Supramolecular architectures for neural prostheses

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2007.Includes bibliographical references (leaves 213-230).Neural prosthetic devices offer a means of restoring function that have been lost due to neural damage. The first part of this thesis investigates the design of a 15-channel, low-power, fully implantable stimulator chip. The chip is powered wirelessly and receives wireless commands. The chip features a CMOS only ASK detector, a single-differential converter based on a novel feedback loop, a low-power adaptive bandwidth DLL and 15 programmable current sources that can be controlled via four commands. Though it is feasible to build an implantable stimulator chip, the amount of power required to stimulate more than 16 channels is prohibitively large. Clearly, there is a need for a fundamentally different approach. The ultimate challenge is to design a self-sufficient neural interface. The ideal device will lend itself to seamless integration with the existing neural architecture. This necessitates that communication with the neural tissue should be performed via chemical rather than electrical messages. However, catastrophic destruction of neural tissue due to the release of large quantities of a neuroactive species, like neurotransmitters, precludes the storage of quantities large enough to suffice for the lifetime of the device. The ideal device then should actively sequester the chemical species from the body and release it upon receiving appropriate triggers in a power efficient manner. This thesis proposes the use of ionic gradients, specifically K+ ions as an alternative chemical stimulation method. The required ions can readily be sequestered from the background extracellular fluid. The parameters of using such a stimulation technique are first established by performing in-vitro experiments on rabbit retinas. The results show that modest increases (~~10mM) of K+ ions are sufficient to elicit a neural response.(cont.) The first building block of making such a stimulation technique possible is the development of a potassium selective membrane. To achieve low-power the membranes must be ultrathin to allow for efficient operation in the diffusive transport limited regime. One method of achieving this is to use lyotropic self-assembly; unfortunately, conventional lipid bilayers cannot be used since they are not robust enough. Furthermore, the membrane cannot be made potassium selective by simply incorporating ion carriers since they would eventually leach away from the membrane. A single solution that solves all the above issues was then investigated in this thesis. A novel facile synthesis of self-assembling receptor functionalized polymers was achieved. By combining the properties of hydrophobic and hydrophilic interactions of two polymers a triblock co-polymer was synthesized. The middle hydrophobic block was composed of biocompatible polysiloxanes and further derivatized to posses ion recognition capabilities via pendant crown ether chains. The hydrophilic blocks were composed of biocompatible polyoxazolines. The self-assembling properties of the membrane were then studied by electroforming them into vesicular structures. The ion responsive properties of these polymers were then examined. These polymers show emergent behavior such as, spontaneous fusion and shape transformation to ionic stimuli due to the synergy between form and function. The results from the thesis show that it is feasible to build a renewable chemically based neural prosthesis based on supramolecular architectures. However, there remains a lot of fundamental work that needs to be pursued in the future to bring the idea to complete fruition.by Luke Satish Kumar Theogarajan.Ph.D

CMOS-compatible Ising and Potts Annealing Using Single Photon Avalanche Diodes

Massively parallel annealing processors may offer superior performance for a wide range of sampling and optimization problems. A key component dictating the size of these processors is the neuron update circuit, ideally implemented using special stochastic nanodevices. We leverage photon statistics using single photon avalanche diodes (SPADs) and temporal filtering to generate stochastic states. This method is a powerful alternative offering unique features not currently seen in annealing processors: the ability to continuously control the computational temperature and the seamless extension to the Potts model, a $n$-state generalization of the two-state Ising model. SPADs also offer a considerable practical advantage since they are readily manufacturable in current standard CMOS processes. As a first step towards realizing a CMOS SPAD-based annealer, we have designed Ising and Potts models driven by an array of discrete SPADs and show they accurately sample from their theoretical distributions

Smart-Cut Layer Transfer of Single-Crystal SiC Using Spin-on-Glass

The authors demonstrate “smart-cut”-type layer transfer of single-crystal silicon carbide (SiC) by using spin-on-glass (SoG) as an adhesion layer. Using SoG as an adhesion layer is desirable because it can planarize the surface, facilitate an initial low temperature bond, and withstand the thermal stresses at high temperature where layer splitting occurs (800–900 °C). With SoG, the bonding of wafers with a relatively large surface roughness of 7.5–12.5 Å rms can be achieved. This compares favorably to direct (fusion) wafer bonding, which usually requires extremely low roughness (\u3c2 Å rms), typically achieved using chemical mechanical polishing (CMP) after implantation. The higher roughness tolerance of the SoG layer transfer removes the need for the CMP step, making the process more reliable and affordable for expensive materials like SiC. To demonstrate the reliability of the smart-cut layer transfer using SoG, we successfully fabricated a number of suspended MEMS structures using this technology

Modeling and Experimental Demonstration of a Hopfield Network Analog-to-Digital Converter with Hybrid CMOS/Memristor Circuits

The purpose of this work was to demonstrate the feasibility of building recurrent artificial neural networks with hybrid complementary metal oxide semiconductor (CMOS)/memristor circuits. To do so, we modeled a Hopfield network implementing an analog-to-digital converter (ADC) with up to 8 bits of precision. Major shortcomings affecting the ADC's precision, such as the non-ideal behavior of CMOS circuitry and the specific limitations of memristors, were investigated and an effective solution was proposed, capitalizing on the in-field programmability of memristors. The theoretical work was validated experimentally by demonstrating the successful operation of a 4-bit ADC circuit implemented with discrete Pt/TiO2−x/Pt memristors and CMOS integrated circuit components.National Science Foundation CCF-1028378Air Force Office of Scientific Research FA9550-12-1-0038Ministerio de Economía y Competitividad TEC2012-37868-C04-0

Communication and control system for a 15-channel hermetic retinal prosthesis

A small, hermetic, wirelessly-controlled retinal prosthesis has been developed for pre-clinical studies in Yucatan minipigs. The device was attached conformally to the outside of the eye in the socket and received both power and data wirelessly from external sources. Based on the received image data, the prosthesis drove a subretinal thin-film polyimide array of sputtered iridium oxide stimulating electrodes. The implanted device included a hermetic titanium case containing a 15-channel stimulator and receiver chip and discrete circuit components. Feedthroughs in the hermetic case connected the chip to secondary power- and data-receiving coils, which coupled to corresponding external power and data coils driven by power amplifiers. Power was delivered by a 125 kHz carrier, and data were delivered by amplitude shift keying of a 15.5 MHz carrier at 100 kbps. Stimulation pulse strength, duration and frequency were programmed wirelessly from an external computer system. The final assembly was tested in vitro in physiological saline and in vivo in two minipigs for up to five and a half months by measuring stimulus artifacts generated by the implant's current drivers.United States. Dept. of Veteran AffairsUnited states. Dept. of Veterans Affairs. Boston Healthcare SystemNational Institutes of Health (U.S.)United States. Dept. of DefenseMassachusetts Lions Foundatio

An Integrated-Photonics Optical-Frequency Synthesizer

Integrated-photonics microchips now enable a range of advanced functionalities for high-coherence applications such as data transmission, highly optimized physical sensors, and harnessing quantum states, but with cost, efficiency, and portability much beyond tabletop experiments. Through high-volume semiconductor processing built around advanced materials there exists an opportunity for integrated devices to impact applications cutting across disciplines of basic science and technology. Here we show how to synthesize the absolute frequency of a lightwave signal, using integrated photonics to implement lasers, system interconnects, and nonlinear frequency comb generation. The laser frequency output of our synthesizer is programmed by a microwave clock across 4 THz near 1550 nm with 1 Hz resolution and traceability to the SI second. This is accomplished with a heterogeneously integrated III/V-Si tunable laser, which is guided by dual dissipative-Kerr-soliton frequency combs fabricated on silicon chips. Through out-of-loop measurements of the phase-coherent, microwave-to-optical link, we verify that the fractional-frequency instability of the integrated photonics synthesizer matches the $7.0*10^{-13}$ reference-clock instability for a 1 second acquisition, and constrain any synthesis error to $7.7*10^{-15}$ while stepping the synthesizer across the telecommunication C band. Any application of an optical frequency source would be enabled by the precision optical synthesis presented here. Building on the ubiquitous capability in the microwave domain, our results demonstrate a first path to synthesis with integrated photonics, leveraging low-cost, low-power, and compact features that will be critical for its widespread use.Comment: 10 pages, 6 figure