Curtis Lee

High strain biocompatible polydimethylsiloxane-based conductive graphene and multiwalled carbon nanotube nanocomposite strain sensors

High performance strain sensors were achieved featuring simple, low-cost construction involving the screen printing of combinations of multi-walled carbon nanotube and graphene nano-platelet nanocomposites on biocompatible and flexible polymer substrates. Conductivity and thermal coefficients of resistance of different conductive nanocomposite sensor materials were measured. The zero current resistance and gauge factor of printed sensors was characterized. The combination of high strain operation (up to 40%), high gauge factor (GF > 100), and biocompatible construction pave the way for applications such as minimally invasive in vivo strain measurements.

3D Parylene sheath neural probe for chronic recordings

OBJECTIVE Reliable chronic recordings from implanted neural probes remain a significant challenge; current silicon-based and microwire technologies experience a wide range of biotic and abiotic failure modes contributing to loss of signal quality. APPROACH A multi-prong alternative strategy with potential to overcome these hurdles is introduced that combines a novel three dimensional (3D), polymer-based probe structure with coatings. Specifically, the Parylene C sheath-based neural probe is coated with neurotrophic and anti-inflammatory factors loaded onto a Matrigel carrier to encourage the ingrowth of neuronal processes for improved recording quality, reduce the immune response, and promote improved probe integration into brain tissue for reliable, long-term implementation compared to its rigid counterparts. MAIN RESULTS The 3D sheath structure of the probe was formed by thermal molding of a surface micromachined Parylene C microchannel, with electrode sites lining the interior and exterior regions of the lumen. Electrochemical characterization of the probes via cyclic voltammetry and electrochemical impedance spectroscopy was performed and indicated suitable electrode properties for neural recordings (1 kHz electrical impedance of ∼200 kΩ in vitro). A novel introducer tool for the insertion of the compliant polymer probe into neural tissue was developed and validated both in vitro using agarose gel and in vivo in the rat cerebral cortex. In vivo electrical functionality of the Parylene C-based 3D probes and their suitability for recording the neuronal activity over a 28-day period was demonstrated by maintaining the 1 kHz electrical impedance within a functional range (<400 kΩ) and achieving a reasonably high signal-to-noise ratio for detection of resolvable multi-unit neuronal activity on most recording sites in the probe. Immunohistochemical analysis of the implant site indicated strong correlations between the quality of recorded activity and the neuronal/astrocytic density around the probe. SIGNIFICANCE The provided electrophysiological and immunohistochemical data provide strong support to the viability of the developed probe technology. Furthermore, the obtained data provide insights into further optimization of the probe design, including tip geometry, use of neurotrophic and anti-inflammatory drugs in the Matrigel coating, and placement of the recording sites.

Epoxy-less packaging methods for electrical contact to parylene-based flat flexible cables

We present two methods for establishing rapid epoxy-less electrical connectivity to Parylene-based flat flexible cables (FFC). The first utilizes commercially available zero-insertion-force (ZIF) connector technology and the second utilizes a custom-fabricated connector with an acrylic/polydimethylsiloxane (PDMS) interface featuring screen-printed contacts. A contact pitch of 0.5 mm was achieved using both connector methods. These techniques are simple to implement, reversible, scalable and unlike epoxy-based approaches, do not require manual intervention to secure individual contacts.

Matrigel coatings for Parylene sheath neural probes

The biologically derived hydrogel Matrigel (MG) was used to coat a Parylene-based sheath intracortical electrode to act as a mechanical and biological buffer as well as a matrix for delivering bioactive molecules to modulate the cellular response and improve recording quality. MG was loaded with dexamethasone to reduce the immune response together with nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) to maintain neuronal density and encourage neuronal ingrowth toward electrodes within the sheath. Coating the Parylene sheath electrode with the loaded MG significantly improved the signal-to-noise ratio for neural events recorded from the motor cortex in rat for more than 3 months. Electron microscopy showed even coverage of both the Parylene substrate and the platinum recording electrodes. Electrochemical impedance spectroscopy (EIS) of coated electrodes in 1× phosphate-buffered saline demonstrated low impedance required for recording neural signals. This result was confirmed by in vivo EIS data, showing significantly decreased impedance during the first week of recording. Dexamethasone, NGF, and BDNF loaded into MG were released within 1 day in 1× phosphate-buffered saline. Although previous studies showed that MG loaded with either the immunosuppressant or the neurotrophic factor cocktail provided modest improvement in recording quality in a 1-month in vivo study, the combination of these bioactive molecules did not improve the signal quality over coating probes with only MG in a 3-month in vivo study. The MG coating may further improve recording quality by optimizing the in vivo release profile for the bioactive molecules.

Effects of lattice mismatch on InxGa1−xAs/InP heterojunctions

The conduction‐band discontinuities and interface charge densities of several n‐N isotype InxGa1−xAs/InP (x≂0.53) heterojunctions with lattice mismatches (Δa/a) ranging from +0.26 to −0.24% were measured using capacitance‐voltage techniques. To facilitate these measurements, organic‐on‐inorganic contact barrier diodes were used. Extremely low interface charge densities (<1×1010 cm−2) are obtained for all the samples, which are approximately one order of magnitude lower than previously reported values for these heterojunctions. We find that the interface charge density is independent of the magnitude of lattice mismatch and temperature. All the samples show a clear peak‐and‐notch in their apparent free‐carrier concentration profiles at temperatures as low as 83 K. This is in contrast to results reported previously where the notch is observed to disappear at low temperature. The measured heterojunction conduction‐band discontinuity is also found to be temperature independent, with a value of 0.22±0.02 eV.

Pre-implantation electrochemical characterization of a Parylene C sheath microelectrode array probe

We present the preliminary electrochemical characterization of 3D Parylene C sheath microelectrode array probes towards realizing reliable chronic neuroprosthetic recordings. Electrochemical techniques were used to verify electrode integrity after our novel post-fabrication thermoforming process was applied to flat surface micromachined structures to achieve a hollow sheath probe shape. Characterization of subsequent neurotrophic coatings was performed and accelerated life testing was used to simulate six months in vivo. Prior to probe implantation, crosstalk was measured and electrode surface properties were evaluated through the use of electrochemical impedance spectroscopy.

3D Parylene sheath probes for reliable, long-term neuroprosthetic recordings

Parylene C neural probes with a 3D sheath structure are introduced as a novel interface for long-term intracortical neural recording. 3D sheath structures were assembled from surface micromachined Parylene microchannels by thermoforming the thermoplastic around a solid microwire mold. Multiple Pt electrodes lined the interior and exterior of the sheath. Electrochemical characterization of the electrodes confirmed impedance values (50-250 KΩ at 1 kHz) suitable for neural recordings. A novel insertion approach was developed that temporarily stiffens the neural probes for surgical implantation and optimized in agarose brain tissue model. Sheath probes implanted into rat cortex recorded neural signals for four weeks. To achieve long-term, reliable recordings, the sheath structures will be coated with eluting neurotrophic factors to promote and attract neural ingrowth towards electrode sites.

Mechanical Properties of Thin-Film Parylene–Metal–Parylene Devices

Structures and testing methods for measuring the adhesion strength, minimum bending diameter, and bending fatigue performance of thin film polymer electronic architectures were developed and applied to Parylene-metal-Parylene systems with and without the moisture barrier Al2O3 (deposited using atomic layer deposition (ALD)). Parylene-metal-Parylene interfaces had the strongest average peel test strength and Parylene-Parylene interfaces had the weakest peel. Layers of ALD Al2O3 deposited within the device increased the average peel strength for Parylene-Parylene interfaces when combined with silane A-174, but did not increase the Parylene-metal-Parylene interface. Metal traces in the middle of 24 µm thick Parylene-metal-Parylene devices had a minimum bending diameter of ~130 µm before breaking and being measured as an open circuit. The addition of one layer of Al2O3 above the traces allowed them to be completely creased when bent away from the Al2O3 layer without producing an open circuit, but increased the minimum bending diameter to ~450 µm when bent away from the Al2O3. Although fatigue testing produced cracks in all devcies after 100k bends, the insulation of the Parylene-metal-Parylene devices without Al2O3 performed well with electrochemical impedance spectroscopy (EIS) showing only small decreases in impedance magnitude and small increases of impedance phase at low frequencies. However, devices with Al2O3 failed during EIS due to Al2O3 being deteriorated by water.

Evaluation of post-fabrication thermoforming process for intracortical Parylene sheath electrode

The chemical, mechanical, and electrochemical attributes of the Parylene sheath electrode (PSE) were evaluated following a post-fabrication thermoforming process to determine its impact on both the polymer and thin film platinum materials. The three-dimensional conical shape of the PSE was formed via thermal molding of a surface micromachined Parylene C microchannel using a custom shape-forming microwire having the desired taper at 200°C for 48 hours under vacuum. Contact angle and Fourier transform infrared spectroscopy measurements indicated that the thermoforming process resulted in no significant changes to the surface and bulk chemistry of Parylene. The thermoformed Parylene samples possessed greater Youngs modulus, but retained their flexibility. Electrochemical characterization of electrodes before and after thermoforming revealed a decreased storage charge capacity and increased electrode impedance, however, recording functionality was not lost as resolvable neuronal unit activity was successfully obtained post-implantation.

Drug eluting coating for 3D Parylene sheath electrode

Two methods for incorporating drug eluting coatings consisting of Matrigel (MG) loaded with dexamethasone (DEX) onto the Parylene sheath electrode (PSE) were developed and compared. The purpose of the coatings is to reduce the immune response evoked by tissue damage during electrode insertion into the cortex and subsequent sustained aggravation of tissues by the implant. Parylene surfaces are hydrophobic and repel MG, therefore, both physical and chemical methods were investigated to disrupt surface tension and increase surface energy to facilitate even coating onto the PSE. A gelling step was also investigated to improve loading of coating onto PSE. Spectrophotometry was used to measure the amount of DEX loaded onto the PSE. Loading of up to 563 ng of DEX was achieved by using a combination of surface energy modification and coating gelling, whereas sonication assisted coating methods loaded 205 ng.