SU-8 microprobe with microelectrodes for monitoring electrical impedance in living tissues

Progress on Designing a Chemical Retinal Prosthesis

The last major review of progress toward a chemical retinal prosthesis was a decade ago. Many important advancements have been made since then with the aim of producing an implantable device for animal testing. We review that work here discussing the potential advantages a chemical retinal prosthesis may possess, the spatial and temporal resolutions it might provide, the materials from which an implant might be constructed and its likely effectiveness in stimulating the retina in a natural fashion. Consideration is also given to implant biocompatibility, excitotoxicity of dispensed glutamate and known changes to photoreceptor degenerate retinas.

Recent Progress in Materials Chemistry to Advance Flexible Bioelectronics in Medicine

Designing bioelectronic devices that seamlessly integrate with the human body is a technological pursuit of great importance. Bioelectronic medical devices that reliably and chronically interface with the body can advance neuroscience, health monitoring, diagnostics, and therapeutics. Recent major efforts focus on investigating strategies to fabricate flexible, stretchable, and soft electronic devices, and advances in materials chemistry have emerged as fundamental to the creation of the next generation of bioelectronics. This review summarizes contemporary advances and forthcoming technical challenges related to three principal components of bioelectronic devices: i) substrates and structural materials, ii) barrier and encapsulation materials, and iii) conductive materials. Through notable illustrations from the literature, integration and device fabrication strategies and associated challenges for each material class are highlighted.

Neural tissue-microelectrode interaction: Brain micromotion, electrical impedance, and flexible microelectrode insertion

Insertion of a microelectrode into the brain to record/stimulate neurons damages neural tissue and blood vessels and initiates the brain's wound healing response. Due to the large difference between the stiffness of neural tissue and microelectrode, brain micromotion also leads to neural tissue damage and associated local immune response. Over time, following implantation, the brain's response to the tissue damage can result in microelectrode failure. Reducing the microelectrode's cross-sectional dimensions to single-digit microns or using soft materials with elastic modulus close to that of the neural tissue are effective methods to alleviate the neural tissue damage and enhance microelectrode longevity. However, the increase in electrical impedance of the microelectrode caused by reducing the microelectrode contact site's dimensions can decrease the signal-to-noise ratio. Most importantly, the reduced dimensions also lead to a reduction in the critical buckling force, which increases the microelectrode's propensity to buckling during insertion. After discussing brain micromotion, the main source of neural tissue damage, surface modification of the microelectrode contact site is reviewed as a key method for addressing the increase in electrical impedance issue. The review then focuses on recent approaches to aiding insertion of flexible microelectrodes into the brain, including bending stiffness modification, effective length reduction, and application of a magnetic field to pull the electrode. An understanding of the advantages and drawbacks of the developed strategies offers a guide for dealing with the buckling phenomenon during implantation.

A comparison of insertion methods for surgical placement of penetrating neural interfaces

Many implantable electrode arrays exist for the purpose of stimulating or recording electrical activity in brain, spinal, or peripheral nerve tissue, however most of these devices are constructed from materials that are mechanically rigid. A growing body of evidence suggests that the chronic presence of these rigid probes in the neural tissue causes a significant immune response and glial encapsulation of the probes, which in turn leads to gradual increase in distance between the electrodes and surrounding neurons. In recording electrodes, the consequence is the loss of signal quality and, therefore, the inability to collect electrophysiological recordings long term. In stimulation electrodes, higher current injection is required to achieve a comparable response which can lead to tissue and electrode damage. To minimize the impact of the immune response, flexible neural probes constructed with softer materials have been developed. These flexible probes, however, are often not strong enough to be inserted on their own into the tissue, and instead fail via mechanical buckling of the shank under the force of insertion. Several strategies have been developed to allow the insertion of flexible probes while minimizing tissue damage. It is critical to keep these strategies in mind during probe design in order to ensure successful surgical placement. In this review, existing insertion strategies will be presented and evaluated with respect to surgical difficulty, immune response, ability to reach the target tissue, and overall limitations of the technique. Overall, the majority of these insertion techniques have only been evaluated for the insertion of a single probe and do not quantify the accuracy of probe placement. More work needs to be performed to evaluate and optimize insertion methods for accurate placement of devices and for devices with multiple probes.

Multimaterial and multifunctional neural interfaces: from surface-type and implantable electrodes to fiber-based devices

Development of neural interfaces from surface electrodes to fibers with various type, functionality, and materials.

Non-transdermal microneedles for advanced drug delivery

Microneedles (MNs) have been used to deliver drugs for over two decades. These platforms have been proven to increase transdermal drug delivery efficiency dramatically by penetrating restrictive tissue barriers in a minimally invasive manner. While much of the early development of MNs focused on transdermal drug delivery, this technology can be applied to a variety of other non-transdermal biomedical applications. Several variations, such as multi-layer or hollow MNs, have been developed to cater to the needs of specific applications. The heterogeneity in the design of MNs has demanded similar variety in their fabrication methods; the most common methods include micromolding and drawing lithography. Numerous materials have been explored for MN fabrication which range from biocompatible ceramics and metals to natural and synthetic biodegradable polymers. Recent advances in MN engineering have diversified MNs to include unique shapes, materials, and mechanical properties that can be tailored for organ-specific applications. In this review, we discuss the design and creation of modern MNs that aim to surpass the biological barriers of non-transdermal drug delivery in ocular, vascular, oral, and mucosal tissue.

Control of neural probe shank flexibility by fluidic pressure in embedded microchannel using PDMS/PI hybrid substrate

Implantable neural probes are widely used to record and stimulate neural activities. These probes should be stiff enough for insertion. However, it should also be flexible to minimize tissue damage after insertion. Therefore, having dynamic control of the neural probe shank flexibility will be useful. For the first time, we have successfully fabricated flexible neural probes with embedded microfluidic channels for dynamic control of neural probe stiffness by controlling fluidic pressure in the channels. The present hybrid neural probes consisted of polydimethylsiloxane (PDMS) and polyimide (PI) layers could provide the required stiffness for insertion and flexibility during operation. The PDMS channels were fabricated by reversal imprint using a silicon mold and bonded to a PI layer to form the embedded channels in the neural probe. The probe shape was patterned using an oxygen plasma generated by an inductively coupled plasma etching system. The critical buckling force of PDMS/PI neural probes could be tuned from 0.25-1.25 mN depending on the applied fluidic pressure in the microchannels and these probes were successfully inserted into a 0.6% agarose gel that mimicked the stiffness of the brain tissue. Polymer-based neural probes are typically more flexible than conventional metal wire-based probes, and they could potentially provide less tissue damage after implantation.

Atomic force acoustic microscopy reveals the influence of substrate stiffness and topography on cell behavior

The stiffness and the topography of the substrate at the cell-substrate interface are two key properties influencing cell behavior. In this paper, atomic force acoustic microscopy (AFAM) is used to investigate the influence of substrate stiffness and substrate topography on the responses of L929 fibroblasts. This combined nondestructive technique is able to characterize materials at high lateral resolution. To produce substrates of tunable stiffness and topography, we imprint nanostripe patterns on undeveloped and developed SU-8 photoresist films using electron-beam lithography (EBL). Elastic deformations of the substrate surfaces and the cells are revealed by AFAM. Our results show that AFAM is capable of imaging surface elastic deformations. By immunofluorescence experiments, we find that the L929 cells significantly elongate on the patterned stiffness substrate, whereas the elasticity of the pattern has only little effect on the spreading of the L929 cells. The influence of the topography pattern on the cell alignment and morphology is even more pronounced leading to an arrangement of the cells along the nanostripe pattern. Our method is useful for the quantitative characterization of cell-substrate interactions and provides guidance for the tissue regeneration therapy in biomedicine.

A Mechanically-Adaptive Polymer Nanocomposite-Based Intracortical Probe and Package for Chronic Neural Recording

Mechanical, materials, and biological causes of intracortical probe failure have hampered their utility in basic science and clinical applications. By anticipating causes of failure, we can design a system that will prevent the known causes of failure. The neural probe design was centered around a bio-inspired, mechanically-softening polymer nanocomposite. The polymer nanocomposite was functionalized with recording microelectrodes using a microfabrication process designed for chemical and thermal process compatibility. A custom package based upon a ribbon cable, printed circuit board, and a 3D-printed housing was designed to enable connection to external electronics. Probes were implanted into the primary motor cortex of Sprague-Dawley rats for 16 weeks, during which regular recording and electrochemical impedance spectroscopy measurement sessions took place. The implanted mechanically-softening probes had stable electrochemical impedance spectra across the 16 weeks and single units were recorded out to 16 weeks. The demonstration of chronic neural recording with the mechanically-softening probe suggests that probe architecture, custom package, and general design strategy are appropriate for long-term studies in rodents.

Characterization of SH-SY5Y human neuroblastoma cell growth over glass and SU-8 substrates

The physical properties of substrates can have profound effects on the structure and function of cultured cells. In this study, we aimed to examine the viability, adherence, and morphological and functional variations between SH-SY5Y human neuroblastoma cells cultured on SU-8 surfaces compared with control surfaces composed of borosilicate glass, which are routinely used for cell culture. The SU-8 polymer has been extensively studied for its biocompatibility, but there has been little investigation into the characteristic differences between cells cultured on SU-8 when compared with glass. SH-SY5Y cells were cultured within polydimethylsiloxane wells on both SU-8 and glass substrates for up to 72 h after which flow cytometry and enzyme-linked immunosorbent assay analysis was performed to examine cell viability and neurotoxicity. Immunocytochemistry was also performed to analyze the morphological and functional characteristics of the cells. Atomic force microscopy was performed to measure surface roughness and to map cell-substrate interactions. Nanoindentation testing was used to characterize the mechanical properties of polymer surface. Results showed that SH-SY5Y cells grown on SU-8 have significantly improved viability and increased morphological and functional characteristics of neurodevelopment. The results from this study suggest that the mechanical properties of the polymer are optimal for the study of cultured cell lines, which could account for the increased viability, adherence, and morphological and functional characteristics of neurodevelopment. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 2129-2138, 2017.

Improvement of Depth Profiling into Biotissues Using Micro Electrical Impedance Spectroscopy on a Needle with Selective Passivation

A micro electrical impedance spectroscopy (EIS)-on-a-needle for depth profiling (μEoN-DP) with a selective passivation layer (SPL) on a hypodermic needle was recently fabricated to measure the electrical impedance of biotissues along with the penetration depths. The SPL of the μEoN-DP enabled the sensing interdigitated electrodes (IDEs) to contribute predominantly to the measurement by reducing the relative influence of the connection lines on the sensor output. The discrimination capability of the μEoN-DP was verified using phosphate-buffered saline (PBS) at various concentration levels. The resistance and capacitance extracted through curve fitting were similar to those theoretically estimated based on the mixing ratio of PBS and deionized water; the maximum discrepancies were 8.02% and 1.85%, respectively. Depth profiling was conducted using four-layered porcine tissue to verify the effectiveness of the discrimination capability of the μEoN-DP. The magnitude and phase between dissimilar porcine tissues (fat and muscle) were clearly discriminated at the optimal frequency of 1 MHz. Two kinds of simulations, one with SPL and the other with complete passivation layer (CPL), were performed, and it was verified that the SPL was advantageous over CPL in the discrimination of biotissues in terms of sensor output.

Microfabricated, amperometric, enzyme-based biosensors for in vivo applications

Miniaturized electrochemical in vivo biosensors allow the measurement of fast extracellular dynamics of neurotransmitter and energy metabolism directly in the tissue. Enzyme-based amperometric biosensing is characterized by high specificity and precision as well as high spatial and temporal resolution. Aside from glucose monitoring, many systems have been introduced mainly for application in the central nervous system in animal models. We compare the microsensor principle with other methods applied in biomedical research to show advantages and drawbacks. Electrochemical sensor systems are easily miniaturized and fabricated by microtechnology processes. We review different microfabrication approaches for in vivo sensor platforms, ranging from simple modified wires and fibres to fully microfabricated systems on silicon, ceramic or polymer substrates. The various immobilization methods for the enzyme such as chemical cross-linking and entrapment in polymer membranes are discussed. The resulting sensor performance is compared in detail. We also examine different concepts to reject interfering substances by additional membranes, aspects of instrumentation and biocompatibility. Practical considerations are elaborated, and conclusions for future developments are presented. Graphical Abstract ᅟ.

Nano-patterned SU-8 surface using nanosphere-lithography for enhanced neuronal cell growth

Mimicking the nanoscale surface texture of the extracellular matrix can affect the regulation of cellular behavior, including adhesion, differentiation, and neurite outgrowth. In this study, SU-8-based polymer surfaces with well-ordered nanowell arrays were fabricated using nanosphere lithography with polystyrene nanoparticles. We show that the SU-8 surface with nanowells resulted in similar neuronal development of rat pheochromocytoma (PC12) cells compared with an unpatterned poly-L-lysine (PLL)-coated SU-8 surface. Additionally, even after soaking the substrate in cell culture medium for two weeks, cells on the nanowell SU-8 surface showed long-term neurite outgrowth compared to cells on the PLL-coated SU-8 surface. The topographical surface modification of the nanowell array demonstrates potential as a replacement for cell adhesive material coatings such as PLL, for applications requiring long-term use of polymer-based implantable devices.

Implantable neurotechnologies: a review of micro- and nanoelectrodes for neural recording

Electrodes serve as the first critical interface to the biological organ system. In neuroprosthetic applications, for example, electrodes interface to the tissue for either signal recording or tissue stimulation. In this review, we consider electrodes for recording neural activity. Recording electrodes serve as wiretaps into the neural tissues, providing readouts of electrical activity. These signals give us valuable insights into the organization and functioning of the nervous system. The recording interfaces have also shown promise in aiding treatment of motor and sensory disabilities caused by neurological disorders. Recent advances in fabrication technology have generated wide interest in creating tiny, high-density electrode interfaces for neural tissues. An ideal electrode should be small enough and be able to achieve reliable and conformal integration with the structures of the nervous system. As a result, the existing electrode designs are being shrunk and packed to form small form factor interfaces to tissue. Here, an overview of the historic and state-of-the-art electrode technologies for recording neural activity is presented first with a focus on their development road map. The fact that the dimensions of recording electrode sites are being scaled down from micron to submicron scale to enable dense interfaces is appreciated. The current trends in recording electrode technologies are then reviewed. Current and future considerations in electrode design, including the use of inorganic nanostructures and biologically inspired or biocomapatible materials are discussed, along with an overview of the applications of flexible materials and transistor transduction schemes. Finally, we detail the major technical challenges facing chronic use of reliable recording electrode technology.

SAW-based fluid atomization using mass-producible chip devices

Surface acoustic wave (SAW)-based fluid atomizers are ideally suited to generate micrometer-sized droplets without any moving parts or nozzles. Versatile application fields can be found for instance in biomedical, aerosol or thin film technology, including medical inhalators or particle deposition for advanced surface treatment. Such atomizers also show great potential for on-chip integration and can lead to economic production of hand-held and even disposable devices, with either a single functionality or integrated in more complex superior systems. However, this potential was limited in the past by fluid supply mechanisms inadequate for mass production, accuracy and reliability. In this work, we briefly discuss existing fluid supply methods and demonstrate a straightforward new approach suited for reliable and cost-effective mass-scale manufacturing of SAW atomizer chips. Our approach is based on a fluid supply at the boundary of the acoustic beam via SU-8 microchannels produced by a novel one-layer/double-exposure photolithography method. Using this technique, we demonstrate precise and stable fluid atomization with almost ideal aerosol plume geometry from a dynamically stabilized thin fluid film. Additionally, we demonstrate the possibility of in situ altering the droplet size distribution by controlling the amount of fluid available in the active region of the chip.

Experimental study on the mechanical interaction between silicon neural microprobes and rat dura mater during insertion

In vivo insertion experiments are essential to optimize novel neural implants. Our work focuses on the interaction between intact dura mater of rats and as-fabricated single-shaft silicon microprobes realized by deep reactive ion etching. Implantation parameters like penetration force and dimpling through intact dura mater were studied as a function of insertion speed, microprobe cross-section, tip angle and animal age. To reduce tissue resistance, we proposed a unique tip sharpening technique, which was also evaluated in in vivo insertion tests. By doubling the insertion speed (between 1.2 and 10.5 mm/min), an increase of 10-35% in penetration forces was measured. When decreasing the cross-section of the microprobes, penetration forces and dimpling was reduced by as much as 30-50% at constant insertion speeds. Force was noticed to gradually decrease by decreasing tip angles. Measured penetration forces through dura mater were reduced even down to 11±3 mN compared to unsharpened (49±13 mN) probes by utilizing our unique tip sharpening technique, which is very close to exerted penetration force in the case of retracted dura (5±1.5 mN). Our findings imply that age remarkably alters the elasticity of intact dura mater. The decreasing stiffness of dura mater results in a significant rise in penetration force and decrease in dimpling. Our work is the first in vivo comparative study on microelectrode penetration through intact and retracted dura mater.

Development of a new contactless dielectrophoresis system for active particle manipulation using movable liquid electrodes

This study presents a new DEP manipulation technique using a movable liquid electrode, which allows manipulation of particles by actively controlling the locations of electrodes and applying on-off electric input signals. This DEP system consists of mercury as a movable liquid electrode, indium tin oxide (ITO)-coated glass, SU-8-based microchannels for electrode passages, and a PDMS medium chamber. A simple squeezing method was introduced to build a thin PDMS layer at the bottom of the medium chamber to create a contactless DEP system. To determine the operating conditions, the DEP force and the friction force were analytically compared for a single cell. In addition, an appropriate frequency range for effective DEP manipulation was chosen based on an estimation of the Clausius-Mossotti factor and the effective complex permittivity of the yeast cell using the concentric shell model. With this system, we demonstrated the active manipulation of yeast cells, and measured the collection efficiency and the dielectrophoretic velocity of cells for different AC electric field strengths and applied frequencies. The experimental results showed that the maximum collection efficiency reached was approximately 90%, and the dielectrophoretic velocity increased with increasing frequency and attained the maximum value of 10.85 ± 0.95 μm/s at 100 kHz, above which it decreased.

Polymer-based, flexible glutamate and lactate microsensors for in vivo applications

We present a flexible microsensor, based on a polymer substrate, for multiparametric, electrochemical in vivo monitoring. The sensor strip with a microelectrode array at the tip was designed for insertion into tissue, for fast and localized online monitoring of physiological parameters. The microsystem fabrication on a wafer-level is based on a polyimide substrate and includes the patterning of platinum microelectrodes as well as epoxy and dry-film-resist insulation in a cost-effective thin-film and laminate process. A stable, electrodeposited silver/silver chloride reference electrode on-chip and a perm-selective membrane as an efficient interference rejection scheme are integrated on a wafer-level. Amperometric, electrochemical, enzyme-based biosensors for the neurotransmitter L-glutamate and the energy metabolite L-lactate have been developed. Hydrogel membranes or direct cross-linking as stable concepts for the enzyme immobilization are shown. Sensor performance including high selectivity, tailoring of sensitivity and long-term stability is discussed. For glutamate, a high sensitivity of 2.16 nAmm(-2) µM(-1) was found. For lactate, a variation in sensitivity between 2.6 and 32 nAmm(-2)mM(-1) was achieved by different membrane compositions. The in vivo application in an animal model is demonstrated by glutamate measurements in the brain of rats. Local glutamate alterations in the micromolar range and in nanoliter-range volumes can be detected and quantified with high reproducibility and temporal resolution. A novel, versatile platform for the integration of various electrochemical sensors on a small, flexible sensor strip for a variety of in vivo applications is presented.

Fabrication of gold/polypyrrole core/shell nanowires on a flexible substrate for molecular imprinted electrochemical sensors

Gold/polypyrrole core/shell nanowires electrochemically grown on flexible substrates are used as molecular imprinted polymer biosensors for dopamine detection.

Drilling electrode for real-time measurement of electrical impedance in bone tissues

In order to prevent possible damages to soft tissues, reliable monitoring methods are required to provide valuable information on the condition of the bone being cut. This paper describes the design of an electrical impedance sensing drill developed to estimate the relative position between the drill and the bone being drilled. The two-electrode method is applied to continuously measure the electrical impedance during a drill feeding movement: two copper wire brushes are used to conduct electricity in the rotating drill and then the drill is one electrode; a needle is inserted into the soft tissues adjacent to the bone being drilled and acts as another electrode. Considering that the recorded electrical impedance is correlated with the insertion depth of the drill, we theoretically calculate the electrode-tissue contact impedance and prove that the rate of impedance change varies considerably when the drill bit crosses the boundary between two different bone tissues. Therefore, the rate of impedance change is used to determine whether the tip of the drill is located in one of cortical bone, cancellous bone, and cortical bone near a boundary with soft tissue. In vitro experiments in porcine thoracic spines were performed to demonstrate the feasibility of the impedance sensing drill. The experimental results indicate that the drill, used with the proposed data-processing method, can provide accurate and reliable breakthrough detection in the bone-drilling process.

In vitro and in vivo evaluation of SU-8 biocompatibility

SU-8 negative photoresist is a high tensile strength polymer that has been used for a number of biomedical applications that include cell encapsulation and neuronal probes. Chemically, SU-8 comprises, among other components, an epoxy based monomer and antimony salts, the latter being a potential source of cytotoxicity. We report on the in vitro and in vivo evaluation of SU-8 biocompatibility based on leachates from various solvents, at varying temperatures and pH, and upon subcutaneous implantation of SU-8 substrates in mice. MTT cell viability assay did not exhibit any cytotoxic effects from the leachates. The hemolytic activity of SU-8 is comparable to that of FDA approved implant materials such as silicone elastomer, Buna-S and medical steel. In vivo histocompatibility study in mice indicates a muted immune response to subcutaneous SU-8 implants.

Gold nanostructures on flexible substrates as electrochemical dopamine sensors

In this study, we fabricated Au nanowires (NWs), nanoslices (NSs), and nanocorals (NCs) on flexible polyethylene terephthalate (PET) substrates via direct current electrochemical depositions. Without any surface modification, the Au nanostructures were used as the electrodes for dopamine (DA) sensing. Among them, the Au NW electrode performed exceptionally well. The determined linear range for DA detection was 0.2-600 μM (N = 3) and the sensitivity was 178 nA/μM cm(2), while the detection limit was 26 nM (S/N = 3). After 10 repeated measurements, 95% of the original anodic current values were maintained for the nanostructured electrodes. Sequential additions of citric acid (CA, 1 mM), uric acid (UA, saturated), and ascorbic acid (AA, 1 μM) did not interfere the amperometric response from the addition of DA (0.1 μM).

A microfluidic-based frequency-multiplexing impedance sensor (FMIS)

We present a novel technology for the simultaneous and simple impedimetric screening of multiple microfluidic channels with only one electrode pair. We have exploited the frequency dimension to distinguish between up to three channels. Each 'sub-sensor' possesses its corresponding measurement frequency where the sample-specific dielectric properties can be probed. We have shown the validity of our frequency-multiplexing impedance sensor (FMIS) by comparison with conventional 'single sensors'. Our highly sensitive FMIS was proven suitable for life science applications through usage as a cell-based toxicology platform. We are confident that our technology might find great utility in parallelized cell-based analysis systems as well as in biomedical devices where size limitations and spatially distributed probing are important parameters.

Mesoporous silica as a membrane for ultra-thin implantable direct glucose fuel cells

The design, fabrication and characterization of an inorganic catalyst based direct glucose fuel cell using mesoporous silica coating as a functional membrane is reported. The desired use of mesoporous silica based direct glucose fuel cell is for a blood vessel implantable device. Blood vessel implantable direct glucose fuel cells have access to higher continuous glucose concentrations. However, reduction in the implant thickness is required for application in the venous system as part of a stent. We report development of an implantable device with a platinum thin-film (thickness: 25 nm) deposited on silicon substrate (500 μm) to serve as the anode, and graphene pressed on a stainless steel mesh (175 μm) to serve as the cathode. Control experiments involved the use of a surfactant-coated polypropylene membrane (50 μm) with activated carbon (198 μm) electrodes. We demonstrate that a mesoporous silica thin film (270 nm) is capable of replacing the conventional polymer based membranes with an improvement in the power generated over conventional direct glucose fuel cells.