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Micromachines, Volume 10, Issue 10 (October 2019)

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Open AccessArticle
Microneedle Patterning of 3D Nonplanar Surfaces on Implantable Medical Devices Using Soft Lithography
Micromachines 2019, 10(10), 705; https://doi.org/10.3390/mi10100705 (registering DOI) - 16 Oct 2019
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Abstract
Micropatterning is often used to engineer the surface properties of objects because it allows the enhancement or modification of specific functionalities without modification of the bulk material properties. Microneedle arrays have been explored in the past for drug delivery and enhancement of tissue [...] Read more.
Micropatterning is often used to engineer the surface properties of objects because it allows the enhancement or modification of specific functionalities without modification of the bulk material properties. Microneedle arrays have been explored in the past for drug delivery and enhancement of tissue anchoring; however, conventional methods are primarily limited to thick, planar substrates. Here, we demonstrate a method for the fabrication of microneedle arrays on thin flexible polyurethane substrates. These thin-film microneedle arrays can be used to fabricate balloons and other inflatable objects. In addition, these thin-filmed microneedles can be transferred, using thermal forming processes, to more complex 3D objects on which it would otherwise be difficult to directly pattern microneedles. This function is especially useful for medical devices, which require effective tissue anchorage but are a challenging target for micropatterning due to their 3D nonplanar shape, large size, and the complexity of the required micropatterns. Ultrathin flexible thermoplastic polyurethane microneedle arrays were fabricated from a polydimethylsiloxane (PDMS) mold. The technique was applied onto the nonplanar surface of rapidly prototyped soft robotic implantable polyurethane devices. We found that a microneedle-patterned surface can increase the anchorage of the device to a tissue by more than twofold. In summary, our soft lithographic patterning method can rapidly and inexpensively generate thin-film microneedle surfaces that can be used to produce balloons or enhance the properties of other 3D objects and devices. Full article
(This article belongs to the Special Issue New Approaches to Micropatterning)
Open AccessArticle
Experimental Investigation on Rotating Electrochemical Etching of a Micro Spiral Cylindrical Electrode
Micromachines 2019, 10(10), 704; https://doi.org/10.3390/mi10100704 (registering DOI) - 16 Oct 2019
Viewed by 88
Abstract
To realize the electrochemical etching of a micro spiral cylindrical electrode, a new method of rotating electrochemical etching is proposed, and its process is further studied. First, according to the electrochemical etching principle, the machining mechanism of rotating electrochemical etching of a micro [...] Read more.
To realize the electrochemical etching of a micro spiral cylindrical electrode, a new method of rotating electrochemical etching is proposed, and its process is further studied. First, according to the electrochemical etching principle, the machining mechanism of rotating electrochemical etching of a micro spiral cylindrical electrode is introduced. Second, based on the spiral vortex theory in the Taylor-Couette system, the effect of the high-speed rotating cylindrical microelectrode on its external flow field is analyzed. Third, the effects of rotation direction, rotation speed, machining voltage, and machining time on the thread structure are analyzed by experiments. Finally, a spiral cylindrical microelectrode with good surface thread shape is fabricated within two minutes by using the optimized machining parameters. It is proved that the rotating electrochemical etching method is an easy way to fabricate a micro spiral cylindrical electrode with high efficiency and low cost. Full article
(This article belongs to the Section D:Materials and Processing)
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Open AccessArticle
Green Phosphors Based on 9,10-bis((4-((3,7-dimethyloctyl)oxy) phenyl) ethynyl) Anthracene for LED
Micromachines 2019, 10(10), 703; https://doi.org/10.3390/mi10100703 - 15 Oct 2019
Viewed by 144
Abstract
An anthracene aromatic unit was introduced into the phenylethynyl structure by a rigid acetylene linkage at the C-9 and C-10 positions via Sonogashira coupling reactions, resulting in a planar and straight-backbone molecule (9,10-bis((4-((3,7-dimethyloctyl)oxy) phenyl) ethynyl) anthracene) (BPEA). Thermogravimetric analysis demonstrated the good thermal [...] Read more.
An anthracene aromatic unit was introduced into the phenylethynyl structure by a rigid acetylene linkage at the C-9 and C-10 positions via Sonogashira coupling reactions, resulting in a planar and straight-backbone molecule (9,10-bis((4-((3,7-dimethyloctyl)oxy) phenyl) ethynyl) anthracene) (BPEA). Thermogravimetric analysis demonstrated the good thermal stability of the BPEA. Photoluminescence analysis showed that a suitable expanded π-conjugation in the BPEA made its excitation band extend into the visible region, and an intense green emission was observed under blue-light excitation. A bright green light-emitting diode with an efficiency of 18.22 lm/w was fabricated by coating the organic phosphor onto a 460 nm-emitting InGaN chip. All the results indicate that BPEA is a useful green-emitting material which is efficiently excited by blue light, and therefore, that it could be applied in many fields without UV radiation. Full article
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Open AccessArticle
A Cr Anti-Sticking Layer for Improving Mold Release Quality in Electrochemical Replication of PVC Optical Molds
Micromachines 2019, 10(10), 702; https://doi.org/10.3390/mi10100702 - 15 Oct 2019
Viewed by 98
Abstract
This paper addresses the issue of mold release quality in an electrochemical replication of an optical polyvinyl chloride (PVC) mold, which has microlens array or microprisms array on its surface. The main idea is to deposit a nanoscale Cr thin layer as an [...] Read more.
This paper addresses the issue of mold release quality in an electrochemical replication of an optical polyvinyl chloride (PVC) mold, which has microlens array or microprisms array on its surface. The main idea is to deposit a nanoscale Cr thin layer as an antisticking layer on the PVC mold surface, followed by Ag film deposition as the conductive layer using magnetron sputtering, and finally, a nickel layer is electrochemically deposited on the Ag surface. By doing so, the upripping of the nickel mold from the PVC mold becomes easier, resulting in better mold release quality. The experiment results showed that when the Cr antisticking layer was used, the release strength between the nickel mold and the PVC substrate reduced from 1.94 N/cm to 1.43 N/cm, the surface roughness of the PVC substrate after mold release reduced from 0.60 μm to 0.55 μm, the surface roughness of the nickel mold reduced from 0.63 μm to 0.49 μm, the retroreflection coefficient of the nickel mold increased from 1600 cd·lx−1·m−2 to 2100 cd·lx−1·m−2, and the surface energy of the PVC substrate reduced from 31.47 mN/m to 15.53 mN/m. Full article
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Open AccessArticle
A Frequency Up-Converted Hybrid Energy Harvester Using Transverse Impact-Driven Piezoelectric Bimorph for Human-Limb Motion
Micromachines 2019, 10(10), 701; https://doi.org/10.3390/mi10100701 - 15 Oct 2019
Viewed by 197
Abstract
Energy harvesting from human-body-induced motion is mostly challenging due to the low-frequency, high-amplitude nature of the motion, which makes the use of conventional cantilevered spring-mass oscillators unrealizable. Frequency up-conversion by mechanical impact is an effective way to overcome the challenge. However, direct impact [...] Read more.
Energy harvesting from human-body-induced motion is mostly challenging due to the low-frequency, high-amplitude nature of the motion, which makes the use of conventional cantilevered spring-mass oscillators unrealizable. Frequency up-conversion by mechanical impact is an effective way to overcome the challenge. However, direct impact on the transducer element (especially, piezoelectric) increases the risk of damaging it and raises questions on the reliability of the energy harvester. In order to overcome this shortcoming, we proposed a transverse mechanical impact driven frequency up-converted hybrid energy harvester for human-limb motion. It utilizes the integration of both piezoelectric and electromagnetic transducers in a given size that allows more energy to be harvested from a single mechanical motion, which, in turn, further improves the power density. While excited by human-limb motion, a freely-movable non-magnetic sphere exerts transverse impact by periodically sliding over a seismic mass attached to a double-clamped piezoelectric bimorph beam. This allows the beam to vibrate at its resonant frequency and generates power by means of the piezoelectric effect. A magnet attached to the beam also takes part in generating power by inducing voltage in a coil adjacent to it. A mathematical model has been developed and experimentally corroborated. At a periodic limb-motion of 5.2 Hz, maximum 93 µW and 61 µW average powers (overall 8 µW·cm−3 average power density) were generated by the piezoelectric and the electromagnetic transducers, respectively. Moreover, the prototype successfully demonstrated the application of low-power electronics via suitable AC-DC converters. Full article
(This article belongs to the Special Issue Piezoelectric Transducers: Materials, Devices and Applications)
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Open AccessReview
Methods of Delivering Mechanical Stimuli to Organ-on-a-Chip
Micromachines 2019, 10(10), 700; https://doi.org/10.3390/mi10100700 - 14 Oct 2019
Viewed by 164
Abstract
Recent advances in integrating microengineering and tissue engineering have enabled the creation of promising microengineered physiological models, known as organ-on-a-chip (OOC), for experimental medicine and pharmaceutical research. OOCs have been used to recapitulate the physiologically critical features of specific human tissues and organs [...] Read more.
Recent advances in integrating microengineering and tissue engineering have enabled the creation of promising microengineered physiological models, known as organ-on-a-chip (OOC), for experimental medicine and pharmaceutical research. OOCs have been used to recapitulate the physiologically critical features of specific human tissues and organs and their interactions. Application of chemical and mechanical stimuli is critical for tissue development and behavior, and they were also applied to OOC systems. Mechanical stimuli applied to tissues and organs are quite complex in vivo, which have not adequately recapitulated in OOCs. Due to the recent advancement of microengineering, more complicated and physiologically relevant mechanical stimuli are being introduced to OOC systems, and this is the right time to assess the published literature on this topic, especially focusing on the technical details of device design and equipment used. We first discuss the different types of mechanical stimuli applied to OOC systems: shear flow, compression, and stretch/strain. This is followed by the examples of mechanical stimuli-incorporated OOC systems. Finally, we discuss the potential OOC systems where various types of mechanical stimuli can be applied to a single OOC device, as a better, physiologically relevant recapitulation model, towards studying and evaluating experimental medicine, human disease modeling, drug development, and toxicology. Full article
(This article belongs to the Special Issue Organs-on-chips)
Open AccessArticle
InGaN/GaN Distributed Feedback Laser Diodes with Surface Gratings and Sidewall Gratings
Micromachines 2019, 10(10), 699; https://doi.org/10.3390/mi10100699 - 14 Oct 2019
Viewed by 114
Abstract
A variety of potential applications such as visible light communications require laser sources with a narrow linewidth and a single wavelength emission in the blue light region. The gallium nitride (GaN)-based distributed feedback laser diode (DFB-LD) is a promising light source that meets [...] Read more.
A variety of potential applications such as visible light communications require laser sources with a narrow linewidth and a single wavelength emission in the blue light region. The gallium nitride (GaN)-based distributed feedback laser diode (DFB-LD) is a promising light source that meets these requirements. Here, we present GaN DFB-LDs that share growth and fabrication processes and have surface gratings and sidewall gratings on the same epitaxial substrate, which makes LDs with different structures comparable. By electrical pulse pumping, single-peak emissions at 398.5 and 399.95 nm with a full width at half maximum (FWHM) of 0.32 and 0.23 nm were achieved, respectively. The surface and sidewall gratings were fabricated alongside the p-contact metal stripe by electrical beam lithography and inductively coupled plasma etching. DFB LDs with 2.5 µm ridge width exhibit a smaller FWHM than those with 5 and 10 µm ridge widths, indicating that the narrow ridge width is favorable for the narrowing of the line width of the DFB LD. The slope efficiency of the DFB LD with sidewall gratings is higher than that of surface grating DFB LDs with the same ridge width and period of gratings. Our experiment may provide a reliable and simple approach for optimizing gratings and GaN DFB-LDs. Full article
(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications)
Open AccessArticle
Integration of Hierarchical Micro-/Nanostructures in a Microfluidic Chip for Efficient and Selective Isolation of Rare Tumor Cells
Micromachines 2019, 10(10), 698; https://doi.org/10.3390/mi10100698 - 14 Oct 2019
Viewed by 106
Abstract
Circulating tumor cells (CTCs) are important clinical markers for both cancer early diagnosis and prognosis. Various techniques have been developed in the past decade to isolate and quantify these cells from the blood while microfluidic technology attracts significant attention due to better controlled [...] Read more.
Circulating tumor cells (CTCs) are important clinical markers for both cancer early diagnosis and prognosis. Various techniques have been developed in the past decade to isolate and quantify these cells from the blood while microfluidic technology attracts significant attention due to better controlled microenvironment. When combined with advanced nanotechnologies, CTC isolation performance in microfluidic devices can be further improved. In this article, by extending the wavy-herringbone concept developed earlier in our team, we prepared a hierarchical microfluidic chip by introducing a uniform coating of nanoparticles with anti-epithelial cell adhesion molecule (EpCAM) on wavy microgrooves. This hierarchical structured platform not only maintains the capture purity of the wavy-herringbone structure but improves the capture efficiency thanks to the larger surface area to volume ratio brought by nanoparticles. Our results demonstrated a capture efficiency of almost 100% at a low shear rate of 60/s. Even at a higher shear rate of 400/s, the hierarchical micro/nanostructures demonstrated an enhancement of up to ~3-fold for capture efficiency (i.e., 70%) and ~1.5-fold for capture purity (i.e., 68%), compared to wavy-herringbone structures without nanoparticle coating. With these promising results, this hierarchical structured platform represents a technological advancement for CTC isolation and cancer care. Full article
(This article belongs to the Special Issue Microfluidics-based Liquid Biopsies)
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Open AccessArticle
Development of a Class-C Power Amplifier with Diode Expander Architecture for Point-of-Care Ultrasound Systems
Micromachines 2019, 10(10), 697; https://doi.org/10.3390/mi10100697 - 14 Oct 2019
Viewed by 109
Abstract
Point-of-care ultrasound systems are widely used in ambulances and emergency rooms. However, the excessive heat generated from ultrasound transmitters has an impact on the implementation of piezoelectric transducer elements and on battery consumption, thereby affecting the system’s sensitivity and resolution. Non-linear power amplifiers, [...] Read more.
Point-of-care ultrasound systems are widely used in ambulances and emergency rooms. However, the excessive heat generated from ultrasound transmitters has an impact on the implementation of piezoelectric transducer elements and on battery consumption, thereby affecting the system’s sensitivity and resolution. Non-linear power amplifiers, such as class-C amplifiers, could substitute linear power amplifiers, such as class-A amplifiers, which are currently used in point-of-care ultrasound systems. However, class-C power amplifiers generate less output power, resulting in a reduction of system sensitivity. To overcome this issue, we propose a new diode expander architecture dedicated to power amplifiers to reduce the effects of sinusoidal pulses toward the power supply. Thus, the proposed architecture could increase the input pulse amplitudes applied to the main transistors in the power amplifiers, hence increasing the output voltage of such amplifiers. To verify the proposed concept, pulse-echo responses from an ultrasonic transducer were tested with the developed class-C power amplifier using a resistor divider and the designed diode expander architecture. The peak-to-peak amplitude of the echo signals of the ultrasonic transducers when using a class-C power amplifier with a diode expander architecture (2.98 Vp–p) was higher than that for the class-C power amplifier with a resistor divider architecture (2.51 Vp–p). Therefore, the proposed class-C power amplifier with diode expander architecture is a potential candidate for improving the sensitivity performance of piezoelectric transducers for point-of-care ultrasound systems. Full article
(This article belongs to the Special Issue Piezoelectric Transducers: Materials, Devices and Applications)
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Open AccessArticle
Mechanism of Unstable Material Removal Modes in Micro Cutting of Silicon Carbide
Micromachines 2019, 10(10), 696; https://doi.org/10.3390/mi10100696 - 13 Oct 2019
Viewed by 138
Abstract
This study conducts large-scale molecular dynamics (MD) simulations of micro cutting of single crystal 6H silicon carbide (SiC) with up to 19 million atoms to investigate the mechanism of unstable material removal modes within the transitional range of undeformed chip thickness in which [...] Read more.
This study conducts large-scale molecular dynamics (MD) simulations of micro cutting of single crystal 6H silicon carbide (SiC) with up to 19 million atoms to investigate the mechanism of unstable material removal modes within the transitional range of undeformed chip thickness in which either brittle or ductile mode of cutting might occur. Under this transitional range, cracks are always formed in the cutting zone, but the stress states cannot guarantee their propagation. The cutting mode is brittle when the cracks can propagate and otherwise ductile mode cutting happens. Plunge cutting experiment is conducted to produce a taper groove on a 6H SiC wafer. There is a transitional zone between the brittle-cut and ductile-cut regions, which has a mostly smooth surface with a few brittle craters on it. This study contributes to the understanding of the detailed process of brittle-ductile cutting mode transition (BDCMT) as it shows that a transitional range can occur even for single crystals without internal defects and provides guidance for the determination of tcritical from taper grooves made by various techniques, e.g., to adopt larger tcritical around the end of the transitional range to increase machining efficiency for grinding or turning as long as the cracks do not extend below the machined surface. Full article
(This article belongs to the Section D:Materials and Processing)
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Open AccessArticle
A Study on the Trimming Effects on the Quality Factor of Micro-Shell Resonators Vibrating in Wineglass Modes
Micromachines 2019, 10(10), 695; https://doi.org/10.3390/mi10100695 - 12 Oct 2019
Viewed by 138
Abstract
Frequency trimming based on mass and stiffness modification is an important post-fabrication process for micro-shell resonators (MSRs). However, the trimming effects on the quality factor are seldom studied, although they may have great influence on the performance of the resonator. This paper presents [...] Read more.
Frequency trimming based on mass and stiffness modification is an important post-fabrication process for micro-shell resonators (MSRs). However, the trimming effects on the quality factor are seldom studied, although they may have great influence on the performance of the resonator. This paper presents a study on the quality factor (Q-factor) variation of trimmed micro-shell resonators (MSR). Thermoelastic damping (QTED) and anchor loss (Qanchor) are found to be the dominant energy loss mechanisms resulting in the reduction of the overall Q-factor, according to finite element method (FEM). The effects of different trimming methods on QTED and Qanchor are studied here, respectively. It is found that trimming grooves ablated in the rim of the resonator can cause a ~1–10% reduction of QTED, and the length of trimming groove is positively related to the reduction of QTED. The reduction of QTED caused by the mass adding process is mainly related to the thermal expansion coefficient and density of the additive and contact area between the resonator and additive masses. Besides, the first and second harmonic errors caused by asymmetrical trimming can cause a 10–90% reduction of Qanchor. Finally, trimming experiments were conducted on different resonators and the results were compared with FEM simulation. The work presented in this paper could help to optimize the trimming process of MSRs. Full article
Open AccessArticle
Improved Output Power of GaN-based VCSEL with Band-Engineered Electron Blocking Layer
Micromachines 2019, 10(10), 694; https://doi.org/10.3390/mi10100694 - 12 Oct 2019
Viewed by 161
Abstract
The vertical-cavity surface-emitting laser (VCSEL) has unique advantages over the conventional edge-emitting laser and has recently attracted a lot of attention. However, the output power of GaN-based VCSEL is still low due to the large electron leakage caused by the built-in polarization at [...] Read more.
The vertical-cavity surface-emitting laser (VCSEL) has unique advantages over the conventional edge-emitting laser and has recently attracted a lot of attention. However, the output power of GaN-based VCSEL is still low due to the large electron leakage caused by the built-in polarization at the heterointerface within the device. In this paper, in order to improve the output power, a new structure of p-type composition-graded AlxGa1−xN electron blocking layer (EBL) is proposed in the VCSEL, by replacing the last quantum barrier (LQB) and EBL in the conventional structure. The simulation results show that the proposed EBL in the VCSEL suppresses the leaking electrons remarkably and contributes to a 70.6% increase of the output power, compared with the conventional GaN-based VCSEL. Full article
(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications)
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Open AccessArticle
Stability Study of an Electrothermally-Actuated MEMS Mirror with Al/SiO2 Bimorphs
Micromachines 2019, 10(10), 693; https://doi.org/10.3390/mi10100693 - 12 Oct 2019
Viewed by 139
Abstract
Electrothermal actuation is one of the main actuation mechanisms and has been employed to make scanning microelectromechanical systems (MEMS) mirrors with large scan range, high fill factor, and low driving voltage, but there exist long-term drifting issues in electrothermal bimorph actuators whose causes [...] Read more.
Electrothermal actuation is one of the main actuation mechanisms and has been employed to make scanning microelectromechanical systems (MEMS) mirrors with large scan range, high fill factor, and low driving voltage, but there exist long-term drifting issues in electrothermal bimorph actuators whose causes are not well understood. In this paper, the stability of an Al / SiO 2 bimorph electrothermal MEMS mirror operated in both static and dynamic scan mode has been studied. Particularly, the angular drifts of the MEMS mirror plate were measured over 90 h at different temperatures in the range of 50 150 °C. The experiments show that the temporal drift of the mirror plate orientation largely depends on the temperature of the electrothermal bimorph actuators. Interestingly, it is found that the angular drift changes from falling to rising as the temperature increases. An optimal operating temperature between 75 °C to 100 °C for the MEMS mirror is identified. At this temperature, the MEMS mirror exhibited stable scanning with an angular drift of less than 0.0001 °/h. Full article
(This article belongs to the Special Issue Optical MEMS, Volume II)
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Open AccessArticle
Implementation of Hand Gesture Recognition Device Applicable to Smart Watch Based on Flexible Epidermal Tactile Sensor Array
Micromachines 2019, 10(10), 692; https://doi.org/10.3390/mi10100692 - 12 Oct 2019
Viewed by 99
Abstract
Ever since the development of digital devices, the recognition of human gestures has played an important role in many Human-Computer interface applications. Various wearable devices have been developed, and inertial sensors, magnetic sensors, gyro sensors, electromyography, force-sensitive resistors, and other types of sensors [...] Read more.
Ever since the development of digital devices, the recognition of human gestures has played an important role in many Human-Computer interface applications. Various wearable devices have been developed, and inertial sensors, magnetic sensors, gyro sensors, electromyography, force-sensitive resistors, and other types of sensors have been used to identify gestures. However, there are different drawbacks for each sensor, which affect the detection of gestures. In this paper, we present a new gesture recognition method using a Flexible Epidermal Tactile Sensor based on strain gauges to sense deformation. Such deformations are transduced to electric signals. By measuring the electric signals, the sensor can estimate the degree of deformation, including compression, tension, and twist, caused by movements of the wrist. The proposed sensor array was demonstrated to be capable of analyzing the eight motions of the wrist, and showed robustness, stability, and repeatability throughout a range of experiments aimed at testing the sensor array. We compared the performance of the prototype device with those of previous studies, under the same experimental conditions. The result shows our recognition method significantly outperformed existing methods. Full article
(This article belongs to the Special Issue Tactile Sensing Technology and Systems)
Open AccessArticle
Cross-Flow Filtration of Escherichia coli at a Nanofluidic Gap for Fast Immobilization and Antibiotic Susceptibility Testing
Micromachines 2019, 10(10), 691; https://doi.org/10.3390/mi10100691 - 12 Oct 2019
Viewed by 216
Abstract
Infections with antimicrobial-resistant (AMR) bacteria are globally on the rise. In the future, multi-resistant infections will become one of the major problems in global health care. In order to enable reserve antibiotics to retain their effect as long as possible, broad-spectrum antibiotics must [...] Read more.
Infections with antimicrobial-resistant (AMR) bacteria are globally on the rise. In the future, multi-resistant infections will become one of the major problems in global health care. In order to enable reserve antibiotics to retain their effect as long as possible, broad-spectrum antibiotics must be used sparingly. This can be achieved by a rapid microfluidic phenotypic antibiotic susceptibility test, which provides the information needed for a targeted antibiotic therapy in less time than conventional tests. Such microfluidic tests must cope with a low bacteria concentration. On-chip filtering of the samples to accumulate bacteria can shorten the test time. By means of fluorescence microscopy, we examined a novel nanogap filtration principle to hold back Escherichia coli and to perform cultivation experiments with and without antibiotics present. Microfluidic chips based on the nanogap flow principle showed to be useful for the concentration and cultivation of E. coli. With a concentration of 106 cells/mL, a specific growth rate of 0.013 min−1 and a doubling time of 53 min were achieved. In the presence of an antibiotic, no growth was observed. The results prove that this principle can, in future, be used in fast and marker-free antimicrobial susceptibility testing (AST). Full article
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Open AccessArticle
High Lateral Breakdown Voltage in Thin Channel AlGaN/GaN High Electron Mobility Transistors on AlN/Sapphire Templates
Micromachines 2019, 10(10), 690; https://doi.org/10.3390/mi10100690 - 12 Oct 2019
Viewed by 136
Abstract
In this paper, we present the fabrication and Direct Current/high voltage characterizations of AlN-based thin and thick channel AlGaN/GaN heterostructures that are regrown by molecular beam epitaxy on AlN/sapphire. A very high lateral breakdown voltage above 10 kV was observed on the thin [...] Read more.
In this paper, we present the fabrication and Direct Current/high voltage characterizations of AlN-based thin and thick channel AlGaN/GaN heterostructures that are regrown by molecular beam epitaxy on AlN/sapphire. A very high lateral breakdown voltage above 10 kV was observed on the thin channel structure for large contact distances. Also, the buffer assessment revealed a remarkable breakdown field of 5 MV/cm for short contact distances, which is far beyond the theoretical limit of the GaN-based material system. The potential interest of the thin channel configuration in AlN-based high electron mobility transistors is confirmed by the much lower breakdown field that is obtained on the thick channel structure. Furthermore, fabricated transistors are fully functional on both structures with low leakage current, low on-resistance, and reduced temperature dependence as measured up to 300 °C. This is attributed to the ultra-wide bandgap AlN buffer, which is extremely promising for high power, high temperature future applications. Full article
(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications)
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Open AccessArticle
Selective Growth and Contact Gap-Fill of Low Resistivity Si via Microwave Plasma-Enhanced CVD
Micromachines 2019, 10(10), 689; https://doi.org/10.3390/mi10100689 - 12 Oct 2019
Viewed by 108
Abstract
Low resistivity polycrystalline Si could be selectively grown in the deep (~200 nm) and narrow patterns (~20 nm) of 20 nm pitch design rule DRAM (Dynamic Random Access Memory) by microwave plasma-enhanced chemical vapor deposition (MW-CVD). We were able to achieve the high [...] Read more.
Low resistivity polycrystalline Si could be selectively grown in the deep (~200 nm) and narrow patterns (~20 nm) of 20 nm pitch design rule DRAM (Dynamic Random Access Memory) by microwave plasma-enhanced chemical vapor deposition (MW-CVD). We were able to achieve the high phosphorus (CVD gap-fill in a large electrical contact area which does is affected by line pitch size) doping concentration (>2.5 × 1021 cm−3) and, thus, a low resistivity by adjusting source gas (SiH4, H2, PH3) decomposition through MW-CVD with a showerhead controlling the decomposition of source gases by using two different gas injection paths. In this study, a selective growth mechanism was applied by using the deposition/etch cyclic process to achieve the bottom–up process in the L-shaped contact, using H2 plasma that simultaneously promoted the deposition and the etch processes. Additionally, the cyclic selective growth technique was set up by controlling the SiH4 flow rate. The bottom-up process resulted in a uniform doping distribution, as well as an excellent filling capacity without seam and center void formation. Thus, low contact resistivity and higher transistor on-current could be achieved at a high and uniform phosphorus (P)-concentration. Compared to the conventional thermal, this method is expected to be a strong candidate for the complicated deep and narrow contact process. Full article
(This article belongs to the Special Issue NANO KOREA 2019)
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Open AccessArticle
A Cancer Spheroid Array Chip for Selecting Effective Drug
Micromachines 2019, 10(10), 688; https://doi.org/10.3390/mi10100688 - 12 Oct 2019
Viewed by 119
Abstract
A cancer spheroid array chip was developed by modifying a micropillar and microwell structure to improve the evaluation of drugs targeting specific mutations such as phosphor-epidermal growth factor receptor (p-EGFR). The chip encapsulated cells in alginate and allowed cancer cells to grow for [...] Read more.
A cancer spheroid array chip was developed by modifying a micropillar and microwell structure to improve the evaluation of drugs targeting specific mutations such as phosphor-epidermal growth factor receptor (p-EGFR). The chip encapsulated cells in alginate and allowed cancer cells to grow for over seven days to form cancer spheroids. However, reagents or media used to screen drugs in a high-density spheroid array had to be replaced very carefully, and this was a tedious task. Particularly, the immunostaining of cancer spheroids required numerous steps to replace many of the reagents used for drug evaluation. To solve this problem, we adapted a micropillar and microwell structure to a spheroid array. Thus, culturing cancer spheroids in alginate spots attached to the micropillar allowed us to replace the reagents in the microwell chip with a single fill of fresh medium, without damaging the cancer spheroids. In this study, a cancer spheroid array was made from a p-EGFR-overexpressing cell line (A549 lung cancer cell line). In a 12 by 36 column array chip (25 mm by 75 mm), the spheroid over 100 µm in diameter started to form at day seven and p-EGFR was also considerably overexpressed. The array was used for p-EGFR inhibition and cell viability measurement against seventy drugs, including ten EGFR-targeting drugs. By comparing drug response in the spheroid array (spheroid model) with that in the single-cell model, we demonstrated that the two models showed different responses and that the spheroid model might be more resistant to some drugs, thus narrowing the choice of drug candidates. Full article
(This article belongs to the Special Issue Organs-on-chips)
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Open AccessArticle
A Misalignment Optical Guiding Module for Power Generation Enhancement of Fixed-Type Photovoltaic Systems
Micromachines 2019, 10(10), 687; https://doi.org/10.3390/mi10100687 - 11 Oct 2019
Viewed by 150
Abstract
: This study presents a misalignment light-guiding module to increase the effectiveness of absorbing light. For a general fixed-type photovoltaic (PV) panel, the misalignment light decreases the efficiency of the system. A solar tracking system was installed for obtaining higher power generation. However, [...] Read more.
: This study presents a misalignment light-guiding module to increase the effectiveness of absorbing light. For a general fixed-type photovoltaic (PV) panel, the misalignment light decreases the efficiency of the system. A solar tracking system was installed for obtaining higher power generation. However, the cost of the PV system and maintenance was 5–10 times higher than the general type. In this study, this module is composed of an array of misalignment light-guiding units that consist of a non-axisymmetric compound parabolic curve (NACPC) and a freeform surface collimator. The NACPC efficiently collects the misalignment light within ±30° and guides the light to the collimator. The light has a better uniformity and smaller angle at the exit aperture. The simulation results show that the optical efficiency of the unit was above 70% when the misalignment angle was smaller than 20°. The experimental results show that the power generation of the light-guiding unit was 1.8 times higher than the naked PV panel. Full article
(This article belongs to the Special Issue Advanced MEMS/NEMS Technology, Volume II)
Open AccessArticle
Analysis of the Performance Variation Mechanism of MEMS Suspended Inductors under Mechanical Shock
Micromachines 2019, 10(10), 686; https://doi.org/10.3390/mi10100686 - 11 Oct 2019
Viewed by 153
Abstract
Micro-electromechanical system (MEMS) suspended inductors have been widely studied in recent decades because of their excellent radio frequency performance. However, the deformation of the inductor coil and the performance variation usually occur to the MEMS suspended inductors when the inductors are under mechanical [...] Read more.
Micro-electromechanical system (MEMS) suspended inductors have been widely studied in recent decades because of their excellent radio frequency performance. However, the deformation of the inductor coil and the performance variation usually occur to the MEMS suspended inductors when the inductors are under mechanical shock. Few studies have been carried out on the performance variation of MEMS suspended inductors under shock. In this study, the mechanism of the performance variation of MEMS suspended inductors under mechanical shock is analyzed by combining theoretical analysis and experiments. A theoretical analysis based on the lumped-element equivalent model is presented and shock tests are carried out. The shock tests show that the main reason of the MEMS suspended inductor performance variation after mechanical shock is the variation of the substrate parasitic effect, which is caused by the variation of the suspension height of the inductor after shock. The test results agree with the theoretical analysis. Full article
Open AccessArticle
A New Self-Activated Micropumping Mechanism Capable of Continuous-Flow and Real-Time PCR Amplification Inside 3D Spiral Microreactor
Micromachines 2019, 10(10), 685; https://doi.org/10.3390/mi10100685 - 11 Oct 2019
Viewed by 107
Abstract
A self-activated micropump which is capable of stable velocity transport for a liquid to flow a given distance inside a 3D microchannel has been a dream of microfluidic scientists for a long time. A new self-activated pumping mechanism has been proposed in this [...] Read more.
A self-activated micropump which is capable of stable velocity transport for a liquid to flow a given distance inside a 3D microchannel has been a dream of microfluidic scientists for a long time. A new self-activated pumping mechanism has been proposed in this paper. It is different from the authors’ previous research which relied on the fluid resistance of a quartz capillary tube or end-blocked gas-permeable silicone or a polydimethylsiloxane (PDMS) wall to automate the flow. In this research, an end-open stretched Teflon tube was utilized for passive transport for the first time. A new fluid transmission mode was adopted with the assistance of a cheaper easily accessible oil mixture to achieve stable continuous flow. Finally, this novel micropump has been applied to real-time continuous-flow polymerase chain reactions (PCRs), with an amplification efficiency similar to that of a commercial PCR cycler instrument. Full article
(This article belongs to the Special Issue Microfluidic for High-Throughput Screening)
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Open AccessArticle
Capillary Transport of Miniature Soft Ribbons
Micromachines 2019, 10(10), 684; https://doi.org/10.3390/mi10100684 - 11 Oct 2019
Viewed by 97
Abstract
Manipulation of soft miniature devices is important in the construction of soft robots, wearable devices, and biomedical devices. However, transport of soft miniature devices is still a challenging task, and few studies has been conducted on the subject. This paper reports a droplet-based [...] Read more.
Manipulation of soft miniature devices is important in the construction of soft robots, wearable devices, and biomedical devices. However, transport of soft miniature devices is still a challenging task, and few studies has been conducted on the subject. This paper reports a droplet-based micromanipulation method for transporting miniature soft ribbons. We show that soft ribbons can be successfully picked up and released to the target location using water droplets. We analyze the forces involved during the process numerically and investigate the influence of the width of the ribbon on the deformation. We verify that the deformation of a soft ribbon caused by elasto-capillary phenomena can be calculated using a well-known equation for calculating the deflection of a cantilever beam. The experimental and theoretical results show that the deformability of a soft miniature device during manipulation depends on its width. Full article
(This article belongs to the Special Issue Robotic Micromanipulation)
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Open AccessArticle
Evolutionary Computation for Parameter Extraction of Organic Thin-Film Transistors Using Newly Synthesized Liquid Crystalline Nickel Phthalocyanine
Micromachines 2019, 10(10), 683; https://doi.org/10.3390/mi10100683 - 10 Oct 2019
Viewed by 119
Abstract
In this work, the topic of the detrimental contact effects in organic thin-film transistors (OTFTs) is revisited. In this case, contact effects are considered as a tool to enhance the characterization procedures of OTFTs, achieving more accurate values for the fundamental parameters of [...] Read more.
In this work, the topic of the detrimental contact effects in organic thin-film transistors (OTFTs) is revisited. In this case, contact effects are considered as a tool to enhance the characterization procedures of OTFTs, achieving more accurate values for the fundamental parameters of the transistor threshold voltage, carrier mobility and on-off current ratio. The contact region is also seen as a fundamental part of the device which is sensitive to physical, chemical and fabrication variables. A compact model for OTFTs, which includes the effects of the contacts, and a recent proposal of an associated evolutionary parameter extraction procedure are reviewed. Both the model and the procedure are used to assess the effect of the annealing temperature on a nickel-1,4,8,11,15,18,22,25-octakis(hexyl)phthalocyanine (NiPc6)-based OTFT. A review of the importance of phthalocyanines in organic electronics is also provided. The characterization of the contact region in NiPc6 OTFTs complements the results extracted from other physical–chemical techniques such as differential scanning calorimetry or atomic force microscopy, in which the transition from crystal to columnar mesophase imposes a limit for the optimum performance of the annealed OTFTs. Full article
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Open AccessArticle
Enhanced Efficiencies of Perovskite Solar Cells by Incorporating Silver Nanowires into the Hole Transport Layer
Micromachines 2019, 10(10), 682; https://doi.org/10.3390/mi10100682 - 10 Oct 2019
Viewed by 125
Abstract
In this study, we incorporated silver nanowires (AgNWs) into poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as a hole transport layer (HTL) for inverted perovskite solar cells (PVSCs). The effect of AgNW incorporation on the perovskite crystallization, charge transfer, and power conversion efficiency (PCE) of PVSCs were [...] Read more.
In this study, we incorporated silver nanowires (AgNWs) into poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as a hole transport layer (HTL) for inverted perovskite solar cells (PVSCs). The effect of AgNW incorporation on the perovskite crystallization, charge transfer, and power conversion efficiency (PCE) of PVSCs were analyzed and discussed. Compared with neat PEDOT:PSS HTL, incorporation of few AgNWs into PEDOT:PSS can significantly enhance the PCE by 25%. However, the AgNW incorporation may result in performance overestimation due to the lateral charge transfer. The corrosion of AgNWs with a perovskite layer was discussed. Too much AgNW incorporation may lead to defects on the interface between the HTL and the perovskite layer. An extra PEDOT:PSS layer over the pristine PEDOT:PSS-AgNW layer can prevent AgNWs from corrosion by iodide ions. Full article
(This article belongs to the Special Issue Nanostructured Photovoltaic Devices)
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Open AccessArticle
Design of Wearable Headset with Steady State Visually Evoked Potential-Based Brain Computer Interface
Micromachines 2019, 10(10), 681; https://doi.org/10.3390/mi10100681 - 10 Oct 2019
Viewed by 112
Abstract
Brain–computer interface (BCI) is a system that allows people to communicate directly with external machines via recognizing brain activities without manual operation. However, for most current BCI systems, conventional electroencephalography (EEG) machines and computers are usually required to acquire EEG signal and translate [...] Read more.
Brain–computer interface (BCI) is a system that allows people to communicate directly with external machines via recognizing brain activities without manual operation. However, for most current BCI systems, conventional electroencephalography (EEG) machines and computers are usually required to acquire EEG signal and translate them into control commands, respectively. The sizes of the above machines are usually large, and this increases the limitation for daily applications. Moreover, conventional EEG electrodes also require conductive gels to improve the EEG signal quality. This causes discomfort and inconvenience of use, while the conductive gels may also encounter the problem of drying out during prolonged measurements. In order to improve the above issues, a wearable headset with steady-state visually evoked potential (SSVEP)-based BCI is proposed in this study. Active dry electrodes were designed and implemented to acquire a good EEG signal quality without conductive gels from the hairy site. The SSVEP BCI algorithm was also implemented into the designed field-programmable gate array (FPGA)-based BCI module to translate SSVEP signals into control commands in real time. Moreover, a commercial tablet was used as the visual stimulus device to provide graphic control icons. The whole system was designed as a wearable device to improve convenience of use in daily life, and it could acquire and translate EEG signal directly in the front-end headset. Finally, the performance of the proposed system was validated, and the results showed that it had excellent performance (information transfer rate = 36.08 bits/min). Full article
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Open AccessArticle
A Modified Duhem Model for Rate-Dependent Hysteresis Behaviors
Micromachines 2019, 10(10), 680; https://doi.org/10.3390/mi10100680 - 09 Oct 2019
Viewed by 197
Abstract
Hysteresis behaviors are inherent characteristics of piezoelectric ceramic actuators. The classical Duhem model (CDM) as a popular hysteresis model has been widely used, but cannot precisely describe rate-dependent hysteresis behaviors at high-frequency and high-amplitude excitations. To describe such behaviors more precisely, this paper [...] Read more.
Hysteresis behaviors are inherent characteristics of piezoelectric ceramic actuators. The classical Duhem model (CDM) as a popular hysteresis model has been widely used, but cannot precisely describe rate-dependent hysteresis behaviors at high-frequency and high-amplitude excitations. To describe such behaviors more precisely, this paper presents a modified Duhem model (MDM) by introducing trigonometric functions based on the analysis of the existing experimental data. The MDM parameters are also identified by using the nonlinear least squares method. Six groups of experiments with different frequencies or amplitudes are conducted to evaluate the MDM performance. The research results demonstrate that the MDM can more precisely characterize the rate-dependent hysteresis behaviors comparing with the CDM at high-frequency and high-amplitude excitations. Full article
(This article belongs to the Special Issue Piezoelectric Transducers: Materials, Devices and Applications)
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Open AccessArticle
Three-Dimensional Bioprinting of Functional Skeletal Muscle Tissue Using Gelatin Methacryloyl-Alginate Bioinks
Micromachines 2019, 10(10), 679; https://doi.org/10.3390/mi10100679 - 09 Oct 2019
Viewed by 294
Abstract
Skeletal muscle tissue engineering aims to fabricate tissue constructs to replace or restore diseased or injured skeletal muscle tissues in the body. Several biomaterials and microscale technologies have been used in muscle tissue engineering. However, it is still challenging to mimic the function [...] Read more.
Skeletal muscle tissue engineering aims to fabricate tissue constructs to replace or restore diseased or injured skeletal muscle tissues in the body. Several biomaterials and microscale technologies have been used in muscle tissue engineering. However, it is still challenging to mimic the function and structure of the native muscle tissues. Three-dimensional (3D) bioprinting is a powerful tool to mimic the hierarchical structure of native tissues. Here, 3D bioprinting was used to fabricate tissue constructs using gelatin methacryloyl (GelMA)-alginate bioinks. Mechanical and rheological properties of GelMA-alginate hydrogels were characterized. C2C12 myoblasts at the density 8 × 106 cells/mL were used as the cell model. The effects of alginate concentration (0, 6, and 8% (w/v)) and crosslinking mechanism (UV crosslinking or ionic crosslinking with UV crosslinking) on printability, cell viability, proliferation, and differentiation of bioinks were studied. The results showed that 10% (w/v) GelMA-8% (w/v) alginate crosslinked using UV light and 0.1 M CaCl2 provided the optimum niche to induce muscle tissue formation compared to other hydrogel compositions. Furthermore, metabolic activity of cells in GelMA bioinks was improved by addition of oxygen-generating particles to the bioinks. It is hoped that such bioprinted muscle tissues may find wide applications in drug screening and tissue regeneration. Full article
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Open AccessArticle
CO2 Laser Fabrication of PMMA Microfluidic Double T-Junction Device with Modified Inlet-Angle for Cost-Effective PCR Application
Micromachines 2019, 10(10), 678; https://doi.org/10.3390/mi10100678 - 09 Oct 2019
Viewed by 151
Abstract
The formation of uniform droplets and the control of their size, shape and monodispersity are of utmost importance in droplet-based microfluidic systems. The size of the droplets is precisely tuned by the channel geometry, the surface interfacial tension, the shear force and fluid [...] Read more.
The formation of uniform droplets and the control of their size, shape and monodispersity are of utmost importance in droplet-based microfluidic systems. The size of the droplets is precisely tuned by the channel geometry, the surface interfacial tension, the shear force and fluid velocity. In addition, the fabrication technique and selection of materials are essential to reduce the fabrication cost and time. In this paper, for reducing the fabrication cost Polymethyl methacrylate (PMMA) sheet is used with direct write laser technique by VERSA CO2 laser VLS3.5. This laser writing technique gives minimum channel width of about 160   μ m , which limit miniaturizing the droplet. To overcome this, modification on double T-junction (DTJ) channel geometry has been done by modifying the channel inlets angles. First, a two-dimensional (2D) simulation has been done to study the effect of the new channel geometry modification on droplet size, droplets distribution inside the channel, and its throughput. The fabricated modified DTJ gives the minimum droplet diameter of 39 ± 2   μ m , while DTJ channel produced droplet diameter of 48 ± 4   μ m at the same conditions. Moreover, the modified double T-junction (MDTJ) decreases the variation in droplets diameter at the same flow rates by 4.5 13 % than DTJ. This low variation in the droplet diameter is suitable for repeatability of the DNA detection results. The MDTJ also enhanced the droplet generation frequency by 8 25 % more than the DTJ channel. The uniformity of droplet distribution inside the channel was enhanced by 3 20 % compared to the DTJ channel geometry. This fabrication technique eliminates the need for a photomask and cleanroom environment in addition shortening the cost and time. It takes only 20   min for fabrication. The minimum generated droplet diameter is within 40   μ m with more than 1000 droplets per second (at 10   mL / h . oil flow rate). The device is a high-throughput and low-cost micro-droplet formation aimed to be as a front-end to a dynamic droplet digital PCR (ddPCR) platform for use in resource-limited environment. Full article
(This article belongs to the Special Issue Microfluidics for Soft Matter and Mechanobiology)
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Open AccessArticle
Tele–Robotic Platform for Dexterous Optical Single-Cell Manipulation
Micromachines 2019, 10(10), 677; https://doi.org/10.3390/mi10100677 - 08 Oct 2019
Viewed by 152
Abstract
Single-cell manipulation is considered a key technology in biomedical research. However, the lack of intuitive and effective systems makes this technology less accessible. We propose a new tele–robotic solution for dexterous cell manipulation through optical tweezers. A slave-device consists of a combination of [...] Read more.
Single-cell manipulation is considered a key technology in biomedical research. However, the lack of intuitive and effective systems makes this technology less accessible. We propose a new tele–robotic solution for dexterous cell manipulation through optical tweezers. A slave-device consists of a combination of robot-assisted stages and a high-speed multi-trap technique. It allows for the manipulation of more than 15 optical traps in a large workspace with nanometric resolution. A master-device (6+1 degree of freedom (DoF)) is employed to control the 3D position of optical traps in different arrangements for specific purposes. Precision and efficiency studies are carried out with trajectory control tasks. Three state-of-the-art experiments were performed to verify the efficiency of the proposed platform. First, the reliable 3D rotation of a cell is demonstrated. Secondly, a six-DoF teleoperated optical-robot is used to transport a cluster of cells. Finally, a single-cell is dexterously manipulated through an optical-robot with a fork end-effector. Results illustrate the capability to perform complex tasks in efficient and intuitive ways, opening possibilities for new biomedical applications. Full article
(This article belongs to the Special Issue Robotic Micromanipulation)
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Open AccessReview
Engineered Liver-On-A-Chip Platform to Mimic Liver Functions and Its Biomedical Applications: A Review
Micromachines 2019, 10(10), 676; https://doi.org/10.3390/mi10100676 - 07 Oct 2019
Viewed by 235
Abstract
Hepatology and drug development for liver diseases require in vitro liver models. Typical models include 2D planar primary hepatocytes, hepatocyte spheroids, hepatocyte organoids, and liver-on-a-chip. Liver-on-a-chip has emerged as the mainstream model for drug development because it recapitulates the liver microenvironment and has [...] Read more.
Hepatology and drug development for liver diseases require in vitro liver models. Typical models include 2D planar primary hepatocytes, hepatocyte spheroids, hepatocyte organoids, and liver-on-a-chip. Liver-on-a-chip has emerged as the mainstream model for drug development because it recapitulates the liver microenvironment and has good assay robustness such as reproducibility. Liver-on-a-chip with human primary cells can potentially correlate clinical testing. Liver-on-a-chip can not only predict drug hepatotoxicity and drug metabolism, but also connect other artificial organs on the chip for a human-on-a-chip, which can reflect the overall effect of a drug. Engineering an effective liver-on-a-chip device requires knowledge of multiple disciplines including chemistry, fluidic mechanics, cell biology, electrics, and optics. This review first introduces the physiological microenvironments in the liver, especially the cell composition and its specialized roles, and then summarizes the strategies to build a liver-on-a-chip via microfluidic technologies and its biomedical applications. In addition, the latest advancements of liver-on-a-chip technologies are discussed, which serve as a basis for further liver-on-a-chip research. Full article
(This article belongs to the Special Issue Organs-on-chips)
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