Molding and bonding of thermoplastic microfluidic devices

• Main Author: Yu, Hao
• Other Authors: Tor Shu Beng
• Format: Theses and Dissertations
• Language: English
• Published: 2015
• Subjects: DRNTU::Engineering::Manufacturing::Polymers and plastics
• Online Access: https://hdl.handle.net/10356/62932
• Institution: Nanyang Technological University

Over the past two decades, many innovative microfluidic applications have been explored on PDMS-based devices. However, the commercialization demands have driven people to use thermoplastics as alternatives, which are more cost-efficient and amenable to high volume production process, such as micro injection molding. To address the demands of inexpensive, disposable and functional thermoplastic devices, the abilities to replicate high fidelity substrates using robust micromold at high volume, and to bond substrates with desired strength and microchannel integrity are necessary. Thus, this thesis deals with the practical fulfilment of these abilities by optimizing micro injection molding process for high fidelity and reliable replication using robust micromold, and bonding thermoplastic substrates with improved bond strength and microchannel integrity. In this thesis, bulk metallic glass (BMG) micromold, fabricated by micro hot embossing was used in micro injection molding for high volume replication. After molding process optimization, high-fidelity microchannels can be replicated with less than 1% relative error in both depth and width. It was also found that the molding-induced residual stresses around well-formed microchannels were reprehensible on irregular microchannel deformation during the subsequent thermal bonding. Further optimization of molding process with reduced molded-in residual stresses was conducted, which can effectively reduce the deformation irregularity from 42.7% to 8.1%. These investigations have deepened our understandings of micro injection molding parameters towards higher fidelity and more reliable replication of thermoplastic microfluidic substrates. Thermal bonding has been initially used to seal microchannels due to its simplicity and material homogeneity; however, it was always difficult to obtain high bond strength and high microchannel integrity together. Therefore, we proposed two new approaches to address this issue. Firstly, a low temperature thermal bonding method combined with oxygen plasma treatments and low-Tg polyvinyl alcohol (PVA) coating was proposed. The effects of surface modification on surface properties and bonding performance were studied. It was found that with improved surface energy and stable hydrophilicity (over one month), the coated substrates can be bonded at 30 °C lower than the glass transition temperature (Tg) with excellent integrity as well as sufficient bond strength for reliable micro-droplet applications. Secondly, we proposed a rapid bonding enhancement method by applying auxiliary ultrasonic actuation on intimate bonding interface. The effects of ultrasonic actuation on interfacial temperature and bonding performance were evaluated. It was found that ultrasonic energy can elevate the bonding interfacial temperature by about 10 °C and rapidly improved the bond strength by about 80% within 10 seconds, while the microchannels were free from undesired deformation due to low bulk temperature.