Microfluidic Optical Devices
Microfluidic optical devices (MOD) are the emerging technology that combines today’s microfluidics technology with the optics. However, MOD can be classified as the integration of these two technologies rather than combination of them. This integration provides a new approach for using microfluidics for control and manipulation of samples and optics for sensing. In this entry we propose a comprehensive review of emerging applications for microfluidic optical devices.
Electric field induction by light exposure 
Small cell population sorting with high accuracy by tunable optical fibers 
Rapid detection of environmental contaminants 
A platform to study mammalian individual axonal injury 
Variable-focus liquid lens 
Integration of microtoroid whispering gallery mode (WGM) sensor into a microfluidic system for transporting molecules directly towards the most sensitive area of the sensor 
Combination of optical tweezers and microfluidic chip technologies based on dynamic fluid and dynamic light pattern, in which single and multiple laser traps are employed for cell transportation 
Fabrication of a reticulocyte microfluidic cytometer system based on optimized epifluorescence with the advantage in the signal-to-noise ratio 
All-optical and electrode-free approach into microfluidic chips for achieving reconfigurable dielectrophoresis (DEP) particle trapping 
Implementing optical fibers to improve the performance of fluorescent spectroscopy detection on a portable chip 
A neuro-optical microfluidic platform to study mammalian individual axonal injury and subsequent regeneration 
Key Research Findings
Mccio et al.  presented all-optical and electrode-free approach for achieving reconfigurable dielectrophoresis (DEP) particle trapping into microfluidic chips. They used direct laser projection through a holographic spatial light modulator (SLM) onto photorefractive crystal substrates as a fabrication method. As a fabrication process, firstly, they considered an all-optical mold-free approach for fabricating the PDMS microfluidic channel. Secondly, they created geometrical flexible DEP traps onto the substrate by the same SLM holographic projection system. For the first time, they presented the possibility of fabricating microfluidic chips onto lithium niobate (LN) crystals by using the photorefractive effect. In addition, they demonstrated that these PDMS structures can be used as microfluidic devices such as making PDMS chambers and channels in which particles can be trapped by DEP effect.
Chabinyc et al.  succeeded the integration of fluorescence detector based on a microavalanche photodiode into a PDMS-based microfluidic device. Their detective system was reusable, and the microfluidic device was disposable. Elimination of the index matching problem which can be problematic in some micromachined devices, the elimination of the need for collection optics, and the inexpensiveness are the superiority of the design. They also addressed some further improvements for the performance of the device.
Mazurczyk et al.  fabricated an integrated fluorescence detection system with the microfluidic lab-on-a-chip device. Various arrangements were tested for the fluorescence beam detection like free space optics, fiber optics, and fully waveguiding optics. Free space optics was found to have the higher sensitivity, but it needed the bulk microscope-based detection system. Fiber and fully waveguiding optics seemed to be the possible option to overcome the bulk microscope setup, but their sensitivity was not found to be sufficient especially for high sensitivity required applications. Anyhow, they showed the feasibility of their device for electrophoretic separations by performing some preliminary experiments. They proposed usage of the soda lime glass for the fabrication of their device and envisaged to fabricate more sophisticated systems.
Optofluidics is marriage of fluidics with optics for dynamic manipulation of optical properties at the microscale with applications ranging from photonic circuits to fluidically adaptable optics (Optofluidics). Heng et al.  developed a novel on-chip microscope system, which is called optofluidic microscope (OFM). The feasibility of OFM was demonstrated. Their images were comparable to that of a conventional microscope. They suggested the possible use of multiple OFM onto a single microfluidic chip either for increasing imaging throughput in case of parallel usage or sequential imaging of the same target in case of serial usage.
Khosla et al.  demonstrated a microfluidic whispering gallery mode (WGM) biosensing system for easy delivery and detection of target molecules. WGM resonators are optical sensors with unprecedented sensitivity because of their small mode volume. Their results showed that the integration of microfluidics and WGM sensing result in highly tunable system, with yield for a given concentration and optimizable sensing time by changing the input power and flow features of the microfluidic system. By combining microfluidics and WGM, particles and molecules as tiny as a single BSA protein (roughly 6 nm in radius) can be detected, and the analysis can be accomplished for femtomolar concentrations.
Camou et al.  used 2D-optical, PDMS lenses to improve the performance of fluorescent spectroscopy detection performed on a portable chip using optical fibers. The fibers are directly inserted into channels ending with PDMS optical lenses. Compared to conventionally use flat interface, these optical lenses increased the intensity of fluorescent response close to the fiber which leads to a higher sensitivity of the on-chip detection method for fluorescent spectroscopy. Chen et al.  implemented a simple, on-chip arrayed-waveguide excitation and detection scheme based on the scattering. Detected signals were processed, and not only sensitivity enhancement was observed but particles moving with different velocities were also detected accurately.
Kruger et al.  proposed a miniaturized flow cytometer using latest photonics technology to perform detection, enumeration, and sorting of fluorescent species. They successfully performed the sample injection, single file flow through the detection system, sorting of fluorescent microbeads. They could not achieve the fully autonomous cell sorting, but they gave that for the future direction. They also demonstrated the feasibility of high gain avalanche photodiodes for more sensitive measurements of fluorescent signals compared to conventional detection techniques.
Lafleur et al.  improved conventional methods of environmental analysis by demonstrating the vast potential of gold nanoparticle-based microfluidic sensors for the rapid detection of two important categories of environmental contaminants – heavy metals and pesticides. Using gold nanoparticle-based microfluidic sensors integrated with a simple fluorescence detector allows the detection range of concentrations as low as 0.6 μg/L up to at least 200 μg; a simple fluorescence detector allows the detection range of concentrations as low as 0.6 μg/L up to at least 200 μg/L. These results show that, synergistically, combination of the unique optical properties of gold nanoparticle probes with the inherent qualities of microfluidic platforms offers sensitive sensors for environmental contaminants.
Kim et al.  proposed a neuro-optical microfluidic platform to study mammalian individual axonal injury and subsequent regeneration. They succeeded the integration of a compartmentalized neuronal culture microfluidic chip for isolating individual axons, femtosecond laser to have precise and reproducible axotomy, and a mini-incubator to monitor the sequence of neuronal activities and post injury events. In addition to study injury to single axons, the platform can also be used to create unique in vitro neuronal circuit models.
Another interesting application of the microfluidics and the optics is the use of variable-focus liquid lenses, which was developed by Philips Research Eindhoven . The lenses which are composed of two immiscible liquids of different refractive indices can be manipulated by the electrowetting. By electrowetting, the meniscus curvature of the lenses can be changed and so does the effective focal length of the lens.
Future Research Directions
Integration of microfluidics and optics merges many advantages associated with these fields and has found many applications in different areas of science and technology. Although, this merging brings many advantages, it also introduces many challenges, and many researchers will continue to solve these challenges and further develop the MODs in the upcoming decades. Microfluidic and microoptical components can be fabricated during the same fabrication step or they can be fabricated separately and post-assembled. Despite recent demonstrations, complete integration of both components is still in a highly developmental stage, and exploration of optofluidic functionality is the goal of the most current investigation.
In addition to the developmental considerations on combination of these two platforms, further investigations on neuro-optical microfluidic platforms are highly recommended because this platform will enable the study and understanding of neuronal response to injury that is currently not possible with conventional cell culture platform and tools.
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