A Multimodal Multi-Shank Fluorescence Neural Probe for Cell-Type-Specific Electrophysiology in Multiple Regions across a Neural Circuit.

Cell-type-specific, activity-dependent electrophysiology can allow in-depth analysis of functional connectivity inside complex neural circuits composed of various cell types. To date, optics-based fluorescence recording devices enable monitoring cell-type-specific activities. However, the monitoring is typically limited to a single brain region, and the temporal resolution is significantly low. Herein, a multimodal multi-shank fluorescence neural probe that allows cell-type-specific electrophysiology from multiple deep-brain regions at a high spatiotemporal resolution is presented. A photodiode and an electrode-array pair are monolithically integrated on each tip of a minimal-form-factor silicon device. Both fluorescence and electrical signals are successfully measured simultaneously in GCaMP6f expressing mice, and the cell type from sorted neural spikes is identified. The probe's capability of combined electro-optical recordings for cell-type-specific electrophysiology at multiple brain regions within a neural circuit is demonstrated. The new experimental paradigm to enable the precise investigation of functional connectivity inside and across complex neural circuits composed of various cell types is expected.

A 3D flexible neural interface based on a microfluidic interconnection cable capable of chemical delivery.

The demand for multifunctional neural interfaces has grown due to the need to provide a better understanding of biological mechanisms related to neurological diseases and neural networks. Direct intracerebral drug injection using microfluidic neural interfaces is an effective way to deliver drugs to the brain, and it expands the utility of drugs by bypassing the blood-brain barrier (BBB). In addition, uses of implantable neural interfacing devices have been challenging due to inevitable acute and chronic tissue responses around the electrodes, pointing to a critical issue still to be overcome. Although neural interfaces comprised of a collection of microneedles in an array have been used for various applications, it has been challenging to integrate microfluidic channels with them due to their characteristic three-dimensional structures, which differ from two-dimensionally fabricated shank-type neural probes. Here we present a method to provide such three-dimensional needle-type arrays with chemical delivery functionality. We fabricated a microfluidic interconnection cable (µFIC) and integrated it with a flexible penetrating microelectrode array (FPMA) that has a 3-dimensional structure comprised of silicon microneedle electrodes supported by a flexible array base. We successfully demonstrated chemical delivery through the developed device by recording neural signals acutely from in vivo brains before and after KCl injection. This suggests the potential of the developed microfluidic neural interface to contribute to neuroscience research by providing simultaneous signal recording and chemical delivery capabilities.

A minimally invasive flexible electrode array for simultaneous recording of ECoG signals from multiple brain regions.

The minimal invasiveness of electrocorticography (ECoG) enabled its widespread use in clinical areas as well as in neuroscience research. However, most existing ECoG arrays require that the entire surface area of the brain that is to be recorded be exposed through a large craniotomy. We propose a device that overcomes this limitation, i.e., a minimally invasive, polyimide-based flexible array of electrodes that can enable the recording of ECoG signals in multiple regions of the brain with minimal exposure of the surface of the brain. Magnetic force-assisted positioning of a flexible electrode array enables recording from distant brain regions with a small cranial window. Also, a biodegradable organic compound used for attaching a magnet on the electrodes allows simple retrieval of the magnet. We demonstrate with an in vivo chronic recording that an implanted ECoG electrode array can record ECoG signals from the visual cortex and the motor cortex during a rat's free behavior. Our results indicate that the proposed device induced minimal damage to the animal. We expect the proposed device to be utilized for experiments for large-scale brain circuit analyses as well as clinical applications for intra-operative monitoring of epileptic activity.

A MEMS ultrasound stimulation system for modulation of neural circuits with high spatial resolution in vitro.

Neuromodulation by ultrasound has recently received attention due to its noninvasive stimulation capability for treating brain diseases. Although there have been several studies related to ultrasonic neuromodulation, these studies have suffered from poor spatial resolution of the ultrasound and low repeatability with a fixed condition caused by conventional and commercialized ultrasound transducers. In addition, the underlying physics and mechanisms of ultrasonic neuromodulation are still unknown. To determine these mechanisms and accurately modulate neural circuits, researchers must have a precisely controllable ultrasound transducer to conduct experiments at the cellular level. Herein, we introduce a new MEMS ultrasound stimulation system for modulating neurons or brain slices with high spatial resolution. The piezoelectric micromachined ultrasonic transducers (pMUTs) with small membranes (sub-mm membranes) generate enough power to stimulate neurons and enable precise modulation of neural circuits. We designed the ultrasound transducer as an array structure to enable localized modulation in the target region. In addition, we integrated a cell culture chamber with the system to make it compatible with conventional cell-based experiments, such as in vitro cell cultures and brain slices. In this work, we successfully demonstrated the functionality of the system by showing that the number of responding cells is proportional to the acoustic intensity of the applied ultrasound. We also demonstrated localized stimulation capability with high spatial resolution by conducting experiments in which cocultured cells responded only around a working transducer.

A 3D Microscaffold Cochlear Electrode Array for Steroid Elution.

In cochlear implants, the electrode insertion trauma during surgery can cause damage residual hearing. Preserving the residual hearing is an important challenge and the localized administration of drugs, such as steroids, is one of the most promising ways, but remains a challenge. Here, a microscaffold cochlear electrode array (MiSCEA) consisting of a microfabricated flexible electrode array and a 3D microscaffold for steroid reservoir is reported. The MiSCEA without loaded drug is tested by measuring the electrically evoked auditory brainstem response of the cochlea in guinea pigs (n = 4). The scaffold is then coated with steroid (dexamethasone) encapsulated in polylactic-co-glycolic acid and the continuous release of the steroid into artificial perilymph during six weeks is monitored. The steroid-containing scaffolds are then implanted into guinea pigs (n = 4) and threshold shifts are analyzed for four weeks by measuring the acoustically evoked auditory brainstem response. The threshold shifts tend to be lower in the group implanted with the steroid-containing MiSCEAs. The feasibility of 3D MiSCEA opens up the development of potential next-generation cochlear electrode with improved steroid release dynamics into cochlea.

Transformation of 2D Planes into 3D Soft and Flexible Structures with Embedded Electrical Functionality.

Three-dimensional (3D) structures composed of flexible and soft materials have been in demand for implantable biomedical devices. However, the fabrication of 3D structures using microelectromechanical system (MEMS) techniques has limitations in terms of the materials and the scale of the structures. Here, a technique to selectively bond polydimethylsiloxane (PDMS) and parylene-C by plasma treatment is reported, with which two-dimensional structures that are fabricated using MEMS techniques are turned into 3D structures by the inflation of selectively non-bonded patterns. The bonding strength and the bonding mechanism were analyzed by mechanical tests and chemical analyses, respectively. We fabricated soft and flexible 3D structures with various patterns and dimensions, even with embedded electrical functions, including light emitting diodes and electrocorticogram electrodes. Based on these results, the flexible, soft, and MEMS-capable 3D structures that are obtained by the developed selective bonding technique are promising for applications in a wide range of biomedical applications.

An Intrafascicular Neural Interface With Enhanced Interconnection for Recording of Peripheral Nerve Signals.

For implantable devices, Parylene C (hereafter referred to as Parylene) has shown promising properties such as flexibility, biocompatibility, biostability, and good barrier properties. Parylene-based flexible interconnection cable (FIC) was previously developed to connect a flexible penetrating microelectrode array (FPMA) with a recording system. However, Parylene-based FIC was difficult to handle and prone to damage during the implantation surgery because of its low mechanical strength. To improve the mechanical properties of the FIC, we suggest a mechanically enhanced flexible interconnection cable (enhanced FIC) obtained using a combination of Parylene and polyimide. To investigate the long-term stability of the enhanced FIC, Parylene-only FIC, and enhanced FIC were tested and their mechanical properties were compared under an accelerated aging condition. During the course of six months of soaking, the maximum strength of the enhanced FIC remained twice as high as that of the Parylene-only FIC throughout the experiment, although the mechanical strength of both FICs decreased over time. To show the capability of the enhanced FIC in the context of nerve signal recording as a part of a neural interfacing device, it was assembled together with the FPMA and custom-made wireless recording electronics. We demonstrated the feasibility of the enhanced FIC in an in vivo application by recording acute nerve signals from canine sciatic nerves.

A Batteryless, Wireless Strain Sensor Using Resonant Frequency Modulation.

In this study, we demonstrated the feasibility of a wireless strain sensor using resonant frequency modulation through tensile impedance test and wireless sensing test. To achieve a high stretchability, the sensor was fabricated by embedding a copper wire with high conductivity in a silicone rubber with high stretchability, in which the resonant frequency can be modulated according to changes in strain. The characteristics of the sensor and the behavior of wireless sensing were calculated based on equations and simulated using finite element method. As the strain of the sensor increased, the inductance increased, resulting in the modulation of resonant frequency. In experimental measurement, as the strain of the sensor increased from 0% to 110%, its inductance was increased from 192 nH to 220 nH, changed by 14.5%, and the resonant frequency was shifted from 13.56 MHz to 12.72 MHz, decreased by 6.2%. It was demonstrated that using the proposed sensor, strains up to 110% could be detected wirelessly up to a few centimeters.

Development of an intrafascicular neural interface for peripheral nerve implantation.

This paper describes an intrafascicular neural interface for peripheral nerve implantation. The flexible penetrating microelectrode array with varying lengths (vl-FPMA), interconnection cable, wireless recording and stimulator modules were designed and fabricated to detect neural signals from the peripheral nerves or to stimulate them. The vl-FPMA consisted of silicon needles and polydimethylsiloxane (PDMS) platform supporting the needles. The length of electrode needles varied from 600 to 1000 µm. The interconnection cable was fabricated as parylene-metal-parylene sandwiched structure. The wireless recording/stimulation modules were also developed and connected with the electrodes. The integrated system was implanted in the sciatic nerve of beagles and the recording capability of the integrated system was demonstrated successfully.

Annealing Effects of Parylene-Caulked Polydimethylsiloxane as a Substrate of Electrodes.

This paper investigates the effects of annealing of the electrodes based on parylene-caulked polydimethylsiloxane (pc-PDMS) in terms of mechanical strength and long-term electrical property. Previously, the electrodes based on pc-PDMS showed a better ability to withstand in vivo environments because of the low water absorption and beneficial mechanical properties of the substrate, compared to native PDMS. Moreover, annealing is expected to even strengthen the mechanical strength and lower the water absorption of the pc-PDMS substrate. To characterize the mechanical strength and water absorption of the annealed pc-PDMS, tensile tests were carried out and infrared (IR) spectra were measured using Fourier transform infrared spectroscopy over a month. The results showed that annealed pc-PDMS had higher mechanical strength and lower water absorption than non-annealed pc-PDMS. Then, electrochemical impedance spectroscopy was measured to evaluate the electrical stability of the electrodes based on annealed pc-PDMS in phosphate-buffered saline solution at 36.5 °C. The impedance magnitude of the electrodes on annealed pc-PDMS was twice higher than that of the electrodes on non-annealed pc-PDMS in the initial days, but the impedance magnitude of the electrodes based on two different substrates converged to a similar value after eight months, indicating that the annealing effects disappear after a certain period of time in a physiological environment.

Long-term characterization of neural electrodes based on parylene-caulked polydimethylsiloxane substrate.

This study investigates the mechanical and long-term electrical properties of parylene-caulked polydimethylsiloxane (PDMS) as a substrate for implantable electrodes. The parylene-caulked PDMS is a structure where particles of parylene fill the porous surface of PDMS. This material is expected to have low water absorption and desirable mechanical properties such as flexibility and elasticity that are beneficial in many biomedical applications. To evaluate the mechanical property and electrical stability of parylene-caulked PDMS for potential in-vivo uses, tensile tests were conducted firstly, which results showed that the mechanical strength of parylene-caulked PDMS was comparable to that of native PDMS. Next, surface electrodes based on parylene-caulked PDMS were fabricated and their impedance was measured in phosphate-buffered saline (PBS) solution at 36.5 °C over seven months. The electrodes based on parylene-caulked PDMS exhibited the improved stability in impedance over time than native PDMS. Thus, with improved electrical stability in wet environment and preserved mechanical properties of PDMS, the electrodes based on parylene-caulked PDMS are expected to be suitable for long-term in-vivo applications.

A Method to Pattern Silver Nanowires Directly on Wafer-Scale PDMS Substrate and Its Applications.

This study describes a fabrication method of microsized AgNW patterns based on poly dimethylsiloxane (PDMS) substrate using a poly(p-xylylene) (parylene) stencil technique. Various patterns of AgNW conductive sheets were created on the wafer scale area in the forms of straight and serpentine lines, texts, and symbols, which dimensions ranged from a few tens of micrometers to hundreds of micrometers. We demonstrated the electrical performance of straight line and serpentine line patterned AgNW electrodes when subjected to mechanical strains. The gauge factor and stretchability ranged from 0.5 to 55.2 at 2% uniaxial strain and from 4.7 to 55.7%, respectively, depending on the shapes and structures of the AgNW electrodes. Using the developed AgNW patterning technique, we fabricated strain sensors to detect small body signals epidermally such as hand motion, eye blink and heart rate. Also, tactile sensors were fabricated and exhibited the sensitivity of 3.91 MPa(-1) in the pressure range lower than 50 kPa, and 0.28 MPa(-1) in the pressure range greater than 50 kPa up to 1.3 MPa. From these results, we concluded that the proposed technique enables the fabrication of reliable AgNW patterns on wafer-scale PDMS substrate and the potential applications for various flexible electronic devices.

A largely deformable surface type neural electrode array based on PDMS.

This paper describes a largely deformable surface type neural electrode array based on polydimethylsiloxane (PDMS) for cortical use. Noncracked and reliable metal patterns were fabricated successfully on PDMS substrate by employing an intermediate parylene layer. The mechanical and electrical stability of the fabricated electrode arrays was demonstrated by repeatable bending test using a custom-designed bending test module. Also the adhesion of the electrode structure consisting of PDMS, parylene and metal layers was proven by ASTM tape test. The electrode impedance was measured in phosphate buffered saline (PBS) solution at 37 (°) C over three months and analyzed using equivalent circuit models. Based on these results, it is concluded that the suggested electrode array provides a largely deformable structure with mechanical integrity and electrical stability, which can withstand mechanical stresses when inserted through a small trephination hole in the skull and expanded in the small room between the cortex and the skull without damage to the electrode array.