The output strain of a dielectric elastomer actuator is directly proportional to the square of its applied electric field. However, since the likelihood of electric breakdown is elevated with an increased applied field, the maximum operating electric field of the dielectric elastomer is significantly derated in systems employing these actuators so that failure due to breakdown remains unlikely even as the material ages. In an effort to ascertain the dielectric strength so that stronger electric fields can be applied, partial discharge testing is used to assess the health of the actuator by detecting the charge that is released when localized instances of breakdown partially bridge the insulator. Pre-stretched and unstretched samples of VHB4910 tape were submerged in dielectric oil to remove external sources of partial discharges during testing, and the partial discharge patterns were recorded just before failure of the dielectric sample.
Partial discharges (PD) occur in solid insulating materials when the insulating material is partially bridged by an
electrical discharge in response to an applied voltage stress. PDs typically occur at localized points of high field stresses
or at voids and other inhomogeneities within the insulator. The applied field's effect on the frequency of occurrence and
intensity of PDs can be used to assess the electrical breakdown strength and aging characteristics of insulating materials.
PD testing is therefore a promising characterization method to understand the insulating properties of the elastomers and
geometries commonly used in DEAs. Prestretched (~100% and ~230% biaxial) and unstretched acrylic elastomers (3M
VHB tapes) with solid metal electrodes have been tested. We have found the number and intensity of PDs increase with
applied field, and that a significant number of PDs are detected before any actuation was visibly observed, implying that
the fields required for actuation will cause material aging and degradation over time. Most interestingly, the number of
PDs steadily increase as the applied voltage increases up to a sufficiently high voltage, where the PDs suddenly cease.
Since internal voids can cause PDs, this may indicate that the Maxwell stress minimized the thickness of or eliminated
these voids, which could explain how prestretching improves performance.
We constructed a whole-body fluorescence tomography instrument to monitor novel bifunctional phototherapeutic drugs (e.g., HPPH-Cyanine dye conjugate) in small animals. The instrument allows dense source and detector sampling with a fast galvo scanner and a CCD detector for improved resolution and sensitivity (Patwardhan et al., 2005). Here we report tissue phantom measurements to evaluate the imaging performance with a newly constructed tomography instrument. Phantom measurements showed that strong fluorescence generated by HPPH-Cyanine dye (HPPH-CD), having high fluorescence quantum yield and long wavelength fluorescence emission, allowed deep tissue imaging. We also report in vivo fluorescence measurements of the conjugate in Nude mice bearing A549 human non-small cell lung carcinoma (NSCLC) tumors at 24 hr post injection to evaluate tumor detection ability of the conjugate. Our results indicate that the HPPH-CD shows preferential uptake in tumors compared to surrounding normal tissue at 24 hr post injection. This study demonstrates a potential use of HPPH-CD in detection (fluorescence imaging) and treatment (PDT) of deeply seated tumors.