Moisés Piedade

Femtomolar limit of detection with a magnetoresistive biochip.

In this paper the biological limit of detection of a spin-valve-based magnetoresistive biochip applied to the detection of 20 mer ssDNA hybridization events is presented. Two reactional variables and their impact on the biomolecular recognition efficiency are discussed. Both the influence of a 250 nm diameter magnetic particle attached to the target molecule during the hybridization event and the effect of a magnetic focusing system in the hybridization of pre-labeled target DNA (assisted hybridization) are addressed. The particles carrying the target molecules are attracted to the probe active sensor sites by applying a 40 mA DC current on U-shaped aluminium current lines. Experiments comparing pre-hybridization versus post-hybridization magnetic labeling and passive versus magnetically assisted hybridization were conducted. The efficiency of a passive hybridization is reduced by about 50% when constrained to the operational conditions (sample volume, reaction time, temperature and magnetic label) of an on-chip real-time hybridization assay. This reduction has shown to be constant and independent from the initial target concentration. Conversely, the presence of the magnetic label improved the limit of detection when a magnetically assisted hybridization was performed. The use of a labeled target focusing system has permitted a gain of three orders of magnitude (from 1 pM down to 1 fM) in the sensitivity of the device, as compared with passive, diffusion-controlled hybridization.

A Portable and Autonomous Magnetic Detection Platform for Biosensing

This paper presents a prototype of a platform for biomolecular recognition detection. The system is based on a magnetoresistive biochip that performs biorecognition assays by detecting magnetically tagged targets. All the electronic circuitry for addressing, driving and reading out signals from spin-valve or magnetic tunnel junctions sensors is implemented using off-the-shelf components. Taking advantage of digital signal processing techniques, the acquired signals are processed in real time and transmitted to a digital analyzer that enables the user to control and follow the experiment through a graphical user interface. The developed platform is portable and capable of operating autonomously for nearly eight hours. Experimental results show that the noise level of the described platform is one order of magnitude lower than the one presented by the previously used measurement set-up. Experimental results also show that this device is able to detect magnetic nanoparticles with a diameter of 250 nm at a concentration of about 40 fM. Finally, the biomolecular recognition detection capabilities of the platform are demonstrated by performing a hybridization assay using complementary and non-complementary probes and a magnetically tagged 20mer single stranded DNA target.

Defect Characterization With Eddy Current Testing Using Nonlinear-Regression Feature Extraction and Artificial Neural Networks

The estimation of the parameters of defects from eddy current nondestructive testing data is an important tool to evaluate the structural integrity of critical metallic parts. In recent years, several works have reported the use of artificial neural networks (ANNs) to deal with the complex relation between the testing data and the defect properties. To extract relevant features used by the ANN, principal component analysis, wavelet decomposition, and the discrete Fourier transform have been proposed. In this paper, a method to estimate dimensional parameters from eddy current testing data is reported. Feature extraction is based on the modeling of the testing data by a template of additive Gaussian functions and nonlinear regressions to estimate their parameters. An ANN was trained using features extracted from a synthetic data set obtained with finite-element modeling of the eddy current probe. The proposed method was applied to both simulated and measured data, providing good estimates.

A New Hand-Held Microsystem Architecture for Biological Analysis

This paper presents a hand-held microsystem based on new fully integrated magnetoresistive biochips for biomolecular recognition (DNA hybridization, antibody antigen interaction, etc.). Magnetoresistive chip surfaces are chemically treated, enabling the immobilization of probe biomolecules such as DNA or antibodies. Fluid handling is also integrated in the biochip. The proposed microsystem not only integrates the biochip, which is an array of 16times16 magnetoresistive sensors, but it also provides all the electronic circuitry for addressing and reading out each transducer. The proposed architecture and circuits were specifically designed for achieving a compact, programmable and portable microsystem. The microsystem also integrates a hand-held analyzer connected through a wireless channel. A prototype of the system was already developed and detection of magnetic nanoparticles was obtained. This indicates that the system may be used for magnetic label based bioassays

Visual neuroprosthesis: a non invasive system for stimulating the cortex

This paper describes a complete visual neuroprosthesis wireless system designed to restore useful visual sense to profoundly blind people. This visual neuroprosthesis performs intracortical microstimulation through one or more arrays of microelectrodes implanted into the primary visual cortex. The whole system is composed by a primary unit located outside the body and a secondary unit, implanted inside the body. The primary unit comprises a neuromorphic encoder, a forward transmitter, and a backward receiver. The developed neuromorphic encoder generates the spikes to stimulate the cortex by approximating the spatio-temporal receptive fields characteristic response of ganglion cells. Power and stimuli information are carried to inside the cranium by means of a low-coupling transformer, which establishes a wireless inductive link between the two units. The secondary unit comprises a forward receiver, microelectrode stimulation circuitry and a backward transmitter that is used to monitor the implant. Address event representation is used for communicating spike events. Data is modulated with binary frequency-shift keying and differential binary phase-shift keying in the forward and in the backward directions, respectively. A prototype of the proposed system was developed and tested. Experimental results show that the spikes to stimulate the visual cortex are accurately generated and that the efficiency of the inductive link is relatively high, about 28% in average for 1 cm intercoil distance providing a power of about 50 milliwatts to the secondary implanted unit. Application specific integrated circuits were designed for this secondary unit, showing that, with current technology, it is possible to implement such a unit, respecting the power constraints.

Diode/magnetic tunnel junction cell for fully scalable matrix-based biochip

Magnetoresistive biochips have been recently introduced for the detection of biomolecular recognition. In this work, the detection site incorporates a thin-film diode in series with a magnetic tunnel junction (MTJ), leading to a matrix-based biochip that can be easily scaled up to screen large numbers of different target analytes. The fabricated 16×16 cell matrix integrates hydrogenated amorphous silicon (a-Si:H) diodes with aluminum oxide barrier MTJ. Each detection site also includes a U-shaped current line for magnetically assisted target concentration at probe sites. The biochip is being integrated in a portable, credit card size electronics control platform. Detection of 250nm diameter magnetic nanoparticles by one of the matrix cells is demonstrated.

Detection of 130nm magnetic particles by a portable electronic platform using spin valve and magnetic tunnel junction sensors

An integrated biosensor with magnetic tunnel juntions (MTJs) and spin valve (SV) sensor was used for 130nm particle detection. A platform drives an external magnet generating an in-plane dc+ac magnetic field on the sensor at frequencies up to 375Hz, provides a current to bias the sensor, and performs the signal acquisition and treatment. The signal-to-noise ratio of the SV and MTJ was characterized. Bead detection (130nm in diameter) was performed using both sensors leading to a detection limit of 3×108particles∕ml for SV and 3×1010particles∕ml for MTJ.

The behavior of the modified FX-LMS algorithm with secondary path modeling errors

In active noise control there has been some research based in the modified filtered-X least mean square (LMS) algorithm (MFX-LMS). When the secondary path is perfectly modeled, this algorithm is able to perfectly eliminate its effect. It is also easily adapted to allow the use of fast algorithms such as the recursive least square, or algorithms with good tracking performance based on the Kalman filter. This letter presents the results of a frequency domain analysis about the behavior of the MFX-LMS in the presence of secondary path modeling errors and a comparison with the FX-LMS algorithm. Namely, it states that for small values of the secondary path delay both algorithms perform the same, but that the step-size of the FX-LMS algorithm decreases with increasing delay, while the MFX-LMS algorithm is stable for an arbitrary large value for the secondary path delay, as long as the real part of the ratio of the estimated to the actual path is greater than one half (Re{S/spl circ//sub z//S/sub z/}>1/2). This means that for the case of no phase errors the estimated amplitude should be greater than half the real one and for the case of no amplitude errors the phase error should be less than 60/spl deg/. Analytical expressions for the limiting values for the step-size in the presence of modeling errors are given for both algorithms.

Integration of Magnetoresistive Biochips on a CMOS Circuit

Since 2006, fully scalable matrix-based magnetoresistive biochips have been proposed. This integration was initially achieved with thin film switching devices and moved to complementary metal-oxide-semiconductor (CMOS) switching devices and electronics. In this paper, a new microfabrication process is proposed to integrate magnetoresistive sensors on a small CMOS chip (4 mm2). This chip includes a current generator, multiplexers, and a diode in series with a spin valve as matrix element. In this configuration, it is shown that the fabricated spin-valves have similar magnetic characteristics when compared to standalone spin valves. This validates the successfulness of the developed microfabrication process. The noise of each matrix element is further characterized and compared to the noise of a standalone spin valve and a portable electronic platform designed to perform biological assays. Although the noise is still higher, the spin valve integrated on the CMOS chip enables an increase in density and compactness of the measuring electronics.

A reconfigurable digital signal processing system for eddy currents non-destructive testing

This paper presents a digital signal processing system specially designed for eddy currents non-destructive testing. This new system has a field programmable gate array based processing core, communication interfaces, data conversion and analog devices to interface the probes. Communication with personal computers is ensured by Ethernet 10/100 and universal serial bus 2.0 high speed interfaces. The proposed architecture enables to set several combinations of peripherals cards to generate or acquire probe signals. Also, the new system allows the digital generation and analysis of the probe signals through multiple digital signal processing algorithms. Two different peripheral cards have been developed to meet the needs for the new IOnic concept of eddy currents probe. The IOnic acquisition card is composed by a programmable gain amplifier and a high speed analog to digital converter. The current stimulus generation is achieved with a digital to analog converter and a high output current transconductance amplifier. Together, the two peripherals are able to operate the probe from 10 kHz up to 10 MHz. An additional peripheral card to interface stepper motors was designed for sensor positioning.