We propose a 3D-printable soft, stretchable, and transparent hydrogel-elastomer device that is able to detect simulated ‘nerve’ signals. The signal is passed to a conductive hydrogel electrode through a non-contact method of capacitive coupling through polydimethylsiloxane (PDMS). We demonstrate that the device is able to detect sinusoidal waveforms passed through a simulated ‘nerve’ made from conductive hydrogel over a range of frequencies (1 kHz – 1 MHz). Analysis of signal detection showed a correlation to the electrode contact area and a Vin/Vout of larger than 10%. This provides the framework for the future development of a soft, 3D-printable, capacitive coupling device that can be used as a cuff electrode for detecting peripheral nerve signals.

In the emerging field of soft machines, large deformation of soft materials is harnessed to provide functions such as regulating flow in microfluidics, shaping light in adaptive optics, harvesting energy from ocean waves, and stretching electronics to interface with living tissues. Soft materials, however, do not provide all of the requisite functions; rather, soft machines are mostly hybrids of soft and hard materials. In addition to requiring stretchable electronics, soft machines often use soft materials that can deform in response to stimuli other than mechanical forces. Dielectric elastomers deform under a voltage. Hydrogels swell in response to changes in humidity, pH, temperature, and salt concentration. How does mechanics meet geometry, chemistry, and electrostatics to generate large deformation? How do molecular processes affect the functions of transducers? How efficiently can materials convert energy from one form to another? These questions are stimulating intriguing and useful advances in mechanics. This review highlights the mechanics that enables the creation of soft machines.

We use instrumented indentation to characterize the mechanical and transport behavior of a pH-sensitive hydrogel in various aqueous buffer solutions. In the measurement, an indenter is pressed to a fixed depth into a hydrogel disk and the load on the indenter is recorded as a function of time. By analyzing the load–relaxation curve using the theory of poroelasticity, the elastic constants of the hydrogel and the diffusivity of water in the gel can be evaluated. We investigate how the pH and ionic strength of the buffer solution, the hydrogel cross-link density, and the density of functional groups on the polymer backbone affect the properties of the hydrogel. This work demonstrates the utility of indentation techniques in the characterization of pH-sensitive hydrogels.

This work uses a method based on indentation to characterize a polydimethylsiloxane (PDMS) elastomer submerged in an organic solvent (decane, heptane, pentane, or cyclohexane). An indenter is pressed into a disk of a swollen elastomer to a fixed depth, and the force on the indenter is recorded as a function of time. By examining how the relaxation time scales with the radius of contact, one can differentiate the poroelastic behavior from the viscoelastic behavior. By matching the relaxation curve measured experimentally to that derived from the theory of poroelasticity, one can identify elastic constants and permeability. The measured elastic constants are interpreted within the Flory–Huggins theory. The measured permeability indicates that the solvent migrates in PDMS by diffusion, rather than by convection. This work confirms that indentation is a reliable and convenient method to characterize swollen elastomers.

In a lithium-ion battery, both electrodes are atomic frameworks that host mobile lithium ions. When the battery is being charged or discharged, lithium ions diffuse from one electrode to the other. Such an insertion reaction deforms the electrodes and may cause the electrodes to crack. This paper uses fracture mechanics to determine the critical conditions to avert insertion-induced cracking. The method is applied to cracks induced by the mismatch between phases in LiFePO4.

For a flexible electronic device integrating inorganic materials on a polymer substrate, the polymer can deform substantially, but the inorganic materials usually fracture at small strains. This paper describes an approach to make such a device highly stretchable. A polyimide substrate is first coated with a thin layer of an elastomer, on top of which SiNx islands are fabricated. When the substrate is stretched to a large strain, the SiNx islands remain intact. Calculations confirm that the elastomer reduces the strain in the SiNx islands by orders of magnitude.

In a previous paper, we have demonstrated that a microcrystalline copper film well bonded to a polymer substrate can be stretched beyond 50% without cracking. The film eventually fails through the coevolution of necking and debonding from the substrate. Here we report much lower strains to failure (approximately 10%) for polymer-supported nanocrystalline metal films, the microstructure of which is revealed to be unstable under mechanical loading. We find that strain localization and deformation-associated grain growth facilitate each other, resulting in an unstable deformation process. Film/substrate delamination can be found wherever strain localization occurs. Therefore, we propose that three concomitant mechanisms are responsible for the failure of a plastically deformable but microstructurally unstable thin metal film: strain localization at large grains, deformation-induced grain growth, and film debonding from the substrate.

A thin film of a stiff material, patterned as a serpentine on a flat elastomeric substrate, can elongate substantially when the substrate is pulled. We showed that the film elongates by twisting out of plane, accommodated by the compliance of the substrate and the pattern of the film. Consequently, large elongations of the substrate induce small strains in the film, even when the width of the film is much larger than its thickness. Such a wide serpentine, or other compliant patterns of stiff materials, can serve as a platform on which electronic circuits can be fabricated. This architecture will make electronics elastically stretchable.

Thin metal films deposited on elastomeric substrates can remain electrically conducting at tensile strains up to ~∼00%. We recently used finite-element simulation to explore the rupture process of a metal film on an elastomer. The simulation predicted the highest stretchability on stiff elastomeric substrates [1]. We now report experiments designed to verify this prediction. A ∼15-μm thick silicone elastomer layer with Young's modulus E ∼ 160 MPa is deposited on a 1mm thick membrane of polydimethylsiloxane (PDMS), a silicone elastomer with E ∼3 MPa. Metal stripes consisting of 25-nm thick gold (Au) film sandwiched between two 5-nm thick chromium (Cr) adhesion layers are fabricated either on top of the stiff layer spun onto the soft membrane substrate, or are encapsulated at the interface between the two elastomers. Encapsulated gold films remain electrically conducting beyond 40% strain. But conductors deposited on top of stiff elastomer lose conduction at strains of 3-8%. These results suggest that, in addition to the stiffness of the elastomeric substrate, the initial microstructure of the metal film plays a role in determining its stretchability.

Stretchable, elastic metal interconnects are a key to the fabrication of 3-D conformal circuits and electrotextiles. The basic concept for reversibly stretchable, elastic metallization is a corrugated stripe of thin-film metal that is expanded by stretching. The maximum elongation is reached when the stripe is stretched flat. We prepared wavy metal stripes by evaporating gold onto pre-stretched strips of the elastomer, poly-dimethyl siloxane (PDMS). We experimented with gold metal line width and thickness and substrate elongation. We measured the film structure, amplitude, and wavelength, as well as electrical resistance in relaxed and various stretched states. So far we have reached elastic strains of 15% while maintaining the initial resistance and 80% with a rise in the resistance. We discovered a rich macroscopic and microscopic film morphology. Presented are the fabrication, electro-mechanical performance, and data on the film structure of these wavy metal interconnects.

3-D displays, sensor skins, mechatronic structures, and e-textiles will rely on deformable and stretchable electronic circuits. It is likely that such circuits will be made of rigid semiconductor islands interconnected with one-time deformable or even elastic metallization. However, free-standing metal films fracture at tensile strains in the order of one percent, well short of the approximately ten-percent extension needed for deformable circuits. We have discovered that flat metal lines made on an elastomeric substrate can be stretched reversibly by ten percent without losing electrical conduction. While this phenomenon of “super-elastic” conductive films is as of yet unexplained, it appears to originate in the diversion of mechanical strain around cracks in the film and through the elastomer substrate. We fabricated 1mm thick poly-dimethylsiloxane (PDMS) membranes with up to 50-nm thick, 1-mm wide gold lines deposited by electron beam evaporation. Then we evaluated the structure of the gold films by optical and scanning electron microscopy, and measured the electro-mechanical characteristics in a strain tester, with contact electrodes applied to the film. We find that 50 nm thick lines retain their electrical conduction up to 30 percent strain. Also, when the tensile strain is cycled between 0 and 10 percent, the electrical resistance in the stressed and relaxed states are reproducible. We will describe substrate and conductor preparation, and their structural and electro-mechanical properties.

In the most fundamental approach, e-Textile circuits will be made by weaving component fibers into circuits. The weaving pattern will determine the circuit function. A key requirement of such e-Textile circuits is reliable electrical contact between fibers. Contacts which rely only on the pressure between fibers are preferred since they preserve the drapability of real fabrics. Since thin-film device fabrication technology is planar, the component fibers, made by the slit-film technique, are flat. Thus a slight edge-to-edge curvature (with a radius of curvature as large as 500mm) can either prevent or promote electrical contact. Using fibers with thin-film transistors of amorphous silicon, we study the processes that produce the desired fiber curvature. A layer of stressed silicon nitride is used to create the curvature. The stress in this layer can be controlled by the deposition parameters. We present successful fabrication of curved fibers with vastly improved electrical contact. We also present electrical characterization of woven transistor circuits

The textile industry uses weaving to create very large quantities of fabric very quickly. The goal of our research is to use this well established technology to create complex large-area circuits quickly and efficiently. In our laboratory we have previously shown that amorphous silicon (a-Si) can be used to make thin-Film transistors (TFTs) on Kapton (a highly temperature-resistant polyimide from DuPont). We also previously showed that these TFTs can survive mechanical loads. A process has been designed to make “TFT fibers” by fabricating a-Si TFTs on Kapton. A special TFT geometry has also been developed. The structure consists of 3 large gold contact pads – one for each terminal of the TFT – running along the fiber. These contact pads allow connections to be made between TFT fibers using conductor fibers – Kapton fibers coated only with gold. The TFT fabrication process is based on a low temperature (150°C) Plasma Enhanced Chemical Vapor Deposition (PECVD) process. The TFTs are fabricated on a Kapton sheet from which flat fibers are made by the slit film technique. So far the best method for cutting a Kapton sheet into fibers has been plasma etching. We will describe the electronic characteristics of these TFTs as well as the electrical characteristics of the contacts between TFT fibers.

The electro-mechanical response of thin gold layers evaporated onto silicone substrates is reported. Gold layers are prepared either thin and flat or thin and wavy on the compliant substrate. The electrical resistance of gold/silicone stripes is measured and analyzed during tensile deformation. For a 100-nm thick gold layer evaporated on a 1-mm thick silicone membrane, we have observed electrical continuity up to ∼ 22 % strain. This maximum strain decreases when the gold layer thickness is raised.

There is a growing interest in the application of large area electronics on curved surfaces. One approach towards realizing this goal is to fabricate circuits on planar substrates of thin plastic or metal foil, which are subsequently deformed into arbitrary shapes. The problem that we consider here is the deformation of substrates into a spherical shape, where the strain is determined by geometry and cannot be reduced by simply using a thinner substrate. The goal is to achieve permanent, plastic deformation in the substrates, without exceeding fracture or buckling limits in the device materials.

Our experiments consist of the planar fabrication of amorphous silicon device structures onto stainless steel or Kapton® polyimide substrates, followed by permanent deformation into a spherical shape. We will present empirical experiments showing the dependence of the results on the island/line size of the device materials and the deformation temperature. We have successfully deformed Kapton® polyimide substrates with 100 [.proportional]m wide amorphous silicon islands into a one steradian spherical cap, which subtends 66 degrees, without degradation of the silicon. This work demonstrates the feasibility of building semiconductor devices on plastically deformed substrates despite a 5% average biaxial strain in the substrate after deformation.