1 Introduction
Micropillar arrays as a value-adding component in microfluidic chips have enabled a wide range of advanced applications, including sensing (Chen et al.
2020), diagnosis (Li et al.
2010), separation (Dalili et al.
2019), 3D cell culture (Liu et al.
2017; Kim et al.
2008) and genome amplification (Tian et al.
2018; Panaro et al.
2005). With larger surface area, micropillar arrays as 3D working electrodes have been shown to enhance sensing performance in microchip-based electrochemical detection systems (Chen et al.
2020; Prehn et al.
2011; Liu et al.
2020). Owing to the capillary effect, a well-defined liquid film can be formed between pillars with carefully engineered spacing and aspect ratio in an array (Semprebon et al.
2014), enabling further applications. It has been reported that micropillar arrays, maintaining electrolytes within the pillar zone, can potentially replace microchannels or gels for electrokinetic separation of proteins to achieve on-chip immunodiagnosis (Li et al.
2010). In previous studies, micropillar arrays with the same wicking effect were used as an ultrashort-pathlength (10–20 µm) cuvette for UV–Vis spectroscopic measurement of high-absorptivity chemicals (Holzner et al.
2015), and as a platform to control the growth of orthorhombic crystals (Holzner et al.
2017). Our recent work has further demonstrated the micropillar arrays as a useful analytical tool to generate spontaneous sample flow and achieve analyte concentration for high-sensitivity measurements (Orlowska et al.
2020). Moreover, micropillar arrays are essential for sorting and separating cells and microparticles in microfluidic systems via the principle of deterministic lateral displacement (Inglis
2009; Chandna and Gundabala
2021). There have also been numerous studies demonstrating that micropillars in a microfluidic chip can help sustain a stable microenvironment for 3D cell culture (Liu et al.
2017; Tomecka et al.
2018). In most cases, the accurate fabrication of the structures is essential for their function. While the fabrication of the aforementioned micropillar arrays was mostly carried out by soft lithography or photolithographic technique with reactive ion etching, the future mass-market need for these microfeature-embedded microfluidic chips has been pressing on manufacturing methods with high production rates. Of these, injection moulding using low-cost polymer materials is promising for its ability to mass-produce microdevices (Attia et al.
2009; Maghsoudi et al.
2017).
There have been studies of injection moulding micropillars using thermoplastic polymers such as polypropylene (Weng et al.
2018,
2021; Lu and Zhang
2009), polylactic acid (Yu et al.
2019), polyether-ether-ketone (PEEK) (Zhang et al.
2016), polyethylene (Zhang et al.
2017), polycarbonate (Zhou et al.
2018a) and other block copolymers (Song et al.
2017; Birkar et al.
2016; Sha et al.
2007). The effects of the process parameters on the moulding process were often the focus of these studies. For example, Li and coworkers have designed and fabricated a disposable blood smart diagnostic chip with an array of micropillars in the flowing channel by injection moulding. They used a styrene acrylic copolymer and investigated the effect of process parameters on the replication quality (Song et al.
2017). A similar structure to micropillars, microneedles were heavily studied for their mass production via injection moulding with polymers such as polycarbonate, PEEK, polystyrene, liquid crystal polymer and so on for the applications in transdermal drug delivery (Juster et al.
2019). These studies particularly highlight the challenge of polymer-filling of microscale features. Successful replication of micro/nano-scale features will continue to be a critical manufacturing requirement as increasingly complex chip designs emerge, and addressing chip material choice will remain an important area of research (Lucchetta et al.
2014; Zhang et al.
2018).
Poly(methyl methacrylate) (PMMA) is a polymer with good mechanical strength and optical properties and is suitable for manufacturing microfluidic devices by injection moulding (Ma et al.
2020). The effect of moulding conditions on the replication of microfeatures on PMMA, including mould temperature, injection velocity (speed), injection pressure/time and packing pressure, have been investigated (Liou and Chen
2006; Chien
2006; Kirchberg et al.
2012). Guided by numerical simulation, there also have been attempts to fabricate nanopillars with PMMA by injection moulding (Jiang et al.
2016; Zhou et al.
2018b). Despite being inadequate in producing nanopillars, these studies have demonstrated the potential of PMMA in moulding micropillars. Cyclic olefin copolymer (COC) is an emerging polymer for injection moulding microstructured polymer parts (Nunes et al.
2010). As a material with low moisture absorption, high optical transparency, resistance to chemical solvents, and biocompatibility, COC has been used to prepare microfluidic devices for DNA stretching (Utko et al.
2011), DNA detection (Geissler et al.
2020), biomolecular separation (Kourmpetis et al.
2019) and sensing (Prada et al.
2019). A study back in 2011 reported the micro-injection moulding of a lab-on-a-chip device with micropillar features in COC, and yet those pillars were shallow with an aspect ratio of 1 or much lower (Oh et al.
2011). COC micropillars with an aspect ratio higher than 1 have been prepared using hot embossing (Geissler et al.
2020; Kourmpetis et al.
2019), and there have been studies on the viscosity of COC at high temperatures to help understand the filling of COC during injection moulding (Lu et al.
2020). However, no comparative studies of micropillar array fabrication in PMMA and COC using injection moulding have been reported.
In this paper, we present a systematic study of injection moulding of micropillar arrays using PMMA and COC. The results obtained under different moulding conditions are compared and analysed in conjugation with the physical properties of polymers. COC proves to be more suitable for creating micropillar arrays, compared with PMMA. Furthermore, we show that the nanoroughness of the mould surface is also replicated more completely in COC. With complex microstructures and interfaces critically important in the operation of many microfluidic devices, including those that contain micropillar arrays, the present findings will prove helpful in guiding further development in the mass manufacture of microscale devices via injection moulding.
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