Federico Carpi

Stretching Dielectric Elastomer Performance

Devices using materials that deform in response to electricity are based on a phenomenon that was observed more than two centuries ago. The idea that a solid material can deform when stimulated by electricity originated in the late-18th century with observations of ruptures in overcharged Leyden jars, the first electrical capacitors. In 1776, Italian scientist Alessandro Volta mentioned in a letter that Italian experimenter Felice Fontana had noted volume changes in the Leyden jar upon electrification (1), an observation that launched a new field of investigation—“deformable” materials affected by electricity. More than two centuries later, the concept of “electrically stretchable materials” is at the forefront of devising bioinspired robots, tactile and haptic interfaces, and adaptive optical systems (2, 3).

Electroactive polymer-based devices for e-textiles in biomedicine

This paper describes the early conception and latest developments of electroactive polymer (EAP)-based sensors, actuators, electronic components, and power sources, implemented as wearable devices for smart electronic textiles (e-textiles). Such textiles, functioning as multifunctional wearable human interfaces, are today considered relevant promoters of progress and useful tools in several biomedical fields, such as biomonitoring, rehabilitation, and telemedicine. After a brief outline on ongoing research and the first products on e-textiles under commercial development, this paper presents the most highly performing EAP-based devices developed by our lab and other research groups for sensing, actuation, electronics, and energy generation/storage, with reference to their already demonstrated or potential applicability to electronic textiles

Electromechanical characterisation of dielectric elastomer planar actuators: comparative evaluation of different electrode materials and different counterloads

This work intends to extend the electromechanical characterisation of dielectric elastomer actuators. Planar actuators were realised with a 50m-thick film of an acrylic elastomer coated with compliant electrodes. The isotonic transverse strain, the isometric transverse stress and the driving current, due to a 2 s high voltage impulse, were measured for four electrode materials (thickened electrolyte solution, graphite spray, carbon grease and graphite powder), four transverse prestress values (19.6, 29.4, 39.2 and 49.0 kPa) and different driving voltages (up to the dielectric breakdown voltage). Results showed that the electrode material and prestress strongly influence the electromechanical performances of the devices. Actuators with graphite spray electrodes and transverse prestress of 39.2 kPa exhibited an isotonic transverse strain of 6% at 49 V/m, with a driving current per unit electrode area of 3.5 A/cm 2 , and an isometric transverse stress of 49 kPa at 42 V/m. An electromechanical coupling efficiency of 10% at 21 V/m was calculated for actuators with thickened electrolyte solution electrodes and a transverse prestress of 29.4 kPa. The presented data permits to choose the best electrode material and the best prestress value (among those tested), to obtain the maximum isotonic transverse strain, the maximum isometric transverse stress or the maximum efficiency for different ranges of applied electric field.

Improvement of electromechanical actuating performances of a silicone dielectric elastomer by dispersion of titanium dioxide powder

This paper presents the first reported data on the embedding of highly dielectric ceramic inclusions in a rubbery host medium as a means to increase the electromechanical material response for dielectric elastomer actuation. The studied polymer/ceramic composite, consisting of a silicone matrix in which titanium dioxide powder was dispersed, exhibited, in comparison with pure silicone, a decreased elastic modulus, as well as an increased dielectric constant. The measured low frequency permittivity resulted in accordance with several classical dielectric mixing rules. The use of this material as elastomeric dielectric for planar actuators enabled a reduction of the driving electric fields, so that a transverse strain of 11% at 10 V//spl mu/m and a transverse stress of 16.5 kPa at 9 V//spl mu/m were obtained. These levels of strain and stress were respectively more than eight and four times higher than the corresponding values generated with the pure polymer matrix for analogous electrical stimuli.

Folded dielectric elastomer actuators

Polymer-based linear actuators with contractile ability are currently demanded for several types of applications. Within the class of dielectric elastomer actuators, two basic configurations are available today for such a purpose: the multi-layer stack and the helical structure. The first consists of several layers of elementary planar actuators stacked in series mechanically and parallel electrically. The second configuration relies on a couple of helical compliant electrodes alternated with a couple of helical dielectrics. The fabrication of both these configurations presents some specific drawbacks today, arising from the peculiarity of each structure. Accordingly, the availability of simpler solutions may boost the short-term use of contractile actuators in practical applications. For this purpose, a new configuration is here described. It consists of a monolithic structure made of an electroded sheet, which is folded up and compacted. The resulting device is functionally equivalent to a multi-layer stack with interdigitated electrodes. However, with respect to a stack the new configuration is advantageously not discontinuous and can be manufactured in one single phase, avoiding layer-by-layer multi-step procedures. The development and preliminary testing of prototype samples of this new actuator made of a silicone elastomer are presented here.

Magnetically Controllable Gastrointestinal Steering of Video Capsules

Wireless capsule endoscopy (WCE) allows for comfortable video explorations of the gastrointestinal (GI) tract, with special indication for the small bowel. In the other segments of the GI tract also accessible to probe gastroscopy and colonscopy, WCE still exhibits poorer diagnostic efficacy. Its main drawback is the impossibility of controlling the capsule movement, which is randomly driven by peristalsis and gravity. To solve this problem, magnetic maneuvering has recently become a thrust research area. Here, we report the first demonstration of accurate robotic steering and noninvasive 3-D localization of a magnetically enabled sample of the most common video capsule (PillCam, Given Imaging Ltd, Israel) within each of the main regions of the GI tract (esophagus, stomach, small bowel, and colon) in vivo, in a domestic pig model. Moreover, we demonstrate how this is readily achievable with a robotic magnetic navigation system (Niobe, Stereotaxis, Inc, USA) already used for cardiovascular clinical procedures. The capsule was freely and safely moved with omnidirectional steering accuracy of 1°, and was tracked in real time through fluoroscopic imaging, which also allowed for 3-D localization with an error of 1 mm. The accuracy of steering and localization enabled by the Stereotaxis system and its clinical accessibility world wide may allow for immediate and broad usage in this new application. This anticipates magnetically steerable WCE as a near-term reality. The instrumentation should be used with the next generations of video capsules, intrinsically magnetic and capable of real-time optical-image visualization, which are expected to reach the market soon.

Helical dielectric elastomer actuators

This paper presents a new type of contractile polymer-based electromechanical linear actuator. The device belongs to the class of dielectric elastomer actuators, which are typically capable of undergoing large deformations induced by an applied electric field. It is based on a novel helical configuration, suitable for the generation of electrically driven axial contractions and radial expansions. The architecture, the principle of operation, a fabrication method and results of a preliminary prototype testing of the new device are described. An axial strain of −5% at about 14 V µm −1 was obtained from first prototypes. (Some figures in this article are in colour only in the electronic version)

Magnetic Maneuvering of Endoscopic Capsules by Means of a Robotic Navigation System

The use of video capsules for noninvasive explorations of the digestive tube is progressively increasing today. At present, the motion of these wireless endoscopic devices cannot be controlled and they proceed by means of visceral peristalsis and gravity. Aimed at enabling a motion control, the technique described here uses an external magnetic field applied to the video capsule, previously coated with a magnetic shell. As a source of controlled magnetic field, a robotic magnetic navigation system, recently introduced in the clinical practice to magnetically steer cardiovascular interventional devices, was suggested in a previous study to be deserving of investigation. The attractive potentialities of this system, along with its current limitations, in order to maneuver endoscopic capsules were studied in this work for the first time, both from a theoretical and an experimental point of view. The actual capabilities were experimentally assessed with preliminary motion control tests on a conventional video capsule inside a human-sized plastic phantom. Results demonstrated the possibility of achieving controlled magnetic maneuvers within the considered environment.

Controlled Navigation of Endoscopic Capsules: Concept and Preliminary Experimental Investigations

This paper describes a technique to control the navigation of traditional wireless endoscopic capsules and reports preliminary proof-of-concept investigations. These capsules are used for noninvasive explorations of the digestive tube. At present, their motion cannot be controlled and they proceed by means of the visceral peristalsis. In order to enable motion controls, the technique proposed here adopts magnetic shells which are to be applied to available capsules, immediately before their use. The shells are used to control the capsule movement by means of external magnetic fields. This solution is readily applicable to any endoscopic capsule, avoiding internal modifications. Prototype elastic shells made of silicone elastomers mixed with magnetic particles were fabricated. They were tested with the most diffused capsule (model M2A, Given Imaging Ltd., Yoqneam, Israel), by studying the performance of the capsule/shell complex in simplified experimental conditions. Bench tests permitted to demonstrate controlled translations, rotations, and rototranslations of the capsule/shell complex within tubular structures coated with pieces of bovine tissues. The use of a new instrumentation, recently developed for cardiovascular treatments, is proposed as a potential means enabling the application of controlled magnetic fields for intrabody navigations.

Standards for dielectric elastomer transducers

Dielectric elastomer transducers consist of thin electrically insulating elastomeric membranes coated on both sides with compliant electrodes. They are a promising electromechanically active polymer technology that may be used for actuators, strain sensors, and electrical generators that harvest mechanical energy. The rapid development of this field calls for the first standards, collecting guidelines on how to assess and compare the performance of materials and devices. This paper addresses this need, presenting standardized methods for material characterisation, device testing and performance measurement. These proposed standards are intended to have a general scope and a broad applicability to different material types and device configurations. Nevertheless, they also intentionally exclude some aspects where knowledge and/or consensus in the literature were deemed to be insufficient. This is a sign of a young and vital field, whose research development is expected to benefit from this effort towards standardisation.