Christoph Keplinger

Stretchable, Transparent, Ionic Conductors

Hydrogel Stretch A range of stretchable, conductive materials can be made either by making an electrical conductor more stretchable or by adding an electrical conductor to a stretchable material. Keplinger et al. (p. 984; see the Perspective by Rogers) have added to the possibilities of an alternative stretchable ionic conductor based on a hydrogel material used to make deformable devices that are fully transparent to light over the visible spectrum and that can withstand high voltages and high frequencies. Stretchable ionic gels are fabricated into transparent actuators and loudspeakers. [Also see Perspective by Rogers] Existing stretchable, transparent conductors are mostly electronic conductors. They limit the performance of interconnects, sensors, and actuators as components of stretchable electronics and soft machines. We describe a class of devices enabled by ionic conductors that are highly stretchable, fully transparent to light of all colors, and capable of operation at frequencies beyond 10 kilohertz and voltages above 10 kilovolts. We demonstrate a transparent actuator that can generate large strains and a transparent loudspeaker that produces sound over the entire audible range. The electromechanical transduction is achieved without electrochemical reaction. The ionic conductors have higher resistivity than many electronic conductors; however, when large stretchability and high transmittance are required, the ionic conductors have lower sheet resistance than all existing electronic conductors.

25th Anniversary Article: A Soft Future: From Robots and Sensor Skin to Energy Harvesters

Scientists are exploring elastic and soft forms of robots, electronic skin and energy harvesters, dreaming to mimic nature and to enable novel applications in wide fields, from consumer and mobile appliances to biomedical systems, sports and healthcare. All conceivable classes of materials with a wide range of mechanical, physical and chemical properties are employed, from liquids and gels to organic and inorganic solids. Functionalities never seen before are achieved. In this review we discuss soft robots which allow actuation with several degrees of freedom. We show that different actuation mechanisms lead to similar actuators, capable of complex and smooth movements in 3d space. We introduce latest research examples in sensor skin development and discuss ultraflexible electronic circuits, light emitting diodes and solar cells as examples. Additional functionalities of sensor skin, such as visual sensors inspired by animal eyes, camouflage, self-cleaning and healing and on-skin energy storage and generation are briefly reviewed. Finally, we discuss a paradigm change in energy harvesting, away from hard energy generators to soft ones based on dielectric elastomers. Such systems are shown to work with high energy of conversion, making them potentially interesting for harvesting mechanical energy from human gait, winds and ocean waves.

Harnessing snap-through instability in soft dielectrics to achieve giant voltage-triggered deformation

A soft dielectric membrane is prone to snap-through instability. We present theory and experiment to show that the instability can be harnessed to achieve giant voltage-triggered deformation. We mount a membrane on a chamber of a suitable volume, pressurize the membrane into a state near the verge of the instability, and apply a voltage to trigger the snap without causing electrical breakdown. For an acrylic membrane we demonstrate voltage-triggered expansion of area by 1692%, far beyond the largest value reported in the literature. The large expansion can even be retained after the voltage is switched off.

A highly stretchable autonomous self-healing elastomer

It is a challenge to synthesize materials that possess the properties of biological muscles-strong, elastic and capable of self-healing. Herein we report a network of poly(dimethylsiloxane) polymer chains crosslinked by coordination complexes that combines high stretchability, high dielectric strength, autonomous self-healing and mechanical actuation. The healing process can take place at a temperature as low as -20 °C and is not significantly affected by surface ageing and moisture. The crosslinking complexes used consist of 2,6-pyridinedicarboxamide ligands that coordinate to Fe(III) centres through three different interactions: a strong pyridyl-iron one, and two weaker carboxamido-iron ones through both the nitrogen and oxygen atoms of the carboxamide groups. As a result, the iron-ligand bonds can readily break and re-form while the iron centres still remain attached to the ligands through the stronger interaction with the pyridyl ring, which enables reversible unfolding and refolding of the chains. We hypothesize that this behaviour supports the high stretchability and self-healing capability of the material.

Dielectric Elastomer Generators: How Much Energy Can Be Converted?

Dielectric elastomers are being developed as generators to harvest energy from renewable sources, such as human movements and ocean waves. We model a generator as a system of two degrees of freedom, represented on either the stress-stretch plane or the voltage-charge plane. A point in such a plane represents a state of the generator, a curve represents a path of operation, a contour represents a cycle of operation, and the area enclosed by the contour represents the energy of conversion per cycle. Each mechanism of failure is represented by a curve in the plane. The curves of all the known mechanics of failure enclose the region of allowable states. The area of this region defines the maximum energy of conversion. This study includes the following mechanisms of failure: material rupture, loss of tension, electrical breakdown, and electromechanical instability. It is found that natural rubber outperforms VHB elastomer as a generator at strains less than 15%. Furthermore, by varying material parameters, energy of conversion can be increased above 1.0 J/g.

Röntgen’s electrode-free elastomer actuators without electromechanical pull-in instability

Electrical actuators made from films of dielectric elastomers coated on both sides with stretchable electrodes may potentially be applied in microrobotics, tactile and haptic interfaces, as well as in adaptive optical elements. Such actuators with compliant electrodes are sensitive to the pull-in electromechanical instability, limiting operational voltages and attainable deformations. Electrode-free actuators driven by sprayed-on electrical charges were first studied by Röntgen in 1880. They withstand much higher voltages and deformations and allow for electrically clamped (charge-controlled) thermodynamic states preventing electromechanical instabilities. The absence of electrodes allows for direct optical monitoring of the actuated elastomer, as well as for designing new 3D actuator configurations and adaptive optical elements.

Flexible ferroelectret field-effect transistor for large-area sensor skins and microphones

Ferroelectrets generate an electric field large enough to modulate the conductance of the source-drain channel of a thin-film field-effect transistor. Integrating a ferroelectret with a thin-film transistor produces a ferroelectret field-effect transistor. The authors made such transistors by laminating cellular polypropylene films and amorphous silicon thin-film transistors on polyimide substrates. They show that these ferrroelectret field-effect transistors respond in a static capacitive or dynamic piezoelectric mode. A touch sensor, a pressure-activated switch, and a microphone are demonstrated. The structure can be scaled up to large-area flexible transducer arrays, such as roll-up steerable compliant sensor skin.

Flexible-foam-based capacitive sensor arrays for object detection at low cost

Polymer foams are used in the automotive and construction industries for thermal insulation, vibration attenuation, and pressure absorption, due to their lightweight structure, thermal characteristics and low manufacturing costs. These foams have higher elasticity in their cross sections than bulk polymers, which makes them the preferred mount for capacitive sensor arrays. The authors describe a flexible pressure-sensitive surface mounted on packaging foam. The elastic properties of the foam are presented along with the sensor array’s weight sensitivity. The authors illustrate an inventory management application where objects on display can be detected through their weights.

Capacitive extensometry for transient strain analysis of dielectric elastomer actuators

Dielectric elastomer actuators (DEAs) are promising structural units for artificial muscles and robotic elements. Understanding the safe and failure mode regimes of such DEAs is essential for controlling the actuator. We develop an electrical characterization technique for obtaining information on the transient strain in the actuator and analyze the behavior of the actuator in safe and failure operation regimes, in particular in the pull-in instability mode. Additionally, the technique allows the strain-dependent measurement of the electrode resistance. The current measurement based technique can be also applied for actuator control with feedback loops.

Method for measuring energy generation and efficiency of dielectric elastomer generators

Dielectric elastomer generators convert mechanical into electrical energy at high energy density, showing promise for large and small scale energy harvesting. We present an experiment to monitor electrical and mechanical energy flows separately and show the cycle of energy conversion in work-conjugate planes. A specific electrical energy generated per cycle of 102mJ/g, at a specific average power of 17mW/g, is demonstrated with an acrylic elastomer in a showcase generation cycle. The measured mechanical to electrical energy conversion efficiency is 7.5%. The experiment may be used to assess the aptitude of specifically designed elastomers for energy harvesting.