This study reports on a novel passive ceramic-based semi-dry electrode prototype for electroencephalography (EEG) applications. With the help of capillary forces of the porous ceramics pillars, the semi-dry electrodes build a stable electrode/scalp interface by penetrating hair and releasing a small amount saline in a controlled and sustained manner. The semi-dry electrode/scalp impedances were low and stable (44.4 ± 16.9 kΩ, n = 10), and the variation between nine different positions was less 5 kΩ. The semi-dry electrodes have shown non-polarization characteristics and the maximum difference of equilibrium potential between eight electrodes was 579 μV. The semi-dry electrodes demonstrated long-term stability, and the impedance only increased by 20 kΩ within 8 h. EEG signals were simultaneously recorded using a 9-channel gel-based electrode and semi-dry electrode arrays setup on ten subjects. The average temporal cross-correlation between them in the eyes open/closed and the steady state visually evoked potentials (SSVEPs) paradigm were 0.938 ± 0.037 and 0.937 ± 0.027 respectively. Spectral analyses revealed similar response patterns with expected functional responses. Together with the advantages of quick setup, self-application and cleanliness, the result suggests the semi-dry electrode is suitable for emerging real-world EEG applications, such as brain-computer interfaces and wearable EEGs.

Methacrylate silsesquioxane terminated by urethane acrylate (MASSQ-UA) was prepared via a combination of a sol–gel process and urethane chemistry. The organic and inorganic segments were hybridized at the molecular level. The methacrylate silsesquioxane (MASSQ) was first prepared by hydrolysis and condensation reactions. The molar ratio of water to silane was specifically quantified to control the molecular structure, molecular weight and the distribution of MASSQ. MASSQ-UA was then synthesized by terminating the residual silanol groups in the incomplete MASSQ with pre-prepared mono-adducts of isophorone diisocyanate and 2-hydroxylethyl acrylate. In the second step of the reaction, the residual, but still active, silanol groups were effectively reduced and sterically hindered, overcoming the instability problems that frequently occur in traditional sol–gel-derived silsesquioxane. In addition to the methacrylate groups, isophorone diisocyanate and 2-hydroxylethyl acrylate were hybridized with MASSQ. Unlike traditional heterogeneous filler systems, these hybrids are molecular systems and are homogeneous, optically clear fluids in which the inorganic and organic components are linked by covalent bonds. The molecular structures, molecular weight and polydispersity of the final products were investigated by various methods, including matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and electrospray ionization time-of-flight mass spectrometry. The thermal stability of MASSQ-UA and the water resistance of UV-cured MASSQ-UA-based coatings were improved compared with the MASSQ and UV-cured MASSQ-based coatings. The process developed here can be monitored and controlled and could therefore be used industrially for commercial large-scale production.

A nano-structured porous platinum (Pt-p) electrode is developed in order to facilitate a low-cost, fully implantable cortical electrical stimulation (CES) device for animal experiments. The Pt-p electrode is made by electrochemical surface modification, which results in a nano-structured porous top layer (∼1.5 μm thick) revealed by scanning electron microscopy (SEM). A roughness factor (R) of 35.9 indicated by cyclic voltammetry (CV) implies expanded interface area up to 35.9 times. The impedance of the Pt-p microelectrode decreased by 77% at 1 kHz measured using electrochemical impedance spectroscopy (EIS). Moreover, the voltage of the electrode under monopolar stimulation in response to stimuli has been reduced by approximately one third, which favors lower compliance voltages and power saving for the implant. Importantly, the Pt-p microelectrode demonstrates excellent mechanical stability and electrochemical stability during ultrasonic bath and CV in vitro. Finally, the implant has been implanted in the cortex of rats for CES study up to 16 days. There were no significant statistical differences in neuronal survival data between CES group and non-stimulation group, suggesting satisfied neuron viability under long-term electrical stimulation with Pt-p electrodes. This shows that the Pt-p electrode is safe and effective in the application as a low cost CES implant.

Aiming to improve the charge transfer ability at the neural electrode interface, a new and simple method for amino-functionalization of carbon nanotubes (CNTs) has been attempted, and the amino-functionalized CNTs after quaternization were electrodeposited onto pretreated Pt microelectrodes using an 80 V cell voltage in anhydrous solvent. The formation of amino-functionalized CNTs and electrodeposition of the functionalized CNTs were confirmed by Fourier transform infrared (FT-IR) spectroscopy and SEM images respectively. The deposited electrodes exhibited a reduction by 91% in the electrode impedance at 1 kHz compared to smooth Pt electrodes. In addition, the CNT deposited electrodes possessed a large charge storage capacity, and provided an increased safe charge injection (Qinj) limit (1.6–2.0 mC/cm2) that is ten times higher than that of Pt electrodes. Electrochemical stability is another important characteristic required by neural electrodes. It was found that the CNT deposited microelectrodes were very stable during electrical stimulation, which is highly desirable for implantable neural electrodes. In summary, this study developed an easy and mild coating method for elevating the safe charge injection limit of neural microelectrodes, which can be applied to neural prostheses such as visual prostheses.Highlights► A new and simple amine-functionalized method for CNTs was investigated. ► Amine-functionalized CNTs was electrodeposited on the pretreated Pt microelectrodes firstly. ► The CNTs deposited electrode reveals a good electrochemical stability. ► The modification provides a new method to improve implantable neural microelectrodes.

The study investigated the influence of poly(vinyl alcohol)/poly(acrylic acid) interpenetrating polymer networks (PVA/PAA IPNs) hydrogel film coatings on interface stability between electrodeposited iridium oxide films (EIROF) microelectrodes and neural tissue by monitoring changes in impedance. The impedance of EIROF and PtIr microelectrodes (d = 75 μm) with and without the coatings was measured weekly, while the microelectrodes were implanted into the motor cortex of rats (n = 48) for 28 days. The results show that without PVA/PAA coatings the impedance of EIROF microelectrodes was higher than that of PtIr microelectrodes in vivo, however, the impedance of EIROF microelectrodes with coatings resulted in the lowest impedance compared to the impedance of both PtIr microelectrodes with coatings and EIROF microelectrodes without coatings through the whole test period. Moreover, compared to the EIROF microelectrodes without coatings, the impedance of EIROF microelectrodes with coatings reduced by ∼40% at day 21 after implantation. The PVA/PAA IPNs hydrogel coating acted as a stable ions conductive layer which enabled the EIROF microelectrodes to maintain its electrochemical superiority in vivo and enhanced the mechanical stability of the EIROF microelectrodes. The study demonstrates a concept of neural microelectrode modification, in which the hydrophilic property is crucial.

Platinum (Pt) implants coated with poly (3, 4-ethylenedioxythiophene)/carbon nanotube (PEDOT/CNT) composite films were implanted into the brain of rats, and the brain response was evaluated 6 weeks after the implantation. The surface morphology of Pt implants with and without the PEDOT/CNT coating was studied using scanning electron microscopy (SEM). After 6 weeks post-implantation, the expression of laminin (vascular endothelial marker) and neuronal nuclei (NeuN, neuronal marker) were evaluated by immnohistochemistry. It is revealed that the obvious improvements of the surface density of blood vessels and neurons aound the Pt implants with the coating, which were evidenced by laminin and NeuN staining in the zone within the distance of 150 μm to the implant interface. These results suggest the PEDOT/CNT composite films can improve the biocompatibility of the Pt electrodes while it is implanted in brain.

An efficient and reliable electrochemical method for preparing electrodeposited iridium oxide film (EIROF) microelectrodes by an anodic electrochemical process is reported. The EIROF microelectrodes exhibited remarkably high safe charge injection (Qinj) limits of ∼2.6 mC/cm2 for anodic-first pulses and ∼1.4 mC/cm2 for cathodic-first pulses, and the electrode impedance at 1 kHz was significantly reduced by ∼92%. The EIROF microelectrodes also exhibited good mechanical and electrochemical stability, as well as an excellent super-Nernstian slope in a broad pH range (1–13). All of these characteristics are greatly desired for neural sensor applications including electrical neural microstimulation, neural signal recording as well as pH monitoring in vivo. Moreover, the electrodepositing method can be facilely incorporated into thin-film technology and Microelectromechanical Systems (MEMS).

A nano-structured porous platinum (Pt-p) electrode is developed in order to facilitate a low-cost, fully implantable cortical electrical stimulation (CES) device for animal experiments. The Pt-p electrode is made by electrochemical surface modification, which results in a nano-structured porous top layer (∼1.5 μm thick) revealed by scanning electron microscopy (SEM). A roughness factor (R) of 35.9 indicated by cyclic voltammetry (CV) implies expanded interface area up to 35.9 times. The impedance of the Pt-p microelectrode decreased by 77% at 1 kHz measured using electrochemical impedance spectroscopy (EIS). Moreover, the voltage of the electrode under monopolar stimulation in response to stimuli has been reduced by approximately one third, which favors lower compliance voltages and power saving for the implant. Importantly, the Pt-p microelectrode demonstrates excellent mechanical stability and electrochemical stability during ultrasonic bath and CV in vitro. Finally, the implant has been implanted in the cortex of rats for CES study up to 16 days. There were no significant statistical differences in neuronal survival data between CES group and non-stimulation group, suggesting satisfied neuron viability under long-term electrical stimulation with Pt-p electrodes. This shows that the Pt-p electrode is safe and effective in the application as a low cost CES implant.

In this study, we investigated whether fully implantable CES with low current density and varying low-frequency burst impulse train enhances functional recovery and promotes brain remodeling in both the ipsilesional and contralesional cortex. Adult rats received occlusion of the right middle cerebral artery for 120 min. One week after ischemia, electrodes were implanted to rats with CES lasting 2 weeks followed by 4-week observation period. After 2-week stimulation and 4-week observation period, body weight (BW) of the rats in CES group was higher than that in no stimulation (NS) group. Limb placement test, foot-fault test and beam walking test demonstrate that CES significantly enhanced functional recovery. Immunohistochemical study has shown that CES enhanced angiogenesis and dendritic sprouting, and suppressed inflammatory response in the ischemic cortex. CES also promoted dendritic sprouting and suppressed inflammatory response in the contralesional cortex. These results suggest the stimulation protocol is safe, and greatly improves functional recovery and brain remodeling in the 4 weeks following 2 weeks stimulation.Highlights► We investigate the efficacy of cortical electrical stimulation (CES) in stroke rats. ► CES with varied low frequencies promotes functional recovery. ► CES promotes perilesional brain tissue remodeling. ► CES promotes dendritic sprouting and inhibits astrogliosis in contralateral cortex. ► The stimulation protocol is safe and effective.