Xurography as a tool for fabrication of microfluidic devices

Microfluidic devices have been conventionally fabricated using traditional photolithography or through the use of soft lithography both of which require multiple complicated steps and a clean room setup. Xurography is an alternative rapid prototyping method which has been used to fabricate microfluidic devices in less than 20–30 minutes. The method is used to pattern two-dimensional pressure-sensitive adhesives, polymer sheets, and metal films using a cutting plotter and these layers are bonded together using methods including adhesive, thermal, and solvent bonding. This review discusses the working principle of xurography along with a critical analysis of parameters affecting the patterning process, various materials patterned using xurography, and their applications. Xurography can be used in the fabrication of microfluidic devices using four main approaches: making multiple layered devices, fabrication of micromolds, making masks, and integration of electrodes into microfluidic devices. We have also briefly discussed the bonding methods for assembling the two-dimensional patterned layers. Due to its simplicity and the ability to easily integrate multiple materials, xurography is likely to grow in prominence as a method for fabrication of microfluidic devices.


Introduction
Microfluidics is the field that deals with the flow of fluids in structures at the sub-millimeter scale. This field has been investigated intensely over the past few decades and has been applied in several areas from biology, medicine, chemistry, 4 Both authors have equal contributions.
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Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. engineering, and the environment (Faustino et al 2016, Kojic et al 2019. Various components such as micromixers (Hessel et al 2005, Nguyen and Wu 2005, Suh and Kang 2010, Cai et al 2017, micropumps (Laser and Santiago 2004, Woias 2005, Au et al 2011, Wang and Fu 2018, microvalves (Oh andAhn 2006, Au et al 2011), biosensors (Choi et al 2011, Luka et al 2015, droplet generators (Seemann et al 2011, Chong et al 2016, Zhu and Wang 2017, and separation devices (Lenshof and Laurell 2010, Sajeesh and Sen 2014, Sonker et al 2017 have been developed to control and handle fluid flow at the sub-scale in microfluidic devices. Microfluidic devices have several advantages such as low cost, small size, ability to handle small sample volume and low power consumption that can improve their analytical performance and sensitivity compared to the conventional size systems (Faustino et al 2016, Scott andAli 2021).
Photolithography is an established microfabrication technique in which light is used to create patterns on a photosensitive material coated on a silicon or glass surface. This process is mostly performed in a clean room environment. The photosensitive material itself can be either used as the structural features of the microfluidic device or as the master mold for transferring the patterns onto a soft polymeric material such as polydimethylsiloxane (PDMS). This process is called soft lithography Brannon-Peppas 2006, Damodara andShahriari et al 2021). To date, photolithography followed by soft lithography has been the most popular method for the fabrication of microfluidic devices (Faustino et al 2016). Although this method is versatile and rapid with resolution sufficient for microfluidic applications, it still requires expensive equipment, specialized personnel, and clean room facilities for the fabrication of the master mold. These limitations led to the search for rapid, simple, and low-cost alternatives for the fabrication of microfluidic devices that do not require the use of cleanroom facilities. Micro milling, 3D printing, laser ablation, and xurography are some examples of rapid and lower cost microfabrication and prototyping techniques that have been used both to fabricate microfluidic devices by themselves or to make molds for soft lithography (Pinto et al 2014, Taylor andHarris 2019). Although rapid prototyping techniques such as xurography do not provide as high a resolution as photolithography methods, they enable scalable manufacturing (Walsh III et al 2017). Moreover, they provide the ability to test new device designs much faster and subsequently enhance the speed of development cycles (Pinto et al 2014).
Xurography also known as craft cutting (or writing) or razor writing, was first introduced as a microfabrication method by Bartholomeusz et al (2005). Xurography utilizes a cutting plotter for cutting various materials such as thin adhesives, polymer films, metal foils, and paper using a physical blade. Xurography is a rapid, simple, and low-cost (both in equipment and materials) microfabrication technique which eliminates the need for clean rooms facilities and specialized personnel (Damodara and Shahriari et al 2021). Full fabrication using the xurography method takes a matter of minutes. Therefore, various repetitions of a design can be tested quickly and efficiently.
Cutting plotters compared to laser cutters which are also employed for cutting layers of tapes, plastic, and paper for creating microdevices, provide less resolution and have limitation in the selection of the material and its thickness, however, their price is significantly less and they are easier to setup (Gale et al 2018). Moreover, laser cutters require a vacuum pump for clearing debris. Leaving burn residue is another issue associated with laser cutters which have been shown to inhibit some reactions such as polymerase chain reaction (PCR). However, cutting plotters need no vacuum pump and leave no burn residues (Walsh et al 2017).
Xurography was first performed using expensive industrial cutters which cost around $4000 USD for macroscale applications such as the production of large advertisement signs mostly from vinyl adhesive, however, in recent years they have become broadly commercialized, and the cost of equipment has dropped to almost $200 USD for a desktop sized cutting equipment. This price reduction has not affected the cutting resolution and is more associated with scaling down the cutting area (Martínez-López et al 2016. For instance, Silhouette and Cricut are two of the desktop size cutting plotter providers. The price of their machines starts from $200 USD and the cutting performance is comparable to more expensive ones (Martínez-López et al 2017). Moreover, the cost of materials used in xurography is much less than the other techniques. For example, in the case of soft lithography; silicon wafers are $6-20 each (University Wafer), UV masks are $84 (Fine Line Imaging), and a PDMS kit is $92/kg (Krayden); whereas in rapid prototyping methods such use xurography, plastics and adhesives are ∼$5 ft −2 (McMaster Carr) and $2 ft −2 (Amazon.com) respectively (Walsh et al 2017). Therefore, microdevices fabricated using this method are cheaper. For example, it was estimated that six tapebonded microchips cost less than $2.00 (Greer et al 2007).
Several approaches have been taken in using xurography to fabricate microfluidic devices such as creating laminated multi-layer microfluidic devices, fabricating micromolds, producing masks, and integration of electrodes into microdevices (figure 1). Various materials such as pressure-sensitive adhesives (PSAs), polymers, paper, and metal films have been used in these fabrication techniques. Most of the xurography-based microfluidic devices are created by bonding and laminating multiple cut layers using PSAs. The simplest microfluidic device consists of three layers; the top one which composes inlet and outlet, the middle one forms the flow channel, and the bottom layer encloses the device. The thickness of the material used defines the height of microchannels. After cutting, individual layers are aligned mostly using an alignment tool. Alignment holes are created in each layer in the cutting process (Gale et al 2018).
Xurography has also been used to make micromolds for the fabrication of microchannels out of PDMS. This can be a lower cost alternative to photolithography for making master molds. In this method, micromolds are created by cutting the adhesives based on the mold design using the cutting plotter. Then, the final device is created using the soft lithography process.
Xurography is also used to create low-cost masks for various applications such as electrode sputtering, electroplating, and wet chemical etching. Electrode fabrication and integration can be conducted directly using xurography as well. Metal films are directly cut with a cutting plotter and integrated into devices.
This review is a broad overview of xurography as a tool for microfabrication from making multi-layered microfluidic devices to the fabrication of micromolds, and masks for creating the devices. The working principle, performance parameters, and bonding methods in this technique are discussed. The diverse range of materials used in this method to fabricate microfluidic devices and their elements have also been detailed. This review consists of six sections. Section 2 describes the working principle of xurography along Reprinted from Do et al (2011) Copyright (2011, with permission from Elsevier. (b) Schematic diagram demonstrating the layers of a pump-free membrane-controlled perfusion microfluidic device fabricated using a craft cutter (Goral et al 2015). B. Fabrication of micromolds; Reprinted from Goral et al (2015), with the permission of AIP Publishing.(c) fabrication steps of microfluidic devices out of xurographically made micromolds (Pinto et al 2014). Reproduced with permission from Pinto et al (2014) (d) Schematic of the process for mold fabrication using a cutting plotter (Nagai et al 2018). C. Fabrication of masks for micropatterning; Reproduced with permission from Nagai et al (2018).(e) fabricating hot embossed plastic microfluidic devices using vinyl adhesive stickers as masks for electroplating (Novak et al 2013). Reproduced from Novak et al (2013) Mohammadzadeh et al. (2019) with the effect of performance parameters like resolution, and cutting quality, on the patterning process. Section 3 provides an overview of various materials patterned using this method and section 4 provides details about the different bonding methods used to convert the 2D patterned films into 3D microfluidic devices. Section 5 summarizes the methods that xurography can be utilized for fabrication of microfluidic devices such as fabricating multilayered devices, micromolds, shadow masks, and integration of electrodes and the applications of these devices into areas such as sensing, micromixing, preconcentrating, and fuel cells. Section 6 discusses the current challenges and future directions.

Working principle
Xurography, the word originates from two Greek words Xuron and graphē, which means razor and writing, respectively (Aeinehvand et al 2017). A cutter plotter that uses a computercontrolled cutting head to trace the path of the blade on a  sheet of polymer or paper can be an effective tool to automate the xurographic fabrication of the design in 2D. The cutter plotter controls the feed of the material either using sprocket feed spools or friction rollers. A cutter plotter may use different blades depending on the cut material. The cutting blades have various angles which are measured between the cutting edge of the blade and the surface of the material. The most used blade angles are 30 • , 45 • and 60 • . The user can control the depth of the cut using the force or depth setting in the cutter.
The cutting methods used by plotter cutters are classified into three types: drag knife, true tangential, and emulated tangential. The Drag knife method uses a swivel blade that introduces a lateral force by the blade. The blade follows the cut pattern, but it can damage the blade when cutting thicker or harder materials. Therefore, the true tangential method is used to cut thick materials where the cutter plotter positions the blade precisely on the cut patterns. While cutting the corners, the blade is completely lifted out of the material to change the blade direction. This method results in overcutting at the corners to ensure that the material is cut completely from top to bottom around corners. The emulated tangential method is a hybrid of the previous two methods where the cutter uses a swivel blade that follows the cut pattern. However, the blade is lifted to the surface of the material at the corners to reduce the lateral force on the blade. This reduces the chances of blade damage while cutting thicker or harder materials. Lifting the blade to the surface also reduces the size of overcuts required in the case of true tangential methods (Bartholomeusz et al 2005).
Xurography has been used for patterning single or multilayered material. The blade depth and force can be used to pattern a through cut or partial cut based on the requirement. Figure 2 shows a multilayered material with parafilm, metal leaf, adhesive and PET and Kapton. Here, the gold leaf on the adhesive with PET liner was patterned without affecting the Kapton tape (figure 2).
The pattern can be designed using any computer-aided design (CAD) software. The material is fixed on a glue pad (cutting mat) to avoid any movement during the cutting process. Then, the cutting parameters including speed, force, acceleration, and offset are optimized to obtain a clean pattern. The cutting force depends on the material type and its thickness as well as on the type of blade used (Islam et al 2015).
A microfluidic device can be fabricated by patterning multiple 2D layers and these patterned layers are bonded together using various bonding methods like an adhesive, vacuum, and thermal, to assemble into a 3D device. For example, a microfluidic device was fabricated using xurographically patterned PSA films (blue) that were cut through and laser-cut acrylic sheets (brown) that formed the top and bottom surfaces (figure 3(a)). The material properties of the PSA such as Young's modulus and Poisson's ratio were found to determine the smallest features that can be cut in a material (Bartholomeusz et al 2005). This method was shown to cut various patterns including geometrical features, straight and serpentine channels ( figure 3(b)).
The limits of the xurographic fabrication method were characterized by cutting geometries such as straight, curved, square serpentines, and zig-zag channels (Islam et al 2015). The parameters used for the characterization of the various patterns include the width of the channel for straight channels, the width and radius of curvature for curved serpentine channels, the width, and the angle of aperture for zigzag channels and the width and the spacing between the arms for square serpentine channels. The study showed that type of blade was important for accuracy and precision of the width of the channels (at <700 µm). For instance, the error in accuracy of the 45 • blade in replicating a design was 26% for 200 µm channel as compared with 9.09% for a 30 • blade. Although the resolution of the used cutting plotter was 25 µm, channels with a width of less than 200 µm were not obtained which can be due to other limitations. The higher error obtained for the 45 • blade could be related to the greater footprint compared to the 30 • blade. Hence, the geometry of the blade determines the practical limit of xurography.
Cutter plotters can be compared based on parameters like resolution, repeatability, and cutting blades.

Resolution
Resolution is defined as the minimum pattern size achieved using the cutter. The resolution of the cutter plotters is specified using two measures: addressable and mechanical resolution. The addressable resolution is defined as the minimum programmable step size which depends on the resolution limit of the driver to the print plotter files. The mechanical resolution depends on the minimum step size of the motors. The best resolution was achieved in a study that demonstrated the cutting of smooth and uniform channels as small as 75 µm out of polyimide tape (Taylor and Harris 2019). The achieved resolution is sufficient for most microfluidic applications such as micromixers, droplet generators, and multilayer pressure actuators. In this study, the fabricated devices were used to perform the Taylor dispersion experiment by investigating the solute dispersion to determine the cut-out channel accuracy. The results agreed with the prediction from the classical Taylor-Aris dispersion theory. Another study obtained features as small as 100 µm, which was found to depend on the resolution of the cutter (25 µm) and the thickness of the PS films (Cao et al 2015).
The minimum feature size also depends on the geometrical shape of the channel. In a study, the cutting plotter was able to pattern square shape microchannels down to 107 ± 13 µm in width whereas well-defined circular patterns of dimension 447 ± 12 µm in diameter or above were only achievable (de Santana et al 2012).

Cutting quality and limitation
In xurography, several parameters affect the cutting quality such as the sharpness of the cutting blade, blade protruding length, cutting material, cutting settings such as speed and force and cutting mat condition. For example, by lowering the cutting speed, the cutting quality can be improved. Before cutting a new material, it is recommended to perform a test cut to make sure that the material can be completely cut. The blade length from the cap should not be more than the thickness of the material to prevent damage to the cutting mat or the blade (Yuen and Goral 2010). The cutting process was evaluated by fabricating microfluidic devices with serpentine channels of various widths using double-sided PSA films (Yuen and Goral 2010). The serpentine microchannel device was designed with a width ranging from 200 µm to 700 µm. It was shown that the cutting resolution was better in the horizontal cutting direction, however, cut quality in the vertical cutting direction was higher. They could cut microchannels as small as 200 µm wide out of double-sided PSA tape (thickness of 150 µm, 50 µm of adhesive tape and two plastic covers with 50 µm thickness each). The failure in cutting small features was mostly due to the adherence of PSA cutting debris to the cutting blade which will scratch the cut pattern and result in cutting failure.

Material
Various materials including PSAs, polymer films, metal films, paper, and PDMS have been used in this fabrication technique. A list of these materials along with the cutting parameters and details of the cutter plotter is summarized in table 1.

PSA
PSAs are the main group of material used in the fabrication of microfluidic devices using xurography. Cutting tools such as xurography and laser cutters can be used for patterning adhesives (Nath et al 2010, Yuen and Goral 2010, Walsh et al 2017. However, one of the drawbacks of using adhesives in microfluidic device fabrication is the inadvertent attachment of particulate debris from the xurography process (Gale et al 2018). The depth of the microchannels is defined by the thickness of the adhesive layer (Nath et al 2010). PSAs used in this method can be categorized into three categories: transfer tapes, onesided and double-sided tapes. Mohammadzadeh et al (2018). Transfer tapes are completely made of PSA material. Doublesided tapes have a carrier layer with both sides coated with adhesive material while for one-sided tapes only one side is coated with an adhesive (figure 4(a)). There are several companies such as 3M and Adhesives Research which provide various PSAs (Walsh et al 2017).
In another categorization, PSAs can be divided into three groups in terms of material types: natural rubber, acrylics and silicone . Natural rubber is the oldest and cheapest group of PSAs. They are usually mixed with a tackifying resin and cross-linked to prevent flow. Acrylic PSAs are mostly made of random copolymers of a long side-chain acrylic and a short side-chain acrylic for adjusting T g , and acrylic acid for improving adhesion (Creton 2003). Solvent-based, water-based, and solvent-free acrylic PSAs are the three groups of acrylics which are widely used currently (Czech 2007). Silicone PSAs usually consist of highmolecular-weight silanol, silicone polymers and siloxane resins. Silicone PSAs provide some advantages such as high thermal stability, low surface tension, and high UV transparency, however, they are more expensive (Lin et al 2007).
There are several criteria for selection of the suitable adhesives such as application, fabrication process, required thickness, and cost (Walsh et al 2017). For instance, in the fabrication of a xurographic-based PCR microdevice, acrylic tape was avoided as it induces an acute inhibition of PCR and silicone adhesive was used instead (Pješčić et al 2010).

Polymers
Polymer materials are used in the fabrication of microfluidic devices as they are low-cost, disposable, and suitable for various applications. Thermoplastics and polydimethylsiloxane (PDMS) are the two main groups of polymers that have been extremely used for the fabrication of microfluidics. Thermoplastics such as polymethyl methacrylate (PMMA), polycarbonate (PC), polyvinyl chloride (PVC), polyimide (PI), and cyclic olefin polymers such as COC and COP, have been used in microfluidics. Thermoplastics have been integrated into microfluidic devices using different methods such as computer numerical controlled (CNC) milling and laser ablation (Tsao 2016). Xurography is another method that has been used to create microchannels in polymer films made of thermoplastics. Polymer films with thicknesses ranging from 25 µm to 1.5 mm can be easily patterned with a cutting plotter (Mohammadzadeh et al 2018).
For example, fluoropolymers were used for prototyping microfluidic devices using xurography (Hizawa et al 2018). Fluoropolymers show good solvent resistance but due to the fabrication complexity, they have not been used in microfluidics. PTFE and FEP films were used as two common fluoropolymers. In their fabrication process, first the films were patterned using a cutting plotter and then laminated using a heat press to get the final device (Hizawa et al 2018).
PDMS is one of the polymers which has been extensively used in the fabrication of microfluidic devices using soft lithography technique. Thin PDMS membranes can be cut and patterned using xurography as well. In some cases, such as observing a live sample, high oxygen permeability and biocompatibility is required. Therefore, PDMS is preferred over other polymer films for the construction of microfluidic channels (Nagai et al 2018). Cosson et al demonstrated the fabrication of simple and cheap microfluidic devices with the combination of PDMS and xurography (2015). A thin PDMS layer was first spin coated on a plastic foil and then cut based on the design using a craft cutter. Furthermore, multiple layers of PDMS can be patterned and bonded using plasma bonding to build multi-layered microfluidic devices (figure 4(b)). It was reported that the resolution of this method using a cutter plotter (ROBO Pro CE5000-40-CRP) and 5:1 ratio of elastomer to curing agent of PDMS was 100 µm.
Shrink films have also been used for the fabrication of microchannels using xurography. For example, an immunoassay microfluidic device was fabricated by cutting polyolefin shrink film (Taylor et al 2010). Then the device was heated and as a result the layers were shrunk (figure 4(c)). The pattern of the channels was maintained while the channel width was decreased, and the height was increased. Therefore, the lateral feature resolution was improved.

Paper
Paper is a widely used microfluidic substrate especially for developing ultra-low-cost point-of-care devices. Recently, paper based microfluidic devices have advanced significantly. Xurography has also been used for fabrication of paper based microfluidic devices , Jafry et al 2017. Paper cutting provides an effective way to route fluid flow due to the porosity and hydrophilicity of paper. Lamination of the cut paper on either side by plastic seals the channel path and lamination pressure can be used to control the flow rate in these channels . Paper has been patterned and cut using a craft cutter to fabricate pressure-driven microfluidic devices for various applications such as diagnostic chemical assays, immunoassays (i.e. lateral flow assays) and PCR. The main drawback of paper microfluidics is its opacity, which limits signal from a fluorescent or colorimetric assay.
GE Healthcare Life Sciences Whatman line provides a wide variety of paper substrates with different thicknesses for integration into microfluidics (Walsh et al 2017). In one study, an omniphobic paper-based open channel microfluidic system with lateral channels of dimensions 45-300 µm was carved using a craft cutter (Glavan et al 2013). A silhouette craft cutter was used to carve microchannels on cardstock paper (figure 4(d)). Cardstock paper was selected as it is flexible, inexpensive, thick, and smooth to be bonded with a tape layer. They showed that a combination of xurography and omniphobic paper provided new capabilities for fluid flow control such as fold valves and porous switches besides other advantages such as low cost, flexibility, and high gas permeability.
In another example, paper was used in the fabrication of a low-cost and disposable multi-layer xurography-based passive micromixer (Samae et al 2020). Paper was cut in the form of a straight or zig-zag microchannel and immersed in wax to prevent lateral flow leakage and then laminated between two PVC layers. A mixing efficiency of 50% in the zig-zag microchannel was obtained which is comparable to previous micromixers. In another example, a cutting plotter was employed for patterning glass fiber membranes for the fabrication of paper microfluidics for the detection of glucose, nitrite, and pH in urine.
Microchannels as small as ∼137 µm were obtained (Fang et al 2014). Microfluidic paper-based devices are used to control reagent transport or storage in microfluidic channels. However, the fluid transport in these devices is dependent on the intrinsic properties of paper like the uniformity of paper, the viscosity of the liquid, and environmental factors (Noh and Phillips 2010). A new method was proposed in a study where open channels were patterned on paper in either longitudinal or perpendicular direction of the fluid flow, to control the fluid flow (Giokas et al 2014). The orthogonal channels create a barrier while the longitudinal channel supports the fluid flow. A precision knife was used to engrave patterns on paper. The method was used to program fluid delivery time in microfluidic devices.

Metal films
Metal films such as thin copper films and metal leaves have been patterned using xurography (Mohammadzadeh et al 2019, Stojanović et al 2019, Patel et al 2022a. Metal foil can be integrated into microfluidics for various applications such as electrical sensors, electro mixers and electrical heaters. In one example, Mohammadzadeh et al used a combination of xurography and cold lamination for the fabrication and integration of electrodes into microfluidics. They could obtain feature sizes as small as 66 µm from a copper-polyimide composite foil (9 µm Cu and 12 µm PI thickness) Mohammadzadeh et al (2019). Later, a study demonstrated direct patterning of thin metal leaves using xurography (figure 4(e)) (Patel et al 2022a). These patterned electrodes can be directly integrated into microfluidic channels and used as sensors (Patel et al 2021, Patel et al 2022b.

Bonding methods
Most of the manufacturing methods employed to create polymer microfluidic devices fabricate open channels. Therefore, these open two-dimensional structures must be bonded to each other to manufacture a closed device. Bonding of the layers to form the final device is a prominent step in xurographically fabricated multi-layer microfluidic devices. The bonding methods can be classified into three broad groups: thermal bonding, adhesive bonding, and solvent bonding. The selection of the bonding method depends on the material used in the fabrication process. The bonding method is a predominant factor which defines the pressure that the device can tolerate. Poor bonding can lead to some issues such as the formation of air bubbles between the layers and deformation of the microfluidic device features (Gale et al 2018).

Adhesive bonding
Adhesive bonding using PSAs enables bonding of materials which may not have been possible with other bonding techniques. PSAs have been used to seal microfluidic devices fabricated with varied materials such as PDMS, COC, PMMA, polycarbonate (PC), and glass. In one example PCR tape was used to seal a microfluidic device and it was shown that the bonding was able to sustain pressure up to 5 bar, even at relatively high temperatures (Serra et al 2017). However, maintaining higher pressures (above 5 bar) is still a challenge in adhesive bonding. It has been shown that plasma treatment of the surfaces before bonding or use of heat and pressure following the bonding are some ways to increase the sustainable pressure (Gale et al 2018). In another example, adhesive bonding was used in fabrication of a seven-layered microfluidic cartridge for electrochemical biosensing (Kim et al 2014). The layers consisting patterned plastic films and double-sided PSA tapes were assembled and bonded layer-by-layer (figure 5(a)).
Adhesive bonding is also capable of being integrated with soft lithography for bonding PDMS. PDMS is known to be difficult to bond with adhesive tapes due to its hydrophobic nature. Therefore, a PDMS/tape composite structure using xurography for bonding PDMS was developed (Kim et al 2009). To create PDMS/tape composite, PDMS was spin coated on a double-sided tape and then patterned using cutting plotter for creating microchannels. Then, the patterned layer can be bonded to almost any substrate such as glass, silicon, and polymers. It was reported that this method showed a higher bonding strength compared to plasma bonding (kim et al 2009).

Thermal bonding
Thermal bonding is a popular bonding method used in fabrication of thermoplastic microfluidic devices (Cassano et al 2015). In thermal bonding the temperature of one or both materials goes up to or above glass-transition temperature (T g ). Following that a force is applied across the layers. One drawback of thermal bonding is the possibility of deformation of the features by heating and formation of bubbles between the layers (Gale et al 2018).
Thermal bonding has been used for bonding multi-layered xurographically based microfluidic devices as well. In one example, COP films were patterned using a cutting plotter and after alignment, the layers were bonded using a hot press at 125 • C for 5 min. It was reported that the complete fabrication process was less than 30 min (Do et al 2011). In another example, thermopress bonding that is a thermal bonding method, was used for fabrication multilayer microfluidic devices made from layers of polystyrene (PS) films using craft cutter ( figure 5(b)) (Cao et al 2015). After alignment of patterned PS layers, a hydraulic hot press was used to bond the layers with sufficient bonding strength. A L-shaped passive mixer was used to test the validity of this fabrication method and characterize the bonding. Bonding strength tests revealed sufficient tensile pressure results (ranging from 43.5 KPa to 375.5 KPa) for microfluidics devices. Small bonding deformations were reported as the result of heat (Cao et al 2015).
A novel thermal bonding technique based on vacuum bagging has also been used for bonding thermoplastic microfluidic devices. Vacuum bagging is a widely used technique used in automobile and aerospace industry to manufacture robust laminated composite materials (Williams et al 1996). The process uses a pressure differential on two sides of flexible membrane to create uniform lamination. It forces a conformal interaction between substrates resulting in more surface contact to produce a strong and optically transparent bond. The technique was used to bond cyclic olefin copolymers (COCs) substrates or multiple layered devices ( figure 5(c)). This technique is simpler and faster compared to other thermal bonding methods (Cassano et al 2015). However, the method requires a clean and smooth polymer surface for strong bonding. Moreover, the vacuum intensity and its uniformity both play a critical role in bond strength. So, non-uniform or unstable vacuum can result in batch-to-batch variations.

Solvent bonding
Solvent bonding is a simple and low-cost bonding method which is mostly used to bond the identical polymeric material layers with each other. It involves addition of solvents like ethanol, and cyclohexane, to partially dissolve and bond polymer layers ( figure 5(d)). Solvent bonding provides high quality bonding with minimized channel deformation (Azouz et al 2014). This bonding method has been used for making xurography based microfluidic devices as well. A COC based microfluidic device was fabricated using xurography and solvent vapor bonding. In solvent vapor bonding, solvent vapor exposure increases the polymer chains mobility and enables them to diffuse at the bonding interface to form covalent bonds. It has been shown that surface roughness of the polymeric channels, fabricated by rapid prototyping techniques, decreased using solvent vapor bonding (Azouz et al 2014).

Xurography for fabrication of microfluidic devices
Xurography has been used to pattern multiple twodimensional materials as discussed in section 3. In this section, we have focused on the application of these patterned materials in microfluidic device fabrication. Xurography can be utilized for fabrication of microfluidic devices with different approaches such as making multilayered devices, micromolds, shadow masks, and integration of electrodes into microfluidic devices.

Fabrication of multi-layer microfluidic devices
Fabrication of microfluidic devices can require integration of multiple layers either made of the same or different materials. These multiple xurographically patterned layers are stacked, aligned, and bonded together to manufacture the final device. A list of examples of multi-layered microfluidic devices fabricated by xurography along with the cutting plotter, material, bonding type, and application information is summarized in table 2. In one of the first studies, two and three-dimensional channels were cut out from a thermal laminate film (thickness 127 µm) (Bartholomeusz et al 2005). Multiple straight (width >50 µm) and coiled channels were patterned along with coupling holes, access ports and alignment holes. The coupling holes were used to connect the channels if needed while the alignment holes were made to align the layers while assembling the device. Alignment holes were able to align the channels within 60 µm accuracy using glass capillary tubes as alignment pins. After alignment, layers were attached together and laminated using a heat laminator. The study demonstrated the fabrication of a 7-layered microfluidic device in less than 30 min. Later, a study provided minimum energy-based models for simulation of liquid filling in planar microfluidic systems and validated the results with experiments (Treise et al 2005). A plotter was used to fabricate planar microfluidic devices out of sheets of vinyl, Plexiglas, and scotch tape layers. This fabrication method was chosen to provide the ability to test different component shapes rapidly.
Multi-layered microfluidic devices have been utilized for various applications such as micromixers (Do et  A microchannel mixer has been fabricated using a desktop craft cutter, doublesided PSA tape and laser printer transparency film (Yuen and Goral 2010). They demonstrated channels as thin as 200 µm in width. Another study fabricated 3D microfluidic devices using a maskless direct writing technique by xurography (Do et al 2011). A cutting plotter was used to write the designs on polymer substrates. The depth of the features was controlled by cutting force. They were able to fabricate microchannels as small as 20 µm width. Another study demonstrated COC film based multilayer microfluidic devices, made using xurography and lamination using cyclohexane vapor exposure (Azouz et al 2014). COC films were chosen because of their properties such as chemical resistance and high optical transparency, which makes them suitable in case of using organic solvents and optical sensing. They also optimized the exposure and compression time for a strong bonding and showed the capability of the microfluidic device to withstand back pressures as high as 23 MPa. A microfluidic mixer and a microfluidic device integrating a polymer monolithic column within the channel, were fabricated to illustrate the functionality of this fabrication method, especially for UV-transparent microfluidic devices.
A new study assessed the use of xurography as a rapid and low-cost tool for the fabrication of point-of-care micromixing devices (figure 6(a)) (Martínez-López et al 2016). They also studied various cutting conditions and materials by fabricating a T mixer design as a sample. An absolute dimensional error was reported to be less than 8% even after changing experimental setup such as the blade and plotter. Later, the same group showed that xurography had better dimensional accuracy for the fabrication of micromixer arrays compared to laser ablation (Martínez-López et al 2017).

Microfluidic devices for sensing applications.
Xurography has been utilized for making multi-layered microfluidic devices used for sensing purposes. In one study, xurography was used for the fabrication of a simple, low-cost, and disposable microfluidic device for ultrasensitive Salmonella typhimurium (S. typhi) detection by magneto-immunoassay integrated with electrochemical detection using gold nanoparticles (AuNPs) as a label (de Oliveira et al 2018). The device was composed of an array of screen-printed carbon electrodes for the detection of S. typhi in milk samples with a total assay time of 1.2 h. A cutting plotter was used to cut the vinyl adhesive sheet based on the layout of eight working electrodes, a pseudo-reference electrode, and a counter electrode. Then the undesired parts were peeled off and a negative stencil forming the layout for electrodes was obtained. Next, this stencil adhered to a polyester sheet and carbon ink was deposited on the sheet and cured to create the working electrodes. Then the stencil was removed, leaving behind the printed electrodes. The same process was used for the  fabrication of the pseudo-reference electrode. In the end, a double-sided adhesive polystyrene card, pre-cut in the shape of a microchannel, was used to attach these two electrode sheets after alignment. The magnets were also attached to the working electrodes using a double-sided adhesive. Another study used xurography to fabricate a low-cost microfluidic chip for detection of microalgae using fluorescence (Gosset et al 2018). The microalgae were detected using a sample volume of 15 µl. A recent study has demonstrated a five-layer xurographically made microfluidic device for an aptamer-based chemiluminescence assay. The device exhibited a broad range of detection 0.01-1000 ng ml −1 due to the efficient mixing of gold nanoparticles, aptamers, and the analyte (Wang et al 2022).
In a later study, xurography has been used to integrate hydrogels into microfluidic devices using a porous membrane by a multi-layer stacking approach (Shahriari and Selvaganapathy 2022). In their method, a porous membrane was sandwiched between two layers of double-sided Kapton tape which were cut based on the defined design for hydrogel. Then, open sections of the membrane were filled with hydrogel ( figure 6(b)). This sandwiched layer containing hydrogel was attached to other adhesive layers which were also cut using xurography to form the final microfluidic device. The manufactured device was used to concentrate and quantify DNA in the hydrogel.
Cutting plotters have been used to fabricate paper-based microfluidic devices for colorimetric assays (Fenton et al 2009, Koesdjojo et al 2015. One of the earliest studies has reported the use of xurography to pattern multiplexed lateral flow assay devices (Fenton et al 2009). The study demonstrated the patterning of nitrocellulose membrane and chromatography paper to fabricate a multiplexed device. A three-channel dipstick was fabricated to measure glucose and albumin in the urine. Another study patterned parafilm and papers using a cutter plotter to manufacture a paper-based colorimetric device (Koesdjojo et al 2015). The paper-based device was fabricated using alternate layers of paper and parafilm and the final assembly was laminated using a thermal laminator to melt the wax for creating hydrophobic barriers. The device was used for quantitative colorimetric analysis of Fe 2+ and Cu 2+ using a phone camera.

Microfluidic devices for cell culture studies.
Diffusion-based microfluidic devices are used for cell culture studies because they can be used to create chemical gradients without any shear stress on cells. Xurography has been used as the fabrication tool for a construction of simple microfluidic chip for the generation of controlled diffusive chemical gradients for dynamic cell assays (Atencia et al 2012). This microfluidic device was used to monitor mammalian cell response to the concentration gradient of a toxin. The microfluidic chip consists of two sections: the bottom layer with buried channels and the top layer with the main channel. The bottom part of the device includes a double-sided adhesive containing the buried channels attached to a glass slide. The upper liner of the adhesive was partially removed. The polystyrene layer with cut-off holes to form the vias and alignment tab was placed over the buried channels. After alignment, the tab was pressed back and attached to exposed tape and the remaining liner was peeled off and the polystyrene layer was folded down. Similarly, the top section was fabricated using tabs to adhere to double-sided tape and mylar film. Alignment tabs were necessary to keep the top and bottom sections open for loading the cells in the buried channels. Another study demonstrated a xurographically patterned passive perfusion device for end-point cell migration assays and other cell culture studies (Goral et al 2015).

Microfluidic devices as fuel cells.
Microfluidic enzymatic fuel cells have been reported to show higher power densities compared to macro fuel cells. Xurography has been used for the fabrication of microfluidic fuel cells (Rewatkar et al 2019). A study demonstrated the use of pyrolyzed photoresist film and polymers like PET, polypropylene, and cyclic olefin, patterned using xurography to manufacture a microfluidic enzymatic fuel cell (González-Guerrero et al 2013). They fabricated the pyrolyzed photoresist film electrodes on silicon wafers and integrated the electrodes into the microfluidic channel fabricated by xurography. Later, another study introduced a xurographically fabricated miniaturized and low-cost microbial fuel cell for sensing biological oxygen demand (figure 6(c)) (Nan et al 2020). The sensor was fabricated in less than 10 min and cost less than 0.5$ USD. The microfabricated sensor reduced the response time from 5 d (conventional BOD test) to 1.1 min.
These microfluidic biofuel cells have also been used to power sensors. A study demonstrated a wireless sensor powered by a 3D membraneless enzymatic glucose biofuel cell by stacking patterned polyethylene naphthalate and doublesided adhesive layers to form two T-shaped microchannels (Desmaële et al 2015). On the walls of both channels, gold electrodes have been coated. Renaud et al extended these 2D T-shaped microfluidic enzymatic fuel cells to multi-level, including several single microchannels (2015).

Microfluidic devices as preconcentrators.
Microfluidic devices fabricated using xurography were also used for biomolecule preconcentration (figure 6(d)) (Yuan et al 2015, Phan et al 2016. Preconcentration within a microfluidic device can be performed using processes including controlled evaporation (Zhang et al 2013), and concentration polarization (Yuan et al 2015, Phan et al 2016. A disposable and portable microfluidic device was fabricated using the xurography method for viral sample concentration in less than 30 min. A multilayer device was fabricated to preconcentrate biomolecules based on ion concentration polarization (Yuan et al 2015). The device was constructed using a patterned doublesided adhesive, two glass slides and a nafion membrane. The material was patterned, and the device was assembled in less than 15 min which makes the method suitable for rapid prototyping preconcentrators with different designs. Later, a study introduced a paper-based channel along with nafion for controlled fluid handling and effective reagent storage (Phan et al 2016). The device was fabricated using xurography in less than 20 min and can attain preconcentration within 200 s.
A recent study has fabricated miniaturized isoelectric gates using agarose to separate biomarker proteins and concentrate and quantify them within 20 min. The device was fabricated using patterned PET sheets bonded with adhesive tapes and the agarose was pipetted into the channels to create the isoelectric gates . Later, a three-reservoir device was fabricated using PET sheets and adhesive tape to separate protein C from human plasma samples. The device was modified for two-stage separation by immobilizing beads on a polyester membrane (Damodara et al 2022).
Xurography has been combined with other methods such as CNC milling and laser micromachining for the fabrication of multilayer microfluidic devices. CNC was used to create highaspect-ratio parts like polymer substrates for reservoirs, and the cutting plotter was for patterning low-aspect ratio materials like adhesives. These layers were aligned and laminated together to fabricate the device (Kido et al 2007). Later, multiple studies have used similar CNC and xurography combination to fabricate a novel seven-layered microfluidic centrifugal device for the preconcentration of aqueous samples Salin 2009, Lafleur et al 2010).

Other applications.
Microfluidic devices have been used to study the dissolution of transparent solid reactive substrates because these devices are suitable to control the flow parameters (Neuville et al 2017). A microfluidic cell was fabricated using xurography to study the dissolution behavior of transparent reactive materials. Using this microfluidic cell, the dissolution rate of a calcite window by water and hydrochloric acid was quantified. Another study has combined xurography with laser micromachining for patterning PVC foils and Ceram tapes (Kojic et al 2019). They determined optical, mechanical, and temperature properties of the fabricated devices were similar to the devices fabricated using conventional techniques.

Fabrication of micromolds
Xurography was also used for the rapid prototyping of micromolds. A list of examples of xurography application for fabrication of micromolds along with the cutter plotter, materials, minimum feature, and bonding type information is summarized in table 3 Photolithography is the most popular method for fabricating micromolds. However, the process is time-consuming and expensive and requires clean room facilities (Kang et al 2017). Fabrication of micromolds by xurography can be a simple, rapid, and low-cost substitute for photolithography and making PDMS-based microfluidic devices. Master molds were created by cutting various adhesive tapes such as 3M Blue Platinum, Kapton, PVC (figure 7(a)) (Speller et al 2019), and plastic films (Ling et al 2017).
One of the earliest studies used xurographically patterned thermal laminate sheets and Rubylith to fabricate the micromolds (Bartholomeusz et al 2005). PDMS was cast using the conventional soft lithography technique on micromolds. The fabricated thermal laminate micromolds were reusable (Bartholomeusz et al 2005).  Later, the cutting plotter was used for creating a mold for analyzing the effect of microbubbles on the distribution of RBCs in flow in microchannels to study the effects of gas embolism on the flow of the cells such as RBCs and the cellfree layers (Bento et al 2017). The mold was cut based on the design of the channels and it was transferred into a petri dish using an adhesive. Next, PDMS was poured into the petri dish and cured in the oven. Another layer of PDMS was cured on a glass slide and it was used to seal the channel. It was reported that the whole mold fabrication process using xurography takes less than 5 min and features as small as 200 µm with a height of 60 µm can be obtained (Speller et al 2019). The geometrical feature of the PDMS microchannels fabricated out of master molds fabricated using this method was also studied (Pinto et al 2014). It has been shown that the difference between the actual dimensions of PDMS channels and  Bartholomeusz et al (2005) NR: Not reported. designed dimensions increases as the size of the geometries decreases ( figure 7(b)). This can be due to the limitation of the cutting plotter for cutting the smaller features. Multilevel master molds for creating multilevel microfluidic devices can also be fabricated by xurography (Hwang et al 2013). Multistep photolithography, which is mostly required for the fabrication of multilevel master molds is an expensive and complicated process. Xurography can decrease the cost and complexity of the fabrication process. In one example, a multilevel mold by cutting and stacking multiple layers of adhesives was created. Next, a PDMS-based device was fabricated out of this mold for 3D cell culture and cell-based assays (figure 7(c)) (Hwang et al 2013).
In this technique for the fabrication of molds, there is the possibility of deformation of small cut mold pieces. Therefore, using an engraved sheet and double casting was proposed. First, the mold was fabricated by patterning 0.3 mm thick films of cast-coated paper or silicone rubber using a cutting plotter. Then, the secondary mold was replicated from this mold using PDMS. Finally, the PDMS chip was made from this negative PDMS mold. The cutting accuracy and pattern transition were characterized. It was shown that the minimum feature width was 0.4 mm for a 0.3 mm thick castcoated paper or silicone rubber (figure 7(d)) (Nagai et al 2018). A recent study has used the xurography-based PDMS micromolding to create superhydrophobic microchannels. The method demonstrated the fabrication of superhydrophobic microchannels with a water contact angle of 152 • and a minimum feature size of 300 µm (Phan and Kim 2022).

Fabrication of masks
Xurography is a simple and low-cost tool for the fabrication of masks for micropatterning as well. In the table 4, a list of examples of xurography application in making masks for micropatterning along with the cutter plotter, materials, minimum feature, and application information is presented. In the very first study, a shadow mask was created by patterning Rubylith (used as a photomask for screen printing; consists of a UV opaque emulsion on a clear polyester backing) using xurography (Bartholomeusz et al 2005). An application tape was used to transfer the cut patterns to a glass slide and then the application tape was peeled away. Next, a layer of silicon was deposited on glass slides using a sputtering system and channels were formed after peeling off the Rubylith layer (Bartholomeusz et al 2005). In another example, a cutting plotter was used to fabricate a structured electrode layer (Sonney et al 2015). First, xurography was used to cut a vinyl electrode mask that was attached to a PS substrate based on the shape and dimensions of the electrodes. Then, a 50 nm thick gold film sputtered and features as small as 100 µm were created on the film ( figure 8(a)).
Xurography-based masks have also been used for making glass microchannels by wet chemical etching (de Santana et al 2013). Vinyl adhesive film was used as the mask material and it could resist HF. After patterning the adhesive film, it was transferred to the surface of a borosilicate glass slide using an application tape. Next, the exposed parts of the glass were etched with HF solution. Figure 8(b) shows images of a vinyl mask with 220 µm of width and a glass microchannel with widths of 270 µm at the top and 190 µm at the bottom. The performance of the fabricated microchannel was tested by an electrophoresis microchip. It was shown that this fabrication technique can generate reproducible glass microchannels while the electrophoretic separation quality was not affected by the roughness created in the channel walls. Xurography made the mask fabrication process for glass wet chemical etching faster and cheaper compared to existing methods. Mask fabrication is performed in a smaller number of steps without the need for a thorough cleaning process of the glass substrate and long baking steps compared to photolithography and toner-mediated methods. Mask preparation using this method only takes a few minutes (about 3 min) which is much less compared to other methods. Moreover, the cost of each mask has been estimated to be about 0.02$ which is the least compared to other methods. This method has also been used for the fabrication of flexible microfluidic devices for CO 2 gas generation and absorption (Yuen and DeRosa 2011). A craft cutter was used for cutting a protective mask from a white vinyl self-adhesive sheet to create 3D interconnected microporous structures on a polystyrene film using a solvent/non-solvent mixture. This method has also been used for making masks for electroplating (Novak et al 2013). Xurography was used to pattern vinyl adhesive to produce masks for electroplating nickel molds on steel wafers ( figure 8(c)). These nickel molds were used to produce hot-embossed plastic microfluidic devices. Using this mask fabrication method, nickel molds were prepared in a rapid and low-cost manner (the required time was as low as 1 h and a single mold cost about 2$). Microfluidic devices such as valves and droplet generators with 100 µm resolution were produced using this method.

Integration of conductive electrodes in microfluidic devices
Conductive electrodes play a significant role in microfluidic devices for heating, electrokinetic transportation, and electrochemical sensing purposes. Most of the current techniques for the integration of electrodes into microfluidics are based on expensive and precise electronic industry fabrication techniques such as evaporation and sputtering combined with photolithography. However, microfluidic devices in lab-on-chip applications do not require high resolution and feature sizes smaller than a micrometer. Therefore, a lowcost method with sufficient resolution for the integration of electrodes can be promising. Xurography can provide these requirements without the need for a clean room environment (Mohammadzadeh et al 2019, Patel et al 2022a. A study demonstrated the fabrication of a seven-layered microfluidic cartridge for an electrochemical biosensor with three porous membranes (Kim et al 2014). The layers consist of a gold electrode substrate which is made by coating gold on PET film, adhesive tapes, and acrylic sheets. However, the electrodes were patterned indirectly by patterning the passivation films. Later, a study integrated 3D structured electrodes within PDMS microfluidic devices (Sonney et al 2015). They used xurography to cut vinyl masks on a polystyrene substrate for gold sputtering. Also, they used xurography to create molds for the PDMS microfluidic channel. The electrode integration was beneficial for electrochemical analyte detection.
Metal wires can also be integrated as electrodes in the microfluidic channels. One such study demonstrated the integration of microwires (diameter 15 µm) into PDMS microfluidic devices (figure 9(a)) (Liu et al 2017). A slit was cut on cured PDMS surface using the cutting plotter and the metal microwire was incorporated into the slit using a specialized tip. The microwire-integrated PDMS layer was sealed using a wet bonding method. The study used this method for a variety of applications such as microfluidic heaters, electrochemical sensors, mixers, and the optical waveguide. This method is limited to the integration of wires in PDMS and is not applicable for planar forms metals.
Another low-cost method to integrate electrode in microfluidic channels is fabricating electrodes using screen printing on the polymer. A study constructed multiplexed channels (8-16 channels) with each channel integrated with one electrode microfluidic devices for electrochemical sensing applications (Fava et al 2019) (figure 9(b)).
Xurography was used in combination with cold lamination for the integration of electrodes into microfluidics applications such as electro sensors, electro mixer and   The metal films such as copper composite foils (thickness 9 µm) were laminated between two layers of dicing tape. The dicing tape becomes non-sticky after exposure to UV light which assists in keeping the electrode pattern intact and peeling off the unwanted parts by exposure to UV. A minimum feature size of 66 µm was obtained in the study. The patterned electrode layer was attached to the other layers of the microfluidic device using transfer adhesive.
Later, a study demonstrated the fabrication of microfluidicbased fundamental circuit elements in electronics; resistor, inductor, capacitor, and memristor (Stojanović et al 2019). They applied xurography for patterning the design of each component in polyvinyl chloride (PVC) foils and gold leaves as the conductive material. After cutting each layer, they were stacked and thermally laminated. Later, another study demonstrated the direct patterning of metal leaves using xurography to manufacture high-quality metal electrodes (Patel et al 2022a). The study patterned flat metal leaves electrodes with a line width of <100 µm and pitch of <100 µm. The study also demonstrated the patterning of metal leaves in geometric shapes and interdigitated electrodes.
Recently, xurography in combination with screen printing was used to fabricate electrochemical microfluidic sensing devices (figure 9(d)) (Hernández-Rodríguez et al 2020). Carbon electrodes were screen printed with dimensions >300 µm and these electrodes were integrated into a microchannel with dimensions 250 µm (width) and 75 µm (height). The process used a cover layer (laminating pouches) to fill the gaps created by the thick screen-printed electrodes to avoid any leakage in the device. The process was used to manufacture 50 devices in less than 60 min and a manufacturing cost of <0.02 USD per device. Another study integrated interdigitated electrodes in the microfluidic channel (800 µm width × 200 µm depth) to fabricate an AC electroosmotic device. The study showed an increase in mixing with an increase in the number of digits in the interdigitated electrodes. Xurography provides a rapid alternative to optimize the mixing performance of AC electro-osmosis by rapid patterning of different interdigitated electrodes (Wu et al 2022). Table 5 summerizes some examples of electrode integration in microfluidic devcies using xurography along with the cutter plotter, material, bonding, metal coating, and application information.

Conclusion
Xurography has been demonstrated as a low-cost and rapid patterning method which can be used without the need for a clean room setup. This method is simple to operate, but it has limitations such as a larger average minimum feature size compared to the other cutting methods like laser cutting. Moreover, xurography is not able to cut material with higher thickness and the material thickness can adversely affect the minimum resolution. On the other hand, cutting plotter does not have the issues associated with laser cutters such as burn residues and has been used to pattern common materials including PET, PDMS, PVA etc.
Despite widespread utilization and advances made, the method currently has several limitations. The primary one is of the feature sizes that can be produced. This restricts its application in cases where small volume of samples need to be collected and processed such as in neonatal and infant screening. One way to overcome this limitation can be in investigation of better and smaller blades that can be positioned more precisely to cut finer feature. Another can be in integration of ultrasonic vibration into the cutting process so that smoother and finer cutting resolutions can be obtained. Finally, blade tips can be structured using processes such as focused ion beam machining to create sharper tips that can lead to higher resolution in the cutting process and lower forces required for cutting.
Flexible microfluidic devices are also an emerging area for making wearable devices. Xurography fabricated devices are currently not suited for this application because most of the common materials used in the process are not flexible or their flexible is compromised due to multiple layers. Furthermore, thicker and more rigid films are easier to cut than thinner and flexible films. In addition, multiple layers also increase the thickness of the devices that further limits their applications in wearables. Development of new methods to cut ultrathin flexible films using xurography needs to be developed for their application in wearable devices. Functional and active materials such as thermoresponsive, piezoelectric, piezoresistive, chromogenic and semiconducting can be developed in a film form so that they can be amenable to xurographic integration.
Xurography provides flexibility to introduce hydrogels before or during the fabrication process. A few studies have introduced hydrogels in devices fabricated using xurography but it needs to be explored extensively in various applications like reagent storage of assays and cell culture. The process needs to be improved to mass produce these hydrogel loaded devices.
In summary, xurography offers an alternative to the expensive and tedious photolithography method to manufacture microfluidic channels larger than 20 µm which is sufficient for multiple applications like concentration polarization, micro mixing, electrochemical sensing, colorimetric sensing, and others. The method can be scaled easily to manufacture lowcost microfluidic devices.

Data availability statement
No new data were created or analysed in this study.