Allison E. Hess

Diamond-on-Polymer Microelectrode Arrays Fabricated Using a Chemical Release Transfer Process

This paper reports the design, fabrication, and characterization of diamond-on-polymer microelectrode arrays. A “diamond-first” chemical release transfer process was implemented to integrate diamond electrodes, grown at temperatures >; 700 °C, on a temperature-sensitive polynorbornene-based (PNB) substrate, allowing for the advantageous neural interfacing properties of diamond to be utilized in a flexible device. Intracortical probes with two electrodes and peripheral nerve electrode arrays with ten electrodes ranging in area from 800 to 41 000 μm2 were fabricated. Mechanical testing showed that the structures were flexible, with the composite structure having mechanical characteristics similar to bare PNB. Electrical testing confirmed that ohmic contacts were formed without a postanneal step and determined a diamond-on-polymer electrode impedance of ~ 1.5 MΩ at 1 kHz.

A bio-inspired, chemo-responsive polymer nanocomposite for mechanically dynamic microsystems

This paper reports the development of a biologically-inspired, variable-modulus nanocomposite material for mechanically dynamic biomedical microsystems. This nanocomposite is comprised of a poly(vinyl acetate) matrix polymer that is reinforced with rigid cellulose nanofibers, and becomes very flexible when exposed to water. A direct-write CO2 laser was used to pattern structures in this chemical- and temperature-sensitive material. Tensile testing of laser-cut, micron-scale nanocomposite beams was performed using a custom-built tensile tester. These samples displayed a significant reduction in Youngs modulus from 4.1 GPa to 6.1 MPa when the nanocomposite was exposed to phosphate buffered saline. Additionally, the modulus change was observed to be reversible upon drying of soaked tensile samples. As a well-suited application of this nanocomposite, cortical probes utilizing this material as a substrate were fabricated. Gold-coated, dual-shank cortical probes utilizing this nanocomposite as a substrate were shown to record action potentials from a single neuron in a cockroach brain.

Development of a Microfabricated Flat Interface Nerve Electrode Based on Liquid Crystal Polymer and Polynorbornene Multilayered Structures

This paper reports on the development of a mechanically-flexible microfabricated flat interface nerve electrode using liquid crystal polymer (LCP) and polynorbornene (PNB) as the structural materials. The device consists of two electrode arrays each fabricated on a LCP base with thin film Pt electrodes and a photolithographically patterned PNB capping layer. The two arrays are inserted into a silicone housing designed to create a flat interface between the electrodes and the nerve bundle. Electrical tests showed that the resistance of the thin film Pt electrode interconnect traces are unaffected by flexing around a 1.5 mm radius. Electrical testing in PBS shows that the resistance of the traces is about 1 kOmega. A 10 day leakage current test in PBS indicates that the PNB absorbs moisture but still maintains its insulating behavior. These and other tests indicate that the LCP/PNB multilayer may be a viable material system for microfabricated electrodes.

Environmentally-controlled Microtensile Testing of Mechanically-adaptive Polymer Nanocomposites for ex vivo Characterization

Implantable microdevices are gaining significant attention for several biomedical applications. Such devices have been made from a range of materials, each offering its own advantages and shortcomings. Most prominently, due to the microscale device dimensions, a high modulus is required to facilitate implantation into living tissue. Conversely, the stiffness of the device should match the surrounding tissue to minimize induced local strain. Therefore, we recently developed a new class of bio-inspired materials to meet these requirements by responding to environmental stimuli with a change in mechanical properties. Specifically, our poly(vinyl acetate)-based nanocomposite (PVAc-NC) displays a reduction in stiffness when exposed to water and elevated temperatures (e.g. body temperature). Unfortunately, few methods exist to quantify the stiffness of materials in vivo, and mechanical testing outside of the physiological environment often requires large samples inappropriate for implantation. Further, stimuli-responsive materials may quickly recover their initial stiffness after explantation. Therefore, we have developed a method by which the mechanical properties of implanted microsamples can be measured ex vivo, with simulated physiological conditions maintained using moisture and temperature control. To this end, a custom microtensile tester was designed to accommodate microscale samples with widely-varying Youngs moduli (range of 10 MPa to 5 GPa). As our interests are in the application of PVAc-NC as a biologically-adaptable neural probe substrate, a tool capable of mechanical characterization of samples at the microscale was necessary. This tool was adapted to provide humidity and temperature control, which minimized sample drying and cooling. As a result, the mechanical characteristics of the explanted sample closely reflect those of the sample just prior to explantation. The overall goal of this method is to quantitatively assess the in vivo mechanical properties, specifically the Youngs modulus, of stimuli-responsive, mechanically-adaptive polymer-based materials. This is accomplished by first establishing the environmental conditions that will minimize a change in sample mechanical properties after explantation without contributing to a reduction in stiffness independent of that resulting from implantation. Samples are then prepared for implantation, handling, and testing (Figure 1A). Each sample is implanted into the cerebral cortex of rats, which is represented here as an explanted rat brain, for a specified duration (Figure 1B). At this point, the sample is explanted and immediately loaded into the microtensile tester, and then subjected to tensile testing (Figure 1C). Subsequent data analysis provides insight into the mechanical behavior of these innovative materials in the environment of the cerebral cortex.

Mechanical behavior of microstructures from a chemo-responsive polymer nanocomposite based on cotton cellulose nanofibers

This paper reports on the fabrication and characterization of MEMS-scale devices from a mechanically dynamic polymer nanocomposite, consisting of a cotton-derived cellulose nanofibers encased in a poly(vinyl acetate) matrix, with a stiffness modulated by the presence or absence of water. Microtensile testing showed that the Youngs modulus (E) of the nanocomposite is initially ∼2.7 GPa, but reduces to ∼4 MPa upon immersion in water for 7 minutes. A combination of laser-micromachining and lithographic processing was used to produce an intracortical probe with switchable stiffness based on the dynamic nanocomposite. An electrode with area 2800 µm2 was found to have an impedance of 156 kΩ at 1 kHz. This investigation was the first time that a chemoresponsive nanocomposite based on cotton cellulose nanofibers was used in MEMS-scale structures.

A Polynorbornene-Based Microelectrode Array for Neural Interfacing

This paper reports the development of a mechanically-flexible microfabricated flat interface nerve electrode using polynorbornene (PNB) as the structural material. The device consists of two electrode arrays each fabricated on a photolithographically-defined PNB base with thin film Pt electrodes and a photolithographically patterned PNB capping layer. The two arrays are inserted into a silicone housing designed to create a flat interface between the electrodes and the nerve bundle. Electrical tests showed that the resistance of the Pt electrode interconnect traces are unaffected by flexing around a 1.8 mm radius. Electrical testing in phosphate-buffered saline (PBS) shows that resistance of the traces is about 3 kOmega. A 10 day leakage current test in PBS did not produce a detectable change in leakage current. These and other tests indicate that a PNB multilayer system may be viable for microfabricated electrodes.