High Aspect Ratio Micro-features Research Articles

Improving assay feasibility and biocompatibility of 3D cyclic olefin copolymer microwells by superhydrophilic modification via ultrasonic spray deposition of polyvinyl alcohol

Sample partitioning is a crucial step towards digitization of biological assays on polymer microfluidic platforms. However, effective liquid filling into microwells and long-term hydrophilicity remain a challenge in polymeric microfluidic devices, impeding the applicability in diagnostic and cell culture studies. To overcome this, a method to produce permanent superhydrophilic 3-dimensional microwells using cyclic olefin copolymer (COC) microfluidic chips is presented. The COC substrate is oxidized using UV treatment followed by ultrasonic spray coating of polyvinyl alcohol solution, offering uniform and long-term coating of high-aspect ratio microfeatures. The coated COC surfaces are UV-cured before bonding with a hydrophobic pressure-sensitive adhesive to drive selective filling into the wells. The surface hydrophilicity achieved using this method remains unchanged (water contact angle of 9°) for up to 6 months and the modified surface is characterized for physical (contact angle & surface energy, morphology, integrity of microfeatures and roughness), chemical composition (FTIR, Raman spectroscopy) and coating stability (pH, temperature, time). To establish the feasibility of the modified surface in biological applications, PVA-coated COC microfluidic chips are tested for DNA sensing (digital LAMP detection of CMV), and biocompatibility through protein adsorption and cell culture studies (cell adhesion, viability, and metabolic activity). Kidney and breast cells remained viable for the duration of testing (7 days) on this modified surface, and the coating did not affect the protein content, morphology or quality of the cultured cells. The ultrasonic spray coated system, coating with 0.25 % PVA for 15 cycles with 0.12 A current after UV oxidation, increased the surface energy of the COC (naturally hydrophobic) from 22.04 to 112.89 mJ/m2 and improved the filling efficiency from 40 % (native untreated COC) to 94 % in the microwells without interfering with the biocompatibility of the surface, proving to be an efficient, high-throughput and scalable method of microfluidic surface treatment for diagnostic and cell growth applications.

Laser-induced oxidation assisted micro milling of high aspect ratio microgroove on WC-Co cemented carbide

Severe tool wear and poor surface quality are the main problems during micro machining of cemented carbide. In this work, an innovative hybrid process of laser-induced oxidation assisted micro milling (LOMM) was proposed to solve the problems. A nanosecond laser was utilized to induce oxidation of the WC-20%Co material, producing loose oxide which was easy to remove. The micro machinability of the material was improved by laser-induced oxidation. The oxidation mechanisms of cemented carbide were studied. A microgroove with a depth of 2.5 mm and aspect ratio of 5 was fabricated successfully. The milling force, surface quality and tool wear mechanisms were investigated. For comparison, a microgroove was also fabricated with conventional micro milling (COMM) using identical milling parameters. Results revealed that in LOMM the milling force and tool wear rate were extremely low during removing the oxide. The machined surface quality and dimensional accuracy achieved by LOMM were superior to those obtained by COMM. The surface roughness Sa of the microgroove bottom reached 88 nm in LOMM, while the cross-sectional geometry of the microgroove was a trapezoid. Perpendicularity of the microgroove sidewall machined by LOMM was better than that by COMM. The tool wear forms in LOMM were coating spalling and slight tool nose breakage. Compared with COMM, the tool life in LOMM was prolonged significantly. It indicates that the proposed hybrid process is an effective and efficient way to fabricate high aspect ratio micro-features with high dimensional accuracy.

Compressed Air-Film Encircling Jet Electrodeposition with High Deposition Accuracy

To improve the deposition accuracy and surface quality of the deposited micro-features, a novel compressed air-film encircling Jet ECD was proposed. In the proposed Jet ECD, a high-speed compressed air-film is coaxially encircling the impinging electrolytic jet. Numerical model describing the coupled field of electric field and flow field was established, and some auxiliary observations and measurement experiments were conducted to investigate the distribution characteristics of electric field and hydrodynamics characteristics in the concerned regions as well as the change of the electrolyte jet diameter. And the effect of air-film formation parameters and hydrodynamic parameters of the electrolyte jet on the electrodeposition behaviors during forming patterns and high aspect ratio micro-features were studied. Deposition accuracy and surface quality of the microstructures fabricated by the compressed air-film encircling Jet ECD were evaluated. It was demonstrated that, compared with the traditional Jet ECD, the proposed Jet ECD has a higher deposition accuracy and faster deposition rate (up to 1 μm s−1) as well as better surface quality. In addition, the newly developed Jet ECD has an admirable additive manufacturing ability and a 370 ± 3 μm-diameter smooth column with the aspect ratio of about 20 was successfully manufactured.

High aspect ratio channel fabrication with near-infrared laser-induced backside wet etching

Abstract Laser-induced backside wet etching (LIBWE) is one of the most promising manufacturing processes for fabricating high aspect ratio microfeatures of glass. However, cracks inside the glass during the LIBWE process due to high thermal stress limits the manufacturable shape. In this study, we propose a new technique to prevent the crack generation in the near-infrared (NIR) LIBWE process by adding phosphoric acid to the absorbent. It was possible to increase the maximum fabricable depth by approximately five times compared to the absorbent without phosphoric acid. The nature of the crack suppression mechanism in the absorbent with added phosphoric acid was investigated by in situ observation of the process and chemical analysis. To investigate the effects of phosphoric acid on LIBWE with added phosphoric acid, the influences of phosphoric acid concentration on maximum fabricable depth, fabrication speed and sidewall roughness were studied. Based on the investigated principle and process characteristics, we found that high aspect ratio microchannels of different shapes could be produced.

An inverse method for the abrasive jet micro-machining of high aspect ratio channels of desired topography – Part II, experiments

Part I of this two-part paper presented an inverse technique to sculpt high aspect ratio (AR) micro-channel cross-sections of desired topography. In this paper, the methodology is experimentally verified for a variety of topographies machined in both brittle and ductile erosive materials. Micro-channels with trapezoidal and semi-circular cross-sections, and semi-circular protrusions were machined in borosilicate glass. Micro-channels with symmetric and asymmetric wedge-shaped cross sections were machined in poly-methyl-methacrylate PMMA. The chosen topographies represent the most challenging cases because they run counter to the natural surface evolution tendency for these two materials. Overall, the average error between the machined and desired profiles was 6.4% in borosilicate glass and 9% in PMMA. The methodology opens up new possibilities for the micro-fabrication of higher efficiency devices containing high-aspect ratio micro-features of virtually any desired shape.

STUDIES ON THE EFFECT OF TOOL ROTATION IN MICRO-ULTRASONIC DRILLING OF HIGH ASPECT RATIO HOLES

Miniaturization is the need of hour as it saves the space, time, material, and energy as well. Therefore the demand of miniaturized features, components and products is rapidly increasing in industries. These micro-products includes the features such as microholes, and microchannels which are extensively used in microfluidics, Bio-MEMS, microelectronic chip (heat transfer) applications and Lab-on-a-chip. These micro-features are fabricated by conventional as well as non-conventional methods. Conventional methods include micro drilling whereas non-conventional techniques are Lithography, Chemical etching, Embossing, LIGA and Laser fabrication etc. Glass is commonly used material in these kind of application because it is chemically inert to most of the materials, have good optical properties, hydrophilic, non-porous suitable for prototyping and inexpensive as well. However being a fragile material, it is very difficult to machine by conventional method. Whereas LIGA, lithography and chemical etching are unable to generate high aspect ratio micro-features with good dimensional accuracy. Moreover these methods are time consuming and costly as well. Laser fabrication have the limitation to produce heat affected zone due to which workpiece properties change which further affect the useful life of product. Therefore there is a need to develop such process which can develop 3D complex micro-features at faster rate without altering the properties of workpiece.

Analysis of the measurement sensitivity of multidimensional vibrating microprobes

A comparison is made between tactile and vibrating microprobes regarding the measurement of typical high aspect ratio microfeatures. It is found that vibrating probes enable the use of styli with higher aspect ratios than tactile probes and are still capable of measuring with high sensitivity. In addition to the one dimensional sensitivity, the directional measurement sensitivity of a vibrating probe is investigated. A vibrating microprobe can perform measurements with high sensitivity in a space spanned by its mode shapes. If the natural frequencies that correspond to these mode shapes are different, the probe shows anisotropic and sub-optimal measurement sensitivity. It is shown that the closer the natural frequencies of the probe are, the better its performance is when regarding optimal and isotropic measurement sensitivity. A novel proof-of-principle setup of a vibrating probe with two nearly equal natural frequencies is realized. This system is able to perform measurements with high and isotropic sensitivity.

Surface finishing of intricate metal mould structures by large-area electron beam irradiation

The advancement of polymer moulding tools is increasingly focused on imparting not only form but also surface texture for functionality to the surfaces of parts that are created. Furthermore, the increasing demand for inexpensive and higher quality micro-components means that tools for replication processes must take advantage of advanced manufacturing techniques. Tools created by processes such as micro-investment casting, as in this case, may often suffer from excessive surface roughness, malformed edges and general deformation. This results in higher de-moulding forces and a reduction in fidelity of moulded parts to design intent. In this study, large-area electron beam irradiation (EB) is shown to be an effective technique for improving these metrics. For the first time, large population, high aspect ratio micro-features are subject to this process and the mechanisms of smoothing and key enhancement phenomena are demonstrated. The possibility of including EB irradiation in an integrated process chain for arriving at net shape is also discussed. Surfaces of protruding features are shown to have surface roughness reduced significantly from 126 to 22 nm Ra value, with bottom substrate also similarly improving from 150 to 27 nm Ra. Bottoms of recessed features are also observed to have much improved surface finishes. ‘Doming’ of tops of column features is also demonstrated, further enhancing form. These features would be far too fragile to be polished by any other mechanical method.

Localized mold heating with the aid of selective induction for injection molding of high aspect ratio micro-features

High-frequency induction is an efficient, non-contact means of heating the surface of an injection mold through electromagnetic induction. Because the procedure allows for the rapid heating and cooling of mold surfaces, it has been recently applied to the injection molding of thin-walled parts or micro/nano-structures. The present study proposes a localized heating method involving the selective use of mold materials to enhance the heating efficiency of high-frequency induction heating. For localized induction heating, a composite injection mold of ferromagnetic material and paramagnetic material is used. The feasibility of the proposed heating method is investigated through numerical analyses in terms of its heating efficiency for localized mold surfaces and in terms of the structural safety of the composite mold. The moldability of high aspect ratio micro-features is then experimentally compared under a variety of induction heating conditions.

Numerical analyses of peel demolding for UV embossing of high aspect ratio micro-patterning

Ultraviolet (UV) embossing, involving molding against micro-structured molds, is a quick and efficient method to mass produce high aspect ratio micro-features. A crucial challenge to the repeatability and large-scale application of this technique is successful demolding, which escalates in difficulty with increasing aspect ratio, due to increased polymer-mold mechanical interlocking. Some of the key factors affecting UV embossing include the crosslinked polymer shrinkage and material properties, interfacial strength between polymer to mold and the demolding method. This paper presents a new method to simulate the demolding of UV cured polymer from a nickel mold. Hyperelastic material model and rate-independent cohesive zone modeling were employed in the numerical simulation; linear elastic polymer response, although relatively easy to apply, was not adequate. Progressive shrinkage was implemented, leading to delamination of the polymer-mold interface. The subsequent peeling of the cured polymer from the mold was modeled with increasing prescribed displacement. The optimal shrinkage degree was found to increase from 0.92 to 1.9% with increasing mold aspect ratio (aspect ratio is defined as height to width ratio) from 5 to 10.

Enhanced machining of micron-scale features in microchip molding masters by CNC milling

An improved approach for the generation of aluminum masters used in the replication of polymer-based microfluidic devices is proposed. It is commonly assumed that conventional computer numerical controlled (CNC)-milling yields insufficient specifications for use in micro-milling. As molding masters suitable for microfluidic device replication require features whose dimensions are on the order of tens to hundreds of microns, the previous limitations of CNC-milling include machining tolerances that exceed 25% of the machined feature and inability to mill intersecting features. Here, we present several novel and advanced machining techniques to overcome existing limitations of a CNC-milling based approach. To achieve microfeatures with tolerances up to a factor of ten better than expected from the CNC-mill used, external calibration and measurement of the milling process was employed. To reduce the unwanted volume occurring from two intersecting features, 102 μm dia. high-density micrograin carbon endmills were used with all milling steps externally calibrated and measured. To generate masters with abutting features of different widths, keyway slots were milled into the master that could accommodate various size molding master keys. In addition to adding flexibility to the downstream embossing step, the use of keys eliminated dead-volume effects. A variety of molding masters have been machined and characterized in this work. Taken together, these approaches illustrate that conventional CNC-milling using equipment commonly found in university machine shops is a viable alternative to other more expensive master generation processes. Molding masters can be machined to yield high aspect ratio microfeatures with a tolerance of ±4 μm, and these masters can then be used in conjunction with casting or hot embossing to produce polymer microchip platforms suitable for analytical use.