Bojan Ilic

Mechanical resonant immunospecific biological detector

We have demonstrated high-sensitivity detection of bacteria using an array of bulk micromachined resonant cantilevers. The biological sensor is a micromechanical oscillator that consists of an array of silicon-nitride cantilevers with an immobilized antibody layer on the surface of the resonator. Measured resonant frequency shift as a function of the additional cell loading was observed and correlated to the mass of the specifically bound Escherichia coli O157:H7 cells. Deposition and subsequent detection of E. coli cells was achieved under ambient conditions.

Label-free biomarker detection from whole blood.

Label-free nanosensors can detect disease markers to provide point-of-care diagnosis that is low-cost, rapid, specific and sensitive. However, detecting these biomarkers in physiological fluid samples is difficult because of ionic screening. Here, we overcome this limitation by using distinct components within the sensor to perform purification and detection.1 A microfluidic purification chip captures multiple biomarkers simultaneously from blood samples and releases them, after washing, into purified buffer for sensing by a silicon nanoribbon detector. This two-stage approach isolates the detector from the complex environment of whole blood, and reduces its minimum required sensitivity by effectively pre-concentrating the biomarkers. We show specific and quantitative detection of two model cancer antigens from a 10 uL sample of whole blood in less than 20 minutes.

The pull-in behavior of electrostatically actuated bistable microstructures

The results of theoretical and experimental investigation of an initially curved clamped–clamped microbeam actuated by a distributed electrostatic force are presented. Reduced-order Galerkin and consistently constructed lumped models of the shallow Euler–Bernoulli arch were built and verified by numerical analysis, and the influence of various parameters on the stability was investigated. Due to the unique combination of generic mechanical and electrostatic nonlinearities, the voltage–deflection characteristic of the device may have two maxima implying the existence of sequential snap-through buckling and pull-in instability and of bistability of the beam. The first critical voltage can be higher or lower than the second one, while the stable deflections are significantly larger than in a straight beam. The minimal initial elevation required for the appearance of the snap-through in the electrostatically actuated beam is smaller than in the case of uniform deflection-independent loading; a closed-form approximation of this elevation was evaluated. The devices were fabricated from silicon on insulator (SOI) wafer using deep reactive ion etching and in-plane responses were characterized by means of optical and scanning electron microscopy. Model results obtained for the actual dimensions of the device were in good agreement with the experimental data.

On-chip manipulation of protein-coated magnetic beads via domain-wall conduits.

For this reasonmanipulationatthenanoscaleofsurfacefunctionalizedmagneticbeads in suspension is of paramount importance in biotechnol-ogy, nanochemistry, and nanomedicine as it leads to a precisecontrol of the tagged biological entity.In the past few years many approaches have been developedboth for the manipulation and transport of a massive particlepopulation or of a single particle, e.g., microfabricated current-carrying wires,

Nanosized corners for trapping and detecting magnetic nanoparticles

We present a device concept based on controlled micromagnetic configurations in a corner-shaped permalloy nanostructure terminated with two circular disks, specifically designed for the capture and detection of a small number of magnetic beads in suspension. A transverse head-to-head domain wall (TDW) placed at the corner of the structure plays the role of an attracting pole for magnetic beads. The TDW is annihilated in the terminating disks by applying an appropriate magnetic field, whose value is affected by the presence of beads chemically bound to the surface. In the case where the beads are not chemically bound to the surface, the annihilation of the TDW causes their release into the suspension. The variation of the voltage drop across the corner, due to the anisotropic magnetoresistance (AMR) while sweeping the magnetic field, is used to detect the presence of a chemically bound bead. The device response has been characterized by using both synthetic antiferromagnetic nanoparticles (disks of 70 nm diameter and 20 nm height) and magnetic nanobeads, for different thicknesses of the protective capping layer. We demonstrate the detection down to a single nanoparticle, therefore the device holds the potential for the localization and detection of small numbers of molecules immobilized on the particles functionalized surface.

Spin waves in circular soft magnetic dots at the crossover between vortex and single domain state

We report on linear spin dynamics in the vortex state of the Permalloy dots subjected to stratified (magnetic) field. We demonstrate experimentally and by simulations the existence of two distinct dynamic regimes corresponding to the vortex stable and metastable states. Breaking cylindrical symmetry leads to unexpected eigenmodes frequency splitting in the stable state and appearance of new eigenmodes in the metastable state above the vortex nucleation field. Dynamic response in the metastable state strongly depends on relative orientation of the external rf pumping and the bias magnetic fields. These findings may be relevant for different vortex states in confined and stratified conditions.

A micromechanical flow sensor for microfluidic applications

We fabricated a microfluidic flow meter and measured its response to fluid flow in a microfluidic channel. The flow meter consisted of a micromechanical plate, coupled to a laser deflection system to measure the deflection of the plate during fluid flow. The 100 /spl mu/m square plate was clamped on three sides and elevated 3 /spl mu/m above the bottom surface of the channel. The response of the flow meter was measured for flow rates, ranging from 2.1 to 41.7 /spl mu/L/min. Several fluids, with dynamic viscosities ranging from 0.8 to 4.5/spl times/10/sup -3/ N/m, were flowed through the channels. Flow was established in the microfluidic channel by means of a syringe pump, and the angular deflection of the plate monitored. The response of the plate to flow of a fluid with a viscosity of 4.5/spl times/10/sup -3/ N/m was linear for all flow rates, while the plate responded linearly to flow rates less than 4.2 /spl mu/L/min of solutions with lower dynamic viscosities. The sensitivity of the deflection of the plate to fluid flow was 12.5/spl plusmn/0.2 /spl mu/rad/(/spl mu/L/min), for a fluid with a viscosity of 4.5/spl times/10/sup -3/ N/m. The encapsulated plate provided local flow information along the length of a microfluidic channel.

A microfabricated electrochemical actuator for large displacements

A large-displacement electrochemical actuator was designed, fabricated, and tested. The large displacement is obtained by using a corrugated membrane made by physical vapor deposition of Parylene sandwiched with an intermediate layer of sputtered platinum. The layered structure is approximately 8-/spl mu/m thick, with 26 grooves approximately 120-/spl mu/m deep, and with a radial period of 350 /spl mu/m. The electrochemical cell consists of platinum electrodes with a 1 M H/sub 2/SO/sub 4/ solution. Hydrogen and oxygen gas is generated to displace the membrane. Although the actuator can operate at a voltage as low as 1.23 V, the experimentally determined efficiency of converting electrical energy to mechanical work is only 0.37%. The governing equations for the conservation of mass, momentum (equilibrium), energy, and the entropy generation rate were formulated assuming that the gas bubbles either nucleate without growth or grow without nucleation. For the nucleation case, simulations were performed for constant pressure isothermal actuation, and the average experimental efficiency was bounded by simulations with gas bubble radii between 1/spl times/10/sup -6/ m and 1/spl times/10/sup -6/ m. The predicted ratio of the power dissipated to the electrical power supplied is 1.37 for isothermal actuation.

Efficient parametric excitation of silicon-on-insulator microcantilever beams by fringing electrostatic fields

Large amplitude flexural vibrations have been excited in single layer silicon-on-insulator micromechanical cantilever beams in ambient air environment. Our driving approach relies on a single co-planar electrode located symmetrically around the actuated grounded cantilever. Electrostatic forces are created via tailored asymmetries in the fringing fields of deformed mechanical states during their electric actuation, with strong restoring forces acting in a direction opposite to the deflection. This results in an effective increase in the structure stiffness in its elastic regime. The devices had been fabricated using deep reactive ion etching based process and their responses were characterized in a laser Doppler vibrometer under ambient conditions. Harmonic voltages applied to the electrode result in the periodic modulation of the effective stiffness and lead to strong parametric excitation of the structure. As opposed to close gap actuators, where high-amplitude drives are severely limited by pull-in ins...

CMOS-Integrated RF MEMS Resonators

We present a design approach that enables monolithic integration of high-quality-factor (Q) radio-frequency (RF) microelectromechanical systems (MEMS) resonators with CMOS electronics. Commercially available CMOS processes that feature two polysilicon layers and field oxide isolation can be used to implement this approach. By using a nonplanar resonator geometry in conjunction with mechanical stress in polycrystalline silicon (poly) gate layers, we create rigid and robust mechanical structures with efficient electromechanical transduction. We demonstrate polysilicon domes with capacitive pickup and arch-bridge resonators with piezoresistive readout. The small footprint of our MEMS structures enables on-chip integration of large arrays of resonators for RF signal processing or sensing applications. Their large surface-to-volume ratio in combination with high rigidity (that alleviates stiction associated with wet chemistry processing) can make these resonators particularly useful for sensors that require surface functionalization.