Peter J. Gilgunn

Chronic tissue response to carboxymethyl cellulose based dissolvable insertion needle for ultra-small neural probes.

Implantable neural electrodes must drastically improve chronic recording stability before they can be translated into long-term human clinical prosthetics. Previous studies suggest that sub-cellular sized and mechanically compliant probes may result in improved tissue integration and recording longevity. However, currently these design features are restricted by the opposing mechanical requirements needed for minimally damaging insertions. We designed a non-cytotoxic, carboxymethylcellulose (CMC) based dissolvable delivery vehicle (shuttle) to provide the mechanical support for insertion of ultra-small, ultra-compliant microfabricated neural probes. Stiff CMC-based shuttles rapidly soften immediately after being placed ∼1 mm above an open craniotomy as they absorb vapors from the brain. To address this, we developed a sophisticated targeting, high speed insertion (∼80 mm/s), and release system to implant these shuttles. After implantation, the goal is for the shuttle to dissolve away leaving only the electrodes behind. Here we show the histology of chronically implanted shuttles of large (300 μm × 125 μm) and small (100 μm × 125 μm) size at discrete time points over 12 weeks. Early time points show the CMC shuttle expanded after insertion as it absorbed moisture from the brain and slowly dissolved. At later time points neuronal cell bodies populate regions within the original shuttle tract. The large CMC shuttles show that the CMC expansion can cause extended secondary damage. On the other hand, the smaller CMC shuttles show limited secondary damage, wound closure by 4 weeks, absence of activated microglia at 12 weeks, as well as evidence suggesting neural regeneration at the implant site. This shuttle, therefore, shows great promise facilitating the implantation of nontraditional ultra-small, and ultra-compliant probes.

CMOS–MEMS Lateral Electrothermal Actuators

In this paper, a type of lateral electrothermal (ET) actuator fabricated with post-CMOS micromachining is presented. The actuator is a beam with a multimorph structure, composed of CMOS dielectric and metal interconnect. Following structural release, the actuators demonstrate self-assembly under the moments arising from residual stress. Actuation is achieved through the imbalanced thermal expansion of internal interconnect members, whose relative positions and widths determine the magnitude and direction of actuation. Joule heating in discrete polysilicon resistors is used to convert energy from the electrical to thermal domain. For a 1.3-mum-wide 100-mum-long 4.2-mum-thick actuator composed of two driving metal layers, thermal sensitivities up to 18 nm/K are demonstrated with a force of 0.27 muN, given a 112-K temperature change. The analytic model and finite-element-analysis simulation output for thermal sensitivity agree with experiment to within 6%. Increasing thermal isolation is shown to give a diminishing return on thermal sensitivity and to reduce the thermal cutoff frequency of an actuator from 800 to 150 Hz. A figure of merit called the efficiency-volume ratio is presented and used to compare this paper with several actuators taken from the literature. Lateral ET multimorph actuators are shown to provide an advantage in applications where area is constrained and the load is small.

An ultra-compliant, scalable neural probe with molded biodissolvable delivery vehicle

This paper describes an ultra-compliant parylene-platinum neural probe embedded in a biodissolvable delivery vehicle. High probe compliance is achieved using thin wires (width of 10.0 μm and thickness of 2.7 μm) and by meandering the probe. The insertion of the ultra-compliant probe is achieved by encasing it in a dissolvable delivery vehicle made from molded carboxy-methylcellulose. In vivo implantations of delivery vehicles with 1.5 mm long shanks, widths of 100 μm and 300 μm and a targeted thickness of 135 μm have been done through the dura in the cortex of Sprague-Dawley rats at a speed of 80 mm-s-1. The delivery vehicle becomes a gel over a period of less than three minutes, after which the handling portions of the delivery vehicle are removed leaving the shanks embedded in the brain.

Structural analysis of explanted microelectrode arrays

Structural analysis of explanted and Utah microelectrode arrays (MEA) was performed using scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) to determine the impact of prolonged exposure to the in vivo environment. The MEAs, one designed for recording and one designed for stimulation, had been implanted in the pre- and postcentral gyri, respectively, of a rhesus macaque for 362 days, prior to explantation. Possible processing, handling or implantation damage was observed on individual electrodes on each MEA. Metal degradation/delamination was observed on the stimulating MEA while residual carbon-based matter on the electrode sites and small fissures in the insulation were observed on the recording MEA. The electrode area of the recording MEA had a 5.0x range and the stimulating MEA had a 7.6x range, which did not appear to be related to long-term exposure to tissue.

On the origin of selectivity and anisotropy loss during microstructure release etch

The origin of the loss of Si anisotropy and selectivity to barrier metals during the release etch process of a CMOS-MEMS fabrication flow is investigated experimentally using in situ infrared imaging. The release etch is performed in an inductively coupled plasma etch chamber fitted with a CaF2 window to maximize IR transmission. Suspended disk test structures of varying plan area, thickness, transverse suspension cross-section and suspension length are etched at various bias voltages using a SF6/O2/C4F8 time-multiplexed, Bosch-type, anisotropic DRIE process followed by an O2 polymer removal process and an isotropic SF6 process. Temperatures up to 150 °C are reported for suspended disk test structures during the isotropic SF6 process while the temperature rise during the anisotropic DRIE and polymer removal processes is less than 7 °C. A physical model based on the power balance of the suspended disk test structures is developed and used to extract the proportion of etch heat absorbed by the structures, which is determined to be 0.26. The model predicts that exothermic heat of reaction of Si with neutral F species is the dominant source of heat and is supported by the observed temperature trends.

Modulation of Parylene-C to silicon adhesion using HMDS priming

The effect of hexamethyldisilazane (HMDS) on adhesion between Parylene-C (Px) and silicon is investigated through the use of peel tests and contact angle tests. These tests allow measurement of the adhesive forces and investigation of the possible mechanisms for variations in adhesion that can be exploited in processes that require devices to be peeled from their substrate to release them after they are fabricated. An analytical framework is used to quantify the upper limit of adhesion on the basis of the geometric and material properties of a device. It was found that the modulation of adhesion between Px and Si by treatment with HMDS is largely due to the increase in dispersive surface energy of treated Si over the range 5.3–18.1 mJ/m2 based on measured contact angles of 46.3°–74.9° with DI water and 36.2°–55.7° with ethylene glycol. Varying the duration of HMDS vapor prime from 0 min to 2 min modulates the average adhesion between Px and Si from 5 mN/cm to 16 mN/cm.

A CMOS-MEMS rotary microactuator suitable for hard disk drive applications

This work presents an electrothermal rotary actuator fabricated using CMOS-MEMS technology. The device discussed herein shows a rotational stroke of more than 10º and an electrothermal actuation cut-off frequency of 300 Hz. The device footprint is less than 1 mm × 1 mm and power consumption is less than 10 mW. The above performance and the device topology make it attractive for application in hard disk drive (HDD) head actuation applications. This paper details fabrication and characterization of this device and methods for extending its performance to levels needed for HDD applications.

Ultra-compliant neural probes are subject to fluid forces during dissolution of polymer delivery vehicles

Ultra-compliant neural probes implanted into tissue using a molded, biodissolvable sodium carboxymethyl cellulose (Na-CMC)-saccharide composite needle delivery vehicle are subjected to fluid-structure interactions that can displace the recording site of the probe with respect to its designed implant location. We applied particle velocimetry to analyze the behavior of ultra-compliant structures under different implantation conditions for a range of CMC-based materials and identified a fluid management protocol that resulted in the successful targeted depth placement of the recording sites.

Mechanisms of process-induced heating of MEMS structures during plasma release etch

The temperature rise on suspended MEMS structures during a Si plasma release etch process is investigated experimentally using in situ infrared imaging. The process is performed on a commercially available inductively coupled plasma etch chamber and comprises an anisotropic, Bosch-type Si DRIE step, an O 2 inhibitor removal step and a SF 6 isotropic Si etch. Temperatures up to 150°C were observed during the isotropic etch and indicate the exothermic reaction of Si and F is the dominant source of heat. Temperature trends with test structure geometry suggest interplay between radiative cooling and etchant transport determines maximum structure temperature.

Model for aspect ratio dependent etch modulated processing

A time-multiplexed, anisotropic, inductively coupled plasma Si deep reactive ion etch process is characterized in terms of the Si macroload, cross-wafer spatial variation, local pattern density, and feature size. The process regime is established as neutral flux limited, in which material transport occurs in the molecular flow to transition flow regimes. For this process regime, a semiempirical, unified analytic model and a numeric model are developed using the Dushman and Clausing vacuum conductance correction factors, respectively, in the Coburn and Winters model of aspect ratio dependent etching. The experimental reaction probability for etching of Si by F was found to be 0.24 for Dushman’s factor and 0.22 for Clausing’s factor. Each model is validated to ±10% against experimental depth data for microdonut and trench test structures and match each other to within 10% for depths of up to 160 μm. The observed depth range is 64 μm at a depth of 160 μm.