Channels, through holes, and chambers are frequent examples of the application of micro-milling [36,37,38]. Shan et al. Teggert A., Datta H., McIntosh S., Warden B., Bateson S., Abugchem F., Ali Z. Hanemann T., Heckele M., Piotter V. Current Status of Micromolding Technology. Double-sided structure forming has also been demonstrated [92], although alignment remains an issue for further development. Oliveira A.F., Pessoa A.C.S.N., Bastos R.G., de la Torre L.G. Furthermore, the trend to integrate the planar components into 3D structures is also a possible direction to increase the function and to reduce the size of microfluidic devices. Becker H., Grtner C. Polymer microfabrication methods for microfluidic analytical applications. Successive laser pulses cause further material ablation, and thus controlled localised milling into the part, which constitutes the basic act of machining. Mech. They can be fabricated as batch processes as well as volume manufacturing and have versatile material properties. [49], and Ehrfeld [50]). UV roller embossing uses UV-curable resin as the process material [89,90,91,204,205]. Unlike an electron microscope, an FIB is inherently destructive to the specimen. A FIB can also be used to deposit material via ion beam-induced deposition. Optimising the process parameters requires careful examination of the overall process and making trade-offs, and the process is time consuming. Roppolo et al. Fan Y., Liu S., He J., Gao K., Zhang Y. The moulds are pressed against the softened polymer and the features are transferred. To increase the absorption and reduce the reflection, one can utilise methods such as changing surface finish, applying surface coating, and oxidising the workpiece surface [24]. Replications approaches require fabrication of mould or master and we describe different methods of mould manufacture, including mechanical (micro-cutting; ultrasonic machining), energy-assisted methods (electrodischarge machining, micro-electrochemical machining, laser ablation, electron beam machining, focused ion beam (FIB) machining), traditional micro-electromechanical systems (MEMS) processes, as well as mould fabrication approaches for curved surfaces. Mamleyev E.R., Heissler S., Nefedov A., Weidler P.G., Nordin N., Kudryashov V.V., Lnge K., MacKinnon N., Sharma S. Laser-induced hierarchical carbon patterns on polyimide substrates for flexible urea sensors. Electrochemical machining (ECM) utilises the electrochemical dissolution of workpiece in electrolyte upon application of electric potential. The high tool wear results from the abrasive effect on both the tool and the workpiece, and thus deep holes are difficult to machine; constant tool change might be required for prolonged machining. The dielectric fluid is constantly flushed to carry away the burned material and heat generated from the process.
The high surface finish achievable by this method is ideal for master making because it significantly reduces the friction during the demoulding process. [179] deduced that holding pressure provides better replication by offsetting part shrinkage at the expense of residual stress in the parts. [195] and Becker [196] have illustrated, the material cost per surface area dominates the total cost of a microfluidic device for large-volume manufacture. For a product that is produced in large volume, then approximately 80% of the cost comes from the latter two phases. A femtosecond laser has been used for fabrication of metallic inserts for microinjection moulding [51]. Fast fabrication of integrated surface-relief and particle-diffusing plastic diffuser by use of a hybrid extrusion roller embossing process. Microfabricated silicon gas chromatographic microchannels: Fabrication and performance. Hesse et al. The choice of the foil material can be limited because of the UV-curable property, such as foil thickness and optical property for the UV process and chemical compatibility for further processes [93]. Surface microstructure replication in injection molding. A PDMS soft mould on the roller cast by wrapping of PC film is also demonstrated [89,90]. Most of these methods can also be used to fabricate polymer parts directly. It is difficult to achieve replicate accuracy of hot roller embossing, which is as good as planar hot embossing, since the material goes through a more rapid temperature ramp and a shorter contact time with the moulds. Second, the softness and thermal expansion make it difficult for PDMS to achieve high accuracy for a large area; PDMS microstructures also tends to collapse with high aspect ratios, although design rules allow these issues to be ameliorated [83] and use of an SU-8 mould can lead to higher aspect ratio features. [191] showed for replication of micro-pillars that a smaller cavity thickness of the main flow region, which allows quicker filling of micro-cavities before solidification, and a higher mould temperature results in higher replication. Lasers can be used for fabrication of moulds that can then be used for higher volume replication of microfluidic devices.
Vinyl chlorides and similar modifications are toxic and difficult to handle. The upcoming 3D-printing revolution in microfluidics. Marques C., Desta Y.M., Rogers J., Murphy M.C., Kelly K. Fabrication of high-aspect-ratio microstructures on planar and nonplanar surfaces using a modified LIGA process. PDMS casting on the fabricated SU-8 master mould is carried out by first preparing the PDMS liquid polymermixing base elastomer and curing agent or catalystand pouring the mixture onto the SU-8 mould. Within laminate manufacturing, individual layers are first structured and then joined together in a stack to create microfluidic devices for a variety of applications [121]. Song et al. Applications of FIB have been centred on mask repairing, device modification, circuit debugging, online inspection, and fabricating nanostructures or lenses. Careers, Healthcare Innovation Centre, School of Health and Life Sciences, Teesside University, Middlesbrough, Tees Valley TS1 3BX, UK. Technol. Other than direct methods, modified LIGA processes [74,75] use flexible covering as pattern and variants of lithographic methods [76,77] adapt intermittent UV exposure to pattern periodic features on roller surfaces. Upon exposure to laser, the liquid polymer cross-links and solidifies. For continuous manufacturing, however, reels or rollers are usually used for printing and packaging purposes. Robust chemical bonding of PMMA microfluidic devices to porous PETE membranes for reliable cytotoxicity testing of drugs. Licensee MDPI, Basel, Switzerland. Further development has shown promising machining tolerance by applying ultra-short pulses in the nanosecond duration [47]. Michel B., Bernard A., Bietsch A., Delamarche E., Geissler M., Juncker D., Kind H., Renault J.-P., Rothuizen H., Schmid H., et al.
Proc. This alternative is easy for mould mounting and exchanging, and imposes no restriction on mould thickness for flexibility. All authors have read and agreed to the published version of the manuscript. Curing of the PDMS is carried out either at room or at elevated temperature (typically 40 to 70 C) for polymeric crosslinking, and then peeling off the elastomer for demoulding. Microcutting with computer numerical control (CNC) machines and ultrasonic micromachining fall into this category. Velten T., Schuck H., Haberer W., Bauerfeld F. Investigations on reel-to-reel hot embossing.
In: Dobisz E.A., editor. Yeo L.P., Ng S.H., Wang Z., Wang Z., de Rooij N.F. Electrochemical machining is relatively easy for design and setup because of the CNC capability. The applied pressure, embossing temperature, and holding time of the moulds and the orientation of polymer chain are recognised. The liquid polymer is then drained from the vat, leaving the solid model. Lee Y.S., Bhattacharjee N., Folch A. Microthermoforming is especially advantageous for 3D thin-walled structures, such as chambers and cavities, and work well as channels [210]. All processes are conducted in a vacuum chamber. Masato et al. Lin T.Y., Do T., Kwon P., Lillehoj P.B. The early SLA systems utilised a point-by-point fashion to scan across each cross section.
The process time is long and expensive. Mkel T., Haatainen T., Majander P., Ahopelto J., Lambertini V. Continuous Double-Sided Roll-to-Roll Imprinting of Polymer Film. Polymers are classified in terms of the chemistry of monomers (which link up to form polymers) but also a physical classification into three categoriesthermosets, thermoplastic, and elastomerswhich depends on the extent to which the polymers change shape when exposed to heat. We have previously described use of photopolymer inkjet 3D printing (i3DP) for creation of a microbioreactor with different functional layers joined by structured adhesive film [145] as well as fast and low cost prototyping of the mechanical and structural elements for a highly sensitive optical cavity enhanced absorption (CEA) detection system [146] (Figure 5). In the case of negative photoresist, exposure to UV light hardens the material whilst the unexposed areas remain soluble and can be washed away during development (Figure 2A). Kim B.H., Ryu S.H., Choi D.K., Chu C.N. Highly Fluorinated AromaticAliphatic Copolyethers. Del Campo A., Greiner C. SU-8: A photoresist for high-aspect-ratio and 3D submicron lithography. Unlike other lasers, focal point applications are rare; most processing is performed using projection optics, where the beam is used to illuminate a mask, whose de-magnified image is then focussed on the part. In: Smith J.H., editor. Thermal bonding of the parts at 95 C enabled the chip to withstand internal pressures of up to 15.6 MPa. Fabrication of microstructures using machining approaches can be costly because of the capital costs of the equipment, but also the process can be time-consuming. The authors declare no conflict of interest. Focused ion beam (FIB) machining utilises ions with high kinetic energy to remove or add material by momentum transfer. Micro-cutting includes drilling and milling, which uses drill bits less than 3 mm in diameter and rotating at much higher speeds (up to 180,000 rpm) than the traditional methods. The running costs are relatively low and the skill demand of labour is not high [25]. A diamond-coated tool to feed with small depth and slow speed can handle the difficulties, but the wear rate of tools can be high [22]. [206] have described a roll-to-roll (R2R) UV nanoimprint lithography approach with biofunctionalised channels for multiplexed DNA detection. Continuous roll to roll nanoimprinting of inherently conducting polyaniline. A drawback of microthermoformed structures is that the characteristic feature size is limited by the thickness of polymer films because of the wrapping effect around moulds, i.e., a thick film cannot bend around conformally. Lu et al. Experimental study of polymer microlens fabrication using partial-filling hot embossing technique. The advantage of e-beam machining is its high resolution; features in the nanometre range can be easily generated. First, the material swells if in contact with several nonpolar organic solvents, such as diisopropylamine, triethylamine, pentane, and xylenes [105], presenting difficulty for maintaining feature sizes. In: Tien N.C., Huang Q.-A., editors. [97] used a silicon film, of thickness about 40 m achieved by dicing-and-thinning process, which was wrapped around a steel cylinder. [62] have informational review on SU-8. Laser processing for bio-microfluidics applications (part I), Malek C.G.K. Gandarias E., Dimov S., Pham D.T., Ivanov A., Popov K., Lizarralde R., Arrazola P.J. A temperature sensor label (Figure 7A) was demonstrated using a hybrid integration process which comprises screen printing, assembly of surface mount devices (SMD), and 3D integration of foils through lamination of pressure sensitive adhesive (PSA) tape for low-temperature gluing and use of laser to create via holes [224]. Mech. Ultra-small features may be produced by two-photon photopolymerisation. High melt and mould temperatures, and high injection speed have a positive effect on the melt flow in very small cavities [189]. Bartholomeusz D.A., Boutt R.W., Andrade J.D. New Methods for Tool Failure Detection in Micromilling. Enabling Microfluidics: From Clean Rooms to Makerspaces. A shorter embossing contact time also means that the softened material may not have enough time to reach the deep recess of the mould. Certain metal and ceramic parts are also possible to be fabricated with this method with further curing and annealing processes. The additional benefit of microthermoforming is the possibility of incorporating other processes to provide additional functions. Thiol-ene chemistrybased on the reaction of a thiol (comprising sulphurhydrogen groups) and an alkene to form a thioetherhas become important as material for microfluidic devices. Plasticised polymer is injected into a tool, and the mould halves are then pressed together to force the melt into the desired shape [. In contrast, acrylatesa class of vinyl polymer from prop-2-enoyl (also known as acryloyl, acrylyl, or acryl) monomerare generally easier to synthesise than pure vinyl polymers. Microthermoforming can be a promising approach for mass production of microstructures with the optimisation of film heating and the pressure applying, as pointed out by Focke [198]. In: Smith J.H., editor. Melting by e-beam is also used in physical vapour deposition (PVD) or in the application of making micro-masters [52]. High aspect ratio meso-scale parts enabled by wire micro-EDM. Most publications used PDMS or silicone rubbers but other elastomers such as polyurethanes [101,102] and polyimides [103] have also been investigated. If the polymer chains are highly interlinked (crosslinked) then the network is temperature-stable and the material is classified as a thermoset. [98] used a copper/liquid-crystal polymer (LCP)/copper laminate as the flexible mould to wrap around a steel roller; the copper layers are used as both the pattern and backing layers. (A) Hybrid integrated temperature sensor label from [224] under creative commons 3.0. Huang M.-S., Chung C.-F. Injection molding and injection compression molding of thin-walled light-guided plates with V-grooved microfeatures. The technique may be sub-classified into three broad categories: There are several replication imperfections of the injection moulding technique. For more details on LIGA process, readers can refer to the following references for more information [63,64]. Micro-milling can, however, be a very effective and low-cost way of prototyping microfluidic devices. Several process parameters are important for the accuracy of a hot embossed part. Energy densities are typically in the range of 110 J/cm2 at repetition rates up to a few kHz; machining takes place at rates on the order of tenths of micrometres per laser shot. The final machining pattern on the part may be built up from repetition of selected motifs associated with part motion in X, Y, and laser firing; machining depth is mainly controlled by local shot dose. In this approach, the desired 3D object is traced out in a block of gel by a focused laser. Brittle materials, such as ceramics, silicon, and glass, are generally easier to machine because of the brittle-breakage principle of the method; soft or ductile materials are less efficient because much of the vibration is absorbed elastically or the workpiece is not chipped away easily due to ductility. Important parameters for EDM generally include pulse energy, pulse rate, polarity, material and geometry of the tool, type of dielectric fluid, gap between the tool and the workpiece, and the flushing of the dielectric fluid. The vibrational characteristic of the tool cause problems in tool loosing and machining accuracy. Childs W.R., Nuzzo R.G. This is inferred from thermoforming in the macroscopic scale for commercial mass production of products such as disposable cups or containers. Wang S.Q., Chinnasamy T., Lifson M.A., Inci F., Demirci U. Ohlander A., Zilio C., Hammerle T., Zelenin S., Klink G., Chiari M., Bock K., Russom A. Genotyping of single nucleotide polymorphisms by melting curve analysis using thin film semi-transparent heaters integrated in a lab-on-foil system. MEMS processes along this line can be found in numerous books and are not elaborated here. Custom 3D printer and resin for 18 m 20 m microfluidic flow channels. Usually, a higher pulse voltage is used for rough machining, and a lower one is used for finish machining. Electrochemical micromachining (EMM) is a variant of ECM at the micro-scale. Jena et al. Chan-Park M.B., Zhang J., Yan Y., Yue C.Y.
Au A.K., Lee W., Folch A. (A) Microbioreactor comprising i3DP fluidic and headspace layers joined by adhesive layers [145]. http://creativecommons.org/licenses/by/4.0/. Diamond or diamond-coated tools are used for high-surface finish on soft materials and cubic boron nitride (CBN) for hard materials. The achievable pattern resolution, however, is limited; continuous patterns on the surface can also be difficult due to intermittent exposure [78]. In the case of plastics, excimer lasers are the most efficient because the photon energy level is similar to that of the molecular bonds for plastics [22]. Maghsoudi K., Jafari R., Momen G., Farzaneh M. Micro-nanostructured polymer surfaces using injection molding: A review. Microfluidic tools toward industrial biotechnology. Nguyen T., Jung S.H., Lee M.S., Park T.E., Ahn S.K., Kang J.H. Here, we focus on the fabrication of the mould insert, since they determine the final features and functions of the microfluidic devices. Mekaru H., Goto H., Takahashi M. Development of ultrasonic micro hot embossing technology. Moreover, the material from the initial embossing can be wasted before the optimal steady state of the machine is attained. Micromilling of metal alloys with focused ion beamfabricated tools. Fabrication of microfluidic devices: Improvement of surface quality of CO2 laser machined poly(methylmethacrylate) polymer. Theoretical and experimental results agreed with one another on mixing efficiency, indicating that the reproducibility of the parts were sufficient. There is a need to further elaborate general best-practice methodology as guidelines as well as greater standardisation for the fabrication of microfluidic devices [226]. In DLP, the resin is cured one layer at a time, whilst in SLA, the laser beam moves from point-to-point tracing the geometry, and consequently SLA 3D printing is more accurate with a better quality of print. For both of these processes, the polymer is first raised to elevated temperatures, and subsequently cooled down below the glass transition temperature to obtain a solid part. The mask may define a simple motif, e.g., circle, slit, or more complex motifs which are then projected as a whole onto the part, e.g., alphanumeric characters. The minimum lateral size of laser ablation is primarily associated with the optics and the wavelength of light (Table 2); a smaller wavelength generally creates finer features. The film is clamped at its edges and heated to the glass transition temperature of the material; the softened material is then stretched against moulds with the assistance of pneumatic or mechanical pressure; the structure is then cooled and demoulded. [224] have described integration of multiple processes within R2R equipment, including sputtering or metal film on web substrate; web to web lamination e.g., lamination of dry film photoresist; lithographic patterning; electroplating of copper for fabricating wiring lines; screen printing of materials; chip bonding; and laser cutting. To localise the dissolution for machining purposes, one shapes the tool or electrode to achieve inhomogeneous current density; preferential dissolution rate around the tool can be therefore obtained and controlled machining is achieved. Another approach is selective fusing of print media in a granular bed. The scenario is very likely to happen after interface between individual processes are sorted out and standard fabrication sequences are established. Hardness of the workpiece material is not a concern for this process [21] because the materials are essentially burned away with high temperature and thus stainless-steel can be used as the master material. Single-crystal nanowires grown via electron-beam-induced deposition. Schuster R. Electrochemical Micromachining. High-aspect ratio silicon structures may be achieved with deep reactive ion etching (DRIE), but this is an expensive process. The process of combining X-ray lithography, electroplating, and moulding is termed as LIGA (Lithographie, Galvanoformung, Abformung from German). The key parameters which affect the fidelity of the embossing, especially the features in the thickness direction of the foil, are the applied pressure, temperature, speed of the roller, and the pre-heating temperature of the foil. Convery N., Gadegaard N. 30 Years of Microfluidics. For the sputtering operation, factors such as ion energy, ion species, incident angle of the beam, and surface bonding energy of the workpiece material are all important for the removal rate of the workpiece [30]. Benavides G.L., Bieg L.F., Saavedra M.P., Bryce E.A. We focus the discussion here on polymer-based foil and use the definition of foils given by Focke et al. Shan et al. Zhang et al. Investigation of micro-injection moulding: Factors affecting the replication quality. Becker H., Grtner C. Polymer microfabrication technologies for microfluidic systems. LabDisk with complete reagent prestorage for sample-to-answer nucleic acid based detection of respiratory pathogens verified with influenza A H3N2 virus. PDMS is one example of a frequently used elastomer. Additional systems are needed for tool-condition monitoring such as laser tool measuring or tool-workpiece voltage monitoring, and routine off-line tool checks are needed to solve the problem [41]. The gas-assisted reaction is similar to the process of oxygen-acetylene torch cutting. Vinyl monomers are those that contain carboncarbon double bonds with the simplest vinyl polymer, polyethylene (PE), formed from the ethylene monomer. Laminate manufacturing allows a wide variety of joining approaches and consequently an extensive choice of materials for the individual layersincluding polymers (typically polycarbonate, PMMA, COC), adhesive tape, and glasswhich can offer specific benefits such as optical clarity or biocompatibility [122]. Fused deposition modelling uses a nozzle to deposit molten polymer onto a support structure, layer by layer. Sayad A., Ibrahim F., Mukim Uddin S., Cho J., Madou M., Thong K.L. Lasers with longer pulse in the nanosecond and microsecond deliver lower energy. Dynamic masks use, for example, a motorised slit or rectangular variable aperture (RVA). Kim et al. have described fabrication of a micro-mechanical device using projection SL [148]. More advanced systems use projection instead of scanning of the laser to solidify the entire cross-section at once.
Inst. sharing sensitive information, make sure youre on a federal To benefit from the different available techniques, then, designers need to know the minimum feature sizes that these methods are able to achieve, along with a number of other parameters including the surface roughness, aspect ratio, and typical working size. The flushing of the dielectric fluid can be difficult, especially when the feature is deep and the gap between the tool and the workpiece is only several microns. No mould is necessary, and the process is simplified.
When the laser beam strikes the workpiece surface, reflection, absorption, and conduction of the beam occur. have demonstrated use of laser cutting and a tape substate for a wearable device that can be used for sweat collection, sample filtration, as well as biofluidic actuation and sensing [131]. The https:// ensures that you are connecting to the Typical examples include polymerase chain reaction (PCR), centrifugation, or flexible vibrating membranes. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (, The cost of substrate per unit area is high. Higher applied pressure, higher embossing temperature, and longer holding time tend to reproduce micro-features closer to the mould [169]. Jacobson S.C., Hergenroeder R., Koutny L.B., Ramsey J.M. The bonding was carried out by spreading small amounts of ethanol and the different layers aligned using custom holders with pins for alignment of the layers. The Ren et al. Moore A.W., Jacobson S.C., Ramsey J.M. Corresponding techniques, such as alignment, bonding, or surface modification, will need to advance with the development. The inter-changeability lowers the total cost of the master manufacturing since trial-and-error is usually required to achieve the optimised design of a mould [14] and only the inserts are to be re-fabricated, not the whole mould. The resolution is also constrained by the stability of the machine. A soft mould provides good conformal contact with the substrates. Alting L., Kimura F., Hansen H.N., Bissacco G. Micro Engineering. National Library of Medicine Previous discussions usually focus on masters of planar feature faces. The manufacturing cost can be greatly reduced if the processes in micro- or nanometres can be carried out using a replication approach. Jiang et al. In a gallium LMIS, gallium metal is placed in contact with a tungsten needle, and heated gallium wets the tungsten and flows to the tip of the needle where the opposing forces of surface tension and electric field form the gallium into a cusp shaped tip called a Taylor cone. Fabrication of Flexible Binary Amplitude Masks for Patterning on Highly Curved Surfaces. 3D printed micro-mechanical device (MMD) for in situ tensile testing of micro/nanowires. Sci. Typical layer thickness is around 100 micrometres (0.1 mm), although some machines can print layers as thin as 16 m. Ions from an ion source (usually liquid metal ion source or LMIS, typically Gallium) are accelerated and impinge onto the surface of workpiece or gas introduced to the process. Van Der Linden P.J.E.M., Popov A.M., Pontoni D. Accurate and rapid 3D printing of microfluidic devices using wavelength selection on a DLP printer. A modified SEM is the typical practice for many research laboratories to conduct e-beam lithography; the machine usually is equipped with a pattern generator and a beam blanker, which redirects the e-beam to prevent some areas from being exposed to the electron bombardment. Low cost microfluidic cell culture array using normally closed valves for cytotoxicity assay. Design and rapid prototyping of thin-film laminate-based microfluidic devices. The approach was able to fabricate microchannels with widths of 50 m and obviated the need of separate step for sealing the channels. SU-8 has been used as masters for PDMS in microfluidics (Figure 2B) or optics, or directly as structures in many occasions [55,56,57,58,59,60]. Mohammed M.I., Zainal Alam M.N.H., Kouzani A., Gibson I. [137] used a CO2 laser (ILS9.75, Universal Laser Systems, Inc., USA) to create capillary circuit components (trigger valves, retention valves, and retention bursting valves) within a PDMS substrate for sequential liquid delivery and sample reagent mixing. Ko H.C., Stoykovich M.P., Song J., Malyarchuk V., Choi W.M., Yu C.-J., Geddes III J.B., Xiao J., Wang S., Huang Y., et al. Wu J.-T., Lai H.-C., Yang S.-Y., Huang T.-C., Wu S.-H. Dip coating cooperated with stepped rotating lithography to fabricate rigid microstructures onto a metal roller. Study on the postbaking process and the effects on UV lithography of high aspect ratio SU-8 microstructures. When the high-energy gallium ions strike the sample, they will sputter atoms from the surface. Velten et al. Conventional microfabrication, such as lithography and wet/dry etching, is the natural choice when mould inserts with micro or nano structures are required and where the materials are usually limited to silicon, glass, or polymers, particularly photoresists.
The high surface finish achievable by this method is ideal for master making because it significantly reduces the friction during the demoulding process. [179] deduced that holding pressure provides better replication by offsetting part shrinkage at the expense of residual stress in the parts. [195] and Becker [196] have illustrated, the material cost per surface area dominates the total cost of a microfluidic device for large-volume manufacture. For a product that is produced in large volume, then approximately 80% of the cost comes from the latter two phases. A femtosecond laser has been used for fabrication of metallic inserts for microinjection moulding [51]. Fast fabrication of integrated surface-relief and particle-diffusing plastic diffuser by use of a hybrid extrusion roller embossing process. Microfabricated silicon gas chromatographic microchannels: Fabrication and performance. Hesse et al. The choice of the foil material can be limited because of the UV-curable property, such as foil thickness and optical property for the UV process and chemical compatibility for further processes [93]. Surface microstructure replication in injection molding. A PDMS soft mould on the roller cast by wrapping of PC film is also demonstrated [89,90]. Most of these methods can also be used to fabricate polymer parts directly. It is difficult to achieve replicate accuracy of hot roller embossing, which is as good as planar hot embossing, since the material goes through a more rapid temperature ramp and a shorter contact time with the moulds. Second, the softness and thermal expansion make it difficult for PDMS to achieve high accuracy for a large area; PDMS microstructures also tends to collapse with high aspect ratios, although design rules allow these issues to be ameliorated [83] and use of an SU-8 mould can lead to higher aspect ratio features. [191] showed for replication of micro-pillars that a smaller cavity thickness of the main flow region, which allows quicker filling of micro-cavities before solidification, and a higher mould temperature results in higher replication. Lasers can be used for fabrication of moulds that can then be used for higher volume replication of microfluidic devices.
Vinyl chlorides and similar modifications are toxic and difficult to handle. The upcoming 3D-printing revolution in microfluidics. Marques C., Desta Y.M., Rogers J., Murphy M.C., Kelly K. Fabrication of high-aspect-ratio microstructures on planar and nonplanar surfaces using a modified LIGA process. PDMS casting on the fabricated SU-8 master mould is carried out by first preparing the PDMS liquid polymermixing base elastomer and curing agent or catalystand pouring the mixture onto the SU-8 mould. Within laminate manufacturing, individual layers are first structured and then joined together in a stack to create microfluidic devices for a variety of applications [121]. Song et al. Applications of FIB have been centred on mask repairing, device modification, circuit debugging, online inspection, and fabricating nanostructures or lenses. Careers, Healthcare Innovation Centre, School of Health and Life Sciences, Teesside University, Middlesbrough, Tees Valley TS1 3BX, UK. Technol. Other than direct methods, modified LIGA processes [74,75] use flexible covering as pattern and variants of lithographic methods [76,77] adapt intermittent UV exposure to pattern periodic features on roller surfaces. Upon exposure to laser, the liquid polymer cross-links and solidifies. For continuous manufacturing, however, reels or rollers are usually used for printing and packaging purposes. Robust chemical bonding of PMMA microfluidic devices to porous PETE membranes for reliable cytotoxicity testing of drugs. Licensee MDPI, Basel, Switzerland. Further development has shown promising machining tolerance by applying ultra-short pulses in the nanosecond duration [47]. Michel B., Bernard A., Bietsch A., Delamarche E., Geissler M., Juncker D., Kind H., Renault J.-P., Rothuizen H., Schmid H., et al.
Proc. This alternative is easy for mould mounting and exchanging, and imposes no restriction on mould thickness for flexibility. All authors have read and agreed to the published version of the manuscript. Curing of the PDMS is carried out either at room or at elevated temperature (typically 40 to 70 C) for polymeric crosslinking, and then peeling off the elastomer for demoulding. Microcutting with computer numerical control (CNC) machines and ultrasonic micromachining fall into this category. Velten T., Schuck H., Haberer W., Bauerfeld F. Investigations on reel-to-reel hot embossing.
In: Dobisz E.A., editor. Yeo L.P., Ng S.H., Wang Z., Wang Z., de Rooij N.F. Electrochemical machining is relatively easy for design and setup because of the CNC capability. The applied pressure, embossing temperature, and holding time of the moulds and the orientation of polymer chain are recognised. The liquid polymer is then drained from the vat, leaving the solid model. Lee Y.S., Bhattacharjee N., Folch A. Microthermoforming is especially advantageous for 3D thin-walled structures, such as chambers and cavities, and work well as channels [210]. All processes are conducted in a vacuum chamber. Masato et al. Lin T.Y., Do T., Kwon P., Lillehoj P.B. The early SLA systems utilised a point-by-point fashion to scan across each cross section.
The process time is long and expensive. Mkel T., Haatainen T., Majander P., Ahopelto J., Lambertini V. Continuous Double-Sided Roll-to-Roll Imprinting of Polymer Film. Polymers are classified in terms of the chemistry of monomers (which link up to form polymers) but also a physical classification into three categoriesthermosets, thermoplastic, and elastomerswhich depends on the extent to which the polymers change shape when exposed to heat. We have previously described use of photopolymer inkjet 3D printing (i3DP) for creation of a microbioreactor with different functional layers joined by structured adhesive film [145] as well as fast and low cost prototyping of the mechanical and structural elements for a highly sensitive optical cavity enhanced absorption (CEA) detection system [146] (Figure 5). In the case of negative photoresist, exposure to UV light hardens the material whilst the unexposed areas remain soluble and can be washed away during development (Figure 2A). Kim B.H., Ryu S.H., Choi D.K., Chu C.N. Highly Fluorinated AromaticAliphatic Copolyethers. Del Campo A., Greiner C. SU-8: A photoresist for high-aspect-ratio and 3D submicron lithography. Unlike other lasers, focal point applications are rare; most processing is performed using projection optics, where the beam is used to illuminate a mask, whose de-magnified image is then focussed on the part. In: Smith J.H., editor. Thermal bonding of the parts at 95 C enabled the chip to withstand internal pressures of up to 15.6 MPa. Fabrication of microstructures using machining approaches can be costly because of the capital costs of the equipment, but also the process can be time-consuming. The authors declare no conflict of interest. Focused ion beam (FIB) machining utilises ions with high kinetic energy to remove or add material by momentum transfer. Micro-cutting includes drilling and milling, which uses drill bits less than 3 mm in diameter and rotating at much higher speeds (up to 180,000 rpm) than the traditional methods. The running costs are relatively low and the skill demand of labour is not high [25]. A diamond-coated tool to feed with small depth and slow speed can handle the difficulties, but the wear rate of tools can be high [22]. [206] have described a roll-to-roll (R2R) UV nanoimprint lithography approach with biofunctionalised channels for multiplexed DNA detection. Continuous roll to roll nanoimprinting of inherently conducting polyaniline. A drawback of microthermoformed structures is that the characteristic feature size is limited by the thickness of polymer films because of the wrapping effect around moulds, i.e., a thick film cannot bend around conformally. Lu et al. Experimental study of polymer microlens fabrication using partial-filling hot embossing technique. The advantage of e-beam machining is its high resolution; features in the nanometre range can be easily generated. First, the material swells if in contact with several nonpolar organic solvents, such as diisopropylamine, triethylamine, pentane, and xylenes [105], presenting difficulty for maintaining feature sizes. In: Tien N.C., Huang Q.-A., editors. [97] used a silicon film, of thickness about 40 m achieved by dicing-and-thinning process, which was wrapped around a steel cylinder. [62] have informational review on SU-8. Laser processing for bio-microfluidics applications (part I), Malek C.G.K. Gandarias E., Dimov S., Pham D.T., Ivanov A., Popov K., Lizarralde R., Arrazola P.J. A temperature sensor label (Figure 7A) was demonstrated using a hybrid integration process which comprises screen printing, assembly of surface mount devices (SMD), and 3D integration of foils through lamination of pressure sensitive adhesive (PSA) tape for low-temperature gluing and use of laser to create via holes [224]. Mech. Ultra-small features may be produced by two-photon photopolymerisation. High melt and mould temperatures, and high injection speed have a positive effect on the melt flow in very small cavities [189]. Bartholomeusz D.A., Boutt R.W., Andrade J.D. New Methods for Tool Failure Detection in Micromilling. Enabling Microfluidics: From Clean Rooms to Makerspaces. A shorter embossing contact time also means that the softened material may not have enough time to reach the deep recess of the mould. Certain metal and ceramic parts are also possible to be fabricated with this method with further curing and annealing processes. The additional benefit of microthermoforming is the possibility of incorporating other processes to provide additional functions. Thiol-ene chemistrybased on the reaction of a thiol (comprising sulphurhydrogen groups) and an alkene to form a thioetherhas become important as material for microfluidic devices. Plasticised polymer is injected into a tool, and the mould halves are then pressed together to force the melt into the desired shape [. In contrast, acrylatesa class of vinyl polymer from prop-2-enoyl (also known as acryloyl, acrylyl, or acryl) monomerare generally easier to synthesise than pure vinyl polymers. Microthermoforming can be a promising approach for mass production of microstructures with the optimisation of film heating and the pressure applying, as pointed out by Focke [198]. In: Smith J.H., editor. Melting by e-beam is also used in physical vapour deposition (PVD) or in the application of making micro-masters [52]. High aspect ratio meso-scale parts enabled by wire micro-EDM. Most publications used PDMS or silicone rubbers but other elastomers such as polyurethanes [101,102] and polyimides [103] have also been investigated. If the polymer chains are highly interlinked (crosslinked) then the network is temperature-stable and the material is classified as a thermoset. [98] used a copper/liquid-crystal polymer (LCP)/copper laminate as the flexible mould to wrap around a steel roller; the copper layers are used as both the pattern and backing layers. (A) Hybrid integrated temperature sensor label from [224] under creative commons 3.0. Huang M.-S., Chung C.-F. Injection molding and injection compression molding of thin-walled light-guided plates with V-grooved microfeatures. The technique may be sub-classified into three broad categories: There are several replication imperfections of the injection moulding technique. For more details on LIGA process, readers can refer to the following references for more information [63,64]. Micro-milling can, however, be a very effective and low-cost way of prototyping microfluidic devices. Several process parameters are important for the accuracy of a hot embossed part. Energy densities are typically in the range of 110 J/cm2 at repetition rates up to a few kHz; machining takes place at rates on the order of tenths of micrometres per laser shot. The final machining pattern on the part may be built up from repetition of selected motifs associated with part motion in X, Y, and laser firing; machining depth is mainly controlled by local shot dose. In this approach, the desired 3D object is traced out in a block of gel by a focused laser. Brittle materials, such as ceramics, silicon, and glass, are generally easier to machine because of the brittle-breakage principle of the method; soft or ductile materials are less efficient because much of the vibration is absorbed elastically or the workpiece is not chipped away easily due to ductility. Important parameters for EDM generally include pulse energy, pulse rate, polarity, material and geometry of the tool, type of dielectric fluid, gap between the tool and the workpiece, and the flushing of the dielectric fluid. The vibrational characteristic of the tool cause problems in tool loosing and machining accuracy. Childs W.R., Nuzzo R.G. This is inferred from thermoforming in the macroscopic scale for commercial mass production of products such as disposable cups or containers. Wang S.Q., Chinnasamy T., Lifson M.A., Inci F., Demirci U. Ohlander A., Zilio C., Hammerle T., Zelenin S., Klink G., Chiari M., Bock K., Russom A. Genotyping of single nucleotide polymorphisms by melting curve analysis using thin film semi-transparent heaters integrated in a lab-on-foil system. MEMS processes along this line can be found in numerous books and are not elaborated here. Custom 3D printer and resin for 18 m 20 m microfluidic flow channels. Usually, a higher pulse voltage is used for rough machining, and a lower one is used for finish machining. Electrochemical micromachining (EMM) is a variant of ECM at the micro-scale. Jena et al. Chan-Park M.B., Zhang J., Yan Y., Yue C.Y.
Au A.K., Lee W., Folch A. (A) Microbioreactor comprising i3DP fluidic and headspace layers joined by adhesive layers [145]. http://creativecommons.org/licenses/by/4.0/. Diamond or diamond-coated tools are used for high-surface finish on soft materials and cubic boron nitride (CBN) for hard materials. The achievable pattern resolution, however, is limited; continuous patterns on the surface can also be difficult due to intermittent exposure [78]. In the case of plastics, excimer lasers are the most efficient because the photon energy level is similar to that of the molecular bonds for plastics [22]. Maghsoudi K., Jafari R., Momen G., Farzaneh M. Micro-nanostructured polymer surfaces using injection molding: A review. Microfluidic tools toward industrial biotechnology. Nguyen T., Jung S.H., Lee M.S., Park T.E., Ahn S.K., Kang J.H. Here, we focus on the fabrication of the mould insert, since they determine the final features and functions of the microfluidic devices. Mekaru H., Goto H., Takahashi M. Development of ultrasonic micro hot embossing technology. Moreover, the material from the initial embossing can be wasted before the optimal steady state of the machine is attained. Micromilling of metal alloys with focused ion beamfabricated tools. Fabrication of microfluidic devices: Improvement of surface quality of CO2 laser machined poly(methylmethacrylate) polymer. Theoretical and experimental results agreed with one another on mixing efficiency, indicating that the reproducibility of the parts were sufficient. There is a need to further elaborate general best-practice methodology as guidelines as well as greater standardisation for the fabrication of microfluidic devices [226]. In DLP, the resin is cured one layer at a time, whilst in SLA, the laser beam moves from point-to-point tracing the geometry, and consequently SLA 3D printing is more accurate with a better quality of print. For both of these processes, the polymer is first raised to elevated temperatures, and subsequently cooled down below the glass transition temperature to obtain a solid part. The mask may define a simple motif, e.g., circle, slit, or more complex motifs which are then projected as a whole onto the part, e.g., alphanumeric characters. The minimum lateral size of laser ablation is primarily associated with the optics and the wavelength of light (Table 2); a smaller wavelength generally creates finer features. The film is clamped at its edges and heated to the glass transition temperature of the material; the softened material is then stretched against moulds with the assistance of pneumatic or mechanical pressure; the structure is then cooled and demoulded. [224] have described integration of multiple processes within R2R equipment, including sputtering or metal film on web substrate; web to web lamination e.g., lamination of dry film photoresist; lithographic patterning; electroplating of copper for fabricating wiring lines; screen printing of materials; chip bonding; and laser cutting. To localise the dissolution for machining purposes, one shapes the tool or electrode to achieve inhomogeneous current density; preferential dissolution rate around the tool can be therefore obtained and controlled machining is achieved. Another approach is selective fusing of print media in a granular bed. The scenario is very likely to happen after interface between individual processes are sorted out and standard fabrication sequences are established. Hardness of the workpiece material is not a concern for this process [21] because the materials are essentially burned away with high temperature and thus stainless-steel can be used as the master material. Single-crystal nanowires grown via electron-beam-induced deposition. Schuster R. Electrochemical Micromachining. High-aspect ratio silicon structures may be achieved with deep reactive ion etching (DRIE), but this is an expensive process. The process of combining X-ray lithography, electroplating, and moulding is termed as LIGA (Lithographie, Galvanoformung, Abformung from German). The key parameters which affect the fidelity of the embossing, especially the features in the thickness direction of the foil, are the applied pressure, temperature, speed of the roller, and the pre-heating temperature of the foil. Convery N., Gadegaard N. 30 Years of Microfluidics. For the sputtering operation, factors such as ion energy, ion species, incident angle of the beam, and surface bonding energy of the workpiece material are all important for the removal rate of the workpiece [30]. Benavides G.L., Bieg L.F., Saavedra M.P., Bryce E.A. We focus the discussion here on polymer-based foil and use the definition of foils given by Focke et al. Shan et al. Zhang et al. Investigation of micro-injection moulding: Factors affecting the replication quality. Becker H., Grtner C. Polymer microfabrication technologies for microfluidic systems. LabDisk with complete reagent prestorage for sample-to-answer nucleic acid based detection of respiratory pathogens verified with influenza A H3N2 virus. PDMS is one example of a frequently used elastomer. Additional systems are needed for tool-condition monitoring such as laser tool measuring or tool-workpiece voltage monitoring, and routine off-line tool checks are needed to solve the problem [41]. The gas-assisted reaction is similar to the process of oxygen-acetylene torch cutting. Vinyl monomers are those that contain carboncarbon double bonds with the simplest vinyl polymer, polyethylene (PE), formed from the ethylene monomer. Laminate manufacturing allows a wide variety of joining approaches and consequently an extensive choice of materials for the individual layersincluding polymers (typically polycarbonate, PMMA, COC), adhesive tape, and glasswhich can offer specific benefits such as optical clarity or biocompatibility [122]. Fused deposition modelling uses a nozzle to deposit molten polymer onto a support structure, layer by layer. Sayad A., Ibrahim F., Mukim Uddin S., Cho J., Madou M., Thong K.L. Lasers with longer pulse in the nanosecond and microsecond deliver lower energy. Dynamic masks use, for example, a motorised slit or rectangular variable aperture (RVA). Kim et al. have described fabrication of a micro-mechanical device using projection SL [148]. More advanced systems use projection instead of scanning of the laser to solidify the entire cross-section at once.
Inst. sharing sensitive information, make sure youre on a federal To benefit from the different available techniques, then, designers need to know the minimum feature sizes that these methods are able to achieve, along with a number of other parameters including the surface roughness, aspect ratio, and typical working size. The flushing of the dielectric fluid can be difficult, especially when the feature is deep and the gap between the tool and the workpiece is only several microns. No mould is necessary, and the process is simplified.
When the laser beam strikes the workpiece surface, reflection, absorption, and conduction of the beam occur. have demonstrated use of laser cutting and a tape substate for a wearable device that can be used for sweat collection, sample filtration, as well as biofluidic actuation and sensing [131]. The https:// ensures that you are connecting to the Typical examples include polymerase chain reaction (PCR), centrifugation, or flexible vibrating membranes. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (, The cost of substrate per unit area is high. Higher applied pressure, higher embossing temperature, and longer holding time tend to reproduce micro-features closer to the mould [169]. Jacobson S.C., Hergenroeder R., Koutny L.B., Ramsey J.M. The bonding was carried out by spreading small amounts of ethanol and the different layers aligned using custom holders with pins for alignment of the layers. The Ren et al. Moore A.W., Jacobson S.C., Ramsey J.M. Corresponding techniques, such as alignment, bonding, or surface modification, will need to advance with the development. The inter-changeability lowers the total cost of the master manufacturing since trial-and-error is usually required to achieve the optimised design of a mould [14] and only the inserts are to be re-fabricated, not the whole mould. The resolution is also constrained by the stability of the machine. A soft mould provides good conformal contact with the substrates. Alting L., Kimura F., Hansen H.N., Bissacco G. Micro Engineering. National Library of Medicine Previous discussions usually focus on masters of planar feature faces. The manufacturing cost can be greatly reduced if the processes in micro- or nanometres can be carried out using a replication approach. Jiang et al. In a gallium LMIS, gallium metal is placed in contact with a tungsten needle, and heated gallium wets the tungsten and flows to the tip of the needle where the opposing forces of surface tension and electric field form the gallium into a cusp shaped tip called a Taylor cone. Fabrication of Flexible Binary Amplitude Masks for Patterning on Highly Curved Surfaces. 3D printed micro-mechanical device (MMD) for in situ tensile testing of micro/nanowires. Sci. Typical layer thickness is around 100 micrometres (0.1 mm), although some machines can print layers as thin as 16 m. Ions from an ion source (usually liquid metal ion source or LMIS, typically Gallium) are accelerated and impinge onto the surface of workpiece or gas introduced to the process. Van Der Linden P.J.E.M., Popov A.M., Pontoni D. Accurate and rapid 3D printing of microfluidic devices using wavelength selection on a DLP printer. A modified SEM is the typical practice for many research laboratories to conduct e-beam lithography; the machine usually is equipped with a pattern generator and a beam blanker, which redirects the e-beam to prevent some areas from being exposed to the electron bombardment. Low cost microfluidic cell culture array using normally closed valves for cytotoxicity assay. Design and rapid prototyping of thin-film laminate-based microfluidic devices. The approach was able to fabricate microchannels with widths of 50 m and obviated the need of separate step for sealing the channels. SU-8 has been used as masters for PDMS in microfluidics (Figure 2B) or optics, or directly as structures in many occasions [55,56,57,58,59,60]. Mohammed M.I., Zainal Alam M.N.H., Kouzani A., Gibson I. [137] used a CO2 laser (ILS9.75, Universal Laser Systems, Inc., USA) to create capillary circuit components (trigger valves, retention valves, and retention bursting valves) within a PDMS substrate for sequential liquid delivery and sample reagent mixing. Ko H.C., Stoykovich M.P., Song J., Malyarchuk V., Choi W.M., Yu C.-J., Geddes III J.B., Xiao J., Wang S., Huang Y., et al. Wu J.-T., Lai H.-C., Yang S.-Y., Huang T.-C., Wu S.-H. Dip coating cooperated with stepped rotating lithography to fabricate rigid microstructures onto a metal roller. Study on the postbaking process and the effects on UV lithography of high aspect ratio SU-8 microstructures. When the high-energy gallium ions strike the sample, they will sputter atoms from the surface. Velten et al. Conventional microfabrication, such as lithography and wet/dry etching, is the natural choice when mould inserts with micro or nano structures are required and where the materials are usually limited to silicon, glass, or polymers, particularly photoresists.