Electronics that are capable of intimate, non-invasive integration with the soft, curvilinear surfaces of biological tissues offer important opportunities for diagnosing and treating disease and for improving brain-machine interfaces. This paper describes a material strategy for a type of biointerfaced system that relies on ultrathin electronics supported by bioresorbable substrates of silk fibroin. Mounting such devices on tissue and then allowing the silk to dissolve and resorb initiates a spontaneous, conformal wrapping process driven by capillary forces at the biotic/abiotic interface. Specialized mesh designs and ultrathin forms for the electronics ensure minimal stresses on the tissue and highly conformal coverage, even for complex curvilinear surfaces, as confirmed by experimental and theoretical studies. In vivo, neural mapping experiments on feline animal

We have developed methods for creating microscale inorganic light-emitting diodes (LEDs) and for assembling and interconnecting them into unusual display and lighting systems. The LEDs use specialized epitaxial semiconductor layers that allow delineation and release of large collections of ultrathin devices. Diverse shapes are possible, with dimensions from micrometers to millimeters, in either flat or "wavy" configurations. Printing-based assembly methods can deposit these devices on substrates of glass, plastic, or rubber, in arbitrary spatial layouts and over areas that can be much larger than those of the growth wafer. The thin geometries of these LEDs enable them to be interconnected by conventional planar processing techniques. Displays, lighting elements, and related systems formed in this manner can offer interesting mechanical and optical properties.

Silicon is a promising candidate for electrodes in lithium ion batteries due to its large theoretical energy density. Poor capacity retention, caused by pulverization of Si during cycling, frustrates its practical application. We have developed a nanostructured form of silicon, consisting of arrays of sealed, tubular geometries that is capable of accommodating large volume changes associated with lithiation in battery applications. Such electrodes exhibit high initial Coulombic efficiencies (i.e., >85%) and stable capacity-retention (>80% after 50 cycles), due to an unusual, underlying mechanics that is dominated by free surfaces. This physics is manifested by a strongly anisotropic expansion in which 400% volumetric increases are accomplished with only relatively small (<35%) changes in the axial dimension. These experimental results and associated theoretical mechanics models demonstrate the extent to which nanoscale engineering of electrode geometry can be used to advantage in the design of rechargeable batteries with highly reversible capacity and long-term cycle stability.

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Owing to their superior mechanical and physical properties, carbon nanotubes seem to hold a great promise as an ideal reinforcing material for composites of high-strength and low-density. In most of the experimental results up to date, however, only modest improvements in the strength and stiffness have been achieved by incorporating carbon nanotubes in polymers. In the present paper, the stiffening effect of carbon nanotubes is quantitatively investigated by micromechanics methods. Especially, the effects of the extensively observed waviness and agglomeration of carbon nanotubes are examined theoretically. The Mori-Tanaka effective-field method is first employed to calculate the effective elastic moduli of composites with aligned or randomly oriented straight nanotubes. Then, a novel micromechanics model is developed to consider the waviness or curviness effect of nanotubes, which are assumed to have a helical shape. Finally, the influence of nanotube agglomeration on the effective stiffness is analyzed. Analytical expressions are derived for the effective elastic stiffness of carbon nanotube-reinforced composites with the effects of waviness and agglomeration. It is found that these two mechanisms may reduce the stiffening effect of nanotubes significantly. The present study not only provides the relationship between the effective properties and the morphology of carbon nanotube-reinforced composites, but also may be useful for improving and tailoring the mechanical properties of nanotube composites.

Stretchable electronic devices that exploit inorganic materials are attractive due to their combination of high performance with mechanical deformability, particularly for applications in biomedical devices that require intimate integration with human body. Several mechanics and materials schemes have been devised for this type of technology, many of which exploit deformable interconnects. When such interconnects are fully bonded to the substrate and/or encapsulated in a solid material, useful but modest levels of deformation (<30–40%) are possible, with reversible and repeatable mechanics. Here, the use of prestrain in the substrate is introduced, together with interconnects in narrow, serpentine shapes, to yield significantly enhanced (more than two times) stretchability, to more than 100%. Fracture and cyclic fatigue testing on structures formed with and without prestrain quantitatively demonstrate the possible enhancements. Finite element analyses (FEA) illustrates the effects of various material and geometric parameters. A drastic decrease in the elastic stretchability is observed with increasing metal thickness, due to changes in the buckling mode, that is, from local wrinkling at small thicknesses to absence of such wrinkling at large thicknesses, as revealed by experiment. An analytic model quantitatively predicts the wavelength of this wrinkling, and explains the thickness dependence of the buckling behaviors.

We present various stretchable high‐performance CMOS circuit demonstrations on unconventional substrates, such as fabric, vinyl, leather, and paper. Electronics on such substrates, especially paper, open up new and important application possibilities for electronics. Theoretical analysis reveals the underlying mechanics of these systems; electrical tests under mechanical cycling demonstrate the robustness of the designs.

Lithographically defined electrical interconnects with thin, filamentary serpentine layouts have been widely explored for use in stretchable electronics supported by elastomeric substrates. We present a systematic and thorough study of buckling physics in such stretchable serpentine microstructures, and a strategic design of serpentine layout for ultra-stretchable electrode, via analytical models, finite element method (FEM) computations, and quantitative experiments. Both the onset of buckling and the postbuckling behaviors are examined, to determine scaling laws for the critical buckling strain and the limits of elastic behavior. Two buckling modes, namely the symmetric and anti-symmetric modes, are identified and analyzed, with experimental images and numerical results that show remarkable levels of agreement for the associated postbuckling processes. Based on these studies and an optimization in design layout, we demonstrate routes for application of serpentine interconnects in an ultra-stretchable electrode that offer, simultaneously, an areal coverage as high as 81%, and a biaxial stretchability as large as ~170%.