Cerebral astrocyte response to micromachined silicon implants

Electrochemically controlled release of dexamethasone from conducting polymer polypyrrole coated electrode

Chronic recordings from micromachined neural electrode arrays often fail a few weeks after implantation primarily due to the formation of an astro-glial sheath around the implant. We propose a drug delivery system, from conducting polymer (CP) coatings on the electrode sites, to modulate the inflammatory implant-host tissue reaction. In this study, polypyrrole (PPy) based coatings for electrically controlled and local delivery of the ionic form of an anti-inflammatory drug, dexamethasone (Dex), was investigated. The drug was incorporated in PPy via electropolymerization of pyrrole and released in PBS using cyclic voltammetry (CV). FTIR analysis of the surface showed the presence of Dex and polypyrrole on the coated electrode. The thickness of the coated film was estimated to be approximately 50 nm by ellipsometry. We are able to release 0.5 mug/cm(2) Dex in 1 CV cycle and a total of almost 16 mug/cm(2) Dex after 30 CV cycles. In vitro studies and immunocytochemistry on murine glial cells suggest that the released drug lowers the count of reactive astrocytes to the same extent as the added drug. In addition, the released drug is not toxic to neurons as seen by healthy neuronal viability in the released drug treated cells.

Reliability of signals from a chronically implanted, silicon-based electrode array in non-human primate primary motor cortex

Multiple-electrode arrays are valuable both as a research tool and as a sensor for neuromotor prosthetic devices, which could potentially restore voluntary motion and functional independence to paralyzed humans. Long-term array reliability is an important requirement for these applications. Here, we demonstrate the reliability of a regular array of 100 microelectrodes to obtain neural recordings from primary motor cortex (MI) of monkeys for at least three months and up to 1.5 years. We implanted Bionic (Cyberkinetics, Inc., Foxboro, MA) silicon probe arrays in MI of three Macaque monkeys. Neural signals were recorded during performance of an eight-direction, push-button task. Recording reliability was evaluated for 18, 35, or 51 sessions distributed over 83, 179, and 569 days after implantation, respectively, using qualitative and quantitative measures. A four-point signal quality scale was defined based on the waveform amplitude relative to noise. A single observer applied this scale to score signal quality for each electrode. A mean of 120 (+/- 17.6 SD), 146 (+/- 7.3), and 119 (+/- 16.9) neural-like waveforms were observed from 65-85 electrodes across subjects for all recording sessions of which over 80% were of high quality. Quantitative measures demonstrated that waveforms had signal-to-noise ratio (SNR) up to 20 with maximum peak-to-peak amplitude of over 1200 microv with a mean SNR of 4.8 for signals ranked as high quality. Mean signal quality did not change over the duration of the evaluation period (slope 0.001, 0.0068 and 0.03; NS). By contrast, neural waveform shape varied between, but not within days in all animals, suggesting a shifting population of recorded neurons over time. Arm-movement related modulation was common and 66% of all recorded neurons were tuned to reach direction. The ability for the array to record neural signals from parietal cortex was also established. These results demonstrate that neural recordings that can provide movement related signals for neural prostheses, as well as for fundamental research applications, can be reliably obtained for long time periods using a monolithic microelectrode array in primate MI and potentially from other cortical areas as well.

Development of a cortical visual neuroprosthesis for the blind: the relevance of neuroplasticity

Clinical applications such as artificial vision require extraordinary, diverse, lengthy and intimate collaborations among basic scientists, engineers and clinicians. In this review, we present the state of research on a visual neuroprosthesis designed to interface with the occipital visual cortex as a means through which a limited, but useful, visual sense could be restored in profoundly blind individuals. We review the most important physiological principles regarding this neuroprosthetic approach and emphasize the role of neural plasticity in order to achieve desired behavioral outcomes. While full restoration of fine detailed vision with current technology is unlikely in the immediate near future, the discrimination of shapes and the localization of objects should be possible allowing blind subjects to navigate in a unfamiliar environment and perhaps even to read enlarged text. Continued research and development in neuroprosthesis technology will likely result in a substantial improvement in the quality of life of blind and visually impaired individuals.

Multi-site incorporation of bioactive matrices into MEMS-based neural probes

Methods are presented to incorporate polymer-based bioactive matrices into micro-fabricated implantable microelectrode arrays. Using simple techniques, hydrogels infused with bioactive molecules are deposited within wells in the substrate of the device. This method allows local drug delivery without increasing the footprint of the device. In addition, each well can be loaded individually, allowing spatial and temporal control over diffusion gradients in the microenvironment of the implanted neural interface probe. In vivo testing verified the following: diffusion of the bioactive molecules, integration of the bioactive molecules with the intended neural target and concurrent extracellular recording using nearby electrodes. These results support the feasibility of using polymer gels to deliver bioactive molecules to the region close to microelectrode shanks. This technique for microdrug delivery may serve as a means to intervene with the initial phases of the neuroinflammatory tissue response to permanently implanted microelectrode arrays.

Model-based analysis of cortical recording with silicon microelectrodes

OBJECTIVE

The purpose of this study was to use computational modeling to better understand factors that impact neural recordings with silicon microelectrodes used in brain-machine interfaces.

METHODS

A non-linear cable model of a layer V pyramidal cell was coupled with a finite-element electric field model with explicit representation of the microelectrode. The model system enabled analysis of extracellular neural recordings as a function of the electrode contact size, neuron position, edema, and chronic encapsulation.

RESULTS

The model predicted spike waveforms and amplitudes that were consistent with experimental recordings. Small (< 1000 microm2) and large (10 k microm2) electrode contacts had similar volumes of recording sensitivity, but small contacts exhibited higher signal amplitudes (approximately 50%) when neurons were in close proximity (50 microm) to the electrode. The model results support the notion that acute edema causes a signal decrease ( approximately 24%), and certain encapsulation conditions can result in a signal increase (approximately 17%), a mechanism that may contribute to signal increases observed experimentally in chronic recordings.

CONCLUSIONS

Optimal electrode design is application-dependent. Small and large contact sizes have contrasting recording properties that can be exploited in the design process. In addition, the presence of local electrical inhomogeneities (encapsulation, edema, coatings) around the electrode shank can substantially influence neural recordings and requires further theoretical and experimental investigation.

SIGNIFICANCE

Thought-controlled devices using cortical command signals have exciting therapeutic potential for persons with neurological deficit. The results of this study provide the foundation for refining and optimizing microelectrode design for human brain-machine interfaces.

Controlled release of anti-inflammatory agent alpha-MSH from neural implants

Si-multi-electrode arrays implanted into brain tissue for long-term recording lose electrical connectivity due to the post-implantation inflammatory reaction. This inflammatory reaction creates a physical and electrical gap between the electrode and the surrounding neurons. In this study, novel nitrocellulose-based coatings were developed for the sustained delivery of the anti-inflammatory neuropeptide alpha-melanocyte stimulating hormone (alpha-MSH). alpha-MSH was incorporated in micron-scale nitrocellulose coatings and slow, sustained release over 21 days was attained in vitro. The alpha-MSH released on day 21 was still bioactive, and successfully inhibited nitric oxide (NO) production by LPS-stimulated microglia. The amount of initial drug loading directly affected the release rate, with higher initial loading increasing the mass released but not the percent of drug released. The surface morphology and thickness of the coatings were examined by scanning electron microscopy (SEM) and profilometry. In addition, impedance measurement showed that the alpha-MSH loaded nitrocellulose coatings reduced the magnitude of electrode impedance at the biologically relevant frequency of 1 kHz. In conclusion, nitrocellulose-based, bioactive coatings that release anti-inflammatory agents without increasing the impedence of the electrode were successfully fabricated. These coatings have the potential to reduce inflammation at the electrode-brain interface in vivo, and facilitate long-term recordings from Si-multi-electrode arrays.

A finite-element model of the mechanical effects of implantable microelectrodes in the cerebral cortex

The viability of chronic neural microelectrodes for electrophysiological recording and stimulation depends on several factors, including the encapsulation of the implant by a reactive tissue response. We postulate that mechanical strains induced around the implant site may be one of the leading factors responsible for the sustained tissue response in chronic implants. The objectives of this study were to develop a finite-element model of the probe-brain tissue interface and analyze the effects of tethering forces, probe-tissue adhesion and stiffness of the probe substrate on the interfacial strains induced around the implant site. A 3D finite-element model of the probe-brain tissue microenvironment was developed and used to simulate interfacial strains created by 'micromotion' of chronically implanted microelectrodes. Three candidate substrates were considered: (a) silicon, (b) polyimide and (c) a hypothetical 'soft' material. Simulated tethering forces resulted in elevated strains both at the tip and at the sharp edges of the probe track in the tissue. The strain fields induced by a simulated silicon probe were similar to those induced by a simulated polyimide probe, albeit at higher absolute values for radial tethering forces. Simulations of poor probe-tissue adhesion resulted in elevated strains at the tip and delamination of the tissue from the probe. A tangential tethering force results in 94% reduction in the strain value at the tip of the polyimide probe track in the tissue, whereas the simulated 'soft' probe induced two orders of magnitude smaller values of strain compared to a simulated silicon probe. The model results indicate that softer substrates reduce the strain at the probe-tissue interface and thus may also reduce tissue response in chronic implants.

Response of brain tissue to chronically implanted neural electrodes

Chronically implanted recording electrode arrays linked to prosthetics have the potential to make positive impacts on patients suffering from full or partial paralysis. Such arrays are implanted into the patient's cortical tissue and record extracellular potentials from nearby neurons, allowing the information encoded by the neuronal discharges to control external devices. While such systems perform well during acute recordings, they often fail to function reliably in clinically relevant chronic settings. Available evidence suggests that a major failure mode of electrode arrays is the brain tissue reaction against these implants, making the biocompatibility of implanted electrodes a primary concern in device design. This review presents the biological components and time course of the acute and chronic tissue reaction in brain tissue, analyses the brain tissue response of current electrode systems, and comments on the various material science and bioactive strategies undertaken by electrode designers to enhance electrode performance.

Biomechanical analysis of silicon microelectrode-induced strain in the brain

The ability to successfully interface the brain to external electrical systems is important both for fundamental understanding of our nervous system and for the development of neuroprosthetics. Silicon microelectrode arrays offer great promise in realizing this potential. However, when they are implanted into the brain, recording sensitivity is lost due to inflammation and astroglial scarring around the electrode. The inflammation and astroglial scar are thought to result from acute injury during electrode insertion as well as chronic injury caused by micromotion around the implanted electrode. To evaluate the validity of this assumption, the finite element method (FEM) was employed to analyze the strain fields around a single Michigan Si microelectrode due to simulated micromotion. Micromotion was mimicked by applying a force to the electrode, fixing the boundaries of the brain region and applying appropriate symmetry conditions to nodes lying on symmetry planes. Characteristics of the deformation fields around the electrode including maximum electrode displacement, strain fields and relative displacement between the electrode and the adjacent tissue were examined for varying degrees of physical coupling between the brain and the electrode. Our analysis demonstrates that when physical coupling between the electrode and the brain increases, the micromotion-induced strain of tissue around the electrode decreases as does the relative slip between the electrode and the brain. These results support the use of neuro-integrative coatings on electrode arrays as a means to reduce the micromotion-induced injury response.

Visualization of the intact interface between neural tissue and implanted microelectrode arrays

This research presents immunohistochemical strategies for assessing the interactions at the immediate interface between micro-scale implanted devices and the surrounding brain tissue during inflammatory astrogliotic reactions. This includes preparation, microscopy and analysis techniques for obtaining images of the intimate contact between neural cells and the surface of implantable micro-electromechanical systems (MEMS) devices. The ability to visualize the intact interface between an implant and the surrounding tissue allows researchers to examine tissue that is unchanged from its native implanted state. Conversely, current popular techniques involve removing the implant. This tends to cause damage to the tissue immediately surrounding the implant and can hinder one's ability to differentiate inflammatory responses to the implant versus physical damage occurring from removal of the implant from the tissue. Due to advances in microscopy and staining techniques, it is now possible to visualize the intact tissue-implant interface. This paper presents the development of imaging techniques for visualizing the intact interface between neural tissue and implanted devices. This is particularly important for understanding both the acute and chronic neuroinflammatory responses to devices intended for long-term use in a prosthetic system. Non-functional, unbonded devices were imaged in vitro and in vivo at different times post-implantation via a range of techniques. Using these techniques, detailed interactions could be seen between delicate cellular processes and the electrode surface, which would have been destroyed using conventional histology processes.

Dexamethasone treatment reduces astroglia responses to inserted neuroprosthetic devices in rat neocortex

Microfabricated neural prosthetic devices hold great potential for increasing knowledge of brain function and treating patients with lost CNS function. Time-dependent loss of brain-device communication limits long-term use of these devices. Lost CNS function is associated with reactive responses that produce an encapsulating cellular sheath. Since early reactive responses may be associated with injuries produced at the time of device insertion, for example, vascular damage and disruption of the blood-brain barrier, we tested the effectiveness of the synthetic glucocorticoid, dexamethasone, in controlling insertion- and device-associated reactive responses. Dexamethasone (200 microg/kg) was administered as subcutaneous injections for 1 or 6 days beginning on the day of device insertion. Single shank microfabricated silicon devices were inserted into pre-motor cortex of adult rats. Reactive responses were assessed by immunohistochemistry for glial fibrillary acidic protein (astrocytes), CD11b (microglia), and laminin that labeled extracellular protein deposited around the insertion site and in association with vascular elements. Data were collected by confocal microscopy imaging of 100-microm-thick tissue slices. Reactive responses in vehicle control animals were similar to non-injected control animals. Dexamethasone treatment profoundly effected early and sustained reactive responses observed 1 and 6 weeks following device insertion, respectively. Dexamethasone treatment greatly attenuated astroglia responses, while microglia and vascular responses appeared to be increased. The 6-day treatment was more effective than the single injection regime. These results suggest that anti-inflammatory agents can be used to control reactive responses around inserted neural prosthetic devices and may provide a means to insure their long-term function.

Neuronal cell loss accompanies the brain tissue response to chronically implanted silicon microelectrode arrays

Implantable silicon microelectrode array technology is a useful technique for obtaining high-density, high-spatial resolution sampling of neuronal activity within the brain and holds promise for a wide range of neuroprosthetic applications. One of the limitations of the current technology is inconsistent performance in long-term applications. Although the brain tissue response is believed to be a major cause of performance degradation, the precise mechanisms that lead to failure of recordings are unknown. We observed persistent ED1 immunoreactivity around implanted silicon microelectrode arrays implanted in adult rat cortex that was accompanied by a significant reduction in nerve fiber density and nerve cell bodies in the tissue immediately surrounding the implanted silicon microelectrode arrays. Persistent ED1 up-regulation and neuronal loss was not observed in microelectrode stab controls indicating that the phenotype did not result from the initial mechanical trauma of electrode implantation, but was associated with the foreign body response. In addition, we found that explanted electrodes were covered with ED1/MAC-1 immunoreactive cells and that the cells released MCP-1 and TNF-alpha under serum-free conditions in vitro. Our findings suggest a potential new mechanism for chronic recording failure that involves neuronal cell loss, which we speculate is caused by chronic inflammation at the microelectrode brain tissue interface.

Influence of nanoscale surface roughness on neural cell attachment on silicon

The adherence and viability of neural cells (primary cortical cells) from rat embryo on silicon wafers with varying surface roughness (10 to 250 nm) at the nano scale were investigated. The roughnesses were achieved by using chemical etching. Atomic force microscopy was utilized to determine surface roughness. We examined the adherence and viability of neural cells by using scanning electron microscopy and fluorescence immunoassaying. Antineuron-specific enolase antibody was used for immunostaining. Results from this investigation show that for these specific neural cells, there is an optimum surface roughness range, R(a) = 20 to 100 nm, that promotes cell adhesion and longevity. For silicon-based devices, this optimum surface roughness will be desirable as a suitable material/neuron interface.

Repeated voltage biasing improves unit recordings by reducing resistive tissue impedances

Reactive tissue encapsulation of chronically implanted microelectrode probes can preclude long-term recording of extracellular action potentials. We investigated an intervention strategy for functionally encapsulated microelectrode sites. This method, known as "rejuvenation," involved applying a +1.5 V dc bias to an iridium site for 4 s. Previous studies have demonstrated that rejuvenation resulted in higher signal-to-noise ratios (SNRs) by decreasing noise levels, and reduced 1-kHz site impedances by decreasing the tissue interface resistances. In this study, we have investigated: 1) the duration of a single-voltage bias session and 2) the efficacy of multiple sessions. These questions were addressed through electrophysiological recordings, cyclic voltammetry, and modeling the electrode-tissue interface via an equivalent circuit model fit to impedance spectroscopy data. In the six implants studied, we found SNRs improved for 1-7 days with a peak typically occurring within 24 h of the voltage bias. Root-mean square (RMS) noise of the extracellular recordings decreased for 1-2 days, which paralleled a similar decrease in the adsorbed tissue resistance (Ren) from the model. Implants whose SNR effects lasted more than a day showed stabilized reductions in the extracellular tissue resistance (Rex) and cellular membrane area (Am). Subsequent stimulus sessions were found to drop neural tissue parameters consistently to levels observed immediately after surgery. In most cases, these changes did parallel an improvement in SNR. These findings suggest that rejuvenation may be a useful intervention strategy to prolong the lifetime of chronically implanted microelectrodes.

Acoustic sensor for monitoring adhesion of Neuro-2A cells in real-time

Neuronal adhesion plays a fundamental role in growth, migration, regeneration and plasticity of neurons. However, current methods for studying neuronal adhesion cannot monitor this phenomenon quantitatively in real-time. In this work, we demonstrate the use of an acoustic sensor to measure adhesion of neuro-blastoma cells (Neuro-2A) in real-time. An acoustic sensor consisting of a quartz crystal sandwiched between gold electrodes was placed in a flow cell and filled with 600 microl of phosphate buffered saline (PBS). Two sets of in vitro experiments were performed using sensors that had uncoated gold electrodes and sensors that were coated with a known neuronal adhesion promoter (poly-l-lysine or PLL). The instantaneous resonant frequency and the equivalent motional resistance of the acoustic sensor were monitored every second. Cell Tracker was used to confirm neuronal adhesion to the surface. Addition of 10 microl of media and Neuro-2A cells into the above set-up elicited exponential changes in the resonant frequency and motional resistance of the quartz crystal with time to reach steady state in the range of 2-11 h. The steady-state change in resonant frequency in response to addition of neurons was linearly related to the number of Neuro-2A cells added (R2=0.94). Acoustic sensors coated with the adhesion promoter, PLL showed a much higher change in resonant frequency for approximately the same number of neurons. We conclude that the acoustic sensor has sufficient sensitivity to monitor neuronal adhesion in real-time. This has potential applications in the study of mechanisms of neuron-substrate interactions and the effect of molecular modulators in the extra cellular matrix.

Cognitive neural prosthetics

Research on neural prosthetics has focused largely on using activity related to hand trajectories recorded from motor cortical areas. An interesting question revolves around what other signals might be read out from the brain and used for neural prosthetic applications. Recent studies indicate that goals and expected value are among the high-level cognitive signals that can be used and will potentially enhance the ability of paralyzed patients to communicate with the outside world. Other new findings show that local field potentials provide an excellent source of information about the cognitive state of the subject and are much easier to record and maintain than spike activity. Finally, new movable probe technologies will enable recording electrodes to seek out automatically the best signals for decoding cognitive variables.

Electrical stimulation of excitable tissue: design of efficacious and safe protocols

The physical basis for electrical stimulation of excitable tissue, as used by electrophysiological researchers and clinicians in functional electrical stimulation, is presented with emphasis on the fundamental mechanisms of charge injection at the electrode/tissue interface. Faradaic and non-Faradaic charge transfer mechanisms are presented and contrasted. An electrical model of the electrode/tissue interface is given. The physical basis for the origin of electrode potentials is given. Various methods of controlling charge delivery during pulsing are presented. Electrochemical reversibility is discussed. Commonly used electrode materials and stimulation protocols are reviewed in terms of stimulation efficacy and safety. Principles of stimulation of excitable tissue are reviewed with emphasis on efficacy and safety. Mechanisms of damage to tissue and the electrode are reviewed.

Surface functionalization of ultrananocrystalline diamond films by electrochemical reduction of aryldiazonium salts

The surface functionalization of ultrananocrystalline diamond (UNCD) thin films via the electrochemical reduction of aryl diazonium cations is described. The one-electron-transfer reaction leads to the formation of solution-based aryl radicals, which in turn react with the UNCD surface forming stable covalent C-C bonds. Cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), ac impedance spectroscopy, and contact angle measurements have been employed to characterize the organic overlayer and estimate the surface coverage. The grafting of 3,5-dichlorophenyl groups renders the UNCD surface hydrophobic, whereas the attachment of 4-aminophenyl groups makes the surface relatively hydrophilic. The surface coverage, estimated from the electrochemical and XPS measurements, is as high as 70% of a compact monolayer. The aminophenyl terminated surface was obtained by electrochemical reduction of the tethered nitrophenyl groups. This two-step approach yields a UNCD surface with functional moieties available for the potential covalent coupling of a wide variety of biomolecules (e.g., DNA and proteins).

Model neural prostheses with integrated microfluidics: a potential intervention strategy for controlling reactive cell and tissue responses

Model silicon intracortical probes with microfluidic channels were fabricated and tested to examine the feasibility of using diffusion-mediated delivery to deliver therapeutic agents into the volume of tissue exhibiting reactive responses to implanted devices. Three-dimensional probe structures with microfluidic channels were fabricated using surface micromachining and deep reactive ion etching (DRIE) techniques. In vitro functional tests of devices were performed using fluorescence microscopy to record the transient release of Texas Red labeled transferrin (TR-transferrin) and dextran (TR-dextran) from the microchannels into 1% w/v agarose gel. In vivo performance was characterized by inserting devices loaded with TR-transferrin into the premotor cortex of adult male rats. Brain sections were imaged using confocal microscopy. Diffusion of TR-transferrin into the extracellular space and uptake by cells up to 400 microm from the implantation site was observed in brain slices taken 1 h postinsertion. The reactive tissue volume, as indicated by the presence of phosphorylated mitogen-activated protein kinases (MAPKs), was characterized using immunohistochemistry and confocal microscopy. The reactive tissue volume extended 600, 800, and 400 microm radially from the implantation site at 1 h, 24 h, and 6 weeks following insertion, respectively. These results indicate that diffusion-mediated delivery can be part of an effective intervention strategy for the treatment of reactive tissue responses around chronically implanted intracortical probes.

Anin vitromodel for investigating impedance changes with cell growth and electrical stimulation: implications for cochlear implants

The impedance of stimulating electrodes used in cochlear implants and other neural prostheses often increases post-implantation, and is thought to be due to fibrous tissue encapsulation of the electrode array. Increased impedance results in higher power requirements to stimulate target neurons at set charge densities. We developed an in vitro model to investigate the electrode-tissue interface in a highly controlled environment. This model was tested using three cell types, with and without charge-balanced biphasic electrical stimulation. Under standard tissue culture conditions, a monolayer of cells was grown over the electrode surface. Electrode impedance increased in proportion to the extent of cell coverage of the electrode. Cell type was a significant factor in the amount of impedance increase, with kidney epithelial cells (MDCK) creating the greatest impedance, followed by dissociated rat skin fibroblasts and then macrophages (J774). The application of electrical stimulation to cell-covered electrodes caused impedance fluctuations similar to that seen in vivo, with a lowering of impedance immediately following stimulation, and a recovery to pre-stimulation levels during inactive periods. Examination of these electrodes suggests that the stimulation-induced impedance changes were due to the amount of cell cover over the electrodes. This in vitro technique accurately models the changes in impedance observed with neural prostheses in vivo, and shows the close relationship between impedance and tissue coverage adjacent to the electrode surface. We believe that this in vitro approach holds great promise to further our knowledge of the mechanisms contributing to electrode impedance.

XPS and AFM analysis of antifouling PEG interfaces for microfabricated silicon biosensors

In the past two decades, the biological and medical fields have seen great advances in the development of biosensors capable of quantifying biomolecules. Many of these biosensors have micro- and nano-scale features, are fabricated using biochip technology, and use silicon as a base material. The creation of antifouling sensor interfaces is critical to avoid serious consequences that arise due to their contact with biological fluids. To this end, we have created thin PEG interfaces of various grafting densities on silicon using a single-step PEG-silane coupling reaction scheme. Initial PEG concentration (5-50 mM) and coupling time (0.5-24 h) were varied to attain different grafting densities, and different PEG interfaces so created were analyzed using XPS and AFM. Furthermore, all the PEG interfaces were evaluated using XPS and AFM for their antifouling abilities using fibrinogen as the model protein. Results indicated that PEG interfaces created in this investigation are appropriate for biosensors with micro- and nano-scale features, and are efficient in controlling protein fouling.

Decreased functions of astrocytes on carbon nanofiber materials

Carbon nanofibers possess excellent conductivity properties, which may be beneficial in the design of more effective neural prostheses; however, limited evidence on their cytocompatibility properties currently exists. The objective of the present in vitro study was to determine cytocompatibility properties of formulations containing carbon nanofibers pertinent to neural implant applications. Substrates were prepared from four different types of carbon fibers, two with nanoscale diameters (nanophase, or less than or equal to 100 nm) and two with conventional diameters (or greater than 100 nm). Within these two categories, both a high and a low surface energy fiber were investigated and tested. Carbon fibers were compacted in a manual hydraulic press via a uniaxial loading cycle. Astrocytes (glial scar tissue-forming cells) were seeded onto the substrates for adhesion, proliferation, and long-term function studies (such as total intracellular protein and alkaline phosphatase activity). Results provided the first evidence that astrocytes preferentially adhered and proliferated on carbon fibers that had the largest diameter and the lowest surface energy. Based on these results, composite substrates were also formed using different weight percentages (0-25 wt%) of the nanophase, high surface energy fibers in a polycarbonate urethane matrix. Results provided the first evidence of decreased adhesion of astrocytes with increasing weight percents of the high surface energy carbon nanofibers in the polymer composite; this further demonstrates that formulations containing carbon fibers in the nanometer regime may limit astrocyte functions leading to decreased glial scar tissue formation. Positive interactions with neurons, and, at the same time, limited astrocyte functions leading to decreased gliotic scar tissue formation are essential for increased neuronal implant efficacy.

An ex vivo method for evaluating the biocompatibility of neural electrodes in rat brain slice cultures

Failure of neural recording electrodes implanted in the brain is often attributed to the formation of glial scars around the implant. A leading cause of scar formation is the electrode material. Described below is an approach to evaluate the biocompatibility of novel electrode materials in a representative three-dimensional model. The model, brain slice culture, accounts for the response of the neural tissue in the absence of the systemic response. While limitations of any in vitro model exist, brain slice culture provides an indication of the response of neurons and glia in an environment more indicative of the in vivo environment than two-dimensional cell culture of glia or neurons alone. Polybenzylcyclobutene (BCB) electrodes were developed as test materials for flexible electrodes due to ease of processing, low water uptake, and inherent flexibility when formed in thin sheets. Biocompatibilty of the BCB neural electrodes was evaluated using living brain slices derived from the hippocampal regions of 100 g CD rats. Importantly, fewer animals can be used in brain slice culture to evaluate the neural tissue response than when using live animals, since several slices can be obtained per animal. Cellular response to the electrodes was evaluated at 0, 7, and 14 days. At all time points living cells, both neurons and glia, were observed in the vicinity of the electrode. In addition, cells were observed migrating out from the brain slices onto the shank of the BCB electrode. Brain slice culture is shown to be a viable alternative to in vivo evaluation, in that the response of both neurons and glia can be evaluated in a native three-dimensional state, while sacrificing fewer animals. Future in vivo evaluation with BCB will provide definitive answers on the degree of glial scarring in response to this new and biocompatible electrode material.

CORTICAL NEURAL PROSTHETICS

Control of prostheses using cortical signals is based on three elements: chronic microelectrode arrays, extraction algorithms, and prosthetic effectors. Arrays of microelectrodes are permanently implanted in cerebral cortex. These arrays must record populations of single- and multiunit activity indefinitely. Information containing position and velocity correlates of animate movement needs to be extracted continuously in real time from the recorded activity. Prosthetic arms, the current effectors used in this work, need to have the agility and configuration of natural arms. Demonstrations using closed-loop control show that subjects change their neural activity to improve performance with these devices. Adaptive-learning algorithms that capitalize on these improvements show that this technology has the capability of restoring much of the arm movement lost with immobilizing deficits.

Chronic response of adult rat brain tissue to implants anchored to the skull

Using quantitative immunohistological methods, we examined the brain tissue response to hollow fiber membranes (HFMs) that were either implanted intraparenchymally, as in a cell encapsulation application, or were attached to the skull as in a biosensor application (transcranially). We found that the reaction surrounding transcranially implanted HFMs was significantly greater than that observed with intraparenchymally implanted materials including increases in immunoreactivity against GFAP, vimentin, ED-1 labeled macrophages and microglia, and several extracellular matrix proteins including collagen, fibronectin, and laminin. In general, these markers were elevated along the entire length of transcranially implanted HFMs extending into the adjacent parenchyma up to 0.5 mm from the implant interface. Intraparenchymal implants did not appear to have significant involvement of a fibroblastic component as suggested by a decreased expression of vimentin, fibronectin and collagen-type I at the implant tissue interface. The increase in tissue reactivity observed with transcranially implanted HFMs may be influenced by several mechanisms including chronic contact with the meninges and possibly motion of the device within brain tissue. Broadly speaking, our results suggest that any biomaterial, biosensor or device that is anchored to the skull and in chronic contact with meningeal tissue will have a higher level of tissue reactivity than the same material completely implanted within brain tissue.

Evaluation of the stability of nonfouling ultrathin poly(ethylene glycol) films for silicon-based microdevices

The creation of nonfouling surfaces is one of the major prerequisites for microdevices for biomedical and analytical applications. Poly(ethylene glycol) (PEG), a water soluble, nontoxic, and nonimmunogenic polymer has the unique ability of reducing nonspecific protein adsorption and cell adhesion and, therefore, is generally coupled with a wide variety of surfaces to improve their biocompatibility. The performance of these modified surfaces for long-term biomedical applications largely depends on the stability of these PEG films. To this end, we have investigated the stability of covalently coupled ultrathin PEG films on silicon in aqueous in vivo like conditions for a period of 4 weeks. The PEG-modified silicon substrates were incubated in PBS (37 degrees C, pH 7.4, 5% CO2) for different periods of time and then characterized using the techniques of ellipsometry, contact angle measurement, X-ray photoelectron spectroscopy, and atomic force microscopy. The ability of the PEG-modified surfaces to control protein fouling was examined by protein adsorption studies using fluorescein isothiocyanate labeled bovine serum albumin and ellipsometry. Furthermore, the ability of these films to control fibroblast adhesion was examined. Studies suggest that the PEG-modified surfaces retain their protein and cell repulsive nature even though the PEG film thickness decreases for the period of investigation.

Properties of crosslinked protein matrices for tissue engineering applications synthesized by multiphoton excitation

We demonstrate the fabrication of model scaffolds and extracellular matrices using multiphoton excited photochemistry. This method is three-dimensional in nature and has excellent biocompatibility. Crosslinked matrices were fabricated from the proteins fibrinogen, fibronectin, and concanavalin A using two-photon rose bengal photoactivation and the relatives rates were determined. Immunofluorescence labeling of fibrinogen and fibronectin indicated retention of bioactivity following the multiphoton crosslinking process. Using the fluorescence recovery after photobleaching method, we measured the lateral mobility of fluorescent dyes of different mass and chemistry in order to model the behavior of therapeutic agents and bioactive molecules and found diffusion coefficients within these fabricated structures to be on the order of 10(-9)-10(-10) cm(2)/s, or approximately three to four orders of magnitude slower than in free solution. The precise diffusion coefficients can be smoothly tuned by varying the laser exposure during the fabrication of the matrix, which results in both an increase in crosslink density as well as protein concentration in the matrix. Terminal crosslink density is achieved at integrated high exposure dose and the relative fabrication rates were determined for these proteins. For all the proteins, the range of diffusion coefficients between the threshold for fabrication and the terminal limit is correlated with the change in matrix mesh size as determined by Flory-Rehner swelling analysis. Both normal Fickian as well as hindered anomalous diffusion is observed depending on specific molecular interactions of the tracer dyes and protein host. (c) 2004 Wiley Periodicals, Inc. J Biomed Mater Res 71A: 359-368, 2004.

In vivo multiphoton microscopy of deep brain tissue

Although fluorescence microscopy has proven to be one of the most powerful tools in biology, its application to the intact animal has been limited to imaging several hundred micrometers below the surface. The rest of the animal has eluded investigation at the microscopic level without excising tissue or performing extensive surgery. However, the ability to image with subcellular resolution in the intact animal enables a contextual setting that may be critical for understanding proper function. Clinical applications such as disease diagnosis and optical biopsy may benefit from minimally invasive in vivo approaches. Gradient index (GRIN) lenses with needle-like dimensions can transfer high-quality images many centimeters from the object plane. Here, we show that multiphoton microscopy through GRIN lenses enables minimally invasive, subcellular resolution several millimeters in the anesthetized, intact animal, and we present in vivo images of cortical layer V and hippocampus in the anesthetized Thy1-YFP line H mouse. Microangiographies from deep capillaries and blood vessels containing fluorescein-dextran and quantum dot-labeled serum in wild-type mouse brain are also demonstrated.

Brain responses to micro-machined silicon devices

Micro-machined neural prosthetic devices can be designed and fabricated to permit recording and stimulation of specific sites in the nervous system. Unfortunately, the long-term use of these devices is compromised by cellular encapsulation. The goals of this study were to determine if device size, surface characteristics, or insertion method affected this response. Devices with two general designs were used. One group had chisel-shaped tips, sharp angular corners, and surface irregularities on the micrometer size scale. The second group had rounded corners, and smooth surfaces. Devices of the first group were inserted using a microprocessor-controlled inserter. Devices of the second group were inserted by hand. Comparisons were made of responses to the larger devices in the first group with devices from the second group. Responses were assessed 1 day and 1, 2, 4, 6, and 12 weeks after insertions. Tissues were immunochemically labeled for glial fibrillary acidic protein (GFAP) or vimentin to identify astrocytes, or for ED1 to identify microglia. For the second comparison devices from the first group with different cross-sectional areas were analyzed. Similar reactive responses were observed following insertion of all devices; however, the volume of tissue involved at early times, <1 week, was proportional to the cross-sectional area of the devices. Responses observed after 4 weeks were similar for all devices. Thus, the continued presence of devices promotes formation of a sheath composed partly of reactive astrocytes and microglia. Both GFAP-positive and -negative cells were adherent to all devices. These data indicate that device insertion promotes two responses-an early response that is proportional to device size and a sustained response that is independent of device size, geometry, and surface roughness. The early response may be associated with the amount of damage generated during insertion. The sustained response is more likely due to tissue-device interactions.

Controlling cellular reactive responses around neural prosthetic devices using peripheral and local intervention strategies

While chronic use of indwelling micromachined neural prosthetic devices has great potential, the development of reactive responses around them results in a decrease in electrode function over time. Since the cellular events responsible for these responses may be anti-inflammatory in nature, we have tested the effectiveness of dexamethasone and cyclosporin A as potential drugs for developing intervention strategies following insertion of single-shank micromachined silicon devices. Peripheral injection of dexamethasone was effective in attenuating increased expression of glial fibrillary acidic protein and astrocyte hyperplasia observed during both initial- and sustained-reactive responses observed at one and six weeks post insertion, respectively. Peripheral injection of cyclosporin A had no positive effect. If anything, application of this drug increased the early reactive response. Effectiveness of local release of dexamethasone in rat neocortex was tested by inserting ribbons of poly (ethyl-vinyl) acetate containing 35% (w/w) dexamethasone. Initial concentrations of dexamethasone were similar to those obtained by peripheral injection. Local drug release provided continued control of cellular reactive responses during the six-week study period. These results demonstrate that peripheral delivery of dexamethasone can be used to control reactive responses and that local drug delivery by slow-release from biocompatible polymers may be a more effective method of drug intervention. Incorporating these strategies on micromachined devices may provide an intervention strategy that will insure the chronic functioning of electrodes on intracortical neuroprosthetic devices.

Biocompatibility and biofouling of MEMS drug delivery devices

The biocompatibility and biofouling of the microfabrication materials for a MEMS drug delivery device have been evaluated. The in vivo inflammatory and wound healing response of MEMS drug delivery component materials, metallic gold, silicon nitride, silicon dioxide, silicon, and SU-8(TM) photoresist, were evaluated using the cage implant system. Materials, placed into stainless-steel cages, were implanted subcutaneously in a rodent model. Exudates within the cage were sampled at 4, 7, 14, and 21 days, representative of the stages of the inflammatory response, and leukocyte concentrations (leukocytes/microl) were measured. Overall, the inflammatory responses elicited by these materials were not significantly different than those for the empty cage controls over the duration of the study. The material surface cell density (macrophages or foreign body giant cells, FBGCs), an indicator of in vivo biofouling, was determined by scanning electron microscopy of materials explanted at 4, 7, 14, and 21 days. The adherent cellular density of gold, silicon nitride, silicon dioxide, and SU-8(TM) were comparable and statistically less (p<0.05) than silicon. These analyses identified the MEMS component materials, gold, silicon nitride, silicon dioxide, SU-8(TM), and silicon as biocompatible, with gold, silicon nitride, silicon dioxide, and SU-8(TM) showing reduced biofouling.

Effect of reactive cell density on net [2-14C]acetate uptake into rat brain: labeling of clusters containing GFAP+- and lectin+-immunoreactive cells

Astrocytic proliferation is a hallmark of brain injury, but the biological functions and metabolic activities of reactive astrocytes in vivo are poorly understood. [2-14C]Acetate, which is preferentially transported into and, therefore, metabolized by astrocytes, was used to assess injury- and trophic factor-induced changes in astrocyte metabolic activity. Local rates of net [2-14C]acetate uptake and glucose utilization (CMR(glc)), determined with [14C]deoxyglucose to assay overall metabolic activity of all brain cells, were assayed 7 days after a cannula placement; adjacent brain sections were immunostained to identify glial fibrillary acidic protein-positive (GFAP(+)) astrocytes and microglia plus macrophages (lectin-positive cells). GFAP(+) cells were abundant in tissue surrounding the cannula compared to the contralateral hemisphere, whereas lectin(+) cells were restricted to the wound boundary. CMR(glc) fell 25% in regions enriched in reactive astrocytes compared to the homologous contralateral hemisphere, whereas [14C]acetate uptake increased slightly (6%) but statistically significantly; metabolism of both tracers in 13 other brain structures was unchanged. Injection of basic fibroblast growth factor (b-FGF) into cerebral cortex or superior colliculus produced fiber-rich cell clusters containing both GFAP(+) and lectin(+) cells that had a 37% increase in [14C]acetate uptake; GFAP(+)-cell density rose in the nearby neuropil but the corresponding change in [14C]acetate uptake was small (6-8%). Sensory stimulation did not alter [14C]acetate uptake into the clusters. Thus, [14C]acetate uptake was relatively stable with respect to changes in the density of reactive astrocytes that are dispersed throughout the neuropil and to changes in cellular activity arising from sensory stimulation. In contrast, b-FGF-induced cell clusters that contain mixed cell types and numerous fibers accumulated higher levels of [14C]acetate, raising the possibility that increased uptake might be due to high numbers of activated astrocytes and, perhaps, acetate metabolism by other cell types.

Retinal prosthesis for the blind

Most of current concepts for a visual prosthesis are based on neuronal electrical stimulation at different locations along the visual pathways within the central nervous system. The different designs of visual prostheses are named according to their locations (i.e., cortical, optic nerve, subretinal, and epiretinal). Visual loss caused by outer retinal degeneration in diseases such as retinitis pigmentosa or age-related macular degeneration can be reversed by electrical stimulation of the retina or the optic nerve (retinal or optic nerve prostheses, respectively). On the other hand, visual loss caused by inner or whole thickness retinal diseases, eye loss, optic nerve diseases (tumors, ischemia, inflammatory processes etc.), or diseases of the central nervous system (not including diseases of the primary and secondary visual cortices) can be reversed by a cortical visual prosthesis. The intent of this article is to provide an overview of current and future concepts of retinal and optic nerve prostheses. This article will begin with general considerations that are related to all or most of visual prostheses and then concentrate on the retinal and optic nerve designs. The authors believe that the field has grown beyond the scope of a single article so cortical prostheses will be described only because of their direct effect on the concept and technical development of the other prostheses, and this will be done in a more general and historic perspective.

Evaluation of MEMS materials of construction for implantable medical devices

Medical devices based on microelectro-mechanical systems (MEMS) platforms are currently being proposed for a wide variety of implantable applications. However, biocompatibility data for typical MEMS materials of construction and processing, obtained from standard tests currently recognized by regulatory agencies, has not been published. Likewise, the effects of common sterilization techniques on MEMS material properties have not been reported. Medical device regulatory requirements dictate that materials that are biocompatibility tested be processed and sterilized in a manner equivalent to the final production device. Material, processing, and sterilization method can impact the final result. Six candidate materials for implantable MEMS devices, and one encapsulating material, were fabricated using typical MEMS processing techniques and sterilized. All seven materials were evaluated using a baseline battery of ISO 10993 physicochemical and biocompatibility tests. In addition, samples of these materials were evaluated using a scanning electron microscope (SEM) pre- and post-sterilization. While not addressing all facets of ISO 10993 testing, the biocompatibility and SEM data indicate few concerns about use of these materials in implant applications.

Topographical and physicochemical modification of material surface to enable patterning of living cells

Precise control of the architecture of multiple cells in culture and in vivo via precise engineering of the material surface properties is described as cell patterning. Substrate patterning by control of the surface physicochemical and topographic features enables selective localization and phenotypic and genotypic control of living cells. In culture, control over spatial and temporal dynamics of cells and heterotypic interactions draws inspiration from in vivo embryogenesis and haptotaxis. Patterned arrays of single or multiple cell types in culture serve as model systems for exploration of cell-cell and cell-matrix interactions. More recently, the patterned arrays and assemblies of tissues have found practical applications in the fields of Biosensors and cell-based assays for Drug Discovery. Although the field of cell patterning has its origins early in this century, an improved understanding of cell-substrate interactions and the use of microfabrication techniques borrowed from the microelectronics industry have enabled significant recent progress. This review presents the important early discoveries and emphasizes results of recent state-of-the-art cell patterning methods. The review concludes by illustrating the growing impact of cell patterning in the areas of bioelectronic devices and cell-based assays for drug discovery.

Visual prostheses

The development of man-made systems to restore functional vision in the profoundly blind has recently undergone a renaissance that has been fueled by a combination of celebrity and government interest, advances in the field of bioengineering, and successes with existing neuroprosthetic systems. This chapter presents the underlying physiologic principles of artificial vision, discusses three contemporary approaches to restoring functional vision in the blind, and concludes by presenting several relevant questions to vision prostheses. While there has been significant progress in the individual components constituting an artificial vision system, the remaining challenge of integrating these components with each other and the nervous system does not lie strictly in the realm of neuroscience, medicine, or engineering but at the interface of all three. In spite of the apparent complexity of an artificial vision system, it is not unreasonable to be optimistic about its eventual success.

Selective adhesion of astrocytes to surfaces modified with immobilized peptides

Under serum-free conditions, rat skin fibroblasts, but not cortical astrocytes, selectively adhered to glass surfaces modified with the integrin-ligand peptide RGDS. In contrast, astrocytes, but not fibroblasts, exhibited enhanced adhesion onto substrates modified with KHIFSDDSSE, a peptide that mimics a homophilic binding domain of neural cell adhesion molecule (NCAM). Astrocyte and fibroblast adhesion onto substrates modified with the integrin ligands IKVAV and YIGSR as well as the control peptides RDGS and SEDSDKFISH were similar to that observed on aminophase glass (reference substrate). This study is the first to demonstrate the use of immobilized KHIFSDDSSE in selectively modulating astrocyte and fibroblast adhesion on material surfaces, potentially leading to materials that promote specific functions of cells involved in the response(s) of central nervous system tissues to injury. This information could be incorporated into novel biomaterials designed to improve the long-term performance of the next generation of neural prostheses.

Characterization of rat meningeal cultures on materials of differing surface chemistry

To better understand the interactions of cells derived from meningeal tissues with the surfaces of devices used for the treatment of central nervous system disorders, the behavior of primary postnatal day 1 rat meningeal cultures was evaluated on biomaterials of differing surface chemistry. Meningeal cultures in serum containing media were analyzed for attachment, spread cell area, proliferation, the production of extracellular matrix (ECM), and neuronal outgrowth. In general, both cell attachment as well as cell spread area decreased with increasing substrate hydrophobicity, whereas cell division as indicated by BrdU incorporation and time to confluence, was lower on the most hydrophobic materials. We suggest that such differences immediately after cell seeding were most likely mediated by differences in surface adsorption of proteins. In longer-term experiments, most of the materials were colonized by meningeal cultures irrespective of surface chemistry, and all cultures were equally inhibitory to neuronal outgrowth suggesting that over time, cells can modify the substrate perhaps by secretion of extracellular matrix molecule proteins. Our data suggests that cell type-specific differences in response to different biomaterials may play an important role in determining the ultimate nature and composition of the CNS at the host-biomaterial interface.

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.

Daily variation and appetitive conditioning-induced plasticity of auditory cortex receptive fields

Long-term modification of cortical receptive field maps follows learning of sensory discriminations and conditioned associations. In the process of determining whether appetitive - as opposed to aversive - conditioning is effective in causing such plastic changes, it was discovered that multineuron receptive fields, when measured in rats under ketamine-sedation, vary substantially over the course of a week, even in the absence of classical conditioning and electrode movement. Specifically, a simple correlation analysis showed that iso-intensity frequency response curves of multiunit clusters and local field potentials recorded from auditory cortex are nonstationary over 7 days. Nevertheless, significant plastic changes in receptive fields, due to conditioned pairing of a pure tone and electrical stimulation of brain reward centres, are detectable above and beyond these spontaneous daily variations. This finding is based on a novel statistical plasticity criterion which compares receptive fields recorded for three days before and three days after conditioning. Based on a more traditional criterion (i.e. one day before and after conditioning), the prevalence of learning-induced changes caused by appetitive conditioning appears to be comparable to that described in previous studies involving aversive conditioning.

Attachment of astroglial cells to microfabricated pillar arrays of different geometries

We studied the attachment of astroglial cells on smooth silicon and arrays of silicon pillars and wells with various widths and separations. Standard semiconductor industry photolithographic techniques were used to fabricate pillar arrays and wells in single-crystal silicon. The resulting pillars varied in width from 0. 5 to 2.0 micrometer, had interpillar gaps of 1.0-5.0 micrometer, and were 1.0 micrometer in height. Arrays also contained 1.0-micromter-deep wells that were 0.5 micrometer in diameter and separated by 0.5-2.0 micrometer. Fluorescence, reflectance, and confocal light microscopies as well as scanning electron microscopy were used to quantify cell attachment, describe cell morphologies, and study the distribution of cytoskeletal proteins actin and vinculin on surfaces with pillars, wells, and smooth silicon. Seventy percent of LRM55 astroglial cells displayed a preference for pillars over smooth silicon, whereas only 40% preferred the wells to the smooth surfaces. Analysis of variance statistics performed on the data sets yielded values of p > approximately.5 for the comparison between pillar data sets and < approximately.0003 in the comparison between pillar and well data sets. Actin and vinculin distributions were highly polarized in cells found on pillar arrays. Scanning electron microscopy clearly demonstrated that cells made contact with the tops of the pillars and did not reach down into the spaces between pillars even when the interpillar gap was 5.0 microm. These experiments support the use of surface topography to direct the attachment, growth, and morphology of cells. These surfaces can be used to study fundamental cell properties such as cell attachment, proliferation, and gene expression. Such topography might also be used to modify implantable medical devices such as neural implants and lead to future developments in tissue engineering.

A technique to prevent dural adhesions to chronically implanted microelectrode arrays

Minimizing relative movements between neural tissues and arrays of microelectrodes chronically implanted into them is expected to greatly enhance the capacity of the microelectrodes to record from single cortical neurons on a long-term basis. We describe a new surgical technique to minimize the formation of adhesions between the dura and an implanted electrode array using a 12 microm (0.5 mil) thick sheet of Teflon film positioned between the array and the dura. A total of 15 cats were implanted using this technique. Gross examination of 12 implant sites at the time of sacrifice failed to find evidence of adhesions between the arrays and the dura when the Teflon(R) film remained in its initial position. In six implants from which recordings were made, an average of nine of the 11 (81%) connected electrodes in each array recorded evoked neural activity after 180 days post implantation. Further, on average, two separable units were identified on each of the implanted electrodes in these arrays. No significant change was found in the density of cell bodies around implanted electrodes of four of the implanted electrode arrays. However, histological evaluation of the implant sites revealed evidence of meningeal proliferation beneath the arrays. The technique described is shown to be effective at preventing adhesions between implanted electrode arrays and improve the characteristics of chronic recordings obtained with these structures.

Correlation of astroglial cell function on micro-patterned surfaces with specific geometric parameters

Microcontact printing techniques were used to modify silicon substrates with arrays of hexagonal features of N1[3-(trimethoxysilyl) propyl]diethylenetriamine (DETA) surrounded by octadecyltrichlorosilane (OTS), which are hydrophilic, cell-adhesive and hydrophobic, non-adhesive organosilanes, respectively. In the presence of serum proteins, LRM55 cell adhesion and morphology on these modified surfaces were best correlated to the width of the cell-adhesive features. On surfaces modified with small (5 microm in width) cell-adhesive features, LRM55 cells elaborated only thin processes. As feature width was increased, cells on these surfaces exhibited increased cell spreading and elaborated wide processes. On surfaces modified with large (>35 microm in width) features, single cells adhered to and spread upon individual DETA features. In a similar fashion, LRM55 cell adhesion density increased with increasing feature width; this correlation could be represented by a simple, second-order relation, and was independent of all other measures of pattern geometry. The results of this study provide evidence that micro-patterning may be effective in controlling astrocyte interaction with implant materials.