Graphene active sensor arrays for long-term and wireless mapping of wide frequency band epicortical brain activity.

Graphene active sensors have demonstrated promising capabilities for the detection of electrophysiological signals in the brain. Their functional properties, together with their flexibility as well as their expected stability and biocompatibility have raised them as a promising building block for large-scale sensing neural interfaces. However, in order to provide reliable tools for neuroscience and biomedical engineering applications, the maturity of this technology must be thoroughly studied. Here, we evaluate the performance of 64-channel graphene sensor arrays in terms of homogeneity, sensitivity and stability using a wireless, quasi-commercial headstage and demonstrate the biocompatibility of epicortical graphene chronic implants. Furthermore, to illustrate the potential of the technology to detect cortical signals from infra-slow to high-gamma frequency bands, we perform proof-of-concept long-term wireless recording in a freely behaving rodent. Our work demonstrates the maturity of the graphene-based technology, which represents a promising candidate for chronic, wide frequency band neural sensing interfaces.

Online oxygen monitoring using integrated inkjet-printed sensors in a liver-on-a-chip system.

The demand for real-time monitoring of cell functions and cell conditions has dramatically increased with the emergence of organ-on-a-chip (OOC) systems. However, the incorporation of co-cultures and microfluidic channels in OOC systems increases their biological complexity and therefore makes the analysis and monitoring of analytical parameters inside the device more difficult. In this work, we present an approach to integrate multiple sensors in an extremely thin, porous and delicate membrane inside a liver-on-a-chip device. Specifically, three electrochemical dissolved oxygen (DO) sensors were inkjet-printed along the microfluidic channel allowing local online monitoring of oxygen concentrations. This approach demonstrates the existence of an oxygen gradient up to 17.5% for rat hepatocytes and 32.5% for human hepatocytes along the bottom channel. Such gradients are considered crucial for the appearance of zonation of the liver. Inkjet printing (IJP) was the selected technology as it allows drop on demand material deposition compatible with delicate substrates, as used in this study, which cannot withstand temperatures higher than 130 °C. For the deposition of uniform gold and silver conductive inks on the porous membrane, a primer layer using SU-8 dielectric material was used to seal the porosity of the membrane at defined areas, with the aim of building a uniform sensor device. As a proof-of-concept, experiments with cell cultures of primary human and rat hepatocytes were performed, and oxygen consumption rate was stimulated with carbonyl-cyanide-4-(trifluoromethoxy)phenylhydrazone (FCCP), accelerating the basal respiration of 0.23 ± 0.07 nmol s-1/106 cells up to 5.95 ± 0.67 nmol s-1/106 cells s for rat cells and the basal respiration of 0.17 ± 0.10 nmol s-1/106 cells by up to 10.62 ± 1.15 nmol s-1/106 cells for human cells, with higher oxygen consumption of the cells seeded at the outflow zone. These results demonstrate that the approach of printing sensors inside an OOC has tremendous potential because IJP is a feasible technique for the integration of different sensors for evaluating metabolic activity of cells, and overcomes one of the major challenges still remaining on how to tap the full potential of OOC systems.

Flexible probe for in vivo quantification of corneal epithelium permeability through non-invasive tetrapolar impedance measurements.

Studies concerning the functional status of the corneal epithelium are of special interest due to its key role in preventing ocular surface disease and corneal infections. In particular, quantitative measurements of the epithelium permeability translayer electrical resistance (TER) have been proven as a sensitive in vitro test for evaluation of the corneal barrier function. In a recent work from the authors (Guimera et al. Biosens. Bioelectron. 31:55-61, 2012), a novel method to non-invasively assess the corneal epithelial permeability by using tetrapolar impedance measurements, based on the same TER theoretical principles, was presented and validated using a rigid sensing device. In this work, the usability of this method has been dramatically improved by using SU-8 photoresist as a substrate material. The flexibility of this novel sensing device makes no need to apply pressure on the cornea to ensure the electrical contact between the electrodes and the corneal surface. The feasibility of this flexible sensor has been evaluated in vivo by increasing the permeability of rabbit corneal epithelium. For that, different concentrations of benzalkonium chloride (BAC) solution were instilled on different rabbit corneas. The obtained results have been compared with measurements of the permeability to sodium fluorescein of different excised corneas, a well-known method used to evaluate the corneal barrier function, to demonstrate the feasibility of this novel flexible sensor for quantifying the corneal epithelium permeability in vivo in a non-invasive way.

Modeling the viscosity and aggregation of suspensions of highly anisotropic nanoparticles.

The rheology of nanofiber suspensions is studied solving numerically the Population Balance Equations (PBE). To account for the anisotropic nature of nanofibers, a relation is proposed for their hydrodynamic volume. The suspension viscosity is calculated using the computed aggregate size distributions together with the Krieger-Dougherty constitutive equation. The model is fitted to experimental flow curves for Carbon NanoFibers (CNF) and for NanoFibrillated Cellulose (NFC), giving a first estimation of the microscopic anisotropy parameter, and yielding information on the structural properties and rheology of each system.