Surface Treatments for Improving the Applicability of Polydimethylsiloxane in Microfluidics
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These days polydimethylsiloxane (PDMS) is one of the most used materials in microfluidic research laboratories because of the easy and rapid fabrication techniques that it supports. PDMS also offers many attractive properties such as biocompatibility, optical transparency, and good thermal and chemical stability. However, the large-scale commercial breakthrough of PDMS is still awaited, owing to certain shortcomings of the material. The main disadvantages of PDMS are its low surface energy and heterogeneous surface charge. These properties limit the separation efficiency, and cause bubble trapping, leading to sample loss and reduced device performance. Given these drawbacks, much effort has been devoted to the development of surface treatments to improve the wetting properties of PDMS. An extensive review of the treatments is included in this thesis. According to the review, present PDMS surface modification techniques are not without their shortcomings. This thesis tackles the problem by developing simple and rapid methods for producing biocompatible and long-term hydrophilic PDMS surfaces. Thirty-nine surface treatments were investigated by measuring static contact angles over a six month period. Surface topography and chemistry were also analyzed. Seven coatings supported a contact angle of 30° or less for at least six months. Only one commercially available polymer and oxygen plasma were needed for a treatment. According to the literature survey, no simpler and more rapid (requiring only minutes to prepare) methods to achieve constantly (contact angle 9° - 22°) hydrophilic PDMS surfaces for at least six-months have been published. The results were utilized to develop a method for fabrication of hydrophilic PDMS structures that have increased separation efficiency. One oxygen plasma treatment was used for bonding of two PDMS components and to tethering of hydrophilic polymer onto their surfaces. The modification reduced the amount of adsorbed bovine serum albumin (BSA) i) for 98.4 %, ii) produced uniform coatings along the channel, and iii) was repeatable. Developed treatment is among the simplest, quickest, and most effective techniques to improve protein repellence of PDMS. The surface treatments of PDMS were developed for Tekes funded NanoFlow and Human Spare Parts projects. The aim was to improve the performance of a PDMS liquid handling system in a commercial surface plasmon resonance (SPR) device, and to improve usability of PDMS in microfluidic cell perfusion systems.