We have developed a novel micro optical diffusion sensor (MODS) with a newly proposed comb-driven–micro Fresnel mirror (CD–MFM) scanner to detect structural changes in biological samples. By controlling the fringe spacing of the excitation laser beam, we can tune the decay time to obtain quick and precise measurements. In this study, the pre-tilted mirror is rotated by vertical comb-driven actuators; the resulting change in the mirror angle alters the fringe spacing. The validity of the proposed mirror scanner is confirmed in simulations and in an experiment using a fabricated prototype device.
© 2015 Optical Society of America
The diffusion coefficients of biological molecules, such as proteins, reflect their size, molecular structure, conformational changes, and inter-molecular interactions. Therefore, how the diffusion coefficient of biological samples changes in liquid solution is essential knowledge for clinical diagnosis and drug discovery. For example, from measurements of protein diffusion by the transient grating method, Terazima et al. deduced the conformational changes of the proteins , their intermolecular interactions , and the protein folding . Wareley et al. measured the diffusion coefficient of antibody functionalized nanoparticles using the particle tracking method . The present study proposes a micro optical diffusion sensor (MODS) with a micro Fresnel mirror (MFM) as a small sensing device with miniaturized optical components. There are many conventional methods for measuring diffusion coefficients, such as Si micro-ring resonators , dynamic light scattering (DLS) [6–8], fluorescence correlation spectroscope (FCS) [9–11], nuclear magnetic resonance (NMR) , and phase-shifting interferometry [13–15]. However, the proposed device and mirror demonstrates superior high-speed sensing of small-volume samples, which will benefit point-of-care testing (POCT) and massively parallel processing in drug discovery.
The proposed device interferometrically manipulates the sample (strictly, a lattice of nano- or micro-particles) by laser-induced dielectrophoresis [16–18]. In recent years, micro-interferometric devices have been applied to optical spectrometry, nanomachining, and bio-sensing. Khirallah et al. developed an integrated MEMS interferometer with a vertical V-shaped micro mirror and comb-driven micro corner prisms . In this configuration, a Michelson interferometer can be fully integrated onto the substrate. We developed the first passive MFM with a fixed mirror angle in 2012 ; subsequently, Wallrabe et al. developed a tunable piezoelectric Fresnel mirror . Such horizontal V-shaped micro mirrors can be layered into a stack and integrated into micro fluidic devices. The present article proposes a novel adoptive comb-driven (CD)–MFM compatible with an MODS, which realizes lower voltage actuation than piezoelectric actuation. The study also demonstrates decay-time control by changing the fringe spacing of the excitation beam in the proposed system.
2. Principle of CD–MFM
The diffusion coefficient of the sample is measured by observing the mass diffusion of the lattice shaped concentration distribution of the sample, which is manipulated by a laser-induced dielectrophoresis (DEP). The mass diffusion phenomenon of our configuration follows a single exponential decay, and the diffusion coefficient can be expressed as a function of the fringe spacing and decay time constant ; therefore, by changing the fringe spacing, the sensing time can be tuned for an ideal measurement condition when a samplestructure change occurs.
The interference pattern with adjustable fringe spacing is formed by a novel CD–MFM (see Fig. 1(a)). The excitation laser beam is split into two beams that intersect and generate an interference pattern. This configuration admits a low-coherency laser source because the optical path difference is negligibly small. The CD–MFM comprises stiction anchors, torsion spring connected to a stiction anchor, torsion bar for a support of the slanted mirror, and electrostatic comb drive (see Fig. 1(b)). When the stiction anchor reaches down to the substrate due to the capillary force after releasing the sacrificial layer, the mirror is inclined with respect to the fixed support, and its angle can be changed by the electrostatic comb drive located at the mirror edge. The initial inclined angle of the CD–MFM is defined by the thickness of the sacrificial layer, and by the length and position of the torsion bars when the stiction anchor is affixed to the substrate. Moreover, the electrostatic force-dependent angular shift of the mirror is defined by the spring constant of the torsion bars and springs.
3. Analysis of CD–MFM
To understand the mirror behavior under the electrostatic force and to realize a precise design, an electro–mechanical simulation was conducted using the finite element method (FEM, CoventorWare). In this simulation, the voltage was applied to the fixed comb electrodes, and the pre-tilted comb electrodes were connected to the ground. Figure 2(a) shows thespring-length dependence of the mirror angle. When the torsion spring is stiff (length = 100 μm), the mirror angle is narrowed by the deflection of the torsion bar. A soft torsion spring (length = 400 μm) also narrows the deflection by the uplift of the torsion spring. Figure 2(b) plots the relationship between the mirror angle and the applied voltage for two comb lengths. According to the simulations, long comb-drives achieve steep angle operation.
Next, the comb-drive operation of the MFM (with torsion spring and comb lengths of 100 μm and 200 μm, respectively) was investigated under an applied voltage of 150 V. No curvature or surface deformation was observed in the initial or driven conditions (see Fig. 3).
4. Fabrication of CD–MFM
The CD–MFM is fabricated by a standard photolithography, a plasma etching and a chemical wet etching process, which is similar to the previous reported process for the passive MFM . In this study, the starting material is a silicon-on-insulator (SOI) wafer comprising a 30-μm silicon device layer, a 5-μm buried oxide layer (BOX) layer, and a 500-μm silicon substrate. The mirrors, stiction anchors, torsion springs and bars, and comb drives on the device layer are formed by deep reactive ion etching (DRIE). After releasing the sacrificial layer by HF wet etching, the anchors are pulled down and affixed to the substrate, inducing a torsional moment in the springs that slant the mirrors at the appropriate initial angle. Finally, the electrodes for the electrostatic comb drive are connected to a high-voltage amplifier by using a wire bonding technique.
A scanning electron microscope (SEM) image of the fabricated CD–MFM is shown in Fig. 4. The 5-μm gap comb drives and 5-μm wide torsion bars and springs were successfully fabricated without breakage of the comb teeth or contamination (which causes electrical shorts). Prior to actuation, the fabricated CD–MFM was pre-characterized as shown in Fig. 5. The fringe pattern generated by the laser irradiation was observed by the CCD camera, and the visibility (i.e., the fringe contrast) was calculated as approximately 0.86. Thus, a high-contrast interference pattern was successfully generated by the fabricated device.
Figure 6 shows the angular shifts of the CD–MFM as functions of the voltage applied to the comb drives. The shifts were acquired by a white-light interferometer (WYKO NT9100, Veeco). The left and right mirrors were appropriately actuated by the comb drives (differing by less than 0.1°). Furthermore, the voltage dependences of the angular shifts reasonably agreed with the simulated results. The difference between the experimental and simulation results probably results from the nonuniformity of the stiction condition due to the chemical residue and the underetching of the electrostatic comb drive by DRIE. This result confirms the validity of the proposed CD–MFM.
5. Mass diffusion measurements using CD–MFM
The mass diffusion phenomenon was investigated by the DEP cell and the fabricated CD–MFM in a preliminary trial. Figure 7 illustrates the bench top apparatus for generating the interferometric DEP force in the sample media and observing mass diffusion of the sample. The CD–MFM splits the excitation laser (operating at 532 nm, 5.22 mW over a diameter of 1.7 mm) into two beams. These beams are collimated by lens 3 and intersected on the DEP cell by lens 4. This setup extends the working distance of the CD–MFM, which is specified as short in the packaging conditions. The DEP cell is irradiated with the 635-nm probe laser emitted from a laser diode, and the first-order diffracted beam is detected by the photo diode. The DEP cell comprises two Pyrex glass substrates (a tin-doped indium oxide sputtered substrate and a hydrogenated amorphous silicon on aluminum doped zinc oxide sputtered substrate) bonded together. The channel height is controlled by the thickness of the SU-8 photoresist. Table 1 summarizes the measurement conditions.
The mass diffusion of polystyrene beads (diameter = 500 nm) was observed by CD–MFM, varying the applied voltage. The intensities (normalized by the peak intensity) of five time-averaged signals are plotted as functions of time in Fig. 8. As the applied voltage increases, the fringe spacing widens and the first-order diffracted signal decays more slowly. The evaluated value of the diffusion coefficient is in the order of 10−12 m2/s, corresponding to the theoretical value. The decay changes are well-described by the theory , confirming the applicability of CD–MFM to MODS. Consequently, the fringe spacing of CD–MFM can be actively controlled to suit the dynamics of the sample characteristics.
We have proposed a novel CD–MFM for rapid diffusion coefficient measurements within small sample volumes. The angular shifts of CD–MFM were analyzed by the finite element method, and the CD–MFM design was determined. The prototype device was successfully fabricated, and the measured rotation angles agreed with the calculated values. The CD–MFM generated a high-contrast fringe pattern. The fringe spacing is controllable at the micro-scale by changing the applied voltage. Finally, for various fringe spacings, the mass diffusion of the lattice-shaped concentration distribution of polystyrene beads was measured by CD–MFM. The results experimentally validated the proposed device.
This study was partially supported by the Japan Society for the Promotion of Science (JSPS), a Grant-in-Aid for Scientific Research (S, No. 24226006), a Ministry of Education, Culture, Sports, Science and Technology (MEXT) Grant-in-Aid for young scientists (A, No. 23686036), and Kawasaki city subsidy for promoting R&D of nano–micro technology by SMEs based on academia-industry cooperation. Part of this study was conducted at the Keio satellite center of the “Low-Carbon Research Network” funded by MEXT, Japan. The fabrication of CD–MFM has been conducted at the clean room in “Global nano micro technology business incubation center (NANOBIC), Kawasaki city, Japan” supported by the academic consortium for nano and micro fabrication of four universities (Keio university, Waseda university, Tokyo institute of technology, and the University of Tokyo). We gratefully acknowledge fruitful discussions on the fabrication of our device with K. Yamada (Kyodo International, Inc.). Furthermore, we deeply thank M. Kamata for his help in fabricating DEP cell.
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