Abstract—
The advent of digital microfluidic lab-on-a-chip (LoC)
technology offers a platform for developing diagnostic applications
with the advantages of portability, increased automation,
low-power consumption, compatibility with mass manufacturing,
and high throughput. However, most digital microfluidic platforms
incorporate limited optical capabilities (e.g., optical transmission)
for integrated sensing, because more complex optical functions are
difficult to integrate into the digital microfluidic platform. This
follows since the sensor must be compatible with the hydrophobic
surfaces on which electrowetting liquid transport occurs. With
the emergence of heterogeneous photonic component integration
technologies such as those described herein, the opportunity
for integrating advanced photonic components has expanded
considerably. Many diagnostic applications could benefit from
the integration of more advanced miniaturized optical sensing
technologies, such as index of refraction sensors (surface plasmon
resonance sensors, microresonator sensors, etc.). The advent of
these heterogeneous integration technologies, that enable the
integration of thin-film semiconductor devices onto arbitrary
host substrates, enables more complex optical functions, and in
particular, planar optical systems, to be integrated into microfluidic
systems. This paper presents an integrated optical sensor
based upon the heterogeneous integration of an InGaAs-based
thin-film photodetector with a digital microfluidic system. This
demonstration of the heterogeneous integration and operation of
an active optical thin-film device with a digital microfluidic system
is the first step toward the heterogeneous integration of entire
planar optical sensing systems on this platform.
Index Terms—Digital microfluidics, electrowetting, lab-on-achip
(LoC), optical detection.
I. INTRODUCTION
MUCH OF THE reported work on electrowetting-based
lab-on-a-chip (LoC) microfluidic devices has focused on
miniaturization of analytical methods and protocols for the purpose
of improving performance and throughput. The benefits of
miniaturization, such as smaller sample requirements, reduced
reagent consumption, decreased analysis time, and higher levels
of throughput and automation, have been demonstrated. However,
to date little work has been reported on integrating the
backend function of optical detection, in part because of the
difficult requirement that detector integration must not interfere
with electrowetting-based droplet transport, and in part because
the heterogeneous integration of thin-film (microns thick) photonic
components is an emerging technology that is just beginning
to be applied to microfluidic systems.
Currently, almost all microfluidic devices are based on continuous
fluid flow in permanent microchannels in glass, plastic,
or other polymers. Numerous papers have been published that
describe optical sensor integration with continuous flowdevices.
[1]–[21] One particularly interesting paper that is related to the
research herein also underscores the importance of integrating
photonic components with microfluidics. [21] This paper describes
the heterogeneous integration of a thin-film surface
normal LED with a microchannel fluidics system. However,
continuous-flow-based microfluidic devices offer very little
flexibility in terms of scalability and reconfigurability, and they
are usually application specific.[22]–[25] This paper outlines
the heterogeneous integration of a thin-film compound semiconductor
photodetector with a digital microfluidics system. This
integration technology has been used to demonstrate complex
photonic systems, for example, that integrate lasers, optical
waveguides and photodetectors [43], and thus, the heterogeneous
integration of the planar optical component described
herein presages the integration of complex photonic systems
with digital microfluidics systems to achieve integrated systems
with complex optical functions and digital fluidic control.

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