Implantable biosensing is attractive for both medical monitoring and diagnostic applications. It is possible to monitor phenomena such as physical loads on joints or implants, vital signs, or osseointegration in vivo and in real time. Microelectromechanical (MEMS)-based generation techniques can allow for the autonomous operation of implantable biosensors by generating electrical power to replace or supplement existing battery-based power systems. By supplementing existing battery-based power systems for implantable biosensors, the operational lifetime of the sensor is increased. In addition, the potential for a greater amount of available power allows additional components to be added to the biosensing module, such as computational and wireless and components, improving functionality and performance of the biosensor. Photovoltaic, thermovoltaic, micro fuel cell, electrostatic, electromagnetic, and piezoelectric based generation schemes are evaluated in this paper for applicability for implantable biosensing. MEMS-based generation techniques that harvest ambient energy, such as vibration, are much better suited for implantable biosensing applications than fuel-based approaches, producing up to milliwatts of electrical power. High power density MEMS-based approaches, such as piezoelectric and electromagnetic schemes, allow for supplemental and replacement power schemes for biosensing applications to improve device capabilities and performance. In addition, this may allow for the biosensor to be further miniaturized, reducing the need for relatively large batteries with respect to device size. This would cause the implanted biosensor to be less invasive, increasing the quality of care received by the patient.

In this article, we report on the new design of a miniaturized strain microsensor. The proposed sensor utilizes the piezoresistive properties of doped single crystal silicon. Employing the Micro Electro Mechanical Systems (MEMS) technology, high sensor sensitivities and resolutions have been achieved. The current sensor design employs different levels of signal amplifications. These amplifications include geometric, material and electronic levels. The sensor and the electronic circuits can be integrated on a single chip, and packaged as a small functional unit. The sensor converts input strain to resistance change, which can be transformed to bridge imbalance voltage. An analog output that demonstrates high sensitivity (0.03mV/me), high absolute resolution (1μe) and low power consumption (100μA) with a maximum range of ±4000μe has been reported. These performance characteristics have been achieved with high signal stability over a wide temperature range (±50^oC), which introduces the proposed MEMS strain sensor as a strong candidate for wireless strain sensing applications under harsh environmental conditions. Moreover, this sensor has been designed, verified and can be easily modified to measure other values such as force, torque…etc. In this work, the sensor design is achieved using Finite Element Method (FEM) with the application of the piezoresistivity theory. This design process and the microfabrication process flow to prototype the design have been presented.

Hydrogen plays a significant role in various industrial applications, but careful handling and continuous monitoring are crucial since it is explosive when mixed with air. Surface Acoustic Wave (SAW) sensors provide desirable characteristics for hydrogen detection due to their small size, low fabrication cost, ease of integration and high sensitivity. In this paper a finite element model of a Surface Acoustic Wave sensor is developed using ANSYS12© and tested for hydrogen detection. The sensor consists of a YZ-lithium niobate substrate with interdigital electrodes (IDT) patterned on the surface. A thin palladium (Pd) film is added on the surface of the sensor due to its high affinity for hydrogen. With increased hydrogen absorption the palladium hydride structure undergoes a phase change due to the formation of the β-phase, which deteriorates the crystal structure. Therefore with increasing hydrogen concentration the stiffness and the density are significantly reduced. The values of the modulus of elasticity and the density at different hydrogen concentrations in palladium are utilized in the finite element model to determine the corresponding SAW sensor response. Results indicate that with increasing the hydrogen concentration the wave velocity decreases and the attenuation of the wave is reduced.

This paper presents the experimental evaluation of a new piezoresistive MEMS strain sensor. Geometric characteristics of the sensor silicon carrier have been employed to improve the sensor sensitivity. Surface features or trenches have been introduced in the vicinity of the sensing elements. These features create stress concentration regions (SCRs) and as a result, the strain/stress field was altered. The improved sensing sensitivity compensated for the signal loss. The feasibility of this methodology was proved in a previous work using Finite Element Analysis (FEA). This paper provides the experimental part of the previous study. The experiments covered a temperature range from −50 °C to +50 °C. The MEMS sensors are fabricated using five different doping concentrations. FEA is also utilized to investigate the effect of material properties and layer thickness of the bonding adhesive on the sensor response. The experimental findings are compared to the simulation results to guide selection of bonding adhesive and installation procedure. Finally, FEA was used to analyze the effect of rotational/alignment errors.

This paper presents the experimental evaluation of a new piezoresistive microelectromechanical systems strain sensor. The sensing chip is highly capable of measuring biaxial state of strain/stress. The sensing elements are p-type piezoresistors on (100) single crystal silicon aligned along [110] and its in-plane transverse. The concept of introducing geometric features to enhance the sensor sensitivity is investigated. The results of experimental evaluation and finite-element analysis (FEA) proved the viability of this concept to improve the sensor sensitivity. The microfabrication process utilizes five doping concentrations to explore the effect of doping level on the sensor performance. The sensor is developed considering applications under varying temperature conditions. Therefore, high doping concentration (more than 1 ×10¹⁹ atoms/cm³) is favorable to reduce the sensor thermal drift. As a result, the sensor sensitivity is significantly reduced. Hence, geometric features are introduced in the sensor silicon carrier to compensate for the signal loss through stress concentration effect, which magnified the strain field in the proximity of the sensing elements. In addition, the use of full-bridge configuration reduced the overall temperature coefficient of resistance (TCR). At doping concentration of ~5 ×10¹⁹ atoms/cm³, the measured strain sensitivity is 0.035 mV/με for input voltage of 5 volts, which corresponds to an effective gauge factor of ~7 and piezoresistive gauge factor of ~44. The effective gauge factor includes all the signal losses and the effect of bonding adhesive. Design and analysis, prototyping, and experimental evaluation are presented. Finally, guidelines to select the bonding adhesive and packaging scheme are provided.

Energy harvesting is the process of capturing the local ambient energy and converting it into useful energy. Piezoelectric energy harvesters (PEHs) are one of the solutions to convert vibration energy to electrical energy. An electrical interface is needed to bridge between the piezoelectric energy harvester and the energy storage element. A high-efficient AC/DC converter is designed for a low vibration power harvester with the milli- or micro-watt range power. This paper reports a novel self-powered high-efficiency interface to rectify the AC voltage generated by a vibration energy harvester to DC voltage. The reported interface consists of a triggering circuit and a Negative Voltage Converter (NVC) combined with a Parallel Synchronized Switch Harvesting on Inductor (PSSHI) forming the NVC-PSSHI. The analytical model and simulation of the designed NVC-PSSHI interface were derived and performed, respectively. The targeted input voltage, frequency and DC loading conditions were 3 Vpp to 7 Vpp, 100 Hz to 500 Hz and 5 kΩ to 30 kΩ, respectively. Experiments with a PEH were also performed to validate the analytical and simulation results. The maximum efficiency of the designed NVC-PSSHI interface reached 82.1% from the PEH experiment. The NVC-PSSHI interface efficiency was higher than the traditional PSSHI interface by up to 23.4%.