Piezoaktoren

Ultrasound Transducers and Energy Transfer | Mikel Gorostiaga

In medical implants, batteries used to power the device will eventually run out of energy and will require of a costly, risky and unpleasant surgery for their replacement. Hence, wireless ultrasonic energy transfer offers an attractive approach to recharge those empty batteries.

Different parameters must be taken into account for an optimized energy transmission. Not only must the piezoelectric material and transducer design be chosen carefully, but also the electric impedance and acoustic matching of the transducers have to be adequately selected. Furthermore, transmission frequency is of paramount importance, since it influences factors such as penetration depth, losses in the propagation medium and the angular tilt sensitivity. Finally, future generations of smart implants will most likely require data exchange between emitter and receiver to fully exploit their potential, provide system upgrades or read out monitoring information.

Our group has developed a novel rapid-prototyping process to fabricate spring-mass matching layers for bandwidth enhancement of emitters, based on spin-coated PDMS on top of metal foils. Currently our research is focused on improving the spring-mass fabrication process, and on the simultaneous ultrasonic energy and information transmission.

Topological piezo actuation | Dr. Matthias Wapler

Most people may think that micro actuators have nothing to do with general relativity or string theory. In our activities on topological piezo actuators, however, we use essentially the differential geometry of surfaces embedded in higher dimensions that is used in the theory of black holes and even quantum gravity to predict the buckling deformation of piezoelectric films with anisotropic strains. 

In practice, this means that we can build out-of-plane piezo actuators out of single piezo layers, with large displacements and strong forces. The scaling behavior of the displacements and forces is more suitable for miniaturization than in bending actuators and currently, the curvatures of sharp features are only limited by the mechanical breakdown strength of the material. We can either produce simple conical displacements or calculate (for rotationally symmetric configurations) custom electrode structures to obtain desired free-form surface deformations with a single control voltage. We currently apply them to drive free-form deformable mirrors and adaptive lenses, but there exist potentially many more applications, e.g. peristaltic micro pumps or micro valves.

Adjustable effective piezoelectric coupling tensors | Dr. Matthias Wapler

Piezo foils and films are usually driven with planar electrodes on their surfaces, such that a driving voltage in the polarization direction causes an isotropic in-plane contraction. Less common in-plane polarized piezo films with interdigitated electrodes have an expansion in the direction of the field and a contraction orthogonal to it, along the direction of the electrodes. For corresponding bending actuators, this means that one can choose between a spherical and a saddle-shaped displacement. In some situations it may however be desirable to have, e.g., unidirectional or elliptical deformations.

Hence, we are developing methods to create effective deformations with an adjustable aspect ratio or ellipticity from piezo films with micro-structured in-plane polarization patterns. For a fixed aspect ratio with a single control voltage, we have developed methods to calculate and fabricate electrode patterns with suitable doubly interdigitated electrodes. Alternatively, we can control the strain in both directions independently using multi-layered configurations, for example for asymmetric focusing mirrors.