An introduction to heat transfer

Electronics that are capable of intimate, non-invasive integration with the soft, curvilinear surfaces of biological tissues offer important opportunities for diagnosing and treating disease and for improving brain/machine interfaces. This article describes a material strategy for a type of bio-interfaced system that relies on ultrathin electronics supported by bioresorbable substrates of silk fibroin. Mounting such devices on tissue and then allowing the silk to dissolve and resorb initiates a spontaneous, conformal wrapping process driven by capillary forces at the biotic/abiotic interface. Specialized mesh designs and ultrathin forms for the electronics ensure minimal stresses on the tissue and highly conformal coverage, even for complex curvilinear surfaces, as confirmed by experimental and theoretical studies. In vivo, neural mapping experiments on feline animal models illustrate one mode of use for this class of technology. These concepts provide new capabilities for implantable and surgical devices. Electronics that are capable of intimate integration with the surfaces of biological tissues create opportunities for improving animal/machine interfaces. A bio-interfaced system of ultrathin electronics supported by bioresorbable silk-fibroin substrates is now presented. Mounting such devices on tissue and then allowing the silk to dissolve initiates a conformal wrapping process that is driven by capillary forces.

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Dissolvable films of silk fibroin for ultrathin conformal bio-integrated electronics

Our understanding of the cellular implementation of systems-level neural processes like action, thought and emotion has been limited by the availability of tools to interrogate specific classes of neural cells within intact, living brain tissue. Here we identify and develop an archaeal light-driven chloride pump (NpHR) from Natronomonas pharaonis for temporally precise optical inhibition of neural activity. NpHR allows either knockout of single action potentials, or sustained blockade of spiking. NpHR is compatible with ChR2, the previous optical excitation technology we have described, in that the two opposing probes operate at similar light powers but with well-separated action spectra. NpHR, like ChR2, functions in mammals without exogenous cofactors, and the two probes can be integrated with calcium imaging in mammalian brain tissue for bidirectional optical modulation and readout of neural activity. Likewise, NpHR and ChR2 can be targeted together to Caenorhabditis elegans muscle and cholinergic motor neurons to control locomotion bidirectionally. NpHR and ChR2 form a complete system for multimodal, high-speed, genetically targeted, all-optical interrogation of living neural circuits.

Multimodal fast optical interrogation of neural circuitry

The neural bases of behavior are often discussed in terms of perceptual, cognitive, and motor stages, defined within an information processing framework that was originally inspired by models of human abstract problem solving. Here, we review a growing body of neurophysiological data that is difficult to reconcile with this influential theoretical perspective. As an alternative foundation for interpreting neural data, we consider frameworks borrowed from ethology, which emphasize the kinds of real-time interactive behaviors that animals have engaged in for millions of years. In particular, we discuss an ethologically-inspired view of interactive behavior as simultaneous processes that specify potential motor actions and select between them. We review how recent neurophysiological data from diverse cortical and subcortical regions appear more compatible with this parallel view than with the classical view of serial information processing stages.

https://www.cisek.org/pavel/Pubs/CisekKalaska2010.pdf

Neural Mechanisms for Interacting with a World Full of Action Choices

Recent work in field of neuroprosthetics has demonstrated that by observing the simultaneous activity of many neurons in specific regions of the brain, it is possible to produce control signals that allow animals or humans to drive cursors or prosthetic limbs directly through thoughts. As neuroprosthetic devices transition from experimental to clinical use, there is a need for fully-implantable amplification and telemetry electronics in close proximity to the recording sites. To address these needs, we developed a prototype integrated circuit for wireless neural recording from a 100-channel microelectrode array. The design of both the system-level architecture and the individual circuits were driven by severe power constraints for small implantable devices; chronically heating tissue by only a few degrees Celsius leads to cell death. Due to the high data rate produced by 100 neural signals, the system must perform data reduction as well. We use a combination of a low-power ADC and an array of "spike detectors" to reduce the transmitted data rate while preserving critical information. The complete system receives power and commands (at 6.5 kb/s) wirelessly over a 2.64-MHz inductive link and transmits neural data back at a data rate of 330 kb/s using a fully-integrated 433-MHz FSK transmitter. The 4.7times5.9 mm2 chip was fabricated in a 0.5-mum 3M2P CMOS process and consumes 13.5 mW of power. While cross-chip interference limits performance in single-chip operation, a two-chip system was used to record neural signals from a Utah Electrode Array in cat cortex and transmit the digitized signals wirelessly to a receiver

/pdf/a-low-power-integrated-circuit-for-a-wireless-100-electrode-33b774psdo.pdf

A Low-Power Integrated Circuit for a Wireless 100-Electrode Neural Recording System

We present benchtop and in vivo experimental results from an integrated circuit designed for wireless implantable neural recording applications. The chip, which was fabricated in a commercially available 0.6- mum 2P3M BiCMOS process, contains 100 amplifiers, a 10-bit analog-to-digital converter (ADC), 100 threshold-based spike detectors, and a 902-928 MHz frequency-shift-keying (FSK) transmitter. Neural signals from a selected amplifier are sampled by the ADC at 15.7 kSps and telemetered over the FSK wireless data link. Power, clock, and command signals are sent to the chip wirelessly over a 2.765-MHz inductive (coil-to-coil) link. The chip is capable of operating with only two off-chip components: a power/command receiving coil and a 100-nF capacitor.

/pdf/wireless-neural-recording-with-single-low-power-integrated-30jglaevqt.pdf

Wireless Neural Recording With Single Low-Power Integrated Circuit

Electronic neural interfaces have been developed to restore function to the nervous system for patients with neural disorders. A conformal and chronically stable dielectric encapsulation is required to protect the neural interface device from the harsh physiological environment and localize the active electrode tips. Chemical vapor deposited Parylene-C films were studied as a potential implantable dielectric encapsulation material using impedance spectroscopy and leakage current measurements. Both tests were performed in 37degC saline solution, and showed that the films provided an electrically insulating encapsulation for more than one year. Isotropic and anisotropic oxygen plasma etching processes were compared for removing the Parylene-C insulation to expose the active electrode tips. Also, the relationship between tip exposure and electrode impedance was determined. The conformity and the uniformity of the Parylene-C coating were assessed using optical microscopy, and small thickness variations on the complex 3-D electrode arrays were observed. Parylene C was found to provide encapsulation and electrical insulation required for such neural interface devices for more than one year. Also, oxygen plasma etching was found to be an effective method to etch and pattern Parylene-C films.

Encapsulation of an Integrated Neural Interface Device With Parylene C

Gas sensors operating at elevated temperatures (T > 500 °C) near combustion engines and power plants are needed to improve combustion control and to monitor and control emission. We investigated the effects of film thickness and operating temperature on the gas sensitivity of NiO films. 50 nm films had the best sensor characteristics and were further characterized for repeatability and selectivity. We present data on the structure, surface morphology, and chemical composition of 30–130 nm thick NiO films using X-ray diffraction (XRD), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS), respectively. This report includes gas sensor responses (GSRs) at operating temperatures from 300 to 650 °C for H2 test gas concentrations ranging from 500 to 10,000 ppm in a synthetic air carrier gas. After annealing, the 50 nm thick films showed the highest surface roughness of 14.6 nm and the highest GSR at all operating temperatures tested. Surface roughness and area appear to be correlated to the gas sensor response of NiO thin films. An average GSR of 55 for 5000 ppm H2 was found at 600 °C with a standard deviation of ±6.23. Repeated measurements for gas sensitivity were collected at 600 °C for 8 h where NiO was exposed to H2 11 times at concentrations varying from 500 to 10,000 ppm and each exposure lasted for 20 min, yielding GSRs that follow a power law behavior. Cross sensitivity of NiO for 1100 ppm CO2, 150 ppm NH3 and 50 ppm NOx was investigated for individual gases and mixtures, all using synthetic air carrier gas. Selectivity was observed to decrease as the operating temperature increased.

H2 gas sensor performance of NiO at high temperatures in gas mixtures

This report presents results from research towards a fully integrated, wireless neural interface consisting of a 100-channel microelectrode array, a custom-designed signal processing and telemetry IC, an inductive power receiving coil, and SMD capacitors. An integration concept for such a device was developed, and the materials and methods used to implement this concept were investigated. We developed a multi-level hybrid assembly process that used the Utah Electrode Array (UEA) as a circuit board. The signal processing IC was flip-chip bonded to the UEA using Au/Sn reflow soldering, and included amplifiers for up to 100 channels, signal processing units, an RF transmitter, and a power receiving and clock recovery module. An under bump metallization (UBM) using potentially biocompatible materials was developed and optimized, which consisted of a sputter deposited Ti/Pt/Au thin film stack with layer thicknesses of 50/150/150 nm, respectively. After flip-chip bonding, an underfiller was applied between the IC and the UEA to improve mechanical stability and prevent fluid ingress in in vivo conditions. A planar power receiving coil fabricated by patterning electroplated gold films on polyimide substrates was connected to the IC by using a custom metallized ceramic spacer and SnCu reflow soldering. The SnCu soldering was also used to assemble SMD capacitors on the UEA. The mechanical properties and stability of the optimized interconnections between the UEA and the IC and SMD components were measured. Measurements included the tape tests to evaluate UBM adhesion, shear testing between the Au/Sn solder bumps and the substrate, and accelerated lifetime testing of the long-term stability for the underfiller material coated with a a-SiCx:H by PECVD, which was intended as a device encapsulation layer. The materials and processes used to generate the integrated neural interface device were found to yield a robust and reliable integrated package.

Florian Solzbacher

Papers

A Low-Power Integrated Circuit for a Wireless 100-Electrode Neural Recording System

Wireless Neural Recording With Single Low-Power Integrated Circuit

Encapsulation of an Integrated Neural Interface Device With Parylene C

H2 gas sensor performance of NiO at high temperatures in gas mixtures

Integrated wireless neural interface based on the Utah electrode array.