Prototyping portable high speed large-scale magneto-nanosensor array and system [electronic resource]
Keywords: Electrical Engineering.

In recent decades, numerous biomarkers have been discovered and many of which have been extensively utilized in biomedical applications such as diagnosis, prognosis and drug discovery. With plentiful information of putative or clinically validated biomarkers, there are increasing demands for rapid and cost effective bioassay platforms which can accommodate a multitude of biomarkers in a single biological assay. Additionally, portable point-of-care (POC) devices featuring multiple biomarkers are highly desired for early or easy diagnosis of many prevalent diseases. From this perspective, giant magnetoresistive (GMR) biosensor platform is a promising candidate to be expanded to large-scale biomarker assays as well as POC applications because they feature multiplexing capability and smaller sample volume requirement. GMR biosensors are easily integrated with electronics, which enables the miniaturization of the GMR sensor platforms. In this dissertation we propose a novel design methodology to enhance both scalability and portability of GMR sensor arrays and the corresponding data acquisition (DAQ) system. It involves designing an entire platform from sensors to readout systems, prototyping the key concepts and components, and demonstrating the proof of concepts. A concept of in-pixel switching in a GMR sensor array is proposed to prevent excess noise or narrowed bandwidth when the size of the arrays is scaled up to the large-scale. The large-scale array necessitates fast DAQ methods to enable real-time analysis during bioassays with a large-scale GMR sensor array. The conventional spectral analysis such as the previously adopted double modulation DAQ system, which requires a sinusoidal modulated magnetic field at one frequency and a sinusoidal modulated bias current (or voltage) at another frequency for each sensor, suffers from timing penalty in reading out individual sensors in a large-scale sensor array and post-processing of readout signals. As a potential solution to the above problem, a correlated double sampling (CDS) method with magnetic field modulation is introduced to achieve fast readout in a large-scale array of GMR sensors. It relaxes the requirement of the modulated magnetic field from pure sinusoidal to pulsed waveform. As the pulsed in-plane magnetic field can be generated by on-chip metal wires with a driving current, we propose to implement on-chip field wires underneath GMR sensor strips in order to 1) support high speed DAQ and 2) minimize the form factor of readout system by eliminating a bulk electromagnet and its associated power supply and power-hungry amplifier. Since GMR sensors are susceptible to the signal drift resulting from ambient temperature changes, a novel temperature correction technique suitable for the CDS operation is proposed. A prerequisite of the correction technique is to obtain 1/f noise and offset-reduced baseline and MR signals from GMR sensors. In order to accomplish the prerequisite, the duplex CDS and the global chopper stabilization techniques are invented. A series of experiments have confirmed that the proposed DAQ and temperature correction techniques performed seamlessly. Proposed techniques are implemented in a prototype GMR spin valve (SV) sensor array consisting of 12 individual sensors, which I fabricated at Stanford Nanofabrication Facility. The sensor array is then connected to the corresponding CDS-based readout system which I designed and assembled. Simulated bioassay results assure that the proposed design methods are readily applicable for large-scale GMR biosensor platforms as well as highly portable biomedical applications. The prototype has demonstrated that the minimum signal detection level of 13.7 ppm in magnetoresistive ratio (ΔMR) changes, equivalent to the detection limit of 7629 superparamagnetic nanoparticles in simulated bioassays. Importantly, the prototype system exhibits a readout speed that is 24.4 times faster than the prior double modulation approach, and this advantage will be even more pronounced as we scale up the sensor array. By eliminating the off-chip bulk electromagnet in the prior approach, the new CDS system demonstrates a power efficiency which is more than 41.2 times better than the prior approach when generating a modulated magnetic field of 23 Oerms. These proof-of-concept experiments demonstrate that the GMR biosensor platform with CDS DAQ methods can be used for either large-scale bioassays or portable POC applications.