Anton Guimerà

SU-8 based microprobes with integrated planar electrodes for enhanced neural depth recording

Here, we describe new fabrication methods aimed to integrate planar tetrode-like electrodes into a polymer SU-8 based microprobe for neuronal recording applications. New concepts on the fabrication sequences are introduced in order to eliminate the typical electrode-tissue gap associated to the passivation layer. Optimization of the photolithography technique and high step coverage of the sputtering process have been critical steps in this new fabrication process. Impedance characterization confirmed the viability of the electrodes for reliable neuronal recordings with values comparable to commercial probes. Furthermore, a homogeneous sensing behavior was obtained in all the electrodes of each probe. Finally, in vivo action potential and local field potential recordings were successfully obtained from the rat dorsal hippocampus. Peak-to-peak amplitude of action potentials ranged from noise level to up to 400-500 μV. Moreover, action potentials of different amplitudes and shapes were recorded from all the four recording sites, suggesting improved capability of the tetrode to distinguish from different neuronal sources.

SU-8-based microneedles for in vitro neural applications

This paper presents novel design, fabrication, packaging and the first in vitro neural activity recordings of SU-8-based microneedles. The polymer SU-8 was chosen because it provides excellent features for the fabrication of flexible and thin probes. A microprobe was designed in order to allow a clean insertion and to minimize the damage caused to neural tissue during in vitro applications. In addition, a tetrode is patterned at the tip of the needle to obtain fine-scale measurements of small neuronal populations within a radius of 100 µm. Impedance characterization of the electrodes has been carried out to demonstrate their viability for neural recording. Finally, probes are inserted into 400 µm thick hippocampal slices, and simultaneous action potentials with peak-to-peak amplitudes of 200–250 µV are detected.

Prototype of a Nanostructured Sensing Contact Lens for Noninvasive Intraocular Pressure Monitoring

PURPOSE To present the application of a new sensor based on a flexible, highly piezoresistive, nanocomposite, all-organic bilayer (BL) adapted to a contact lens (CL) for non-invasive monitoring intraocular pressure (IOP). METHODS A prototype of a sensing CL, adapted to a pig eyeball, was tested on different enucleated pig eyes. A rigid, gas-permeable CL was designed as a doughnut shape with a 3-mm hole, where the BL film-based sensor was incorporated. The sensor was a polycarbonate film coated with a polycrystalline layer of the highly piezoresistive molecular conductor β-(ET)₂I₃, which can detect deformations caused by pressure changes of 1 mm Hg. The pig eyeballs were subjected to controlled-pressure variations (low-pressure transducer) to register the electrical resistance response of the CL sensor to pressure changes. Similarly, a CL sensor was designed according to the anatomic characteristics of the eye of a volunteer on the research team. RESULTS A good correlation (r² = 0.99) was demonstrated between the sensing CL electrical response, and IOP (mm Hg) changes in pig eyes, with a sensitivity of 0.4 Ω/mm Hg. A human eye test also showed the high potential of this new sensor (IOP variations caused by eye massage, blinking, and eye movements were registered). CONCLUSIONS A new nanostructured sensing CL for continuous monitoring of IOP was validated in an in vitro model (porcine eyeball) and in a human eye. This prototype has adequate sensitivity to continuously monitor IOP. This device will be useful for glaucoma diagnosis and treatment.

Non-invasive assessment of corneal endothelial permeability by means of electrical impedance measurements

The permeability of the corneal endothelial layer has an important role in the correct function of the cornea. Since ionic permeability has a fundamental impact on the passive electrical properties of living tissues, here it is hypothesized that impedance methods can be employed for assessing the permeability of the endothelial layer in a minimally invasive fashion. Precisely, the main objective of the present study is to develop and to analyze a minimally invasive method for assessing the electrical properties of the corneal endothelium, as a possible diagnostic tool for the evaluation of patients with endothelial dysfunction. A bidimensional model consisting of the main corneal layers and a four-electrode impedance measurement setup placed on the epithelium has been implemented and analyzed by means of the finite elements method (FEM). In order to obtain a robust indicator of the permeability of the endothelium layer, the effect of the endothelium electrical properties on the measured impedance has been studied together with reasonable variations of the other model layers. Simulation results show that the impedance measurements by means of external electrodes are indeed sufficiently sensitive to the changes in the electrical properties of the endothelial layer. It is concluded that the method presented here can be employed as non-invasive method for assessing endothelial layer function.

A rapid and reliable means of assessing hepatic steatosis in vivo via electrical bioimpedance.

Background. In liver transplantation, macrovesicular steatosis is a major determinant of graft outcome. Visual assessment of steatosis by the donor surgeon is highly inaccurate, whereas hepatic biopsy is user dependent and cumbersome. Our objective was to validate a novel bioelectrical impedance sensor as a means of objectively quantifying macrovesicular hepatic steatosis and to correlate the results with another surrogate measure of macrosteatosis, hepatic microcirculation. Methods. Fatty (n=36) and lean (n=18) male Zucker rats, 250 to 450 g, were used to achieve varying degrees of steatosis. After a bilateral subcostal incision, hepatic microcirculation was measured using laser Doppler microflowmetry. Low-frequency bioelectrical impedance (LF-BEI) was measured at 1 kHz using a custom-made sensor and instrumentation system. Complete hepatectomy was performed. Hepatic tissue was preserved and stained with hematoxylin-eosin for histology. Results and Conclusion. Both microflow and LF-BEI correlated well with macrosteatosis and each other: Pearson correlation coefficients −0.71, 0.73, and −0.81, respectively. Livers were grouped according to the degree of macrosteatosis: mild (<30%), moderate (30%–60%), and severe (>60%). Both LF-BEI and microflow varied significantly among groups on one-way analysis of variance, although only LF-BEI was capable of discriminating between mild and moderate macrosteatosis on post hoc analysis. Regarding their individual capacities to detect the presence of severe macrosteatosis, both tests were excellent classifiers: receiver operating curve area under the curve 0.885 and 0.9 for LF-BEI and microflow, respectively. However, the bioimpedance apparatus is more rapid and less susceptible to local factors and background noise and could more easily be used in the clinical liver transplantation setting.

A non-invasive method for an in vivo assessment of corneal epithelium permeability through tetrapolar impedance measurements

The permeability of the cornea epithelial layer has an important role in optimal function of the cornea. To assess this property quantitatively, methods must be based on the passive electrical properties of living tissues, as they can take advantage of the fundamental role that ionic permeability plays in such properties. For such techniques, measurement of the translayer electrical resistance (TER) has been consistently used to examine the ion transport mechanisms in the corneal epithelial cells; however, this technique has been only possible in vitro. To enhance the applications of this method, in this work we present a novel sensor to perform non-invasive in vivo TER measurements. Herein, the epithelial permeability was assessed using non-invasive tetrapolar impedance measurements that were performed with four electrodes placed on the corneal surface. The geometry of these electrodes was previously optimized to maximize the sensitivity of the corneal epithelium. To evaluate the feasibility of this sensor, the permeability of a rabbit corneal epithelium was monitored by applying a solution of benzalkonium chloride (0.05% BAC). The results validate the capability of the sensor to evaluate the cornea epithelial permeability in vivo.

Portable 4 Wire Bioimpedance Meter with Bluetooth Link

To allow some clinical research based on bioimpedance measurements a small and easy to use device that can be used in the clinical environment is needed. A portable device for 4 wire bioimpedance monitoring is presented. It consists of a battery powered 100% autonomous device including Bluetooth capability. It complies with IEC60601-1 electrical safety and electromagnetic compatibility requirements. The device is able to measure impedances from 1 Ω to 1 MΩ within 100 Hz to 400 kHz frequency range with a maximum error of 3%. The accuracy of the device has been evaluated with dummy cells and the effects of the electrode impedance have been studied.

Geometric correction factor for transepithelial electrical resistance measurements in transwell and microfluidic cell cultures

Transepithelial electrical resistance (TEER) measurements are regularly used in in vitro models to quantitatively evaluate the cell barrier function. Although it would be expected that TEER values obtained with the same cell type and experimental setup were comparable, values reported in the literature show a large dispersion for unclear reasons. This work highlights a possible error in a widely used formula to calculate the TEER, in which it may be erroneously assumed that the entire cell culture area contributes equally to the measurement. In this study, we have numerically calculated this error in some cell cultures previously reported. In particular, we evidence that some TEER measurements resulted in errors when measuring low TEERs, especially when using Transwell inserts 12 mm in diameter or microfluidic systems that have small chamber heights. To correct this error, we propose the use of a geometric correction factor (GCF) for calculating the TEER. In addition, we describe a simple method to determine the GCF of a particular measurement system, so that it can be applied retrospectively. We have also experimentally validated an interdigitated electrodes (IDE) configuration where the entire cell culture area contributes equally to the measurement, and it also implements minimal electrode coverage so that the cells can be visualized alongside TEER analysis.

Simultaneous monitoring of Staphylococcus aureus growth in a multi-parametric microfluidic platform using microscopy and impedance spectroscopy.

We describe the design, construction, and characterization of a scalable microfluidic platform that allows continuous monitoring of biofilm proliferation under shear stress conditions. Compared to other previous end-point assay studies, our platform offers the advantages of integration into multiple environments allowing simultaneous optical microscopy and impedance spectroscopy measurements. In this work we report a multi-parametric sensor that can monitor the growth and activity of a biofilm. This was possible by combining two interdigitated microelectrodes (IDuEs), and punctual electrodes to measure dissolved oxygen, K+, Na+ and pH. The IDuE has been optimized to permit sensitive and reliable impedance monitoring of Staphylococcus aureus V329 growth with two- and four-electrode measurements. We distinguished structural and morphological changes on intact cellular specimens using four-electrode data modeling. We also detected antibiotic mediated effects using impedance. Results were confirmed by scanning electrode microscopy and fluorescence microscopy after live/dead cell staining. The bacitracin mediated effects detected with impedance prove that the approach described can be used for guiding the development of novel anti-biofilm agents to better address bacterial infection.

A novel strategy to monitor microfluidic in-vitro blood-brain barrier models using impedance spectroscopy

In this work, we present the use of interdigitated electrodes (IDEs) for performing electrical impedance spectroscopy (EIS) measurements to monitor a microfluidic blood brain barrier model. In particular, an electrode configuration which would not impair the optical visualization of the cell culture is proposed. Numerical studies have been performed to evaluate the electrical impedance sensitivity of the proposed tetrapolar configuration along the cell barrier in a given microfluidic chamber geometry. The system has been validated using a home-made cyclo olefin polymer (COP) bioreactor and perforated poly (methyl methacrylate) (PMMA) sheets with different pore densities in order to simulate different cell barrier impedances.