Microsphere-assisted fabrication of high aspect-ratio elastomeric micropillars and waveguides

Elastic Organic Crystals as Bioinspired Hair-Like Sensors

One of the typical haptic elements are natural hairy structures that animals and plants rely on for feedback. Although these hair sensors are an admirable inspiration, the development of active flow sensing components having low elastic moduli and high aspect ratios remains a challenge. Here, we report a new sensing approach based on a flexible, thin and optically transmissive organic crystal of high aspect ratio, which is stamped with fluorescent dye for tracking. When subjected to gas flow and exposed to laser, the crystal bends due to exerted pressure and acts as an optical flow (hair) sensor with low detection limit (≈1.578 m s^-1 ) and fast response time (≈2.70 s). The air-flow-induced crystal deformation and flow dynamics response are modelled by finite element analysis. Due to having a simple design and being lightweight and mechanically robust this prototypical crystal hair-like sensor opens prospects for a new class of sensing devices ranging from wearable electronics to aeronautics.

Modeling and analysis of nature-inspired branched micropillars for enhanced dynamic bio-sensing

Research evidence abounds on the effectiveness of micropillar-based microelectromechanical systems for the detection of a wide variety of ultrasmall biological objects for clinical and non-clinical applications. However, the standard micropillar-based sensing platforms rely on a single-column micropillar with a spot at the tip for binding of objects. Although this long-standing form has shown immense potential, performance improvement is hindered by the fundamental limits enforced by physical laws. Moreover, the single-column micropillar has a lower sensing area and is ill-suited for a simultaneous differential sensing of chemical/biological objects of different mass. Here, we report a new set of nature-inspired, branched micropillar-based sensing resonators to address the highlighted issues. The characteristics of the newly proposed branched micropillars are comprehensively examined with three payloads (Bartonella Bacilliformis, Escherichia coli, and Micro magnetic beads). Anchored on the capability of continuum theoretical framework, the mathematical model of the micropillar is formulated through the synthesis of the modified couple stress, the Rayleigh-Love, and the Timoshenko theories. The finite element method is employed to shed light on the variability of the structures' resonant response under performance reduction factors (payload's rotary inertia, damaged substrate, and density of a surrounding fluid). The results obtained indicate superior performance indicators for the triply-branched micropillar: enhanced response sensitivity for multiple payloads and less susceptibility to deterioration in resonant frequencies due to fluid immersion.

Potential of CO2-laser processing of quartz for fast prototyping of microfluidic reactors and templates for 3D cell assembly over large scale

Carbon dioxide (CO2)-laser processing of glasses is a versatile maskless writing technique to engrave micro-structures with flexible control on shape and size. In this study, we present the fabrication of hundreds of microns quartz micro-channels and micro-holes by pulsed CO2-laser ablation with a focus on the great potential of the technique in microfluidics and biomedical applications. After discussing the impact of the laser processing parameters on the design process, we illustrate specific applications. First, we demonstrate the use of a serpentine microfluidic reactor prepared by combining CO2-laser ablation and post-ablation wet etching to remove surface features stemming from laser-texturing that are undesirable for channel sealing. Then, cyclic olefin copolymer micro-pillars are fabricated using laser-processed micro-holes as molds with high detail replication. The hundreds of microns conical and square pyramidal shaped pillars are used as templates to drive 3D cell assembly. Human Umbilical Vein Endothelial Cells are found to assemble in a compact and wrapping way around the micro-pillars forming a tight junction network. These applications are interesting for both Lab-on-a-Chip and Organ-on-a-Chip devices.

In Situ Assembling of Glass Microspheres and Bonding Force Analysis by the Ultraviolet-Near-Infrared Dual-Beam Optical Tweezer System

Microresonators show great potential as interlayer routing solutions for multilayered three-dimensional (3D) photonic communication networks. New techniques are needed for the convenient and in situ manipulation and immobilization of glass microspheres into functional structures. Herein, near-infrared (NIR) and ultraviolet (UV) lasers were used as optical tweezers to precisely arrange silica microspheres and UV-initiated immobilization in a 3D space. The NIR laser was used to trap targeted microspheres, and the UV laser was focused to immobilize the trapped microspheres in 3-methacryloxypropyltrimethoxysilane (MOPS) in ∼6 s. Optical force spectroscopy was performed using the optical tweezers to measure individual bond strength. Next, functional triangular pedestals were designed to flexibly control the gap space for vertical router applications in 3D photonic networks. Thus, the designed UV-NIR dual-beam optical tweezer system can be used to assemble arbitrary functional 3D structures, making it a valuable tool for microfabrication, photonics, and optical communication applications.

Biomimetic Soft Polymer Microstructures and Piezoresistive Graphene MEMS Sensors Using Sacrificial Metal 3D Printing

Recent advances in 3D printing technology have enabled unprecedented design freedom across an ever-expanding portfolio of materials. However, direct 3D printing of soft polymeric materials such as polydimethylsiloxane (PDMS) is challenging, especially for structural complexities such as high-aspect ratio (>20) structures, 3D microfluidic channels (∼150 μm diameter), and biomimetic microstructures. This work presents a novel processing method entailing 3D printing of a thin-walled sacrificial metallic mold, soft polymer casting, and acidic etching of the mold. The proposed workflow enables the facile fabrication of various complex, bioinspired PDMS structures (e.g., 3D double helical microfluidic channels embedded inside high-aspect ratio pillars) that are difficult or impossible to fabricate using currently available techniques. The microfluidic channels are further infused with conductive graphene nanoplatelet ink to realize two flexible piezoresistive microelectromechanical (MEMS) sensors (a bioinspired flow/tactile sensor and a dome-like force sensor) with embedded sensing elements. The MEMS force sensor is integrated into a Philips 9000 series electric shaver to demonstrate its application in "smart" consumer products in the future. Aided by current trends in industrialization and miniaturization in metal 3D printing, the proposed workflow shows promise as a low-temperature, scalable, and cleanroom-free technique of fabricating complex, soft polymeric, biomimetic structures, and embedded MEMS sensors.

Rapid Design and Analysis of Microtube Pneumatic Actuators Using Line-Segment and Multi-Segment Euler-Bernoulli Beam Models

Soft material-based pneumatic microtube actuators are attracting intense interest, since their bending motion is potentially useful for the safe manipulation of delicate biological objects. To increase their utility in biomedicine, researchers have begun to apply shape-engineering to the microtubes to diversify their bending patterns. However, design and analysis of such microtube actuators are challenging in general, due to their continuum natures and small dimensions. In this paper, we establish two methods for rapid design, analysis, and optimization of such complex, shape-engineered microtube actuators that are based on the line-segment model and the multi-segment Euler–Bernoulli’s beam model, respectively, and are less computation-intensive than the more conventional method based on finite element analysis. To validate the models, we first realized multi-segment microtube actuators physically, then compared their experimentally observed motions against those obtained from the models. We obtained good agreements between the three sets of results with their maximum bending-angle errors falling within ±11%. In terms of computational efficiency, our models decreased the simulation time significantly, down to a few seconds, in contrast with the finite element analysis that sometimes can take hours. The models reported in this paper exhibit great potential for rapid and facile design and optimization of shape-engineered soft actuators.

Probing in-mouth texture perception with a biomimetic tongue

An experimental biomimetic tongue-palate system has been developed to probe human in-mouth texture perception. Model tongues are made from soft elastomers patterned with fibrillar structures analogous to human filiform papillae. The palate is represented by a rigid flat plate parallel to the plane of the tongue. To probe the behaviour under physiological flow conditions, deflections of model papillae are measured using a novel fluorescent imaging technique enabling sub-micrometre resolution of the displacements. Using optically transparent Newtonian liquids under steady shear flow, we show that deformations of the papillae allow their viscosity to be determined from 1 Pa s down to the viscosity of water (1 mPa s), in full quantitative agreement with a previously proposed model (Lauga et al. 2016 Front. Phys.4, 35 (doi:10.3389/fphy.2016.00035)). The technique is further validated for a shear-thinning and optically opaque dairy system.

Comprehensive analysis of efficient swimming using articulated legs fringed with flexible appendages inspired by a water beetle

Drag-based swimming is usually accompanied with the shape change of rowing appendages to generate asymmetric force during the power stroke and recovery stroke. To implement this in an aquatic robot, one may actively control the surface area of its legs during the swimming. However, a small sized robot with a limited number of actuators should adjust the surface area of legs in passive manner. For this reason, we proposed a novel articulated leg with flexible appendages inspired by a water beetle. These leg structures were designed to implement an efficient recovery stroke with less resistive force during the recovery stroke, while its surface area was increased again if suitable relaxation time was applied to perform improved power stroke. To identify an optimal leg design, 36 different types were fabricated by changing the passive joint thickness, appendage materials, length, and morphology. Several correlations and dominant parameters were identified, and it was shown that the swimming leg with fixed joint and appendage stiffness cannot always generate the largest torque in all the swimming frequency. Also, a two-dimensional dynamic model was proposed based on an underactuated manipulator, and the model validation was proceeded by comparing with two selected leg designs. In addition, a 5.5 cm long robot with one pair of legs was built to further investigate their locomotory performance. By varying the beating frequency and relaxation time, thorough analysis was addressed in terms of the position, velocity, non-dimensional traveled distance, Strouhal number, and quasi-propulsive efficiency. Here, some important relationships between dimensionless numbers were established. Furthermore, it was found that introducing a relaxation phase between the power stroke and recovery stroke can increase the traveled distance per stroke with slight expense of propulsive efficiency.

Micro-/Nanofluidics for Liquid-Mediated Patterning of Hybrid-Scale Material Structures

Various materials are fabricated to form specific structures/patterns at the micro-/nanoscale, which exhibit additional functions and performance. Recent liquid-mediated fabrication methods utilizing bottom-up approaches benefit from micro-/nanofluidic technologies that provide a high controllability for manipulating fluids containing various solutes, suspensions, and building blocks at the microscale and/or nanoscale. Here, the state-of-the-art micro-/nanofluidic approaches are discussed, which facilitate the liquid-mediated patterning of various hybrid-scale material structures, thereby showing many additional advantages in cost, labor, resolution, and throughput. Such systems are categorized here according to three representative forms defined by the degree of the free-fluid-fluid interface: free, semiconfined, and fully confined forms. The micro-/nanofluidic methods for each form are discussed, followed by recent examples of their applications. To close, the remaining issues and potential applications are summarized.

Polymeric biomaterials for biophotonic applications

With the growing importance of optical techniques in medical diagnosis and treatment, there exists a pressing need to develop and optimize materials platform for biophotonic applications. Particularly, the design of biocompatible and biodegradable materials with desired optical, mechanical, chemical, and biological properties is required to enable clinically relevant biophotonic devices for translating in vitro optical techniques into in situ and in vivo use. This technological trend propels the development of natural and synthetic polymeric biomaterials to replace traditional brittle, nondegradable silica glass based optical materials. In this review, we present an overview of the advances in polymeric optical material development, optical device design and fabrication techniques, and the accompanying applications to imaging, sensing and phototherapy.

Locomotion of arthropods in aquatic environment and their applications in robotics

Many bio-inspired robots have been developed so far after careful investigation of animals' locomotion. To successfully apply the locomotion of natural counterparts to robots for efficient and improved mobility, it is essential to understand their principles. Although a lot of research has studied either animals' locomotion or bio-inspired robots, there have only been a few attempts to broadly review both of them in a single article. Among the millions of animal species, this article reviewed various forms of aquatic locomotion in arthropods including relevant bio-inspired robots. Despite some previous robotics research inspired by aquatic arthropods, we found that many less-investigated or even unexplored areas are still present. Therefore, this article has been prepared to identify what types of new robotics research can be carried out after drawing inspiration from the aquatic locomotion of arthropods and to provide fruitful insights that may lead us to develop an agile and efficient aquatic robot.

Microdroplet-based On-Demand Drawing of High Aspect-Ratio Elastomeric Micropillar and Its Contact Sensing Application

High aspect-ratio elastomeric micropillars play important roles as the platform for microscale sensing and actuation. Many soft-lithographic techniques have been developed for their facile realization but most of the techniques are limited to build the micropillars only on totally flat, widely accessible substrate areas with the micropillar’s structural characteristics completely predetermined, leaving little room for in situ control. Here we demonstrate a new technique which overcomes these limitations by directly drawing micropillars from pipette-dispensed PDMS microdroplets using vacuum-chucked microspheres. The combined utilization of PDMS microdroplets and microspheres not only enables the realization of microsphere-tipped PDMS micropillars on non-flat, highly space-constrained substrate areas at in situ controllable heights but also allows arraying of micropillars with dissimilar heights at a close proximity. To validate the new technique’s utility and versatility, we realize PDMS micropillars on various unconventional substrate areas in various configurations. We also convert one of them, the optical fiber/micropillar hybrid, into a soft optical contact sensor. Both the fabrication technique and the resulting sensing scheme will be useful for future biomedical microsystems.

Measurement of near-wall 3D flow velocity from wave-guiding micro-pillars

The measurement of near-wall flow in a plane close to the wall is achieved using the wave-guiding feature of transparent flexible micro-pillars which are attached in a 2D array to a surface and bend with the flow. Optical detection of bending from below the surface and application of auto-correlation methods provide mean and fluctuating part of the components of the wall-parallel velocity components. In addition, the wall-normal fluid motion is determined from spatial gradients in the array. The data provide the three-component velocity vector field in a plane close to the wall as well as their statistics.

Chemical vapour deposition of zeolitic imidazolate framework thin films

Integrating metal-organic frameworks (MOFs) in microelectronics has disruptive potential because of the unique properties of these microporous crystalline materials. Suitable film deposition methods are crucial to leverage MOFs in this field. Conventional solvent-based procedures, typically adapted from powder preparation routes, are incompatible with nanofabrication because of corrosion and contamination risks. We demonstrate a chemical vapour deposition process (MOF-CVD) that enables high-quality films of ZIF-8, a prototypical MOF material, with a uniform and controlled thickness, even on high-aspect-ratio features. Furthermore, we demonstrate how MOF-CVD enables previously inaccessible routes such as lift-off patterning and depositing MOF films on fragile features. The compatibility of MOF-CVD with existing infrastructure, both in research and production facilities, will greatly facilitate MOF integration in microelectronics. MOF-CVD is the first vapour-phase deposition method for any type of microporous crystalline network solid and marks a milestone in processing such materials.

Synthetic microfluidic paper: high surface area and high porosity polymer micropillar arrays

We introduce Synthetic Microfluidic Paper, a novel porous material for microfluidic applications that consists of an OSTE polymer that is photostructured in a well-controlled geometry of slanted and interlocked micropillars. We demonstrate the distinct benefits of Synthetic Microfluidic Paper over other porous microfluidic materials, such as nitrocellulose, traditional paper and straight micropillar arrays: in contrast to straight micropillar arrays, the geometry of Synthetic Microfluidic Paper was miniaturized without suffering capillary collapse during manufacturing and fluidic operation, resulting in a six-fold increased internal surface area and a three-fold increased porous fraction. Compared to commercial nitrocellulose materials for capillary assays, Synthetic Microfluidic Paper shows a wider range of capillary pumping speed and four times lower device-to-device variation. Compared to the surfaces of the other porous microfluidic materials that are modified by adsorption, Synthetic Microfluidic Paper contains free thiol groups and has been shown to be suitable for covalent surface chemistry, demonstrated here for increasing the material hydrophilicity. These results illustrate the potential of Synthetic Microfluidic Paper as a porous microfluidic material with improved performance characteristics, especially for bioassay applications such as diagnostic tests.

Ultrasound-assisted recovery of free-standing high-aspect-ratio micropillars

High-aspect-ratio polymer micropillar arrays are widely employed in microfluidics and microdevices.

Microsphere integrated microfluidic disk: synergy of two techniques for rapid and ultrasensitive dengue detection

The application of microfluidic devices in diagnostic systems is well-established in contemporary research. Large specific surface area of microspheres, on the other hand, has secured an important position for their use in bioanalytical assays. Herein, we report a combination of microspheres and microfluidic disk in a unique hybrid platform for highly sensitive and selective detection of dengue virus. Surface engineered polymethacrylate microspheres with carefully designed functional groups facilitate biorecognition in a multitude manner. In order to maximize the utility of the microspheres' specific surface area in biomolecular interaction, the microfluidic disk was equipped with a micromixing system. The mixing mechanism (microballoon mixing) enhances the number of molecular encounters between spheres and target analyte by accessing the entire sample volume more effectively, which subsequently results in signal amplification. Significant reduction of incubation time along with considerable lower detection limits were the prime motivations for the integration of microspheres inside the microfluidic disk. Lengthy incubations of routine analytical assays were reduced from 2 hours to 5 minutes while developed system successfully detected a few units of dengue virus. Obtained results make this hybrid microsphere-microfluidic approach to dengue detection a promising avenue for early detection of this fatal illness.

Microrobotic tentacles with spiral bending capability based on shape-engineered elastomeric microtubes

Microscale soft-robots hold great promise as safe handlers of delicate micro-objects but their wider adoption requires micro-actuators with greater efficiency and ease-of-fabrication. Here we present an elastomeric microtube-based pneumatic actuator that can be extended into a microrobotic tentacle. We establish a new, direct peeling-based technique for building long and thin, highly deformable microtubes and a semi-analytical model for their shape-engineering. Using them in combination, we amplify the microtube’s pneumatically-driven bending into multi-turn inward spiraling. The resulting micro-tentacle exhibit spiraling with the final radius as small as ~185 μm and grabbing force of ~0.78 mN, rendering itself ideal for non-damaging manipulation of soft, fragile micro-objects. This spiraling tentacle-based grabbing modality, the direct peeling-enabled elastomeric microtube fabrication technique, and the concept of microtube shape-engineering are all unprecedented and will enrich the field of soft-robotics.

Bio-inspired flow sensor from printed PEDOT:PSS micro-hairs

This paper reports on the creation of a low-cost, disposable sensor for low flow velocities, constructed from extruded micro-sized 'hair' of conducting polymer PEDOT. These microstructures are inspired by hair strands found in many arthropods and chordates, which play a prime role in sensing air flows. The paper describes the fabrication techniques and the initial prototype testing results toward employing this sensing mechanism in applications requiring sensing of low flow rates such as a flow sensor in neonatal resuscitators. The fabricated 1000 μm long, 6 μm diameter micro-hairs mimic the bending movement of tactile hair strands to sense the velocity of air flow. The prototype sensor developed is a four-level direct digital-output sensor and is capable of detecting flow velocities of up to 0.97 m s(-1).