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An analog VLSI-based low-power neural tissue stimulator is presented as a part of the MIT and Massachusetts Eye and Ear Infirmary Retinal Implant Project to develop a prosthesis for restoring some useful vision to patients blinded by retinal degeneration. Such a prosthesis would receive image data from an external camera and electrically stimulate...
Citations
... To develop Class E Amplifier we employed single ended Class E configuration with multi-frequency load network topology. The advantage of using this structure is the ease of inclusion of parasitic drain-source capacitance of the switch transistor into the design optimization as compared to Class C or Class D amplifiers [17,18]. The parameters of proposed class E PA are presented in Table 2. ...
p>In this work a 2 MHz on-off keying (OOK) transmitter/receiver for inductive power and data transmission for biomedical implant system is presented. Inductive link, driven by a Class E power amplifier (PA) is the most PA used to transfer data and power to the internal part of biomedical implant system. Proposed transmitter consists of a digital control oscillator (DCO) and a class E PA which uses OOK modulation to transfer both data and power to a biomedical implant. In proposing OOK transmitter when the transmitter sends binary value “0” the DCO and PA are turned off. With this architecture and 2 MHz carrier wave we have implemented a wireless data and power transfer link which can transmit data with data rate 1Mbps and bit error rate (BER) of 10-5. The efficiency of power transfer is 42% with a 12.7 uH transmitter coil and a 2.4 uH receiver coil and the power delivered to the load is about 104.7 mW. Proposed transmitter is designed for output power 4.1V. OOK receiver consists of an OOK demodulator, powered by rectified and regulated 5V p-p RF signal across the receiver coil. The supply voltage of proposed voltage regulator is 5 V with 9mV/V line regulation of. All circuits proposed in this paper were designed and simulated using Cadence in 0.18 um CMOS process.</p
... Implanted neural stimulation electrodes may be modeled by an electrochemical cell consisting of the stimulating (working) and return (counter) electrodes, and tissue, which may be modeled as a 0.9% NaCl electrolyte (physiological saline). A more detailed treatment of electrodes as they apply to neural stimulation may be found in (Kelly, 2003;Roach, 2003), and a more general reference on electrodes and electrochemistry may be found in (Bard and Faulkner, 2001). ...
Electrical stimulation of tissues is an increasingly valuable tool for treating a variety of disorders, with applications including cardiac pacemakers, cochlear implants, visual prostheses, deep brain stimulators, spinal cord stimulators, and muscle stimulators. Brain implants for paralysis treatments are increasingly providing sensory feedback via neural stimulation. Within the field of neuroscience, the perturbation of neuronal circuits wirelessly in untethered, freely-behaving animals is of particular importance. In implantable systems, power consumption is often the limiting factor in determining battery or power coil size, cost, and level of tissue heating, with stimulation circuitry typically dominating the power budget of the entire implant. Thus, there is strong motivation to improve the energy efficiency of implantable electrical stimulators. In this thesis, I present two examples of low-power tissue stimulators. The first type is a wireless, low-power neural stimulation system for use in freely behaving animals. The system consists of an external transmitter and a miniature, implantable wireless receiver-and-stimulator utilizing a custom integrated chip built in a standard 0.5 ptm CMOS process. Low power design permits 12 days of continuous experimentation from a 5 mAh battery, extended by an automatic sleep mode that reduces standby power consumption by 2.5x. To test this device, bipolar stimulating electrodes were implanted into the songbird motor nucleus HVC of zebra finches. Single-neuron recordings revealed that wireless stimulation of HVC led to a strong increase of spiking activity in its downstream target, the robust nucleus of the arcopallium (RA). When this device was used to deliver biphasic pulses of current randomly during singing, singing activity was prematurely terminated in all birds tested. The second stimulator I present is a novel, energy-efficient electrode stimulator with feedback current regulation. This stimulator uses inductive storage and recycling of energy based on a dynamic power supply to drive an electrode in an adiabatic fashion such that energy consumption is minimized. Since there are no explicit current sources or current limiters, wasteful energy dissipation across such elements is naturally avoided. The stimulator also utilizes a shunt current-sensor to monitor and regulate the current through the electrode via feedback, thus enabling flexible and safe stimulation. The dynamic power supply allows efficient transfer of energy both to and from the electrode, and is based on a DC-DC converter topology that is used in a bidirectional fashion. In an exemplary electrode implementation, I show how the stimulator combines the efficiency of voltage control and the safety and accuracy of current control in a single low-power integrated-circuit built in a standard 0.35 pm CMOS process. I also perform a theoretical analysis of the energy efficiency that is in accord with experimental measurements. In its current proof-of-concept implementation, this stimulator achieves a 2x-3x reduction in energy consumption as compared to a conventional current-source-based stimulator operating from a fixed power supply.
... The IEEE and ANSI have a joint standard which recommends an occupational limit for magnetic-field exposure that decreases as frequency increases between 100 kHz and 100 MHz [9]. This system and others already exceed this recommended field-frequency product of 16.3 MHz A/m [10], [11], and greater power consumption in the stimulating electronics necessarily means a greater need for power transmitted via magnetic fields. Of the 125 W consumed in our stimulator system, 49 W is consumed in the electrodes, calculated directly from the product of electrode current and voltage. ...
This paper presents a power-efficient neural stimulator integrated circuit, designed to take advantage of our understanding of iridium-oxide electrode impedance. It efficiently creates a programmable set of voltage supplies directly from a secondary power telemetry coil, then switches the target electrode sequentially through the voltage steps. This sequence of voltages mimics the voltage of the electrode under the constant current drive, resulting in approximately constant current without the voltage drop of the more commonly used linear current source. This method sacrifices some precision, but drastically reduces the series losses seen in traditional current sources and attains power savings of 53%-66% compared to these designs. The proof-of-concept circuit consumes 125 μW per electrode and was fabricated in a 1.5-μm CMOS process, in a die area of 4.76 mm(2).
... The Biot-Savart Law (1) is numerically integrated across a secondary coil using a script for MATLAB (The MathWorks, Natick, MA, USA) written by the authors and provided in the appendix of [13]. For each small segment of primary coil, the script integrates the normal portion of the resulting field across the area of the secondary coil. ...
... The first step is to integrate the Biot-Savart expression across a secondary coil to determine the error from using the central field approximation in (2). This is the calculation performed in [13], and is performed here with a secondary coil radius of 9.75 mm, primary coil radius of 18.5 mm, and displacement between coils of 15 mm. The data are displayed in Fig. 5 in the Results section. ...
... To learn more about the effects of eye movement on inductive coupling, we have created a non-conductive plastic test jig for measuring coupling between coils [13], and affixed to it a rotating arm with a radius of 12 mm, the approximate radius of the human eye (Fig. 4). Our secondary coil is affixed to this arm. ...
A retinal prosthesis telemetry system is examined, and several methods are explored to optimize the size of the external primary telemetry coil to maximize the wireless delivery of power to an implanted secondary coil of constrained size. A simplified version of the Biot-Savart Law is used to give a first-pass optimal primary coil size for a small secondary coil. Numerical integration is then used to improve the optimization for larger secondary coils, and this calculation is repeated across a range of secondary coil radii. Finally, the effects of eye rotation angle are explored, with the future goal of expanding the optimization techniques to cover the predicted range of angular eye excursions.
... Next, primary coil should be designed such that it is neither too small and neither unwieldy large. If the diameter of the primary coil is too small, the field strength would drop off quickly with axial displacement [9]. ...
... IV. CLASS E POWER AMPLIFIER "Class E" refers to a tuned power amplifier (PA) composed of a single-pole switch and a load network [11]. It is also known as switched mode PA because the active device or transistor acts as a switch [9]. On the other hand, Classes A, B and C refer to amplifiers in which the transistor acts as a current source [11]. ...
This paper describes design principles to design and develop a transcutaneous link for medical implants using inductively coupled coils. Parameters which optimize link efficiency have been discussed in the light of previous studies and a simple design methodology to find optimized parameters for Class E amplifier and inductive coils is outlined. Paper also describes design of an indigenously developed transcutaneous link from commercial off-the-shelf components to demonstrate the design process. Simulation and practical results of the link developed at 2.5 MHz for 100 mW output power are provided. We were able to achieve 40% link efficiency with data rate of 128 kbps from laboratory-based discrete electronic components.
... Next, primary coil should be designed such that it is neither too small and neither unwieldy large. If the diameter of the primary coil is too small, the field strength would drop off quickly with axial displacement [9]. ...
... IV. CLASS E POWER AMPLIFIER "Class E" refers to a tuned power amplifier (PA) composed of a single-pole switch and a load network [11]. It is also known as switched mode PA because the active device or transistor acts as a switch [9]. On the other hand, Classes A, B and C refer to amplifiers in which the transistor acts as a current source [11]. ...
This paper describes design principles to design and develop a transcutaneous link for medical implants using inductively coupled coils. Parameters which optimize link efficiency have been discussed in the light of previous studies and a simple design methodology to find optimized parameters for Class E amplifier and inductive coils is outlined. Paper also describes design of an indigenously developed transcutaneous link from commercial off-the-shelf components to demonstrate the design process. Simulation and practical results of the link developed at 2.5 MHz for 100 mW output power are provided. We were able to achieve 40% link efficiency with data rate of 128 kbps from laboratory-based discrete electronic components.
... Unlike a hard-gated design, the energy stored in the MOSFET capacitances is recycled (resonated) each cycle back to the gate drive power supply (minus the amount lost due to conduction through parasitic resistances). It can be shown that such a quasi-trapezoidal waveform is the most efficient possible for a specified transition time [16, 37, 52, 53] To complete the design, the sinusoidal signal applied to the gate of S aux in the simulation must now be replaced by a suitable self-oscillation network, as illustrated inFigure 5.4. The next section treats the synthesis of such a self-oscillation network. ...
Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2007. This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. Includes bibliographical references (p. 139-143). This thesis presents a resonant boost topology suitable for very high frequency (VHF, 30-300 MHz) dc-dc power conversion. The proposed design is a fixed frequency, fixed duty ratio resonant converter featuring low device stress, high efficiency over a wide load range, and excellent transient performance. A 110 MHz, 23 W experimental converter has been built and evaluated. The input voltage range is 8-16 V (14.4 V nominal), and the selectable output voltage is between 22-34 V (33 V nominal). The converter achieves higher than 87% efficiency at nominal input and output voltages, and maintains efficiency above 80% for loads as small as 5% of full load. Furthermore, efficiency is high over the input and output voltage range. In addition, a resonant gate drive scheme suitable for VHF operation is presented, which provides rapid startup and low-loss operation. The converter regulates the output using high-bandwidth on-off hysteretic control, which enables fast transient response and efficient light load operation. The low energy storage requirements of the converter allow the use of coreless inductors, thereby eliminating magnetic core loss and introducing the possibility of integration. The target application of the converter is the automotive industry, but the design presented here can be used in a broad range of applications where size, cost, and weight are important, as well as high efficiency and fast transient response. by Robert C.N. Pilawa-Podgurski. M.Eng.
... In Section 4.2, we will look at electrode modeling, and in Section 4.3, we will look at some electrode measurements. A more detailed treatment of electrodes as they apply to neural stimulation may be found in [9,14], and a more general reference on electrodes and electrochemistry may be found in [1]. ...
In this thesis, I present the design of a wireless neural stimulation system. The system consists of an external transmitter, controllable through a computer interface, and a miniature, implantable wireless receiver and stimulator. The implant is tailored for use in zebra finches - small birds weighing just 12-15g - as part of ongoing research into the neural mechanisms of sequence generation and learning. The implant, assembled on a miniature printed circuit board, contains a receiver coil, battery, electrodes, and a custom integrated circuit for data demodulation and neural stimulation. The chip, fabricated in a standard 0.5[mu]m CMOS process, is capable of delivering biphasic current pulses to 4 addressable electrode sites at 16 selectable current levels ranging from 100[mu]A to mA. Additionally, the biphasic pulses may be inverted. The entire implant weighs less than 1.5g and occupies a footprint smaller than 1.5cm2. A miniaturized neural stimulator such as this one also has applications in neural prostheses for blindness, Parkinson's disease, and paralysis. Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2006. Includes bibliographical references (p. 103-105).
... [39], compared to ~0.9 for standard transformers), great care must be taken to optimize the receiving radiation. The optimization of coil geometry and receiving circuitry to maximize Q has been the subject of numerous studies [39][40][41][42][43][44][45][46][47]. Since Q is proportional to the transmission frequency, high frequency operation yields higher Q values; however, tissue's RF absorption increases exponentially beyond a few MHz [48] limiting transmission frequency t f to 1-10 MHz. ...
... Thus, while CHAPTER 1: INTRODUCTION 22 received power is directly proportional to coil Q , the attainable data rate is inversely proportional to Q . For this reason, many visual prosthesis designs use two coil pairs: one for power, and one for data, where data transmission is accomplished at a higher frequency [39,42] or with a lower-Q coil [36]. In addition, complex single-coil systems capable of delivering both power and data over one coil pair have also been developed [40,51,52], in one case achieving a data rate in excess of 1 Mb/s [34]. ...
This chapter discusses neural stimulator circuits, focusing on the power consumed in such circuits. The basis of neural communication, the action potential, involves the movement of ions across the nerve membrane, and externally applied electrical currents create electric fields that can modulate that ion movement to induce action potentials. These currents are generally applied by a pulsed current source circuit, but these circuits waste a large amount of electrical power. An architecture is put forth here that uses a series of stepped voltage sources to drive charge onto an electrode in a manner similar to that used in adiabatic digital circuits. A sample system is described that creates five voltage supplies on capacitors from a single secondary telemetry coil voltage. Test results from this system show a power reduction of 53 % compared to a current source using the same chip voltage supplies and a power reduction of 66 % compared to a current source using the lowest reported voltage supplies for the same type of electrode.
