Andrew J. DeRouin

A Varactor-Based, Inductively Coupled Wireless pH Sensor

An inductively coupled, wireless sensor was fabricated for remote measurement of pH. The sensor consisted of a planar spiral inductor connected to a surface mount varactor, which was a voltage controlled capacitor, forming an inductive-capacitive (LC) resonant circuit. The pH electrodes, made of a thick-film antimony/antimony oxide sensing electrode and a thick-film silver/silver chloride reference electrode, were connected in parallel to the varactor. A voltage change across the electrodes due to the pH variation in the test medium would change the capacitance of the varactor, shifting the resonant frequency of the LC circuit. By inductively coupling the spiral inductor with a detection coil, the resonant frequency of the LC circuit was remotely monitored, allowing measurement of the pH. The advantage of the described pH sensor is its wireless and passive nature, which allows for long-term pH monitoring in inaccessible area. The sensor will be useful for remote monitoring of pH during industrial or food processes. When miniaturized, the sensors can also be used for biomedical applications such as remote tracking of gastric or esophageal pH on patients suffering from gastroesophageal reflux disease.

Biodegradation and biocompatibility of mechanically active magnetoelastic materials

Magnetoelastic (ME) materials have many advantages for use as sensors and actuators due to their wireless, passive nature. This paper describes the application of ME materials as biodegradable implants with controllable degradation rates. Experiments have been conducted to show that degradation rates of ME materials are dependent on the material compositions. In addition, it was shown that the degradation rates of the ME materials can be controlled remotely by applying a magnetic field, which causes the ME materials to generate low-magnitude vibrations that hasten their degradation rates. Another concern of ME materials for medical applications is biocompatibility. Indirect cytotoxicity analyses were performed on two types of ME materials: Metglas™ 2826 MB (FeNiMoB) and iron–gallium alloy. While results indicate Metglas is not biocompatible, the degradation products of iron–gallium materials have shown no adverse effects on cell viability. Overall, these results present the possibility of using ME materials as biodegradable, magnetically-controlled active implants.

Development and Application of the Single-Spiral Inductive-Capacitive Resonant Circuit Sensor for Wireless, Real-Time Characterization of Moisture in Sand

A wireless, passive embedded sensor was designed and fabricated for monitoring moisture in sand. The sensor, consisted of an inductive-capacitive (LC) resonant circuit, was made of a printed spiral inductor embedded inside sand. When exposed to an electromagnetic field, the sensor resonated at a specific frequency dependent on the inductance of the inductor and its parasitic capacitance. Since the permittivity of water was much higher than dry sand, moisture in sample increased the parasitic capacitance, thus decreasing the sensor’s resonant frequency. Therefore, the internal moisture level of the sample could be easily measured through tracking the resonant frequency using a detection coil. The fabrication process of this sensor is much simpler compared to LC sensors that contain both capacitive and inductive elements, giving it an economical advantage. A study was conducted to investigate the drying rate of sand samples of different grain sizes. The experimental data showed a strong correlation with the actual moisture content in the samples. The described sensor technology can be applied for long term monitoring of localized water content inside soils and sands to understand the environmental health in these media, or monitoring moisture levels within concrete supports and road pavement.

Magnetoelastic-Harmonic Stress Sensors With Tunable Sensitivity

A wireless and passive stress sensor was developed by measuring changes in the induced magnetic field of a magnetoelastic strip under varying force loadings. The magnetoelastic strip was made of an amorphous ferromagnetic alloy and was attached to a solid body at its two longitudinal ends to form a bridge-like structure. When subjected to a lateral loading, the bridge-like structure allows the magnetoelastic strip to deflect, creating a longitudinal stress and changing its magnetic property. To remotely monitor the force loading, an AC magnetic field is applied and the magnetic fields induced by the magnetoelastic strip are recorded at multiple frequencies of the excitation frequency (higher-order harmonic fields). Experimental results showed that the second-order harmonic field (at twice the excitation frequency) amplitude produced by the sensor increased with applied compressive stress. It was also illustrated that sensor sensitivity with applied stress could be controlled by changing the length of the magnetoelastic strip. Good repeatability and stability were demonstrated with the highest signal drift of 9.45% occurring at a 9.7 kPa compressive load. Furthermore, a theoretical model was developed and evaluated to show the correlation between mechanical loading and second-order harmonic fields. Potential applications of this wireless and passive sensor technology include the monitoring of pressure in sphincter of Oddi, aortic aneurysms, and knee joint prostheses.

Partially Loaded Magnetoelastic Sensors With Customizable Sensitivities for Large Force Measurements

Magnetoelastic sensors are typically made of strips of magnetostrictive materials that efficiently convert magnetic energy into mechanical energy, and vice versa. When exposed to an ac magnetic field, the sensor vibrates, producing a secondary magnetic flux that can be remotely detected. If the frequency of the ac magnetic field matches the sensors resonant frequency, the magnetic-mechanical energy conversion is optimal, resulting in a large secondary magnetic flux. The magnetoelastic sensor has been used to monitor physical parameters relevant to force, such as mass or stress, since its resonant frequency, indirectly through the

A Wireless, Passive Magnetoelastic Force–Mapping System for Biomedical Applications

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Design, Fabrication, and Implementation of a Wireless, Passive Implantable Pressure Sensor Based on Magnetic Higher-Order Harmonic Fields

effect, is dependent on the magnitude of an applied force. Typically, the applied force must be significantly less than the weight of the sensor or it completely dampens the sensors resonance. Presented here is the design and operation of a magnetoelastic sensor capable of monitoring large forces by applying partial loading to strategic points on a sensor. The characterization and analysis of this new magnetoelastic sensor is presented along with numerical modeling to illustrate the proposed sensing mechanism. Additionally, an array of magnetoelastic sensors were deployed to demonstrate monitoring of force loading on the lock-in portion of a lock-in style lower limb prosthetic sleeve.

Monitoring the Long-Term Degradation Behavior of Biomimetic Bioadhesive Using Wireless Magnetoelastic Sensor

A wireless, passive force–mapping system based on changes in magnetic permeability of soft, amorphous Metglas 2826MB strips is presented for long-term force/stress monitoring on biomedical devices. The presented technology is demonstrated for use in lower-limb prosthetics to ensure proper postoperative fitting by providing real-time monitoring of the force distribution at the body-prosthesis interface. The sensor system consisted of a force-sensitive magnetoelastic sensing strip array that monitored applied loading as an observed change in the peak amplitude of the measured magnetic higher-order harmonic signal of each array element. The change in higher-order harmonic signal is caused by the change in the magnetic permeability of the sensing strips that corresponds to an increase in strip magnetization. After loading, the measured higher-order harmonic signals were fed into an algorithm to determine the applied forces, allowing for determination of the real-time loading profile at the body prosthesis interface.

A Wireless Sensor for Real-Time Monitoring of Tensile Force on Sutured Wound Sites

A passive and wireless sensor was developed for monitoring pressure in vivo. Structurally, the pressure sensor, referred to as the magneto-harmonic pressure sensor, is an airtight chamber sealed with an elastic pressure membrane. A strip of magnetically-soft material is attached to the bottom of the chamber and a permanent magnet strip is embedded inside the membrane. Under the excitation of an externally applied AC magnetic field, the magnetically-soft strip produces a higher-order magnetic signature that can be remotely detected with an external receiving coil. As ambient pressure varies, the pressure membrane deflects, altering the separation distance between the magnetically-soft strip and the permanent magnet. This shifts the higher-order harmonic signal, allowing for detection of pressure change as a function of harmonic shifting. The wireless, passive nature of this sensor technology allows for continuous long-term pressure monitoring, particularly useful for biomedical applications such as monitoring pressure in aneurysm sac and sphincter of Oddi. In addition to demonstrating its pressure sensing capability, an animal model was used to investigate the efficacy and feasibility of the pressure sensor in a biological environment.

Multi-parameter sensing with a single magnetoelastic sensor by applying loads on the null locations of multiple resonant modes

The degradation behavior of a tissue adhesive is critical to its ability to repair a wound while minimizing prolonged inflammatory response. Traditional degradation tests can be expensive to perform, as they require large numbers of samples. The potential for using magnetoelastic resonant sensors to track bioadhesive degradation behavior was investigated. Specifically, biomimetic poly (ethylene glycol)- (PEG-) based adhesive was coated onto magnetoelastic (ME) sensor strips. Adhesive-coated samples were submerged in solutions buffered at multiple pH levels (5.7, 7.4 and 10.0) at body temperature (37 °C) and the degradation behavior of the adhesive was tracked wirelessly by monitoring the changes in the resonant amplitude of the sensors for over 80 days. Adhesive incubated at pH 7.4 degraded over 75 days, which matched previously published data for bulk degradation behavior of the adhesive while utilizing significantly less material (~103 times lower). Adhesive incubated at pH 10.0 degraded within 25 days while samples incubated at pH 5.7 did not completely degrade even after 80 days of incubation. As expected, the rate of degradation increased with increasing pH as the rate of ester bond hydrolysis is higher under basic conditions. As a result of requiring a significantly lower amount of samples compared to traditional methods, the ME sensing technology is highly attractive for fully characterizing the degradation behavior of tissue adhesives in a wide range of physiological conditions.