface Profile. Journal of Applied Mechanics-Transactions of the ASME 77, 011003 (2010).