Microfluidic molding of photonic microparticles with engraved elastomeric membranes.

A microfluidic approach to prepare photonic microparticles by repeated molding of photocurable colloidal suspension is reported. An elastomeric membrane with negative relieves which vertically separates two microfluidic channels is integrated; bottom channel is used for suspension flow, whereas water-filled top channel is used for pneumatic actuation of the membrane. Upon pressurization of the top channel, membrane is deformed to confine the suspension into its negative relieves, which is then polymerized by UV irradiation, making microparticles with mold shape. The microparticles are released from the mold by relieving the pneumatic pressure and flows through the bottom channel. This one cycle of molding, polymerization, and release can be repeatedly performed in microfluidic device of which pneumatic valves are actuated in a programmed manner. The microparticles exhibit structural colors when the suspension contains high concentration of silica nanoparticles; the nanoparticles form regular arrays and the microparticles reflect specific wavelength of light as a photonic crystals. The silica nanoparticles can be selectively removed to make pronounced structural colors. In addition, the microparticles can be further functionalized by embedding magnetic particles in the matrix of the microparticles, enabling the remote control of rotational motion of microparticles.

Monolithic photonic crystals created by partial coalescence of core-shell particles.

Colloidal crystals and their derivatives have been intensively studied and developed during the past two decades due to their unique photonic band gap properties. However, complex fabrication procedures and low mechanical stability severely limit their practical uses. Here, we report stable photonic structures created by using colloidal building blocks composed of an inorganic core and an organic shell. The core-shell particles are convectively assembled into an opal structure, which is then subjected to thermal annealing. During the heat treatment, the inorganic cores, which are insensitive to heat, retain their regular arrangement in a face-centered cubic lattice, while the organic shells are partially fused with their neighbors; this forms a monolithic structure with high mechanical stability. The interparticle distance and therefore stop band position are precisely controlled by the annealing time; the distance decreases and the stop band blue shifts during the annealing. The composite films can be further treated to give a high contrast in the refractive index. The inorganic cores are selectively removed from the composite by wet etching, thereby providing an organic film containing regular arrays of air cavities. The high refractive index contrast of the porous structure gives rise to pronounced structural colors and high reflectivity at the stop band position.

Ordered packing of emulsion droplets toward the preparation of adjustable photomasks.

Monodisperse emulsion droplets with a high volume fraction form crystalline phases that can potentially serve as adjustable photomasks in photolithography. Such photomasks were prepared using a microfluidic device in which a flow-focusing junction, side channels, and a reservoir were connected in series. Transparent oil droplets were generated in a dye-containing continuous water phase at the flow-focusing junction. The droplets were then concentrated through the selective removal of the continuous phase using the side channels. This process led to the formation of a regular array of droplets in the reservoir with a configuration that depended on the relative height of the reservoir to the droplet diameter. The configurations could be selected among a single-layered hexagonal array, a bilayered square array, and a bilayered hexagonal array. The droplet arrays were used as a photomask to create hexagonal or square arrays of microdots. The transmittance profile of the ultraviolet (UV) light from each droplet was parabolic, which enabled the dot size to be tuned by controlling the UV irradiation time. This mask effect is otherwise difficult to achieve using conventional photomasks. The dot size and array periodicity could be adjusted by the in-situ control of the droplet size at the flow-focusing droplet maker. The combination of droplet size adjustments and the UV irradiation time provided independent control over the dot size and array periodicity to enable the preparation of a series of hexagonal microarrays with a wide spectrum of array parameters using a single microfluidic device.

Droplet microfluidics for producing functional microparticles.

Isotropic microparticles prepared from a suspension that undergoes polymerization have long been used for a variety of applications. Bulk emulsification procedures produce polydisperse emulsion droplets that are transformed into spherical microparticles through chemical or physical consolidation. Recent advances in droplet microfluidics have enabled the production of monodisperse emulsions that yield highly uniform microparticles, albeit only on a drop-by-drop basis. In addition, microfluidic devices have provided a variety of means for particle functionalization through shaping, compartmentalizing, and microstructuring. These functionalized particles have significant potential for practical applications as a new class of colloidal materials. This feature article describes the current state of the art in the microfluidic-based synthesis of monodisperse functional microparticles. The three main sections of this feature article discuss the formation of isotropic microparticles, engineered microparticles, and hybrid microparticles. The complexities of the shape, compartment, and microstructure of these microparticles increase systematically from the isotropic to the hybrid types. Each section discusses the key idea underlying the design of the particles, their functionalities, and their applications. Finally, we outline the current limitations and future perspectives on microfluidic techniques used to produce microparticles.

Fabrication of three-dimensional nanostructured titania materials by prism holographic lithography and the sol-gel reaction.

We present a simple, easy method for fabricating high-quality titania inverted replicas of 3D holographically featured structures. A combination of single-prism holographic lithography and sol-gel chemistry was used to prepare 3D titania inverse structures with flat and completely open surfaces without the use of additional postprocessing steps, such as reactive ion etching, ion-beam milling, and/or polishing steps. A hydrophobic, stable liquid titania precursor facilitated the complete infiltration of the precursor into the hydrophobic 3D SU-8 polymer template, which produced very uniform high-quality titania inverse structures. Although the degree of film shrinkage during the calcination process was large (∼34%), the optical strength of the 3D titania inverse photonic crystals doubled because of the high-refractive-index contrast. Compared to titania inverse opal structures, the filling fraction (∼27%) of titania materials has been doubled. This is the first work to fabricate titania inverse photonic crystals with a high filling fraction by utilizing prism holographic lithography and the sol-gel chemistry reaction of a stable titania precursor. The X-ray diffraction patterns indicated the presence of a crystalline anatase or rutile phase depending on the calcination temperature.

Liquid waveguide-based evanescent wave sensor that uses two light sources with different wavelengths.

We demonstrate a prototypic optofluidic evanescent wave sensor made of poly(dimethylsiloxane) (PDMS) elastomer in which two light sources with different wavelengths are coupled into an optofluidic liquid-core/liquid-cladding (L(2)) waveguide. The exponentially decaying evanescent wave interacts with analyte molecules dissolved in the cladding fluids or products formed by in situ reactions at the core-cladding interface. The analyte molecules exhibit distinctly different light absorbance at the two wavelengths during the light-analyte interaction. Therefore, by using the normalized absorbance calculated from the intensity ratio of the two wavelengths instead of the absolute magnitude of either signal, unwanted effects from omnipresent external noise sources can be reduced. In addition, the differential absorption of the two beams by the analyte solutions can be used to enhance the resolution of sample analysis. The evanescent wave sensor based on a liquid waveguide can also be used for real-time monitoring of chemical reactions, because the core and cladding fluids in the L(2) waveguide are slightly miscible at the core-cladding interface due to the diffusional mixing.

High-fidelity optofluidic on-chip sensors using well-defined gold nanowell crystals.

Recent advances in nanofabrication techniques have enabled the creation of various metallic nanostructures in order to engineer the location and properties of electromagnetic hot spots in a controlled manner. However, most previous methods usually require complicated and time-consuming techniques, and the integration of metallic nanostructures into simple, low-cost devices for chemical or biological sensing is still challenging. Here, we report a promising new strategy for the fabrication of large-area gold nanowell arrays with novel geometric features that makes use of the trapping of self-assembled colloidal particles on a polymer surface. Through both systematic experimental and theoretical analysis, we confirm that the strong plasmon resonances of the proposed nanowell structures are associated with localized surface plasmon resonance (LSPR) on the brims of the nanoholes in the top gold films as well as in the bottom gold disks. In addition, we demonstrate a novel optofluidic platform with built-in subwavelength nanowell arrays that exhibits strong plasmon resonances within microfluidic chips. In our optofluidic systems, the plasmon coupling between the brims and the disks of nanowells makes the plasmon resonance more sensitive to surrounding materials. The dependence of the plasmon resonance on the refractive index of the surrounding medium is found to be as high as 570 nm RIU(-1) (refractive index units). These data lead to a figure of merit (FOM), the slope of refractive index sensitivity in eV RIU(-1)/line width (eV), as high as 4.1.

Development of reflective biosensor using fabrication of functionalized photonic nanocrystals.

Photonic crystals (PCs) are periodic dielectric structures that have a band-gap that forbids propagation of a certain range of wavelengths of light. This property enables control of light with remarkable facility by modification of the band-gaps and produce effects that are impossible with conventional optics. Using chemically functionalized PCs, where the chemical functional group consists of amine and carboxyl group, in conjunction with a biomolecular probe material, the detection of pathogens and viral disease is possible, indicated by the shift in wavelength signal. Moreover, this system using the bioinspired PCs allows specific target detection in biosensor chip fields through control of the PCs. In this study, we demonstrated that two bacterial pathogens (Fusobacterium necrophorum and Acinetobacter baumannii) causing sepsis were detected by DNA-probe hybridization and a severe acute respiratory syndrome coronavirus was detected by antigen-antibody interaction using the functional PCs. Optical readout with the integrated sensor detecting the signals from PCs, allows for low cost and robust readout of resonance peak shift. This biosensor system using the functional PCs on the photonic crystal-fabricated chip can efficiently and effectively detect various targets, and be easily prepared with high productivity and economic property.

Designed pneumatic valve actuators for controlled droplet breakup and generation.

The dynamic breakup of emulsion droplets was demonstrated in double-layered microfluidic devices equipped with designed pneumatic actuators. Uniform emulsion droplets, produced by shearing at a T-junction, were broken into smaller droplets when they passed downstream through constrictions formed by a pneumatically actuated valve in the upper control layer. The valve-assisted droplet breakup was significantly affected by the shape and layout of the control valves on the emulsion flow channel. Interestingly, by actuating the pneumatic valve immediately above the T-junction, the sizes of the emulsion droplets were controlled precisely in a programmatic manner that produced arrays of uniform emulsion droplets in various sizes and dynamic patterns.

Perfectly hydrophobic surfaces with patterned nanoneedles of controllable features.

In this Letter, we present a simple and reproducible method for generating polystyrene (PS) nanoneedle arrays by utilizing the trapping of inorganic silica particles at the polystyrene/air interface via capillary wetting of a thermoplastic polystyrene polymer and SF(6) reactive-ion etching. A monolayer of silica microspheres was directly formed and trapped on the smooth PS film, and subsequent wet etching with HF and reactive-ion etching with SF(6) left behind hexagonal arrays of protruding tips with tip diameters around 20 nm. The patterned PS surface possessed a well-defined nanoneedle array with the pattern density as high as 2.5 x 10(8)/cm(2) and exhibited advancing and receding water contact angles of 180 degrees. The surface showed no affinity for water as confirmed by a series of contact, compression, and release tests. Finally, the perfect hydrophobicity of the fabricated surface is explained in terms of its surface morphology and chemical composition.

Bioinspired holographically featured superhydrophobic and supersticky nanostructured materials.

In this Letter, we present an intriguing method for fabricating polymeric superhydrophobic surfaces by reactive-ion etching of holographically featured three-dimensional structures. Using the proposed strategy, we generated both lotus and gecko surfaces by simply controlling the incident angle of the laser beam during holographic lithography. The adhesion force of the gecko-state superhydrophobic surfaces was the highest yet reported for an artificial superhydrophobic surface. The well-controlled patterns enable an in-depth understanding of superhydrophobic and superadhesive surfaces. In particular, the present observations provide direct evidence of a high adhesive force resulting from surface-localized wetting, which is quite different from previously suggested mechanisms.

Rigiflex lithography-based nanodot arrays for localized surface plasmon resonance biosensors.

We present a facile and robust means of fabricating metallic nanodot arrays for localized surface plasmon resonance (LSPR) biosensors through the strategic coupling of a polymeric template prepared with rigiflex lithography and a subsequent metallization via electrodeposition. Rigiflex lithography provides the capability to realize large-scale nanosized features as well as process flexibility during contact molding. In addition, the electrodeposition process enables wet-based nanoscale metallization with high pattern fidelity and geometric controllability. Generated metallic nanodot arrays can be used as a general platform for LSPR biosensors via the sequential binding of chemicals and biomolecules. Extinction spectra of the corresponding LSPR signal are measured with UV-vis-NIR spectroscopy, from which the pattern size and shape dependence of LSPR are readily confirmed. The feasibility of a very sensitive biosensor is demonstrated by the targeted binding of human immunoglobulin G, yielding subnanomolar detection capability with high selectivity.

Lithographically-featured photonic microparticles of colloidal assemblies.

We have described a new and promising strategy for the fabrication of composite and porous photonic crystal microparticles that combines the self-assembly of colloidal particles with photolithography techniques. We fabricated silica/SU-8 composite microparticles with photonic bandgaps via four steps: (1) deposition of the silica colloidal crystals on the photoresist, (2) embedding of the colloidal crystals in the photoresist, (3) UV exposure through a photomask and subsequent development, and (4) release of the microparticles from the substrate. Embedding was performed above the glass transition temperature (T(g)) of uncrosslinked SU-8. At such temperatures, capillary forces on the silica particles facilitate the migration of colloidal crystals in the SU-8 matrix. Particle migration ceased when the top colloidal crystal layer was trapped at the interface between air and SU-8. In addition, we also prepared porous microparticles with an inverse opaline structure by dissolving the embedded silica particles from the composite structures. The porous microparticles showed enhanced reflectivity at the bandgap position due to the large refractive index contrast. The bandgap position of the microparticles was controlled by the size of the silica particles, which determined the lattice constant. Bilayered composite and porous microparticles with two distinct photonic bandgaps were also prepared by sequential deposition of colloidal crystals composed of two differently sized silica particles.

Holographic fabrication of three-dimensional nanostructures for microfluidic passive mixing.

In this study, we incorporated mixing units of three-dimensional (3D) interconnected pore network inside microfluidic channels by combining single prism holographic lithography and photolithography. 3D pore network structures were generated by the interference of four laser beams generated by a truncated triangular pyramidal prism. The levelling between the 3D porous structures and the channel walls was greatly improved by employing supercritical drying, which induced negligible internal capillary stresses and reduced substantially anisotropic volume shrinkage of 3D structures. Also, complete sealing of the microfluidic chips was achieved by attaching flexible PDMS cover substrates. Overall mixing performance of the systems with completely sealed mixing units was 84% greater than that obtained without such mixers. Splitting and recombination of flows in the 3D interconnected pore structures enhanced the mixing efficiency by decreasing the diffusion path and increasing the surface contact between two liquid streams. Because the flow splitting and recombination was developed through the 3D interconnected pore network, high mixing efficiency (>0.60) was achieved at low Reynolds numbers (Re < 0.05) and Péclet numbers in the regime of Pe < 1.4 x 10(3).

Arrays of ferromagnetic nanorings with variable thickness fabricated by capillary force lithography.

A new promising strategy is reported for the fabrication of ferromagnetic nanoring arrays with novel geometrical features through the use of capillary force lithography and subsequent reactive ion etching. In particular, we fabricated two different types of elliptic rings with variable width and height: one with pinching zones near the major axes and the other with pinching zones near the minor axes. We used PDMS stamps with either elliptic hole or antihole arrays for creating these elliptic rings with variable thickness by virtue of the uneven capillary rise, which was induced by the distributed Laplace pressure around the walls of elliptic holes or antiholes with nonuniform local curvatures. We transferred the polymer ring patterns to array of elliptical NiFe rings by Ar ion milling and characterized magnetic properties in terms of nonuniform ring width using magnetic force microscopy measurements. Our results demonstrated that the magnetic domain wall can be positioned in a controlled manner by using these novel elliptical ferromagnetic rings with local pinching zones and that the proposed CFL method can be utilized as a simple and effective fabrication tool.

Detection of DNA immobilization and hybridization on gold/silver nanostructures using localized surface plasmon resonance.

In this study, we used localized surface plasmon resonances (LSPR) to observe a phenomenon of binding of DNA on Au and Au/Ag nanostructure arrays. Au and Au/Ag nanostructures of various geometric sizes and metal compositions were fabricated by colloidal lithography technique. The immobilization of capture DNA and subsequent hybridization with target DNA on the nanostructures caused the shift of maximum peak in LSPR spectra of the nanostructures. Using the peak shift, the immobilization of capture DNA was clearly verified in a nondestructive manner and hybridization with complementary target DNA was reliably differentiated from the non-specific binding with noncomplementary DNA. This work firmly implies that the LSPR spectra of the nanostructrues can be efficiently utilized to achieve a novel strategy for the detection of DNA on the nanostructures.

Bidirectional Recurrent Auto-Encoder for Photoplethysmogram Denoising.

Photoplethysmography (PPG) has become ubiquitous with the development of smart watches and the mobile healthcare market. However, PPG is vulnerable to various types of noises that are ever present in uncontrolled environments, and the key to obtaining meaningful signals depends on successful denoising of PPG. In this context, algorithms have been developed to denoise PPG, but many were validated in controlled settings or are reliant on multiple steps that must all work correctly. This paper proposes a novel PPG denoising algorithm based on bidirectional recurrent denoising auto-encoder (BRDAE) that requires minimal pre-processing steps and have the benefit of waveform feature accentuation beyond simple denoising. The BRDAE was trained and validated on a dataset with artificially augmented noise, and was tested on a large open database of PPG signals collected from patients enrolled in intensive care units as well as from PPG data collected intermittently during the daily routine of nine subjects over 24 h. Denoising with the trained BRDAE improved signal-to-noise ratio of the noise-augmented data by 7.9 dB during validation. In the test datasets, the denoised PPG showed statistically significant improvement in heart rate detection as compared with the original PPG in terms of correlation to reference and root-mean-squared error. These results indicate that the proposed method is an effective solution for denoising the PPG signal, and promises values beyond traditional denoising by providing PPG feature accentuation for pulse waveform analysis.

Holographic fabrication of photonic nanostructures for optofluidic integration.

Holographic lithography in combination with photo-lithography provides a novel optofluidic platform through incorporation of periodic photonic units inside the microfluidic chips in a highly compatible and facile way.

Fluorescent liquid-core/air-cladding waveguides towards integrated optofluidic light sources.

We have demonstrated fluorescent liquid-core/air-cladding (LA) waveguides suitable for use as integrated optofluidic light sources. These waveguides were fabricated by conventional soft lithography using poly(dimethylsiloxane) (PDMS). Two-phase stratified flows of air and ethylene glycol with fluorescent dye were generated along the PDMS channel. Compared to the liquid-core/liquid-cladding (L(2)) waveguide, the larger refractive index contrast of the LA waveguide resulted in stronger optical confinement. Specifically, the larger refractive index contrast led to experimentally achievable captured fractions (the amount of light to be coupled into the liquid core) as high as 22.8% and the measured propagation losses as low as 0.14 dB cm(-1). Furthermore, in our LA waveguides, diffusional mixing of the core and cladding fluids did not occur and the size of the core stream could be reversibly tuned simply by adjusting the flow rates of the two contiguous phases.