Inkjet-Printed and Electroplated 3D Electrodes for Recording Extracellular Signals in Cell Culture.

Recent investigations into cardiac or nervous tissues call for systems that are able to electrically record in 3D as opposed to 2D. Typically, challenging microfabrication steps are required to produce 3D microelectrode arrays capable of recording at the desired position within the tissue of interest. As an alternative, additive manufacturing is becoming a versatile platform for rapidly prototyping novel sensors with flexible geometric design. In this work, 3D MEAs for cell-culture applications were fabricated using a piezoelectric inkjet printer. The aspect ratio and height of the printed 3D electrodes were user-defined by adjusting the number of deposited droplets of silver nanoparticle ink along with a continuous printing method and an appropriate drop-to-drop delay. The Ag 3D MEAs were later electroplated with Au and Pt in order to reduce leakage of potentially cytotoxic silver ions into the cellular medium. The functionality of the array was confirmed using impedance spectroscopy, cyclic voltammetry, and recordings of extracellular potentials from cardiomyocyte-like HL-1 cells.

Determination of Stiffness and the Elastic Modulus of 3D-Printed Micropillars with Atomic Force Microscopy-Force Spectroscopy.

Nowadays, many applications in diverse fields are taking advantage of micropillars such as optics, tribology, biology, and biomedical engineering. Among them, one of the most attractive is three-dimensional microelectrode arrays for in vivo and in vitro studies, such as cellular recording, biosensors, and drug delivery. Depending on the application, the micropillar's optimal mechanical response ranges from soft to stiff. For long-term implantable devices, a mechanical mismatch between the micropillars and the biological tissue must be avoided. For drug delivery patches, micropillars must penetrate the skin without breaking or bending. The accurate mechanical characterization of the micropillar is pivotal in the fabrication and optimization of such devices, as it determines whether the device will fail or not. In this work, we demonstrate an experimental method based only on atomic force microscopy-force spectroscopy that allows us to measure the stiffness of a micropillar and the elastic modulus of its constituent material. We test our method with four different types of 3D inkjet-printed micropillars: silver micropillars sintered at 100 and 150 °C and polyacrylate microstructures with and without a metallic coating. The estimated elastic moduli are found to be comparable with the corresponding bulk values. Furthermore, our findings show that neither the sintering temperature nor the presence of a thin metal coating plays a major role in defining the mechanical properties of the micropillar.

Inkjet-printed 3D micro-ring-electrode arrays for amperometric nanoparticle detection.

Chip-based impact electrochemistry can provide means to measure nanoparticles in solution by sensing their stochastic collisions on appropriately-polarized microelectrodes. However, a planar microelectrode array design still restricts the particle detection to the chip surface and does not allow detection in 3D environments. In this work, we report a fast fabrication process for 3D microelectrode arrays by combining ink-jet printing with laser-patterning. To this end, we printed 3D pillars from polyacrylate ink as a scaffold. Then, the metal structures are manufactured via sputtering and laser-ablation. Finally, the chip is passivated with a parylene-C layer and the electrode tips are created via laser-ablation in a vertical alignment. As a proof of principle, we employ our 3D micro-ring-electrode arrays for single impact recordings from silver nanoparticles.

Ultrasoft Silicone Gel as a Biomimetic Passivation Layer in Inkjet-Printed 3D MEA Devices.

Multielectrode arrays (MEAs) are versatile tools that are used for chronic recording and stimulation of neural cells and tissues. Driven by the recent progress in understanding of how neuronal growth and function respond to scaffold stiffness, development of MEAs with a soft cell-to-device interface has gained importance not only for in vivo but also for in vitro applications. However, the passivation layer, which constitutes the majority of the cell-device interface, is typically prepared with stiff materials. Herein, a fabrication of an MEA device with an ultrasoft passivation layer is described, which takes advantage of inkjet printing and a polydimethylsiloxane (PDMS) gel with a stiffness comparable to that of the brain. The major challenge in using the PDMS gel is that it cannot be patterned to expose the sensing area of the electrode. This issue is resolved by printing 3D micropillars at the electrode tip. Primary cortical neurons are grown on the fabricated device, and effective stimulation of the culture confirms functional cell-device coupling. The 3D MEA device with an ultrasoft interface provides a novel platform for investigating evoked activity and drug responses of living neuronal networks cultured in a biomimetic environment for both fundamental research and pharmaceutical applications.

Fully Printed µ-Needle Electrode Array from Conductive Polymer Ink for Bioelectronic Applications.

Microelectrode arrays (MEAs) are widely used platforms in bioelectronics to study electrogenic cells. In recent years, the processing of conductive polymers for the fabrication of three-dimensional electrode arrays has gained increasing interest for the development of novel sensor designs. Here, additive manufacturing techniques are promising tools for the production of MEAs with three-dimensional electrodes. In this work, a facile additive manufacturing process for the fabrication of MEAs that feature needle-like electrode tips, so-called µ-needles, is presented. To this end, an aerosol-jet compatible PEDOT:PSS and multiwalled carbon nanotube composite ink with a conductivity of 323 ± 75 S m-1 is developed and used in a combined inkjet and aerosol-jet printing process to produce the µ-needle electrode features. The µ-needles are fabricated with a diameter of 10 ± 2 µm and a height of 33 ± 4 µm. They penetrate an inkjet-printed dielectric layer to a height of 12 ± 3 µm. After successful printing, the electrochemical properties of the devices are assessed via cyclic voltammetry and impedance spectroscopy. The µ-needles show a capacitance of 242 ± 70 nF at a scan rate of 5 mV s-1 and an impedance of 128 ± 22 kΩ at 1 kHz frequency. The stability of the µ-needle MEAs in aqueous electrolyte is demonstrated and the devices are used to record extracellular signals from cardiomyocyte-like HL-1 cells. This proof-of-principle experiment shows the µ-needle MEAs' cell-culture compatibility and functional integrity to investigate electrophysiological signals from living cells.

On-Chip Stochastic Detection of Silver Nanoparticles without a Reference Electrode.

We report the electrochemical detection of 20 nm silver nanoparticles at a chip-based microelectrode array (MEA) without the need for a conventional reference electrode. This is possible due to the system's open-circuit potential allowing the oxidation of silver nanoparticles in the presence of phosphate-buffered saline (PBS). The hypothesis is confirmed by modulating the open-circuit potential via addition of ascorbic acid in solution, effectively inhibiting the detection of silver nanoparticle events. Employing the reference-free detection concept, we observe a linear relationship between the nanoparticle impact frequency at the microelectrodes and the nanoparticle concentration. This allows for viable quantification of silver nanoparticle concentrations in situ. The presented concept is ideal for the development of simple lab-on-a-chip or point-of-use systems enabling fast and low-cost screening of nanoparticles.