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.

Dynamic designing of microstructures by chemical gradient-mediated growth.

Shape is one of the most important determinants of the properties of microstructures. Despite of a recent progress on microfabrication techniques, production of three-dimensional micro-objects are yet to be fully achieved. Nature uses reaction-diffusion process during bottom-up self-assembly to create functional shapes and patterns with high complexity. Here we report a method to produce polymeric microstructures by using a dynamic reaction-diffusion process during top-down photolithography, providing unprecedented control over shape and composition. In radical polymerization, oxygen inhibits reaction, and therefore diffusion of oxygen significantly alters spatial distribution of growth rate. Therefore, growth pathways of the microstructures can be controlled by engineering a concentration gradient of oxygen. Moreover, stepwise control of chemical gradients enables the creation of highly complex microstructures. The ease of use and high controllability of this technology provide new opportunities for microfabrication and for fundamental studies on the relationships between shape and function for the materials.

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.

Magnetoresponsive discoidal photonic crystals toward active color pigments.

Photonic microdisks with a multilayered structure are designed from photocurable suspensions by step-by-step photolithography. In each step of photolithography, either a colloidal photonic crystal or a magnetic-particle-laden layer is stacked over the windows of a photomask. Sequential photolithography enables the creation of multilayered photonic microdisks that have brilliant structural colors that can be switched by an external magnetic field.

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.

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.

Controlled pixelation of inverse opaline structures towards reflection-mode displays.

Pixelated inverse opals with red, green, and blue colors were prepared by hybridizing convective assembly of colloidal particles and photolithography techniques. The brilliant structural colors, high mechanical stability, and small feature size of the pixels were simultaneously accomplished, thereby providing color reflectors potentially useful for display devices. Moreover, this hybridized method provides a general means to create multi-colored photonic crystals.

3D hierarchical architectures prepared by single exposure through a highly durable colloidal phase mask.

Three-dimensional hierarchical architectures are fabricated using a simple, cost-effective, durable colloidal phase mask containing a colloidal monolayer embedded in a flexible polydimethylsiloxane (PDMS) membrane. These structures give rise to a photonic bandgap that can be tuned over a wide spectral range from the visible to the near-infrared regions.

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.

Bio-inspired nanotadpoles with component-specific functionality.

We report a new class of bio-inspired nanotadpoles (NTPs) with component-specific functionalities. The plasmonic NTPs with a gold-coated head and a reactive ion etching-treated tail showed the tail length dependence of their cellular uptake, enabling the photothermal treatment of cancer cells with high efficacy.

Photothermal control of membrane permeability of microcapsules for on-demand release.

We report the use of a simple microfluidic device for producing microcapsules with reversible membrane permeability that can be remotely controlled by application of near-infrared (NIR) light. Water-in-oil-in-water (W/O/W) double-emulsion drops were prepared to serve as templates for the production of mechanically stable microcapsules with a core-shell structure and highly uniform size distribution. A biocompatible ethyl cellulose shell was formed, containing densely packed thermoresponsive poly(N-isopropylacrylamide) (pNIPAAm) particles in which gold nanorods were embedded. Irradiation with a NIR laser resulted in heating of the hydrogel particles due to the photothermal effect of the gold nanorods, which absorb at that wavelength. This localized heating resulted in shrinkage of the particles and formation of macrogaps between them and the matrix of the membrane. Large encapsulated molecules could then pass through these gaps into the surrounding fluid. As the phase transition behavior of pNIPAAm is highly reversible, this light-triggered permeability could be repeatedly switched on and off by removing the laser irradiation for sufficient time to allow the gold nanorods to cool. This reversible and remote control of permeability enabled the programmed release of encapsulants, with the time and period of the open valve state able to be controlled by adjusting the laser exposure. This system thus has the potential for spatiotemporal release of encapsulated drugs.

Nanoarchitectures with controllable anisotropic features in structures and properties from simple and robust holographic lithography.

Anisotropic nanostructures with precise orientations or sharp corners display unique properties that may be useful in a variety of applications; however, precise control over the anisotropy of geometric features, using a simple and reproducible large-area fabrication technique, remains a challenge. Here, we report the fabrication of highly uniform polymeric and metallic nanostructure arrays prepared using prism holographic lithography (HL) in such a way that the isotropy that can be readily and continuously tuned. The prism position on the sample stage was laterally translated to vary the relative intensities of the four split beams, thereby tuning the isotropy of the resulting polymer nanostructures through the following shapes: circular nanoholes, elliptical nanoholes, and zigzag-shaped nanoarrays. Corresponding large-area, defect-free anisotropic metallic nanostructures could then be fabricated using an HL-featured porous polymer structure as a milling mask. Removal of the polymer mask left zigzag-shaped metallic nanostructure arrays in which nanogaps separated adjacent sharp edges. These structures displayed two distinct optical properties, depending on the direction along which the excitation beam was polarized (longitudinal and transverse modes) incident on the array. Furthermore, bidirectional anisotropic wetting was observed on the anisotropic polymer nanowall array surface.

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.

Durable plasmonic cap arrays on flexible substrate with real-time optical tunability for high-fidelity SERS devices.

Active tunable plasmonic cap arrays were fabricated on a flexible stretchable substrate using a combination of colloidal lithography, lift-up soft lithography, and subsequent electrostatic assembly of gold nanoparticles. The arrangement of the plasmonic caps could be tuned under external strain to deform the substrate in reversible. Real-time variation in the arrangement could be used to tune the optical properties and the electromagnetic field enhancement, thereby a proving a promising mechanism for optimizing the SERS sensitivity.

Multicolor patterning using holographic woodpile photonic crystals at visible wavelengths.

High-quality woodpile structures with photonic band gaps in the visible wavelength range were fabricated using a single prism holographic lithography technique. Woodpile structures were prepared to have a rod diameter of 160 nm, a lateral periodicity of 240 nm, and an axial periodicity of 300 nm, which is the smallest feature size for three-dimensional nanopatterned structures. A multicolor patterning method was used to vary the degree of crosslinking in the negative photoresist. These woodpile structures showed enhanced fluorescence due to a large surface area and the presence of bicontinuous interconnected pores.

Shape control of Ag nanostructures for practical SERS substrates.

Large-area, highly ordered, Ag-nanostructured arrays with various geometrical features were prepared for use as surface-enhanced Raman scattering (SERS)-active substrates by the self-assembly of inorganic particles on an SU-8 surface, followed by particle embedding and Ag vapor deposition. By adjusting the embedding time of the inorganic particles, the size of the Ag nanogap between the geometrically separated hole arrays and bowl-shaped arrays could be controlled in the range of 60 nm to 190 nm. More importantly, the SU-8 surface was covered with hexagonally ordered nanopillars, which were formed as a result of isotropic dry etching of the interstices, leading to triangular-shaped Ag plates on nanopillar arrays after Ag vapor deposition. The size and sharpness of the triangular Ag nanoplates and nanoscale roughness of the bottom surface were adjusted by controlling the etching time. The potential of the various Ag nanostructures for use as practical SERS substrates was verified by the detection of a low concentration of benzenethiol. Finite-difference time-domain (FDTD) methodology was used to demonstrate the SERS-activities of these highly controllable substrates by calculating the electric field intensity distribution on the metallic nanostructures. These substrates, with high sensitivity and simple shape-controllability, provide a practical SERS-based sensing platform.

Microfluidic synthesis of atto-liter scale double emulsions toward ultrafine hollow silica spheres with hierarchical pore networks.

A facile PDMS-glass hybrid microfluidic device is developed for generating uniform submicrometer-scale double emulsion droplets with unprecedented simplicity and controllability. Compared with planar flow-focusing geometries, our three-dimensional flow-focusing geometry is advantageous for stably producing femto- to atto-liter droplets without the retraction problem of the dispersed phase fluid. In addition, this microfluidic platform can withstand the use of strong organic solvents (e.g. tetrahydrofuran (THF) and toluene) as a dispersed phase without deforming PDMS devices because the dispersed phase containing organic solvents does not directly contact the PDMS wall. In particular, monodisperse double emulsions are generated spontaneously via the internal phase separation of single emulsions driven by the diffusion of a co-solvent (tetrahydrofuran) in microfluidic devices. Finally, we demonstrated that the double emulsions can be used as morphological templates of ultrafine spherical silica capsules with controlled hierarchical pore networks via the evaporation-induced self-assembly (EISA) method. During EISA, triblock copolymers (Pluronic F127) act as a surfactant barrier separating the internal droplet from the continuous oil phase, resulting in the 'inverse' morphology (i.e. hydrophobic polymer-in-water-in-oil emulsions). Depending on the precursor composition and kinetic condition, various structural and morphological features, such as mesoporous hollow silica spheres with a single central core, multi-cores, or a combination of these with robust controllability can be seen. Electron microscopy (SEM, STEM, HR-TEM), small angle X-ray scattering (SAXS), and N(2) adsorption-desorption confirm the well-controlled hierarchical pore structure of the resulting particles.

High-throughput optofluidic platforms for mosaicked microfibers toward multiplex analysis of biomolecules.

We describe high-throughput optofluidic platforms for mosaic-patterned microfibers by generating stratified laminar flows. An inert carrier liquid flow near PDMS channel walls conveyed a photopolymerizable liquid which permitted stable production of microfibers with particular morphologies and compositional patterns. Finally, mosaicked microfibers were prepared with desired configurations toward multiplex biomolecular analysis.

Flexible, angle-independent, structural color reflectors inspired by morpho butterfly wings.

Thin-film color reflectors inspired by Morpho butterflies are fabricated. Using a combination of directional deposition, silica microspheres with a wide size distribution, and a PDMS (polydimethylsiloxane) encasing, a large, flexible reflector is created that actually provides better angle-independent color characteristics than Morpho butterflies and which can even be bent and folded freely without losing its Morpho-mimetic photonic properties.