In plasmon nano-optical tweezers, plasmonic nanoantennas are illuminated to generate highly localized and enhanced electromagnetic field in the vicinity of the nanoantenna. The highly localized and enhanced electromagnetic field creates much stronger optical gradient forces and tighter potential wells for confining particles than in conventional optical tweezers, thus providing a means to trap nanoscale objects and molecules. This approach have been successfully applied for trapping small particles such as protein molecules. However a long standing problem in this field is how to rapidly load the potential well without relying on Brownian diffusion. Conventional design rely on Brownian diffusion to load the trap, which is very slow and could take several minutes to hours depending on the concentration of the nanoscale objects. Furthermore since the plasmonic trapping sites are pre-patterned on a substrate, current plasmonic nanotweezers suffer from the problem of lack of dynamic control over the particles in the trap. Recently we have addressed these challenges by introducing a novel design paradigm known as the Hybrid Electrothermoplasmonic Nanotweezer (HENT)1, where the intrinsic photo-induced heating of the plasmonic nanoantenna is combined with an applied AC electric field to induce a large scale microfluidic flow on-demand. The microfluidic flow enables rapid delivery of suspended nanoparticles to an illuminated plasmonic nanoantenna where they are trapped within a few seconds. In this talk I will discuss the working principle of HENT, as well as HENT-based nanotweezers utilizing alternative plasmonic materials.
Plasmonic nanostructures, which support highly localized and enhanced electromagnetic field are now actively researched as a means for efficient trapping of nanoscale objects, not addressable by conventional diffraction-limited optical tweezers. An issue of critical concern is how to efficiently transport and deliver the suspended particles to the illuminated plasmonic nanostructure. There are primarily two main approaches that researchers employ for trapping of particles with plasmonic nanostructure(s) on a substrate. The first approach involves illuminating arrays of closely-spaced plasmonic nanostructures. However resonant illumination of the nanostructures results in collective heating and this produces strong fluid convection that exerts drag forces on the particles. Elucidating the roles of these heating-induced forces and optical gradient forces arising from plasmonic field enhancement have so far remained elusive. The other scheme involves illuminating a single plasmonic nanostructure. However, due to the absence of thermoplasmonic convection in this case, the dynamics of the suspended particle to be trapped becomes dictated by Brownian motion- an inherently slow process. We will discuss a new fluid flow mechanism, which we have termed electrothermoplasmonic (ETP) flow to resolve this dilemma. ETP flow harnesses intrinsic plasmonic heating combined with AC electric field to generate on-demand fluid and particle transport, which means that particles could be rapidly transported for trapping in sub-wavelength plasmonic hotspots only when desired, and without any competition between heating-induced forces and optical gradient forces. These new capabilities certainly provide new directions for research in the field of plasmon-assisted optical trapping, which will be discussed.
Aneurysms are pockets of blood that collect outside blood vessel walls forming dilatations and leaving arterial walls very prone to rupture. There is little information concerning the causes of intracranial aneurysm formation, growth, and rupture. Current treatments include: (1) clipping, and (2) coil embolization, including stent-assisted coiling. Further, the evolution of any aneurysm is assumed to be caused by the remodeling of the affected blood vessel’s material constituents (tunica intima, tunica media, or tunica adventitia). Velocity, pressure, and wall shear stresses aid in the disease development of aneurysmal growth, while the shear force mechanisms effecting wound closure are elusive. To study aneurysm pathogenesis, a lab-on-a-chip device is the key to discovering the underlying mechanisms of these lesions. A two-dimensional microfluidic model, the Aneurysm-on-a-Chip™ (AOC), was the logical answer to study particle flow within an aneurysm “sac”. The AOC apparatus can track particles/cells when it is coupled to particle image velocimetry software (PIV) package. The AOC fluid flow was visualized using standard microscopy techniques with commercial microparticles and human aortic smooth muscle cells (HASMC). Images were taken during fluid flow experiments and PIV was utilized to monitor the flow of particles within the “sac” region, as well as particles entering and exiting the device. Quiver plots were generated from fluid flow experiments using standard 7 μm latex particles and fixed HASMC in PBS. PIV analysis shows that the particles flowed nicely from input to output. Wall shear stress provided evidence that there was some back flow at the edges of the “sac” – an indicator of aneurysm development in human patients.
Lab-on-a-chip (LOC) systems allow complex laboratory assays to be carried out on a single chip using less time,
reagents, and manpower than traditional methods. There are many chips addressing PCR and other DNA assays, but few
that address blood cell analysis. Blood analysis, particularly of the cellular component, is highly important in both
medical and scientific fields. Traditionally blood samples require a vial of blood, then several processing steps to
separate and stain the various components, followed by the preparations for each specific assay to be performed. A LOC
system for blood cell analysis and sorting would be ideal. The microfluidic-based system we have developed requires a
mere drop of blood to be introduced onto the chip. Once on chip, the blood is mixed with both fluorescent and magnetic
labels. The lab-on-a-chip device then uses a syringe drive to push the cells through the chip, while a permanent magnet
is positioned to pull the magnetically labeled white blood cells to a separate channel. The white blood cells, labeled with
different color fluorescent quantum dots (Qdots) conjugated to antibodies against WBC subpopulations, are analyzed
and counted, while a sampling of red blood cells is also counted in a separate channel. This device will be capable of
processing whole blood samples on location in a matter of minutes and displaying the cell count and should eventually
find use in neonatology, AIDS and remote site applications.
We utilize a simple parallel electrode setup on which intense holographic optical landscapes are shone. Intense optical
illumination creates high local gradients in the dielectric properties of the fluid, which in the presence of an electric field
results in a fluid body force. This leads to the creation of toroidal microvortices, which aid the particle concentration
process. Fluid drag aiding low frequency AC electrokinetic forces leads to an aggregation of particles on the illuminated
regions of the electrode surface. With a fine balance of these forces, we show that such optically driven electrokinetic
mechanisms can capture and aggregate nanoparticles (50nm and 100 nm). Particle aggregation is a function of the AC
frequency and by using fluorescent particles we characterize the technique as a function of the applied AC frequency.
Relatively low optical powers (~20 mW) are utilized in this technique.
We show an optically induced AC electrokinetic technique that rapidly and continuously accumulates colloids on an
electrode surface resulting in a crystalline-like monolayer aggregation. We demonstrate colloidal aggregation for
particles ranging from 100 nm to 3 μm. Electrothermal hydrodynamics produce a microfluidic vortex that carries
particles in suspension towards its center where they are trapped by low-frequency AC electrokinetic forces. We
characterize the rate of particle aggregation as a function of the applied AC voltage and hence characterize trapping
kinetics of this technique. We show that inter-particle distance varies with frequency and we explain this in the light of
available theory.
Point-of-care devices represent the future for medical technologies. Current diagnostic tools are cumbersome,
expensive, complicated, and often at risk for contamination. There is a need for cost effective, portable, closed-system,
high-speed cell screening and cell isolating device. A microfabricated, exponentially-staging, BioMEMS microfluidic
cytometer/cell sorting device offers these advantages over current technologies. A two-stage branched architecture
allows the study of inter-particle spacing, flow relations, pressure measurements, and cell behavior in an environment
where fluorescence detection is used to identify and analyze certain cellular characteristics. This device was
microfabricated using the polymer PDMS to transmit light effectively, to be inexpensive and disposable, and to be easy
to manipulate. For initial prototyping, an inverted fluorescent Nikon microscope provided the necessary excitation to
view the particles and cells. For the portable device, avalanche photo diodes (APDs) and light emitting diodes (LEDs)
are being incorporated into the device for the detection and excitation respectively. For low light level applications,
sigma-delta modulation methods are being applied to reduce noise susceptibility and to detect the APD signal more
efficiently. In addition, a data acquisition system (DAQ) has been designed that can effectively track signals from a cell
sorter using a digital signal processing (DSP) board and a laptop computer. Currently elastomeric valves for diverting
flow have been incorporated into the microfluidic chip. Measurements are being made of the effects of the microfluidics
valve structures, or the simple opening and closing of selected channels to divert flow and cells down specific channels
depending on their measured properties.
Trans-cellular calcium currents play a central role in the establishment of polarity in differentiating cells. Typically these
currents are measured and studied experimentally using ion selective glass microelectrodes. We have recently developed
an in silico cell electrophysiology lab-on-a-chip device with the specific science objectives of measuring these
transcellular calcium currents in an advanced throughput format. The device consists of 16 pyramidal pores on a silicon
substrate with four Ag/AgCl electrodes leading into each pore on the four poles. An SU-8 layer is used as the structural
and insulating layer and a calcium ion selective membrane is used to impart ion selectivity to the Ag/AgCl electrodes. In
this paper we demonstrate the utility of the cell electrophysiology biochip in measuring these transcellular calcium
currents from single cells using the model biological system Ceratopteris richardii. We monitored these fern spores
during germination and pharmacologically inhibited biophysical calcium transport. These results demonstrate the utility
and versatility of the in silico cell electrophysiology biochip. While this version of the biochip was engineered to fulfill
the specific science objectives of measuring trans-cellular calcium currents from Ceratopteris fern spores, the chip can
easily be modified for a variety of biomedical and pharmacological applications. Future
Flows in microscale domains are found in many places such as biomedical microdevices (Bio-MEMS), inkjet printer heads, micropropulsion systems, and microchannel networks, among many others. In fact, microfluidic devices represent the fastest growing and what is projected to be the dominant segment of the MEMS market. Because microscopic flows frequently display counter-intuitive behavior due to the different dominating forces at microscopic length scales, experimental diagnostic techniques are essential for characterizing microfluidic MEMS beharior. In recent work, we have developed several micron-scale fluidic diagnostic techniques. Micro-Particle Image Velocimetry (mPIV) measures the velocity of a flow by tracking the motion of small tracer particles seeded into the flow. To measure high velocity, small length scale flows, such as those found inside an inkjet, high-speed lasers and cameras are used in conjunction with a microscope to image the tracer particles with sub microsecond temporal resolution. Two extensions of the mPIV technique allow for flow boundary topology to be measured to with tens of nanometers and for the temperature of the flow to be measured. Combined, these three technique provide experimenters a very complete look at microfluidic device behavior at length scales on the order of 1 micron.
Particle image velocimetry (PIV), a technique commonly used at the macroscopic level to measure velocity vectors of particle-seeded flows, is adapted to measure both instantaneous and ensemble-averaged flow fields in microfluidic MEMS devices, where micro-scale spatial resolution is critical. Adapting PIV to the microscopic level presents a number of challenges, including: (1) visualizing tracer particles that are smaller than the wavelength of light, (2) minimizing errors due to the Brownian motion of the tracer particles, and (3) recording particle images with short exposure times, so that their motion does not cause particle streaking in the image field. The PIV technique is used to measure a low Reynolds number Hele-Shaw flow around a roughly 30 micrometers elliptical obstruction and a low Reynolds number flow through a 20 X 200 micrometers capillary tube. Velocity vector fields are presented with a spatial resolution of 6.9 X 6.9 X 1.5 micrometers . In principle, super-resolution particle tracking velocimetry can be used to extend the spatial resolution of the velocity measurements down to approximately 1.5 X 1.5 X 1.5 micrometers .
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