Recent advances in mechanics and materials provide routes to integrated circuits that can offer the electrical properties of conventional, rigid wafer-based technologies but with the ability to be stretched, compressed, twisted, bent, and deformed into arbitrary shapes. Inorganic and organic electronic materials in microstructured and nanostructured forms, intimately integrated with elastomeric substrates, offer particularly attractive characteristics, with realistic pathways to sophisticated embodiments. Here, we review these strategies and describe applications of them in systems ranging from electronic eyeball cameras to deformable light-emitting displays. We conclude with some perspectives on routes to commercialization, new device opportunities, and remaining challenges for research.

http://science.sciencemag.org/content/sci/327/5973/1603.full.pdf

Materials and mechanics for stretchable electronics

Transparent films of carbon nanotubes can accommodate strains of up to 150% and demonstrate conductivities as high as 2,200 S cm−1 in the stretched state.

Skin-like pressure and strain sensors based on transparent elastic films of carbon nanotubes

There is a growing demand for flexible and soft electronic devices. In particular, stretchable, skin-mountable, and wearable strain sensors are needed for several potential applications including personalized health-monitoring, human motion detection, human-machine interfaces, soft robotics, and so forth. This Feature Article presents recent advancements in the development of flexible and stretchable strain sensors. The article shows that highly stretchable strain sensors are successfully being developed by new mechanisms such as disconnection between overlapped nanomaterials, crack propagation in thin films, and tunneling effect, different from traditional strain sensing mechanisms. Strain sensing performances of recently reported strain sensors are comprehensively studied and discussed, showing that appropriate choice of composite structures as well as suitable interaction between functional nanomaterials and polymers are essential for the high performance strain sensing. Next, simulation results of piezoresistivity of stretchable strain sensors by computational models are reported. Finally, potential applications of flexible strain sensors are described. This survey reveals that flexible, skin-mountable, and wearable strain sensors have potential in diverse applications while several grand challenges have to be still overcome.

Stretchable, Skin-Mountable, and Wearable Strain Sensors and Their Potential Applications: A Review

Human skin is a remarkable organ. It consists of an integrated, stretchable network of sensors that relay information about tactile and thermal stimuli to the brain, allowing us to maneuver within our environment safely and effectively. Interest in large-area networks of electronic devices inspired by human skin is motivated by the promise of creating autonomous intelligent robots and biomimetic prosthetics, among other applications. The development of electronic networks comprised of flexible, stretchable, and robust devices that are compatible with large-area implementation and integrated with multiple functionalities is a testament to the progress in developing an electronic skin (e-skin) akin to human skin. E-skins are already capable of providing augmented performance over their organic counterpart, both in superior spatial resolution and thermal sensitivity. They could be further improved through the incorporation of additional functionalities (e.g., chemical and biological sensing) and desired properties (e.g., biodegradability and self-powering). Continued rapid progress in this area is promising for the development of a fully integrated e-skin in the near future.

25th Anniversary Article: The Evolution of Electronic Skin (E-Skin): A Brief History, Design Considerations, and Recent Progress

Skin plays an important role in mediating our interactions with the world. Recreating the properties of skin using electronic devices could have profound implications for prosthetics and medicine. The pursuit of artificial skin has inspired innovations in materials to imitate skin's unique characteristics, including mechanical durability and stretchability, biodegradability, and the ability to measure a diversity of complex sensations over large areas. New materials and fabrication strategies are being developed to make mechanically compliant and multifunctional skin-like electronics, and improve brain/machine interfaces that enable transmission of the skin's signals into the body. This Review will cover materials and devices designed for mimicking the skin's ability to sense and generate biomimetic signals.

Pursuing prosthetic electronic skin.

A number of materials have been explored for their use as artificial muscles Among these, dielectric elastomers (DEs) appear to provide the best combination of properties for true muscle-like actuation DEs behave as compliant capacitors, expanding in area and shrinking in thickness when a voltage is applied Materials combining very high energy densities, strains, and efficiencies have been known for some time To date, however, the widespread adoption of DEs has been hindered by premature breakdown and the requirement for high voltages and bulky support frames Recent advances seem poised to remove these restrictions and allow for the production of highly reliable, high-performance transducers for artificial muscle applications

Advances in dielectric elastomers for actuators and artificial muscles.

A new compliant electrode-based on silver nanowire-polymer composite has been developed The composite electrode has low sheet resistance (as low as 10 Ω/sq), remains conductive (10(2) -10(3) Ω/sq) at strains as high as 140%, and can support Joule heating The combination of the composite and a bistable electroactive polymer produces electrically-induced, large-strain actuation and relaxation, reversibly without the need of mechanical programming

Compliant Silver Nanowire‐Polymer Composite Electrodes for Bistable Large Strain Actuation

We have studied the electrical and optical properties of transparent and conductive nanotube thin films subjected to extremely large strains, both isotropic and anisotropic. The films maintain electrical conductivity for strains up to 700% and the eventual loss of conductivity is due primarily to the buildup of cracks in the nanotube films. We also measured the change in optical transmittance and explain the observed haziness of the films by considering the micrometer sized cluster. This study of transparent nanotube films as stretchable electrodes is crucial for many applications, in particular, for medical implantation of electronic devices.

Highly stretchable, conductive, and transparent nanotube thin films

Prestrain provides high actuation performance in dielectric elastomers (DEs) but increases the bulk, mass, and fatigue of the resulting actuators. Based on our experiments on prestrain-locked interpenetrating polymer films and the model developed by Zhao and Suo, materials with a certain stress–strain relationship should be capable of high strain without prestrain by suppressing electromechanical instability (EMI). Here, we report the synthesis of an acrylic elastomer capable of achieving high actuation performance without prestrain. DE films were directly fabricated by ultraviolet curing of precursors comprising a mixture of acrylate comonomers. Varying the amount of crosslinker comonomer in the precursor allowed us to tune the stress–strain relationship and completely suppress EMI while maintaining high strain performance. Addition of plasticizing agents increased strain sensitivity. The result is a new DE, synthesized from scratch, capable of high actuation strain (>100%), high energy density (>1 J g−1), and good temperature and frequency response without requiring prestretching. The material can be fabricated using conventional coating techniques and the process can allow for high volume throughput of stacked DE actuators. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013

Synthesizing a new dielectric elastomer exhibiting large actuation strain and suppressed electromechanical instability without prestretching

Thermoplastic poly(tert-butyl acrylate) (PTBA) is reported as an electroactive polymer that is rigid at ambient conditions and turns into a dielectric elastomer above a transition temperature. In the rubbery state, a PTBA thin film can be electrically actuated to strains up to 335% in area expansion. The calculated actuation pressure is 3.2 MPa. The actuation is made bistable by cooling to below glass transition temperature. The PTBA represents the bistable electroactive polymer (BSEP) that can be actuated to various largely strained, rigid shapes. The application of the BSEP for refreshable Braille display, an active tactile display, is also demonstrated.

Paul Brochu

Papers

Advances in dielectric elastomers for actuators and artificial muscles.

Compliant Silver Nanowire‐Polymer Composite Electrodes for Bistable Large Strain Actuation

Highly stretchable, conductive, and transparent nanotube thin films

Synthesizing a new dielectric elastomer exhibiting large actuation strain and suppressed electromechanical instability without prestretching

Large-strain, rigid-to-rigid deformation of bistable electroactive polymers