Gerda Buchberger

Flexible large area ferroelectret sensors for location sensitive touchpads

Cellular polypropylene ferroelectrets are flexible transducer materials with dominant longitudinal piezoelectricity. They can be used for transient pressure sensing applications, due to the low cost of the sensor material. The authors show that ferroelectrets allow for the production of keyboards and touch pads with a customizable layout. Large area resistive electrodes on the ferroelectret with electronics at the edges avoid the preparation of complex sensor matrices. A touch pad of 10×10cm2 area was tested by applying a force of 1.7N on an area with a diameter of 5mm in a grid matrix with 23×23 elements.

Directional, passive liquid transport: the Texas horned lizard as a model for a biomimetic ‘liquid diode’

Moisture-harvesting lizards such as the Texas horned lizard (Iguanidae: Phrynosoma cornutum) live in arid regions. Special skin adaptations enable them to access water sources such as moist sand and dew: their skin is capable of collecting and transporting water directionally by means of a capillary system between the scales. This fluid transport is passive, i.e. requires no external energy, and directs water preferentially towards the lizards snout. We show that this phenomenon is based on geometric principles, namely on a periodic pattern of interconnected half-open capillary channels that narrow and widen. Following a biomimetic approach, we used these principles to develop a technical prototype design. Building upon the Young–Laplace equation, we derived a theoretical model for the local behaviour of the liquid in such capillaries. We present a global model for the penetration velocity validated by experimental data. Artificial surfaces designed in accordance with this model prevent liquid flow in one direction while sustaining it in the other. Such passive directional liquid transport could lead to process improvements and reduction of resources in many technical applications.

Cellular ferroelectrets for flexible touchpads, keyboards and tactile sensors

Cellular polypropylene ferroelectrets are flexible and conformable transducer materials with a dominant longitudinal piezoelectric effect. They can be used in a wide range of pressure sensing applications. We present concepts for position sensitive touchpads and keyboards based on piezoelectric materials with a dominant longitudinal piezoelectric effect. As large area electrodes are superimposed onto the ferroelectret and combined with electronics at the edges, complex sensor matrices are avoided. We use a ferroelectretic detector stripe as a model system for the theoretical description of the touchpad and show that this detector can be treated mathematically analogous to a transmission line. We present the solutions of the corresponding differential equations under the appropriate boundary conditions and experimental results that are in agreement to them. For the flexible, ferroelectretic keyboard we use a binary or balanced ternary coding technique profiting from the polarization of the ferroelectret.

Bioinspired polymer microstructures for directional transport of oily liquids

Nature has always served as an inspiration for scientists, helping them to solve a large diversity of technical problems. In our case, we are interested in the directional transport of oily liquids and as a model for this application we used the flat bug Dysodius lunatus. In this report, we present arrays of drops looking like polymer microstructures produced by the two-photon polymerization technique that mimic the micro-ornamentation from the bugs cuticle. A good directionality of oil transport was achieved, directly controlled by the direction of the pointed microstructures at the surface. If the tips of the drop-like microstructures are pointing towards the left side, the liquid front moves to the right and vice versa. Similar effects could be expected for the transport of oily lubricants. These results could, therefore, be interesting for applications in friction and wear reduction.

Temporal change in the electromechanical properties of dielectric elastomer minimum energy structures

Dielectric elastomer minimum energy structures (DEMES) are soft electronic transducers and energy harvesters with potential for consumer goods. The temporal change in their electromechanical properties is of major importance for engineering tasks. Therefore, we study acrylic DEMES by impedance spectroscopy and by optical methods for a total time period of approx. 4.5 months. We apply either compliant electrodes from carbon black particles only or fluid electrodes from a mixture of carbon black particles and silicone oil. From the measurement data, the equivalent series capacitances and resistances as well as the bending angles of the transducers are obtained. We find that the equivalent series capacitances change in average between −12 %/1000 h and −4.0 %/1000 h, while the bending angles decrease linearly with slopes ranging from −15 %/1000 h to −7 %/1000 h. Transducers with high initial bending angles and electrodes from carbon black particles show the smallest changes of the electromechanical characteri...

Dynamic capacitive extensometry setup for in-situ monitoring of dielectric elastomer actuators

Dielectric elastomer actuators (DEAs) and their subcategory energy minimum structures (DEMES) are promising candidates for biomimetic robotic elements. Especially interesting for their characterization is the transient strain or stretch-response of the deforming elastomer part to high voltage driving signals, and the state of the applied compliant electrodes. However, this information can hardly be obtained by optical measurement techniques only. Especially DEMES with their complex three dimensional shapes are difficult to study. To this end, we present a setup of dynamic capacitive extensometry which is able to monitor DEAs and DEMES in situ; both the stretch dependent capacitance of the actuated elastomer part and the stretch dependent resistance of the compliant electrodes are recorded in situ during dynamic high voltage actuation. With the help of this measurement data the transient stretch response is studied with regard to the frequency, magnitude and shape of the actuation signal. Furthermore, the stretch-dependent resistance of the compliant electrodes and their percolation limit are investigated. The developed measurement technique can be applied also for actuator control with feedback loops.

A comparison of the electromechanical characteristics of dielectric elastomer minimum energy structures (DEMES) and planar dielectric elastomer actuators (p-DEAs)

Dielectric elastomer actuators are considered as promising candidates for robotic elements. To this end, planar dielectric elastomer actuators (p-DEAs) and dielectric elastomer minimum energy structures (DEMES) are applicable. However, the knowledge of their electrical and mechanical characteristics is of major importance for engineering tasks. Therefore we study p-DEAs and DEMES by impedance spectroscopy (IS) and dynamic capacitive extensometry (DCE). We vary the boundary conditions with regard to p-DEAs (free and fixed boundaries) and fabricate various DEMES with one angular degree of freedom. A mixture of carbon black particles and silicone oil serves as compliant electrodes. We present equivalent circuit models of the actuators based on impedance spectroscopy data, the frequency ranges in which they are applicable and effects of aging on the equivalent circuit models. By DCE the electrical characteristics of dielectric elastomer actuators are monitored in situ during dynamic high voltage actuation. These electrical characteristics of the dielectric elastomer actuators such as p-DEAs and DEMES can be related to their transient stretch in response to high voltage driving signals. We study the viscoelastic response of the actuators to square driving signals of different magnitudes; furthermore we monitor the state of the compliant electrodes. By means of the DCE measurement data and the impedance spectra the p-DEAs and DEMES can be compared.

The external scent efferent system of selected European true bugs (Heteroptera): a biomimetic inspiration for passive, unidirectional fluid transport

In this work, we present structured capillaries that were inspired by the microstructures of the external scent efferent system as found in different European true bug species (Pentatomidae and Cydnidae). These make use of small, orientated structures in order to facilitate fluid movement towards desired areas where defensive substances are evaporated. Gland channels and microstructures were investigated by means of scanning electron microscopy and abstracted into three-dimensional models. We used these models to create scent channel replicas from different technical substrates (steel and polymers) by means of laser ablation, laser structuring and casting. Video analysis of conducted fluid-flow experiments showed that bug-inspired, artificial scent fluid channels can indeed transport different fluids (water solutions and oils/lubricants) passively in one direction (velocities of up to 1 mm s−1), while halting the fluid movement in the opposite direction. At the end of this contribution, we present a physical theory that explains the observed fluid transport and sets the rules for performance optimization in future work.

Moisture-Harvesting Reptiles: A Review

Reptiles can live in arid environments due to special adaptations of their integument to such habitats. So called moisture-harvesting reptiles show behavioral and morphological adaptations, as their diet often does not cover the complete water demand and rain is scarce. The collection of water from various sources by moisture-harvesting reptiles is often accompanied by a stereotypical behavior: snakes coil up in the open and show a dorso-ventral flattening of their body to increase the surface area. Lizards also show a flattening of their body, but additionally raise their abdomen by splaying and extending their legs and lowering their head and tail. A similar behavior is observed in tortoises. Though there are several behavioral descriptions of moisture-harvesting reptiles, there are only few investigations about the physical principles enabling a passive collection of water. Special skin structures, comprising a micro structured surface with capillary channels in between imbricate overlapping scales, enable lizards to collect water efficiently. In some lizards, such as the Texas horned lizard Phrynosoma cornutum, water droplets applied to their body surface show a preferred spreading direction, transporting the water towards their mouth for ingestion. This passive directional transport is enabled by asymmetric and interconnected channels between the scales. Elucidation of the physical principles behind the directional water spreading has inspired a biomimetic transfer to optimize future applications in liquid handling, e.g. in fields of microfluidics.

Ferroelectret based flexible keyboards and tactile sensors

Ferroelectrets easily allow for the preparation of multilayer keyboards, where keys are digitally encoded with a ternary code. Contact electrode charging turned out to be a reliable way for the fast preparation of charge patterns on cellular polymers. A three layer keyboard, allowing for encoding 26 keys was fabricated to demonstrate the feasibility of the proposed input device. Further work is necessary to better understand the underlying mechanisms during the preparation of charge patterns on cellular ferroelectrets.