Page 258 - Optofluidics Fundamentals, Devices, and Applications

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computing or low-level light collection, these losses badly degrade
performance. This is especially problematic when creating optical
systems from such “freestanding” lenses mentioned above, where
the total travel distance can become quite large relative to the vertical
beam expansion. Slab waveguiding has been employed in various
material systems to combat this issue [83,84,86]. The necessary lay-
ered structure can pose some physical robustness issues in terms of
the interfaces. For multimaterial systems, some bonding is necessary
[83,92,95]. The “interfaceless” all-PDMS system developed by Lien et
al. [95,96] uses cure-bonding of PDMS waveguides to the PDMS
device body, along with UV/ozone bonding of the two device body
layers, to create a layered system without physical interfaces, allow-
ing for the creation of a robust, slab-waveguided device.
While fluid-filled lenses are practical and sensible for simple and
complete space filling as well as ready control of optical properties,
they are unfortunately somewhat less practical for commercialization.
Some fluids may be absorbed into the surrounding solid material. One
simple solution is often overlooked: the use of curable polymers as a
lens fluid, similar to the way waveguides have been created [88,95].
The precured polymer allows the designer to choose the optical prop-
erties, maintains the ease and completeness of filling (no air gaps),
but then is heated or photo-cured into a solid. In a polymer body, a
polymer-filled lens will bond to the sidewalls, preventing the forma-
tion of air gaps due to thermal cycling or physical stresses. Thus by
utilizing uncured polymer as a lens fluid, all of the benefits of fluid
use can be maintained while sidestepping its pitfalls.
Custom-shaped, replica-molded optics can take a variety of forms
beyond lenses. Not only can this technology be used to create spheri-
cal, parabolic, or aspheric lens profiles, but other elements such as
Fresnel lenses [97], prisms [87,98], reflectors [99], and apertures (or
beam stops) [85,90,100] can also be fabricated, allowing for a com-
plete optical-system-on-a-chip to be molded. These optical elements
can be seamlessly integrated with microfluidic channels and associ-
ated technologies, creating a truly optofluidic device. A frequent test
bed for the marriage of these technologies is the flow cytometer, a
research and biomedical device commonly used for blood tests as
well as for cell and bacteria studies [101,102]. The basic operation
involves illuminating cells as they flow one by one past a laser beam
and then detecting light scatter and/or fluorescence signals from
each individual cell, allowing sample statistics to be acquired. This
device relies heavily on optical detection; indeed, adequate system
performance cannot be obtained with a simple fiber-and-channel
configuration [103]. From the optics standpoint, signal resolution
relies on providing a small, uniform source of illumination and on
collecting light from a well-defined area by minimizing background
illumination levels and collection of background signal. Lenses, aper-
tures, and beam blocks are commonly found in benchtop cytometers.