Xinyan Tracy Cui

Surface modification of neural recording electrodes with conducting polymer/biomolecule blends

The interface between micromachined neural microelectrodes and neural tissue plays an important role in chronic in vivo recording. Electrochemical polymerization was used to optimize the surface of the metal electrode sites. Electrically conductive polymers (polypyrrole) combined with biomolecules having cell adhesion functionality were deposited with great precision onto microelectrode sites of neural probes. The biomolecules used were a silk-like polymer having fibronectin fragments (SLPF) and nonapeptide CDPGYIGSR. The existence of protein polymers and peptides in the coatings was confirmed by reflective microfocusing Fourier transform infrared spectroscopy (FTIR). The morphology of the coating was rough and fuzzy, providing a high density of bioactive sites for interaction with neural cells. This high interfacial area also helped to lower the impedance of the electrode site and, consequently, to improve the signal transport. Impedance spectroscopy showed a lowered magnitude and phase of impedance around the biologically relevant frequency of 1 kHz. Cyclic voltammetry demonstrated the intrinsic redox reaction of the doped polypyrrole and the increased charge capacity of the coated electrodes. Rat glial cells and human neuroblastoma cells were seeded and cultured on neural probes with coated and uncoated electrodes. Glial cells appeared to attach better to polypyrrole/SLPF-coated electrodes than to uncoated gold electrodes. Neuroblastoma cells grew preferentially on and around the polypyrrole/CDPGYIGSR-coated electrode sites while the polypyrrole/CH(3)COO(-)-coated sites on the same probe did not show a preferential attraction to the cells. These results indicate that we can adjust the chemical composition, morphology, electronic transport, and bioactivity of polymer coatings on electrode surfaces on a multichannel micromachined neural probe by controlling electrochemical deposition conditions.

Electrically controlled drug delivery from graphene oxide nanocomposite films.

On-demand, local delivery of drug molecules to target tissues provides a means for effective drug dosing while reducing the adverse effects of systemic drug delivery. This work explores an electrically controlled drug delivery nanocomposite composed of graphene oxide (GO) deposited inside a conducting polymer scaffold. The nanocomposite is loaded with an anti-inflammatory molecule, dexamethasone, and exhibits favorable electrical properties. In response to voltage stimulation, the nanocomposite releases drug with a linear release profile and a dosage that can be adjusted by altering the magnitude of stimulation. No drug passively diffuses from the composite in the absence of stimulation. In vitro cell culture experiments demonstrate that the released drug retains its bioactivity and that no toxic byproducts leach from the film during electrical stimulation. Decreasing the size and thickness of the GO nanosheets, by means of ultrasonication treatment prior to deposition into the nanocomposite, alters the film morphology, drug load, and release profile, creating an opportunity to fine-tune the properties of the drug delivery system to meet a variety of therapeutic needs. The high level of temporal control and dosage flexibility provided by the electrically controlled GO nanocomposite drug delivery platform make it an exciting candidate for on-demand drug delivery.

Poly (3,4-Ethylenedioxythiophene) for Chronic Neural Stimulation

Chronic neural stimulation using microelectrode arrays requires highly stable and biocompatible electrode materials with high charge injection capability. Conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) was electrochemically deposited on thin film Pt electrodes of stimulation electrode arrays to evaluate its properties for chronic stimulation. The coated electrodes demonstrated much lower impedance than thin film Pt due to the high surface area and high ion conductivity across the film. The PEDOT film also presents intrinsic redox activity which contributes to the low impedance as well as a much higher charge storage capacity. The charge injection limit of PEDOT electrode was found to be 2.3 mC/cm2 , comparable to IrOx and much higher than thin film Pt. Under biphasic stimulation, the coated electrodes exhibited lower voltage and linear voltage excursion. Well-coated PEDOT electrodes were stable under chronic stimulation conditions, suggesting that PEDOT is a promising electrode material to be further developed for chronic neural stimulation applications.

Highly stable carbon nanotube doped poly(3,4-ethylenedioxythiophene) for chronic neural stimulation

The function and longevity of implantable microelectrodes for chronic neural stimulation depends heavily on the electrode materials, which need to present high charge injection capability and high stability. While conducting polymers have been coated on neural microelectrodes and shown promising properties for chronic stimulation, their practical applications have been limited due to unsatisfying stability. Here, poly(3,4-ethylenedioxythiophene) (PEDOT) doped with pure carbon nanotubes (CNTs) was electrochemically deposited on Pt microelectrodes to evaluate its properties for chronic stimulation. The PEDOT/CNT coated microelectrodes demonstrated much lower impedance than the bare Pt, and the PEDOT/CNT film exhibited excellent stability. For both acute and chronic stimulation tests, there is no significant increase in the impedance of the PEDOT/CNT coated microelectrodes, and none of the PEDOT/CNT films show any cracks or delamination, which have been the limitation for many conducting polymer coatings on neural electrodes. The charge injection limit of the Pt microelectrode was significantly increased to 2.5 mC/cm(2) with the PEDOT/CNT coating. Further in vitro experiments also showed that the PEDOT/CNT coatings are non-toxic and support the growth of neurons. It is expected that this highly stable PEDOT/CNT composite may serve as excellent new material for neural electrodes.

Carbon nanotube nanoreservior for controlled release of anti-inflammatory dexamethasone

On demand release of anti-inflammatory drug or neurotropic factors have great promise for maintaining a stable chronic neural interface. Here we report the development of an electrically controlled drug release system based on conducting polymer and carbon nanotubes. Drug delivery research using carbon nanotubes (CNTs) has taken advantage of the ability of CNTs to load large amounts of drug molecules on their outer surface. However, the utility of the inner cavity of CNTs, which can increase the drug loading capacity, has not yet been explored. In this paper, the use of multi-wall CNTs as nanoreserviors for drug loading and controlled release is demonstrated. The CNTs are pretreated with acid sonication to open their ends and make their outer and inner surfaces more hydrophilic. When dispersed and sonicated in a solution containing the anti-inflammatory drug dexamethasone, experiments show that the pretreated CNTs are filled with the drug solution. To prevent the unwanted release of the drug, the open ends of the drug-filled CNTs are then sealed with polypyrrole (PPy) films formed through electropolymerization. The prepared electrode coating significantly reduced the electrode impedance, which is desired for neural recording and stimulation. More importantly, the coating can effectively store drug molecules and release the bioactive drug in a controlled manner using electrical stimulation. The dexamethasone released from the PPy/CNT film was able to reduce lipopolysaccharide induced microglia activation to the same degree as the added dexamethasone.

The Surface Immobilization of the Neural Adhesion Molecule L1 on Neural Probes and its Effect on Neuronal Density and Gliosis at the Probe/Tissue Interface

Brain tissue inflammatory responses, including neuronal loss and gliosis at the neural electrode/tissue interface, limit the recording stability and longevity of neural probes. The neural adhesion molecule L1 specifically promotes neurite outgrowth and neuronal survival. In this study, we covalently immobilized L1 on the surface of silicon-based neural probes and compared the tissue response between L1 modified and non-modified probes implanted in the rat cortex after 1, 4, and 8 weeks. The effect of L1 on neuronal health and survival, and glial cell reactions were evaluated with immunohistochemistry and quantitative image analysis. Similar to previous findings, persistent glial activation and significant decreases of neuronal and axonal densities were found at the vicinity of the non-modified probes. In contrast, the immediate area (100 μm) around the L1 modified probe showed no loss of neuronal bodies and a significantly increased axonal density relative to background. In this same region, immunohistochemistry analyses show a significantly lower activation of microglia and reaction of astrocytes around the L1 modified probes when compared to the control probes. These improvements in tissue reaction induced by the L1 coating are likely to lead to improved functionality of the implanted neural electrodes during chronic recordings.

Enhanced catalytic and dopamine sensing properties of electrochemically reduced conducting polymer nanocomposite doped with pure graphene oxide.

Significantly enhanced catalytic activity of a nanocomposite composed of conducting polymer poly (3,4-ethylenedioxythiophene) (PEDOT) doped with graphene oxide (GO) was achieved through a simple electrochemical reduction process. The nanocomposite (PEDOT/GO) was electrodeposited on an electrode and followed by electrochemical reduction, and the obtained reduced nanocomposite (PEDOT/RGO) modified electrode exhibited lowered electrochemical impedance and excellent electrocatalytic activity towards the oxidation of dopamine. Based on the excellent catalytic property of PEDOT/RGO, an electrochemical sensor capable of sensitive and selective detection of DA was developed. The fabricated sensor can detect DA in a wide linear range from 0.1 to 175μM, with a detection limit of 39nM, and it is free from common interferences such as uric acid and ascorbic acid.

Highly sensitive single polyaniline nanowire biosensor for the detection of immunoglobulin G and myoglobin.

A single polyaniline (PANI) nanowire-based biosensor was established to detect immunoglobulin G (IgG) and myoglobin (Myo), which is one of the cardiac biomarkers. The single PANI nanowires were fabricated via an electrochemical growth method, in which single nanowires were formed between a pair of patterned electrodes. The single PANI nanowires were functionalized with monoclonal antibodies (mAbs) of IgG or Myo via a surface immobilization method, using 1-ethyl-3-(3-dimethyaminopropyl) carbodiimide (EDC), and N-hydroxysuccinimde (NHS). The functionalization was then verified by Raman spectroscopy and fluorescence microscopy. The target proteins of IgG and Myo were detected by measuring the conductance change of functionalized single PANI nanowires owing to the capturing of target proteins by mAbs. The detection limit was found to be 3 ng/mL for IgG and 1.4 ng/mL for Myo. No response was observed when single nanowires were exposed to a non-specific protein, demonstrating excellent specificity to expected target detection. Together with the fast response time (a few seconds), high sensitivity, and good specificity, this single PANI nanowire-based biosensor shows great promise in the detection of cardiac markers and other proteins.

Surface immobilization of neural adhesion molecule L1 for improving the biocompatibility of chronic neural probes: In vitro characterization

Silicon-based implantable neural electrode arrays are known to experience failure during long-term recording, partially due to host tissue responses. Surface modification and immobilization of biomolecules may provide a means to improve their biocompatibility and integration within the host brain tissue. Previously, the laminin biomolecule or laminin fragments have been used to modify the neural probes silicon surface to promote neuronal attachment and growth. Here we report the successful immobilization of the L1 biomolecule on a silicon surface. L1 is a neuronal adhesion molecule that can specifically promote neurite outgrowth and neuronal survival. Silane chemistry and the heterobifunctional coupling agent 4-maleimidobutyric acid N-hydroxysuccinimide ester (GMBS) were used to covalently bind these two biomolecules onto the surface of silicon dioxide wafers, which mimic the surface of silicon-based implantable neural probes. After covalent binding of the biomolecules, polyethylene glycol (PEG)-NH(2) was used to cap the unreacted GMBS groups. Surface immobilization was verified by goniometry, dual polarization interferometry, and immunostaining techniques. Primary murine neurons or astrocytes were used to evaluate the modified silicon surfaces. Both L1- and laminin-modified surfaces promoted neuronal attachment, while the L1-modified surface demonstrated significantly enhanced levels of neurite outgrowth (p<0.05). In addition, the laminin-modified surface promoted astrocyte attachment, while the L1-modified surface showed significantly reduced levels of astrocyte attachment relative to the laminin-modified surface and other controls (p<0.05). These results demonstrate the ability of the L1-immobilized surface to specifically promote neuronal growth and neurite extension, while inhibiting the attachment of astrocytes, one of the main cellular components of the glial sheath. Such unique properties present vast potentials to improve the biocompatibility and chronic recording performance of neural probes.

Detection of Cardiac Biomarkers Using Single Polyaniline Nanowire-Based Conductometric Biosensors

The detection of myoglobin (Myo), cardiac troponin I (cTnI), creatine kinase-MB (CK-MB), and b-type natriuretic peptide (BNP) plays a vital role in diagnosing cardiovascular diseases. Here we present single site-specific polyaniline (PANI) nanowire biosensors that can detect cardiac biomarkers such as Myo, cTnI, CK-MB, and BNP with ultra-high sensitivity and good specificity. Using single PANI nanowire-based biosensors integrated with microfluidic channels, very low concentrations of Myo (100 pg/mL), cTnI (250 fg/mL), CK-MB (150 fg/mL), and BNP (50 fg/mL) were detected. The single PANI nanowire-based biosensors displayed linear sensing profiles for concentrations ranging from hundreds (fg/mL) to tens (ng/mL). In addition, devices showed a fast (few minutes) response satisfying respective reference conditions for Myo, cTnI, CK-MB, and BNP diagnosis of heart failure and for determining the stage of the disease. This single PANI nanowire-based biosensor demonstrated superior biosensing reliability with the feasibility of label free detection and improved processing cost efficiency due to good biocompatibility of PANI to monoclonal antibodies (mAbs). Therefore, this development of single PANI nanowire-based biosensors can be applied to other biosensors for cancer or other diseases.