Abstract-The origin of electrical burns under gel-type surface electrodes is a controversial topic that is not well understood. To investigate the phenomenon, we have developed an excised porcine skin-gel model, and used low-frequency current density imaging (LFCDI) to determine the current density (CD) distribution through the skin before and after burns were induced by application of electrical current (200 Hz, 70% duty cycle, 20-35 mA monophasic square waveform applied to the electrodes for 30-135 min). The regions of increased CD correlate well with the gross morphological changes (burns) observed. The measurement is sensitive enough to show regions of high current densities in the pre-burn skin, that correlate with areas were burn welts were produced, thus predicting areas where burns are likely to occur. Statistics performed on 28 skin patches revealed a charge dependency of the burn areas and a relatively uniform distribution. The results do not support a thermal origin of the burns but rather electro-chemical mechanisms. We found a statistically significant difference between burn area coverage during anodic and cathodic experiments.

Polydimethylsiloxane (PDMS) is the most common type of silicone polymer for the fabrication of implantable medical devices. Because of its inherent hydrophobic nature, the PDMS surface does not readily promote cellular adhesion, which leads to diverse clinical issues. Previously, we reported a simple water vapor plasma treatment of PDMS surfaces that resulted in stable long-term wettability and excellent in vitro cell compatibility. In this work, we report investigation of the in vivo local responses to PDMS implants treated by water vapor plasma using a subcutaneous rat model. The local tissue responses were assessed after 2 and 4 weeks of implantation by means of macroscopic and histomorphometric analysis. After 2 weeks of implantation, the plasma-treated implants elicited the formation of fibrous tissue capsules that were significantly thinner, more adherent, and vascularized than the control counterparts. The improved cell adhesion was correlated with an increased amount of cells attached to the implant surface after retrieval. There was no difference in the inflammatory response between untreated and treated samples. This study provides a rational approach to optimize the long-term performance of silicone implants, which is likely to have a significant impact in clinical applications demanding enhanced tissue integration of the implants.

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Polydimethylsiloxane (PDMS) or silicone rubber is a widely used implant material. Approaches to promote tissue integration to PDMS are desirable to avoid clinical problems associated with sliding and friction between tissue and implant. Plasma-etching is a useful way to control cell behavior on PDMS without additional coatings. In this work, different plasma processing conditions were used to modify the surface properties of PDMS substrates. Surface nanotopography and wettability were measured to study their effect on in vitro growth and morphology of fibroblasts. While fluorinated plasma treatments produced nanorough hydrophobic and superhydrophobic surfaces that had negative or little influences on cellular behavior, water vapor/oxygen plasma produced smooth hydrophillic surfaces that enhanced cell growth.

-The origin of electrical burns under gel-type surface electrodes is a controversial topic that is not well understood. To investigate the phenomenon, we have developed an excised porcine skin+gel model. In the present paper, we describe methods to detect these burns in the skin+gel model in an effort to understand the genesis of these burns. Burns were induced by severe electrical stimulation and changes in the impedance spectra and current density measured. We found that the changes in impedance spectrum were characterized by significant drop in the low frequency (<1 kHz) impedance magnitude and the formation of welts in the skin. Low frequency current density imaging (LFCDI) revealed regions of high current density beneath the electrode before burns were induced suggesting the possibility of predicting the locations where welts from burns will form and the importance of current density and local tissue impedance in the formation of these burns.

-Designers of gel-type surface electrodes, used in medical applications such as pain relief and neuromuscular stimulation, require a more thorough understanding of current pathways in tissue in order to design more effective electrical stimulation systems. To investigate these pathways, a finite element model (FEM) was used to compute current density distributions produced by an electrode placed on the surface of a homogeneous, tissue-mimicking gel slab. The gel slab phantom was constructed and the current densities were measured using a recently developed technique called current density imaging (CDI). CDI uses the phase data produced by magnetic resonance imaging (MRI) as a measure of the magnetic fields produced by the externally applied current. The results of the FEM simulation and CDI measurements compare well. CDI has several potential advantages over conventional FEM techniques including: no requirement for knowledge of local tissue conductivities, low and constant computational overhead regardless of tissue complexity, and the potential to p . erform invivo measurements.

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