Elsevier

Journal of Manufacturing Processes

Volume 35, October 2018, Pages 107-117
Journal of Manufacturing Processes

Flexible micro manufacturing platform for the fabrication of PMMA microfluidic devices

https://doi.org/10.1016/j.jmapro.2018.07.030Get rights and content

Abstract

In this work we present a micro manufacturing platform for the production of polymeric microfluidic devices on a mass scale, based on the integration of microinjection moulding and femtosecond laser (fs-laser) micromachining technologies. A mould prototype was designed for the fabrication of polymeric thin plates characterized by simplified microfeatures representative of typical Lab-on-a-Chip (LoC) devices. The injection moulding master tool includes replaceable metallic inserts, which were fabricated by exploiting the extreme flexibility and accuracy of fs-laser milling. Here, the laser process parameters have been studied and properly adjusted to meet the target geometry and surface quality of the mould inserts, which were subsequently characterized by confocal and SEM microscopy. The micro injection moulding (μIM) process parameters for the device production have been defined by complete three-dimensional filling and packing process simulations. Finally, the micro-injection mould with reconfigurable inserts was employed for the production of thin plates with simplified microfeatures using PMMA. The ability to reproduce these microfeatures via μIM is an essential step to approach to the mass-production of a polymeric LoC and the use of replaceable micro-inserts fabricated by direct fs-laser ablation promises high flexibility in the design and manufacturing of such devices.

Introduction

Traditional approaches for LoC fabrication rely largely on the use of conventional inorganic substrates, such as silicon and glass [1]. The increasing demand for low-cost microfluidic chips has driven the technology towards polymeric materials, such as polymethylmethacrylate (PMMA) [2]. Polymers are crucial for the realization of optofluidic devices for their intrinsic properties (mechanical strength, optical transparency, chemical stability and biocompatibility) and their easy processability. Furthermore, due to the ability of polymers to accurately replicate the target surface, small features down to tens of nanometres can be accurately fabricated [3]. Nikcevic et al. [4] produced injection moulded PMMA capillary electrophoresis disposable chips. To strengthen their conclusion the authors tested the optical properties of several selected polymers and chips with laser-induced fluorescence detection and they concluded that PMMA was the right material for injection moulded chips.

However, despite the use of potential low-cost materials and the advantages that LoCs can provide in many fields, these devices are still experiencing a limited market penetration due to complexity in their fabrication, represented by large number of microfeatures with high aspect ratio to be reproduced on the same substrate with tight tolerances [5]. In order to guarantee the commercial success of such devices, which appears to be inevitable due to their many groundbreaking applications, several technical hurdles related to their fabrication must be overcome. In particular, LoCs manufacturing technology is expected to meet the following requirements: (a) to be low cost and flexible in order to be easily adapted to any change of the LoC design; (b) to be suitable for a mass scale production; (c) to allow the on-chip integration of imaging systems. Hence, a flexible manufacturing platform enabling fast verification of different designs would be very beneficial for the development of LoC devices.

The flexibility of the manufacturing technology, in particular with regards to the range of scales that can be incorporated into a single device and adaptability to different designs involving microfeatures, is a master key in LoC device production.

A direct fabrication of LoCs can be accomplished by using micromilling, which is a well-established and quite flexible mechanical tooling process, which starts from a bulk polymeric material [6]. However, the manufactured patterns usually display low resolution and poor surface quality, due to the size of the milling tool.

Laser microstructuring is a novel direct manufacturing platform, which offers large flexibility and shows great potential for LoC fabrication. Laser ablation using nanosecond laser pulse is a state-of-the-art technology in industrial manufacturing. However, the long duration of the pulses and the pulse energy required for ablation yields a strong thermal load on the material. Hence, the microfeatures undergo severe modifications, being affected by cracks, burrs and recast layers. Consequently, the achievable dimensional accuracy does not meet the requirements for the manufacturing of micrometer-sized structures. Femtosecond or picosecond laser pulses can accomplish the target geometrical precision since the reduced time duration ensures minimal thermal damage and enables precise ablation of polymers with minimal thermal damage to the surroundings [7]. Owing to the intrinsic properties of focused ultra-short laser pulses, this approach can be more reliable and accurate than other machining methods like micro-EDM, which is affected by tool wearing [8] and very slow material removal rates [9]. However, fabrication of the whole LoC device by ultrafast laser ablation is slow and very expensive. Therefore, this technology can be suitable for rapid prototyping or fabrication of small series [10] but cannot be employed for the cost-effective production of LoC devices on a mass scale.

Injection moulding and hot embossing are replication technologies that can respond to the previous requirements and are suitable for mass production. However, these conventional technologies present huge limitations in reproducing very small components, due to the amount of material managed with poor metering precision for such small dimensions and low injection speed with the risk of short shots due to the fast cooling of the polymer melt. μIM, instead, is a leading technology for the mass production of a polymeric microcomponent, such as microfluidic devices, due to the very small amount of material managed with high dosing precision and high injection speed. However, this technology appears still limited in flexibility since it requires master tools [11].

The wide range of polymers allows the manufacturer to select the suitable one satisfying the requirements for a specific application and providing more freedom to geometrical designs. One of the main limitations of the μIM process is the low flexibility related to the mould configuration. Once the geometry of the cavity is defined and the negative one reproduced on the mould, any modification or reconfiguration is very expensive so that sometimes it can be more convenient to produce a completely new mould. The main advantage of using a mould with changeable inserts is displayed by the ability to test different part geometries (removable cavities) using the same master mould plates, specifically designed for the injection of micro-components [12]. The fabrication of a reconfigurable mould for μIM is a challenging task due to the small dimensions of the whole master mould and high accuracy required for the features. Chen et al. [13] realized biochip LoC devices via μIM. LIGA-like processes using a UV light aligner were first implemented to prepare a silicon-based SU-8 photoresist, followed by electroforming to make a Ni–Co-based biochip mould insert. PMMA, PC, COC, and PS were selected as suitable polymers for micro injection moulded biochip devices with arrays of micro-channels of about 30 μm depth, 100 μm width and 50 μm of pitch size. The experimentation showed better results with COC compared to PC, PS, and PMMA.

Laser micromachining is a valuable technology for the fabrication of micro-injection mould inserts with complex shapes, displaying a high level of accuracy [14]. Indeed, laser ablation using ultra-short pulses is a versatile tool for machining 3D micro-structures into metals [15]. In particular, laser micro-milling based on laser ablation with ultra-short pulses is gradually emerging as a highly flexible technology enabling material removal in a layer-by-layer fashion with a micrometric precision [16].

In this paper, a novel cost-effective and flexible manufacturing platform for the fabrication of polymeric LoCs, which takes advantage of the combination of microinjection moulding and fs-laser milling technologies, is presented. While microinjection moulding ensures reliability and reduced costs for large-scale production, the use of application-tailored inserts in a master mould provides flexibility to the technology. Very small inserts were effectively structured by high-resolution fs-laser micro-milling. This novel approach overcomes the limitations of each technology taken individually and gives an important contribution to the current state of the art of the manufacturing of polymeric microdevices such as LoC.

In section 2 the layout of a concept polymeric LoC device is described, which has been chosen as a benchmark to prove the capability of the novel manufacturing platform. The main microfeatures, identified as most critical from a micro-manufacturing point of view, are highlighted and the mould design is presented.

In section 3 the employed materials and experimental setups, as well as the μIM process simulation settings are described.

In section 4 results of simulations, fs-laser fabrication of the mould insert and μIM are presented and discussed. Here, verification measurements of the finally produced polymeric device are also presented to validate the functionality of the proposed micro-manufacturing platform.

Section snippets

Design of concept LoC and reconfigurable mould

The polymeric LoC device sketched in Fig. 1 is proposed, in this paper, as a benchmark to demonstrate the feasibility of combining fs-laser and μIM micro-manufacturing technologies. It represents a polymeric optical stretcher for single cell manipulation, consisting of two layers to be aligned and assembled after fabrication through μIM. The bottom layer includes a microchannel and two grooves perpendicular to the channel separated from the latter by small walls. The two grooves have been

Materials

PMMA is one of the hardest thermoplastics exhibiting high scratch resistance, low moisture and water absorption, which guarantee good dimensional stability and excellent optical properties. Due to its good degree of compatibility with human tissue, it can be considered as an ideal candidate for microfluidic applications. For the present work, the polymer Acryrex CM211 PMMA (Polymethylmethacrylate) – CHIMEI has been selected.

The material chosen for the fabrication of the master mould for micro

Simulation analysis

A screening simulation analysis was used to identify a set of process parameters useful for the experimentation. Starting from previous experiences on the PMMA material, we aimed at defining a suitable set of process parameters through a small number of simulation experiments in order to reduce as much as possible the experimentation for the microinjection moulding of the component. According to the foregoing, we set, as fixed parameters, the injection speed Vinj, the duration of the packing

Conclusions

In this work, the improvement in terms of flexibility of the microinjection moulding technology, based on the use of application-tailored fs-laser micro-fabricated inserts in a standardized mould, has been fully demonstrated. The combined technological approach provides an easy and low-cost method to fabricate polymeric microfluidic devices on a mass scale. This modular approach requires the substitution of only small inserts in the mould when the geometrical features of the device are changed.

Conflicts of interest

None.

Acknowledgements

This activity was financed by the Italian Ministry of Education, University and Research (MIUR): “Progetto Bandiera” of the Factory of the Future Programme – Subproject 1: PLUS – “Plastic Lab-on-chips for the optical manipUlation of Single-cells” (2013–2016), http://www.fabbricadelfuturo-fdf.it/progetti/sottoprogetto-1/progetto-plus/.

Gianluca Trotta holds an MSc degree in Mechanical Engineering (2002) from Politecnico di Bari. From 2004 to 2009 he worked as Laser Technology and Application Specialist at PRIMA Industrie S.p.a. and then as Responsible of the Laser Technology Laboratory of the PRIMA Research Centre of Bari. From 2009 he is a researcher at Institute of Industrial Technology and Automation of the National Research Council of Italy. He deals mainly with research activities related with micro manufacturing

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    Gianluca Trotta holds an MSc degree in Mechanical Engineering (2002) from Politecnico di Bari. From 2004 to 2009 he worked as Laser Technology and Application Specialist at PRIMA Industrie S.p.a. and then as Responsible of the Laser Technology Laboratory of the PRIMA Research Centre of Bari. From 2009 he is a researcher at Institute of Industrial Technology and Automation of the National Research Council of Italy. He deals mainly with research activities related with micro manufacturing focusing on micro injection moulding process and on the evaluation and definition of production methods for polymeric micro systems and devices.

    Annalisa Volpe received her degree (cum laude) and PhD in Physics from the University of Bari in 2012 and 2017, respectively. Since January 2017, she is research fellow at the CNR-IFN. Her research activities are mainly focused on the study of ultrafast laser processes for medical applications.

    Antonio Ancona received his Degree (cum laude) and PhD in Physics from the University of Bari in 1997 and 2002, respectively. Since September 2004 he is a full time technologist at CNR-IFN. His research activities are mainly focused on the study of high power laser welding processes and laser ablation with ultrafast fiber lasers as well as on the development of real-time process sensors. In 2006 he worked as DAAD research fellow at the Institut für Angewandte Physik of the Friedrich-Schiller-Universität Jena (Germany), exploiting leading research in the field of ultra-short pulse micromachining and materials modification for industrial and medical applications.

    Irene Fassi holds a Ph.D. (2001) from Politecnico of Milano and a MSc degree in Mechanical Engineering (1997) from Politecnico of Milano. Since 1998 she has been a full time researcher at CNR-ITIA, where she leads the CNR-ITIA research group MEDIS. She has been involved with research and management role in various regional, national and European projects. She is a member of the Executive Board of SIRI (Italian Robotics and Automation Association), of the ASME/DED Technical Committee on Micro and Nano Manufacturing, and of the Executive Committee of the International Institution for MicroManufacturing.

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