There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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

Fundamentals, processes and applications of high-permittivity polymer–matrix composites

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.

Soft robots actuated by infl ation of a pneumatic network (a “pneu-net”) of small channels in elastomeric materials are appealing for producing sophisticated motions with simple controls. Although current designs of pneu-nets achieve motion with large amplitudes, they do so relatively slowly (over seconds). This paper describes a new design for pneu-nets that reduces the amount of gas needed for infl ation of the pneu-net, and thus increases its speed of actuation. A simple actuator can bend from a linear to a quasicircular shape in 50 ms when pressurized at Δ P = 345 kPa. At high rates of pressurization, the path along which the actuator bends depends on this rate. When infl ated fully, the chambers of this new design experience only one-tenth the change in volume of that required for the previous design. This small change in volume requires comparably low levels of strain in the material at maximum amplitudes of actuation, and commensurately low rates of fatigue and failure. This actuator can operate over a million cycles without signifi cant degradation of performance. This design for soft robotic actuators combines high rates of actuation with high reliability of the actuator, and opens new areas of application for them.

/pdf/pneumatic-networks-for-soft-robotics-that-actuate-rapidly-4epg8a3njg.pdf

Pneumatic Networks for Soft Robotics that Actuate Rapidly

An approach for creating complex structures with embedded actuation in planar manufacturing steps is presented. Self-organization and energy minimization are central to this approach, illustrated with a model based on minimization of the hyperelastic free energy strain function of a stretched elastomer and the bending elastic energy of a plastic frame. A tulip-shaped gripper structure illustrates the technological potential of the approach. Advantages are simplicity of manufacture, complexity of final structures, and the ease with which any electroactive material can be exploited as means of actuation.

Energy minimization for self-organized structure formation and actuation

The ElectroMechanical Film (EMFi) is a thin, cellular, biaxially oriented polypropylene film that can be used as an electret. Having a special voided internal structure and high resistivity, it is capable of storing large permanent charge. The charge is injected by a corona method using ∼10 kV cm−1 fields, thus creating internal electrical discharges inside the cellular structure. Films of different thickness and elasticity can be manufactured. The thickness in sensor and actuator applications is typically 30–70 μm. When metallized on both sides, EMFi is capable of measuring pressure and force changes offering large application potential in different fields of technology including microphones and also actuators. New loudspeaker panels based on EMFi are only a few millimeters thick.

ElectroMechanical Film (EMFi) — a new multipurpose electret material

The influence of the corona-charging process on the piezoelectric transducer coefficient d33 of a cellular electret film has been investigated. An increased corona voltage can be considered as a way to enhance the charge density and thus also the resulting piezoelectric effect. Higher corona-charging voltages are possible with increased ambient pressure or in suitable dielectric gases. The effect of the gas inside the voids has also been studied. Enhanced transducer coefficients were obtained by corona charging in N2 or N2O gas atmospheres at 100-450 or 100-140 kPa pressures, respectively. The highest transducer coefficients of about 790 pCN-1 were obtained when N2 gas was filled into the voids of a cellular polymer film by means of consecutive vacuum and high-pressure treatments at 295 or 313 K.

https://iopscience.iop.org/article/10.1088/0022-3727/34/16/313/pdf

Understanding the role of the gas in the voids during corona charging of cellular electret films - a way to enhance their piezoelectricity

Charged closed-cell microporous polypropylene foams are shown to exhibit piezoelectric resonance modes in the dielectric function, coupled with a large anisotropy in the electromechanical and elastic material properties. Strong direct and converse dynamic piezoelectricity with a piezoelectric d33 coefficient of 140 pC/N at 600 kHz is identified. The piezoelectric d33 coefficient exceeds that of the ferroelectric polymer polyvinylidene fluoride by a factor of 5 and compares favorably with ferroelectric ceramics. Applications of similar concepts should provide a broad class of easily fabricated “soft” piezoelectric materials.

Large and broadband piezoelectricity in smart polymer-foam space-charge electrets

When a stretched elastomer is laminated to a flat plastic frame, a complex shape is formed, which is termed a minimum-energy structure. It is shown how self-organized structures can be applied in the development of actuators with complex, out-of-plane actuationmodes. This unusual concept is then demonstrated in the case of dielectric elastomer actuators. Among advantages of this approach are the simplicity in manufacturing, the potential complexity and sophistication of the manufactured structures, and the general benefits of the concept when applied to other electro-mechanically active materials.

Mika Paajanen

Papers

Energy minimization for self-organized structure formation and actuation

ElectroMechanical Film (EMFi) — a new multipurpose electret material

Understanding the role of the gas in the voids during corona charging of cellular electret films - a way to enhance their piezoelectricity

Large and broadband piezoelectricity in smart polymer-foam space-charge electrets

Self-organized minimum-energy structures for dielectric elastomer actuators