Micromachines

14127 KiB

Open AccessArticle

Droplet Microfluidics Approach for Single-DNA Molecule Amplification and Condensation into DNA-Magnesium-Pyrophosphate Particles

by Greta Zubaite, Karolis Simutis, Robertas Galinis, Valdemaras Milkus, Vaidotas Kiseliovas and Linas Mazutis

Micromachines 2017, 8(2), 62; https://doi.org/10.3390/mi8020062 - 20 Feb 2017

Cited by 14 | Viewed by 10347

Abstract

Protein expression in vitro has broad applications in directed evolution, synthetic biology, proteomics and drug screening. However, most of the in vitro expression systems rely on relatively high DNA template concentrations to obtain sufficient amounts of proteins, making it harder to perform in [...] Read more.

Protein expression in vitro has broad applications in directed evolution, synthetic biology, proteomics and drug screening. However, most of the in vitro expression systems rely on relatively high DNA template concentrations to obtain sufficient amounts of proteins, making it harder to perform in vitro screens on gene libraries. Here, we report a technique for the generation of condensed DNA particles that can serve as efficient templates for in vitro gene expression. We apply droplet microfluidics to encapsulate single-DNA molecules in 3-picoliter (pL) volume droplets and convert them into 1 μm-sized DNA particles by the multiple displacement amplification reaction driven by phi29 DNA polymerase. In the presence of magnesium ions and inorganic pyrophosphate, the amplified DNA condensed into the crystalline-like particles, making it possible to purify them from the reaction mix by simple centrifugation. Using purified DNA particles, we performed an in vitro transcription-translation reaction and successfully expressed complex enzyme β-galactosidase in droplets and in the 384-well format. The yield of protein obtained from DNA particles was significantly higher than from the corresponding amount of free DNA templates, thus opening new possibilities for high throughput screening applications. Full article

(This article belongs to the Special Issue Droplet Microfluidics: Techniques and Technologies, Volume II)

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Graphical abstract

1223 KiB

Open AccessErratum

Erratum: Hayakawa, M., et al. Influence of Asymmetry and Driving Forces on the Propulsion of Bubble-Propelled Catalytic Micromotors. Micromachines 2016, 7, 229

by Micromachines Editorial Office

Micromachines 2017, 8(2), 61; https://doi.org/10.3390/mi8020061 - 20 Feb 2017

Viewed by 3385

Abstract

n/a Full article

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3582 KiB

Open AccessArticle

Visible to Infrared Diamond Photonics Enabled by Focused Femtosecond Laser Pulses

by Belén Sotillo, Vibhav Bharadwaj, John Patrick Hadden, Stefano Rampini, Andrea Chiappini, Toney T. Fernandez, Cristina Armellini, Ali Serpengüzel, Maurizio Ferrari, Paul E. Barclay, Roberta Ramponi and Shane M. Eaton

Micromachines 2017, 8(2), 60; https://doi.org/10.3390/mi8020060 - 17 Feb 2017

Cited by 29 | Viewed by 8106

Abstract

Diamond’s nitrogen-vacancy (NV) centers show great promise in sensing applications and quantum computing due to their long electron spin coherence time and because they can be found, manipulated, and read out optically. An important step forward for diamond photonics would be connecting multiple [...] Read more.

Diamond’s nitrogen-vacancy (NV) centers show great promise in sensing applications and quantum computing due to their long electron spin coherence time and because they can be found, manipulated, and read out optically. An important step forward for diamond photonics would be connecting multiple diamond NVs together using optical waveguides. However, the inertness of diamond is a significant hurdle for the fabrication of integrated optics similar to those that revolutionized silicon photonics. In this work, we show the fabrication of optical waveguides in diamond, enabled by focused femtosecond high repetition rate laser pulses. By optimizing the geometry of the waveguide, we obtain single mode waveguides from the visible to the infrared. Additionally, we show the laser writing of individual NV centers within the bulk of diamond. We use µ-Raman spectroscopy to gain better insight on the stress and the refractive index profile of the optical waveguides. Using optically detected magnetic resonance and confocal photoluminescence characterization, high quality NV properties are observed in waveguides formed in various grades of diamond, making them promising for applications such as magnetometry, quantum information systems, and evanescent field sensors. Full article

(This article belongs to the Special Issue Laser Micromachining and Microfabrication)

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6173 KiB

Open AccessReview

Internal Laser Writing of High-Aspect-Ratio Microfluidic Structures in Silicate Glasses for Lab-on-a-Chip Applications

by Ya Cheng

Micromachines 2017, 8(2), 59; https://doi.org/10.3390/mi8020059 - 16 Feb 2017

Cited by 25 | Viewed by 7148

Abstract

Femtosecond laser direct writing is unique in allowing for fabrication of 3D micro- and nanofluidic structures, thereby enabling rapid and efficient manipulation of fluidic dynamics in 3D space to realize innovative functionalities. Here, I discuss the challenges in producing fully functional and highly [...] Read more.

Femtosecond laser direct writing is unique in allowing for fabrication of 3D micro- and nanofluidic structures, thereby enabling rapid and efficient manipulation of fluidic dynamics in 3D space to realize innovative functionalities. Here, I discuss the challenges in producing fully functional and highly integrated 3D micro- and nanofluidic systems with potential applications ranging from chemical and biological analyses to investigations of nanofluidic behaviors. In particular, I review the achievements we have made in the past decade, which have led to 3D microchannels with controllable cross-sectional profiles and large aspect ratios, 3D nanofluidic channels with widths of several tens of nanometers, and smooth inner walls with roughness on the order of ~1 nm. Integration of the microfluidics with other functional microcomponents including microoptics and microelectrodes will also be discussed, followed by conclusions and the future perspective. Full article

(This article belongs to the Special Issue Ultrafast Laser Fabrication for Lab-on-a-Chip)

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2853 KiB

Open AccessArticle

Optical Flow Cell for Measuring Size, Velocity and Composition of Flowing Droplets

by Sammer-ul Hassan, Adrian M. Nightingale and Xize Niu

Micromachines 2017, 8(2), 58; https://doi.org/10.3390/mi8020058 - 16 Feb 2017

Cited by 17 | Viewed by 6959

Abstract

Here an optical flow cell with two light paths is reported that can accurately quantify the size and velocity of droplets flowing through a microchannel. The flow cell can measure the time taken for droplets to pass between and through two conjoined light [...] Read more.

Here an optical flow cell with two light paths is reported that can accurately quantify the size and velocity of droplets flowing through a microchannel. The flow cell can measure the time taken for droplets to pass between and through two conjoined light paths, and thereby is capable of measuring the velocities (0.2–5.45 mm/s) and sizes of droplets (length > 0.8 mm). The composition of the droplet can also be accurately quantified via optical absorption measurements. The device has a small footprint and uses low-powered, low-cost components, which make it ideally suited for use in field-deployable and portable analytical devices. Full article

(This article belongs to the Special Issue Droplet Microfluidics: Techniques and Technologies, Volume II)

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4513 KiB

Open AccessArticle

Droplet Dynamics of Newtonian and Inelastic Non-Newtonian Fluids in Conﬁnement

by Nikolaos Ioannou, Haihu Liu, Mónica S. N. Oliveira and Yonghao Zhang

Micromachines 2017, 8(2), 57; https://doi.org/10.3390/mi8020057 - 15 Feb 2017

Cited by 10 | Viewed by 4977

Abstract

Microﬂuidic droplet technology has been developing rapidly. However, precise control of dynamical behaviour of droplets remains a major hurdle for new designs. This study is to understand droplet deformation and breakup under simple shear ﬂow in conﬁned environment as typically found in microﬂuidic [...] Read more.

Microﬂuidic droplet technology has been developing rapidly. However, precise control of dynamical behaviour of droplets remains a major hurdle for new designs. This study is to understand droplet deformation and breakup under simple shear ﬂow in conﬁned environment as typically found in microﬂuidic applications. In addition to the Newtonian–Newtonian system, we consider also both a Newtonian droplet in a non-Newtonian matrix ﬂuid and a non-Newtonian droplet in a Newtonian matrix. The lattice Boltzmann method is adopted to systematically investigate droplet deformation and breakup under a broad range of capillary numbers, viscosity ratios of the ﬂuids, and conﬁnement ratios considering shear-thinning and shear-thickening ﬂuids. Conﬁnement is found to enhance deformation, and the maximum deformation occurs at the viscosity ratio of unity. The droplet orients more towards the ﬂow direction with increasing viscosity ratio or conﬁnement ratio. In addition, it is noticed that the wall effect becomes more signiﬁcant for conﬁnement ratios larger than 0.4. Finally, for the whole range of Newtonian carrier ﬂuids tested, the critical capillary number above which droplet breakup occurs is only slightly affected by the conﬁnement ratio for a viscosity ratio of unity. Upon increasing the conﬁnement ratio, the critical capillary number increases for the viscosity ratios less than unity, but decreases for the viscosity ratios more than unity. Full article

(This article belongs to the Special Issue Droplet Microfluidics: Techniques and Technologies, Volume II)

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5982 KiB

Open AccessFeature PaperReview

Faraday Waves-Based Integrated Ultrasonic Micro-Droplet Generator and Applications

by Chen S. Tsai, Rong W. Mao, Shirley C. Tsai, Kaveh Shahverdi, Yun Zhu, Shih K. Lin, Yu-Hsiang Hsu, Gerry Boss, Matt Brenner, Sari Mahon and Gerald C. Smaldone

Micromachines 2017, 8(2), 56; https://doi.org/10.3390/mi8020056 - 14 Feb 2017

Cited by 13 | Viewed by 7869

Abstract

An in-depth review on a new ultrasonic micro-droplet generator which utilizes megahertz (MHz) Faraday waves excited by silicon-based multiple Fourier horn ultrasonic nozzles (MFHUNs) and its potential applications is presented. The new droplet generator has demonstrated capability for producing micro droplets of controllable [...] Read more.

An in-depth review on a new ultrasonic micro-droplet generator which utilizes megahertz (MHz) Faraday waves excited by silicon-based multiple Fourier horn ultrasonic nozzles (MFHUNs) and its potential applications is presented. The new droplet generator has demonstrated capability for producing micro droplets of controllable size and size distribution and desirable throughput at very low electrical drive power. For comparison, the serious deficiencies of current commercial droplet generators (nebulizers) and the other ultrasonic droplet generators explored in recent years are first discussed. The architecture, working principle, simulation, and design of the multiple Fourier horns (MFH) in resonance aimed at the amplified longitudinal vibration amplitude on the end face of nozzle tip, and the fabrication and characterization of the nozzles are then described in detail. Subsequently, a linear theory on the temporal instability of Faraday waves on a liquid layer resting on the planar end face of the MFHUN and the detailed experimental verifications are presented. The linear theory serves to elucidate the dynamics of droplet ejection from the free liquid surface and predict the vibration amplitude onset threshold for droplet ejection and the droplet diameters. A battery-run pocket-size clogging-free integrated micro droplet generator realized using the MFHUN is then described. The subsequent report on the successful nebulization of a variety of commercial pulmonary medicines against common diseases and on the experimental antidote solutions to cyanide poisoning using the new droplet generator serves to support its imminent application to inhalation drug delivery. Full article

(This article belongs to the Special Issue Droplet Microfluidics: Techniques and Technologies, Volume II)

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5196 KiB

Open AccessArticle

Ultrasonic Spray-Coating of Large-Scale TiO₂ Compact Layer for Efficient Flexible Perovskite Solar Cells

by Peng Zhou, Wangnan Li, Tianhui Li, Tongle Bu, Xueping Liu, Jing Li, Jiang He, Rui Chen, Kunpeng Li, Juan Zhao and Fuzhi Huang

Micromachines 2017, 8(2), 55; https://doi.org/10.3390/mi8020055 - 14 Feb 2017

Cited by 30 | Viewed by 8730

Abstract

Flexible electronics have attracted great interest in applications for the wearable devices. Flexible solar cells can be integrated into the flexible electronics as the power source for the wearable devices. In this work, an ultrasonic spray-coating method was employed to deposit TiO₂ [...] Read more.

Flexible electronics have attracted great interest in applications for the wearable devices. Flexible solar cells can be integrated into the flexible electronics as the power source for the wearable devices. In this work, an ultrasonic spray-coating method was employed to deposit TiO₂ nanoparticles on polymer substrates for the fabrication of flexible perovskite solar cells (PSCs). Pre-synthesized TiO₂ nanoparticles were first dispersed in ethanol to prepare the precursor solutions with different concentrations (0.5 mg/mL, 1.0 mg/mL, 2.0 mg/mL) and then sprayed onto the conductive substrates to produce compact TiO₂ films with different thicknesses (from 30 nm to 150 nm). The effect of the different drying processes on the quality of the compact TiO₂ film was studied. In order to further improve the film quality, titanium diisopropoxide bis(acetylacetonate) (TAA) was added into the TiO₂-ethanol solution at a mole ratio of 1.0 mol % with respect to the TiO₂ content. The final prepared PSC devices showed a power conversion efficiency (PCE) of 14.32% based on the indium doped tin oxide coated glass (ITO-glass) substrate and 10.87% on the indium doped tin oxide coated polyethylene naphthalate (ITO-PEN) flexible substrate. Full article

(This article belongs to the Special Issue Flexible and Stretchable Electronics)

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7639 KiB

Open AccessArticle

Performance Analysis of Double-Layer Microchannel Heat Sinks under Non-Uniform Heating Conditions with Random Hotspots

by Danish Ansari and Kwang-Yong Kim

Micromachines 2017, 8(2), 54; https://doi.org/10.3390/mi8020054 - 14 Feb 2017

Cited by 46 | Viewed by 7097

Abstract

Performance analysis of double-layer microchannel heat sinks was performed under non-uniform heating conditions having randomly distributed hotspots. Two parallel-channel (parallel-flow and counter-flow) and one cross-channel (transverse-flow) designs of double-layer heat sink were evaluated with three sets of heating schemes. Each set of heating [...] Read more.

Performance analysis of double-layer microchannel heat sinks was performed under non-uniform heating conditions having randomly distributed hotspots. Two parallel-channel (parallel-flow and counter-flow) and one cross-channel (transverse-flow) designs of double-layer heat sink were evaluated with three sets of heating schemes. Each set of heating scheme consisted of eleven randomly distributed hotspots generated by Latin hypercube sampling. The heat flux, area, and location of the hotspots were selected as the design parameters. Conjugate heat transfer analysis of the heat sinks was performed by solving three-dimensional Navier–Stokes and energy equations. Water with temperature-dependent properties was selected as the coolant. The thermal resistance, pressure drop, maximum temperature rise, and temperature variation among hotspots were evaluated for all the heat sinks. The transverse-flow microchannel heat sink exhibited the lowest thermal resistance, temperature rise and temperature variation among the hotspots throughout the specified range of flow rate. The lowest pressure drop was exhibited by the counter-flow heat sink. Full article

(This article belongs to the Special Issue Selected Papers from 2016 International Conference of Microfluidics, Nanofluidics and Lab-on-a-Chip)

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1207 KiB

Open AccessReview

Micro-Hole Drilling on Glass Substrates—A Review

by Lucas A. Hof and Jana Abou Ziki

Micromachines 2017, 8(2), 53; https://doi.org/10.3390/mi8020053 - 13 Feb 2017

Cited by 97 | Viewed by 12880

Abstract

Glass micromachining is currently becoming essential for the fabrication of micro-devices, including micro- optical-electro-mechanical-systems (MOEMS), miniaturized total analysis systems (μTAS) and microfluidic devices for biosensing. Moreover, glass is radio frequency (RF) transparent, making it an excellent material for sensor and energy transmission devices. [...] Read more.

Glass micromachining is currently becoming essential for the fabrication of micro-devices, including micro- optical-electro-mechanical-systems (MOEMS), miniaturized total analysis systems (μTAS) and microfluidic devices for biosensing. Moreover, glass is radio frequency (RF) transparent, making it an excellent material for sensor and energy transmission devices. Advancements are constantly being made in this field, yet machining smooth through-glass vias (TGVs) with high aspect ratio remains challenging due to poor glass machinability. As TGVs are required for several micro-devices, intensive research is being carried out on numerous glass micromachining technologies. This paper reviews established and emerging technologies for glass micro-hole drilling, describing their principles of operation and characteristics, and their advantages and disadvantages. These technologies are sorted into four machining categories: mechanical, thermal, chemical, and hybrid machining (which combines several machining methods). Achieved features by these methods are summarized in a table and presented in two graphs. We believe that this paper will be a valuable resource for researchers working in the field of glass micromachining as it provides a comprehensive review of the different glass micromachining technologies. It will be a useful guide for advancing these techniques and establishing new hybrid ones, especially since this is the first broad review in this field. Full article

(This article belongs to the Special Issue Glass Micromachining)

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4558 KiB

Open AccessArticle

Direct Silver Micro Circuit Patterning on Transparent Polyethylene Terephthalate Film Using Laser-Induced Photothermochemical Synthesis

by Chen-Jui Lan, Song-Ling Tsai and Ming-Tsang Lee

Micromachines 2017, 8(2), 52; https://doi.org/10.3390/mi8020052 - 13 Feb 2017

Cited by 5 | Viewed by 6597

Abstract

This study presents a new and improved approach to the rapid and green fabrication of highly conductive microscale silver structures on low-cost transparent polyethylene terephthalate (PET) flexible substrate. In this new laser direct synthesis and pattering (LDSP) process, silver microstructures are simultaneously synthesized [...] Read more.

This study presents a new and improved approach to the rapid and green fabrication of highly conductive microscale silver structures on low-cost transparent polyethylene terephthalate (PET) flexible substrate. In this new laser direct synthesis and pattering (LDSP) process, silver microstructures are simultaneously synthesized and laid down in a predetermined pattern using a low power continuous wave (CW) laser. The silver ion processing solution, which is transparent and reactive, contains a red azo dye as the absorbing material. The silver pattern is formed by photothermochemical reduction of the silver ions induced by the focused CW laser beam. In this improved LDSP process, the non-toxic additive in the transparent ionic solution absorbs energy from a low cost CW visible laser without the need for the introduction of any hazardous chemical process. Tests were carried out to determine the durability of the conductive patterns, and numerical analyses of the thermal and fluid transport were performed to investigate the morphology of the deposited patterns. This technology is an advanced method for preparing micro-scale circuitry on an inexpensive, flexible, and transparent polymer substrate that is fast, environmentally benign, and shows potential for Roll-to-Roll manufacture. Full article

(This article belongs to the Special Issue Flexible and Stretchable Electronics)

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11797 KiB

Open AccessArticle

Development of MEMS Multi-Mode Electrostatic Energy Harvester Based on the SOI Process

by Bongwon Jeong, Min-Ook Kim, Jae-Ik Lee, Youngkee Eun, Jungwook Choi and Jongbaeg Kim

Micromachines 2017, 8(2), 51; https://doi.org/10.3390/mi8020051 - 13 Feb 2017

Cited by 19 | Viewed by 5411

Abstract

Multi-vibrational-mode electrostatic energy harvesters are designed and micro-machined utilizing a simple silicon-on-insulator (SOI) wafer-based process. Enhanced adaptability to various vibrational environments is achieved in the proposed design by using serpentine springs attached to the fishbone-shaped inertial mass. The experimental results show that the [...] Read more.

Multi-vibrational-mode electrostatic energy harvesters are designed and micro-machined utilizing a simple silicon-on-insulator (SOI) wafer-based process. Enhanced adaptability to various vibrational environments is achieved in the proposed design by using serpentine springs attached to the fishbone-shaped inertial mass. The experimental results show that the developed device could convert an input vibration of 6 g at 1272 Hz to 2.96, 3.28, and 2.30 μW for different vibrational directions of 0°, 30°, and 45° with respect to a reference direction, respectively, when all serpentine springs are identical. An alternative device design using serpentine springs with different stiffnesses between x- and y-axes exhibited resonance frequencies at 1059 and 1635 Hz for an input vibrational direction of 45° and acceleration amplitude of 4 g, successfully generating 0.723 and 0.927 μW of electrical power at each resonance, respectively. Full article

(This article belongs to the Special Issue MEMS Energy Harvesters)

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4722 KiB

Open AccessFeature PaperArticle

The Effects of Profile Errors of Microlens Surfaces on Laser Beam Homogenization

by Axiu Cao, Hui Pang, Jiazhou Wang, Man Zhang, Jian Chen, Lifang Shi, Qiling Deng and Song Hu

Micromachines 2017, 8(2), 50; https://doi.org/10.3390/mi8020050 - 13 Feb 2017

Cited by 11 | Viewed by 4783

Abstract

Microlens arrays (MLAs) are key optical components in laser beam homogenization. However, due to imperfect surface profiles resulting from microfabrication, the functionalities of MLAs in beam modulation could be compromised to some extent. In order to address this issue, the effects of surface [...] Read more.

Microlens arrays (MLAs) are key optical components in laser beam homogenization. However, due to imperfect surface profiles resulting from microfabrication, the functionalities of MLAs in beam modulation could be compromised to some extent. In order to address this issue, the effects of surface profile mismatches between ideal and fabricated MLAs on beam homogenization were analyzed. Four types of surface profile errors of MLAs were modeled theoretically and numerical simulations were conducted to quantitatively estimate the effects of these profile errors on beam homogenization. In addition, experiments were conducted to validate the simulation results, revealing that profile errors leading to optical deviations located on the apex of microlenses affected beam homogenization less than deviations located further away from it. This study can provide references for the further applications of MLAs in beam homogenization. Full article

(This article belongs to the Special Issue Micro/Nano Manufacturing)

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5844 KiB

Open AccessArticle

Analysis of Liquid–Liquid Droplets Fission and Encapsulation in Single/Two Layer Microfluidic Devices Fabricated by Xurographic Method

by Chang Nong Lim, Kai Seng Koh, Yong Ren, Jit Kai Chin, Yong Shi and Yuying Yan

Micromachines 2017, 8(2), 49; https://doi.org/10.3390/mi8020049 - 10 Feb 2017

Cited by 23 | Viewed by 7082

Abstract

This paper demonstrates a low cost fabrication approach for microscale droplet fission and encapsulation. Using a modified xurography method, rapid yet reliable microfluidic devices with flexible designs (single layer and double layer) are developed to enable spatial control of droplet manipulation. In this [...] Read more.

This paper demonstrates a low cost fabrication approach for microscale droplet fission and encapsulation. Using a modified xurography method, rapid yet reliable microfluidic devices with flexible designs (single layer and double layer) are developed to enable spatial control of droplet manipulation. In this paper, two different designs are demonstrated, i.e., droplet fission (single layer) and droplet encapsulation (double layer). In addition, the current fabrication approach reduces the overall production interval with the introduction of a custom-made polydimethylsiloxane (PDMS) aligner. Apart from that, the fabricated device is able to generate daughter droplets with the coefficient of variance (CV) below 5% and double emulsions with CV maintained within 10% without involvement of complex surface wettability modification. Full article

(This article belongs to the Special Issue Insights and Advancements in Microfluidics)

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789 KiB

Open AccessArticle

Laser Fabrication of Miniature Internal Thread in Glass Substrate

by Hiroyuki Degawa, Noriaki Urano and Shigeki Matsuo

Micromachines 2017, 8(2), 48; https://doi.org/10.3390/mi8020048 - 8 Feb 2017

Cited by 7 | Viewed by 5674

Abstract

Miniature internal threads (tapped holes) of S0.5 were fabricated in a glass substrate. Water-assisted laser drilling was applied for fabrication of the threads of S0.5 standard using a subnanosecond laser as a light source. The landscape of the inner surface of the threads [...] Read more.

Miniature internal threads (tapped holes) of S0.5 were fabricated in a glass substrate. Water-assisted laser drilling was applied for fabrication of the threads of S0.5 standard using a subnanosecond laser as a light source. The landscape of the inner surface of the threads was measured by a laser microscope, and showed reasonable agreement with the desired standard. As a proof of concept, a commercial external screw was fitted to the fabricated internal thread. Full article

(This article belongs to the Special Issue Ultrafast Laser Fabrication for Lab-on-a-Chip)

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4504 KiB

Open AccessArticle

Fabrication of Graphene Aerogels with Heavily Loaded Metallic Nanoparticles

by Chen Shen, Elizabeth Barrios, Matthew McInnis, Joseph Zuyus and Lei Zhai

Micromachines 2017, 8(2), 47; https://doi.org/10.3390/mi8020047 - 7 Feb 2017

Cited by 13 | Viewed by 6400

Abstract

Natural biomaterials with hierarchical structures that enable extraordinary capability of detecting chemicals have inspired the interest in producing materials that can mimic these natural structures. This study reports the fabrication of hierarchically-structured, reduced graphene oxide (rGO) aerogels with heavily loaded palladium (Pd), platinum [...] Read more.

Natural biomaterials with hierarchical structures that enable extraordinary capability of detecting chemicals have inspired the interest in producing materials that can mimic these natural structures. This study reports the fabrication of hierarchically-structured, reduced graphene oxide (rGO) aerogels with heavily loaded palladium (Pd), platinum (Pt), nickel (Ni), and tin (Sn) metallic nanoparticles. Metal salts chelated with ethylenediaminetetraacetic acid (EDTA) were mixed with graphene oxide (GO) and then freeze-dried. The subsequent reduction produces rGO/metal nanoparticle aerogels. SEM and EDS results indicated that a loading of 59, 67, 39, and 46 wt % of Pd, Pt, Ni, and Sn nanoparticles was achieved. Pd/rGO aerogels of different Pd nanoparticle concentrations were exposed to H₂ gas to monitor the resistance change of the composites. The results suggest that rGO aerogels can achieve a higher nanoparticle loading by using chelation to minimize electrostatic interactions between metal ions and GO. Higher loading of Pd nanoparticles in graphene aerogels lead to improved hydrogen gas sensing performance. Full article

(This article belongs to the Special Issue Bio-Inspired Micro/Nano Devices and Systems)

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4371 KiB

Open AccessArticle

On-Surface Locomotion of Particle Based Microrobots Using Magnetically Induced Oscillation

by U Kei Cheang, Jamel Ali, Hoyeon Kim, Louis Rogowski and Min Jun Kim

Micromachines 2017, 8(2), 46; https://doi.org/10.3390/mi8020046 - 4 Feb 2017

Cited by 10 | Viewed by 6907

Abstract

The low Reynolds number condition presents a fundamental constraint on designing locomotive mechanisms for microscale robots. We report on the use of an oscillating magnetic field to induce on-surface translational motion of particle based microrobots. The particle based microrobots consist of microparticles, connected [...] Read more.

The low Reynolds number condition presents a fundamental constraint on designing locomotive mechanisms for microscale robots. We report on the use of an oscillating magnetic field to induce on-surface translational motion of particle based microrobots. The particle based microrobots consist of microparticles, connected in a chain-like manner using magnetic self-assembly, where the non-rigid connections between the particles provide structural flexibility for the microrobots. Following the scallop theorem, the oscillation of flexible bodies can lead to locomotion at low Reynolds numbers, similar to the beating motion of sperm flagella. We characterized the velocity profiles of the microrobots by measuring their velocities at various oscillating frequencies. We also demonstrated the directional steering capabilities of the microrobots. This work will provide insights into the use of oscillation as a viable mode of locomotion for particle based microrobots near a surface. Full article

(This article belongs to the Special Issue Bio-Inspired Micro/Nano Devices and Systems)

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3609 KiB

Open AccessArticle

Electrophoretic Concentration and Electrical Lysis of Bacteria in a Microfluidic Device Using a Nanoporous Membrane

by Md. Shehadul Islam, Ali Shahid, Kacper Kuryllo, Yingfu Li, M. Jamal Deen and P. Ravi Selvaganapathy

Micromachines 2017, 8(2), 45; https://doi.org/10.3390/mi8020045 - 3 Feb 2017

Cited by 22 | Viewed by 7054

Abstract

Pathogenic bacteria such as Escherichia coli O157, Salmonella and Campylobacter are the main causes for food and waterborne illnesses. Lysis of these bacteria is an important component of the sample preparation for molecular identification of these pathogens. The pathogenicity of these bacteria [...] Read more.

Pathogenic bacteria such as Escherichia coli O157, Salmonella and Campylobacter are the main causes for food and waterborne illnesses. Lysis of these bacteria is an important component of the sample preparation for molecular identification of these pathogens. The pathogenicity of these bacteria is so high that they cause illness at very low concentrations (1–10 CFU/100 mL). Hence, there is a need to develop methods to collect a small number of such bacterial cells from a large sample volume and process them in an automated reagent-free manner. An electrical method to concentrate the bacteria and lyse them has been chosen here as it is reagent free and hence more conducive for online and automated sample preparation. We use commercially available nanoporous membranes sandwiched between two microfluidic channels to create thousands of parallel nanopore traps for bacteria, electrophoretically accumulate and then lyse them. The nanopores produce a high local electric field for lysis at moderate applied voltages, which could simplify instrumentation and enables lysis of the bacteria as it approaches them under an appropriate range of electric field (>1000 V/cm). Accumulation and lysis of bacteria on the nanoporous membrane is demonstrated by using the LIVE/DEAD BacLight Bacterial Viability Kit and quantified by fluorescence intensity measurements. The efficiency of the device was determined through bacterial culture of the lysate and was found to be 90% when a potential of 300 V was applied for 3 min. Full article

(This article belongs to the Special Issue Application of Microfluidic Methodology for the Analysis of DNA)

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3367 KiB

Open AccessArticle

Suspended Graphene-Based Gas Sensor with 1-mW Energy Consumption

by Jong-Hyun Kim, Qin Zhou and Jiyoung Chang

Micromachines 2017, 8(2), 44; https://doi.org/10.3390/mi8020044 - 1 Feb 2017

Cited by 15 | Viewed by 5961

Abstract

This paper presents NH3 sensing with ultra-low energy consumption for fast recovery and a graphene sheet based on a suspended microheater. Sensitivity and repeatability are important characteristics of functional gas sensors embedded in mobile devices. Moreover, low energy consumption is an essential requirement [...] Read more.

This paper presents NH3 sensing with ultra-low energy consumption for fast recovery and a graphene sheet based on a suspended microheater. Sensitivity and repeatability are important characteristics of functional gas sensors embedded in mobile devices. Moreover, low energy consumption is an essential requirement in flexible and stretchable mobile electronics due to their small dimension and fluctuating resistivity during mechanical behavior. In this paper, we introduce a graphene-based ultra-low power gas detection device with integration of a suspended silicon heater. Dramatic power reduction is enabled by a duty cycle while not sacrificing sensitivity. The new oscillation method of heating improves the sensitivity of 0.049 (ΔR/R0) measured at a flow rate of 18.8 sccm NH3(g) for 70 s. Our experimental tests show that a 60% duty cycle does not sacrifice sensitivity or recovery by dropping the total power consumption from 1.76 mW to 1.05 mW. The aforementioned low energy consuming gas sensor platform not only attracts environmentally-related industries, but also has the potential to be applied to flexible and stretchable mobile electronic devices. Full article

(This article belongs to the Special Issue Flexible and Stretchable Electronics)

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3684 KiB

Open AccessArticle

Wrinkled Graphene–AgNWs Hybrid Electrodes for Smart Window

by Ki-Woo Jun, Jong-Nam Kim, Jin-Young Jung and Il-Kwon Oh

Micromachines 2017, 8(2), 43; https://doi.org/10.3390/mi8020043 - 1 Feb 2017

Cited by 13 | Viewed by 6422

Abstract

Over the past few years, there has been an increasing demand for stretchable electrodes for flexible and soft electronic devices. An electrode in such devices requires special functionalities to be twisted, bent, stretched, and deformed into variable shapes and also will need to [...] Read more.

Over the past few years, there has been an increasing demand for stretchable electrodes for flexible and soft electronic devices. An electrode in such devices requires special functionalities to be twisted, bent, stretched, and deformed into variable shapes and also will need to have the capacity to be restored to the original state. In this study, we report uni- or bi-axially wrinkled graphene–silver nanowire hybrid electrodes comprised of chemical vapor deposition (CVD)-grown graphene and silver nanowires. A CVD-grown graphene on a Cu-foil was transferred onto a biaxially pre-strained elastomer substrate and silver nanowires were sprayed on the transferred graphene surface. The pre-strained film was relaxed uni-(or bi-)axially to produce a wrinkled structure. The bi-axially wrinkled graphene and silver nanowires hybrid electrodes were very suitable for high actuating performance of electro-active dielectric elastomers compared with the wrinkle-free case. Present results show that the optical transparency of the highly stretchable electrode can be successfully tuned by modulating input voltages. Full article

(This article belongs to the Special Issue Flexible and Stretchable Electronics)

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4415 KiB

Open AccessArticle

Z-Axis Micromachined Tuning Fork Gyroscope with Low Air Damping

by Minh Ngoc Nguyen, Nhat Sinh Ha, Long Quang Nguyen, Hoang Manh Chu and Hung Ngoc Vu

Micromachines 2017, 8(2), 42; https://doi.org/10.3390/mi8020042 - 1 Feb 2017

Cited by 18 | Viewed by 6804

Abstract

This paper reports on the design and fabrication of a z-axis tuning fork gyroscope which has a freestanding architecture. In order to improve the performance of the tuning fork gyroscope by eliminating the influence of the squeeze-film air damping, the driving and sensing [...] Read more.

This paper reports on the design and fabrication of a z-axis tuning fork gyroscope which has a freestanding architecture. In order to improve the performance of the tuning fork gyroscope by eliminating the influence of the squeeze-film air damping, the driving and sensing parts of the gyroscope were designed to oscillate in-plane. Furthermore, by removing the substrate underneath the device, the slide-ﬁlm air damping in the gap between the proof masses and the substrate was eliminated. The proposed architecture was analyzed by the finite element method using ANSYS software. The simulated frequencies of the driving and sensing modes were 9.788 and 9.761 kHz, respectively. The gyroscope was fabricated using bulk micromachining technology. The quality factor and sensitivity of the gyroscope operating in atmospheric conditions were measured to be 111.2 and 11.56 mV/°/s, respectively. Full article

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5097 KiB

Open AccessFeature PaperArticle

Design, Manufacture and Testing of Capacitive Pressure Sensors for Low-Pressure Measurement Ranges

by Vasileios Mitrakos, Lisa Macintyre, Fiona C. Denison, Philip J.W. Hands and Marc P.Y. Desmulliez

Micromachines 2017, 8(2), 41; https://doi.org/10.3390/mi8020041 - 1 Feb 2017

Cited by 50 | Viewed by 9295

Abstract

This article presents the design, manufacture and testing of a capacitive pressure sensor with a high, tunable performance to low compressive loads (<10 kPa) and a resolution of less than 0.5 kPa. Such a performance is required for the monitoring of treatment efficacy [...] Read more.

This article presents the design, manufacture and testing of a capacitive pressure sensor with a high, tunable performance to low compressive loads (<10 kPa) and a resolution of less than 0.5 kPa. Such a performance is required for the monitoring of treatment efficacy delivered by compression garments to treat or prevent medical conditions such as deep vein thrombosis, leg ulcers, varicose veins or hypertrophic scars. Current commercial sensors used in such medical applications have been found to be either impractical, costly or of insufficient resolution. A microstructured elastomer film of a polydimethylsiloxane (PDMS) blend with a tunable Young’s modulus was used as the force-sensing dielectric medium. The resulting 18 mm × 18 mm parallel-plate capacitive pressure sensor was characterised in the range of 0.8 to 6.5 kPa. The microstructuring of the surface morphology of the elastomer film combined with the tuning of the Young’s modulus of the PDMS blend is demonstrated to enhance the sensor performance achieving a 0.25 kPa pressure resolution and a 10 pF capacitive change under 6.5 kPa compressive load. The resulting sensor holds good potential for the targeted medical application. Full article

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2757 KiB

Open AccessReview

Ultrafast Laser Fabrication of Functional Biochips: New Avenues for Exploring 3D Micro- and Nano-Environments

by Felix Sima, Jian Xu, Dong Wu and Koji Sugioka

Micromachines 2017, 8(2), 40; https://doi.org/10.3390/mi8020040 - 28 Jan 2017

Cited by 21 | Viewed by 8388

Abstract

Lab-on-a-chip biological platforms have been intensively developed during the last decade since emerging technologies have offered possibilities to manufacture reliable devices with increased spatial resolution and 3D configurations. These biochips permit testing chemical reactions with nanoliter volumes, enhanced sensitivity in analysis and reduced [...] Read more.

Lab-on-a-chip biological platforms have been intensively developed during the last decade since emerging technologies have offered possibilities to manufacture reliable devices with increased spatial resolution and 3D configurations. These biochips permit testing chemical reactions with nanoliter volumes, enhanced sensitivity in analysis and reduced consumption of reagents. Due to the high peak intensity that allows multiphoton absorption, ultrafast lasers can induce local modifications inside transparent materials with high precision at micro- and nanoscale. Subtractive manufacturing based on laser internal modification followed by wet chemical etching can directly fabricate 3D micro-channels in glass materials. On the other hand, additive laser manufacturing by two-photon polymerization of photoresists can grow 3D polymeric micro- and nanostructures with specific properties for biomedical use. Both transparent materials are ideal candidates for biochips that allow exploring phenomena at cellular levels while their processing with a nanoscale resolution represents an excellent opportunity to get more insights on biological aspects. We will review herein the laser fabrication of transparent microfluidic and optofluidic devices for biochip applications and will address challenges associated with their potential. In particular, integrated micro- and optofluidic systems will be presented with emphasis on the functionality for biological applications. It will be shown that ultrafast laser processing is not only an instrument that can tailor appropriate 3D environments to study living microorganisms and to improve cell detection or sorting but also a tool to fabricate appropriate biomimetic structures for complex cellular analyses. New advances open now the avenue to construct miniaturized organs of desired shapes and configurations with the goal to reproduce life processes and bypass in vivo animal or human testing. Full article

(This article belongs to the Special Issue Ultrafast Laser Fabrication for Lab-on-a-Chip)

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17059 KiB

Open AccessArticle

A Microfluidic Chip for Liquid Metal Droplet Generation and Sorting

by Lu Tian, Meng Gao and Lin Gui

Micromachines 2017, 8(2), 39; https://doi.org/10.3390/mi8020039 - 27 Jan 2017

Cited by 33 | Viewed by 8257

Abstract

A liquid metal based microfluidic system was proposed and demonstrated for the generation and sorting of liquid metal droplets. This micro system utilized silicon oil as the continuous phase and Ga₆₆In_20.5Sn_13.5 (66.0 wt % Ga, 20.5 wt % [...] Read more.

A liquid metal based microfluidic system was proposed and demonstrated for the generation and sorting of liquid metal droplets. This micro system utilized silicon oil as the continuous phase and Ga₆₆In_20.5Sn_13.5 (66.0 wt % Ga, 20.5 wt % In, 13.5 wt % Sn, melting point: 10.6 °C) as the dispersed phase to generate liquid metal droplets on a three-channel F-junction generator. The F-junction is an updated design similar to the classical T-junction, which has a special branch channel added to a T-junction for the supplement of 30 wt % aqueous NaOH solution. To perform active sorting of liquid metal droplets by dielectrophoresis (DEP), the micro system utilized liquid-metal-filled microchannels as noncontact electrodes to induce electrical fields through the droplet channel. The electrode channels were symmetrically located on both sides of the droplet channel in the same horizontal level. According to the results, the micro system can generate uniformly spherical liquid metal droplets, and control the flow direction of the liquid metal droplets. To better understand the control mechanism, a numerical simulation of the electrical field was performed in detail in this work. Full article

(This article belongs to the Special Issue Selected Papers from 2016 International Conference of Microfluidics, Nanofluidics and Lab-on-a-Chip)

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7319 KiB

Open AccessArticle

Thermoplastic Micromodel Investigation of Two-Phase Flows in a Fractured Porous Medium

by Shao-Yiu Hsu, Zhong-Yao Zhang and Chia-Wen Tsao

Micromachines 2017, 8(2), 38; https://doi.org/10.3390/mi8020038 - 26 Jan 2017

Cited by 14 | Viewed by 6493

Abstract

In the past few years, micromodels have become a useful tool for visualizing flow phenomena in porous media with pore structures, e.g., the multifluid dynamics in soils or rocks with fractures in natural geomaterials. Micromodels fabricated using glass or silicon substrates incur high [...] Read more.

In the past few years, micromodels have become a useful tool for visualizing flow phenomena in porous media with pore structures, e.g., the multifluid dynamics in soils or rocks with fractures in natural geomaterials. Micromodels fabricated using glass or silicon substrates incur high material cost; in particular, the microfabrication-facility cost for making a glass or silicon-based micromold is usually high. This may be an obstacle for researchers investigating the two-phase-flow behavior of porous media. A rigid thermoplastic material is a preferable polymer material for microfluidic models because of its high resistance to infiltration and deformation. In this study, cyclic olefin copolymer (COC) was selected as the substrate for the micromodel because of its excellent chemical, optical, and mechanical properties. A delicate micromodel with a complex pore geometry that represents a two-dimensional (2D) cross-section profile of a fractured rock in a natural oil or groundwater reservoir was developed for two-phase-flow experiments. Using an optical visualization system, we visualized the flow behavior in the micromodel during the processes of imbibition and drainage. The results show that the flow resistance in the main channel (fracture) with a large radius was higher than that in the surrounding area with small pore channels when the injection or extraction rates were low. When we increased the flow rates, the extraction efficiency of the water and oil in the mainstream channel (fracture) did not increase monotonically because of the complex two-phase-flow dynamics. These findings provide a new mechanism of residual trapping in porous media. Full article

(This article belongs to the Special Issue Insights and Advancements in Microfluidics)

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1460 KiB

Open AccessArticle

A Rapid Magnetofluidic Micromixer Using Diluted Ferrofluid

by Majid Hejazian and Nam-Trung Nguyen

Micromachines 2017, 8(2), 37; https://doi.org/10.3390/mi8020037 - 25 Jan 2017

Cited by 46 | Viewed by 7160

Abstract

Effective and rapid mixing is essential for various chemical and biological assays. The present work describes a simple and low-cost micromixer based on magnetofluidic actuation. The device takes advantage of magnetoconvective secondary flow, a bulk flow induced by an external magnetic field, for [...] Read more.

Effective and rapid mixing is essential for various chemical and biological assays. The present work describes a simple and low-cost micromixer based on magnetofluidic actuation. The device takes advantage of magnetoconvective secondary flow, a bulk flow induced by an external magnetic field, for mixing. A superparamagnetic stream of diluted ferrofluid and a non-magnetic stream are introduced to a straight microchannel. A permanent magnet placed next to the microchannel induced a non-uniform magnetic field. The magnetic field gradient and the mismatch in magnetic susceptibility between the two streams create a body force, which leads to rapid and efficient mixing. The micromixer reported here could achieve a high throughput and a high mixing efficiency of 88% in a relatively short microchannel. Full article

(This article belongs to the Collection Lab-on-a-Chip)

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4219 KiB

Open AccessArticle

Microfluidic Cell Cycle Analysis of Spread Cells by DAPI Staining

by Jing Sun, Jiayu Zhang, Haibo Yang, Gongzhuo Wang, Yanzhao Li, Xuxin Zhang, Qidan Chen and Ming-Fei Lang

Micromachines 2017, 8(2), 36; https://doi.org/10.3390/mi8020036 - 24 Jan 2017

Cited by 9 | Viewed by 8508

Abstract

Single-cell cell cycle analysis is an emerging technique that requires detailed exploration of the image analysis process. In this study, we established a microfluidic single-cell cell cycle analysis method that can analyze cells in small numbers and in situ on a microfluidic chip. [...] Read more.

Single-cell cell cycle analysis is an emerging technique that requires detailed exploration of the image analysis process. In this study, we established a microfluidic single-cell cell cycle analysis method that can analyze cells in small numbers and in situ on a microfluidic chip. In addition, factors that influenced the analysis were carefully investigated. U87 or HeLa cells were seeded and attached to microfluidic channels before measurement. Cell nucleic DNA was imaged by 4′-6-diamidino-2-phenylindole (DAPI) staining under a fluorescent microscope and subsequently fluorescent intensities of the cell nuclei DNA were converted to depict histograms for cell cycle phases. DAPI concentration, microscopic magnification, exposure time and cell number were examined for optimal cell cycle analysis conditions. The results showed that as few as a few hundred cells could be measured by DAPI staining in the range of 0.4–0.6 μg/mL to depict histograms with typical cell cycle phase distribution. Microscopic magnification during image acquisition, however, could distort the phase distribution. Exposure time did not significantly affect the cell cycle analysis. Furthermore, cell cycle inhibitor rapamycin treatment changed the cell cycle phase distribution as expected. In conclusion, a method for microfluidic single-cell cell cycle analysis of spread cells in situ was developed. Factors such as dye concentration and microscopic magnification had more influence on cell cycle phase distribution. Further studies will focus on detail differentiation of cell cycle phases and the application of such a method for biological meanings. Full article

(This article belongs to the Special Issue Selected Papers from 2016 International Conference of Microfluidics, Nanofluidics and Lab-on-a-Chip)

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2858 KiB

Open AccessFeature PaperArticle

Magnetically Driven Micromachines Created by Two-Photon Microfabrication and Selective Electroless Magnetite Plating for Lab-on-a-Chip Applications

by Tommaso Zandrini, Shuhei Taniguchi and Shoji Maruo

Micromachines 2017, 8(2), 35; https://doi.org/10.3390/mi8020035 - 24 Jan 2017

Cited by 23 | Viewed by 6415

Abstract

We propose a novel method to fabricate three-dimensional magnetic microparts, which can be integrated in functional microfluidic networks and lab-on-a-chip devices, by the combination of two-photon microfabrication and selective electroless plating. In our experiments, magnetic microparts could be successfully fabricated by optimizing various [...] Read more.

We propose a novel method to fabricate three-dimensional magnetic microparts, which can be integrated in functional microfluidic networks and lab-on-a-chip devices, by the combination of two-photon microfabrication and selective electroless plating. In our experiments, magnetic microparts could be successfully fabricated by optimizing various experimental conditions of electroless plating. In addition, energy dispersive X-ray spectrometry (EDS) clarified that iron oxide nanoparticles were deposited onto the polymeric microstructure site-selectively. We also fabricated magnetic microrotors which could smoothly rotate using common laboratory equipment. Since such magnetic microparts can be remotely driven with an external magnetic field, our fabrication process can be applied to functional lab-on-a-chip devices for analytical and biological applications. Full article

(This article belongs to the Special Issue Ultrafast Laser Fabrication for Lab-on-a-Chip)

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2174 KiB

Open AccessArticle

Dynamic Electroosmotic Flows of Power-Law Fluids in Rectangular Microchannels

by Cunlu Zhao, Wenyao Zhang and Chun Yang

Micromachines 2017, 8(2), 34; https://doi.org/10.3390/mi8020034 - 24 Jan 2017

Cited by 28 | Viewed by 5304

Abstract

Dynamic characteristics of electroosmosis of a typical non-Newtonian liquid in a rectangular microchannel are investigated by using numerical simulations. The non-Newtonian behavior of liquids is assumed to obey the famous power-law model and then the mathematical model is solved numerically by using the [...] Read more.

Dynamic characteristics of electroosmosis of a typical non-Newtonian liquid in a rectangular microchannel are investigated by using numerical simulations. The non-Newtonian behavior of liquids is assumed to obey the famous power-law model and then the mathematical model is solved numerically by using the finite element method. The results indicate that the non-Newtonian effect produces some noticeable dynamic responses in electroosmotic flow. Under a direct current (DC) driving electric field, it is found that the fluid responds more inertly to an external electric field and the steady-state velocity profile becomes more plug-like as the flow behavior index decreases. Under an alternating current (AC) driving electric field, the fluid is observed to experience more significant acceleration and the amplitude of oscillating velocity becomes larger as the fluid behavior index decreases. Furthermore, our investigation also shows that electroosmotic flow of power-law fluids under an AC/DC combined driving field is enhanced as compared with that under a pure DC electric field. These dynamic predictions are of practical use for the design of electroosmotically-driven microfluidic devices that analyze and process non-Newtonian fluids such as biofluids and polymeric solutions. Full article

(This article belongs to the Special Issue Insights and Advancements in Microfluidics)

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3522 KiB

Open AccessArticle

A PZT Actuated Triple-Finger Gripper for Multi-Target Micromanipulation

by Tao Chen, Yaqiong Wang, Zhan Yang, Huicong Liu, Jinyong Liu and Lining Sun

Micromachines 2017, 8(2), 33; https://doi.org/10.3390/mi8020033 - 24 Jan 2017

Cited by 33 | Viewed by 6005

Abstract

This paper presents a triple-finger gripper driven by a piezoceramic (PZT) transducer for multi-target micromanipulation. The gripper consists of three fingers assembled on adjustable pedestals with flexible hinges for a large adjustable range. Each finger has a PZT actuator, an amplifying structure, and [...] Read more.

This paper presents a triple-finger gripper driven by a piezoceramic (PZT) transducer for multi-target micromanipulation. The gripper consists of three fingers assembled on adjustable pedestals with flexible hinges for a large adjustable range. Each finger has a PZT actuator, an amplifying structure, and a changeable end effector. The moving trajectories of single and double fingers were calculated and finite element analyses were performed to verify the reliability of the structures. In the gripping experiment, various end effectors of the fingers such as tungsten probes and fibers were tested, and different micro-objects such as glass hollow spheres and iron spheres with diameters ranging from 10 to 800 μm were picked and released. The output resolution is 145 nm/V, and the driven displacement range of the gripper is 43.4 μm. The PZT actuated triple-finger gripper has superior adaptability, high efficiency, and a low cost. Full article

(This article belongs to the Special Issue Piezoelectric MEMS)

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Micromachines, Volume 8, Issue 2 (February 2017) – 32 articles

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