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Magnetism-based systems are widely utilized for sensing and imaging biological phenomena, for example, the activity of the brain and the heart. Magnetomyography (MMG) is the study of muscle function through the inquiry of the magnetic signal that a muscle generates when contracted. It is central in movement neuroscience, in the diagnosis of peripheral muscle and nerve diseases, and in technologies for motor rehabilitation. Within the last few decades, extensive efforts have been invested to identify, characterize and quantify the MMG signals. However, it is still far from highly miniaturized, sensitive, inexpensive, low-power and room-temperature MMG sensors. This thesis presents the development of new magnetic sensing technologies that have the potential to realize a low-profile wearable system. The technical challenges associated with the detection of the MMG signals, including geomagnetic fields and movement artifacts are also discussed. Several research strategies are proposed on how to fill the gap between the conventional and the next-generation MMG sensors that enable sensing pico-Tesla signals to revitalize the MMG measurement. There are two important factors that hinder the effective technology for recoding the muscle behaviours: (1) Sensor: conventionally, in the clinic, the muscle activity is recorded by means of surface (on-skin) or depth (in-muscle tissue) electromyography (EMG). The surface EMG signals suffer from poor spatial resolution. It is difficult, if not impossible, to record from individual motor units within a muscle selectively. Spatial resolution in the measurement of muscle activity can be increased by using the invasive method of intramuscular EMG in which needle electrodes are pushed into the muscle tissue. In addition to being painful, the needle’s penetration into the muscle disturbs the muscle structure and function. Moreover, in chronic implants, such as those used for controlling motor prostheses, the interface between the sensor’s needle contacts and the muscle tissue changes over time, leading to inflammation (or even infection) and rejection by the body. Besides, needle-based approaches put the patients at significant risk and particular patients e.g. children do not tolerate repeated examinations, as in the cases where the clinical picture is used to monitor the treatment efficacy. (2) Active electronic circuit: the miniaturization of the active electronic circuit is a crucial factor for wearable and implantable systems. However, its size is limited due to bulky batteries and inductive coils for powering the sensor readout circuitry. Moreover, the sensors’ resolution has been limited by output instability and noise. There is an urgent need for an innovative alternative paradigm that enables the recording of muscle activity at high spatial resolution with minimal invasiveness. The MMG, which is the magnetic counterpart of the EMG method, is a the only conceivable candidate that can address both limitations target objectives. (1) Sensor: a proof-of-concept of thin-film magnetic sensors, including magnetoelectric and magnetoresistive sensors, which provide better spatial resolution and biomedical compatibility. Spintronic sensors revolutionize magnetic recording owing to their full compatibility with CMOS technology. Modern spintronic sensors with Magnesium-Oxide barrier layer exhibit spin-related magnetoresistive properties at room temperature. The sensors’ resolution has been limited by output instability and noise, thus enhancing sensitivity with novel microelectronics design using application-specific integrated circuits (ASIC) or complementary metal-oxide-semiconductor (CMOS) circuits is highly desirable. The integrated readout circuitry onto a sub-mm diameter silicon substrate realizes the on-chip signal conditioning, including amplification, filtering, noise and drift cancellation. (2) Active electronic circuit: for the first time, a fully integrated chip including the sensor and its readout circuit with miniaturized electronic components is proposed to record the MMG signals. I aim to develop and validate, for the first time, a wireless spintronic-based MMG sensing system. It will integrate readout microelectronic circuit and function in the sub-pico-Tesla range at room temperature. Addressing the technical challenges in an unprecedented level of miniaturization, the detection range of the TMR sensors, with their wireless data transfer, can enable the use of chronic MMG signals for a wide range of applications in neurotechnology systems, neurophysiology, and movement neuroscience. Miniaturizing magnetic sensing systems can offer the prospect to replace the bulky laboratory instruments with easy-to-use implantable clinical platforms. It would bring down the cost, size and noise floor by several orders of magnitude. This new generation integrated system using nanofabricated spintronic-based sensors with a small footprint, excellent sensitivity, ultralow noise and excellent spatial resolution can enable sensing for the detection of low pico-Tesla magnetic fields generated by muscle. Beyond its applications in muscle disease diagnosis and pathological spontaneous activity monitoring, this new technological tool for magnetic field detection will have the potential to be utilized in rehabilitation such as from traumatic nerve injuries, spinal cord lesions or entrapment syndrome. In addition, it can be utilized as wearable and handheld magnetic-based lab-on-a-chip sensing platforms. |
