Jit Muthuswamy

Brain micromotion around implants in the rodent somatosensory cortex

The magnitude of brain tissue micromotion relative to stationary brain implants and its impact on the viability and function of the surrounding brain tissue due to mechanical stresses is poorly understood. The central goal of this study is to characterize surface micromotion in the somatosensory cortex against stationary cylindrical implants. We used a differential variable reluctance transducer (DVRT) in adult rats (n = 6) to monitor micromotion normal to the somatosensory cortex surface. Experiments were performed both in the presence and in the absence of dura mater and displacement measurements were made at three different locations within craniotomies of two different sizes. In anesthetized rats, pulsatile surface micromotion was observed to be in the order of 10-30 microm due to pressure changes during respiration and 2-4 microm due to vascular pulsatility. Brain displacement values due to respiration were significantly lower in the presence of the dura compared to those without the dura. In addition, large inward displacements of brain tissue between 10-60 microm were observed in n = 3 animals immediately following the administration of anesthesia. Such significant micromotion can impact a wide variety of acute and chronic procedures involving any brain implants, precise neurosurgery or imaging and therefore has to be factored in the design of such procedures.

Thin microelectrodes reduce GFAP expression in the implant site in rodent somatosensory cortex

The objective of this study was to test the hypothesis that neural implants with reduced cross-sectional areas will have less glial scarring associated with implantation injury in long-term experiments. In this study, we implanted nine adult rats with two different implants of 12 microm (n = 6), and 25 microm (n = 6) diameters (cross-sectional areas of 68 microm(2), 232 microm(2) respectively) and the expression of glial fibrilliary acidic protein (GFAP) was assessed after 2 weeks and 4 weeks of implantation. In order to facilitate implantation, the 12 microm diameter implants were coated with poly-glycolic acid (PGA), a biodegradable polymer that degraded within minutes of implantation. In n = 3 animals, 25 microm diameter implants also coated with PGA were implanted and assessed for GFAP expression at the end of 4 weeks of implantation. Statistical analysis of the GFAP expression around the different implants demonstrated that after 2 weeks of implantation there is no statistically significant difference in GFAP expression between the 12 microm and the 25 microm diameter implants. However, after 4 weeks of implantation the implant site of 12 microm diameter implants exhibited a statistically significant reduction in GFAP expression when compared to the implant sites of the 25 microm diameter implants (both with and without the PGA coating). We conclude that in neural implants that are tethered to the skull, implant cross-sectional areas of 68 microm(2) and smaller could lead to a reduced glial scarring under chronic conditions. Future studies with longer implant durations can confirm if this observation remains consistent beyond 4 weeks.

Electrostatic microactuators for precise positioning of neural microelectrodes

Microelectrode arrays used for monitoring single and multineuronal action potentials often fail to record from the same population of neurons over a period of time likely due to micromotion of neurons away from the microelectrode, gliosis around the recording site and also brain movement due to behavior. We report here novel electrostatic microactuated microelectrodes that will enable precise repositioning of the microelectrodes within the brain tissue. Electrostatic comb-drive microactuators and associated microelectrodes are fabricated using the SUMMiT V/spl trade/ (Sandias Ultraplanar Multilevel MEMS Technology) process, a five-layer polysilicon micromachining technology of the Sandia National labs, NM. The microfabricated microactuators enable precise bidirectional positioning of the microelectrodes in the brain with accuracy in the order of 1 /spl mu/m. The microactuators allow for a linear translation of the microelectrodes of up to 5 mm in either direction making it suitable for positioning microelectrodes in deep structures of a rodent brain. The overall translation was reduced to approximately 2 mm after insulation of the microelectrodes with epoxy for monitoring multiunit activity. The microactuators are capable of driving the microelectrodes in the brain tissue with forces in the order of several micro-Newtons. Single unit recordings were obtained from the somatosensory cortex of adult rats in acute experiments demonstrating the feasibility of this technology. Further optimization of the insulation, packaging and interconnect issues will be necessary before this technology can be validated in long-term experiments.

Long-term changes in the material properties of brain tissue at the implant–tissue interface

OBJECTIVE Brain tissue undergoes dramatic molecular and cellular remodeling at the implant-tissue interface that evolves over a period of weeks after implantation. The biomechanical impact of such remodeling on the interface remains unknown. In this study, we aim to assess the changes in the mechanical properties of the brain-electrode interface after chronic implantation of a microelectrode. APPROACH Microelectrodes were implanted in the rodent cortex at a depth of 1 mm for different durations-1 day (n = 4), 10-14 days (n = 4), 4 weeks (n = 4) and 6-8 weeks (n = 7). After the initial duration of implantation, the microelectrodes were moved an additional 1 mm downward at a constant speed of 10 µm s(-1). Forces experienced by the microelectrode were measured during movement and after termination of movement. The biomechanical properties of the interfacial brain tissue were assessed from measured force-displacement curves using two separate models-a two-parameter Mooney-Rivlin hyperelastic model and a viscoelastic model with a second-order Prony series. MAIN RESULTS Estimated shear moduli using a second-order viscoelastic model increased from 0.5-2.6 kPa (day 1 of implantation) to 25.7-59.3 kPa (after 4 weeks of implantation) and subsequently decreased to 0.8-7.9 kPa after 6-8 weeks of implantation in 6 of the 7 animals. The estimated elastic modulus increased from 4.1-7.8 kPa on the day of implantation to 24-44.9 kPa after 4 weeks. The elastic modulus was estimated to be 6.8-33.3 kPa in 6 of the 7 animals after 6-8 weeks of implantation. The above estimates suggest that the brain tissue surrounding the microelectrode evolves from a stiff matrix with maximal shear and elastic modulus after 4 weeks of implantation into a composite of two different layers with different mechanical properties-a stiff compact inner layer surrounded by softer brain tissue that is biomechanically similar to brain tissue-during the first week of implantation. Tissue micromotion-induced stresses on the microelectrode constituted 12-55% of the steady-state stresses on the microelectrode on the day of implantation (n = 4), 2-21% of the steady-state stresses after 4 weeks of implantation (n = 4), and 4-10% of the steady-state stresses after 6-8 weeks of implantation (n = 7). SIGNIFICANCE Understanding biomechanical behavior at the brain-microelectrode interface is necessary for the long-term success of implantable neuroprosthetics and microelectrode arrays. Such quantitative physical characterization of the dynamic changes in the electrode-tissue interface will (a) drive the design and development of more mechanically optimal, chronic brain implants, and (b) lead to new insights into key cellular and molecular events such as neuronal adhesion, migration and function in the immediate vicinity of the brain implant.

Flexible Chip-Scale Package and Interconnect for Implantable MEMS Movable Microelectrodes for the Brain

We report here a novel approach called microelectromechanical systems (MEMS) microflex interconnect (MMFI) technology for packaging a new generation of bioMEMS devices that involve movable microelectrodes implanted in brain tissue. MMFI addresses the need for the following: (1) operating space for movable parts and (2) flexible interconnects for mechanical isolation. We fabricated a thin polyimide substrate with embedded bond pads, vias, and conducting traces for the interconnect with a backside dry etch, so that the flexible substrate can act as a thin-film cap for the MEMS package. A double-gold-stud-bump rivet-bonding mechanism was used to form electrical connections to the chip and also to provide a spacing of approximately 15-20 mum for the movable parts. The MMFI approach achieved a chip-scale package that is lightweight and biocompatible and has flexible interconnects and no underfill. Reliability tests demonstrated minimal increases of 0.35, 0.23, and 0.15 mOmega in mean contact resistances under high humidity, thermal cycling, and thermal shock conditions, respectively. High-temperature tests resulted in increases of > 90 and ~ 4.2 mOmega in resistance when aluminum and gold bond pads were used, respectively. The mean time to failure was estimated to be at least one year under physiological conditions. We conclude that MMFI technology is a feasible and reliable approach for packaging and interconnecting bioMEMS devices.

Long-Term Neural Recordings Using MEMS Based Movable Microelectrodes in the Brain

One of the critical requirements of the emerging class of neural prosthetic devices is to maintain good quality neural recordings over long time periods. We report here a novel MEMS (Micro Electro Mechanical Systems) based technology that can move microelectrodes in the event of deterioration in neural signal to sample a new set of neurons. Microscale electro-thermal actuators are used to controllably move microelectrodes post-implantation in steps of approximately 9 μm. In this study, a total of 12 movable microelectrode chips were individually implanted in adult rats. Two of the twelve movable microelectrode chips were not moved over a period of 3 weeks and were treated as control experiments. During the first 3 weeks of implantation, moving the microelectrodes led to an improvement in the average signal to noise ratio (SNR) from 14.61 ± 5.21 dB before movement to 18.13 ± 4.99 dB after movement across all microelectrodes and all days. However, the average root-mean-square values of noise amplitudes were similar at 2.98 ± 1.22 μV and 3.01 ± 1.16 μV before and after microelectrode movement. Beyond 3 weeks, the primary observed failure mode was biological rejection of the PMMA (dental cement) based skull mount resulting in the device loosening and eventually falling from the skull. Additionally, the average SNR for functioning devices beyond 3 weeks was 11.88 ± 2.02 dB before microelectrode movement and was significantly different (p < 0.01) from the average SNR of 13.34 ± 0.919 dB after movement. The results of this study demonstrate that MEMS based technologies can move microelectrodes in rodent brains in long-term experiments resulting in improvements in signal quality. Further improvements in packaging and surgical techniques will potentially enable movable microelectrodes to record cortical neuronal activity in chronic experiments.

Assessment of gliosis around moveable implants in the brain

Repositioning microelectrodes post-implantation is emerging as a promising approach to achieve long-term reliability in single neuronal recordings. The main goal of this study was to (a) assess glial reaction in response to movement of microelectrodes in the brain post-implantation and (b) determine an optimal window of time post-implantation when movement of microelectrodes within the brain would result in minimal glial reaction. Eleven Sprague-Dawley rats were implanted with two microelectrodes each that could be moved in vivo post-implantation. Three cohorts were investigated: (1) microelectrode moved at day 2 (n = 4 animals), (2) microelectrode moved at day 14 (n = 5 animals) and (3) microelectrode moved at day 28 (n = 2 animals). Histological evaluation was performed in cohorts 1-3 at four-week post-movement (30 days, 42 days and 56 days post-implantation, respectively). In addition, five control animals were implanted with microelectrodes that were not moved. Control animals were implanted for (1) 30 days (n = 1), (2) 42 days (n = 2) and (3) 56 days (n = 2) prior to histological evaluation. Quantitative assessment of glial fibrillary acidic protein (GFAP) around the tip of the microelectrodes demonstrated that GFAP levels were similar around microelectrodes moved at day 2 when compared to the 30-day controls. However, GFAP expression levels around microelectrode tips that moved at day 14 and day 28 were significantly less than those around control microelectrodes implanted for 42 and 56 days, respectively. Therefore, we conclude that moving microelectrodes after implantation is a viable strategy that does not result in any additional damage to brain tissue. Further, moving the microelectrode downwards after 14 days of implantation may actually reduce the levels of GFAP expression around the tips of the microelectrodes in the long term.

Vulnerability of the thalamic somatosensory pathway after prolonged global hypoxic-ischemic injury.

The aim of this study was to test the hypothesis that under prolonged global ischemic injury, the somatosensory thalamus and the cortex would manifest differential susceptibility leading to varying degrees of thalamo-cortical dissociation. The thalamic electrical responses displayed increasing suppression with longer durations of ischemia leading to a significant thalamo-cortical electrical dissociation. The data also point to a selective vulnerability of the network oscillations involving the thalamic relay and reticular thalamic neurons. An adult rat model of asphyxial cardiac arrest involving three cohorts with 3 min (G1, n=5), 5 min (G2, n=5) and 7 min (G3, n=5) of asphyxia respectively was used. The cortical evoked response, as quantified by the peak amplitude at 20 ms in the cortical evoked potential, recovers to more than 60% of baseline in all the cases. The multi-unit responses to the somatosensory stimuli recorded from the thalamic ventral posterior lateral (VPL) nuclei consists typically of three components: (1). the ON response (<30 ms after stimulus), (2). the OFF response (period of inhibition, from 30 ms to 100 ms after stimulus) and (3). rhythmic spindles (beyond 100 ms after stimulus). Asphyxia has a significant effect on the VPL ON response at 30 min (P<0.025), 60 min (P<0.05) and 90 min (P<0.05) after asphyxia. Only animals in G3 show a significant suppression (P<0.05) of the VPL ON response when compared to the sham group at 30 min, 60 min and 90 min after asphyxia. There was no significant reduction in somatosensory cortical N20 (negative peak in the cortical response at 20 ms after stimulus) amplitude in any of the three groups with asphyxia indicating a thalamo-cortical dissociation in G3. Further, rhythmic spindle oscillations in the thalamic VPL nuclei that normally accompany the ON response recover either slowly after the recovery of ON response (in the case of G1 and G2) or do not recover at all (in the case of G3).We conclude that there is strong evidence for selective vulnerability of thalamic relay neurons and its network interactions with the inhibitory interneurons in the somatosensory pathway leading to a thalamo-cortical dissociation after prolonged durations of global ischemia.

Single neuronal recordings using surface micromachined polysilicon microelectrodes

Bulk micromachining techniques of silicon have been used successfully in the past several years to microfabricate microelectrodes for monitoring single neurons in acute and chronic experiments. In this study we report for the first time a novel surface micromachining technique to microfabricate a very thin polysilicon microelectrode that can be used for monitoring single-unit activity in the central nervous system. The microelectrodes are 3 mm long and 50 microm x 3.75 microm in cross-section. Excellent signal to noise ratios in the order of 25-35 dB were obtained while recording neuronal action potentials. The microelectrodes successfully penetrated the brains after a microincision of the dura mater. Chronic implantation of the microprobe for up to 33 days produced only minor gliosis. Since the polysilicon shank acts as a conductor, additional processing steps involved in laying conductor lines on silicon substrates are avoided. Further, surface micromachining allows for fabricating extremely thin microelectrodes which could result in decreased inflammatory responses. We conclude that the polysilicon microelectrode reported here could be a complementary approach to bulk-micromachined silicon microelectrodes for chronic monitoring of single neurons in the central nervous system.

Artificial dural sealant that allows multiple penetrations of implantable brain probes.

This study reports extensive characterization of the silicone gel (3-4680, Dow Corning, Midland, MI), for potential use as an artificial dural sealant in long-term electrophysiological experiments in neurophysiology. Dural sealants are important to preserve the integrity of the intracranial space after a craniotomy and in prolonging the lifetime and functionality of implanted brain probes. In this study, we report results of our tests on a commercially available silicone gel with unique properties that make it an ideal dural substitute. The substitute is transparent, elastic, easy to apply, and has re-sealing capabilities, which makes it desirable for applications where multiple penetrations by the brain probe is desirable over an extended period of time. Cytotoxicity tests (for up to 10 days) with fibroblasts and in vivo tests (for 12 weeks) show that the gel is non-toxic and does not produce any significant neuronal degeneration when applied to the rodent cortex even after 12 weeks. In vivo humidity testing showed no sign of CSF leakage for up to 6 weeks. The gel also allows silicon microprobes to penetrate with forces less than 0.5 mN, and a 200-microm diameter stainless steel microprobe with a blunt tip to penetrate with a force less than 2.5 mN. The force dependency on the velocity of penetration and thickness of the gel was also quantified and empirically modeled. The above results demonstrate that the silicone gel (3-4680) can be a viable dural substitute in long-term electrophysiology of the brain.