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Topic Name: Harvard Engineers Demonstrate a new Type of Optical Tweezer to Make Biological and Microfluidic Force Measurements
Category: Optoelectronics
Research persons: A Research Team
Location: Harvard University, United States
Details
Researchers at the
Harvard School of
Engineering and Applied Sciences (SEAS) demonstrated a new type of optical
tweezer with the potential to make biological and microfluidic force
measurements in integrated systems such as microfluidic chips. The tweezer,
consisting of a Fresnel Zone Plate microfabricated on a glass slide, has the
ability to trap particles without the need for high performance objective
lenses.
The device was designed, fabricated, and tested by
postdoctoral fellow Ethan Schonbrun and undergraduate researcher Charles Rinzler
under the direction of Assistant Professor of Electrical Engineering Ken Crozier
(all are affiliated with SEAS). The team's results were published in the
February 18th edition of Applied Physics Letters and the researchers have filed
a U.S. provisional patent covering this new device.
"The microfabricated nature of the new optical tweezer offers
an important advantage over conventional optical tweezers based on microscope
objective lenses," says Crozier. "High performance objective lenses usually have
very short working distances -- the trap is often ~200 mm or less from the front
surface of the lens. This prevents their use in many microfluidic chips since
these frequently have glass walls that are thicker than this."
The researchers note that the Fresnel Zone Plate optical
tweezers could be fabricated on the inner walls of microfluidic channels or even
inside cylindrical or spherical chambers and could perform calibrated force
measurements in a footprint of only 100x100μm.
Traditional tweezers, by contrast, would suffer from
crippling aberrations in such locations. Moreover, in experimental trials, the
optical tweezers exhibited trapping performance comparable to conventional
optical tweezers when the diffraction efficiency was taken into account.
The researchers envision using their new tweezer inside
microfluidic chips to carry out fluid velocity, refractive index, and local
viscosity measurements. Additional applications include biological force
measurements and sorting particles based on their size and refractive index.
Particle-sorting chips based on large arrays of tweezers could be used to
extract the components of interest of a biological sample in a high-throughput
manner.
Note for Optical Tweezer
An optical tweezer is a scientific instrument that uses a focused laser beam to
provide an attractive or repulsive force, depending on the index mismatch
(typically on the order of piconewtons) to physically hold and move microscopic
dielectric objects. Optical tweezers have been particularly successful in
studying a variety of biological systems in recent years.
Optical tweezers are capable of manipulating nanometer and micrometer-sized
dielectric particles by exerting extremely small forces via a highly focused
laser beam. The beam is typically focused by sending it through a microscope
objective. The narrowest point of the focused beam, known as the beam waist,
contains a very strong electric field gradient. It turns out that dielectric
particles are attracted along the gradient to the region of strongest electric
field, which is the center of the beam. The laser light also tends to apply a
force on particles in the beam along the direction of beam propagation. It is
easy to understand why if you imagine light to be a group of tiny particles,
each impinging on the tiny dielectric particle in its path. This is known as the
scattering force and results in the particle being displaced slightly downstream
from the exact position of the beam waist, as seen in the figure.
Optical traps are very sensitive instruments and are capable of the manipulation
and detection of sub-nanometer displacements for sub-micrometre dielectric
particles. For this reason, they are often used to manipulate and study single
molecules by interacting with a bead that has been attached to that molecule.
DNA and the proteins and enzymes that interact with it are commonly studied in
this way.
For quantitative scientific measurements, most optical traps are operated in
such a way that the dielectric particle rarely moves far from the trap center.
The reason for this is that the force applied to the particle is linear with
respect to its displacement from the center of the trap as long as the
displacement is small. In this way, an optical trap can be compared to a simple
spring, which follows Hooke's law.
Note for Microfluidics
Microfluidics deals with the behavior, precise control and manipulation of
fluids that are geometrically constrained to a small, typically sub-milimeter,
scale. It is a multidisciplinary field intersecting engineering, physics,
chemistry, microtechnology and biotechnology, with practical applications to the
design of systems in which such small volumes of fluids will be used.
Microfluidics has emerged in the beginning of the 1980s and is used in the
development of inkjet printheads, DNA chips, lab-on-a-chip technology,
micro-propulsion, and micro-thermal technologies.
The behavior of fluids at the microscale can differ from 'macrofluidic' behavior
in that factors such as surface tension, energy dissipation, and fluidic
resistance start to dominate the system. Microfluidics studies how these
behaviors change, and how they can be worked around, or exploited for new uses.
At small scales (channel diameters of around 100 nanometers to several hundred
micrometers) some interesting and unintuitive properties appear. The Reynolds
number, which characterizes the presence of turbulent flow, is extremely low,
thus the flow will remain laminar. Thus, two fluids joining will not mix readily
via turbulence, so diffusion alone must cause the two fluids to mingle.
Microfluidic structures include on one hand micropneumatic systems, i.e.
microsystems for the handling of off-chip fluids (liquid pumps, gas valves,
etc), and on the other hands microfluidic structures for the on-chip handling of
nano- and picolitre volumes. The commercially most successful application today
is the inkjet printhead.
Advances in microfluidics technology are revolutionizing molecular biology
procedures for enzymatic analysis (e.g., glucose and lactate assays), DNA
analysis (e.g., polymerase chain reaction and high-throughput sequencing), and
proteomics. The basic idea of microfluidic biochips is to integrate assay
operations such as detection, as well as sample pre-treatment and sample
preparation on one chip.
An emerging application area for biochips is clinical pathology, especially the
immediate point-of-care diagnosis of diseases. In addition, microfluidics-based
devices, capable of continuous sampling and real-time testing of air/water
samples for biochemical toxins and other dangerous pathogens, can serve as an
always-on "bio-smoke alarm" for early warning.
Note for Fresnel Zone
In optics and radio communications, a Fresnel zone (pronounced FRA-nel Zone),
named for physicist Augustin-Jean Fresnel, is one of a (theoretically infinite)
number of concentric ellipsoids of revolution which define volumes in the
radiation pattern of a (usually) circular aperture. Fresnel zones result from
diffraction by the circular aperture.
The cross section of the first Fresnel zone is circular. Subsequent Fresnel
zones are annular in cross section, and concentric with the first.
To maximize receiver strength, one needs to minimize the effect of the out of
phase signals by removing obstacles from the RF Line of Sight (RF LoS). The
strongest signals are on the direct line between transmitter and receiver and
always lie in the 1st Fresnel Zone.
If unobstructed, radio waves will travel in a straight line from the transmitter
to the receiver. But if there are obstacles near the path, the radio waves
reflecting off those objects may arrive out of phase with the signals that
travel directly and reduce the power of the received signal. On the other hand,
the reflection can enhance the power of the received signal if the reflection
and the direct signals arrive in phase. Sometimes this results in the
counterintuitive finding that reducing the height of an antenna increases the
S+N/N ratio.
Fresnel provided a means to calculate where the zones are where obstacles will
cause mostly in phase and mostly out of phase reflections between the
transmitter and the receiver. Obstacles in the first Fresnel will create signals
that will be 0 to 90 degrees out of phase, in the second zone they will be 90 to
270 degrees out of phase, in third zone, they will be 270 to 450 degrees out of
phase and so on. Odd numbered zones are constructive and even numbered zones are
destructive.
The concept of Fresnel zone clearance may be used to analyze interference by
obstacles near the path of a radio beam. The first zone must be kept largely
free from obstructions to avoid interfering with the radio reception. However,
some obstruction of the Fresnel zones can often be tolerated, as a rule of thumb
the maximum obstruction allowable is 40%, but the recommended obstruction is 20%
or less.
The work was supported by the
Microsystems Technology
Office of the Defense Advanced Research Projects Agency and the Harvard
Nanoscale Science and
Engineering Center of the
National Science Foundation.
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