Design, analysis, simulation and optimization of a MEMS Lorentz force magnetic field sensor for biosensing of biowarfare agents

The alarming terrorist attacks via the exploitation of bacteria and viruses in the history of mankind have set concerns globally and led the governments to identify and recognize biological warfare agents such as bacteria and viruses for taking preventative action. Hence, fast detection and precise identification of these agents have been attributed as key strategies for biodefense globally. In the interest of biological warfare agent sensing, sensitive and optimized tools/sensors are needed that are capable of quantifying the response of physical attachments of the biological agents into measurable values for their detection. For this purpose, MEMS based cantilever biosensors that are capable of prompt, real time, cost effective and label free methods of biomolecular recognition have emanated as promising early warning systems for biological defense and have been opted for studies of mass sensing and detection of biological particles. In this sense, the interdisciplinary nature of MEMS could be exploited through merging and synergizing diverse range of fields such as the Lorentz force actuated MEMS magnetic field sensors with the concept of BioMEMS microcantilever sensors by exploring the best of both worlds for achieving the ultimate objective of bioparticle sensing. In the literature, few studies were undertaken that considered the Lorentz force actuation mechanism with the microcantilever based biosensors for detecting the absorption of biological particles on the functionalized region of the sensor. The concentrations of these studies were found to be more oriented towards the simulation or the experimental demonstration of the subject with mere explorations on the optimization of the process parameters. Hence, in order to fulfill the relevant gap, an investigation has been carried out that aims at the design, simulation and optimization of a MEMS magnetic field sensor consisting of a microcantilever with a bioplate as a transducer that is actuated by exploiting the principle of Lorentz force in the static mode. In this study, the system was viewed as a whole and the design and process parameters that were potentially influential on the Lorentz force actuation mechanism of the proposed MEMS magnetic field sensor were identified and classified into categories of variables related to permanent magnet and the microcantilever biosensor. The parameters in the class of permanent magnet were identified as the type, grade, shape and dimensions of magnet and the positioning of the magnet with respect to the microcantilever biosensor in x, y, and z axes. The input parameters with regard to the microcantilever were distinguished as the sizes, materials and shapes of the microcantilever, the conductive path and the biosensor and the amount of current that could be injected into the ports of the sensor. A vacuum region encapsulating the magnetic field sensor and the permanent magnet was defined for the relative packaging of the device. A design of experiments process was modeled for simulating the Lorentz forces in Ansoft Maxwell® FEA software and an optimization procedure was developed that followed a pattern of fixating some parameters and changing only one parameter at a time to determine the relative effect of the considered parameter on the Lorentz force. The case of the varied parameter that yielded the maximum Lorentz force was opted as the ideal case and the subsequent parameter was investigated in the same manner, repeating until a full set of optimized parameters was obtained. The parameters to be optimized were selected as the shape and the ideal dimensional ratios for the magnet, the x, y and z position of the magnet with respect to the microcantilever and the variation of dc current supply. The dimensions, shapes and materials of the microcantilever, the bio-plate and the conductive path remained untouched throughout the optimization process. In addition to this, the optimization of the dimensions of the vacuum box was not conducted due to the automatic adjustment of the vacuum region with a percent ratio relative to the microcantilever and the permanent magnet. Following the series of optimizations, the resulting Lorentz force was concluded to be 2.65 times higher than that of the initial value. A joule heating analysis was conducted in COMSOL Multiphysics® to explore the volumetric distribution of the heat throughout the magnetic field sensors due to the injection of current in the interval of 0-50 mA. The relationship between the current and the average surface temperature on the sensor due to joule heating was plotted from the simulation results so as to find the potential need to define a safe current margin such that the sensor does not begin misfunctioning due to increased temperature. An exponentially increasing relationship was discovered for every incremental rise in the current and hence the 0-15 mA interval was defined to be the safe current range for the respective sensor. The analogy between the Lorentz force and the mechanical deflection of the magnetic field sensor has also been simulated in structural mechanics module of COMSOL Multiphysics® and a linearly increasing trend has been identified from the relationship. After a thorough examination and construction of relationships of the input parameters with respect to the Lorentz force, including the joule heating and the deflectional analysis, the optimized parameter values obtained from the results were employed for modeling five new designs of the microcantilever based biosensor for deriving further evaluations on the developed Lorentz force. A new optimization procedure was prepared in this respect and the effects of increasing the lengths and the widths of the middle arms of conductive paths on the Lorentz force were examined. The acts of lengthening and widening were found to be hardly affecting the Lorentz force but subsequent undertaking of a joule heating analysis for the increased width of the conductive paths presented improved average surface temperature results for the sensor. The thickness of the conductive paths for the current were also varied between 1 µm, 3 µm and 5 µm that yielded no changes on the Lorentz force in Ansoft Maxwell® while worsening the heat distributions on the microcantilever in COMSOL Multiphysics® Houle heating Analysis, thus downsizing the range of safe current. Also, the implication of increasing the surface area of the microcantilever structure with respect to the joule heating was examined and an indirect proportionality was concluded where the expansion in the surface area led to better dissemination of the heat across the volume of the sensor, maximizing the safe current intervals that would result in higher Lorentz force. A fabrication procedure for the MEMS magnetic field sensor was also described comprehensively for its practical realization. A contact has been established with a local permanent magnet manufacturer to see if the optimized dimensional sizes for the NdFe35 type block magnet was commercially available in case that the study is practically implemented. As a proof of concept, the biological warfare agents such as E. coli, variola virus, Influenza A, B and C viruses were modeled and their respective estimative weights were numerically calculated based on the data found on the literature. The translation of the weights induced by these bioparticles on the functionalized gold site of the microcantilever were simulated and the relevant characteristics of the deflections for differing numbers of the respective particles were established for the sensor. Afterwards, the methodology for mass sensing of the bioparticles by the Lorentz force based magnetic field sensor was presented and elaborated by exemplifying with an optical interferometry. The plot of the results of the simulations for the current supply corresponding to the detection of respective numbers of E. coli bacteria illustrated that the current was in the microampere range. Therefore, a requirement for an instrumentation that feeds dc current at microampere range was pointed in order to realize the detection of the respective biological agents by the sensor. Additionally, the bulkiness and the cost of the optical transduction method encouraged discussions relating to the piezoresistive method of detection through a potential modification of the developed sensor such that a compact, ready to use Lorentz force actuated microcantilever biosensor could be employed for practical field implementations in detecting biological warfare agents.