Design of a wearable microfluidic-based electrostatic mass sensor for sweat analysis in athletes
Subject Areas : Sports Science and HealthyReza Hadjiaghaie Vafaie 1 , Mahnaz Mehdipoor 2
1 - Department of Electronics, Faculty of Electrical Engineering, Bonab University, Bonab, Iran.
2 - Department of Electronics, Faculty of Electrical Engineering, Bonab University, Bonab, Iran
Keywords: Mass sensor, wearable microfluidics, perspiration, sports industry, health diagnostics,
Abstract :
Recently, the use of wearable sensors is widely used in the field of perspiration monitoring and health diagnosis. In this paper, a label-free mass sensor is introduced for lab on a chip and microfluidics application revolutionized the fields of point-of-care medical diagnostics and other rapid in-field testing for various applications including chemical and biological warfare detection to environmental monitoring. All finite element simulations and analysis were structural and according to the available facilities, the desired resonance frequency for rotational in-plane vibration was 330.73 kHz with mass sensitivity and quality factor of 19 (kHz/pgr), 570, respectively which is comparable to the theoretically and experimentally similar works. Rotational comb- drive actuators were applied in design of the sensor that vibrates in-plane azimuthally with low damping. The central part of the sensor was assumed to be covered by a thin layer of gold during the fabrication process which is a proper choice to detect various bio-particles including protein thiol groups, antibody connections, glucose oxidase connections, DNA, bacteria and fructose. By immobilizing the bio-particles on the active area, the total mass of the structure increases so the output resonance frequency decreases consequently. To sense the targets, the proposed sensor was assumed as an oscillator in an electrical circuit, and through the positive feedback loop, the resonance frequency of the system matches the mechanical resonance frequency. Mass changes in the active area causes electrical equivalent value change so the resonance frequency decreases. The proposed method to deliver the bio-particles to the sensor is the use of driven electrodes in an electrowetting-based digital microfluidic platform. The carrier droplet, encapsulating bioparticles, was driven to the central part of the sensor by electrowetting electrodes. The design consists of two rings inner and outer, some com-drive electrodes, a central resonator, and some springs which outer and inner rings have radius of 250um, 140um, respectively connecting the comb drive actuators. To have a structure with more strength and stability, four anchors were used on two sides of the sensor so the probability of vertical resonance would be reduced. The central part of the sensor was applied for sensing aim, where interaction with biological targets occurs, and vibrated parallel to the substrate by actuators. Due to the active area was not surrounded by actuators completely so EWO electrodes can be replaced near to it and deliver the bioparticles to the sensor easily. The advantages of using electrostatic actuators are low power consumption and high operating speed so several actuators were applied to increase the electrostatic force to move the active area with no such complexity to the system. Also, it can be fabricated by the proposed manufacturing technology and their performance depends on the dimensions of the electrodes and the voltage. Also the output signal can be applied to control other microfluidic components like micropump and micromixer. The active area has been connected to the actuators by three springs and each comb-drive has two sets of electrodes, fixed and movable with a minimum gap between them to obtain maximum momentum in low voltage. The proposed sensor preparation and measurement steps are pre-wash, frequency measurement, and post-wash, respectively. In the pre-wash step, the sensor was washed with DI water droplets delivered by EWO electrodes toward the active zoon. As only the top surface of the active zone was immobilized to capture target, so the possible leakage of drops cannot affect the measurement and remove the residue pollution during former measurements. This step can be repeated several times. Then, the reference frequency was obtained in the presence of the droplet and any probable stiction was omitted. During the measurement step, the desired frequency was measured in the presence of the droplet carrying bioparticles and it was compared with the reference frequency to achieve the frequency shift. Because of the fabrication process, 2µ gap is between the structure and the substrate which is connected through four anchors. Appling the actuating voltage to the in-phase comb-drive electrodes, the active area moves around the z-axis and as the target was absorbed on it, the viscous damping decreases the mechanical energy and consequently displacement of the sensor. Considering, the proposed sensor is for fluidic application, to reduce the viscous damping effect, a frequency is chosen that the structure moves in-plane so the damping types are slide and viscous that affect the system less than out-of-plane mode.
In the post-wash step, after each frequency measurement, a DI water droplet was use to wash the active zone again where target were removed through proper washing methods. To extract the main parameter of the sensor including resonance frequency, quality factor, and mass sensitivity, the proposed design was simulated structurally using the finite element method. To calculate the damping coefficient resulting from a thin layer of air between the sensor and the substrate and the viscous damping effect of carrier droplet on the active area, the Couetee-type model and Stokes-type model are used, respectively. The desired resonance frequency is 330.37 kHz that the structure rotates around the z axis. To find the performance of the proposed sensor, quality factor and mass sensitivity, frequency studies have been done using Couette model, stokes model, and several reasonable masses applied to the system in the finite element simulation. The quality factor of the present sensor is almost 570 with mass sensitivity of 19 (kHz/pgr) which is comparable with other sensors with the liquid application.
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