Ee Lim Tan

A Wireless, Passive Sensor for Quantifying Packaged Food Quality

This paper describes the fabrication of a wireless, passive sensor based on an inductive-capacitive resonant circuit, and its application for in situ monitoring of the quality of dry, packaged food such as cereals, and fried and baked snacks. The sensor is made of a planar inductor and capacitor printed on a paper substrate. To monitor food quality, the sensor is embedded inside the food package by adhering it to the packages inner wall; its response is remotely detected through a coil connected to a sensor reader. As food quality degrades due to increasing humidity inside the package, the paper substrate absorbs water vapor, changing the capacitors capacitance and the sensors resonant frequency. Therefore, the taste quality of the packaged food can be indirectly determined by measuring the change in the sensors resonant frequency. The novelty of this sensor technology is its wireless and passive nature, which allows in situ determination of food quality. In addition, the simple fabrication process and inexpensive sensor material ensure a low sensor cost, thus making this technology economically viable.

A Wireless, Passive Embedded Sensor for Real-Time Monitoring of Water Content in Civil Engineering Materials

A wireless, passive embedded sensor was applied for real-time monitoring of water content in civil engineering materials such as sands, subgrade soils, and concrete materials. The sensor, which comprised of a planar inductor-capacitor (LC) circuit, was embedded in test samples so that the internal water content of the samples could be remotely measured with a loop antenna by tracking the changes in the sensors resonant frequency. Since the dielectric constant of water was much higher compared with that of the test samples, the presence of water in the samples increased the capacitance of the LC circuit (capacitance of the capacitor was proportional to the dielectric constant of the medium between its electrodes), thus decreasing the sensors resonant frequency. Using the described sensor, a study was conducted to investigate the drying rate of sand samples of different grain sizes. A study was also conducted to measure the curing rate of a portland cement concrete slab during casting, and its drying rate after it has been soaked in water. 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. In addition, this sensor will be useful for monitoring water content within concrete supports and road pavements. The measurement of water content is important for civil engineering infrastructure since excess water may hasten their degradation.

A wireless, passive strain sensor based on the harmonic response of magnetically soft materials

A wireless, passive strain sensor based on the shift of higher-order harmonic signals of a magnetically soft material is described. The strain sensor consisted of a magnetically soft element, placed over a permanent magnetic element and separated by a deformable layer. As compressive forces were exerted on the strain sensor, the dimension of the deformable layer varied, changing the separation distance between the soft and permanent magnetic elements. This in turn altered the higher-order harmonic field of the magnetically soft element, allowing remote measurement of stress and strain. In the current study, three different types of deformable layers with distinctive material properties were separately incorporated into the sensor. External forces were gradually applied on each sensor and the variations in harmonic signals were measured. The shifts in the magnetic harmonic spectrum of the sensors were linearly correlated with the mechanical alteration. Good stability, linearity and repeatability of the strain sensor were also demonstrated. This passive and wireless sensor is useful for long-term detection of mechanical loading from within an object such as inside a concrete structure or a human body.

Implantable Biosensors for Real-time Strain and Pressure Monitoring

Implantable biosensors were developed for real-time monitoring of pressure and strain in the human body. The sensors, which are wireless and passive, consisted of a soft magnetic material and a permanent magnet. When exposed to a low frequency AC magnetic field, the soft magnetic material generated secondary magnetic fields that also included the higher-order harmonic modes. Parameters of interest were determined by measuring the changes in the pattern of these higher-order harmonic fields, which was achieved by changing the intensity of a DC magnetic field generated by a permanent magnet. The DC magnetic field, or the biasing field, was altered by changing the separation distance between the soft magnetic material and the permanent magnet. For pressure monitoring, the permanent magnet was placed on the membrane of an airtight chamber. Changes in the ambient pressure deflected the membrane, altering the separation distance between the two magnetic elements and thus the higher-order harmonic fields. Similarly, the soft magnetic material and the permanent magnet were separated by a flexible substrate in the stress/strain sensor. Compressive and tensile forces flexed the substrate, changing the separation distance between the two elements and the higher-order harmonic fields. In the current study, both stress/strain and pressure sensors were fabricated and characterized. Good stability, linearity and repeatability of the sensors were demonstrated. This passive and wireless sensor technology may be useful for long term detection of physical quantities within the human body as a part of treatment assessment, disease diagnosis, or detection of biomedical implant failures.

Fabrication of Biocompatible, Vibrational Magnetoelastic Materials for Controlling Cellular Adhesion

This paper describes the functionalization of magnetoelastic (ME) materials with Parylene-C coating to improve the surface reactivity to cellular response. Previous study has demonstrated that vibrating ME materials were capable of modulating cellular adhesion when activated by an externally applied AC magnetic field. However, since ME materials are not inherently biocompatible, surface modifications are needed for their implementation in biological settings. Here, the long-term stability of the ME material in an aqueous and biological environment is achieved by chemical-vapor deposition of a conformal Parylene-C layer, and further functionalized by methods of oxygen plasma etching and protein adsorption. In vitro cytotoxicity measurement and characterization of the vibrational behavior of the ME materials showed that Parylene-C coatings of 10 µm or greater could prevent hydrolytic degradation without sacrificing the vibrational behavior of the ME material. This work allows for long-term durability and functionality of ME materials in an aqueous and biological environment and makes the potential use of this technology in monitoring and modulating cellular behavior at the surface of implantable devices feasible.

A Remote Query Pressure Sensor Based on Magnetic Higher Order Harmonic Fields

The design and fabrication of a wireless, passive pressure sensor based on the change in magnetic higher order harmonic fields is described. The sensor was made of an airtight pressure chamber with two opposite membranes: a rigid membrane attached to a magnetically soft ferromagnetic strip (sensing element) and a flexible membrane attached to a permanent magnetic strip (biasing element). The flexible membrane of the chamber deflected with changing pressure, thus varying the separation distance between the sensing and biasing elements. The change in separation distance in turn altered the biasing field experienced by the sensing element, varying the pattern of its magnetic higher order harmonic fields allowing remote pressure monitoring through a magnetic coil. In this work, different materials were used to fabricate the flexible membranes for sensors of different dynamic ranges. Experimental results showed the shifts in magnetic higher order harmonic fields were linear for all sensors, but with different sensitivity depending on the elasticity of the flexible membrane. The novelty of this sensor is its wireless, passive nature, which is ideal for applications where wire connections are prohibited. In addition, the simple sensor design reduces cost, allowing disposable use. Potential applications of such a sensor technology include long-term structural monitoring (concrete, asphalt) and in vivo pressure monitoring inside the human body.

A Wireless Embedded Sensor Based on Magnetic Higher Order Harmonic Fields: Application to Liquid Pressure Monitoring

A wireless sensor based on the magnetoelastic, magnetically soft ferromagnetic alloy was constructed for remote measurement of pressure in flowing fluids. The pressure sensor was a rectangular strip of ferromagnetic alloy Fe40Ni38Mo4B18 adhered on a solid polycarbonate substrate and protected by a thin polycarbonate film. Upon excitation of a time-varying magnetic field through an excitation coil, the magnetically soft sensor magnetized and produced higher order harmonic fields, which were detected through a detection coil. Under varying pressures, the sensors magnetoelastic property caused a change in its magnetization, altering the amplitudes of the higher order harmonic fields. A theoretical model was developed to describe the effect of pressure on the sensors higher order harmonic fields. Experimental observations showed the second-order harmonic field generated by the pressure sensor was correlated to the surrounding fluid pressure, consistent with the theoretical results. Furthermore, it was demonstrated that the sensor exhibited good repeatability and stability with minimal drift. Sensors with smaller dimensions were shown to have greater sensitivity but lower pressure range as compared to their larger counterparts. Since the sensor signal was also dependent on the location of the sensor with respect to the excitation/detection coil, a calibration algorithm was developed to eliminate signal variations due to the changing sensor location. Because of its wireless and passive nature, this sensor is useful for continuous and long-term monitoring of pressure at inaccessible areas. For example, sensors with these capabilities are suitable to be used in biomedical applications where permanent implantation and long-term monitoring are needed.

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.

A wireless flow sensor based on magnetic higher-order harmonic fields

The design and fabrication of a wireless, passive flow sensor based on changes in magnetic higher-order harmonic fields is described. The sensor consisted of a flow channel, a permanent magnetic strip (biasing element) and a magnetically soft ferromagnetic strip (sensing element). The biasing element was attached on the channels wall in parallel to the flow direction, while the sensing element was applied on the opposite wall at a small angle to the flow direction. Flowing water in the channel created a pressure on the sensing elements surface, causing a deflection that varied its separation distance from the biasing element. The change in the separation distance in turn altered the biasing field experienced by the sensing element, causing a shift in its higher-order harmonic fields that could be measured remotely through a magnetic coil. The novelty of this sensor is its wireless, passive nature, which is ideal for applications where wire connections are prohibited. In addition, this sensor can be used on a disposable basis due to its simple design and relatively low material cost.

Magnetoelastic vibrational biomaterials for real-time monitoring and modulation of the host response

Magnetoelastic (ME) biomaterials are ferromagnetic materials that physically deform when exposed to a magnetic field. This work describes the real-time control and monitoring capabilities of ME biomaterials in wound healing. Studies were conducted to demonstrate the capacity of the materials to monitor changes in protein adsorption and matrix stiffness. In vitro experiments demonstrated that ME biomaterials can monitor cell adhesion and growth in real-time, and a long-term in vivo study demonstrated their ability to monitor the host response (wound healing) to an implant and control local cell density and collagen matrix production at the soft tissue-implant interface. This approach represents a potentially self-aware and post-deployment activated biomaterial coating as a means to monitor an implant surface and provide an adjuvant therapy for implant fibrosis.